Search

We report magnetic and thermodynamic properties of a 4d1 (Mo5+) magnetic insulator MoOPO4 single crystal, which realizes a J1−J2 Heisenberg spin-1/2 model on a stacked square lattice. The specific-heat measurements show a magnetic transition at 16 K which is also confirmed by magnetic susceptibility, ESR, and neutron diffraction measurements. Magnetic entropy deduced from the specific heat corresponds to a two-level degree of freedom per Mo5+ ion, and the effective moment from the susceptibility corresponds to the spin-only value. Using ab initio quantum chemistry calculations, we demonstrate that the Mo5+ ion hosts a purely spin-1/2 magnetic moment, indicating negligible effects of spin-orbit interaction. The quenched orbital moments originate from the large displacement of Mo ions inside the MoO6 octahedra along the apical direction. The ground state is shown by neutron diffraction to support a collinear Neel-type magnetic order, and a spin-flop transition is observed around an applied magnetic field of 3.5 T. The magnetic phase diagram is reproduced by a mean-field calculation assuming a small easy-axis anisotropy in the exchange interactions. Our results suggest 4d molybdates as an alternative playground to search for model quantum magnets.

Detailed investigation of ${\mathrm{Ca}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. The crystal and magnetic structures well below the antiferromagnetic transition temperature of ${T}_{\text{N}}\ensuremath{\approx}5.7$ K were determined using neutron diffraction. Neutron diffraction data imply no structural phase transition from 10 K down to 2.5 K and are well described within the orthorhombic space group $P{2}_{1}{2}_{1}2$ with a $3\ifmmode\times\else\texttimes\fi{}3\ifmmode\times\else\texttimes\fi{}1$ supercell compared with the high-temperature unmodulated state (tetragonal space group $P\overline{4}{2}_{1}m$). We found that in zero magnetic field the magnetic space group is $P{2}_{1}{2}_{1}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ with antiferromagnetic order along the [100] or [010] axes for two types of ${90}^{\ensuremath{\circ}}$ twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the $ab$ plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below ${T}_{\text{N}}$. The ordered moment with a magnitude of about 2.8 ${\ensuremath{\mu}}_{\text{B}}/{\mathrm{Co}}^{2+}$ at 2.5 K lies in the $ab$ plane. Distinct differences between the magnetic structure of ${\mathrm{Ca}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ as compared to those of ${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Sr}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ are discussed.

The $1/4$-filled organic compound, $\ensuremath{\delta}$-(EDT-TTF-${\mathrm{CONMe}}_{2}$)${}_{2}{\mathrm{AsF}}_{6}$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40 K, but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4 K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.

We report a study of the electrical transport properties of single crystals of $\mathrm{P}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}\mathrm{T}{\mathrm{e}}_{1\ensuremath{-}x}{\mathrm{O}}_{4}$, a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic $c$ axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of $\mathrm{P}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}\mathrm{T}{\mathrm{e}}_{1\ensuremath{-}x}{\mathrm{O}}_{4}$. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

We report on spherical neutron polarimetry and unpolarized neutron diffraction in zero magnetic field as well as flipping ratio and static magnetization measurements in high magnetic fields on the multiferroic square lattice antiferromagnet Ba2CoGe2O7. We found that in zero magnetic field the magnetic space group is Cm'm2' with sublattice magnetization parallel to the [100] axis of this orthorhombic setting. The spin canting has been found to be smaller than 0.2 degrees in the ground state. This assignment is in agreement with the field-induced changes of the magnetic domain structure below 40 mT as resolved by spherical neutron polarimetry. The magnitude of the ordered moment has been precisely determined. Above the magnetic ordering temperature short-range magnetic fluctuations are observed. Based on the high-field magnetization data, we refined the parameters of the recently proposed microscopic spin model describing the multiferroic phase of Ba2CoGe2O7.

