Session Z45: Mostly Spinels

Ba$_{2}$CoO$_{4}$: Crystal Growth, Structure Refinement, and Physical Properties

Single crystalline Ba$_{2}$CoO$_{4}$ is grown for the first time using the floating-zone technique. Single-crystal refinement of X-ray diffraction data indicates that Ba$_{2}$CoO$_{4}$ has a monoclinic structure with $a$ = 5.9176(13) {\AA}, $b$ = 7.6192(16) {\AA}, $c$ = 10.3790(22) {\AA}, and {\ss} = 91.734(4)$^{o}$ at room temperature. Magnetic susceptibility, specific heat, and thermal conductivity show anomalies at T$_{N}$ = 25 K, due to antiferromagnetic ordering. However, the magnitude of the N\'{e}el temperature T$_{N}$ is significantly lower than the Curie-Weiss temperature ($\vert \Theta \vert \sim $ 110 K), suggesting either reduced-dimensional magnetic interactions and/or the existence of magnetic frustration. The latter may be induced by disorder evident from the variable-range-hopping behavior in electrical resistivity, non-zero T term in specific heat and T$^{2}$ dependence in thermal conductivity at low temperatures.   

Spin disordered state of the triangular lattice antiferromagnet NiGa$_{2}$S$_{4}$

We have recently found a spin disordered state in two dimensions in NiGa$_{2}$S$_{4}$, a bulk insulating antiferromagnet on a triangular lattice.Despite strong antiferromagnetic interactions of $\sim $ 80 K, no magnetic long-range order has been observed down to 0.35 K where we instead find nano-scale quasi-static correlation. The spin disordered state appears on cooling through highly degenerate states with an entropy plateau, and exhibits gapless linearly dispersive modes, suggesting coherence beyond the two-spin correlation length. A possible ground state will be discussed.   

Incommensurate spin correlations in a spin-1 triangular lattice antiferromagnet

Spin correlations in the triangular lattice antiferromagnet NiGa2S4 were investigated as a function of temperature and magnetic field through neutron scattering. At T=1.5 K the in plane correlations are incommensurate with a wave vector $(\frac{1}{6}-\delta ,\frac{1}{6}-\delta ,0)$ where $\delta =$ 0.00866. The in-plane correlation length is 6.9(8) lattice spacings while inter-plane correlations cannot be detected beyond the second nearest plane. These correlations persist on a time scale that exceeds 0.3 ns. Application of an in-plane magnetic field of 10 Tesla only slightly reduces the inter-plane correlations with no appreciable effect on intra-plane correlations, while heating reduces the frozen moment, the in-plane correlation length, and the correlation time. We shall discuss what can be inferred about the spin Hamiltonian for NiGa2S4 as well as the spin-1 triangular lattice antiferromagnet from these data.   

Nonmagnetic impurity effects of the spin disordered state in NiGa$_{2}$S$_{4}$

Nonmagnetic impurity effects of the spin disordered state in the triangular antiferromagnet NiGa$_{2}$S$_{4}$[1] was studied through magnetic and thermal measurements for Zn substituted insulating materials Ni$_{1-x}$Zn$_{x}$Ga$_{2}$S$_{4}$ (0.0 $\le \quad x \quad \le $ 0.3)[2]. Only 1 {\%} Zn substitution is enough to strongly suppress the coherence observed in the spin disordered state. However, suppression is not complete and the robust feature of the quadratic temperature dependent specific heat and its scaling behavior with the Weiss temperature indicate the existence of a coherent Nambu-Goldstone mode. Absence of either conventional magnetic long-range order or bulk spin freezing suggests a novel symmetry breaking of the ground state. [1] Satoru Nakatsuji, Yusuke Nambu, Hiroshi Tonomura, Osamu Sakai, Seth Jonas, Collin Broholm, Hirokazu Tsunetsugu, Yiming Qiu and Yoshiteru Maeno, Science \textbf{309}, 1697 (2005). [2] Yusuke Nambu, Satoru Nakatsuji and Yoshiteru Maeno, preprint.   

