DOW-UAP-D129, AAWSAP DIRD, Metallic Spintronics, March 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys metallic spintronics, a branch of electronics that seeks to use both the electric charge and the magnetic spin of electrons to store, detect, and manipulate information, and argues that the field could lead to faster, lower-power, and more radiation-resistant devices than conventional semiconductor electronics. The report focuses on two main effects: giant magnetoresistance (GMR), which allows magnetic states to be read through changes in electrical resistance, and spin-transfer torque (STT), which allows electrical currents to change those magnetic states. The DIRD reviews their underlying physics, the experimental work then available, and possible applications in memory, sensors, oscillators, and logic devices. The document treats metallic spintronics as a promising field while emphasizing that many of its more advanced proposed applications still require substantial further development.

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
23 March 2010
ICOD: 1 December 2009
DIA-08-1003-011
Metallic Spintronics
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Metallic Spintronics
Prepared by:
Acquisition Support Division (DW0-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:
AAP
Person 71
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs In this publication Is not a.uthorlzed.
This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or questions pertaining to
I
this document should be addressed to!AAP Person 1 AAWSA Program
Manager, Defense Intelligence Agency, ATTN: CLAR/DW0-3, Bldg 6000, Washington,
DC 20340-5100.
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Contents
1. Introduction ......................................................................................................iv
2. Giant Magnetoresistance .................................................................................... 1
2.1 GMR Basics.........................................................................................1••········· 1
2.2 GMR Applications ........................................................................................ 3
3. Spin-Transfer-Torque ......................................................................................... 4
3.1 STT Basics ........................................................................................,........... 4
3.2 STT Experiments ............................................................................................... 7
3.3 STT Applications............................................................................................ 10
3.4 STT-Driven Motion of Magnetic Domain Walls ........................................... 12
4, Antiferromagnetic Metal Spintronics ............................................................... 12
4.1 Antiferromagnetic GMR .........,.............,....................................................... 13
4.2 Antiferromagnetic STT............................................................................... 14
5. Summary and Conclusions ............................................................................... 16
6. References ..............................................................................................,......... 18
Figures
Figure 1. In a Magnetic Multilayer, Several Atomic Layers of Magnetic Material
Alternate With Layers of Nonmagnetic Material ...................................... 1
Figure 2. Resistance of a Magnetic Multilayer R Versus Magnetic Field .................. 2
Figure 3. Differential Resistance dV/dI of a Mechanical Point Contact as a Function
of Current for a Series of Magnetic Fields................................................ 4
Figure 4. Device Schematics for STT Experiments ................................................. 5
Figure 5. Qualitative Picture of STT ....................................................................... 6
Figure 6. Torques on a Magnetic Moment in a Magnetic Field and Subject to an
Electrical Current......................................................................... 7
, 1•••••••••••
Figure 7. Spin-Torque-Driven Magnetic Switching .................................................. 8
Figure 8. Oscillatory Voltage .................................................................................. 9
Figure 9. Scanning Transmission X-ray Microscopy Images ................................. 10
Figure 10. Conventional MRAM Cell ....................................................................... 11
Figure 11. Racetrack Memory Concept .................................................................. 12
Figure 12. Schematic of Point Contact to Sample Geometry ................................. 15
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Metallic Spintroni·cs
1. Introduction
The rapid pace of progress in the computer industry over the past 40
years has been based on the miniaturization of chips and other
computer components. Further miniaturization, however, faces
serious challenges-for example, increasingly high power dissipation.
To continue on pace, the industry must g,o beyond incremental
improvements and embrace radically new technologies. A promising
nanoscale technology known as spintronics (a neologism for "spin­
based electronics"} has emerged. Spintronics refers to the role an
electron spin plays in solid-state physics. Spintronics researchers
aim to develop a revolutionary new class of electronic devices based
on the spin of electrons in addition to the charge. In spintronic
devices, information is carried not by the electron's charge, as in
conventional microchips, but by the electron's intrinsic spin.
