Magnetic-field tuning of the spin dynamics in the magnetic topological insulators (MnBi₂Te₄)(Bi₂Te₃₎<i>n</i>

We report a high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi2Te4)(Bi2Te3)n with n = 0 and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of T=4K combined with the calculations of the resonance modes showed that the spin dynamics in MnBi4Te7 is typical for an anisotropic easy-axis type ferromagnet whereas MnBi2Te4 demonstrates excitations of an anisotropic easy-axis type antiferromagnet. However, by applying the field stronger than a threshold value ∼6T, we observed in MnBi2Te4 a crossover from the antiferromagnetic (AFM) resonance modes to the ferromagnetic (FM) modes, whose properties are very similar to the FM response of MnBi4Te7. We attribute this remarkably unusual effect unexpected for a canonical easy-axis antiferromagnet, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic, featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in MnBi4Te7, or by applying a moderate magnetic field, as in MnBi2Te4, which may have an effect on the surface topological band structure of these compounds.</p

The kabachnik-fields and pudovik reactions on the basis of E,Z-citral and its imines and (R,S)-citronellal

The Kabachnik-Fields reaction of E,Z-citral with diethyl phosphite in the presence of isobutylamine has been found to form α-aminophosphonates. The Pudovik reactions of diethyl phosphite with prenyl imines prepared on the basis of E,Z-citral with isobutylamine also allowed us to obtain the same α-aminophosphonates. We have managed to synthesize α- aminophosphonates by the reaction of O,O-dialkyl trimethylsilyl phosphites with prenyl imines in the presence of water or diethylamine. α- Aminophosphonates were also synthesized by the reaction of diethyl phosphite with (R,S)-citronellal in the presence of alkylamines. α-Aminophosphonates obtained showed bacteriostatic activity against Staphylococcus aureus and Bacillus cereus. © 2012 Wiley Periodicals, Inc

Evolution of the spin dynamics in the van der Waals system M2M_{\text{2}}P2_{\text{2}}S6_{\text{6}} (M2\boldsymbol{M}_{\text{2}} = Mn2_{\text{2}}, MnNi, Ni2_{\text{2}}) series probed by electron spin resonance spectroscopy

In this work we report a detailed ESR spectroscopic study of the single-crystalline samples of the van der Waals compounds $M_{\text{2}}$P$_{\text{2}}$S$_{\text{6}}$ ($M_{\text{2}}$ = Mn$_{\text{2}}$, MnNi, Ni$_{\text{2}}$), performed at an excitation frequency of 9.56 GHz, in a broad range of temperatures above the magnetic order, and at different orientations of the magnetic field with respect to the sample. Analyzing temperature and angular dependences of the resonance field and of the linewidth of the Mn$_2$P$_2$S$_6$ compound we have observed a significant change of the spin dynamics from the dominance of the 3D-like fluctuations close to the magnetic order to a relative increase of the 2D-like spin fluctuations at higher temperatures. Such a behavior, which is opposite to the development of the low-D signatures in the previously studied Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$ compound, can be explained by the difference in the type of magnetic order in Mn$_2$P$_2$S$_6$ and Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$. On the other hand, MnNiP$_2$S$_6$ compound exhibits angular dependences of the linewidth typical for the system with 3D-like spin correlations in the whole measurement temperature range, however the 2D-like correlations can be seen in the temperature dependences of the resonance field and the linewidth. Ni$_2$P$_2$S$_6$, in turn, does not show any 2D signatures. This suggests that varying the Ni content in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ one can control the exchange interaction, possibly also in the third dimension

Magnetic anisotropy and low-energy spin dynamics in the van der Waals compounds Mn2_{2}P2_{2}S6_{6} and MnNiP2_{2}S6_{6}

We report the detailed high-field and high-frequency electron spin resonance (HF-ESR) spectroscopic study of the single-crystalline van der Waals compounds Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$. Analysis of magnetic excitations shows that in comparison to Mn$_{2}$P$_{2}$S$_{6}$ increasing the Ni content yields a larger magnon gap in the ordered state and a larger g-factor value and its anisotropy in the paramagnetic state. The studied compounds are found to be strongly anisotropic having each the unique ground state and type of magnetic order. Stronger deviation of the g-factor from the free electron value in the samples containing Ni suggests that the anisotropy of the exchange is an important contributor to the stabilization of a certain type of magnetic order with particular anisotropy. At temperatures above the magnetic order, we have analyzed the spin-spin correlations resulting in a development of slowly fluctuating short-range order. They are much stronger pronounced in MnNiP$_{2}$S$_{6}$ compared to Mn$_{2}$P$_{2}$S$_{6}$. The enhanced spin fluctuations in MnNiP$_{2}$S$_{6}$ are attributed to the competition of different types of magnetic order. Finally, the analysis of the temperature dependent critical behavior of the magnon gaps below the ordering temperature in Mn$_{2}$P$_{2}$S$_{6}$ suggests that the character of the spin wave excitations in this compound undergoes a field induced crossover from a 3D-like towards 2D XY regime