Magnetic Anisotropy: Hard and Soft Materials

Dear Colleagues,

In ferromagnetic materials, the experimental evidence indicates a directional dependence of the magnetic properties. This phenomenon is called magnetic crystalline anisotropy. The magnetic response to a ferromagnetic crystal depends on the direction of the magnetic field with respect to the crystallographic axes. This implies that there are preferred orientations in space along which the spontaneous magnetization of the magnetic system is to be directed in equilibrium conditions. These are the easy-directions for spontaneous magnetization, which are observed at all temperatures below the Curie temperature in ferromagnets. For ferromagnetic crystals with hexagonal close-packed (hcp) structure (e.g., cobalt), the room-temperature magnetization at equilibrium can be found to be aligned along the positive or negative easy axis (i.e., uniaxial anisotropy). Other types of anisotropy include the stress anisotropy related to magnetoelastic coupling and the magnetostriction, and the magnetic shape anisotropy linked to the magnetostatic energy of a ferromagnetic body with anisotropic shape. Magneto-elastic, random, and induced anisotropies are the anisotropy contributions relevant for soft magnetic materials. Regarding the surface or interface anisotropy in nanostructured and/or multilayered thin-film systems, the origin of surface anisotropy stems from the stability of the crystal symmetry at the surface and plays a dominant role in defining the effective anisotropy of a magnetic nanoparticle or ultra-thin film. On the other hand, interfacial anisotropy arises at the interface between hard and soft ferromagnetic layers or between a ferromagnetic (FM) and an antiferromagnetic (AFM) layer, and is related to the exchange interaction between layers. Typically, in FM/AFM systems this type of anisotropy is unidirectional. The AFM layers can remarkably modify the response of an adjacent FM layer by inducing an exchange bias field with an important application in magnetic hard disk technology. One other high-technology priority is the demand to increase the storage density of magnetic recording media, which leads to the continuous development of new advanced magnetic nanostructures and the engineering of their anisotropy. The latter can be achieved by tuning the growth properties to promote perpendicular anisotropy in bilayer thin films using a combination of low- and high-anisotropy layers, or by continuously varying the anisotropy along the length of a columnar grain that could lead to an even better graded anisotropy, where the coercivity is further reduced with the anisotropy gradient.

Dr. Thanassis Speliotis
Guest Editor

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• magnetic anisotropy
• magnetic stress anisotropy
• magnetic graded anisotropy