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- Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
- Kategorie: Fachliteratur
- Serie: Nanotechnology Innovation & Applications
- Sprache: Englisch
This first book to focus on the applications of nanomagnetism presents those already realized while also suggesting bold ideas for further breakthroughs.
The first part is devoted to the concept of spin electronics and its use for data storage and magnetic sensing, while the second part concentrates on magnetic nanoparticles and their use in industrial environment, biological and medical applications. The third, more prospective part goes on to describe emerging applications related to spin current creation and manipulation, dynamics, spin waves and binary logic based on nano-scale magnetism.
With its unique choice of topics and authors, this will appeal to academic as well as corporate researchers in a wide range of disciplines from physics via materials science to engineering, chemistry and life science.
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Seitenzahl: 642
Veröffentlichungsjahr: 2017
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Contents
Cover
Further Volumes of the Series “Nanotechnology Innovation & Applications”
Title Page
Copyright
Dedication
Series Editor Preface
About the Series Editor
Part One: Spin Electronics and Magnetic Sensing Applications
Chapter 1: Introduction on Magnetic Sensing and Spin Electronics
1.1 Magnetic Fields
1.2 Magnetic Field Sensing
1.3 Introduction to Spin Electronics
1.4 Main Applications of Spin Electronics
References
Chapter 2: Spin Electronics for Biomagnetism and Nuclear Magnetic Resonance
2.1 Introduction
2.2 Biomagnetic Signals Detection with Spin Electronics Sensors
2.3 Nuclear Magnetic Resonance
2.4 Conclusion and Perspectives
References
Chapter 3: Large-Volume Applications of Spin Electronics-Based Sensors
3.1 Introduction
3.2 General Concepts
3.3 Read Heads
3.4 Current Sensors
3.5 Angle and Compass Sensors
3.6 Speed Sensors
3.7 Switches and Position Sensors
3.8 Conclusion and Perspectives
References
Chapter 4: Magnetic Random Access Memories
4.1 Introduction
4.2 MRAM General Principles
4.3 Field-induced Switching MRAM
4.4 Spin Transfer Torque Switching MRAM
4.5 Emerging MRAM Technologies
4.6 Conclusions
Acknowledgment
References
Chapter 5: Spin Electronics for Non Destructive Testing
5.1 Introduction
5.2 Basic Concepts of Electromagnetic Testing Methods
5.3 GMR in MFL Testing
5.4 MR and Eddy Current Testing
5.5 Concluding Remarks
Acknowledgment
References
Chapter 6: Diamond Spin Sensors: A New Way to Probe Nanomagnetism
6.1 Introduction
6.2 Magnetic Sensing with Nitrogen Vacancy Defects in Diamond
6.3 Experimental Implementations for Sensing and Imaging
6.4 Applications
6.5 Conclusions
References
Part Two: Magnetic Nanoparticles
Chapter 7: Introduction to Magnetic Nanoparticles
7.1 Introduction
7.2 Main Properties of Magnetic Nanoparticles
7.3 Synthesis of Magnetic Nanoparticles
7.4 Main Classes of Applications of Magnetic Nanoparticles
7.5 Conclusions and Perspectives
References
Chapter 8: Use of Magnetic Nanoparticles in Biomedical Applications
8.1 Introduction
8.2 The Physics of Magnetic Nanoparticles Used in Biomedical Applications
8.3 Applied Nanotechnology: Biomedical Applications of MNP
8.4 Preparation of Magnetic Nanoparticles for Biomedical Applications
8.5 MNP Imaging in Biomedicine
8.6 Summary and Conclusions
References
Chapter 9: Spintronic Biochips: From the Laboratory to Pre-Clinical Applications
9.1 Introduction
9.2 Static Multiplexed Biosensors
9.3 Magnetoresistive Cytometers and the Detection of Rare Cells in Blood/Serum
9.4 Lateral Flow Magnetoresistive Biochips
9.5 Conclusions
Acknowledgment
References
Part Three: Future Applications
Chapter 10: Promising Prospects for Chiral Domain Walls and Magnetic Skyrmions as a New Way to Manipulate and Store Information
10.1 Introduction
10.2 Origin and Consequences of an Antisymmetric Exchange Interaction
10.3 Chiral Néel Walls in Systems with Perpendicular Magnetic Anisotropy and Dzyaloshinskii–Moriya Interaction
10.4 Magnetic Skyrmions in Noncrystalline Materials for Stabilization at Room Temperature
10.5 New Device Concepts Based on Chiral Magnetic Objects
10.6 Conclusions and Perspectives
Acknowledgments
References
Chapter 11: Nanomagnetic Devices
11.1 Introduction
11.2 Memory and Storage-Class Memory
11.3 Logic Devices
11.4 Concluding Remarks
References
Chapter 12: Microwave Nanomagnetism: Spin Torque Oscillators and Magnonics
12.1 Introduction
12.2 Basics of Magnetization Dynamics
12.3 Spin Torque Oscillators
12.4 Magnonics
12.5 Conclusions
References
Chapter 13: Applications of Magnetic Oxides Thin Films and Nanostructures
13.1 Introduction
13.2 Magnetism of Oxides: Theory
13.3 Interest in Oxides: Strong Coupling Between Properties
13.4 Conclusions
References
Index
End User License Agreement
List of Tables
Table 1.1
Table 1.2
Table 1.3
Table 1.4
Table 1.5
Table 2.1
Table 3.1
Table 5.1
Table 9.1
Table 9.2
Table 9.3
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 3.7
Figure 3.8
Figure 3.9
Figure 3.10
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Figure 4.10
Figure 4.11
Figure 4.12
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8
Figure 5.9
Figure 5.10
Figure 5.11
Figure 5.12
Figure 5.13
Figure 5.14
Figure 5.15
Figure 5.16
Figure 5.17
Figure 6.1
Figure 6.2
Figure 6.3
Figure 6.4
Figure 6.5
Figure 6.6
Figure 6.7
Figure 6.8
Figure 6.9
Figure 6.10
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Figure 8.6
Figure 8.7
Figure 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Figure 9.7
Figure 9.8
Figure 9.9
Figure 9.10
Figure 9.11
Figure 9.12
Figure 9.13
Figure 9.14
Figure 9.15
Figure 9.16
Figure 10.1
Figure 10.2
Figure 10.3
Figure 10.4
Figure 10.5
Figure 10.6
Figure 10.7
Figure 10.8
Figure 10.9
Figure 10.10
Figure 10.11
Figure 10.12
Figure 10.13
Figure 10.14
Figure 10.15
Figure 10.16
Figure 10.17
Figure 10.18
Figure 10.19
Figure 10.20
Figure 10.21
Figure 10.22
Figure 10.23
Figure 10.24
Figure 11.1
Figure 11.2
Figure 11.3
Figure 11.4
Figure 11.5
Figure 11.6
Figure 11.7
Figure 11.8
Figure 11.9
Figure 12.1
Figure 12.2
Figure 12.3
Figure 12.4
Figure 12.5
Figure 12.6
Figure 12.7
Figure 12.8
Figure 12.9
Figure 12.10
Figure 12.11
Figure 13.1
Figure 13.2
Figure 13.3
Figure 13.4
Figure 13.5
Figure 13.6
Figure 13.7
Figure 13.8
Figure 13.9
Figure 13.10
Figure 13.11
Figure 13.12
Figure 13.13
Figure 13.14
Guide
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Table of Contents
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Further Volumes of the Series “Nanotechnology Innovation & Applications”
Axelos, M. A. V. and Van de Voorde, M. (eds.)
