Frustration induced disordered magnetism in Ba3RuTi2O9

The title compound Ba3RuTi2O9 crystallizes with a hexagonal unit cell. It contains layers of edge shared triangular network of Ru4+ (S=1) ions. Magnetic susceptibility chi(T) and heat capacity data show no long range magnetic ordering down to 1.8K. A Curie-Weiss (CW) fitting of chi(T) yields a large antiferromagnetic CW temperature theta_CW=-166K. However, in low field, a splitting of zero field cooled (ZFC) and field cooled (FC) chi(T) is observed below ~30K. Our measurements suggest that Ba3RuTi2O9 is a highly frustrated system but only a small fraction of the spins in this system undergo a transition to a frozen magnetic state below ~30K.Comment: 5 pages, 6 figures (accepted in EPJB

A S=1/2 vanadium-based geometrically frustrated spinel system Li2ZnV3O8

We report the synthesis and characterization of Li2ZnV3O8, which is a new Zn-doped LiV2O4 system containing only tetravalent vanadium. A Curie-Weiss susceptibility with a Curie-Weiss temperature of CW ~214 K suggests the presence of strong antiferromagnetic correlations in this system. We have observed a splitting between the zero-field cooled ZFC and field cooled FC susceptibility curves below 6 K. A peak is present in the ZFC curve around 3.5 K suggestive of spin-freezing . Similarly, a broad hump is also seen in the inferred magnetic heat capacity around 9 K. The consequent entropy change is only about 8% of the value expected for an ordered S = 1=2 system. This reduction indicates continued presence of large disorder in the system in spite of the large CW, which might result from strong geometric frustration in the system. We did not find any temperature T dependence in our 7Li nuclear magnetic resonance NMR shift down to 6 K (an abrupt change in the shift takes place below 6 K) though considerable T-dependence has been found in literature for LiV2O4- undoped or with other Zn/Ti contents. Consistent with the above observation, the 7Li nuclear spin-lattice relaxation rate 1/T1 is relatively small and nearly T-independent except a small increase close to the freezing temperature, once again, small compared to undoped or 10% Zn or 20% Ti-doped LiV2O4.Comment: 7 pages, 8 figures, accepted in JPCM (Journal of Physics condensed matter

Tunable temperature induced magnetization jump in a GdVO3 single crystal

We report a novel feature of the temperature induced magnetization jump observed along the a-axis of the GdVO3 single crystal at temperature TM = 0.8 K. Below TM, the compound shows no coercivity and remanent magnetization indicating a homogenous antiferromagnetic structure. However, we will demonstrate that the magnetic state below TM is indeed history dependent and it shows up in different jumps in the magnetization only when warming the sample through TM. Such a magnetic memory effect is highly unusual and suggesting different domain arrangements in the supposedly homogenous antiferromagnetic phase of the compound.Comment: 17 pages, 8 Figure

BaV3O8: A possible Majumdar-Ghosh system with S=1/2

BaV3O8 contains both magnetic V4+(S=1/2) ions and non-magnetic V5+(S=0) ions. The V4+ ions are arranged in a coupled Majumdar-Ghosh chain like network. Our magnetic susceptibility chi(T) data fit well with the Curie-Weiss formula in the temperature range of 80-300K and it yields a Curie constant C=0.39cm3K/mole-V4+ and an antiferromagnetic Weiss temperature theta=-26K. The chi(T) curve shows a broad maximum at T~25K indicative of short-range order (SRO) and an anomaly corresponding to long-range order (LRO) at TN~6K. The value of the frustration index (f=mod[theta/TN]~5) suggests that the system is moderately frustrated. Above the LRO temperature the experimental magnetic susceptibility data match well with the coupled Majumdar-Ghosh chain model with the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic coupling alpha=2 and Jnnn/kB=40K. In a mean-field approach when considering the inter-chain interactions, we obtain the total inter-chain coupling to be about 16K. The LRO anomaly at TN is also observe in the specific heat Cp(T) data and is not sensitive to an applied magnetic field up to 90kOe. A 51V NMR signal corresponding to the non-magnetic vanadium was observed. Anomalies at 6K were observed in the variation with temperature of the 51V NMR linewidth and in the spin-lattice relaxation rate 1/T1, indicating that they are sensitive to the LRO onset and fluctuations at the magnetic V sites. The existence of two components (one short and another long) is observed in the spin-spin relaxation rate 1/T2 data in the vicinity of TN. The shorter component seems to be intimately connected with the magnetically ordered state. We suggest that both magnetically ordered and non-long range ordered (non-LRO) regions coexist in this compound below the long range ordering temperature.Comment: Accepted in Phys. Rev.

89Y NMR Probe of Zn Induced Local Magnetism in YBa2(Cu(1-y)Zn(y))3O(6+x)

We present detailed data and analysis of the effects of Zn substitution on the planar Cu site in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO$_{6+x}$) as evidenced from our $^{89}$Y NMR measurements on oriented powders. For $x<<1$ we find additional NMR lines which are associated with the Zn substitution. From our data on the intensities and temperature dependence of the shift, width, and spin-lattice relaxation rate of these resonances, we conclude that the spinless Zn 3$d$$^{10}$ state induces local moments on the near-neighbour ($$) Cu atoms. Additionally, we conjecture that the local moments actually extend to the farther Cu atoms with the magnetization alternating in sign at subsequent $nn$ sites. We show that this analysis is compatible with ESR data taken on dilute Gd doped (on the Y site) and on neutron scattering data reported recently on Zn substituted YBCO$_{6 + x}$. For optimally doped compounds $^{89}$Y $nn$ resonances are not detected, but a large $T$% -dependent contribution to the $^{89}$Y NMR linewidth is evidenced and is also attributed to the occurence of a weak induced local moment near the Zn. These results are compatible with macroscopic magnetic measurements performed on YBCO$_{6 + x}$ samples prepared specifically in order to minimize the content of impurity phases. We find significant differences between the present results on the underdoped YBCO$_{6 + x}$ samples and $$Al NMR data taken on Al$^{3+}$ substituted on the Cu site in optimally doped La$_2$CuO$_4$. Further experimental work is needed to clarify the detailed evolution of the impurity induced magnetism with hole content in the cuprates.Comment: To be published in EPJB 15 pages of text and figures in eps forma

75As NMR local probe study of magnetism in (Eu1-xKx)Fe2As2

75As NMR measurements were performed as a function of temperature and doping in (Eu1-xKx)Fe2As2 (x=0,0.38,0.5,0.7) samples. The large Eu2+ moments and their fluctuations are found to dominate the 75As NMR properties. The 75As nuclei close to the Eu2+ moments likely have a very short spin-spin relaxation time (T2) and are wiped out of our measurement window. The 75As nuclei relatively far from Eu2+ moments are probed in this study. Increasing the Eu content progressively decreases the signal intensity with no signal found for the full-Eu sample (x=0). The large 75As NMR linewidth arises from an inhomogeneous magnetic environment around them. The spin lattice relaxation rate (1/T1) for x=0.5 and 0.7 samples is nearly independent of temperature above 100K and results from a coupling to paramagnetic fluctuations of the Eu2+ moments. The behavior of 1/T1 at lower temperatures has contributions from the antiferromagnetic fluctuations of the Eu2+ moments as also the fluctuations intrinsic to the FeAs planes and from superconductivity.Comment: 6 pages, 6 figures (to appear in EPJB