Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Abstract

Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right‐hand side). The cubic austenitic phase at high temperature shows long‐range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non‐modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long‐range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non‐magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape‐memory and exchange‐bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone‐depleting and greenhouse chemicals compared with conventional fluid‐compression technology. Experimental phase diagram of Ni50−x Cox Mn39 Sn11 for 0≤x≤10. Here, TC is the Curie temperature of austenite; at TM the system transforms to paramagnetic martensite and at TS to superparamagnetic martensite (SPM) and then to superspin‐glass martensite (SSG) at TP. The possible strain‐glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field‐cooling/zero‐field‐cooling experiments.

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

“…Figure 2 shows that the materials do not form homogeneous alloys, instead we observe a multiplicity of coexisting non-modulated (L1 0 ) and modulated (5M, 7M) intermediate martensitic structures because of chemical clustering effects in the alloys [23,24]. This also leads to multiple spin configurations.…”

“…With increasing temperature (which can be simulated using the fixed spin moment method allowing to calculate binding curves at reduced magnetization) "ferro" and "ferri" solutions move relative to each other until the "ferro" solution becomes lower in energy at the martensitic to austenitic transition. The magnetic exchange coupling constants can be obtained from ab initio calculations [24] and are plotted in Fig. 4 for the case of Mn-excess Ni 50 Mn 30 Ga 20 compared to stoichiometric Ni 2 MnGa.…”

“…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]).…”

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

“…In order to evaluate the magnetic exchange coupling constants, magnetic moments, and equilibrium magnetic reference states in Co-and In-doped alloys, we first performed ab initio calculations for the cubic L2 1 structure with lattice parameter a 0 = 5.85 Å and the tetragonal L1 0 structure with different tetragonal ratios, c/a, assuming that the volume of the unit cell does not practically change with structural distortion. The equilibrium lattice parameter a 0 was taken from our recent ab initio calculations [66] using the Vienna ab initio simulation package (VASP). In that work, we have done equilibrium energy calculations with tetragonal distortion c/a using 16- The calculation of exchange coupling constants has been performed for "ferro" and "ferri" configurations in austenite and martensite using the SPR-KKR package [69,70], where the Heisenberg coupling constants J ij are determinated by employing Liechtenstein's method [71] and the spin-polarized scalar-relativistic mode and atomic sphere approximation (ASA).…”

Magnetic states of C-doped Ni43.75Co6.25Mn37.5In12.5Heusler alloys

“…The value of 5.96 Å has been estimated theoretically from ab initio relaxation calculations presented in Refs. [21,22]. The equilibrium lattice constants and bulk moduli of cubic C-doped Ni 7 Co 1 Mn 6 In 2 alloy calculated for different supercells and spin configurations are listed in Table 1.…”