Changing the spin of an electron is faster and requires less power
than moving it. Therefore, if a reliable way could be found to control
and manipulate spins, spintronic devices could offer higher data
processing speeds, lower electricity consumption, and many other
advantages over conventional chips, perhaps including the ability to
carry out radically new quantum computations.
Spintronics in ferromagnetic systems is built on a complementary
set of phenomena in which the magnetic configuration of the system
influences its transport properties and vice versa. Giant
magnetoresistance (GMR} (Reference 1, 2} and spin-transfer-torque
(STT} (Reference 3-5} phenomena exemplify such interconnections
in multilayers composed of ferromagnetic (F} and nonmagnetic (N)
layers. The physics and applications of metallic spintronics are
discussed in this report from the perspec:tive of these two
phenomena. GMR, research on which was awarded the Nobel Prize in
Physics in 2007, refers to a large change in resistance of magnetic
multilayers when the relative orientation of magnetic moments in
their constituent ferromagnetic layers is altered by an applied
magnetic field. The inverse effect, STT, in which a large electrical
current density j can perturb the magnetic state of a multilayer, has
also been predicted (Reference 3, 4} and observed in experiments on
current-induced reversal and precession of magnetization
(Reference 5-9} and magnetic domain wall motion (Reference 10,
11}.
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Spintronics is a broad research field with (currently) three major
subfields: (1) materials research that is attempting to create new
materials that are both magnetic and semiconductors, (2) research
on novel magnetotransport effects in ferromagnetic metals, and (3)
research on techniques that can be used to manipulate individual
electron spins. The first subfield is targeting magnetic
semiconductors because devices based on such materials would be
the easiest to integrate with the present semiconductor device
technology and processing capabilities. However, despite extensive
research, most semiconductor spintronic devices are still theoretical
concepts awaiting experimental demonstrations. This report focuses
on spintronics research in metallic systems within the scope of the
second and third subfields. The second subfield has experienced an
unprecedented period of new discovery over the past 20 years,
including the discovery of GMR, and has already spawned major
technological change in the information storage industry with the
use of GMR sensors and read heads. The third subfield is vital for
spintronic devices, as virtually any processing of information in such
devices is associated with transport and manipulation of spins.
New and efficient methods for manipulating spins that stimulate
active research programs in spintronics at a large number of
academic institutions and a half-dozen industrial research labs
around the world are highly desirable. The prize to be gained is
active control and manipulation of spin distributions (magnetic
moments) for new and improved functionality in
electronic/spintronic devices. The confluence of intense basic
science and industry interest in ferromagnetic metal spintronics has
not occurred on this scale in physics in a long time.
The report is arranged as follows: Section 2 is dedicated to
magnetotransport effects in magnetic systems where magnetic
configuration can influence the system's transport properties. It
discusses GMR in magnetic multilayers and related phenomena,
highlights basic physical principles responsible for GMR, and
describes technological applications of the effect. Section 3 focuses
on the reverse connection between the system's magnetic
configuration and its transport properties-the so-called STT effect.
The physical origin and potential applications are discussed. Section
4 discusses other new directions in metallic spintronics, with a
particular focus on spintronics with antiferromagnetic materials.
Section 5 summarizes, with an eye to the future, the development of
spintronic technologies and their aerospace applications.
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2. Giant Magnetoresistance
2.1 GMR BASICS
This section discusses the phenomenon of giant magnetoresistance (GMR). Excellent
reviews of GMR are available elsewhere (Reference 12-22). The focus on physical
concepts important for the sections to follow are discussed.
GMR in magnetic multilayers refers to a dramatic reduction in the resistance of the
multilayers when subjected to an external magnetic field. GMR's size is usually defined
as the resistance change in magnetic field relative to its peak value. The effect can be
distinguished from the ordinary magnetoresistance (MR) coming from the direct action
of the magnetic field on the electron trajectories via the Lorentz force (Reference 23),
and from the anisotropic MR, which comes from dependence of the resistivity on the
relative orientation of magnetic moment to the current (Reference 24 ).
