Local magnetovolume effects inFe65Ni35alloys

Abstract

A systematic ab initio study of static ionic displacements in a face-centered-cubic Fe 65 Ni 35 alloy has been carried out. Theoretical results for magnitudes of average Fe-Fe, Fe-Ni, and Ni-Ni ͗110͘ bond vectors agree well with experimental measurements. In addition, we have observed that in collinear ferrimagnetic states, iron-iron nearest-neighbor pairs with antiparallel local magnetic moments are shorter on average than those with parallel moments. Furthermore, having considered different states ͑ferromagnetic, nonmagnetic, and collinear ferrimagnetic states͒ for the same lattice spacing, we have shown that the magnetic structure strongly influences local geometrical properties of the alloy. For example, a transition from a ferromagnetic state to a collinear ferrimagnetic state induces a significant contraction of the volume associated with an iron site where the moment flips. A model system based on a Hamiltonian written as the sum of Lennard-Jones energies and a classical Heisenberg Hamiltonian has been introduced. It yields structural properties which are qualitatively similar to those obtained ab initio. We have found that some of the phenomena can be classified as magnetovolume effects.

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

“…This bears similarities to the situation in the Fe-based Invar and magnetic shape-memory materials, where considerable static displacements are also encountered. 50,51 In this case, for the ordered Fe 3 Pt MSMA, a strong anomalous softening of the TA 1 branch in ͓110͔ direction is found at the zone boundary, which disappears in the disordered case in favor of a smeared out anomaly much closer to the ⌫ point. [52][53][54][55] Thus, the type and degree of disorder of a Co-Ni-Ga alloy, which is controlled experimentally by the annealing conditions, should be considered very important for the martensitic transformation and the ͑magnetic͒ shape-memory properties of the sample.…”

Interatomic Interactions in F.C.C.-Ni–Fe Alloys

“…2) at c > 0.55 is apparently conditioned by both the presence of heterogeneous (atomic and/or magnetic) states [14][15][16][17][18][19][20][21][22][23][24]66] in f.c.c.-Ni-Fe alloys and the simultaneously increasing contribu-tion of itinerant electron magnetism into 'effective' 'exchange' interactions [75,76]. In general case, in the measurands such as T C (c) (25), there is a combination of all parameters such as follow:…”

“…Conditionally, all these theories and models can be divided into two groups: (i) models, which are based on the primary contribution of itinerant electron magnetism [64,75,76] to magnetism of an alloy, and (ii) so-called local magnetic moment model [66][67][68], which implies the carriers of uncompensated magnetic moments as atoms located at the effectively-periodic lattice sites. As for magnetism of f.c.c.-Ni-Fe alloys, the basic complexity for developing such quantitative model is the simultaneous quantification of, firstly, the magnetism of both constituents of an alloy (Ni and Fe), secondly, the significant difference between Ni and Fe magnetic moments, and, thirdly, the availability of two magnetic states of Fe atoms, namely, two so-called Weiss γ-states [18][19][20][21][22][23][24]66], namely, the low-spin (LS) and high-spin (HS) states. There are some methods and approaches for definition of 'exchange' 'integrals' for magnetic interactions in alloys, in particular, MSCF (or MF) approximations [69][70][71][72][73][74]77], cluster methods in the mean-field theory [79], Monte Carlo Ising-type approximation [80], ab initio models [18-24, 75, 76], etc.…”

“…Many theoretical investigations [15][16][17][18][19][20][21][22][23][24][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81] have been devoted to studies of effects of ferro-and (or) antiferromagnetic order in alloys with atomic LRO (SRO). Conditionally, all these theories and models can be divided into two groups: (i) models, which are based on the primary contribution of itinerant electron magnetism [64,75,76] to magnetism of an alloy, and (ii) so-called local magnetic moment model [66][67][68], which implies the carriers of uncompensated magnetic moments as atoms located at the effectively-periodic lattice sites.…”

“…In a concentration region of Fe atoms, 0.45 < c < 0.55, Elinvars are formed (with atomic long-rangeordered L1 0 -CuAuI-type (NiFe) layered structure with temperature factor of electrical resistance possessing high values) [2]. A special attention should be paid to Fe-Ni Invars [1-3, [18][19][20][21][22][23][24] with relative concentration of Fe atoms close to c ≅ 0.65. Near this chemical composition, such an alloy has microheterogeneous structure, which may be probably atomic long-range-ordered of L1 2 -Cu 3 Au type (Fe 3 Ni) in part.…”