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Register a new userTuning the magnetic exchange via a control of orbital hybridization in Cr2(Te1-xWx)O6
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We report the complex magnetic phase diagram and electronic structure of Cr2(Te1-xWx)O6 systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above xc = 0.7, where both the transition temperature TN and sublattice magnetization (Ms) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab-initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.
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The asymmetry of chemical nature at the hetero-structural interface offers an unique opportunity to design desirable electronic structure by controlling charge transfer and orbital hybridization across the interface. However, the control of hetero-interface remains a daunting task. Here, we report the modulation of interfacial coupling of (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattices by manipulating the periodic thickness with n unit cells of SrTiO3 and n unit cells La0.67Sr0.33MnO3. The easy axis of magnetic anisotropy rotates from in-plane (n = 10) to out-of-plane (n = 2) orientation at 150 K. Transmission electron microscopy reveals enlarged tetragonal ratio > 1 with breaking of volume conservation around the (La0.67Sr0.33MnO3)n/(SrTiO3)n interface, and electronic charge transfer from Mn to Ti 3d orbitals across the interface. Orbital hybridization accompanying the charge transfer results in preferred occupancy of 3d3z2-r2 orbital at the interface, which induces a stronger electronic hopping integral along the out-of-plane direction and corresponding out-of-plane magnetic easy axis for n = 2. We demonstrate that interfacial orbital hybridization in superlattices of strongly correlated oxides may be a promising approach to tailor electronic and magnetic properties in device applications.
We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-delta}$. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysis, we have synthesized a series of samples with precisely controlled $delta=0.00-0.49$. These samples show three structural phases at $delta=0.00-0.15$, $approx0.25$, $approx0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{rm C}=274$ K. The increase of $delta$ to 0.15 is followed by a linear decrease of $T_{rm C}$ to $approx$ 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase of $delta$ results in formation of a tetragonal $2a_ptimes 2a_p times 4a_p$ phase for $deltaapprox 0.25$ and a brownmillerite phase for $deltaapprox0.5$. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at $T_{rm m}approx230$ and 120 K, respectively. At higher temperatures, the $2a_ptimes 2a_p times 4a_p$ phase is $G$-type antiferromagnetic between 230 K and $approx$360 K. Low temperature magnetic properties of this system for $delta<1/3$ can be described in terms of a mixture of Co$^{3+}$ ions in the low-spin state and Co$^{4+}$ ions in the intermediate-spin state and a possible spin transition of Co$^{3+}$ to the intermediate-spin state above $T_{rm C}$. For $delta>1/3$, there appears to be a combination of Co$^{2+}$ and Co$^{3+}$ ions, both in the high-spin state with dominating antiferromagnetic interactions.
We present the results of an LDA and LDA+U band structure study of the monoclinic and the corundum phases of V2O3 and argue that the most prominent (spin 1/2) models used to describe the semiconductor metal transition are not valid. Contrary to the generally accepted assumptions we find that the large on site Coulomb and exchange interactions result in a total local spin of 1 rather than 1/2 and especially an orbital occupation which removes the orbital degeneracies and the freedom for orbital ordering. The calculated exchange interaction parameters lead to a magnetic structure consistent with experiment again without the need of orbital ordering. While the low-temperature monoclinic distortion of the corundum crystal structure produces a very small effect on electronic structure of v2o3, the change of magnetic order leads to drastic differences in band widths and band gaps. The low temperature monoclinic phase clearly favors the experimentally observed magnetic structure, but calculations for corundum crystal structure gave two consistent sets of exchange interaction parameters with nearly degenerate total energies suggesting a kind of frustration in the paramagnetic phase. These results strongly suggest that the phase transitions in V2O3 which is so often quoted as the example of a S=1/2 Mott Hubbard system have a different origin. So back to the drawing board!
We demonstrate a direct correlation between the domain structure of multiferroic BiFeO3 thin films and exchange bias of Co0.9Fe0.1/BiFeO3 heterostructures. Two distinct types of interactions, an enhancement of the coercive field (exchange enhancement) and an enhancement of the coercive field combined with large shifts of the hysteresis loop (exchange bias), have been observed in these heterostructures, which depend directly on the type and crystallography of the nanoscale (2 nm) domain walls in the BiFeO3 film. We show that the magnitude of the exchange bias interaction scales with the length of 109 degree ferroelectric domain walls in the BiFeO3 thin films which have been probed via piezoresponse force microscopy and x-ray magnetic circular dichroism.
In spin-density-functional theory for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. We obtain exact benchmark solutions for two electrons on four-site extended Hubbard lattices over a wide range of interaction strengths, and compare exact xc potentials and magnetic fields with approximations obtained from orbital-dependent xc functionals. The xc magnetic fields turn out to play an increasingly important role as systems becomes more and more correlated and the electrons begin to localize; the effects of the xc torques, however, remain relatively minor. The approximate xc functionals perform overall quite well, but tend to favor symmetry-broken solutions for strong interactions.
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