Exchange bias
Encyclopedia
Exchange bias or exchange anisotropy occurs in bilayers (or multilayers) of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exchange bias phenomenon is of tremendous utility in magnetic recording, where it is used to pin the state of the readback heads of hard disk
drives at exactly their point of maximum sensitivity; hence the term "bias."
The essential physics underlying the phenomenon is the exchange interaction between the antiferromagnet and ferromagnet at their interface. Since antiferromagnets have a small or no net magnetization, their spin orientation is only weakly influenced by an externally applied magnetic field. A soft ferromagnetic film which is strongly exchange-coupled to the antiferromagnet will have its interfacial spins pinned. Reversal of the ferromagnet's moment will have an added energetic cost corresponding to the energy necessary to create a Néel domain wall
within the ferromagnetic film. The added energy term implies a shift in the switching field of the ferromagnet. Thus the magnetization curve of an exchange-biased ferromagnetic film looks like that of the normal ferromagnet except that is shifted away from the H=0 axis by an amount Hb.
In most well-studied ferromagnet/antiferromagnet bilayers, the Curie temperature of the ferromagnet is larger than the Néel temperature
TN of the antiferromagnet. This inequality means that the direction of the exchange bias can be set by cooling through TN in the presence of an applied magnetic field. The moment of the magnetically ordered ferromagnet will apply an effective field to the antiferromagnet as it orders, breaking the symmetry and influencing the formation of domains.
Exchange anisotropy
has long been poorly understood due to the difficulty of studying the dynamics of domain walls in thin antiferromagnetic films. A naive approach to the problem would suggest the following expression for energy per unit area:
where n is the number of interfacial spins interactions per unit area, Jex is the exchange constant at the interface, S refers to the spin vector, M refers to the magnetization, t refers to film thickness and H is the external field. The subscript F describes the properties of the ferromagnet and AF to the antiferromagnet. The expression omits magnetocrystalline anisotropy, which is unaffected by the presence of the antiferromagnet. At the switching field of the ferromagnet, the pinning energy represented by the first term and the Zeeman dipole coupling represented by the second term will exactly balance. The equation then predicts that the exchange bias shift Hb will be given by the expression
Many experimental findings regarding the exchange bias contradict this simple model. For example, the magnitude of measured Hb values is typically 100 times less than that predicted by the equation for reasonable values of the parameters. The amount of hysteresis shift Hb is not correlated with the density n of uncompensated spins in the plane of the antiferromagnet that appears at the interface. In addition, the exchange bias effect tends to be smaller in epitaxial bilayers than in polycrystalline ones, suggesting an important role for defects. In recent years progress in fundamental understanding has been made via synchrotron
radiation based element-specific magnetic linear dichroism
experiments that can image antiferromagnetic domains and frequency-dependent magnetic susceptibility
measurements that can probe the dynamics. Experiments on the Fe/FeF2 and Fe/MnF2 model systems have been particularly fruitful.
(AMR) effect. Without the stabilization, the magnetic domain state of the head could be unpredictable, leading to reliability problems. Currently exchange bias is used to pin the harder reference layer in spin valve
readback heads and MRAM
memory circuits that utilize the giant magnetoresistance or magnetic tunneling
effect. Similarly the most advanced disk media are antiferromagnetically coupled, making use of interfacial exchange to effectively increase the stability of small magnetic particles whose behavior would otherwise be superparamagnetic.
Desirable properties for an exchange bias material include a high Néel temperature
, a large magnetocrystalline anisotropy
and good chemical and structural compatibility with NiFe and Co, the most important ferromagnetic films. The most technologically significant exchange bias materials have been the rocksalt-structure antiferromagnetic oxides like NiO, CoO and their alloys and the rocksalt-structure intermetallics like FeMn, NiMn, IrMn and their alloys.
in 1956. The first commercial device to employ the exchange bias was IBM's
anisotropic magnetoresistance
(AMR) disk drive recording head
, which was based on a design by Hunt in the 1970s but which didn't fully displace the inductive readback head until the early 1990s. By the mid-1990s, the spin valve
head using an exchange-bias layer was well on its way to displacing the AMR head.
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
drives at exactly their point of maximum sensitivity; hence the term "bias."
Fundamental science
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
within the ferromagnetic film. The added energy term implies a shift in the switching field of the ferromagnet. Thus the magnetization curve of an exchange-biased ferromagnetic film looks like that of the normal ferromagnet except that is shifted away from the H=0 axis by an amount Hb.
In most well-studied ferromagnet/antiferromagnet bilayers, the Curie temperature of the ferromagnet is larger than the Néel temperature
Néel temperature
The Néel temperature or magnetic ordering temperature , TN, is the temperature above which an antiferromagnetic material becomes paramagnetic—that is, the thermal energy becomes large enough to destroy the macroscopic magnetic ordering within the material....
TN of the antiferromagnet. This inequality means that the direction of the exchange bias can be set by cooling through TN in the presence of an applied magnetic field. The moment of the magnetically ordered ferromagnet will apply an effective field to the antiferromagnet as it orders, breaking the symmetry and influencing the formation of domains.
