We present the Raman scattering results on layered 2D semiconducting ferromagnetic compound CrSiTe3. Four Raman active modes, predicted by symmetry, have been observed and assigned. The experimental results are supported by DFT calculations. The self-energies of the A 3 g and the E 3 g symmetry modes exhibit unconventional temperature evolution around 180 K. In addition, the doubly degenerate E 3 g mode shows clear change of asymmetry in the same temperature region. The observed behaviour is consistent with the presence of the previously reported short-range magnetic order and the strong spin-phonon coupling.

Easy axis antiferromagnets usually exhibit a first order spin-flop transition when the magnetic field is applied along the easy axis. Recently a colossal magnetoelectric effect was discovered in Ni3TeO6, suggesting a continuous spin-flop transition across a narrow phase in this material [Y. S. Oh, et al., Nature Comm. 5, 3201 (2014)]. Additional evidence is, however, desirable to verify this mechanism. Here we measure the infrared vibrational properties of Ni3TeO6 in high magnetic fields and demonstrate that the phonon anomalies are consistent with a second-order mechanism.

Nonreciprocal directional dichroism is an unusual light-matter interaction that gives rise to diode-like behavior in low-symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin-orbit coupling, broken time-reversal symmetry, and a chiral axis. Here we bring together magneto-optical spectroscopy and first-principles calculations to reveal highenergy, broadband nonreciprocal directional dichroism in Ni 3 TeO 6 with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and, by so doing, uncover an additional configuration with a unique nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first-principles methods to analyze how the Ni 2+ d-to-d on-site excitations develop magneto-electric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.

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Infrared reflectance analysis is facilitated via the comparison of spectra recorded in situ to a databank of actual or synthetic infrared reflectance spectra. It has recently been shown that reference spectra corresponding to the many different morphological forms of the same chemical can be generated synthetically using the imaginary, k, and real, n, components of the complex refractive index, [Formula: see text] =  n + i k. One method to obtain the n and k vectors is infrared ellipsometry, which measures the changes in amplitude, tan Ψ, and phase, Δ, of polarized light reflected from the sample both as a function of wavenumber and angle of incidence. The method requires specularly reflected light, so best results are usually obtained with polished planar samples of large surface area. Due to the difficulties of obtaining such samples, however, we investigate the possibility of pressing powders of neat materials and obtaining the corresponding optical constants from the pellets. In this paper, variability in the sample pellet and preparation method is investigated, as is variability in the fitting procedure for the derived optical constants. The n/k vectors are derived from the measured ellipsometric parameters, tan ψ and Δ, as they are fit by an oscillator model which yield n([Formula: see text]) and k([Formula: see text]) vectors as a function of wavenumber, [Formula: see text]. Construction of the oscillator model is not automatic and depends on significant input from the analyst as well as the sample’s physical characteristics. For pellet pressing, the experimental variability was found to be minimized for size-selected powdered samples as gauged by the minimal variance in ψ and Δ for three different pellets; similarly, the analytical precision for multiple measurements of the same pellet was also quite good, suggesting that a pressed pellet is a viable sample preparation method. Experimental variabilities were comparatively small; the greatest variability came in the analytic fitting procedure with differences in the k-peak values up to 10% for only the sharpest bands arising from four different fits to the same data set. The final ellipsometric n/k data are compared to literature values obtained from crystalline ammonium sulfate ((NH4)2SO4) samples as well as single-angle reflectance measurements that also used pressed pellets. Comparison with the previous literature values shows generally good agreement, although larger k-values are observed for the independent sets of data derived from pressed pellets. These data are suggested as an improved set of optical constants for (NH4)2SO4.

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH)NH] M(HCOO) ( M = Mn, Co, Ni) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH)NH]Mn(HCOO) takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across T reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH)NH]Co(HCOO) reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

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We document an artifact associated with the back reflection from samples or sampling accessories in Fourier transform spectrometers. Samples oriented normal to the incoming modulated beam can reflect light back to the interferometer and this light (the percentage dependent on the sample’s refractive index) is modulated by the interferometer a second time resulting in light erroneously appearing at twice its true frequency. The phenomenon occurs across the spectrum but is most apparent when positive-going narrow absorption peaks at 1 f display as negative-going peaks at 2 f. We have redressed the artifact by implementing a rotation stage directly beneath the sample holder: As the stage is rotated through small angles relative to beam normal, the back-reflected light does not enter the interferometer and the artifact disappears. The observation is relevant to several IR sampling methods: gas/liquid cells, alkali halide pellets, reflectance accessories, etc.