A high frequency (111.2-420 GHz) electron spin resonance study of the interlayer spin diffusion is presented in the conducting phases of the layered organic compounds, kappa-( BEDT-TTF)(2)Cu[N(CN)(2)]X (kappa-ET2-X), X = Cl or Br. The interlayer spin cross relaxation time T-x and the intrinsic spin relaxation time T-2 of single layers are measured as a function of temperature and pressure. Spin diffusion is two dimensional in the high temperature bad-metal phase (i.e., electrons are confined to a single molecular layer for longer than T-2). The interlayer electron hopping frequency nu(perpendicular to) = 1/(2T(x)) decreases along the bad-metal to Mott insulator crossover and increases along the bad-metal to normal metal (or superconductor) crossover. The density of states (DOS) is determined from a comparison of T-x and the interlayer resistivity. In the bad-metal phase it is four to five times larger than the DOS calculated from the electronic structure neglecting electron correlations. In kappa-ET2-X the DOS increases with pressure along the bad-metal to normal metal crossover. Results are compared with predictions of the dynamical mean field theory.

We present a multifrequency electron spin resonance study in the range of 4-420 GHz of the quasi-one-dimensional, nondimerized, quarter-filled Mott insulators, delta-(EDT-TTF-CONMe2)(2)X (X=AsF6, Br). In the high-temperature orthorhombic phase above T similar to 190 K, the magnitude and the temperature dependence of the high-temperature spin susceptibility are described by a S=1/2 Heisenberg antiferromagnetic chain with J(AsF6)=298 K and J(Br)=474 K coupling constants for X=AsF6 and Br, respectively. We estimate from the temperature dependence of the linewidth (Delta H) an exchange anisotropy, J'/J of similar to 2 X 10(-3). The frequency dependence of Delta H and the g shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii-Moriya) interaction.

The radical cation salt, ET2MnCu[N(CN)(2)](4), [ET=bis(ethylenedithio)tetrathiafulvalene] with an unusual three-dimensional anionic polymeric network is studied by x-ray diffraction, static susceptibility measurements, and electron spin resonance (ESR) at frequencies between 9 and 420 GHz. The magnetic properties are determined by the alternating two-dimensional layers of the Mn2+ ions of the network and the partially charged ET molecules. At ambient temperature the overlap between Mn2+ ions and ET molecules is weak and an exchange integral vertical bar J(Mn-ET)vertical bar approximate to 4.10(-2) K is estimated from their resolved ESR lines. At lower temperatures, ET2MnCu[N(CN)(2)](4) is not a simple system of weakly interacting paramagnetic ions in spite of the isotropic, Curie-like static susceptibility. There are first-order phase transitions at 292 K and in the range of 120-180 K. One of the lattice constants shows anomalous temperature dependence below 292 K. Anisotropic ESR shifts appear below 150 K, which we explain by demagnetizing fields of the platelike crystals and an exchange-narrowed fine structure. The latter contributes significantly to the shift when the populations of Zeeman levels are altered in high magnetic fields at low temperatures. We estimated the exchange coupling between Mn2+ ions within a layer, J(Mn-Mn)approximate to-48 K and determined the fine structure parameters below 150 K, showing a distortion in the plane of the Mn2+ ions.

We studied the temperature stability of the endohedral fullerene molecule N@C-60 inside single-wall carbon nanotubes using electron-spin-resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time T-1 of the encapsulated molecule is significantly shorter than that of the crystalline material, which is explained by the interaction of the nitrogen spin with the host nanotubes.

The transition metal tellurium oxychloride, Ni-5(TeO3)(4)Cl-2, has been investigated by high-field electron-spin resonance for frequencies up to 3 THz, at temperatures well below the magnetic ordering at 23 K. At zero external field several resonance modes have been identified. When the applied magnetic field is perpendicular to both the a and b crystallographic directions, one of the magnetic-resonance modes softens, and a spin-flop transition occurs around 10 T. The results are discussed in terms of the crystal structure, and compared to other magnetically ordered materials with multiple magnetic sublattices, including orthoferrites and triangular antiferromagnets.