We give general arguments that show that the linear magnetoelectric effect in antiferromagnetic materials gives rise to a magnetocapacitance anomaly-a divergence of the dielectric constant at the magnetic ordering temperature T N that appears in an applied magnetic field. The measurement of magnetodielectric response thus provides a definitive and experimentally accessible method to recognize antiferromagnetic linear magnetoelectric materials, circumventing the experimental difficulties often involved in measuring electric polarization. We confirm this result experimentally using the example of MnTiO 3 , which we show to exhibit the linear magnetoelectric effect. No dielectric anomaly is observed at T N in the absence of an applied magnetic field. However, a sharp peak in the dielectric constant appears here when a magnetic field is applied along the c axis, reflecting a linear coupling of the polarization P with the antiferromagnetic order parameter L. In accordance with our theoretical analysis, the dielectric constant close to T N increases with the square of the magnetic field.
We demonstrate, using a combination of experiment and density functional theory, that orbital ordering drives the formation of a one-dimensional (1D) S=1/2 antiferromagnetic spin chain in the 3D rocksalt structure of cesium superoxide (CsO2). The magnetic superoxide anion (O2(-)) exhibits degeneracy of its 2p-derived molecular orbitals, which is lifted by a structural distortion on cooling. A spin chain is then formed by zigzag ordering of the half-filled superoxide orbitals, promoting a superexchange pathway mediated by the p(z) orbitals of Cs(+) along only one crystal direction. This scenario is analogous to the 3d-orbital-driven spin chain found in the perovskite KCuF3 and is the first example of an inorganic quantum spin system with unpaired p electrons.
Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO 2 ). The anion vacancies in the resulting phase with composition RbO 1.72 provide greater structural flexibility compared to RbO 2 and facilitate a Jahn-Teller-driven order-disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides.
A Raman spectroscopy study on the half-doped single-layer manganite Pr 0.5 Ca 1.5 MnO 4 has been performed in combination with x-ray diffraction, resistivity, magnetization, and specific heat measurements. The results provide insight into the underlying mechanisms of phenomena that arise from correlations between lattice, charge, orbital, and spin degrees of freedom. The appearance of a new Raman mode at 366 cm −1 , a visible jump in the resistivity, and a doubling of the unit cell signify the onset of charge/orbital ordering at 320 K. This transition is also marked by a sharp peak in the magnetic susceptibility and specific heat, suggesting strong spin-charge coupling. Our structural analysis suggests that the charge disproportionation below 320 K is small. Orbital fluctuations below 320 K are evidenced by the evolution with temperature of the Jahn-Teller Raman mode (situated at 533 cm −1 at 320 K). A coincidence between the onset of two-dimensional short-range antiferromagnetic order at 215 K and anomalies in both the temperature dependence of the Jahn-Teller mode and the Mn-O bonding pattern in the ab plane indicate that the short-range magnetic order plays a role in stabilizing the orbital fluctuations. Below the Néel temperature of 127 K, the softening of both the 366 cm −1 mode and an octahedral tilting mode at 214 cm −1 mark the onset of three-dimensional antiferromagnetic ordering. The estimated spin-phonon coupling constants for these two modes are 2.6 and 6.8 cm −1 , respectively. This study highlights the remarkable coupling of charge, orbital, and spin degrees of freedom to the lattice in single-layer Pr 0.5 Ca 1.5 MnO 4 .
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