In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

The magnetic and structural transformations in the Heusler-based system ${\mathrm{Ni}}_{0.50}{\mathrm{Mn}}_{0.50\ensuremath{-}x}{\mathrm{In}}_{x}$ are studied in the composition range $0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.25$. While the cubic phase is preserved in the range $0.165\ensuremath{\leqslant}x\ensuremath{\leqslant}0.25$, we find the presence of martensitic transformations in alloys with $x\ensuremath{\leqslant}0.16$. In a critical composition range $0.15\ensuremath{\leqslant}x\ensuremath{\leqslant}0.16$, the magnetic coupling in both austenitic and martensitic states is ferromagnetic. Magnetic field-induced structural transitions are also found in the $x=0.16$ alloy, whereby the structural transition temperature shifts by $42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in a field of $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$.

/pdf/ferromagnetism-in-the-austenitic-and-martensitic-states-of-2ixgukki05.pdf

Ferromagnetism in the austenitic and martensitic states of Ni-Mn-In alloys

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

https://iopscience.iop.org/article/10.1088/0022-3727/39/5/S13/pdf

Modelling the phase diagram of magnetic shape memory Heusler alloys

Ferromagnetic shape memory alloys Ni$_{2+x}$Mn$_{1-x}$Ga were studied in the range of compositions $0.16 \le x \le 0.36$. Experimental phase diagram, constructed from differential scanning calorimetry, transport and magnetic measurements, exhibits distinctive feature in a compositional interval $0.18 \le x \le 0.27$, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic martensite $\leftrightarrow$ paramagnetic austenite. Observed in this interval of compositions a non-monotonous behavior of the magnetostructural phase transition temperature was ascribed to the difference in the exchange interactions of martensitic and austenitic phase and to the competition between increasing number of valence electron and progressive dilution of the magnetic subsystem which occur in the presence of a strong magnetoelastic interaction. Based on the experimental phase diagram, the difference between Curie temperature of martensite $T_C^M$ and Curie temperature of austenite $T_C^A$ was estimated. Influence of volume magnetostriction was considered in theoretical modeling in order to account for the existence of the magnetostructural phase transition over a wide range of compositions.

Phase transitions in Ni2+xMn1-xGa with a high Ni excess

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

Composition dependence on the martensitic structures of the Mn-rich NiMnGa alloys

Giant heterogeneous magnetostriction in Fe–Ga alloys: Effect of trace element doping

Giant magnetostriction is achieved in lightly Tb-doped Fe83Ga17 melt-spun ribbons. The average perpendicular magnetostriction λ⊥ is −886 ppm along the melt-spun ribbon direction in the Fe82.89Ga16.88Tb0.23 alloy and the calculated parallel magnetostriction λ|| is 1772 ppm. These values are more than four times as large as those found in binary Fe83Ga17. The enhanced magnetostriction is attributed to a small amount of Tb entering solution in the A2 matrix phase during rapid solidification. The strong localized magnetocrystalline anisotropy of terbium is thought to cause the giant magnetostriction.

Wei Wu

Papers

Composition dependence on the martensitic structures of the Mn-rich NiMnGa alloys

Giant heterogeneous magnetostriction in Fe–Ga alloys: Effect of trace element doping

Giant magnetostriction in Tb-doped Fe83Ga17 melt-spun ribbons

Tb solid solution and enhanced magnetostriction in Fe83Ga17 alloys

Improved magnetostriction of Fe83Ga17 ribbons doped with Sm