Nanotechnology in Agriculture and Food Science
2017
Print ISBN: 9783527339891
Cornier, J., Kwade, A., Owen, A., Van de Voorde, M. (eds.)
Pharmaceutical Nanotechnology
Innovation and Production
2017
Print ISBN: 9783527340545
Mansfield, E., Kaiser, D. L., Fujita, D., Van de Voorde, M. (eds.)
Metrology and Standardization for Nanotechnology
Protocols and Industrial Innovations
2017
Print ISBN: 9783527340392
Meyrueis, P., Sakoda, K., Van de Voorde, M. (eds.)
Micro- and Nanophotonic Technologies
2017
Print ISBN: 9783527340378
Müller, B. and Van de Voorde, M. (eds.)
Nanoscience and Nanotechnology for Human Health
2017
Print ISBN: 978-3-527-33860-3
Puers, R., Baldi, L., van Nooten, S. E., Van de Voorde, M. (eds.)
Nanoelectronics
Materials, Devices, Applications
2017
Print ISBN: 9783527340538
Raj, B., Van de Voorde, M., Mahajan, Y. (eds.)
Nanotechnology for Energy Sustainability
2017
Print ISBN: 9783527340149
Sels, B. and Van de Voorde, M. (eds.)
Nanotechnology in Catalysis
Applications in the Chemical Industry, Energy Development, and Environment Protection
2017
Print ISBN: 9783527339143
Edited by Claude Fermon and Marcel Van de Voorde
Nanomagnetism
Applications and Perspectives
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de.
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-33985-3ePDF ISBN: 978-3-527-69905-6ePub ISBN: 978-3-527-69906-3Mobi ISBN: 978-3-527-69907-0oBook ISBN: 978-3-527-69850-9
Thanks to my wife for her patience with me spending many hours working on the book series through the nights and over weekends. The assistance of my son Marc Philip related to the complex and large computer files with many sophisticated scientific figures is also greatly appreciated.
Marcel Van de Voorde
Series Editor Preface
Since years, nanoscience and nanotechnology have become particularly an important technology areas worldwide. As a result, there are many universities that offer courses as well as degrees in nanotechnology. Many governments including European institutions and research agencies have vast nanotechnology programmes and many companies file nanotechnology-related patents to protect their innovations. In short, nanoscience is a hot topic!
Nanoscience started in the physics field with electronics as a forerunner, quickly followed by the chemical and pharmacy industries. Today, nanotechnology finds interests in all branches of research and industry worldwide. In addition, governments and consumers are also keen to follow the developments, particularly from a safety and security point of view.
This books series fills the gap between books that are available on various specific topics and the encyclopedias on nanoscience. This well-selected series of books consists of volumes that are all edited by experts in the field from all over the world and assemble top-class contributions. The topical scope of the book is broad, ranging from nanoelectronics and nanocatalysis to nanometrology. Common to all the books in the series is that they represent top-notch research and are highly application-oriented, innovative, and relevant for industry. Finally they collect a valuable source of information on safety aspects for governments, consumer agencies and the society.
The titles of the volumes in the series are as follows:
Human-related nanoscience and nanotechnology
Nanoscience and Nanotechnology for Human Health
Pharmaceutical Nanotechnology
Nanotechnology in Agriculture and Food Science
Nanoscience and nanotechnology in information and communication
Nanoelectronics
Micro- and Nanophotonic Technologies
Nanomagnetism: Perspectives and Applications
Nanoscience and nanotechnology in industry
Nanotechnology for Energy Sustainability
Metrology and Standardization of Nanomaterials
Nanotechnology in Catalysis: Applications in the Chemical Industry, Energy Development, and Environmental Protection
The book series appeals to a wide range of readers with backgrounds in physics, chemistry, biology, and medicine, from students at universities to scientists at institutes, in industrial companies and government agencies and ministries.
Ever since nanoscience was introduced many years ago, it has greatly changed our lives – and will continue to do so!
March 2016 Marcel Van de Voorde
About the Series Editor
Marcel Van de Voorde, Prof. Dr. ir. Ing. Dr. h.c., has 40 years' experience in European Research Organisations, including CERN-Geneva and the European Commission, with 10 years at the Max Planck Institute for Metals Research, Stuttgart. For many years, he was involved in research and research strategies, policy, and management, especially in European research institutions.
He has been a member of many Research Councils and Governing Boards of research institutions across Europe, the United States, and Japan. In addition to his Professorship at the University of Technology in Delft, the Netherlands, he holds multiple visiting professorships in Europe and worldwide. He holds a doctor honoris causa and various honorary professorships.
He is a senator of the European Academy for Sciences and Arts, Salzburg, and Fellow of the World Academy for Sciences. He is a member of the Science Council of the French Senate/National Assembly in Paris. He has also provided executive advisory services to presidents, ministers of science policy, rectors of Universities, and CEOs of technology institutions, for example, to the president and CEO of IMEC, Technology Centre in Leuven, Belgium. He is also a Fellow of various scientific societies. He has been honored by the Belgian King and European authorities, for example, he received an award for European merits in Luxemburg given by the former President of the European Commission. He is author of multiple scientific and technical publications and has coedited multiple books, especially in the field of nanoscience and nanotechnology.
