Processing of Nanocrystalline Alloys Induced Magnetic Anisotropy

The magnetization characteristics of Finemet-type nanocrystalline magnets (FeCuNbSiB alloy) similar to those of metallic glasses, also can be well controlled by the magnetic anisotropy induced by field annealing, stress annealing, and stress plus field annealing. Magnetic field annealing induces uniaxial anisotropy with the easy axis parallel to the direction of the magnetic field applied during the heat treatment. The magnitude of the field-induced anisotropy in soft nanocrystalline alloys depends upon the annealing conditions (that is, if the magnetic field is applied during the nanocrystallization process or firstly the sample is nano-crystallized and then submitted to field annealing) [63, 66] and on the alloy composition (relative percentage content of Fe and metalloids) [67]. Nevertheless, this field-induced anisotropy is induced at a temperature range of 300 to 600 ° C (above the Curie temperature of the residual amorphous matrix and below that of the Curie temperature of the a-Fe(Si) grains. Thus, the anisotropy induced during nano-crystallization should primarily originate from the bcc grains. The amorphous matrix has a rather inactive part, since its Curie temperature is far below the typical field annealing temperature.

The evolutions of the different types of anisotropy induced in a typical alloy susceptible to being nano-crystallized, as a function of current density (thermal treatment carried out by current annealing technique under action of stress and/or field) are shown in Figure 7. As can be seen, stress and stress plus field induced anisotropies increase with the current density (temperature) up to a maximum value at 45 A/mm2, which may be related to a maximum of the coercive field. The increase of induced magnetic anisotropy up to 45 A/mm2 could be ascribed to an increase in the intensity of the interactions between the metallic atoms, and, consequently, an increase of the induced anisotropy could be expected. This argument is linked to the internal stress relaxation produced by thermal treatment in the metallic glasses. Similarly, field-induced

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