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Register a new userStructural and magnetic properties of the new cobaltate series (BaSr)4-xLa2xCo4O15
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We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.
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Magnetization, neutron diffraction and X-ray diffraction of Zn doped MnV2O4 as a function of temperature have been measured and the critical exponents and magnetocaloric effect of this system have been estimated. It is observed, that with increase in Zn substitution the noncollinear orientation of Mn spins with the V spins decreases which effectively leads to the decrease of structural transition temperature more rapidly than Curie temperature. It has been shown that the obtained values of {beta}, {gamma} and {delta} from different methods match very well. These values do not belong to universal class and the values are in between the 3D Heisenberg model and mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperature are unambiguous and intrinsic to the system. The observed double peaks in magneto-caloric curve of Mn0.95Zn0.05V2O4 is due to the strong distortion of VO6 octahedra.
We report the structural and magnetic properties of a new system LiRhMnO$_{4}$ (LRMO) through x-ray diffraction, bulk magnetization, heat capacity and $^{7}$Li nuclear magnetic resonance (NMR) measurements. LRMO crystallizes in the cubic space group $mathit{Fd}$$bar{3}$$mathit{m}$. From the DC susceptibility data, we obtained the Curie-Weiss temperature $mathrm{theta}_{mathrm{CW}}$ = -26 K and Curie constant $mathit{C}$ = 1.79 Kcm$^{3}$/mol suggesting antiferromagnetic correlations among the magnetic Mn$^{4+}$ ions with an effective spin $mathit{S}$ = $frac{3}{2}$. At $mathit{H}$ = 50 Oe, the field cooled and zero-field cooled magnetizations bifurcate at a freezing temperature, $T_{f}$ = 4.45 K, which yields the frustration parameter $mathit{f=frac{midtheta_{CW}mid}{T_{f}}}>$5. AC susceptibility, shows a cusp-like peak at around $T_{f}$, with the peak position shifting as a function of the driving frequency, confirming a spin-glass-like transition in LRMO. LRMO also shows typical spin-glass characteristics such as memory effect, aging effect and relaxation. In the heat capacity, there is no sharp anomaly down to 2 K indicative of long-range ordering. The field sweep $^{7}$Li NMR spectra show broadening with decreasing temperature without any spectral line shift. The $^{7}$Li NMR spin-lattice and spin-spin relaxation rates also show anomalies due to spin freezing near $T_{f}$.
We have synthesized single crystals of CeZnAl$_3$, which is a new member of the family of the Ce-based intermetallics Ce$TX_3$ ($T$ = transition metal, $X$= Si, Ge, Al), crystallizing in the non-centrosymmetric tetragonal BaNiSn$_3$-type structure. Magnetization, specific heat and resistivity measurements all show that CeZnAl$_3$ orders magnetically below around 4.4 K. Furthermore, magnetization measurements exhibit a hysteresis loop at low temperatures and fields, indicating the presence of a ferromagnetic component in the magnetic state. This points to a different nature of the magnetism in CeZnAl$_3$ compared to the other isostructural Ce$T$Al$_3$ compounds. Resistivity measurements under pressures up to 1.8 GPa show a moderate suppression of the ordering temperature with pressure, suggesting that measurements to higher pressures are required to look for quantum critical behavior.
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below about 14 K. In addition to this orbitally driven ferroelectricity, lacunar spinels are an interesting material class, as the vanadium ions form V4 clusters representing stable molecular entities with a common electron distribution and a well-defined level scheme of molecular states resulting in a unique spin state per V4 molecule. Here we report detailed x-ray, magnetic susceptibility, electrical resistivity, heat capacity, thermal expansion, and dielectric results to characterize the structural, electric, dielectric, magnetic, and thermodynamic properties of this interesting material, which also exhibits strong electronic correlations. From the magnetic susceptibility, we determine a negative Curie-Weiss temperature, indicative for antiferromagnetic exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular clusters. The low-temperature heat capacity provides experimental evidence for gapped magnon excitations. From the entropy release, we conclude about strong correlations between magnetic order and lattice distortions. In addition, the observed anomalies at the phase transitions also indicate strong coupling between structural and electronic degrees of freedom. Utilizing dielectric spectroscopy, we find the onset of significant dispersion effects at the polar Jahn-Teller transition. The dispersion becomes fully suppressed again with the onset of spin order. In addition, the temperature dependencies of dielectric constant and specific heat possibly indicate a sequential appearance of orbital and polar order.
Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in the magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems.
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