Part OneSpin Electronics and Magnetic Sensing Applications
1Introduction on Magnetic Sensing and Spin Electronics
Claude Fermon
DRF/IRAMIS/SPEC/LNO, CEA CNRS Paris Saclay, 91191 Gif sur Yvette Cedex, France
This introductory chapter provides the basic knowledge of magnetism and spin electronics, which will help the reader to understand the contents of the book. Then, after a brief introduction to magnetic fields, some bases of magnetic sensing and spin electronics are proposed. The last part of the chapter provides definitions that are useful for understanding spin electronics applications. More in-depth information can be found [1,2]. A number of books have been published on nanomagnetism [3], spin electronics [4,5], GMR [6], and spin dynamics [7], where each particular topic is discussed in detail.
1.1 Magnetic Fields
1.1.1 Introduction
Magnetism and magnetic field are known since thousands of years. First magnetic sensors were compass made of magnetite stones in China during the Han dynasty rule and later used by sailors to navigate. Today, magnetic objects, such as fridge magnets, are used as ornaments or for health purpose. In parallel, electricity is associated with electrons flowing in conductors and its use in domestic applications. Rotating magnetic fields seen by a coil is today the major source of electricity and, inversely, current in a coil produces magnetic fields like in MRI devices. The fundamental reason is that both are, in fact, identical depending on the reference frame taken. This has been highlighted by the well-known Maxwell equations that link electric fields and magnetic fields, one being the derivative of the other.
In parallel to the enormous importance of electricity in our life, electromagnetism has a fundamental property that justifies the billions of magnetic sensors and antennas produced each year: it is the only long-range interaction that we can create, modify, and detect. This long-range interaction property takes various forms. Light is an electromagnetic wave. Radiofrequency transmissions used for radio, TV, or mobiles are electromagnetic waves at lower frequencies. Static or low-frequency magnetic fields are the extremely low or zero frequency aspect of the same interaction.
1.1.2 Magnetic Field, Magnetic Induction, and Units
Historically, the magnetic field has been described by two different quantities. The first one is the field created by a magnet that has been called , the magnetic field intensity. The second one is the field created by a current that has been called , the magnetic induction.
It took some time to reconcile the two quantities that are proportional in the vacuum.
Magnetic field intensity H is given in A/m or in Oersted and magnetic field induction is given in Tesla or in Gauss. They are related by the following relation:
where is the magnetization of the material at the point where the field is measured. In the presence of vacuum or in nonmagnetic materials that quantity is 0. is a constant equal to
A/m is not a very useful quantity for a common comparison, and now nearly everybody is using Tesla or Gauss as a unit both for magnetic field intensity and induction. In this book, we will follow the same use knowing that this is just a commodity.
The relationship between these quantities is given in Table 1.1.
Table 1.1 Main fields units.
Quantity
Designation
Unit
Link
Magnetic field intensity
H
A/m (MKS)
In vacuum
.
Oe: Oersted (CGS)
In vacuum1Oe = 1G
Magnetic field induction
B
T
: Tesla (MKS)
In vacuum
.
G
: Gauss (CGS)
1
G
= 10
−4
T
1.1.3 Magnetic Materials
Materials present various states of magnetism and they are classified into three main classes: diamagnetic materials, paramagnetic materials, and ordered magnetic materials. The first one, diamagnetic materials, corresponds to the large majority of materials. These materials present a very weak magnetization that is proportional and opposite of the applied magnetic field. This magnetization is due to the reaction of electrons. Their magnetization is then simply:
where the magnetic susceptibility is negative of the order of 10−6.
Superconducting materials like Niobium at very low temperature are also diamagnetic, but in that case, the susceptibility is nearly equal to −1.
Other materials, called magnetic materials, present an internal magnetization much higher than diamagnetic materials. That magnetization is created by unpaired electrons.
Magnetic materials are disordered at high temperature and become ordered below a critical temperature. When they are disordered, they are called paramagnetic materials and their magnetization can be written as (Eq. 1.2) with χ positive and relatively large, typically 10−3. Magnetic ordered materials are ferromagnetic, antiferromagnetic, or ferromagnetic. Table 1.2 gives a list of the materials you will encounter in this book with their order type and ordering temperature.
Table 1.2 Main magnetic materials found in this book.
Material
Order
Temperature of ordering (K)
Comment
Co
Ferromagnetic
1388 K
3D metal
Fe
Ferromagnetic
1043 K
3D metal
Ni
Ferromagnetic
627 K
3D metal
Ni79Fe21
Ferromagnetic
553–871
Very soft alloy called micrometal. Ordering temperature depends on crystal structure
CoFe
Ferromagnetic
1360
Used due to its large spin polarization
CoFeB
Ferromagnetic
1300
Used due to its large spin polarization and very soft material
PtMn
Antiferromagnetic
1000 K
Used for spin electronics
IrMn
Antiferromagnetic
700 K
Unsed for spin electronics
Fe3O4
Ferrimagnetic
948 K
Called magnetite
YIG (yttrium garnet)
Ferromagnetic
560 K
Soft magnetic insulator used for its dynamic properties
Nd
2
FeB
Ferromagnetic
593–673
Rare earth-based hard magnet
Co
2
Sm
17
Ferromagnetic
720
Rare earth-based hard magnet
Here, we do not consider pure rare earths that exhibit a larger variety of magnetic ordering. Some of them have a different kind of order as function of the temperature.
1.1.4 Magnetic Field Created by a Magnet
The magnetic field created by a magnet is the sum of the fields created by the individual components of the material. This principle of superposition is very important and is included in the Maxwell equations. This principle applies for both magnetic materials and fields created by electrical currents. However, in the determination of the field created by a magnetic material, one has to take care of the magnetization induced by the field created by the other parts of the magnetic material or by external currents. This field-induced effect is very important when you have magnetic cores inserted in coils.
The field created by a small magnet having a homogeneous magnetization taken, for example, along z at a large distance from it decreases at 1/r3 and has a shape given in Figure 1.1. This shape, called dipolar shape, will appear very often in this book. The formula of this field is as follows:
where is the distance from the small magnet considered as a point (Figure 1.1).
Figure 1.1 Dipolar shape created by a small magnet.
The main features to retain are this rapid decrease, the fact that the field created along has the same direction to , and the field created perpendicular is opposite to it and for the same value of r equal to ½ of the longitudinal field.
1.1.5 Magnetic Fields Created by Electrical Currents
In 1819, Hans Christian Oersted discovered that an electric current is able to generate a magnetic field. One year later, Jean-Baptiste Biot and Félix Savart wrote the famous Biot–Savart law that gave the magnetic field intensity as function of the current in an elementary element. This law is always used to calculate the field created by an arbitrary conductor. If we consider an element of length dl with a current I, the field created at a distance r is given by
For having in mind an order of magnitude, useful for understanding the various concepts described in this book, we are giving here two simple examples.
