Frustration Of Magnetic Moments Research Articles

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the Roverline{3 }c crystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

Magnetoluminescence of CdTe/MnTe/CdMgTe heterostructures with ultrathin MnTe layers

CdTe/MnTe/CdMgTe quantum-well structures with one or two monolayers of MnTe inserted at CdTe/CdMgTe interfaces were fabricated. The spectra of the excitonic luminescence from CdTe quantum wells and their variation with temperature indicate that introduction of ultrathin MnTe layers improves the interface quality. The effect of a magnetic field in the Faraday configuration on the spectral position of the exciton-emission peaks indicates that frustration of magnetic moments in one-monolayer MnTe insertions is weaker than in two-monolayer insertions. The effect of a magnetic field on the exciton localization can be explained in terms of the exciton wave-function shrinkage and obstruction of the photoexcited charge-carrier motion in the quantum well.

Magnetic features of the (U 0.50Tm 0.50)Ni 2B 2C solid solution

The quaternary borocarbides TmNi 2B 2C and UNi 2B 2C crystallize in the body-centered tetragonal LuNi 2B 2C-type structure, and exhibit antiferromagnetic order below 1.53 and 218 K, respectively. The magnetic structure of TmNi 2B 2C is characterized by incommensurate, long-range, spin-density-wave in the basal plane, with Tm moments along the c-axis, and does not involve ferromagnetic basal planes, while that of UNi 2B 2C is assumed to contain U moments along the c-axis in ferromagnetic basal planes. In the system (U,Tm)Ni 2B 2C the different magnetic structures of the end compounds result in a frustration of magnetic moments, which is not just directional. Indeed, the newly-prepared intermediate solid solution (U 0.50Tm 0.50)Ni 2B 2C is paramagnetic down to 2 K, for both as-cast and annealed samples, with no indication for any cooperative phenomenon. The paramagnetic values of the annealed sample [ θ=−5.6(5) K, μ eff=5.4(1) μ B] are compatible with those of the end compounds.