Yuki Higuchi

Internal magnetic field in the zigzag-chain family (Na,Ca)Cr2O4

In order to elucidate the magnetic nature for a novel one-dimensional zigzag chain compound, NaCr2O4, we have measured μ+SR spectra using a powder sample in the temperature range between 2 and 200 K. Weak transverse field (wTF-) μ+SR measurements indicated that the whole volume of the sample enters into an antiferromagnetic (AF) phase below TN = 125 K. The zero field (ZF-) μ+SR spectrum obtained below TN exhibits a clear oscillation with a single muon-spin precession frequency (fμ). This suggests that static AF order is formed below TN and that all the implanted muons sense the same internal magnetic field. The temperature dependence of fμ was found to be very similar to that for the intensity of the magnetic Bragg peak in neutron diffraction (ND) measurements. On the other hand, the ZF-μ+SR spectrum for the isostructural compound, β-CaCr2O4, showed a rapidly damped oscillation below TN = 21 K, supporting the formation of incommensurate AF order, as proposed by ND.

Na-ion dynamics in Quasi-1D compound NaV2O4

We have used the pulsed muon source at ISIS to study high-temperature Na-ion dynamics in the quasi-one-dimensional (Q1D) metallic antiferromagnet NaV2O4. By performing systematic zero-field and longitudinal-field measurements as a function of temperature we clearly distinguish that the hopping rate increases exponentially above Tdiff=250 K. The data is well fitted to an Arrhenius type equation typical for a diffusion process, showing that the Na-ions starts to be mobile above Tdiff . Such results makes this compound very interesting for the tuning of Q1D magnetism using atomic-scale ion-texturing through the periodic potential from ordered Na-vacancies. Further, it also opens the door to possible use of NaV2O4 and related compounds in energy related applications.

Li-ion diffusion in Li intercalated graphite C6Li and C12Li probed by mu+SR

In order to study a diffusive behavior of Li+ in Li intercalated graphites, we have measured muon spin relaxation (μ+SR) spectra for C6Li and C12Li synthesized with an electrochemical reaction between Li and graphite in a Li-ion battery. For both compounds, it was found that Li+ ions start to diffuse above 230 K and the diffusive behavior obeys a thermal activation process. The activation energy (Ea) for C6Li is obtained as 270(5) meV, while Ea = 170(20) meV for C12Li. Assuming a jump diffusion of Li+ in the Li layer of C6Li and C12Li, a self-diffusion coefficient DLi at 310 K was estimated as 7.6(3) × 10-11 (cm2 s-1) in C6Li and 14.6(4) × 10-11 (cm2 s-1) in C12Li.

Magnetic phase diagram of Sr1-xCaxCo2P2

In order to study the phase diagram from a microscopic viewpoint, we have measured wTF- and ZF-μ+SR spectra for the Sr1-xCaxCo2P2 powder samples with x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1. Due to a characteristic time window and spatial resolution of μ+SR, the obtained phase diagram was found to be rather different from that determined by magnetization measurements. That is, as x increases from 0, a Pauli-paramagnetic phase is observed even at the lowest T measured (1.8 K) until x = 0.4, then, a spin-glass like phase appears at 0.5 ≤ x ≤ 0.6, and then, a phase with wide field distribution probably due to incommensurate AF order is detected for x = 0.8, and finally, a commensurate A-type AF ordered phase (for x = 1) is stabilized below TN ~ 80 K. Such change is most likely reasonable and connected to the shrink of the c-axis length with x, which naturally enhances the magnetic interaction between the two adjacent Co planes.

Structural, magnetic, and diffusive nature of olivine-type NaxFePO4

In order to investigate the microscopic magnetic nature of a sodium phospho- olivine, we have measured μ+SR spectra for NaxFePO4 using a powder sample in the temperature range between 2 and 500 K. ZF-μ+ SR measurements below 200 K at TRIUMF showed the appearance of static magnetic order below TN ~ 61 K, where the susceptibility-vs.- temperature curve exhibits a sharp maximum due to an antiferromagnetic (AF) transition. The wide distribution found for the internal magnetic field (Hint) was explained by the formation of AF order with the spread in Hint due to random distribution of Na vacancies. At higher temperatures, the ZF-spectrum for Na0.7FePO4, obtained at J-PARC, is found to change from a low-T static behavior to a high-T dynamic behavior above ~ 300 K. This is consistent with the fact that Na+ ions are reversibly extracted from and intercalated into the NaxFePO4 lattice, while there is no NMR study on Na-diffusion in NaxFePO4 due to large Fe moments.

Inductively Coupled Plasma Mass Spectrometry Study on the Increase in the Amount of Pr Atoms for Cs-Ion-Implanted Pd/CaO Multilayer Complex with Deuterium Permeation

To investigate the nuclear transmutation of Cs into Pr reported in this journal by Iwamura and coworkers, we have measured the amount of Pr atoms in the range as low as ~1×1010 cm-2 using inductively coupled plasma mass spectrometry for Cs-ion-implanted Pd/CaO multilayer complexes before and after deuterium permeation. The amount of Pr was initially at most 2.0×1011 cm-2 and it increased up to 1.6×1012 cm-2 after deuterium permeation. The increase in the amount of Pr could be explained neither by deuterium permeation-stimulated segregation of Pr impurities nor by external contamination from the experimental environment during the permeation. No increase in Pr was observed for permeation with hydrogen. These findings suggest that the observed increase in Pr with deuterium permeation can be attributed to a nuclear origin, as reported by Iwamura and coworkers, although the amount of the increase in Pr is two orders of magnitude less than that reported by them.