The first one is the field created by a long wire, assumed as infinite in its neighborhood (see Figure 1.2). The integration of the formula (1.3) is then
is the orthoradial component of the field. The two other components are 0 due to symmetry.
Figure 1.2 Field created by a wire and by a circular coil.
The field created by a circular loop can also be calculated by the (1.3) formula. Along the axis, the field is perpendicular to the coil plane and varies as follows:
Outside of the axis, the field has a dipolar shape, similar to the field created by a small magnet.
1.1.6 Magnetic Thin Films
Nearly all devices presently fabricated are composed of thin films deposited on flat surfaces, typically silicon wafers. Industrial tools are now able to deposit these films on surfaces up to 300 mm with accuracy better than 0.1 nm and homogeneity on the whole surface better than 1 % of the thickness. Properties of these thin films are in general similar to bulk properties, but thin films may exhibit new features. For example, some films can be crystallized in a structure impossible to achieve with bulk materials. The second effect of thin-film geometry is to modify strongly the magnetic anisotropy of the magnetic materials.
Some films can be crystallized in relation to the wafer underneath, we are hence speaking about epitaxy. A lot of films are textured, that is, they are partially crystallized with a preferred direction imposed by the thin-film geometry. Some are nearly amorphous: an assembly of small grains with random directions. Conditions of deposition (method, temperature, and pressure) and annealing have a large impact on the final structure.
1.1.6.1 Magnetic Anisotropy
A magnetic material may have preferential axis of magnetization induced either by the crystalline anisotropy or by its shape. The crystalline anisotropy is due to the coupling between spin orientation and crystalline electric field. The minimization of the corresponding energy gives in general some preferred orientation.
That anisotropy may be very strong in crystalline materials. Rare earth-based materials present usually a very high magnetic anisotropy due to their orbital shape. It is the reason why the strongest permanent magnets are rare earth based.
A specific magnetic anisotropy appears also at the surface of the magnetic material. This is due to the breaking of the crystalline electric field symmetry at the interface. That anisotropy can be larger than the shape anisotropy and help to create magnetic thin films with a magnetization perpendicular to the plane. This is the case of, for example, a thin Co layer on Pt.
The shape anisotropy is simply due to the field created by each individual atom of the layer to the others. This field, called dipolar field or demagnetizing field, has a dipolar shape given in (Eq. 1.3). This field decreases as 1/r3, but as the number of atoms varies as r3 its impact at long distance is huge for ferromagnetic or ferrimagnetic materials. The first main effect of this shape anisotropy is to force magnetization to be in the plane of the film. This can be counteracted only by using very thin films having an additional surface anisotropy. The second effect of this shape anisotropy is to create domains, that is, parts of the films, where the magnetization has the same direction.
1.1.6.2 Magnetic Domains
Dipolar interactions responsible for the shape anisotropy impose an overall magnetic configuration of the thin film that tends to minimize the overall energy. If the film is infinite, a uniform magnetization is the lowest energy state, but as soon as lateral dimensions are reduced, it costs dipolar energy to have a magnetization perpendicular to the edge more than rotating smoothly the magnetization inside the layer. For that reason, patterned objects in thin films acquire specific magnetic configurations that you will encounter in this book. Figure 1.3 gives examples of some classical shapes you will see with their stable state.
Figure 1.3 Typical magnetic domains observed in small objects. Arrows indicate the direction of magnetization and blue lines domain walls. In the cross of domain walls or in the center of the disk, the magnetization goes out of plane. This is called a vortex.
1.2 Magnetic Field Sensing
There is a large variety of magnetic sensors and it would take several books to describe all of them. Here, we are just giving some indications that will help the teacher to find more information. Some sensors such as Hall effect or inductive sensors have been developed since decades and now main innovations for these sensors are mainly coming from the integration of sophisticated electronics able to perform in real-time complicated algorithms. Others, such as NV sensors (Chapter 6), are very promising for specific applications and are at the stage of research and development. We decided to focus a part of this book on magnetoresistive sensors because they illustrate the dynamism of research in magnetism and are reaching large-volume applications that were mainly covered by Hall sensors. Table 1.3 provides some characteristic properties of the main magnetic sensors technologies.
Table 1.3 Main magnetic sensors technologies with some properties.
Principle
Scalar/Vectorial
Operating temperature range
Field range
Frequency range
Linearity
Size
Material
Hall
Vect.
−200 °C/150 °C
1 µT–10 T
DC-1 MHz
Good
µm–mm
Semiconductor
AMR
Vect.
−275 °C/200 °C
1 nT–1 mT
DC-10 MHz
Limited
µm–mm
Ferromagnet
Optical
Vect. or scalar
Room temp.
1 fT–1 µT
DC
Requires feedback
mm–cm
Alkali gas
GMI
Vect.
−50–150 °C
10 pT–0.1 mT
DC-10 kHz
Requires feedback
mm–cm
Soft ferromagnet
Magnetoelectric
Vect.
−50–150 °C
100 pT–1 mT
DC-1 kHz
Limited
0.1 mm–cm
Composite
GMR/TMR
Vect.
−273–180 °C
100 pT–10 mT
DC-GHz
Limited
µm
Multilayer
Coils
Vect.
−273–600 °C
1 fT–10 T
AC
Excellent
0.1 mm–m
Metal
Search coil
Vect
−50–200 °C
1 fT–10 mT
AC
Excellent
0.1 mm–1 m
Ferrite core
Fluxgate
Vect.
−50–200 °C
5 pT–100 µT
DC-5 kHz
Good
0.1 mm–5 cm
Ferrite core
SQUID
Vect.
−273–200 °C
1 fT–10 µT
DC-100 kHz
Requires feedback
0.1 mm–1 cm
Metallic
1.2.1 Magnetic Sensors for DC and Low-Frequency Applications
The main sensor used for DC and low-frequency applications is the Hall sensor based on the Hall effect. When a field is applied on a material where a current is flowing, a voltage appears perpendicular to the current direction due to Lorentz force. This voltage is proportional to the field and the applied current through a factor RH called Hall resistance.
Today, Hall sensors represent 85 % of the world production of magnetic sensors for DC and low-frequencies applications with a growth of about 3 % per year. The main competitors are magnetoresistive sensors (AMR, GMR, and TMR) described in this book that represent only 10 % but are growing at an annual rate of about 10 %. Magnetoelectric sensors also appear in some commercial products. They present the advantage to be passive, but they cannot be integrated. Fluxgates are mainly used for very sensitive applications such as earth field mapping for field monitoring.
