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Register a new userNMR study of the high-field magnetic phase of LiCuVO$_4$
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We report on NMR studies of the quasi--1D antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$, focusing on the high--field spin--modulated phase observed recently in applied magnetic fields $H > H_{rm c2}$ ($mu_0H_{rm c2} approx 7.5$ T). The NMR spectra of $^7$Li and $^{51}$V around the transition from the ordered to the paramagnetic state were measured. It is shown that the spin--modulated magnetic structure forms with ferromagnetic interactions between spins of neighboring chains within the {bf ab}--plane at low temperatures 0.6 K $ < T < T_{rm N}$. The best fit provides evidence that the mutual orientation between spins of neighboring {bf ab}--planes is random. For elevated temperatures $T_{rm N} < T lesssim 15$ K, short--range magnetic order occurs at least on the characteristic time scale of the NMR experiment.
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We report on NMR studies of the quasi one--dimensional (1D) antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$ in magnetic fields $H$ up to $mu_0H$ = 30 T ($approx 70$% of the saturation field $H_{rm sat}$). NMR spectra in fields higher than $H_{rm c2}$ ($mu_0H_{rm c2} approx 7.5$ T) and temperatures $T<T_{rm N}$ can be described within the model of a spin-modulated phase in which the magnetic moments are aligned parallel to the applied field $H$ and their values alternate sinusoidally along the magnetic chains. Based on theoretical concepts about magnetically frustrated 1D chains, the field dependence of the modulation strength of the magnetic structure is deduced from our experiments. Relaxation time $T_2$ measurements of the $^{51}$V nuclei show that $T_2$ depends on the particular position of the probing $^{51}$V nucleus with respect to the magnetic copper moments within the 1D chains: the largest $T_2$ value is observed for the vanadium nuclei which are very next to the magnetic Cu$^{2+}$ ion with largest ordered magnetic moment. This observation is in agreement with the expectation for the spin-modulated magnetic structure. The $(H,T)$ magnetic phase diagram of LiCuVO$_4$ is discussed.
Due to the strong coupling between magnetism and ferroelectricity, $(mathrm{ND}_4)_2mathrm{FeCl}_5cdotmathrm{D}_2mathrm{O}$ exhibits several intriguing magnetic and electric phases. In this letter, we include high-order onsite spin anisotropic interactions in a spin model that successfully captures the ferroelectric phase transitions of $(mathrm{ND}_4)_2mathrm{FeCl}_5cdotmathrm{D}_2mathrm{O}$ under a magnetic field and produces the large weights of high-order harmonic components in the cycloid structure that are observed from neutron diffraction experiments. Moreover, we predict a new ferroelectric phase sandwiched between the FE II and FE III phases in a magnetic field. Our results emphasize the importance of the high-order spin anisotropic interactions and provide a guideline to understand multiferroic materials with rich phase diagrams.
High-$T_{rm{c}}$ cuprate superconductors host spin, charge and lattice instabilities. In particular, in the antiferromagnetic glass phase, over a large doping range, lanthanum based cuprates display a glass-like spin freezing with antiferromagnetic correlations. Previously, sound velocity anomalies in La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) for hole doping $pgeq 0.145$ were reported and interpreted as arising from a coupling of the lattice to the magnetic glass [Frachet, Vinograd et al., Nat. Phys. 16, 1064-1068 (2020)]. Here we report both sound velocity and attenuation in LSCO $p=0.12$, i.e. at a doping level for which the spin freezing temperature is the highest. Using high magnetic fields and comparing with nuclear magnetic resonance (NMR) measurements, we confirm that the anomalies in the low temperature ultrasound properties of LSCO are produced by a coupling between the lattice and the spin glass. Moreover, we show that both sound velocity and attenuation can be simultaneously accounted for by a simple phenomenological model originally developed for canonical spin glasses. Our results point towards a strong competition between superconductivity and spin freezing, tuned by the magnetic field. A comparison of different acoustic modes suggests that the slow spin fluctuations have a nematic character.
We report on the electric field control of magnetic phase transition temperatures in multiferroic Ni3V2O8 thin films. Using magnetization measurements, we find that the phase transition temperature to the canted antiferromagnetic state is suppressed by 0.2 K in an electric field of 30 MV/m, as compared to the unbiased sample. Dielectric measurements show that the transition temperature into the magnetic state associated with ferroelectric order increases by 0.2 K when the sample is biased at 25 MV/m. This electric field control of the magnetic transitions can be qualitatively understood using a mean field model incorporating a tri-linear coupling between the magnetic order parameters and spontaneous polarization.
[Ni(HF$_2$)(3-Clpyridine)$_4$]BF$_4$ (NBCT) is a one-dimensional, $S = 1$ spin chain material that shows no magnetic neutron Bragg peaks down temperatures of 0.1 K. Previous work identified NBCT to be in the Haldane phase and near a quantum phase transition as a function of $D/J$ to the large-$D$ quantum paramagnet phase (QPM), where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Herein, inelastic neutron scattering results are presented on partially deuterated, $^{11}$B enriched NBCT polycrystalline samples in zero magnetic field and down to temperatures of 0.3 K. Comparison to density matrix renormalization group calculations yields $D/J = 1.51$ and a significant rhombic single-ion anisotropy $E$ ($E/D approx 0.03$, $E/J approx 0.05$). These $D$, $J$, and $E$ values place NBCT in the large-$D$ QPM phase but precipitously near a quantum phase transition to a long-range ordered phase.
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