Novel spin structures in Mn and Co chromite

The ferrimagnetic or conical spiral (FS), a variational approximation to the ground state of the classical Heisenberg model with competing AB and BB interactions in cubic spinels$^1$, is in qualitative to quantitative agreement with neutron diffraction (ND) results for MnCr$_2$O$_4$ and CoCr$_2 $O$_4$.$^2 $ This despite its local instability for the experimental parameter values.$^1$ It also was used to interpret related NMR studies.$^3$ Understanding recent ND experiments$^4$ on both materials again was generally based on the FS. However, these measurements, done on single crystals, and with highly improved resolution, uncovered a subtle but important modification of the FS: while the fundamental Bragg peaks (originating from the spin components along the cone axes) are typical, the satellite peaks (coming from the transverse or spiral components) are broadened.$^4$ Efforts to understand this based on the classical spin model will be discussed. \newline 1. D. H. Lyons et al., Phys. Rev. \textbf{126}, 540 (1962). \newline 2. J. Hastings et al., Phys. Rev. \textbf{126}, 556 (1962). N. Menyuk et al., J. de Physique \textbf{25}, 528 (1964). \newline 3. A. J. Heeger et al., J. Phys. Chem Solids \textbf{29}, 1085 (1968); T. Tsuda et al., Solid State Comm. \textbf{9}, 2207 (1971). \newline 4. K. Tomiyasu et al., Phys. Rev. B \textbf{70}, 214434 (2004).   

Competing phases in a frustrated magnet: a study with neutron scattering.

Zero-field magnetic neutron diffraction on the pyrochlore magnet ZnCr$_{2}$S$_{4}$ reveals successive phase transitions from a paramagnet into a helical (T$_{N1}$ = 13 K) then collinear (T$_{N2}$ = 8 K) spin ordered state as the system is cooled. This occurs as the system crosses a critical line in J$_{1}$-J$_{2}$ space, where J$_{1}$ and J$_{2}$ are the first and second nearest neighbor exchange constants respectively. We report the magnetic field dependence of the dc susceptibility and neutron scattering of ZnCr$_{2}$S$_{4}$ in the form of a temperature-field phase diagram. We also study the low energy excitation spectrum of ZnCr$_{2}$S$_{4}$ which was found to be consistent with the J$_{1}$-J$_{2}$ model.   

Quantum effects in a half-polarized pyrochlore antiferromagnet

We study quantum effects in a spin-$3/2$ antiferromagnet on the pyrochlore lattice in an external magnetic field, focusing on the vicinity of a plateau in the magnetization at half the saturation value, observed in CdCr$_2$O$_4$\, and HgCr$_2$O$_4$. Our theory, based on quantum fluctuations, results in an effective quantum dimer model of the dynamics. We predict the existence of a symmetry-broken ground state on the plateau, even with only nearest-neighbor microscopic exchange. This symmetry broken state consists of a particular arrangement of spins polarized parallel and antiparallel to the field in a 3:1 ratio on each tetrahedron. It quadruples the lattice unit cell, and reduces the space group from $Fd\overline{3}m$ to $P4_332$. We also predict that for fields just above the plateau, the low temperature phase has transverse spin order, describable as a Bose-Einstein condensate of magnons. Other comparisons to and suggestions for experiments are discussed.   

Ordering in a frustrated pyrochlore antiferromagnet proximate to a spin liquid

We perform a general study of spin ordering on the pyrochlore lattice with a 3:1 proportionality of two spin polarizations[1]. Equivalently, this describes valence bond solid conformations of a quantum dimer model on the diamond lattice. We determine the set of likely low temperature ordered phases, on the assumption that the ordering is weak, i.e the system is close to a ``U(1)'' quantum spin liquid in which the 3:1 proportionality is maintained but the spins are strongly fluctuating. The nature of the 9 ordered states we find is determined by a ``projective symmetry'' analysis. All the phases exhibit translational and rotational symmetry breaking, with an enlarged unit cell containing 4 to 64 primitive cells of the underlying pyrochlore. The simplest of the 9 phases is the same ``R'' state found earlier in a theoretical study[2] of the ordering on the magnetization plateau in the $S=3/2$ materials CdCr$_2$0$_4$ and HgCr$_2$0$_4$. We suggest that the spin/ dimer model proposed therein undergoes a direct transition from the spin liquid to the R state, and describe a field theory for the universal properties of this critical point, at zero and non-zero temperatures. [1] D. L. Bergman, G. A. Fiete, and L. Balents, cond-mat/0511176. [2] D. L. Bergman, R. Shindou, G. A. Fiete, and L. Balents, cond-mat/0510202.   