Exchange anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
has long been poorly understood due to the difficulty of studying the dynamics of domain walls in thin antiferromagnetic films. A naive approach to the problem would suggest the following expression for energy per unit area:
where n is the number of interfacial spins interactions per unit area, Jex is the exchange constant at the interface, S refers to the spin vector, M refers to the magnetization, t refers to film thickness and H is the external field. The subscript F describes the properties of the ferromagnet and AF to the antiferromagnet. The expression omits magnetocrystalline anisotropy, which is unaffected by the presence of the antiferromagnet. At the switching field of the ferromagnet, the pinning energy represented by the first term and the Zeeman dipole coupling represented by the second term will exactly balance. The equation then predicts that the exchange bias shift Hb will be given by the expression
Many experimental findings regarding the exchange bias contradict this simple model. For example, the magnitude of measured Hb values is typically 100 times less than that predicted by the equation for reasonable values of the parameters. The amount of hysteresis shift Hb is not correlated with the density n of uncompensated spins in the plane of the antiferromagnet that appears at the interface. In addition, the exchange bias effect tends to be smaller in epitaxial bilayers than in polycrystalline ones, suggesting an important role for defects. In recent years progress in fundamental understanding has been made via synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
radiation based element-specific magnetic linear dichroism
Dichroism
Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths , or one in which light rays having different polarizations are absorbed by different amounts.The original meaning of...
experiments that can image antiferromagnetic domains and frequency-dependent magnetic susceptibility
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
measurements that can probe the dynamics. Experiments on the Fe/FeF2 and Fe/MnF2 model systems have been particularly fruitful.
Technological impact
Exchange bias was initially used to stabilize the magnetization of soft ferromagnetic layers in readback heads based on the anisotropic magnetoresistanceMagnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
(AMR) effect. Without the stabilization, the magnetic domain state of the head could be unpredictable, leading to reliability problems. Currently exchange bias is used to pin the harder reference layer in spin valve
Spin valve
A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on...
readback heads and MRAM
MRAM
Magnetoresistive Random-Access Memory is a non-volatile computer memory technology that has been under development since the 1990s. Continued increases in density of existing memory technologies – notably flash RAM and DRAM – kept it in a niche role in the market, but its proponents...
memory circuits that utilize the giant magnetoresistance or magnetic tunneling
Tunnel magnetoresistance
The Tunnel magnetoresistance is a magnetoresistive effect that occurs in magnetic tunnel junctions . This is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough , electrons can tunnel from one ferromagnet into the other...
effect. Similarly the most advanced disk media are antiferromagnetically coupled, making use of interfacial exchange to effectively increase the stability of small magnetic particles whose behavior would otherwise be superparamagnetic.
Superparamagnetism
Superparamagnetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel relaxation time...
Desirable properties for an exchange bias material include a high Néel temperature
Néel temperature
The Néel temperature or magnetic ordering temperature , TN, is the temperature above which an antiferromagnetic material becomes paramagnetic—that is, the thermal energy becomes large enough to destroy the macroscopic magnetic ordering within the material....
, a large magnetocrystalline anisotropy
Magnetocrystalline anisotropy
Magnetocrystalline anisotropy is the dependence of the internal energy of a ferromagnet on the direction of its magnetization. As a result, certain crystallographic directions are preferred directions, or easy axes, for the magnetization. It is a special case of magnetic anisotropy...
and good chemical and structural compatibility with NiFe and Co, the most important ferromagnetic films. The most technologically significant exchange bias materials have been the rocksalt-structure antiferromagnetic oxides like NiO, CoO and their alloys and the rocksalt-structure intermetallics like FeMn, NiMn, IrMn and their alloys.
History
Exchange anisotropy was discovered by Meiklejohn and Bean of General ElectricGeneral Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
in 1956. The first commercial device to employ the exchange bias was IBM's
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
anisotropic magnetoresistance
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
(AMR) disk drive recording head
Recording head
A recording head is the physical interface between a recording apparatus and a moving recording medium. Recording heads are generally classified according to the physical principle that allows them to impress their data upon their medium...
, which was based on a design by Hunt in the 1970s but which didn't fully displace the inductive readback head until the early 1990s. By the mid-1990s, the spin valve
Spin valve
A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on...
head using an exchange-bias layer was well on its way to displacing the AMR head.
See also
- S. Chikazumi and S. H. Charap, Physics of Magnetism, ASIN B0007DODNA.
- A. E. Berkowitz and K. Takano, "Exchange anisotropy: a review," J. Magn. Magn. Matls. 200, 552 (1999).
- John C. Mallinson, Magneto-Resistive and Spin Valve Heads: Fundamentals and Applications, ISBN 0-12-466627-2.
-
- Ivan K. SchullerIvan K. SchullerIvan K. Schuller is an American condensed matter experimental physicist. He is best known for his work on superlattices. His interests are focused on thin films, nanostructures, novel materials, magnetism, and superconductivity....
and G. Guntherodt, "The Exchange Bias Manifesto," 2002. - Jung-Il Hong, Titus Leo, David J. SmithDavid J. SmithDavid J. Smith is a Regents' Professor of physics at Arizona State University. He is an Australian experimental physicist and his research is focussed on using the electron microscope to study the microstructure of different materials. He is a pioneer in high-resolution relectron microscopy...
, and Ami E. Berkowitz, "Enhancing Exchange Bias with Diluted Antiferromagnets," Phys. Rev. Lett. 96, 117204 (2006).
- Ivan K. Schuller