Short-range Orbital Ordering Research Articles

Multiscale investigation of magnetic field distortion on surface of ferromagnetic materials caused by stress concentration for metal magnetic memory method

In this paper, we use multiscale modeling to explore the coupling of the atomic orbital and magnetic moment and their evolutions under the excitations of external stress and magnetic fields. Our results of density functional theory (DFT) calculations show that strain-induced spin reorientation and redistribution of orbital coupling are the fundamental reasons for altering the atomic magnetic moment, magnetic anisotropy, and exchange interactions. Also, magnetic anisotropy and exchange interactions are found to play a more crucial role in the magnetization reversal process than the atomic magnetic moment. By using micromagnetics approach and the finite element method (FEM), the macroscopic magnetic properties such as coercivity, magnetic permeability, and remanence alter as a consequence of modified electronic structure, which eventually leads to the distortion of the macroscopic magnetic field of the stress concentration area. Through DFT, micromagnetics and FEM approaches, the cross-scale simulations herein highlight the remarkable short-range orbital ordering and coupling in ferromagnetic materials which provide the fresh insight in the stress-magnetic coupling for MMM applications.

Local Structural Analysis of Half-Metallic Ferromagnet CrO2

We have performed powder neutron diffraction on the half-metallic ferromagnet CrO2 which has a rutile-type crystal structure with a tetragonal unit cell. Although the powder diffraction pattern can be fitted by the reported crystal structure including a single Cr site, the atomic pair distribution function (PDF) can be fitted by the structural model with an orthorhombic unit cell including two kinds of inequivalent Cr sites. The difference between the valences of the two inequivalent Cr sites, δ of Cr+4±δ, estimated from the local structural parameters is about 0.06. The shapes of the two CrO6 octahedra are slightly different, suggesting the short-range orbital ordering of the Cr 3d orbitals. The lattice distortion and the improvement of the fitting to the PDF obtained using the locally distorted structure model are apparent in the region below about 10 A, suggesting that the domain size or correlation length of the locally distorted structure is about 10 A, roughly corresponding to the size of two unit c...

Spin re-orientation in FeCr2S4

The spinel FeCr2S4has been studied intensely for its peculiar magnetic and local structural changes which are sensitively influenced by the Jahn–Teller properties of Fe2 + in tetrahedral sulfur coordination. Recent muon spin rotation data give strong evidence that the commonly assumed collinear magnetic structure of this compound is only found between the Curie temperature TC = 165 K and 50 K. For lower temperatures a helical structure has been proposed. We present new Mossbauer spectroscopic data taken on the same sample as used for muon spin rotation. Also the hyperfine spectra revealing non-equivalent iron sites support the appearance of a spin re-orientation around 50 K which may be related to the onset of short-range orbital order. Below 20 K severe dynamic broadenings are found which may indicate orbital fluctuations. Orbital order occurs around 11 K accompanied by severe changes in the crystalline electric field ground state as traced from quadrupole interaction.

Structural anomalies and the orbital ground state inFeCr2S4

We report on high-resolution x-ray synchrotron powder-diffraction, magnetic-susceptibility, sound-velocity, thermal-expansion, and heat-capacity studies of the stoichiometric spinel ${\text{FeCr}}_{2}{\text{S}}_{4}$. We provide clear experimental evidence of a structural anomaly which accompanies an orbital-order transition at low temperatures due to a static cooperative Jahn-Teller effect. At 9 K, magnetic susceptibility, ultrasound velocity, and specific heat reveal pronounced anomalies that correlate with a volume contraction as evidenced by thermal-expansion data. The analysis of the low-temperature heat capacity using a mean-field model with a temperature-dependent gap yields a gap value of about 18 K and is interpreted as the splitting of the electronic ground state of ${\text{Fe}}^{2+}$ by a cooperative Jahn-Teller effect. This value is close to the splitting of the ground state due to spin-orbit coupling for isolated ${\text{Fe}}^{2+}$ ions in an insulating matrix, indicating that Jahn-Teller and spin-orbit coupling are competing energy scales in this system. We argue that due to this competition, the spin-reorientation transition at around 60 K marks the onset of short-range orbital ordering accompanied by a clear broadening of Bragg reflections, an enhanced volume contraction compared to usual anharmonic behavior, and a softening of the lattice observed in the ultrasound measurements.

