1 Univ. Grenoble Alpes, Inst NEEL, F-38000 Grenoble, France
2 CNRS, Inst NEEL, F-38000 Grenoble, France
3 Instituto de Fisica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Abstract:
The two main parameters characterizing the properties of hard magnetic materials are the remanent magnetization, Mr, and the coercive field, Hc. The former of these two quantities is directly linked to the spontaneous magnetization, it depends also on parameters characterizing the microstructure such as grain alignment. Most important is the coercive field, which depends on intrinsic physical parameters (the anisotropy field) but also on extrinsic parameters, such as grain size or grain decoupling. Coercivity is a defect property and this is the very reason why the analysis of magnetization reversal mechanisms is very difficult.
Measurements of Mr and Hc are important to characterize a material’s functional properties, but it is difficult to extract information on reversal mechanisms from them. Simple models suggest that various reversal processes, including coherent rotation and domain wall propagation, have similar temperature dependences to Hc.
The second parameter is the activation volume, v, derived from the comparison between the time dependence of the magnetization and its field dependence. By relating the size of the activation volume and its temperature dependence to intrinsic physical parameters, indirect information may be extracted on the nature of reversal mechanisms.
The third physical parameter to characterize coercivity is the angular dependence of the coercive field. Coherent rotation and domain wall propagation exhibit fundamentally different angular dependencies, thus providing special interest to such measurements.
A number of observations indicate that dipolar interactions are very large in hard magnets, a property related to the intrinsically heterogeneous character of the microstructure. In materials made of sufficiently large grains, such as most sintered magnets, the size of the dipolar interactions may be derived from the measurements of the field required to render initially multi-domain grains single domain.
The same demagnetizing field corrections as in soft magnetic materials are usually applied to permanent magnets. There is however a fundamental difference between both types of materials since the divergence of the magnetization is zero in soft materials but not in hard ones. An approximate expression for demagnetizing field corrections in hard magnets will be proposed. It will be shown that the unexplained over-skewing effect, found in a number of thin films magnets, disappears when appropriate corrections are applied.
Measurements of Mr and Hc are important to characterize a material’s functional properties, but it is difficult to extract information on reversal mechanisms from them. Simple models suggest that various reversal processes, including coherent rotation and domain wall propagation, have similar temperature dependences to Hc.
The second parameter is the activation volume, v, derived from the comparison between the time dependence of the magnetization and its field dependence. By relating the size of the activation volume and its temperature dependence to intrinsic physical parameters, indirect information may be extracted on the nature of reversal mechanisms.
The third physical parameter to characterize coercivity is the angular dependence of the coercive field. Coherent rotation and domain wall propagation exhibit fundamentally different angular dependencies, thus providing special interest to such measurements.
A number of observations indicate that dipolar interactions are very large in hard magnets, a property related to the intrinsically heterogeneous character of the microstructure. In materials made of sufficiently large grains, such as most sintered magnets, the size of the dipolar interactions may be derived from the measurements of the field required to render initially multi-domain grains single domain.
The same demagnetizing field corrections as in soft magnetic materials are usually applied to permanent magnets. There is however a fundamental difference between both types of materials since the divergence of the magnetization is zero in soft materials but not in hard ones. An approximate expression for demagnetizing field corrections in hard magnets will be proposed. It will be shown that the unexplained over-skewing effect, found in a number of thin films magnets, disappears when appropriate corrections are applied.
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