Unusual Low-Temperature Phase in VO2_2 Nanoparticles

We present a systematic investigation of the crystal and electronic structure and the magnetic properties above and below the metal-insulator transition of ball-milled VO$_2$ nanoparticles and VO$_2$ microparticles. For this research, we performed a Rietveld analysis of synchrotron radiation x-ray diffraction data, O $K$ x-ray absorption spectroscopy, V $L_3$ resonant inelastic x-ray scattering, and magnetic susceptibility measurements. This study reveals an unusual low-temperature phase that involves the formation of an elongated and less-tilted V-V pair, a narrowed energy gap, and an induced paramagnetic contribution from the nanoparticles. We show that the change in the crystal structure is consistent with the change in the electronic states around the Fermi level, which leads us to suggest that the Peierls mechanism contributes to the energy splitting of the $a_{1g}$ state. Furthermore, we find that the high-temperature rutile structure of the nanoparticles is almost identical to that of the microparticles.Comment: 7 pages, 8 figures, 2 table

Synthesis, Structural and Magnetic Properties of La0.5Ba0.5CoO2.75+x

Hole doping to the Co3+ ion in cobaltite perovskites can significantly modify the electromagnetic properties. We prepared the hole-doped cobaltite perovskites La0.5Ba0.5CoO2.75+x (x = 0.06 and x = 0.15). Neutron powder diffraction and dc-magnetization experiments were performed to investigate the crystal structure and magnetic properties. We found that both of the samples have the cubic crystal structure with space group Pmm in all the measured temperatures. Ferromagnetic transition occurs at 160K in x = 0.15 sample. For x = 0.06, we found the short-range ordered ferromagnetic state and a long-range ordered antiferromagnetic state coexist in low temperature

Magnetic-field-induced instability of the cooperative paramagnetic state in Zn xCo 4-x(OD) 6Cl 2

Using elastic and inelastic neutron scattering techniques with and without application of an external magnetic field H, the magnetic ground states of Zn xCo 4-x(OD) 6Cl 2 (x=0,1) were studied. Our results show that for x=0, the ground state is a magnetic long-range ordered (LRO) state where each tetrahedron forms an "umbrella"-type structure. On the other hand, for x=1, no static ordering was observed down to 1.5 K, which resembles the behavior found in the isostructural quantum system Zn xCu 4-x(OD) 6Cl 2. When H field is applied, however, the x=1 system develops the same LRO state as x=0. This indicates that the x=1 disordered state is in the vicinity of the x=0 ordered state. © 2012 American Physical Society