Orbital domain dynamics in a doped manganite

We explore a number of novel effects near the orbital-order phase transition in a half-doped manganite, Pr0.5Ca0.5MnO3. To probe the unusual short-range orbital order in this system, we have performed coherent soft x-ray resonant scattering measurements in a Bragg geometry to measure dynamics. Near the transition temperature, we observe a small fluctuating component in the scattered signal that is correlated with three effects: a rapidly decreasing total signal and orbital domain size, as well as an abrupt onset of a broad background intensity that we attribute to the thermal production of correlated polarons. Our speckle results suggest that the transition is characterized by a competition between a pinned orbital domain topology that remains static and mobile domain boundaries that exhibit slow, temporal fluctuations.

Orbital-Ordering Transition inSr2VO4

Temperature-dependent dc susceptibility and x-ray powder diffraction on a pure tetragonal sample of Sr(2)VO(4) show an antiferromagnetic orbital-ordering transition at T(oo) = 97 K, in which the occupied orbitals lie along the c axis. The unusual broadening of the x-ray Bragg peaks throughout the orbital-ordering transition temperature region indicates that this process occurs in stages, and the onset of short-range orbital ordering occurs at T(1) approximately 122 K. The study of the order parameter associated with this transition by analyzing the spontaneous strain results in a critical exponent beta = 0.35(2) consistent with 3D Heisenberg behavior. These results experimentally confirm the orbital-ordering state in Sr(2)VO(4) predicted by first-principles calculations using combinations of the local-density approximation and the GW method.

Absence of true long-range orbital order in a two-leg Kondo ladder

We investigate, through the density-matrix renormalization group and the Lanczos technique, the possibility of a two-leg Kondo ladder presenting an incommensurate orbital order. Our results indicate staggered short-range orbital order at half-filling. Away from half-filling our data are consistent with incommensurate quasi-long-range orbital order. We also observed that an interaction between the localized spins enhances the rung-rung current correlations.

Orbital degree of freedom in single-layeredLa1−xSr1+xMnO4: Doping- and temperature-dependent rearrangement of orbital states

Oxygen $1s$ near-edge x-ray absorption spectroscopy and x-ray diffraction investigations on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5)$ single crystals evidence a doping- and temperature-dependent redistribution of ${e}_{g}$ electrons between ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, and ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals: For undoped $\mathrm{La}\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{4}$ the ${e}_{g}$ electrons predominantly reside on ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals, nevertheless a small amount of charge carriers is found on in-plane ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals. These in-plane electrons are gradually transferred to the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ states upon cooling. Sr doping, however, does not only provide holes to the system but also induces a continuous transfer of electrons from ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ to ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals and, finally long-range charge-exchange-type orbital ordering develops for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ below $T\ensuremath{\leqslant}225\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The data indicate a short-range orbital ordering for the intermediate doping range.

Local order and frustration in the geometrically frustrated spinelsCd1−xZnxV2O4

Orbitally degenerate frustrated spinels, ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$ were investigated using elastic and inelastic neutron scattering techniques. In the end members with $x=0$ and 1, a tetragonal distortion $(c<a)$ has been observed upon cooling mediated by a Jahn-Teller distortion that gives rise to orbital ordering. This leads to the formation of spin chains in the $ab$-plane that upon further cooling, N\'eel ordering is established due to interchain coupling. In the doped compositions, however, the bulk susceptibility, $\ensuremath{\chi}$, shows that the macroscopic transitions to cooperative orbital ordering and long-range antiferromagnetic ordering are suppressed. However, the inelastic neutron scattering measurements suggest that the dynamic spin correlations at low temperatures have similar one-dimensional characteristics as those observed in the pure samples. The pair density function analysis of neutron diffraction data shows that the local atomic structure does not become random with doping but rather consists of two distinct environments corresponding to $\mathrm{Zn}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{Cd}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. This indicates that short-range orbital ordering is present which leads to the one-dimensional character of the spin correlations even in the low temperature cubic phase of the doped compositions.

Anisotropic CE-type orbital correlations in the ferromagnetic metallic phase ofNd1/2Sr1/2MnO3

We present hard x-ray scattering studies of the charge and orbital ordering in ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ together with magnetization and thermal expansion measurements as a function of temperature and magnetic field. Superstructure reflections corresponding to the orbital ordering below ${T}_{\mathrm{CO}}\ensuremath{\simeq}160\mathrm{K}$ are observed in the ferromagnetic metallic phase up to 200 K. The correlation length of the observed structural modulations is found to be highly anisotropic; i.e., the correlations exist predominantly in the $\mathrm{ab}$ planes. Moreover, the periodicity of the superstructure modulations is the same above and below ${T}_{\mathrm{CO}}.$ As a consequence, the modulations found in the ferromagnetic metallic phase are incommensurate to the average lattice structure. The results from hard x-ray scattering, thermal expansion, and magnetization studies can be consistently interpreted in terms of short-range orbital ordering in the ferromagnetic metallic phase of ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$.