Contradictory role of the magnetic contribution in inverse magnetocaloric Heusler materials

In this paper, we illustrate the dilemma of inverse magnetocaloric materials using the example of Heusler alloys. For such materials, the magnetic and lattice contribution to the total entropy change are competing with each other. For the two paradigmatic Heusler systems of Ni-Mn-In and Ni-Mn-In-Co, we provide a systematic comparison of experimental data under different magnetic fields and hydrostatic pressures with magnetic and the magnetocaloric properties obtained from the Heisenberg model. This allows us to separate the lattice and the magnetic contribution to the total entropy of the martensitic transition. Our analysis reveals that a large magnetization change is parasitic, but at the same time it is necessary to drive the magnetocaloric effect. This contradicting role of the magnetic contribution—the dilemma—is a general characteristic of inverse magnetocaloric Heusler materials.

Figure 1

$M-T$ dependences of various Ni-Mn-In (a) and Ni-Mn-In-Co samples (b) measured in ${\mu }_{0}H=1\mathrm{T}$, both for heating and cooling. The dashed and the dotted lines are calculated curves of the saturation magnetization in $1\mathrm{T}$ of austenite and martensite.

Figure 2

Measurements of the entropy change of the transition of Co free (a) and Co containing samples (b) obtained by DSC. The maximum entropy change of the complete transition $\mathrm{\Delta }{S}_{\mathrm{t}}$ from Eq. (3) of the two systems is plotted in dashed lines. For selected samples of each system, the isothermal entropy change $\mathrm{\Delta }{S}_T$ (orange) and the adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}$ (blue) in a magnetic field change of $2\mathrm{T}$ are shown as bars.

Figure 3

Magnetization versus temperature of the compound ${\mathrm{Ni}}_{45.7}{\mathrm{Mn}}_{36.6}{\mathrm{In}}_{13.5}{\mathrm{Co}}_{4.2}$ in magnetic fields up to $14\mathrm{T}$.

Figure 4

Shift of transition temperature of Ni-Mn-In (squares) and Ni-Mn-In-Co (circles) determined from experiment data (literature values are plotted as open symbols). The dashed and solid lines represent $\frac{\mathrm{\Delta }M}{\mathrm{\Delta }{S}_{\mathrm{t}}}$ which were determined combining Eqs. (1), (3), and (4) in magnetic fields of $0\mathrm{T}$ and $2\mathrm{T}$, respectively.

Figure 5

(a) Magnetization as a function of temperature of ${\mathrm{Ni}}_{45.7}{\mathrm{Mn}}_{36.6}{\mathrm{In}}_{13.5}{\mathrm{Co}}_{4.2}$ in a magnetic field of $1\mathrm{T}$ in different pressures of 0, 4.5, and $8.4\mathrm{kbar}$. (b) Magnetization in a magnetic field of $0.2$ and $2\mathrm{T}$, in zero pressure and in $8.4\mathrm{kbar}$. (c) Isothermal entropy change $\mathrm{\Delta }{S}_T$ as a function of temperature in a magnetic field change of 1 (circles) and $2\mathrm{T}$ (triangles) under different pressure.