日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Journal of Physics Condensed Matter: Preface

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.

Spintronics: a challenge for materials science and solid-state chemistry.

A remarkably high saturation magnetization of ~0.4mu_B/Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4-40 nm) multiferroic BiFeO_3 which in bulk form exhibits weak magnetization (~0.02mu_B/Fe) and an antiferromagnetic order. The magnetic hysteresis loops, however, exhibit exchange bias as well as vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, like in bulk BiFeO_3, both the calorimetric and dielectric permittivity data in nanoscale BiFeO_3 exhibit characteristic features at the magnetic transition point. These features establish formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO_3.

Ferromagnetism in nanoscale BiFeO3

A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV(2)O(4), we analyze the orbital and magnetic ground state of this system within an ab initio density functional theory approach. While local density approximation+Hubbard U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit (SO) effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the SO coupling along with electronic correlations which play a significant role in determining the orbital structure in these materials.

https://arxiv.org/pdf/cond-mat/0701528.pdf

Orbital order in ZnV(2)O(4).

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Proposed orbital ordering in MnV2O4 from first-principles calculations.

/pdf/proposed-orbital-ordering-in-mnv-2-o-4-from-first-principles-2zte9a75gn.pdf

Proposed Orbital Ordering in MnV$_2$O$_4$ from First-principles Calculations

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide-band-gap spinel $\mathrm{Zn}{\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange-correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.

Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa2O4: A systematic ab initio study

The three-dimensional perovskite manganites R 1 - x A x MnO 3 in the range of hole doping x>0.5 are studied in detail using a double-exchange model with degenerate e g orbitals including intraorbital and interorbital correlations and near-neighbor Coulomb repulsion. We show that such a model captures the observed phase diagram and orbital ordering in the intermediate- to large-bandwidth regimes. It is argued that the Jahn-Teller effect, considered to be crucial for the region x<0.5, does not play a major role in this region, particularly for systems with moderate to large bandwidths. The anisotropic hopping across the degenerate eg orbitals is essential for the understanding of the ground-state phases of this region, an observation emphasized earlier by Brink and Khomskii. Based on calculations using a realistic limit of finite Hund's coupling, we show that the inclusion of interactions stabilizes the C phase, and the antiferromagnetic metallic A-phase moves closer to x=0.5 while the ferromagnetic phase shrinks, in agreement with recent observations. The charge ordering close to x=0.5 and the effect of reduction of bandwidth are also outlined. The effect of disorder and the possibility of inhomogeneous mixture of competing states are discussed.

Tulika Maitra

Papers

Orbital order in ZnV(2)O(4).

Proposed orbital ordering in MnV2O4 from first-principles calculations.

Proposed Orbital Ordering in MnV$_2$O$_4$ from First-principles Calculations

Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa2O4: A systematic ab initio study

Magnetic, orbital, and charge ordering in the electron-doped manganites