Giant Magneto-Resistance Devices

187 Pages·2002·7.258 MB·English

by  Professor Dr. Eiichi Hirota, Professor Dr. Hirosi Sakakima, Professor Dr. Koichiro Inomata (auth.)

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40 Giant Magneto-Resistance Devices
By E. Hirota, H. Sakakima, and K. Inomata
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E. Hirota H. Sakakima K. Inomata
Giant
Magneto-Resistance
Devices
With 167 Figures
Springer
Professor Dr. Eiichi Hirota
Professor Dr. Hirosi Sakakima
Advanced Technology Reserach Laboratories, Matushita Electric Industrial Co., Ltd.
3-4 Hikaridai, Seika, Souraku, Kyoto, 619-0237, Japan
E-mail: [email protected], [email protected]
Professor Dr. Koichiro Inomata
Department of Materials Science, Tohoku University
02 Aza-aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
E-mail: [email protected]
Series Editors:
Professor Dr. Gerhard Ertl
Fritz-Haber-Institute der Max-Planck-Gesellschaft, Faradayweg 4-6,
14195 Berlin, Germany
Professor Robert Gomer, Ph.D.
The James Franck Institute, The University of Chicago, 5640 Ellis Avenue,
Chicago, IL 60637, USA
Professor Dr. Hans Liith
Institut fiir Schicht-und Ionentechnik
Forschungszentrum Jiilich GmbH,
52425 Ji ilich, Germany
Professor Douglas L. Mills, Ph.D.
Department of Physics, University of California,
Irvine, CA 92717, USA
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Hirota, Eiichi:
Giant magneto-resistance devices I E. Hirota ; H. Sakakima ; K. Inomata.
(Springer series in surface sciences ; 40)
(Physics and astronomy library)
ISBN 978-3-642-07526-1 ISBN 978-3-662-04777-4 (eBook)
DOI 10.1007/978-3-662-04777-4
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ISBN 978-3-642-07526-1
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Preface
In the last decade after the discovery of "giant magnetoresistance effects" in me
tallic multilayers worldwide developments in basic research and in engineering
applications have been achieved, and various remarkable results have been ob
tained in both fields. On the basic research into the GMR effects an excellent re
view book edited by Hartmann was published in 1999, entitled "Magnetic Multi
layers and Giant Magnetoresistance", and it describes the experimental and
theoretical aspects of GMR studies and the magneto-optics in metallic multilayers,
including applications in electronic data storage fields. This book aims to give an
overview on the electronics applications of the GMR in metallic multilayers, espe
cially on the sensors and memories in magnetic data storage, the main concerns of
electronic engineers.
If the mean free path of conduction electrons is longer than the period of the
multilayer, and for a low applied magnetic field magnetizations in the adjacent
magnetic layers are in antiparallel, the GMR effect in magnetic metallic multilay
ers occurs due to the resistance change in the electric current flowing in the layer
plain, with a change in the magnetization from antiparallel to parallel alignment
by an applied magnetic field (CIP mode), i.e., in the saturation field magnetiza
tions are in parallel and the resistance is lowered. The saturation field H, is given
by the RKKY-like magnetic coupling between adjacent magnetic layers through a
nonmagnetic conductive layer between the layers. In early GMR multilayers H,
was larger than 0.1 T and they could not be used in electronic devices. Electronic
applications need multilayer systems with low H,, such as the [NiFe/Cu/Co/Cu]
multilayer with a H, of 1 mT. These low-H, systems and their sensor applications
are described. After proposal of "spin valve" or "pseudo-spin-valve" techniques
for novel devices, which are essentially trilayer devices composed of a nonmag
netic layer sandwiched between two magnetic layers, the development research
shifts to these spin-valve-type devices which are now used in the market in read
heads in hard disk drives (HDDs ). This is one of the main subjects of this book.
When an electric current flows perpendicular to the layer plain (CPP mode), the
GMR effect becomes larger than that in the CIP mode. But devices in this mode
show impedances too low for use. A practical GMR device in CPP mode is con
structed by using a ferromagnetic tunnel junction (MTJ), i.e., the nonmagnetic
conductive layer in the spin valve is replaced by an insulator film. This so called
TMR device has high potential for future electronics applications. A probable ap
plication is magnetic random access memory (MRAM) and this is another main
subject in this book. The forecast for other possible spin electronics devices is also
given in this book.
