Epitaxial Bain path of tetragonal Fe

Abstract

The epitaxial Bain paths ͑EBP's͒ of tetragonal Fe are found by minimizing the total energy with respect to c at each a using first-principles total-energy calculations with the local-spin-density approximation plus a generalized gradient approximation and relativistic corrections. The energy minima along the EBP's give the equilibrium states of the ferromagnetic, nonmagnetic and two antiferromagnetic phases-the type-I phase, called here AF1, and a phase in which pairs of ferromagnetically coupled ͑001͒ layers alternate in moment, called here AF2. The AF2 phase is found to be lower in energy than AF1 when each is in equilibrium and also when the ͑001͒ plane lattice constant is strained to the Cu͑001͒ lattice constant. Calculations of the six elastic constants of AF2 at equilibrium inserted in the four tetragonal stability conditions show that it is unstable for ͓100͔ and ͓010͔ shears in the ͑001͒ plane, but is stabilized by epitaxy on Cu͑001͒. The AF2 phase is indicated to be the phase of the bulk of epitaxial Fe films on Cu͑001͒. The bulk structure of Fe films on Cu͑001͒ found by quantitative low-energy electron diffraction analysis is shown to be close to the EBP's of the AF1 phase, but farther from the AF2 phase. On substrates with slightly larger lattice constants than Cu͑001͒, the ferromagnetic phase is found to be more stable than AF2.

Prediction of first-order martensitic transitions in strained epitaxial films

“…In the case of magnetic systems, high-spin to low-spin transitions [9] or a change of the ground state magnetic order [17] are usually accompanied by discontinuities of the lattice geometry. An example might be the a-dependence of the magnetic ground state of BCT Fe found by Qiu et al [17]. These authors did not, however, discuss the consequence of a transition into a different magnetic state for the lattice parameter c of coherent films.…”

“…In the past, the EBP model was used to predict metastable states in transition metals [11,[13][14][15][16], to investigate the magnetic order in strained overlayers [17][18][19][20], and to identify the mother phases of strained bulklike films [11,[21][22][23]. In the same context, we recently suggested Ru, Os, and U films to be ferromagnetic in certain ranges of epitaxial strain [24].…”

Effects of Co doping on the metamagnetic states of the ferromagnetic fcc Fe–Co alloy

“…Furthermore, the most stable state predicted by ab initio state of the art calculations is a spin-spiral configuration [28][29][30], which is very close to a collinear antiferromagnetic double-layer arrangement (AF-II). Nevertheless, an elastic stability analysis of fcc Fe in the FM, AF-I, and AF-II configurations show that all these magnetic configurations are unstable [31]. Thus, the epitaxial film is not a true metastable phase but might be called a constrained metastable phase.…”