Direct observation of spin-quadrupolar excitations in Sr2_2CoGe2_2O7_7 by high field ESR

Exotic spin-multipolar ordering in spin transition metal insulators has so far eluded unambiguous experimental observation. A less studied, but perhaps more feasible fingerprint of multipole character emerges in the excitation spectrum in the form of quadrupolar transitions. Such multipolar excitations are desirable as they can be manipulated with the use of light or electric field and can be captured by means of conventional experimental techniques. Here we study single crystals of multiferroic Sr$_2$CoGe$_2$O$_7$, and show that due to its nearly isotropic nature a purely quadrupolar bimagnon mode appears in the electron spin resonance (ESR) spectra. This non-magnetic spin-excitation couples to the electric field of the light and becomes observable for a specific experimental configuration, in full agreement with a theoretical analysis of the selection rules.Comment: 19 pages, 13 figures, , accepted for publication in Phys. Rev.

Degenerate ground state in the classical pyrochlore antiferromagnet Na3_3Mn(CO3_3)2_2Cl

In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy at a ground state leads to absence of a magnetic order and spin-glass transition. Here we present Na$_3$Mn(CO$_3$)$_2$Cl as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice, and report its structural and magnetic properties.The temperature dependences of the magnetic susceptibility and heat capacity, and the magnetization curve are consistent with those of an $S$ = 5/2 pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 K. Neither an apparent signature of a spin-glass transition nor a magnetic order is detected in magnetization and heat capacity measurements, or powder neutron diffraction experiments. On the other hand, an antiferromagnetic short-range order from the nearest neighbors is evidenced by the $Q$-dependence of the diffuse scattering which develops around 0.85 \AA$^{-1}$. A high degeneracy near the ground state in Na$_3$Mn(CO$_3$)$_2$Cl is supported by the magnetic entropy estimated as almost 4 J K$^{-2}$ mol$^{-1}$ at 0.5 K.Comment: 9 pages, 7 figures, accepted to PR

Magnetic Resonance in the Chiral Helimagnet CrNb3S6

Recently, magnetic substances with chirality, namely the handedness of the magnetic structure, have attracted considerable attention because of the anomalous phenomena which appear in magnetic fields. CrNb3S6 is one of the chiral magnets formed by exchange and Dzyaloshinsky-Moriya (DM) interactions. Electron spin resonance (ESR) measurements of CrNb3S6 in magnetic fields parallel to the c-axis (helical axis) have been performed to evaluate the exchange and the DM constants that determine the helical structure. Fitting the ESR data to a calculated mode based on a spin wave theory yields values for the ferromagnetic inter-plane exchange constant J/kB = 16.2 K, the DM constant D/kB = 1.29 K, and the single-ion anisotropy constant K /kB = 1.02 K. From the Curie-Weiss temperature θCW ~ 145 K, large intra-plane ferromagnetic exchange interactions are suggested.20th International Conference on Magnetism(ICM 2015), 5th to 10th July, 2015, Barcelona, Spai

Zero-point entropies of spin-jam and spin-glass states in a frustrated magnet

Thermodynamics of glassy states in a quasi-two-dimensional frustrated magnet Ba$_2$Sn$_2$ZnCr$_{7p}$Ga$_{10-7p}$O$_{22}$ where $p$ is the spin density are investigated experimentally. The system features a triangular network of bipyramids of spins with the quantum spin number $s = 3/2$. The DC magnetic susceptibility measurements on a series of samples with $0.44 \le p \le 0.98$ show a freezing transition with the transition temperature $T_\mathrm{f} \le 1.2$ K. $T_\mathrm{f}$ is found to decrease with decreasing $p$. The low-lying excitations in the glassy state of the system are examined via the temperature dependence of the magnetic heat capacity and are shown to consist of two components: the hydrodynamic Halperin-Saslow modes characteristic of a spin jam and the two-level systems of a spin glass. A continuous crossover between the two glassy states is observed via the varying weights of the two components as the spin density is varied. The $p$ dependence of the spin jam's zero-point entropy determined from the exotic perimeter-scaling behavior combined with the observed zero-point entropy of the samples provides the $p$ dependence of the spin glass's zero-point entropy. The obtained result shows that the correlations between orphan spins begin below $p \sim 0.8$, the limit that was also found using a neutron scattering technique in a previous report on the isostructural compound SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$. The domain size of the spin-jam state estimated from the value of the zero-point entropy for the cleanest sample is approximately $4 \times 4$ bipyramids, about 2.5 times the measured spin correlation length

Anomalous spiked structures in ESR signals from the chiral helimagnet CrNb3S6

We have performed X-band electron spin resonance (ESR) measurements on a single crystal of the metallic chiral helimagnet CrNb3S6 from 3.5 to 180 K and for the external magnetic fields Hext , up to 4 kOe, perpendicular to the c axis (the helical axis of CrNb3S6). This field-crystalline configuration is expected to provide the chiral soliton lattice (CSL) state in this system. The main resonance line can be fit with a Dysonian function above Tc = 127 K, but additional features in the spectra were observed below 105 K. Specifically, spiked anomalies superposed on the main signals were observed for magnetic fields between Hc1 and Hc2 that are the appearing and disappearing fields of the spiked anomalies, respectively. The resulting magnetic field vs temperature phase diagram possesses three regions, which are interpreted as different dynamical responses in the CSL phase. In addition, the values of Hc2 are close to those reported by the d2M/dH2 curve [Tsuruta et al., Phys. Rev. B 93, 104402 (2016)]. Furthermore, the field range between Hc1 and Hc2, where the spiked anomalies exist, depends on the field direction and shifts to higher fields when turning to the c axis, thereby providing additional evidence that these spiked anomalies must be related to the chiral soliton dynamics