Nature of magnetic structure in a geometrically frustrated manganite spinel

Unlike the pyrochlore geometrically frustrated magnets, several normal spinels such as ZnCr$_{2}$O$_{4}$ and ZnV$_{2}$O$_{4}$ undergo magnetic long-range order as well as cubic to tetragonal structural transitions at low temperatures. By comparison, ZnMn$_{2}$O$_{4}$, although tetragonal at room temperature, does not undergo a magnetic transition in spite of the rather large Curie-Weiss temperature of $\sim $ -800K indicative of strong antiferromagnetic fluctuations. The absence of an antiferromagnetic transition is surprising considering the nature of the Mn$^{3+}$ ion that is Jahn-Teller active and its effect on the ground state degeneracy. With the orbital degree of freedom active and a crystal anisotropy created by the Jahn-Teller octahedral distortions the system is tetragonally distorted just as in the vanadates where the V ion is orbitally active, but no magnetic transition is observed. Using neutron scattering we investigated how the magnetic structure changes as a function of temperature. We observed the presence of incommensurate magnetic peaks close to nuclear Bragg peaks that exhibit a first order phase transition. The FWHM of the magnetic peaks is much wider than that of nuclear peaks that indicates that the magnetic order is short range. Considering the 1-dimensional Boner-Fisher like behavior of the bulk susceptibility, it is proposed that the spin structure is helical and one-dimensional.   

Local structural disorder by chemical pressure in geometrically frustrated magnets, Cd$_{1-x}$Zn$_{x}$V2O4

The orbital degree of freedom plays an important role in the physics of frustration in Cd$_{x}$Zn$_{1-x}$V2O4 spinels. The magnetic and structural properties for $0<=x<=1$ compounds were investigated using neutron scattering. While the end members, x = 0 and 1, are magnetically frustrated in the high temperature cubic phase, the frustration is relieved with the transformation to the tetragonal N\'{e}el state through a spin-orbital coupling mechanism. In the doped compounds, both macroscopic transitions are absent due to the local randomness induced by the difference in the chemical pressure imposed by Zn and Cd ions. This leads to the loss of cooperative orbital ordering and suppression of any long range antiferromagnetic coupling. However, from the local atomic structure it is deduced that in the mixed compounds the local environments around Zn and Cd ions correspond to ZnV2O4 and CdV2O4, respectively. Thus locally short-range orbital ordering is possible that explains why the magnetic exchange interactions remain largely unchanged with doping.   

Magnetic ordering in EuNiO$_{3}$ and NdNiO$_{3}$

We report M\"{o}ssbauer and magnetization studies on $^{57}$Fe:EuNiO$_{3}$ and $^{57}$Fe:NdNiO$_{3}$ between 1.5 K and 300 K. Below T$_{N} \quad \approx $ 190 K and 132 K for the Eu and the Nd compound, resp., the M\"{o}ssbauer spectra reveal two magnetically inequivalent Ni (Fe) sites. Roughly 50{\%} of the spectral area split into a sextet (A) with a well defined magnetic hyperfine field. The spectra A are due to the antiferromagnetically ordered Ni sublattices. The other 50{\%} of the spectral area remain unsplit and only gradually broaden on decreasing temperature (spectra B). The observation of two kinds of magnetically inequivalent Ni(Fe) sites is both compatible with charge ordering or orbital ordering leaving 50{\%} of Ni sites frustrated. Spectrum B is typical for a slowing down magnetic fluctuations. A best approach is yielded by a model allowing for stochastic jumps of the hyperfine field in random directions. Both compounds yield nearly identical fluctuation rates with a roughly linear temperature dependence indicating that the influence of the magnetic moment of the rare earth on Ni is negligible.   

Ultrasonic Study of Critical Behavior in the Magnetic Phase Diagram of CsNiCl$_{3}$

It is now well established that many quasi-one-dimensional frustrated triangular antiferromagnets of the ABX$_{3}$ family exhibit exotic magnetic phase diagrams with new types of multicritical points [1]. Nevertheless, investigations into the nature of some phase boundaries remain controversial. While some theoretical and numerical studies suggest new chiral universality classes, others conclude these transitions are weakly first order [2]. Results are presented here of a systematic investigation of the critical behavior associated with the magnetic field - temperature phase diagram of CsNiCl$_{3}$ using high resolution ultrasonic measurements of the elastic constants, along with analysis based on a Landau-type model of magnetoelastic coupling [3]. In particularly, we will show how it is possible to obtain very reliable values for the critical exponents associated with the transitions to elliptical and the spin-flop ordered states. Exponent estimates very close to the multicritical point are also presented. [1] M.L. Plumer et al, in Magnetic Systems with Competing Interactions, Ed. H.T. Diep (World Scientific, Singapore, 1994). [2] A. Peles et al, Phys. Rev. B 69, 220408 (R) (2004). [3] G. Quirion, T. Taylor and M. Poirier, Phys. Rev. B 72, 094403 (2005).