VI Preface
The purpose of this book is to provide a text on GMR devices for electronic
material scientists and/or device engineers, and for graduate students in these
fields, and give an overview of applications and elemental physics models related
to GMR and TMR devices. The authors' backgrounds are in solid-state physics
where we accumulated knowledge and experience through research and develop
ment into electronic materials and devices, especially from magnetic materials and
related devices.
This book started with a recommendation from Dr. S. Y osimura and he intro
duced us to his friend Dr. Claus E. Ascheron of Springer-Verlag. We would like to
express our thanks for their continuous concern and support.
Osaka, Kyoto and Sendai, Japan E. Hirota
May 2001 H. Sakakima
K. lnomata
Contents
1 Introduction (E. Hirota) ........................................... . 1
1.1 Magnetic Transducers (Sensor and Read Head) .................... . 3
1.1.1 MR Sensor ............................................... . 4
1.1.2 HDD Head ............................................... . 5
1.2 Magnetic Random Access Memory (MRAM). . . . . . . . . . . . . . . . . . . . . . . 9
References......................................................... 10
2 Physics of GMR and TMR Devices (E. Hirota) . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Spin Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Phenomenological Theory of Spin Pinning . . . . . . . . . . . . . . . . . . . . 12
2.1.2 Micromagnetic Theory of Spin Pinning . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Free-Electron Model of GMR Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Conductivity of Metals and Metal Films . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 Conductivity of a Ferromagnetic Metal (Two-Current Model). . . . 26
2.2.3 Two-Current Model for the GMR Effect . . . . . . . . . . . . . . . . . . . . . . 29
2.3 Free-Electron Model of the TMR Decices . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.3.1 Simmon' Formula for the Tunnel Current. . . . . . . . . . . . . . . . . . . . . . 36
2.3.2 Ferromagnetic Tunnel Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3 GMR Devices of Metallic Multilayers (H. Sakakima). . . . . . . . . . . . . . . . . . 47
3.1 Exchange Coupled Metallic Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.1.1 Interlayer Coupling in Metallic Multilayers .................... 47
3.1.2 MR Properties of Metallic Multilayers . . . . . . . . . . . . . . . . . . . . . . . . 51
3.1.3 Dependence of the MR Properties on the Film Structures. . . . . . . . 55
3.1.4 Thermal Stability of the MR Ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.2 Magnetic Sensors ............................................... 65
3.2.1 Static Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.2.2 Dynamic Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
References ......................................................... 70
4 Spin-Valve Devices (H. Sakakima) ................................... 72
4.1 Spin Valves ................................................... 72
4.1.1 Exchange-Biased Spin Valves ............................... 72
4.1.2 Pseudo-Spin-Valves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.2 Enhancement of GMR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2.1 Back-Layer Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2.2 Specular Scattering Effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
VIII Contents
4.3 Magnetic Read Heads........................................... 99
4.3.1 Head Design .............................................. 100
4.3.2 Spin Valves for Read Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
5 Tunnel-Type GMR (TMR) Devices (K Inomata and H. Sakakima) . . . . 115
5.1 Spin-Dependent Tunneling ...................................... 115
5 .1.1 Tunneling Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.1.2 Fabrication of Spin Tunneling Junctions. . . . . . . . . . . . . . . . . . . . . . 120
5.1.3 Microfabrication of Spin Tunneling Junctions. . . . . . . . . . . . . . . . . 122
5.1.4 Annealing Properties ....................................... 124
5.1.5 Temperature Dependence ofTMR ........................... 125
5.1.6 Bias Voltage Dependence ofTMR ........................... 127
5.1.7 Double Tunneling Junctions ................................ 129
5.2 Magnetic Read Heads ofTMR ................................... 131
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6 Magnetic Random Access Memory (MRAM) (K. Inomata) . . . . . . . . . . . 135
6.1 MRAM Devices and their Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.1.1 Early MRAM Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.1.2 MRAM Devices Using GMR Cells . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.1.3 MRAM Devices Using MTJ Cells ........................... 138
6.2 MRAM Architectures Using MTJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.3 Features ofMRAM ............................................. 140
