Robust half-metallic antiferromagnetsLaAVOsO6andLaA

Abstract

We have theoretically designed three families of the half-metallic (HM) antiferromagnets (AFM), namely, LaAVOsO 6 , LaAMoTcO 6 and LaAMoReO 6 (A = Ca, Sr, Ba), based on a systematic ab initio study of the ordered double perovskites LaABB ′ O 6 with the possible B and B ′ pairs from all the 3d, 4d and 5d transtion metal elements being considered. Electronic structure calculations based on first-principles density-functional theory with generalized gradient approximation (GGA) for more than sixty double perovskites LaCaBB ′ O 6 have been performed using the all-electron fullpotential linearized augmented-plane-wave method. The found HM-AFM state in these materials survives the full ab initio lattice constant and atomic position optimizations which were carried out using frozen-core full potential projector augmented wave method. It is found that the HM-AFM properties predicted previously in some of the double perovskites would disappear after the full structural optimizations. The AFM is attributed to both the superexchange mechanism and the generalized double exchange mechanism via the B (t 2g ) -O (2p π ) -B ′ (t 2g ) coupling and the latter is also believed to be the origin of the HM. Finally, in our search for the HM-AFMs, we find LaACrTcO 6 and LaACrReO 6 to be AFM insulators of an unconventional type in the sense that the two antiferromagnetic coupled ions consist of two different elements and that the two spin-resolved densities of states are no longer the same. It is hoped that our interesting predictions would stimulate further experimental searches for the HM-AFMs which have so far been unsuccessful.

Temperature dependence of the direct piezoelectric effect in relaxor-ferroelectric single crystals: Intrinsic and extrinsic contributions

“…46 Noting the similarities 47 between relaxorferroelectrics and martensites the authors propose a model based on the fine-scale twinning of tetragonal ͑or rhombohedral͒ domains. In their model, the lattice parameters of the observed monoclinic and orthorhombic phases, derived from x-ray diffraction measurements with a finite beam size, are actually the volume-averaged parameters of the finely twinned tetragonal ͑or rhombohedral͒ phase.…”

“…Such a model is indeed in agreement with the experimental evolution of the lattice parameters as a function of electric field and temperature, which satisfy the required "invariance conditions." 45,46 Importantly, in the adaptive phase model, the piezoelectric response when a stress or electric field is applied along a nonpolar direction is inherently coupled to local ferroelastic domain wall motion, albeit on a very fine scale. That is, it will be irreversible wherever domain walls are pinned and Rayleigh-law hysteresis will be expected as a result.…”

Temperature-driven evolution of hierarchical nanodomain structure in tetragonal-like BiFeO3 films

“…In our case, the M C phase of BFO, has a hierarchical nanodomain structure, but consists of the same M C phase in both the stripe domains and the lamellar substructure, which is unlike the adaptive structure of the PMN-PT single crystals. Furthermore, the large tetragonal ratio (c/a = 1.25) in T-like M C phase of BFO wouldn't encourage a domains at large compressive stresses and we did not detect the a/c/a/c… type polydomain structure, which is reportedly in the adaptive domain picture proposed in cubiclike PMN-PT and PZT near their MPB compositions [20, 24,25]. The nanodomain walls in PMN-PT and PZT are along <110> or <100> direction [20,22] while, in BFO, the substructure nanodomain walls do not appear to have any principle crystallographic orientation.…”

“…One also note that the LSAO substrate may not be completely removed in the relatively thick area of plan-view TEM sample and thus is still able to stabilize the M C phase. PT, besides a intrinsic monoclinic M C structure reported by Noheda et al [23], others suggested that the monoclinic phase could be an adaptive phase [24,25] where the tetragonal c/a nanodomains assembled together into submicrodomains, and thus the overall microdomain exhibited a monoclinic nature due to an averaging effect [20,24,25]. In our case, the M C phase of BFO, has a hierarchical nanodomain structure, but consists of the same M C phase in both the stripe domains and the lamellar substructure, which is unlike the adaptive structure of the PMN-PT single crystals.…”

Correlation between non-180° domain structures in (1−x)PbA1∕3Nb2∕3O3−xPbTiO3 single crystals (A=Mg or Zn) under an applied [001] electric field

“…Recently, Wang 28 carried out a systematic analysis of the lattice parameters and their changes at the T − M C phase transition in PMN− xPT and PZN− xPT compositions near the MPB. This approach was based on three intrinsic relationships previously derived assuming an adaptive ferroelectric phase transformation: 19 there the lattice parameters of the intermediate M C phase were related to those of the T and C ones.…”