First-principles calculation of the instability leading to giant inverse magnetocaloric effects

The structural and magnetic properties of functional Ni-Mn-$Z$ ($Z=\text{Ga}$, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting $d$-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

Figure 1

(a)–(f) Element and orbital resolved magnetic exchange parameters of austenite and martensite Ni${}_{50}$Mn${}_{30}$Ga${}_{20}$ from ab initio calculations (not all contributions are shown). It is obvious that the effect of ferromagnetic interactions is largely compensated by the influence of the antiferromagnetic interactions. (Austenite: $a=5.85$ Å, martensite: $a=b=5.47$ Å, $c=6.68$, Å, $c/a=1.22$.)

Figure 2

Element resolved density of states of (a) austenite (L2${}_1$, $a=5.96$ Å) and (b) martensite (L1${}_0$, $a=b=5.6$ Å, $c=6.76$ Å, $c/a=1.21$) of Ni${}_{45}$Co${}_5$Mn${}_{37}$In${}_{13}$ showing the stabilization of martensite due to the formation of a pseudogap at $E_F$ (“ferri” means that the spin of Mn on the In sites is reversed).

Figure 3

(a) Monte Carlo magnetization curves (solid lines) of Ni${}_{50}$Mn${}_{25}$Ga${}_{25}$ from the Potts model using ab initio magnetic exchange and magnetocrystalline anisotropy parameters, in comparison with experiment (Ref. [34]). The jump of the magnetization curves vanishes for sufficiently large fields. (b) Theoretical and experimental magnetization curves of Ni${}_{45}$Co${}_5$Mn${}_{37}$In${}_{13}$ in a small and large external magnetic field of 10 mT and 2 T [20]. Here, the jump $\Delta M$ does not vanish for increasing magnetic fields. The inset shows the hysteresis from the first-order cubic-tetragonal transformation.

Figure 4

Adiabatic temperature change of the direct (red circles) and inverse (blue squares) MCE of polycrystalline Ni${}_{45}$Co${}_5$Mn${}_{37}$In${}_{13}$ as obtained from the extended Potts model (diamonds: results for single-domain state; inset: temperature variation of the entropy).