1.2.2 Magnetic Sensors for High-Frequency Applications
When the frequency is increased, the sensor used universally is the coil or antenna. The radiofrequency field creates a current inside a metallic wire that can be amplified and detected. As this current is proportional to the frequency, higher the frequency, the higher the sensitivity of a coil is. The electromagnetic wave is both an electric field and a magnetic field and antennas are designed to be more sensitive to electric fields, whereas coils are designed to be more sensitive to magnetic fields. There is, however, one specific case where coils/antennas are less competitive than magnetoresistive sensors: when the size becomes so small that it is impossible to build a performing coil. This has created two application cases for magnetoresistive sensors: nondestructive evaluation (see Chapter 5) and integrated position sensors.
1.2.3 Very Sensitive Magnetic Sensors
The development of very sensitive sensors for low-frequency magnetic field detection is a domain where a very active research work is going on across the world. Table 1.4 describes the main sensors technologies for subpicotesla detection. Applications of very sensitive sensors are brain/body imaging like biomagnetism and low-field MRI (Chapter 2), magnetic particles detection (Chapters 8 and 9) and earth field mapping.
Table 1.4 Very sensitive magnetic sensors with their working temperature and field equivalent noise.
Sensor type
Working temperature
Minimal detectivity for 1 cm
2
Comments
SQUIDs
4 K
1 fT/sqrt(Hz)
Extensively developed and used. This is the reference sensor
HTS SQUIDs
77 K
30 fT/sqrt(Hz)
Atomic magnetometers
150 °C
10 fT/sqrt(Hz)
Absolute magnetometers. Need the suppression of DC fields
Fluxgates
RT
1 pT/sqrt(Hz)
Superconducting/GMRMixed sensors
4 K
3 fT/sqrt(Hz)
Large 1/f noise
Superconducting/GMRMixed sensors
77 K
7 fT/sqrt(Hz)
Large 1/f noise
1.3 Introduction to Spin Electronics
Spin electronics is based on the fact that electrons have not only a charge but also a magnetic moment, called spin, which is quantified. The aim is to use this magnetic moment to filter electrons, to manipulate macroscopic magnetization, and in some cases to transport information. Historically, the first spin electronic effect, called the GMR effect, was discovered by P. Grunberg and A. Fert, who were awarded the Nobel Prize in 2007 [8]. The TMR effect was proposed earlier by Julière in 1975 [9] and observed later. Spin electronics applications are today mainly magnetic sensing with GMR and TMR sensors and magnetic storage with MRAMs and magnetic logics. Both are now in their commercial phase and are still being improved in terms of their performance.
1.3.1 Bases
1.3.1.1 Spin Polarization
The base of spin electronics is the fact that conduction electrons in magnetic materials are polarized, that is, the direction of the spin is not arbitrary but has a preferred direction imposed by the magnetization of the material. That polarization strongly depends on the nature of the material and on its crystalline structure. CoFe is the 3D alloy mainly used in devices as it is easy to deposit and presents a large spin polarization, around 70 %.
1.3.1.2 Spin Diffusion Length
When a polarized electron is sent inside a material, it experiences collisions. A lot of collisions are elastic and the spin is conserved, while some are inelastic and may conduct to a change in its spin orientation. The typical length on which the memory of the spin is lost is few nanometers at room temperature. This implies that in a nonmagnetic material, a spin polarization cannot be maintained beyond that distance. The impact is that all spin electronics devices have to be engineered with at least one dimension at nanometer scale. The thin-film technology and micronanofabrication techniques have hence played an essential role in the development of spin electronics.
1.3.1.3 Spin Currents and Spin Hall Effects
A spin current is the propagation of a net magnetic moment. Two kinds of spin currents can be proposed. The first one is the spin wave that propagates magnetic information by elementary excitations of the magnetic material. This is discussed in Section 1.4.3. The second way is to use polarized electrons. A polarized electron current propagates magnetic information that can be used, for example, to rotate a magnetic layer through a spin torque effect. This effect is described in detail in Chapter 4.
More recently, pure spin currents carried by electrons have been created. These currents are created by two flows of polarized electrons with opposite polarities and opposite directions: there is a net magnetic moment transferred and no charge.
These spin currents can be created by a spin Hall effect where an electrical current can create a transverse spin current through spin–orbit coupling. A very recent review of spin Hall effects has been published [10], which discusses the main aspects of this subfield of spintronics.
1.4 Main Applications of Spin Electronics
1.4.1 GMR and TMR Sensors
1.4.1.1 Principle
The GMR device was the first spin electronics device proposed 25 years ago. The principle is to have magnetic thin layers separated by nonmagnetic layers having a large enough spin diffusion length. Electrons traveling inside the first layer have a spin polarization that depends on the magnetization direction. When they arrive inside the nonmagnetic layer, electrons conserve their polarization to a distance of the order of the spin diffusion length. If there is another magnetic material nearby, electrons enter it. But that entrance will be easier if the magnetization direction of this second magnetic layer is identical to the first one. Hence, electrical resistance of the stack will depend on the relative orientations of the magnetization of each magnetic layer.
1.4.1.2 Spin Valve Devices
The simplest GMR device is called spin valve where there are only two magnetic layers, one with a large coercivity, that is, reasonably blocked in an external field, and another one that can easily rotate in an external field. A typical spin valve is given in Figure 1.4. This device is composed of a blocked magnetic layer. The blocking is obtained by using an antiferromagnet, typically PtMn or IrMn, coupled with a CoFe layer. The antiferromagnet has the property of being insensitive to the external magnetic field. Furthermore, it has typically a blocking temperature below its ordering temperature. Below the blocking temperature, it is very hard to move it and above this temperature it becomes easy to move it with a field. The blocking temperature for IrMn is typically 240 °C and for PtMn about 340 °C. For that reason, PtMn is more interesting for high-temperature applications such as in automotive. Often an extra synthetic antiferromagnet (SAF) is added to increase the field stability.
Figure 1.4 GMR simple spin valve (a). TMR spin valve with an SAF configuration (b).
The second magnetic layer is usually a free layer, that is, able to rotate easily in external magnetic fields. It is in general composed of a bilayer of NiFe and CoFe. NiFe, called permalloy, is a very soft material, whereas CoFe ensures a high spin polarization.
The GMR spacer is generally a Cu thin layer. The typical Cu thickness is about 2 nm, and this insures a magnetic decoupling of the two magnetic layers and a low enough spin depolarization.
