Soft magnetic properties and field induced anisotropies in Fe-metalloid based nanocrystalline materials with high saturation magnetisation

Abstract

The effect of planar flow casting parameters on the microstructural development and soft magnetic properties of Fe-metalloid nanocrystalline materials with a high saturation magnetisation has been investigated. Furthermore, the size and potential origin of a field induced anisotropy for this class of materials has also been examined. Fe-metalloid nanocrystalline magnetically soft materials, such as the Fe-B-(Si)-Cu alloy system, can offer the unique combination of a large saturation magnetisation while still maintaining relatively low core-losses. However, the nanocrystallisation process is known to be strongly influenced by the microstructure present in the precursor materials, which may differ considerably between batches due to the natural variability in the planar flow casting process used for its production. This work therefore investigates the origin of this variability and the effect that different processing parameters have on the final microstructure and magnetic softness for a nanocrystalline Fe₈₂.₅B₁₄Si₂Cu₁.₅ alloy. It is found that the ribbon geometry and the casting substrate speed both strongly influence the magnetic softness after nanocrystallisation and that by closely controlling these parameters the variability between batches may be reduced. Furthermore, these findings also suggest that variations in the ribbon-substrate contact and ribbon geometry may lead to significant fluctuations in the cooling rate experienced by different regions within an as-cast ribbon and that this might lead to an inhomogeneous microstructure. Therefore, these findings suggest that by tailoring the substrate speed and ribbon geometry for a given composition, it may be possible to achieve a far larger and more consistent cooling rate than that currently achievable by planar flow casting. It is known that a substantial field induced anisotropy has the potential to degrade magnetic softness in nanocrystalline materials at small grain sizes. Therefore, the size and origin of a field induced anisotropy located within the crystalline phase of an Fe-rich, Si-poor Fe-metalloid nanocrystalline alloy is also investigated. It is seen that a substantial field induced anisotropy exists within the crystalline phase of a Fe-B-(Si)-Cu alloy system and that this may negatively impact its magnetic softness. It is also found that a small uniaxial lattice distortion exists within the crystalline phase after field annealing for nanocrystalline Fe₉₄₋ₓNb₆Bₓ (x = 10, 12 and 14). The size of this lattice distortion is shown to agree well with predictions made by the magnetoelastic model and that this is the likely origin of the field induced anisotropy seen to exist in Si-free Fe-metalloid nanocrystalline alloys. Therefore, with this new understanding it may now be possible to develop novel methods for the control or removal of field induced anisotropies from nanocrystalline Fe-rich materials which may potentially improve their soft magnetic properties.