Singular point detection of discontinuous magnetization processes

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Abstract
Ferromagnetic crystals can show discontinuous jumps in the magnetization curve for certain combinations of the anisotropy constants. These are called first-order magnetization processes (FOMP) and have been recently observed in a large variety of compounds. We analyze the problem of how this singularity in the magnetization curve is transformed in the case of a polycrystalline specimen. The problem is the same as that of the singular point detection theory (SPD) that was originally concerned with the singularities of the anisotropy fields and with the way of enhancing them. SPD is now a widely used method for measuring anisotropy fields. In the present work it is demonstrated that SPD can be extended to FOMP, thus allowing extremely clear and accurate measurements of critical fields using polycrystalline specimens. This provides in principle a powerful method for deep investigations in the anisotropy properties of ferromagnetic and ferrimagnetic materials. In Sec. II we examine how the critical parameters of FOMP are modified when the magnetizing field is rotated out of symmetry directions. It is demonstrated that an additional transition is possible within certain conditions of anisotropy constants and field orientations (extraordinary FOMP). Section III is an extension of the Stoner–Wohlfarth model including anisotropy constants up to the sixth order (K1, K2, and K3). In Sec. IV we determine by analytical methods the order of derivative dnM/dHn at which the singularity becomes apparent. For FOMP of type P a discontinuity Δχ appears in the differential susceptibility, while FOMP of type A give rise to a discontinuity in the second derivative of the magnetization d2M/dH2.

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