TMR (tunnel magnetoresistance), also called MTJ (magnetic tunnel junctions), has the same structure as GMR's, but the spacer is a very thin insulating layer called barrier. The transport through this spacer is no longer a diffusive path but requires a tunnel transport, and the TMR ratio mainly depends on the electrode spin polarization at the interface. This has several consequences. The first consequence is that the resistance of the device increases exponentially as the thickness of the barrier increases and hence has to be very well controlled; the second is that the effect can be much higher than the GMR effect; the third is that for practical devices, resistance and size are partly decoupled; and finally the current has to flow through the barrier, so it requires top and down contacts. MgO insulating barriers create a symmetry filtering that increases the TMR ratios and thus are generally used.
1.4.1.3 Electric Response
Response
A spin valve gives an angular response: the resistance varies with the angle between the free layer and the hard layer. This is, for example, very different from a Hall sensor where the response is mainly linear with field. Figure 1.5 gives a typical response of a GMR sensor as function of the external field direction. For that reason, except for angle sensors (Chapter 3) the GMR response is linearized. This can be done either by a closed-loop scheme, in which a coil creates a field on the GMR device that cancels the external field, or by applying a bias field on the GMR along an axis perpendicular to the sensitive axis. The bias field has to be larger than the field range required for the device. This bias field can be created in three different ways: the first one is an external magnet, the second is to play on the shape, and the third is to use a second antiferromagnet to partly pin the free layer perpendicular to the sensitive layer. All these approaches are more thoroughly described in Chapters 3 and 9.
Figure 1.5 (a) Response of a GMR as function of external field and (b) response of a linearized GMR.
The typical variation of magnetoresistance is 6–12 % for GMR and 200 % for MgO based TMR.
Noise
Reig et al. [6] give a detailed description of noise of GMR and TMR devices. Here, we give only the main features. Electric noise in GMR and TMR has mainly three sources. The first one is a frequency-independent noise that in GMR is only the thermal noise related to the resistance of the devices. The second source is a 1/f noise that comes from resistance fluctuations in the device. For GMR, the fluctuations are related to defects and 1/f noise is comparable to usual metals and follows the classical Hooge [11] empirical formula. For TMR, the noise is mainly related to conductance fluctuations of the barrier. The 1/f noise in TMR strongly depends on the resistance and materials and is usually much higher than the noise in GMR, and this mitigates the advantages of TMR versus GMR for low-frequency applications. The third source of noise is the magnetic noise that comes mainly from domain fluctuations. This noise can be avoided by giving the device a proper shape that does not allow domain formation in the active region. The C shape, also called yoke shape, is very often used.
Sensitivity and Detectivity
The sensitivity of a device is strongly related to its linearization. In practice, if a device has a magnetoresistance of 10 %, 5 % can be used as a linear range. If you bias your device so that this 5 % variation covers 1 mT, the sensitivity will be 5 %/mT. If you apply a much stronger bias on the spin valve, say 20 mT, the sensitivity decreases to 0.25 %/mT. So, the sensitivity can be tailored as function of the applications. In the various chapters, you will see how sensitivity and field dynamic range have been chosen for the targeted applications.
Very often, a magnetic sensor is characterized by the detectivity that is the field corresponding to a signal-to-noise ratio of one. It corresponds to the noise divided by the sensitivity given in V/T. The detectivity of GMR and TMR sensors depend on the size because of the 1/f noise . Table 1.5 gives a short summary of classical detectivity achieved today for magnetic sensing.
Table 1.5 Detectivity of spin electronics sensors as function of size and frequency.
Device
Detectivity at 1 Hz
Detectivity at 1 kHz
1/f corner
Detectivity at high frequency
GMR 1 mm
2
30 pT/sqrt(Hz)
10 pT/sqrt(Hz)
10 Hz
10 pT/sqrt(Hz)
GMR 3 µm × 20 µm
1 nT/sqrt(hz)
30 pT/sqrt(hz)
10 khz
10 pT/sqrt(Hz)
Array of TMR1 mm
2
100 pT/sqrt(Hz)
3 pT/sqrt(Hz)
10 kHz
1 pT/sqrt(hz)
TMR3 µm × 20 µm
1 nT/sqrt(Hz)
30 pT/sqrt(Hz)
1 MHz
1 pT/sqrt(hz)
1.4.2 Spin Electronics Devices for Storage, MRAM, and Magnetic Logics
The basic block of a spin valve is used for magnetic storage. At present, MgO-based magnetic tunnel junctions present a typical magnetoresistance of 150–200 % at room temperature. This large magnetoresistance induces a good separation between the 0 and the 1 level. Contrary to magnetic sensors, magnetic tunnel junctions used for storage are not at all linear but should present a high stability against external field while preserving the writing possibility. This aspect of spin electronics is developed in Chapters 4 and 11.
1.4.3 Spin Dynamics and Magnonics
The dynamics of small magnetic objects has been a field of intense research for the past two decades.
We can separate different main regimes: a low-frequency regime, typically below 1 GHz where the magnetization of the thin magnetic layer in small devices is able to follow the external magnetic field. In this regime, static description works well. It has, in particular, been demonstrated that the GMR effect does not decrease up to 20 GHz. The second regime corresponds to the same domain of frequency, but the size of the device is increased. Then, domains are appearing, and domain propagation, reversal, and creation dominate the dynamics of these objects. This is partly described in Chapters 10 and 11. Nevertheless, if domain propagation has been extensively studied, no devices based on their use are presently commercialized.
The third domain is the high-frequency domain between GHz and THz. In this frequency domain, natural thermal excitations of magnetic ferromagnetic order appear; these excitations, called spin waves or magnons, are the magnetic equivalent of phonons, the vibrations of crystals. They are able to propagate, to be diffused, and could be used for magnetic logic. This aspect of spintronics called magnonics is a fast-growing field, where Europe has conducted a lot a pioneering work.
Spin waves are used for small RF devices in particular circulators and dephasing devices. The two new main applications forecast today are:
- STNO (spin torque nano-oscillators). It has been demonstrated that a DC current is able to induce spontaneous precession of small magnetic pillars through a torque applied by polarized electrons on thin layers. This can be used to generate locally GHz frequency with a large agility.
- Spin wave logics based on the combination of propagating spin waves to perform fast logic computing.
These aspects of spintronics are described in Chapters 11 and 12.
References
1
Kronmüller, H. and Parkin, S. (eds) (2007)
Handbook of Magnetism and Advanced Magnetic Materials
, vol.
5
, John Wiley & Sons, Ltd, Chichester.
