Hiroyuki Chudo

Evidence for Appearance of an Internal Field in the Ordered State of CeRu2Al10 by µ+SR

We report zero-field µ + SR measurements made at J-PARC on CeRu 2 Al 10 . Below the phase transition temperature T 0 ∼27 K, a clear modulation of µ + polarization due to an internal field is observed, indicating that magnetic ordering takes place. Compared with the internal field expected from the effective magnetic moment in the disordered state, the observed internal field at the µ + site is quite small. This indicates that the ordered Ce magnetic moment is small (∼10 -2 µ B ) and/or the nearest pair of Ce magnetic moments are mutually anti-parallel. The T -dependence of the internal field is somewhat peculiar, suggesting that another ordering is taking place as well.

Observation of Barnett fields in solids by nuclear magnetic resonance

A magnetic field is predicted to emerge on a particle in a rotating material body even if the body is electrically neutral. This emergent field is called a Barnett field. We show that nuclear magnetic resonance (NMR) enables direct measurement of the Barnett field in solids. We rotated both a sample and an NMR coil synchronously at high speed and found an NMR shift whose sign reflects that of the nuclear magnetic moments. This result provides direct evidence of the Barnett field. The use of NMR for Barnett field measurement enables the unknown signs of nuclear magnetic moments in solids to be determined.

NMR Evidence for the 8.5 K Phase Transition in Americium Dioxide

We report here the first NMR study of americium dioxide (AmO 2 ). More than 30 years ago, a phase transition was suggested to occur in this compound at 8.5 K based on magnetic susceptibility data, while no evidence had been obtained from microscopic measurements. We have prepared a powder sample of 243 AmO 2 containing 90 at. % 17 O and have performed 17 O NMR at temperatures ranging from 1.5 to 200 K. After a sudden drop of the 17 O NMR signal intensity below 8.5 K, at 1.5 K we have observed an extremely broad spectrum covering a range of ∼14 kOe in applied field. These data provide the first microscopic evidence for a phase transition as a bulk property in this system. In addition, the 17 O NMR spectrum has been found to split into two peaks in the paramagnetic state, an effect which has not been reported for actinide dioxides studied up to now. We suggest that the splitting is induced by self-radiation damage from the alpha decay of 243 Am.

Spin pumping efficiency from half metallic Co2MnSi

We present spin pumping using a Heusler alloy Co2MnSi/Pt bilayer film. A spin current is produced by a ferromagnetic resonance (FMR) technique. The pure spin current injected into the Pt layer from the Co2MnSi layer is detected by the inverse spin-Hall effect (ISHE), which converts the spin current into an electric current. We estimated a damping constant of the Co2MnSi/Pt bilayer film from an angular dependence of FMR spectra. Using the damping constant efficiency of spin pumping from the Co2MnSi layer is evaluated. We found that a mixing conductance at the Co2MnSi/Pt interface is comparable to that at a permalloy/Pt interface.

Rotational Doppler Effect and Barnett Field in Spinning NMR

We report the observation of the rotational Doppler effect using nuclear magnetic resonance (NMR). We have developed a coil-spinning technique that enables measurements by rotating a detector and fixing a sample. We found that the rotational Doppler effect gives rise to NMR frequency shifts equal to the rotation frequency. We formulate the rotational Doppler effect and the Barnett field using a vector model for the nuclear magnetic moment. This formulation reveals that, with just the sample rotating, both effects cancel each other, thereby explaining the absence of an NMR frequency shift in conventional sample-spinning NMR measurements.

Gyroscopic g factor of rare earth metals

We develop the in situ magnetization measurement apparatus for observing the Barnett effect consisting of a fluxgate sensor, a high speed rotor with frequencies of up to 1.5 kHz, and a magnetic shield at room temperature. The effective magnetic field (Barnett field) in a sample arising from rotation magnetizes the sample and is proportional to the rotational frequency. The gyroscopic g factor, g ′, of rare earth metals, in particular, Gd, Tb, and Dy, was estimated to be 2.00 ± 0.08, 1.53 ± 0.17, and 1.15 ± 0.32, respectively, from the slopes of the rotation dependence of the Barnett field. This study provides a technique to determine the g ′ factor even in samples where the spectroscopic method may not be available.

Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor NpPd5Al2

We present 27 Al NMR measurements of the Knight shift and nuclear spin–lattice relaxation rate 1/ T 1 in the normal state for a single crystal of the heavy-fermion superconductor NpPd 5 Al 2 ( T c = 4.9 K). We report here that 1/ T 1 shows a large anisotropy with a different temperature dependence for H ∥ a and H ∥ c . 1/ T 1 for H ∥ c shows a power-law temperature dependence ∼ T 0.38 , while 1/ T 1 for H ∥ a shows a T -linear dependence at low temperatures in fields well above H c2 . From the analysis of 1/ T 1 , X Y -type anisotropy of AFM spin fluctuations is found to become prominent below 20 K.

Line splitting by mechanical rotation in nuclear magnetic resonance

Nuclear magnetic resonance (NMR) measurements for a nuclear spin 1/2 system in an external magnetic field and mechanical rotation in kHz range are reported. NMR signals are measured by the synchronous rotation of a sample and a detector coil under the condition that the rotation axis is perpendicular to the external field. NMR line splits into two despite the system having a single Zeeman level separation. We analytically show that the splitting originates in the nuclear spin motion under mechanical rotation, and is determined by only the angular velocity of the rotation without any material parameters.

Fabrication and magnetic control of Y3Fe5O12 cantilevers

We have fabricated ferrite cantilevers in which their vibrational properties can be controlled by external magnetic fields. Submicron-scale cantilever structures were made from Y3Fe5O12 films by physical etching combined with the use of a focused ion beam milling technique. We found that the cantilevers exhibit two resonance modes which correspond to horizontal and vertical vibrations. Under external magnetic fields, the resonance frequency of the horizontal mode increases, while that of the vertical mode decreases, quantitatively consistent with our numerical simulation for magnetic forces. The changes in resonance frequencies with magnetic fields reach a few percent, showing that an efficient magnetic control of resonance frequencies was achieved.

La-NMR study on a La dilute system of PrPb3

We report the first 139 La-NMR measurements on a La dilute system of PrPb3. We have succeeded to detect 139La-NMR signal on a powder sample of Pr0.97La0.03Pb3 and extracted the temperature dependence of the Knight shift139 K(T) in a temperature range between 1.5 and 220 K. K(T) has been found to maintain a linear relation with the bulk-susceptibility χ(T)bulk in wide temperature range, except for a small deviation in the temperature region below 6 K. The slopes of the139 K vs ν bulk plots yield for the hyperfine coupling constants Ahf the value 375 Oe/μB. From the NMR results, we discuss the effect of the La substitutions on microscopic magnetic properties of PrPb3.