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Parity Broken Chiral Spin Dynamics in Ba3_3NbFe3_3Si2_2O14_{14}

The spin wave excitations emerging from the chiral helically modulated 120$^{\circ}$ magnetic order in a langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ enantiopure crystal were investigated by unpolarized and polarized inelastic neutron scattering. A dynamical fingerprint of the chiral ground state is obtained, singularized by (i) spectral weight asymmetries answerable to the structural chirality and (ii) a full chirality of the spin correlations observed over the whole energy spectrum. The intrinsic chiral nature of the spin waves elementary excitations is shown in absence of macroscopic time reversal symmetry breaking

Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of about 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.Comment: 26 pages, 5 figure

Antiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7

We present an experimental study of the quantum spin ice candidate pyrochlore coumpound \przr\ by means of magnetization measurements, specific heat and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1\bar{1}0]$ directions, ${\bf k}=0$ field induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H \parallel [1\bar{1}0]$, the ordered moment appears on the so called $\alpha$ chains only. The spin excitation spectrum is essentially {\it inelastic} and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H \parallel [1\bar{1}0]$ (at least up to 2.5 T), we find that a well defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific heat anomaly. We finally discuss these results in the light of mean field calculations and propose a new interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those new interactions. We then propose a range of acceptable parameters for \przr\, that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice like excitations

Incommensurate antiferromagnetic order in the manifoldly-frustrated SrTb2_2O4_4 with transition temperature up to 4.28 K

The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector \textbf{Q}$^{\rm 0.5 K}_{\rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $\mu_{\rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the \emph{bc} plane, 11.9(66)$^\circ$ away from the \emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations.Comment: 19 pages, 8 Figures, 1 Tabl

Conical antiferromagnetic order in the ferroelectric phase of Mn0.8Co0.2WO4 resulting from the competition between collinear and cycloidal structures

Evolution of competing commensurate collinear (AF4) and incommensurate cycloidal (AF2) magnetic structures in Mn0.8Co0.2WO4 multiferroic was studied by neutron diffraction, magnetic, and pyroelectric characterization measurements. In contrast to pure and slightly Co doped MnWO4, the antiferromagnetic AF4 collinear phase [k1=(1/2,0,0)] inherent to the pure CoWO4 was observed below Néel temperature TN≈20 K in Mn0.8Co0.2WO4. This collinear order survives down to the lowest temperature reached in the experiments (2 K) even after the appearance of the second (cycloidal AF2) spin order below TFE≈8.5 K [k2=(−0.211,1/2,0.452)]. Ferroelectric polarization along b axis was revealed below TFE in the low temperature conical phase resulting from the superposition of the AF4 and AF2 spin structures. The arrangement of the spins after the two successive magnetic transitions are thoroughly described. In particular, we found that spins in the AF4 phase are aligned along the easy direction in the ac plane (∼142∘ with respect to the c* axis), while the cycloidal AF2 spin order is developed in the magnetically hard directions, perpendicular to the easy one, and consequently the TFE decreases compared to the pure MnWO4

Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of E50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions

Field-induced States and Excitations in the Quasicritical Spin-1/2 Chain Linarite

The mineral linarite, PbCuSO$_4$(OH)$_2$, is a spin 1/2 chain with frustrating nearest neighbor ferromagnetic and next-nearest neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. However, the measured complex magnetic phase diagram depends strongly on the magnetic field direction. The field-dependent phase sequence is explained by our classical simulations of a nearly critical model with tiny orthorhombic exchange anisotropy. The simulations also capture qualitatively the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field

Neutron diffraction, magnetic, and magnetoelectric studies of phase transitions in multiferroic Mn0.90Co0.10WO4

We have studied various spontaneous and magnetic-field-induced phase transitions in single crystals of multiferroic Mn0.9Co0.1WO4 using magnetic and magnetoelectric measurements and neutron diffraction. Compared to pure MnWO4, our data consistently confirm that the anisotropic Co2+ ions induce reorientation of the spin cycloid structure to the ac plane and reveal Pa and Pc components of spontaneous electric polarization.Field-induced phase transitions accompanied by anomalies of magnetic susceptibility and suppression of both Pa and Pc polarizations have been observed for H∥c (∼3 T) and H∥a (∼8.5 T). Neutron diffraction has revealed that in both cases the spin cycloid plane flops in direction almost perpendicular to H, i.e., close to the ab and bc planes, respectively. Parameters describing the magnetic structures including wave vectors, orientations of the main elliptical axes, etc., have been determined in all spontaneous and field-induced states