AAWSAP DIRD, Metallic Spintronics, March 2010

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r) 2/(R+r) 2 ~ 1. The other definition ti.R/R = (RAF-RF)/RF= (R-r)2/4Rr (unbounded from
above) is also in use. Figure 2a shows a magnetoresistance curve typical for magnetic
multilayers. The resistance is constant at a minimum value RF above a saturation field
Bs (parallel Fs) and rises to a maximum value RAF as the applied magnetic field B
approaches zero (antiparallel Fs).
GMR occurs in two different geometries (see Figure 1): namely when the current flows
in the plane of the layers, or CIP geometry, or when current flows perpendicular to the
layers, or CPP geometry. Most of experiments on GMR are carried out in the CIP
geometry because measuring the fairly large resistance of a thin film is quite easy (film
length is typically orders of magnitude larger than its thickness). Experiments in the
CPP geometry are more d'ifficult (Reference 33) and require special techniques for
precision measurements of very small resistances ~10-7- 10-s Q resulting from the
"short and wide" geometry of a 1-mm2 "wide" and 1-~Lm "long" sample. In order to
increase the resistances to easily observable values, microfabrication techniques can be
used to reduce the sample's cross-sectional area (Reference 34-36). Finally, a simple
and inexpensive point-contact technique (Reference 37) may also be suitable for this
purpose. The samples with a reduced cross-sectional area will be of interest for spin­
transfer-torque experiments presented in Section 3.
2.2 GMR APPLICATIONS
GMR is currently used in magnetic field sensors, including those in read heads for
computer hard drives, in galvanic isolators, and in nonvolatile random access memory
devices. Reading information stored on magnetic hard disk drives in computers was the
first large-sca le commercial application of GMR. The information is stored by
magnetizing small regions (magnetic domains) of a magnetic recording disk in different
directions. The stray magnetic fields from these domains are detected by a GMR
sensing element called spin valve. The simplest type of spin valve consists of two
ferromagnetic layers separated by a thin, nonmagnetic spacer. The spin-valve
resistance is smallest when the magnetizations of the two ferromagnetic layers are
parallel and largest when the magnetizations are antiparallel. The antiparallel alignment
is achieved by making the two layers respond differently to an external magnetic field;
an antiferromagnet in contact with one of the layers is used to effectively "pin" the
magnetization in this layer through an effect called "exchange bias." The exceptional
responsiveness of spin valves to magnetic fields has enabled very high areal packing
densities in hard drives.
Other sensor applications using GMR elements include monitoring of a ferrous gear
rotation in machinery operation (Reference 38) via detection of a changing magnetic
flux when a gear tooth passes near the sensor, man itoring of electrical current via
detection of the current-induced Oersted magnetic field, and transferring high­
frequency signals between isolated circuits (Reference 39) via magnetic fields
generated by a high-frequency inductor in one circuit and replicated in another circuit
by a GMR sensor.
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