Effect of configurational order on the magnetic characteristics of Co-Ni-Ga ferromagnetic shape memory alloys

Abstract

In most of the ternary (and higher order) ferromagnetic shape memory alloys (FSMAs) with compositions close to the A2BC stoichiometry, the austenite phase exhibits L21-type ordering. Recent investigations of the Co-Ni-Ga FSMA system, however, suggest that the austenite phase has B2-type ordering, although definite confirmation remains elusive. In this work, we present a theoretical investigation of the effect of configurational order on the magnetic properties of ordered (L21) and disordered (B2) FSMA Co2NiGa. Through the use of calculations based on density functional theory, we predict the structural and magnetic properties (including magnetic exchange constants) of ordered and disordered Co2NiGa alloys. We validate our calculation of the magnetic exchange constants by extracting the Curie temperatures of the austenite and martensite structures and comparing them to experiments. By constructing a q-state Potts magnetic Hamiltonian and through the use of lattice Monte Carlo simulation, we predict the finite temperature behavior of the magnetization, magnetic susceptibility as well as the magnetic specific heat and entropy. The role of configurational order on the magnetic properties of the phases involved in the martensitic phase transformation is discussed and predictions of the magnitude of the magnetic contributions to the transformation entropy are presented.The calculations are compared to experimental information available in the literature as well as experiments performed by the authors. It is concluded that in FSMAs, magnetism plays a fundamental role in determining the relative stability of the austenite and martensite phases, which in turn determines the martensitic transformation temperature MS, irrespective of whether magnetic fields are used to drive the transformation.

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“…Figure 8 contains a comparison of the phonon dispersions of Ni 2 MnGa and Co 2 NiGa along the [110] direction while Figure 9 highlights the dominant ferromagnetic interactions in CoNiGa for both the ordered and disordered cases in contrast to the pronounced antiferromagnetic tendencies in NiMnIn shown in Figure 5. For further discussion of CoNiGa alloys we refer to ref 15…”

“…Therefore, an intensive search for new materials with transformation temperatures higher than room temperature and better elastic and mechanical properties has been undertaken during the last decade. In this context, a variety of different compounds have been targeted, ranging from the Heusler Ni‐based magnetic shape‐memory alloys (MSMAs) like NiMnZ (with Z = Ga, In, Sn, Sb),6 Fe‐based alloys like FePt or FePd,7, 8 Co‐based alloys like CoNiAl9 and CoNiGa,10–15 to Cu‐based alloys like CuMnGa,16 and also quaternary alloys such as NiMnInCo17 and PtNiMn(Ga,Sn) 18. The necessity of expanding the repertoire of shape memory alloys (SMAs) has recently been highlighted by Ma and Karaman 19.…”

Multiple ferroic glasses via ordering

“…By holding the composition constant in the alloy under investigation, i.e. Ni 45 Co 5 Mn 36.6 In 13.4 magnetic shape memory alloy (MSMA), the strain glass / cluster spin glass is examined in this work without affecting the alloy"s electron-to-atom ratio (e/a ratio) or magnetic valence that have considerable influence on its properties [26][27][28]. Different degrees of configurational order, which produce different defect concentrations and distributions, were achieved in this off-stoichiometric Heusler alloy through heat treatments between 673 K and 1073 K. It is will be shown here in the present work that these heat treatments affect the material"s configurational order and point defect concentration of the material which enhances T C and T f , suppresses T Ms and ultimately stabilizes the formation of a strain glass phase.…”

First-principles computation of structural, elastic and magnetic properties of Ni2FeGa across the martensitic transformation

“…The critical issues related to the martensitic transformation are the changes in the crystallographic and electronic structures. Though these issues have had extensive theoretical study for Co-Ni-Ga alloys [9][10][11][12], not much attention has been paid to Ni-Fe-Ga systems except in the works of Liu et al [13], Bai et al [14] and Qawasmeh and Hamad [15]. Employing a full-potential linearized-augmented plane-wave (FP-LAPW) method, Liu et al [13] considered the martensitic phase to be orthorhombic in structure and focused on the electronic and magnetic properties of the system as it undergoes the martensitic transformation.…”