2
Coey, J.M.D. (2009)
Magnetism and Magnetic Materials
, Cambridge University Press, Cambridge.
3
Rosa, A. (2015)
Handbook of Nanomagnetism: Applications and Tools
, Pan Standford Publishing, Lukaszew.
4
Ziese, M. and Thornton, M.J. (eds) (2013)
Spin Electronics
, Springer, Berlin.
5
Shinjo, T. (ed.) (2013)
Nanomagnetism and Spintronics
, Elsevier.
6
Reig, C., Cardoso, S., and Mukhopadhyay, S. (eds) (2013)
Giant Magnetoresistance (GMR) Sensors
, Springer, Berlin.
7
Hillebrands, B. and Ounadjela, K. (eds) (2003)
Spin Dynamics in Confined Magnetic Structures
, Springer, Berlin.
8
Baibich, M.N., Broto, J.M., Fert, A., and Van Dau, F.N. (1988)
Phys. Rev. Lett.
,
61
(1), 2472–2475.
9
Julliere, M. (1975)
Phys. Lett.
,
54A
, 225–226.
10
Sinova, J., Valenzuela, S.O., Wunderlich, J., Back, C.H., and Jungwirth, T. (2015)
Rev. Mod. Phys.
,
87
, 1213.
11
Hooge, F.N. (1976)
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,
83
, 14–23.
2Spin Electronics for Biomagnetism and Nuclear Magnetic Resonance
Myriam Pannetier Lecoeur, Reina Ayde, and Claude Fermon
CEA-CNRS, Université Paris-Saclay, SPEC/LNO, CEA Saclay, 91191 Gif sur Yvette Cedex, France
2.1 Introduction
Detection of magnetic signals is used in clinical applications for four main modalities. The first one and most known is MRI (magnetic resonance imaging), which is based on detection of the magnetic resonance of protons. The second one is MEG (magnetoencephalography), which is used for 3D mapping of the brain activity. The third one is MPI (magnetic particle imaging) based on the mapping of magnetic nanoparticles for flow imaging and cancer detection. The last one is related to cell detection and counting.
Spin electronics plays a role in the first two applications through the development of ultrasensitive sensors. MPI is described in Chapter 8 and cell detection and counting are described in Chapter 9. In this chapter, we show how spin electronics introduced in Chapter 1 can be used not only for large-scale but also for local detection of both biomagnetic signals and nuclear magnetic resonance (NMR) signals.
2.2 Biomagnetic Signals Detection with Spin Electronics Sensors
2.2.1 Biomagnetism
Biomagnetism refers to magnetic signals produced by the tissues. Excitable cells such as muscle cells, nerves, or neurons convey information through their electrical activity, generated by local changes in ionic concentrations at the membrane's inner and outer surfaces. These local potential variations in conductive tissues lead to currents that exhibit not only an electrical signature but also a related magnetic signal.
As magnetic fields travel through the tissues without deformation, only attenuated due to the distance to the source, they can provide insights into the information transmission of the cells without a direct physical contact to the tissues. Furthermore, magnetic information because of its vectorial nature indicates the direction of the current dipole generating the resulting signal and, therefore, conveys additional information compared to electrical recordings that are scalar only.
Biomagnetism addresses cardiac, nerve, muscle, or brain activity at the level of the whole organ or at the cell level. Magnetoencephalography is related to brain activity imaging and magnetocardiography (MCG) is related to cardiac activity imaging.
The requirements for biomagnetic signal detection is first to use sensors with sensitivity in the low-frequency range of the biological signals (typically from few hertz to few kilohertz) and high enough for the very low amplitude of signals. At the surface of the body, the signals emitted by the cardiac or the brain activity are in the tens of femtotesla to hundreds of picotesla range (see Figure 2.1). At few millimeters from the tissues, the signals can be in the picotesla range, while within the tissues, it is expected to be of the order of few nanotesla.
Figure 2.1 Magnetic field scale in Tesla, indicating on the left-hand side the amplitude of biomagnetic signals or low-field MRI signal, showing the range of field sensitivity of various magnetic sensors, based on Hall effect or superconducting interference (SQUIDs) and spin electronics magnetometers (including mixed sensors and magnetrodes described in this chapter).
2.2.2 Sensors for Biomagnetism at Large Scale
Biomagnetic signals can be detected noninvasively close to the surface of the body, which allows recording signals from humans. Since the measurement is passive, it is routinely applied to various populations (healthy subject or patients, babies, etc.). Large arrays of centimeter-size sensors are used. Commercial devices are based on typically 300 SQUIDs. Atomic magnetometers and mixed sensors are being investigated as alternative technologies.
2.2.2.1 SQUIDs and Atomic Magnetometers
If the first recorded magnetic signal from the human body has been achieved with coils [1], the reference magnetometer for this type of measurements has rapidly been SQUID (superconducting quantum interference device), which is now used in commercial MEG systems installed worldwide.
SQUID sensors are thin-film devices exploiting the properties of superconducting materials and exhibiting an exquisite sensitivity to the variation of a magnetic flux [2]. Two superconducting materials are used to develop SQUIDs for biomagnetism; Nb, which is a low critical temperature (Tc) material, operating below 9 K and thus needing a cooling at liquid helium temperature (4.2 K), and YBa2Cu3O7−δ (YBCO), whose superconducting transition around 90 K allows operating at liquid nitrogen temperature (77 K). The fabrication of high Tc SQUIDs is, compared to low Tc SQUIDs, more difficult than Nb SQUIDs and can hardly be realized at wafer scale. Furthermore, their noise is higher by an order of magnitude to low Tc SQUIDs of the same size.
As a consequence, commercial MEG systems require a liquid helium cooling, which is expensive and imply a very high thermal insulation between the sensor and the room, inducing a distance from the sensor to the surface of the scalp of typically 2–3 cm. A nitrogen cooling system would be much cheaper in terms of coolant consumption, easier to install (liquid nitrogen being available on many sites, in particular hospitals), and the distance from sensor to source can be reduced, leading to a larger signal at the sensing site.
Since the early 2000s, new concepts for high-sensitivity magnetometers dedicated to biomagnetism have been proposed.
The first one, SERF magnetometer, is based on the change of absorption of polarized alkali atoms' vapor as function of the magnetic field, where the sensitivity is improved by reducing the spin exchange collision of the atoms (spin-exchange relaxation free – SERF). MEG and MCG recordings have been achieved over the last decade by SERF magnetometers [3,4]. The main advantage of this technique of detection is that it operates at room temperature. The main drawback is that since the sensitivity is achieved on a very narrow resonance, a very strong magnetic shielding is necessary to maintain the sensitivity. Therefore, MCG and MEG recordings have been successfully achieved in highly shielded installation, such as the MSR2 in Berlin, and adaptation to clinical environment, where such a high level of shielding is excluded, remains a challenge.
