Giant magnetoresistance (GMR) occurs in thin-film structures of alternating ferromagnetic and non-magnetic layers. This effect causes a significant change in electrical resistance, and is exploited to reduce space in hard disk drives. Electrons generate magnetic fields as they rotate. These fields are oriented either up or down. Some electrons scatter as they conduct electricity, resulting in resistance. The scattering occurs mostly in the interface between magnetic and non-magnetic materials. Electrical current running perpendicular through layers of materials experience high or low resistance.
A material exhibiting the GMR effect is very sensitive to weak magnetic fields, which can cause a large change in the resistance of the layers. This change is resistance can be recorded using a current and fed to a circuit to generate a signal. Therefore data can be more densely written to a hard disk drive as a weaker magnetic field means more data can be written before needing ti worry about the magnetic domains cancelling each other. The material can be used for a rapidly fluctuating magnetic field as electrons can change their direction quickly. 
Hard disk drives store information by magnetizing a thin film of ferromagnetic material on a disk. Data is written to the drive as a pattern of sequential magnetic transitions on the disk. Data is read from the disk by detecting the transitions and decoding the data. Hard disk drives usually contain flat circular disks where data is written to. The disk are make of a non-magnetic material such as aluminum alloy and coated with a thin layer of magnetic material about 10-20 nm in depth. The data is written to the disk as it rotates past read-and-write heads, which detect and modify magnetization of the material under it. The magnetic surface of the disk is divided into different regions known as magnetic domains. Each domain forms a magnetic dipole which generates a magnetic field. To store data accurately, the disk drive must be able to resist self-demagnetization. Magnetic domains to close to each other will degrade over time as the domains rotate and cancel out the forces. The read-and-write head magnetizes a region by generating a local magnetic field. A data density increased, read heads first used electromagnetic induction to do their job, then magnetoresistance, and then GMR. 
Electrical conductivity in metals occurs through two independent conducting channels, which are the up-spin and down-spin electrons. Since the probability of spin-flip is small relative to the probability of scattering where spin is conserved, up-spin and down-spin electrons do not mix. In ferromagnetic metals, the scattering rates of up-spin and down-spin electrons are different as the density of states is not the same for up-spin and down-spin electrons at the Fermi energy. Scattering rates are proportional to the density of states, hence the scattering rates are different.
Assume that scattering rates are large for electrons with antiparallel spin relative to the magnetization direction, and weak for electrons with spin parallel to the magnetization direction, in as the density of states at the Fermi level is asymmetric. Up-spin electrons pass through parallel-aligned magnetic layers without much scattering as their spin is parallel to the magnetization of the layers. Down-spin electrons are scattered a lot within the ferromagnetic layers as their spin is antiparallel to the magnetization of the layers. When the layers are parallel, total resistivity of the material is mostly determined by the up-spin electrons, and is low. When the layers are parallel, total resistivity is determined equally by up-spin and down-spin electrons, and is high.
Current research in spintronics involve two general approaches. Existing GMR technology can be improved by developing new materials with larger spin polarization of elections, or improving existing devices for better spin filtering, Otherwise, new ways to generate and use spin-polarized currents include searching for spin transport in semiconductors and looking for ways where semiconductors can function as spin polarizers and spin valves.
Magnetization reversal in nanomagnetic devices due to the spin-transfer torque (STT) effect have been experimentally verified by numerous experiments. It would be useful for technological applications such as memory devices and dc current tuned microwave oscillators. The first model for this is a single domain rotation model for the moving free layer of the device. There is evidence to the presence of nonuniform intermediate states during magnetization reversal which have been shown by time-resolved x-ray imaging experiments. 
Time-resolved x-ray microscopy was used to study the switching behavior of samples with 45Å degree angle between the free and polarizing magnetic layers. The model was developed in terms of a linearized Landau-Lifshitz-Gilbert equation showing that the initial dynamics is dominated by the balance between the Oersted field and thermal fluctuations. The model was shown to agree with current and previous experimental observations. 
© Lay Kuan Loh. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 V. Chembrolu, et al., "Time-Resolved X-Rray Imaging of Magnetization Dynamics in Spin-Transfer Torque Devices," Phys. Rev. B 80, 024417 (2009).