To prepare the magnetic multilayers,
where several atomic layers of one
(ferromagnetic) material alternate by
layers of another (nonmagnetic) Current
material (see Figure 1), a wide variety In
of deposition methods have been used,
the Plane
such as electrochemical deposition
techniques (Reference 25, 26) and
various vacuum deposition techniques
(Reference 27, 28). The latter shares
mainly between two methods using
either sputter deposition or molecular
beam epitaxy (MBE) systems. Sputter
deposition involves knocking off the Current
atoms of the material of interest from a Perpendicular
target by particle bombardment, to the Plane
followed by the deposition of high­
energetic atoms (N2-30 electronvolts
[eV]) onto the substrate. A principal
advantage of sputter deposition is the
ease with which many different
materials can be deposited at relatively Figure 1. In a Magnetic Multilayer, Several Atomic
Layers of Magnetic Material (shown In grey)
high deposition rates. In contrast,
Alternate With Layers of Nonmagnetic Material
deposition rates in MBE systems are (shown in white). GMR occurs in one of two different
usually much lower than for sputtering geometries: (1) when the current flows in the plane (CIP
geometry) of the layers or (2) when the current flows
systems, but much lower energies
perpendicular (CPP geometry) to the layers.
(~0.1 eV) of the evaporated material
make this technique favorable for growth of highly oriented single-crystalline films.
The original observation of GMR (Reference 1) was made on MBE grown iron-chromium
(Fe/Cr) multilayers with nearly perfect crystallinity. Subsequently, by using sputtered
samples that are grown much more rapidly than MBE samples, it was possible not only
to reproduce these results but also to observe oscillations in the magnetoresistance as
the thickness of the nonmagnetic spacer layers was varied (Reference 29). Subsequent
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studies (Reference 30) on sputtered cobalt-copper (Co/Cu) multilayers revealed
magnetoresistances at room temperatures 3 to 4 times larger than those for iron­
chromium and 13 times greater than those for the permalloy films that were used as
magnetoresistive sensors in magnetic reading heads at that time. The much higher
numbers observed in magnetic multilayers predetermined the fate of GMR in magnetic
recording technology.
The current understanding is that GMR R
observed in magnetic multilayers arises
from the dependence of the resistivity
on their internal magnetic configuration
and the role of the external magnetic
field to change this configuration. Figure
2b illustrates GMR in the simple limit
where the electron mean-free-path is Bs
much longer than the layer thicknesses. J
RF ----
The electrical transport properties of the
system are described in terms of the so­ 0
Magnetic Field
called two-current model (Reference
F1 F2 F1 F2
31), based on the suggestion by Mott
l l
(Reference 32) that, at temperatures
' t- -
lower than the Curie temperature, the
spin-up and spin-down electrons will be
almost independent and carry current in
parallel. Electrons are much more w
strongly scattered by a magnetic layer if t R R t R r t r r
they and the local magnetization spin in ~ ioo}
opposite rather than the same direction
t
r r
(R > r). For simplicity, the figure is R R R
drawn with scattering only at interfaces;
Figure 2. (a) Resistance of a magnetic multilayer R
however, there is also scattering within versus magnetic field. {b) Origin of GMR in terms of
the layers. At zero magnetic field, spin-dependent electron scattering: F1 aInd F2 are
ferromagnetic layers with a nonmagnetic layer in
where the magnetizations of adjacent
between. At zero magnetic field, the magnetizations
magnetic layers are aligned
in F1 and F2 are aligned antiparallel (center panel)
antiparallel-for example, because of and can be switched to parallel orientation by an
exchange coupling between the layers applied field. (c) The equivalent resistance circuits
corresponding to the three magnetic configurations
(Reference 29)-the spin-down
shown in (b). See text for details.
electrons are weakly scattered in layer
Fl but strongly scattered in F2. In contrast, the spin-up electrons are weakly scattered
in layer F2 but strongly scattered in FL As a result, two channels are equivalent,
leading to a total resistance in this "antiferromagnetic" configuration RAF= (R+r)/2 (see
the corresponding resistance circuit in Figure 2c).
When the magnetizations of the two F layers are set into parallel configuration by an
applied magnetic field, the spin-up electrons are weakly scattered in both layers and
form a low-resistivity channel, whereas the spin-down electrons are strongly scattered
in all the