The hydrogenation of Nd2Fe14B under a high pressure of hydrogen has been investigated for the first time. At the heat-treatment temperature of 600 degree C, the almost complete decomposition of Nd2Fe14B into NdH2+x and alpha-Fe is observed, although a rather long heat-treatment time is necessary to achieve the sufficient hydrogenation. The desorption of hydrogen from NdH2+x does not occur in the furnace-cooling process.
In this report, the magnetic behaviour of $NiCr_{2}O_{4}$ bulk and nanoparticle samples under different applied magnetic field has been investigated extensively. Nanoparticles of $NiCr_{2}O_{4}$ were obtained by mechanical milling of polycrystalline powder prepared by polyol method. FC-ZFC measurement of bulk at different applied magnetic field has revealed the existence of a ferrimagnetic transition around 66K followed by an antiferromagnetic transition close to 30K. However, its nano counterpart has shown remarkable change in magnetic properties - a suppression of ferrimagnetic transition accompanied by strengthening low temperature magnetic phase and observation of a new transition at 90K ($T_P$), which is weakly magnetic in nature. The frequency dependent ac susceptibility data of nanoparticle have been fitted to the well known de Almedia-Thouless equation and a $H^{2/3}$ dependence of the low temperature peak is observed with a resulting zero field freezing temperature ($T_f^0$) equal to 10.1K. Further, the dynamical behaviour near freezing temperature has been analysed in terms of critical behaviour and the obtained fitted parameters values being as $tau_0$(relaxation time constant) = $3.6 X 10^{-6}s$, $T_f^0=8.7$K and $z u = 11.1$. Moreover, Vogel-Fulcher law has been used to understand the nature of freezing transition and the parameter after fitting are obtained as $E_a/k_B = 58.9$K, $tau_0 = 5.22 times 10^{-8}$ and $T_0 = 8.03$K. Finally, the spin-glass phase is concluded. Moreover, in contrast to bulk, the $H^{2/3}$ dependence of freezing temperature of nanoparticle sample (75h) does support the 2D surface like spin glass nature.
Determining ground states of correlated electron systems is fundamental to understanding novel phenomena in condensed matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states, potentially prompting the emergence of quantum spin states, such as resonating valence bond (RVB) and valence bond solid (VBS). Although theoretically proposed to exist in a kagome lattice -- one of the most highly frustrated lattices in two dimensions (2D) being comprised of corner-sharing triangles -- such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the pinwheel VBS ground state in the S=1/2 deformed kagome lattice antiferromagnet Rb2Cu3SnF12. In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.
Hexagonal antiferromagnets Cs$_2$Cu$_3$MF$_{12}$ (M = Zr, Hf and Sn) have uniform Kagome lattices of Cu$^{2+}$ with S = 1/2, whereas Rb$_2$Cu$_3$SnF$_{12}$ has a 2a by 2a enlarged cell as compared with the uniform Kagome lattice. The crystal data of Cs$_2$Cu$_3$SnF$_{12}$ synthesized first in the present work are reported. We performed magnetic susceptibility measurements on this family of Kagome antiferromagnet using single crystals. In the Cs$_2$Cu$_3$MF$_{12}$ systems, structural phase transitions were observed at $T_t = 225$ K, 172 K and 185 K for M = Zr, Hf and Sn, respectively. The magnetic susceptibilities observed for $T > T_t$ are almost perfectly described using theoretical results obtained by exact diagonalization for the 24-site Kagome cluster with $J/k_B = 244$ K, 266 K and 240 K, respectively. Magnetic ordering accompanied by the weak ferromagnetic moment occurs at $T_N = 23.5$ K, 24.5 K and 20.0 K, respectively. The origins of the weak ferromagnetic moment should be ascribed to the lattice distortion that breaks the hexagonal symmetry of the exchange network for $T < T_t$ and the Dzyaloshinsky-Moriya interaction. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as a modified Kagome antiferromagnet with four types of neighboring exchange interaction. Neither structural nor magnetic phase transition was observed in Rb$_2$Cu$_3$SnF$_{12}$. Its magnetic ground state was found to be a spin singlet with a triplet gap. Using exact diagonalization for a 12-site Kagome cluster, we analyzed the magnetic susceptibility and evaluated individual exchange interactions. The causes leading to the different ground states in Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$ are discussed.
We synthesized single crystals of the new hexagonal compounds A$_2$Cu$_3$SnF$_{12}$ with A=Cs and Rb, and investigated their magnetic properties. These compounds are composed of Kagom{e} layers of corner-sharing CuF$_6$-octahedra. Cs$_2$Cu$_3$SnF$_{12}$ has the proper Kagom{e} layer at room temperature, and undergoes structural phase transition at $T_mathrm{t}simeq 185$ K. The temperature dependence of the magnetic susceptibility in Cs$_2$Cu$_3$SnF$_{12}$ agrees well with the result of the numerical calculation for $S=1/2$ two-dimensional Heisenberg Kagom{e} antiferromagnet down to $T_mathrm{t}$ with the nearest exchange interaction $J/k_mathrm{B}simeq 240$ K. Although the magnetic susceptibility deviates from the calculated result below $T<T_mathrm{t}$, the rounded maxima were observed at approximately $Tsimeq (1/6)J/k_mathrm{B}$ as predicted by the theory. Cs$_2$Cu$_3$SnF$_{12}$ undergoes three-dimensional magnetic ordering at $T_mathrm{N} = 20$ K.
We investigated the crystal structure of Rb$_2$Cu$_3$SnF$_{12}$ and its magnetic properties using single crystals. This compound is composed of Kagome layers of corner-sharing CuF$_{6}$ octahedra with a 2a x 2a enlarged cell as compared with the proper Kagome layer. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as an $S$=1/2 modified Kagome antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagome cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated.
We report here the synthesis of single-phase bulk samples of CoO2, the x = 0 end member of the AxCoO2 systems (A = Li, Na), from a pristine LiCoO2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetic susceptibility of the CoO2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO2 is a Pauli-paramagnetic metal with itinerant electrons.
Specific heat and the magnetocaloric effect are used to probe the field-induced up-up-down phase of Cs2CuBr4, a quasi-two-dimensional spin-1/2 triangular antiferromagnet with near-maximal frustration. The shape of the magnetic phase diagram shows that the phase is stabilized by quantum fluctuations, not by thermal fluctuations as in the corresponding phase of classical spins. The magnon gaps determined from the specific heat are considerably larger than those expected for a Heisenberg antiferromagnet, probably due to the presence of a small Dzyaloshinskii-Moriya interaction.
