Abstract
The magnetic properties tuning and volume dependence in the series of quaternary full Heusler alloys with formula Co2Cr1−xYxGa (Y=Ti, V, Mn, Fe, Co, Ni) were studied with a detailed first-principles exploration. We employ the density functional KKR method with the coherent potential approximation, estimating effective Heisenberg exchange constants via the magnetic force theorem together with mean-field Curie temperature (TC) and magnetic moment for compositions in the whole concentration range. The volumetric dependency of these magnetic properties is studied, particularly the pressure derivatives of TC at equilibrium. Our ternary alloy calculations show good agreement with local-density and generalized gradient approximations in the literature. The quaternary alloys show a wide range of tunable magnetic properties, where magnetic moments range from 0.8 to 4.9μB, TC from 130K to 1250K, and dTC/dV values range from −7 to +6.3KÅ−3.
Highlights
Heusler alloys are known for the great variety of compositions and properties, since many elements can occupy the X, Y and Z sites in the L21 structure with fractional occupations [1], allowing a great flexibility
A related aspect which benefits from computation but has been less studied is the variation of exchange interactions, and ferromagnetic Curie temperature (TC) with volume, or hydrostatic pressure
With regards to the magnetic moments, there is not one approximation consistently better for all compounds: local density approximation (LDA) is better for Co2TiGa, generalized gradient approximation (GGA) is better for Co2FeGa and slightly better for Co2CrGa, while for the other compounds both approximations have similar accuracies
Summary
Heusler alloys are known for the great variety of compositions and properties, since many elements can occupy the X, Y and Z sites in the L21 structure with fractional occupations [1], allowing a great flexibility. Experimental investigations of TC with pressure have been specially focused in the Mn based systems, due to coupling between structure and magnetism (with, e.g. the shape memory and magnetocaloric effects), including Ni2MnZ (Z = Al, Ga, In, Sn, and Sb) [11, 12], Au2MnAl and Pd2MnZ (Z = Sn and Sb) [13] For all these alloys, wherein Mn has localized moments, the positive dTC/dP (negative dTC/dV ) measured is consistent with an empirical interaction curve as a function of the distance between Mn atoms [12]. Values of the same order of magnitude have been found for Heusler alloys (for example, 6.2 and 5.6 K/GPa respectively for Ni2MnSn and Ni2MnIn [26]) We found it necessary to study the influence of the exchange-correlation approximation in a series of LDA and GGA calculations. Substituting Ti, V, Co, or Ni for Cr gradually decreases dTC/dV and eventually changes its sign
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