The second type of new concept, based on spin electronics, through so-called mixed sensors, is presented in the next section.
2.2.2.2 Mixed Sensors
The main idea behind the mixed sensor principle is to enhance the field sensitivity of spin electronics-based magnetic sensor by using a plain superconducting loop that acts as a magnetic concentrator toward the micron size GMR or TMR element.
As shown in Figure 2.2, when a magnetic field is applied perpendicular to a superconducting loop, the diamagnetic properties of the superconductor lead to the generation of a supercurrent Is running into the loop to prevent magnetic field entrance. This current is directly proportional to the flux applied and thus to the outer side dimensions of the loop. With the supercurrent Is are associated field lines rotating around the width of the superconducting loop. The loop contains a constricted part, such as the current density at the location of the constriction is high, thus creating a high local magnetic field. The GMR or TMR element is placed on top or below the constriction to benefit from this enhanced magnetic field.
Figure 2.2 Mixed sensor principle of operation. The field applied perpendicular to the loop (Bapp) induces a supercurrent IS to flow in the superconducting loop to prevent the flux entrance. A constriction of few microns is designed in the loop. The current density in the constriction is high and generates a locally strong magnetic field around the constriction. A GMR or TMR sensor located on top or below the constriction can detect the locally enhanced magnetic field. The enhancement factor depends mainly on the dimensions of the loop and its constriction width [5].
Such a sensor achieves sensitivity in the femtotesla range at liquid nitrogen and has been successfully used for biomagnetic recording. The main disadvantage is the 1/f noise occurring in the GMR element at low frequency, which limits the detection level for very low-frequency signals. On the opposite, in the thermal range (typically for signals frequency higher than 1 kHz), the sensor detectivity is limited only by the thermal noise.
2.2.2.3 MCG Recordings with Mixed Sensors
MCG recordings have been achieved with mixed sensors on healthy adult volunteers in magnetic shielded room. Recordings have been achieved on Nb-based and YBCO-based sensors [6,7]. These recordings have allowed us to obtain a temporal magnetic mapping of the cardiac activity. From the magnetic recordings at various locations over the torso, it is possible to determine the position and orientation of the current dipole responsible for the magnetic signal (see Figure 2.3).
Figure 2.3 MCG recordings with mixed sensors. (a) Individual MCG output; each trace represents the recordings at the location over the torso. In inset, single MCG compared to a reference electrocardiographic (ECG) recording. (b) Mapping reconstruction of the current dipole at two timing of the cardiac cycle (initial QR peak and T-wave). The dipole is represented by the green arrow, the blue/red curves being the magnetic field isocontours, pointing perpendicular to the x–y plane. (Taken from Ref. [7].)
2.2.3 Sensors for Biomagnetism at Local Scale
While MEG records the magnetic signals generated by thousands or tens of thousands of neurons and at a distance that limits the spatial resolution, local magnetic recordings have remained a technological challenge because of the demanding conditions of miniaturization and high sensitivity required to record at the scale of one or few neurons with good spatial resolution. Furthermore, magnetic detection within tissues exclude technologies that are of large size and/or require cooling – such as SQUIDs or mixed sensors – or heating like some of the SERF optically pumped magnetometers.
Recently efforts have been made to be able to detect single-neuron signal or to measure within tissues. The main motivation toward conventional electrophysiology techniques is to access information transmission in a contactless mode, while not only getting a scalar information on local potentials but also accessing amplitude and direction information, allowing the determination of ionic currents vectors.
2.2.3.1 Specificities and State of the Art
Requirements for local biomagnetic recordings are sensitivity and miniaturization. Recordings inside tissues impose the use of sensors operating at physiological temperatures and biologically compatible in material and dimensions.
First recordings on individual neurons were obtained in the 1980s [8] using a model system of giant invertebrate axon, which exhibits large dimensions. Toroidal coils have been used by winding around the excised structure. SQUIDs have also allowed recording on nerves, with good signal detection but a poor spatial resolution because of the centimeter size of the SQUID pick-up loop [9]. Very recently, NV center-based magnetometers have allowed recording the magnetic response under electrical stimulation of the giant neuron of invertebrates [10].
Because of the sensor size, none of these detection schemes can be used to record on micron-size resolution or inside the tissues.
2.2.3.2 Magnetrodes
A new approach for magnetic local recordings has been proposed using spin electronics sensors. GMR sensors can be achieved at micron size while reaching subnanotesla sensitivity. Planar and sharp probes (Figure 2.4) have been realized to record magnetic action potentials of excitable cells such as muscle or neurons, as well as evoked response fields generated by assembly of neurons inside the tissues. These devices can pave the way for magnetophysiology.
Figure 2.4 (a) Schematic representation of magnetrode containing two GMR sensors and an electrode to achieve local magnetic and electrical recordings in a neuronal structure. (b) Electron microscopy of a magnetrode containing two meander GMR elements and an electrode. Scale bar 100 µm.
2.3 Nuclear Magnetic Resonance
NMR is widely used in physics, chemistry, biology, and more recently in medicine as that technique gives information on the local structure of molecules (NMR spectroscopy) and is able to give noninvasive images of proton density (magnetic resonance imaging). We are giving here first an introduction to NMR and in the second part, we describe how spin electronics can help to resolve major challenges. The first one is to perform NMR spectroscopy at micron scale and the second is to perform MRI at very low fields. As NMR and MRI are invoked in various chapters, we have given here a rather detailed introduction of these techniques.
2.3.1 Introduction to NMR
Nuclei are composed of neutrons and protons. Protons and neutrons have a magnetic moment due to their internal structure (see Table 2.1). This moment is linked to the angular moment of the particle. In quantum mechanics, the angular moment is quantified and called spin. The magnetic moment is written as follows:
The factor γ is the gyromagnetic ratio and is the spin operator with an half integer or integer module. For protons and neutrons, .
Table 2.1 Table of commonly observed, spin = 1/2 nuclei [11].
Nucleus
% abundance
γ (MHz/T)
ω
0
(MHz) at 3 T
99.99
42,58
125
100
39,95
118
100
17,21
50.6
1.1
10,69
31.4
0.37
4,31
12.68
When a magnetic moment is placed in a magnetic field , it acquires a magnetic energy.
It corresponds to the Zeeman interaction term:
