Microscopic origin of the structural phase transitions at the Cr2O3(0001) surface

Abstract

The surface of a Cr 2 O 3 (0001) film epitaxially grown on Cr undergoes an unusual reentrant sequence of structural phase transitions (1 × 1 → √ 3 × √ 3 → 1 × 1). In order to understand the underlying microscopic mechanisms, the structural and magnetic properties of the Cr 2 O 3 (0001) surface are here studied using firstprinciples electronic structure calculations. Two competing surface Cr sites are identified. The energetics of the surface is described by a configurational Hamiltonian with parameters determined using total-energy calculations for several surface supercells. Effects of epitaxial strain and magnetic ordering on configurational interaction are also included. The thermodynamics of the system is studied using Monte Carlo simulations. At zero strain the surface undergoes a 1 × 1 → √ 3 × √ 3 ordering phase transition at T c ∼ 165 K. Tensile epitaxial strain together with antiferromagnetic ordering drive the system toward strong configurational frustration, suggesting the mechanism for the disordering phase transition at lower temperatures.

Finite element analysis of lithiation-induced decohesion of a silicon thin film adhesively bonded to a rigid substrate under potentiostatic operation

“…Under surface concentration loading (c s ¼ 30 fmol/lm 3 ), which is less than half of the limit concentration, Fig. 7(c) shows that interfacial damage occurs even for interfacial energy of 20 J/m 2 , which is larger than the normal values for work of separation, since surface energies of Cr-terminated Cr 2 O 3 (0 0 0 1) and Ni(1 1 1), and work of adhesion of Ni/Cr 2 O 3 interface with aluminum diffusion are less than 3 J/m 2 (Wysocki et al, 2012;Liu et al, 2013). The stress generated during insertion is expected to be large enough to induce material fracture and comminution most likely within the regions near surfaces and corners.…”

Modeling temperature dependent exchange bias in systems with magnetoelectric chromia

“…These are direct consequences of the Bloch T 3/2 law and collective spin oscillation near T N , neither of which are captured by mean-field theory. It is also worth noting that the previous results 18 neglect anisotropy.…”

“…Magnetization vanishes at T N = 307 K. The steep drop in the surface magnetization with temperature may be attributed to the loss by surface sites of half their neighbors, especially the first two nearest neighbors which have the most prominent exchange. Our MC simulation results can be compared with the meanfield calculation by Wysocki et al 18 It can be seen that our bulk sublattice magnetization at high temperature approaches T N more sharply and low temperature magnetization is reduced. These are direct consequences of the Bloch T 3/2 law and collective spin oscillation near T N , neither of which are captured by mean-field theory.…”

Magnetoelectric control of topological phases in graphene

“…1a), assuming the energetically most favorable surface structure terminated with a single Cr layer top and bottom of the slab. 59 Figure S2 shows the DFT-calculated band structure of the free standing graphene layer using the atomic positions of the C atoms in the relaxed graphene/Cr2O3 (0001) interface structure ( Figures 1a,b in the main text). In is seen that the electronic structure projected onto the C pz orbitals exhibits the Dirac-cone dispersions near the Fermi energy at the K and K′ points of the Brillouin zone.…”