Ab Initio and Monte Carlo Approaches For the Magnetocaloric Effect in Co- and In-Doped Ni-Mn-Ga Heusler Alloys

Abstract

Abstract:The complex magnetic and structural properties of Co-doped Ni-Mn-Ga Heusler alloys have been investigated by using a combination of first-principles calculations and classical Monte Carlo simulations. We have restricted the investigations to systems with 0, 5 and 9 at% Co. Ab initio calculations show the presence of the ferrimagnetic order of austenite and martensite depending on the composition, where the excess Mn atoms on Ga sites show reversed spin configurations. Stable ferrimagnetic martensite is found for systems with 0 (5) at% Co and a c/a ratio of 1.31 (1.28), respectively, leading to a strong competition of ferro-and antiferro-magnetic exchange interactions between nearest neighbor Mn atoms. The Monte Carlo simulations with ab initio exchange coupling constants as input parameters allow one to discuss the behavior at finite temperatures and to determine magnetic transition temperatures. The Curie temperature of austenite is found to increase with Co, while the Curie temperature of martensite decreases with increasing Co content. This behavior can be attributed to the stronger Co-Mn, Mn-Mn and Mn-Ni exchange coupling constants in austenite compared to the corresponding ones in martensite. The crossover from a direct to inverse magnetocaloric effect in Ni-Mn-Ga due to the substitution of Ni by Co leads to the appearance of a "paramagnetic gap" in the martensitic phase. Doping with In increases the magnetic jump at the martensitic transition temperature. The simulated magnetic and magnetocaloric properties of Co-and In-doped Ni-Mn-Ga alloys are in good qualitative agreement with the available experimental data.

Critical behavior and magnetocaloric effect in Co50−Ni Cr25Al25 (x= 0 and 5) full Heusler alloy system

“…Therefore, when Ni (3d 8 4s 2 ) substitutes for Co (3d 7 4s 2 ), e/a increases. It is expected that the Curie temperature will decrease because of the weak ferromagnetic nature of Ni as compared with Co [36,37]. It seems that T C is sensitive to Ni addition; however, no direct relationship between T C and Co additions or the valence electron concentration e/a can be established.…”

Field-temperature phase diagrams of freestanding and substrate-constrained epitaxial Ni-Mn-Ga-Co films for magnetocaloric applications

“…37 The transformation intervals into martensite for the substrate-constrained and the freestanding film are both very small and exhibit a two-stage process: first, an abrupt drop and then a gradual decrease of magnetization until a steady magnetization is reached below M F . When heating up again and passing the austenitic start temperature A S , the sample starts to transform back into austenite.…”

“…For the forward transformation, a magnetic field makes the growth process more difficult since the martensite exhibits a substantial magnetocrystalline anisotropy. 37,54 As Zeeman energy is a volume contribution to free energy and not depending on interfaces, which affect nucleation, a magnetic field influences the growth process. Each nucleus consists of variants with different orientations.…”

The metamagnetic behavior and giant inverse magnetocaloric effect in Ni–Co–Mn–(Ga, In, Sn) Heusler alloys

“…On the other hand, recently we have done ab initio equilibrium energy calculations with tetragonal distortion c a / using 16-atom supercell Calculations have been carried out for two spin configurations referred to as "ferro" (i.e., all spins are parallel) and "ferri" (the spin of Mn Z on the Ga-lattice is reversed). It has been found that the "ferri" solution is energetically favorable for both alloys [16,29]. Therefore, we can propose that any small influences from defects, impurities or addition of magnetic atoms (for example, Co, Cr etc.)…”

“…To accomplish this, the three-dimensional lattice with real unit cell was divided into domains with random initial spin configurations. The intra-domain (intra-cluster) as well as inter-domain (inter-cluster) interactions are realized with the probability containing stochastic competition between the magnetic anisotropy field and the external magnetic field [17,29,33,34]. With respect to Co-doped Ni-Mn-Sn compound, in this paper we discuss magnetocaloric properties of Ni 45 Co 5 Mn 39 Sn 11 .…”