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

Abstract

The structural and magnetic properties of functional Ni-Mn-Z (Z = 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 ferroand 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. PACS numbers: 75.50.-y, 75.10.-b, 75.30.SgFollowing the concepts of Hume-Rothery the influence of composition on martensitic and magnetic transformation temperatures is commonly condensed as a dependency of electrons per atom (e/a-ratio) [1]. Experiment and first-principles calculations, however, reveal that the Z element in Ni-Mn-Z Heusler alloys (Z = Ga, In, Sn) also affects the transformation temperatures substantially [2]. Moreover, recent experiments on samples with identical composition but different heat treatment indicate that chemical disorder also plays an important role [3][4][5]. Here, we use first-principles calculations to identify the influence of chemical disorder on the magnetic exchange parameters and derive guidelines for a further systematic improvement of magnetocaloric materials [6].Besides the magnetocaloric effect (MCE) in Gd and other alloys at room temperature [7,8], the metamagnetic Ni-Mn based Heusler materials [9,10], have attracted much interest recently [11,12]. In these alloys the metamagnetic features are responsible for magnetic glass behavior and frustration due to chemical disorder [13][14][15] as well as unusual magnetization behavior under an external magnetic field such as a large jump of the magnetization ∆M (T m ) at the martensitic/magnetostructural transformation temperature T m [16]. This gives rise to the large inverse MCE of the materials [9,10,17,18]. The MCE can be influenced when Ni is substituted in part by Co: It is strongly enhanced in the case of In-based intermetallics [19,20] (with adiabatic temperature change ∆T ad = −6 K in 2 T field [20]) while in the case of Ga the MCE is turned from direct to inverse by decoupling T m and Curie temperature T C [21] (with ∆T ad = −1.6 K in 1.9 T field [22,23]).Chemical disorder in the Mn-rich Heusler alloys is responsible for competing magnetic interactions (ferromagnetic versus antiferromagnetic) because the extra Mn atoms occupy lattice sites of the Z-sublattice which interact antiferromagnetically with the Mn atoms on the Y-sublattice due to RKKY-type interactions. This competition of magnetic interactions leads to the characteristic drop of magnetization curves at T m , wh...

Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies

“…5 for the case of Ni-CoMn-Ga-In [26,27]. This extraordinary behavior is displayed by other Heusler alloys, too [29]. The resulting adiabatic temperature change ∆T ad for this alloy is still quite small (1.6 K) in spite of the large change of M showing that the magnetocaloric effect (MCE) needs further optimization compared to the classical Gd-Si-Ge, Fe-Rh and La-Fe-Si MCE materials, which all display ∆T ad of about 13 K and a large isothermal entropy change ∆S iso .…”

“…When adding Co to the Heusler alloys for getting better magnetocaloric effects, the magnetostructural transition is shifted to lower temperatures because of decrease of e/a and eventually the martensitic phase vanishes altogether for larger Co concentration. We have recently argued that the phenomenon of metamagnetism in Heusler alloys can be understood by considering the complex spin configurations which appear due to spin-spin interactions arising from the orbital-resolved magnetic exchange coupling constants [29]. We have observed that in austenite, the ferromagnetic exchange interactions (between the t 2g orbitals of the Mn atoms on their original sublattice sites) can nearly be compensated by the antiferromagnetic exchange interactions associated with the e g orbitals [29,30].…”

“…In the following we show results for the magnetocaloric effect calculated with the help of ab initio calculations and Monte Carlo simulations [22,29] in comparison to experimental data. In the Monte Carlo study we use an effective spin model which includes elastic and mangetoelastic interactions.…”

“…We have recently argued that the phenomenon of metamagnetism in Heusler alloys can be understood by considering the complex spin configurations which appear due to spin-spin interactions arising from the orbital-resolved magnetic exchange coupling constants [29]. We have observed that in austenite, the ferromagnetic exchange interactions (between the t 2g orbitals of the Mn atoms on their original sublattice sites) can nearly be compensated by the antiferromagnetic exchange interactions associated with the e g orbitals [29,30]. This feature appears in all magnetic Heusler alloys of the NiMn based series [30] (see the highlighted J ij values in Fig.…”

“…Regarding the spin-spin interactions, we have used a Potts model, which allows to keep track of the various spin multiplicities such as S = 5/2 for Mn 3+ , S = 1 for Ni 2+ , S = 3/2 for Co 2+ and S = 4/2 for Cr 2+ . For details, see [22,24,29]. The structural and magnetic properties (including magnetic exchange coupling constants) have been calculated using ab initio method whereas the thermodynamic properties and MCE have been obtained from Monte Carlo simulation, using whereby the entropy change across the magnetostructural transition in an external magnetic field is obtained by integrating the specific heat (using a model Hamiltonian which consists of magnetic, elastic and magnetoelastic terms, see [42]).…”