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Register a new userCluster spin glass behavior in geometrically frustrated Zn3V3O8
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We report the bulk magnetic properties of a yet unexplored vanadium-based multivalued spinel system, Zn3V3O8. A Curie-Weiss fit of our dc magnetic susceptibility data in the temperature region from 140 to 300 K yields a Curie constant C = 0.75cm3K/mole V, theta CW = -370 K. We have observed a splitting between the zero field cooled ZFC and field cooled FC susceptibility curves below a temperature Tirr of about 6.3 K. The value of the frustration parameter nearly equals to 100 suggests that the system is strongly frustrated. From the ac susceptibility measurements we find a logarithmic variation of freezing temperature (Tf ) with frequency attesting to the formation of a spin glass below Tf . However, the value of the characteristic frequency obtained from the Vogel-Fulcher fit suggests that the ground state is closer to a cluster glass rather than a conventional spin glass. We explored further consequences of the spin glass behavior and observed aging phenomena and memory effect (both in ZFC and FC). We found that a positive temperature cycle erases the memory, as predicted by the hierarchical model. From the heat capacity CP data, a hump-like anomaly was observed in CP /T at about 3.75 K. Below this temperature the magnetic heat capacity shows a nearly linear dependence with T which is consistent with the formation of a spin glass state below Tf in Zn3V3O8.
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The results of ac and dc magnetic susceptibility isothermal magnetization and heat-capacity measurements as a function of temperature (T) are reported for Sr3NiRhO6 and Sr3NiPtO6 containing magnetic chains arranged in a triangular fashion in the basal plane and crystallizing in K4CdCl6-derived rhombohedral structure. The results establish that both the compounds are magnetically frustrated, however in different ways. In the case of the Rh compound, the susceptibility data reveal that there are two magnetic transitions, one in the range 10 -15 K and the other appearing as a smooth crossover near 45 K, with a large frequency dependence of ac susceptibility in the range 10 to 40 K; in addition, the features in C(T) are smeared out at these temperatures. The magnetic properties are comparable to those of previously known few compounds with partially disordered antiferromagnetic structure. On the other hand, for Sr3NiPtO6, there is no evidence for long-range magnetic ordering down to 1.8 K despite large value of paramagnetic Curie temperature.
We report neutron scattering studies of the spin correlations of the geometrically frustrated pyrochlore Tb2Mo2O7 using single crystal samples. This material undergoes a spin-freezing transition below Tg~24 K, similar to Y2Mo2O7, and has little apparent chemical disorder. Diffuse elastic peaks are observed at low temperatures, indicating short-range ordering of the Tb moments in an arrangement where the Tb moments are slightly rotated from the preferred directions of the spin ice structure. In addition, a Q-independent signal is observed which likely originates from frozen, but completely uncorrelated, Tb moments. Inelastic measurements show the absence of sharp peaks due to crystal field excitations. These data show how the physics of the Tb sublattice responds to the glassy behavior of the Mo sublattice with the associated effects of lattice disorder.
CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = Theta_CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.
We propose a method to study the magnetic properties of a disordered Ising kagome lattice. The model considers small spin clusters with infinite-range disordered couplings and short-range ferromagnetic (FE) or antiferromagnetic interactions. The correlated cluster mean-field theory is used to obtain an effective single-cluster problem. A finite disorder intensity in FE kagome lattice introduces a cluster spin-glass (CSG) phase. Nevertheless, an infinitesimal disorder stabilizes the CSG behavior in the geometrically frustrated kagome system. Entropy, magnetic susceptibility and spin-spin correlation are used to describe the interplay between disorder and geometric frustration (GF). We find that GF plays an important role in the low-disorder CSG phase. However, the increase of disorder can rule out the effect of GF.
ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K from a cubic spin liquid phase to a tetragonal Neel state. Using powder diffraction and single crystal polarized neutron scattering, we determined the complex spin structure of the Neel phase. This phase consisted of several magnetic domains with different characteristic wave vectors. This indicates that the tetragonal phase of ZnCr2O4 is very close to a critical point surrounded by many different Neel states. We have also studied, using elastic and inelastic neutron scattering techniques, the effect of nonmagnetic dilution on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05, the magnetic correlations do not change qualitatively from those in the pure material, except that the phase transition becomes second order. For x= 0.3, the spin-spin correlations become short range. Interestingly, the spatial correlations of the frozen spins in the x=0.3 material are the same as those of the fluctuating moments in the pure and the weakly diluted materials.
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