6.4 Design of a MTJ Element ....................................... 141
6.5 Spin Switching Field for MTJ Elements ........................... 145
6.5.1 Single-Domain Calculation ................................. 145
6.5.2 Micromagnetic Simulation ................................. 146
6.5.3 Two-Dimensional Magnetic Switching. . . . . . . . . . . . . . . . . . . . . . . 148
6.6 Problems in Fabricating MRAM ................................. 151
6.6.1 Integrated Circuit Compatibility ............................. 151
6.6.2 Uniformity ofMTJ Elements ............................... 151
6. 7 Current Trial of MRAM Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6.8 Scalability ofMRAM: Challenges for High Bit Density ............. 153
6.8.1 Resistance ................................................ 153
6.8.2 Signal Voltage............................................ 153
6.8.3 Writing Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
6.8.4 Superparamagnetism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
7 Other GMR Devices (K. Inomata) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.1 Bipolar Spin Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.1.1 Spin Injection ............................................. 158
7 .1.2 Spin Transistor Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.1.3 Device Properties ......................................... 162
7 .1.4 Demonstration of a Prototype Device . . . . . . . . . . . . . . . . . . . . . . . . 162
7.2 Spin FET ...................................................... 164
Contents IX
7.3 Spin-Valve Transistor ........................................... 165
7.3.1 Principle of the Spin-Valve Transistor ........................ 165
7.3.2 Device Properties .......................................... 167
7.4 Spin-Dependent Single-Electron Tunneling Devices ................. 168
7 .4.1 Structures and Properties .................................... 168
7.4.2 TMR Enhancement by Coulomb Blockade Effect .............. 174
7.4.3 Spin Accumulation ......................................... 175
References ......................................................... 176
1 Introduction
The discovery of the giant magnetoresistance (GMR) effect in metal multilayers
make a strong impact on scientific researchs and also the applied sciences, espe
cially on the development of computer memory technologies. The former is to re
search the origin of this extraordinary large MR effect in artificial materials, such
as, metal multilayers. The latter is the possibility of achieving high-density mem
ory in hard disk drives (HDD) of computer peripherals. At present the introduction
of GMR read heads in the HDD opens the door to the "Gb/in2 memory era" and
allows aerial recording densities (ARD) of up to 100 Gb/in2•
In 1971 Hunt [1] proposed using a magnetoresistance transducer, i.e., MR read
heads and sensors. After his proposal various studies on MR transducers were car
ried out aiming to develop practical devices. Early devices were composed with
anisotropic ferromagnetic thin films, such as Fe2~i80 alloy film, which have a
magnetoresistance (MR) ratio of about 2 % and which were later called aniso
tropic magnetoresistance (AMR) films as distinct from GMR films. Several AMR
devices were put on the market as magnetometric positioning sensors, rotary en
coders, and read heads of digital audio tape recorders etc.
By the beginning of the 1990 inductive thin film heads and thin film MR read
heads had been put on the HDD market, successively, and attained an ARD of
1 Gb/in2 (see Fig. 1.5).
Under these circumstances the GMR effect, which shows a MR ratio larger than
10 %, is very attractive as an application ofread heads to HDD. Intensive studies
of this application have been done in USA, Europe and Japan. Now, GMR heads
of 50 Gb/in2 have been announced and in the near future are sure to be put on the
market.
Successful results in GMR devices act as an incentive to study MR effects in
ferromagnetic tunnel junctions (TMR), which has a MR ratio larger than 20 %.
The TMR head is expected to be the next generation of read heads of HDD with
an ARD of over 100 Gb/in2• The most promising TMR device is the magnetic ran
dom access memory (MRAM) and many studies are now progressing with the aim
of developing random access memories with high-frequency access, are nonvola
tile, have low power consumption and are radiation resistant.
GMR and TMR devices have a basic structure of two magnetic layers separated
by a nonmagnetic spacer layer. These effects are the change in electric resistance
of this basic structure, when the magnetizations of both magnetic layers change
from a parallel configuration to an antiparallel configuration and vice versa, fol
lowed with an applied magnetic field. Thus, the key technologies are to fabricate
an antiparallel configuration, which is easily converted to a parallel configuration
by a small applied field. There are two different methods: one is to use the spin-
E. Hirota et al., Giant Magneto-Resistance Devices
© Springer-Verlag Berlin Heidelberg 2002