Application of the ratio Δλ/ΔM to giant magnetostrictive materials in the <110> easy regime

The parameter Δλ/ΔM is a useful tool for investigating the magnetization process in magnetostrictive materials. This work extends the use of this parameter from that previously explored in the <100> and <111> easy regimes to the <110> easy regime. Theoretical values of Δλ/ΔM are presented for all possible domain wall motions that produce an increase in magnetization. In the <110> easy regime, Δλ/ΔM has contributions from both λ100 and λ111, but for practical purposes it is only necessary to consider the latter for the C15 phase giant magnetostrictive materials, because λ111≫λ100. Under such conditions there are 61 different wall motions but only four (nonzero) values for Δλ/ΔM. We have obtained the theoretically generated magnetostrain versus magnetization curve, derived from a model of distinct domain wall motions with successively higher values of Δλ/ΔM. It is shown that this curve is a good fit to the experimental magnetostrain versus magnetization curve obtained for Tb0.1Ho0.9Fe1.9, measured at intervals over the temperature range 213-373 K. Since it is known that equivalent theoretical curves in the <100> and <111> easy regimes model the magnetization process well, and that TbxHo1-xFe1.9 (0≪x≪0.2) undergoes a spin reorientation from <111- > to <110> at approximately 260 K, and then to <100> as the temperature is reduced further, we conclude that Δλ/ΔM may be used in the <110> easy regime. However, the large number of wall motions makes it more difficult to identify any particular motion.