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." In materials that exhibit antiferromagnetism, the spins of magnetic electrons align in a regular pattern with neighboring spins pointing in opposite directions. ... Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ... Typical hard drives of the mid-1990s. ...

Fundamental science

Easy-axis magnetization curves of a) a soft ferromagnetic film; b) an antiferromagnetic film and c) an exchange-biased bilayer consisting of a ferromagnet and an antiferromagnet. The susceptibility (slope) of the antiferromagnetic's magnetization curve is exaggerated for clarity.

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 H_b. Image File history File links Download high resolution version (837x286, 4 KB)Easy-axis magnetization curves of a) a soft ferromagnetic film; b) an antiferromagnetic film and c) an exchange-biased bilayer consisting of a ferromagnet and an antiferromagnet. ... Image File history File links Download high resolution version (837x286, 4 KB)Easy-axis magnetization curves of a) a soft ferromagnetic film; b) an antiferromagnetic film and c) an exchange-biased bilayer consisting of a ferromagnet and an antiferromagnet. ... Louis Eugène Félix Néel (November 2, 1904 – November 17, 2000), a French physicist born in Lyons, was corecipient (with the Swedish astrophysicist Hannes Alfvén) of the Nobel Prize for Physics in 1970 for his pioneering studies of the magnetic properties of solids. ... A domain wall is a theoretical 2-dimensional singularity. ...

In most well-studied ferromagnet/antiferromagnet bilayers, the Curie temperature of the ferromagnet is larger than the Néel temperature T_N of the antiferromagnet. This inequality means that the direction of the exchange bias can be set by cooling through T_N 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. In physics, the Curie point, or Curie temperature, is the temperature above which a ferromagnet loses its ferromagnetic ability to possess a net (spontaneous) magnetization in the absence of an external magnetic field. ... ...

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: Look up anisotropy in Wiktionary, the free dictionary. ...

where n is the number of interfacial spins interactions per unit area, J_ex 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 H_b will be given by the expression

Il manque un terme d'energie supplémentaire

Many experimental findings regarding the exchange bias contradict this simple model. For example, the magnitude of measured H_b values is typically 100 times less than that predicted by the equation for reasonable values of the parameters. The amount of hysteresis shift H_b 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 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. A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field (to turn the particles so they circulate) and the electric field (to accelerate the particles) are carefully synchronized with the travelling particle beam. ... In optics, the term dichroic has two related but distinct meanings. ... In electrical engineering, the magnetic susceptibility is the degree of magnetization of a material in response to a magnetic field. ...

Technological impact

Exchange bias was initially used to stabilize the magnetization of soft ferromagnetic layers in readback heads based on the anisotropic magnetoresistance (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. Magnetoresistance is the property of some materials to lose or gain electrical resistance when an external magnetic field is applied to them. ... A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance (from low to high or high to low) depending on the alignment of the magnetic layers. ... Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memory (NVRAM) technology, which has been in development since the 1990s. ... The Giant Magnetoresistive Effect (GMR) is a quantum mechanical effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers. ... In physics, the tunnel magnetoresistance effect, commonly abbreviated as TMR, occurs when two ferromagnets are separated by a thin (about 1 nm) insulator. ... Superparamagnetism is a phenomenon by which magnetic materials may exhibit a behavior similar to paramagnetism at temperatures below the Curie or the Neel temperature. ...

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. ... Magnetocrystalline anistropy is the energy cost per atom to align its magnetization from one crystallographic direction to another. ...

History

Exchange anisotropy was discovered by Meiklejohn and Bean of General Electric in 1957. The first commercial device to employ the exchange bias was IBM's AMR head, which was based on a design by Hunt in the 1970's but which didn't fully displace the inductive readback head until the early 1990's. By the mid-1990's, the spin-valve head using an exchange-bias layer was well on its way to displacing the AMR head. The General Electric Company, or GE (NYSE: GE) is a multinational technology and services company. ... International Business Machines Corporation (IBM, or colloquially, Big Blue; NYSE: IBM) is a computer technology firm headquartered in Armonk, NY, USA. The company, which was founded in 1888 and incorporated June 15, 1911, manufactures and sells computer hardware, software, infrastructure services, hosting services, and consulting services. ...

See also

• "New Magnetic Anisotropy," W. H. Meiklejohn and C. P. Bean, Phys. Rev. 105, 904–913 (1957).
• S. Chikazumi and S.H. Charap, Physics of Magnetism, ASIN B0007DODNA.
• J. Nogués and Ivan K. Schuller, "Exchange Bias", J. Magn. Magn Mat. 192, 203 (1999).
• 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 0124666272.
• Ivan K. Schuller and G. Guntherodt, "The Exchange Bias Manifesto," 2002.