Meiro Chiba

Electron Spin Echo Decay Behaviours of Phosphorus Doped Silicon

Electron spin echo decay behaviours of the shallow trapped donor electrons in the phosphorus doped silicon at liquid helium temperatures have been studied. The spin echo signals were observed for the samples whose donor concentrations were less than 2.0×10 17 cm -3 . From the experimental date the region of the concentration has been divided into three. In each region has been observed the characteristic spin echo decay behaviour, which has been analyzed by using the stochastic model.

Magnetic properties of the frustrated diamond chain compound Cu3(CO3)2(OH)2

Abstract Cu3(CO3)2(OH)2 (azurite), in which Cu 2+ (S= 1 2 ) monomers and dimers are arranged alternately along the b-axis to make an infinite chain, is regarded as an actual material for the frustrated diamond chain model. we have measured magnetic susceptibility, high field magnetization and 1 H NMR of a natural mineral single crystal of azurite to investigate the magnetic properties. The experimental results were compared with the theoretical expectation to suggest the ground state of Cu3(CO3)2(OH)2 is in a gapless phase.

Magnetic properties of the diamond chain compound Cu3(CO3)2(OH)2

Azurite (Cu 3 (CO 3 ) 2 (OH) 2 ) is a model substance of a diamond spin chain, one of frustrated antiferromagnetic quantum spin chains. The high field magnetization up to 60 T, magnetic susceptibility, magnetic entropy, muon spin relaxation and 1 H-NMR of azurite have been measured using single crystals. A distinct 1/3 magnetization plateau is confirmed to be present in the magnetization curve. Two resonance peaks of 1 H-NMR. were observed. By analyzing temperature dependencies of their hyperfine shift. we found two different local susceptibilities which correspond to susceptibility of dimer and monomer spins, respectively. New field induced phase transition was found to occur at a critical field where the magnetic plateau onsets. A short range magnetic ordering developed in a peculiar two-stage process, which was revealed by temperature dependence of the magnetic specific heat and susceptibility.

High Field ESR Measurements Using Gyrotron FU Series as Radiation Sources

Submillimeter wave ESR spectrometer using Gyrotron FU series as electromagnetic wave radiation sources and the pulse magnet for high field up to 40 T has been developed. The ESR spectrometer has been successfully applied to several ESR measurements. ESR measurements were performed under the pulsed magnetic fields up to 40 T at the temperature of 4.2 K in CsFeCl 3 single crystal. The frequency-field diagram of ESR absorptions was obtained.

Nuclear Spin-Lattice Relaxation Mechanism of Singlet-Ground-State Magnet with Exchange Coupling: Application to CsFeCl3

A model is proposed on the exchange coupled singlet-ground-state magnet around the level crossing field. The model starts from the energy level of the isolated single spin in paramagnetic spin system. The effect of the exchange coupling is taken into account by modifying the single ion level scheme to the broadened continuum band. On the basis of this model is discussed a mechanism of the nuclear spin-lattice relaxation inherent in the level crossing, due to the direct process between the nuclear spin and the exchange coupled electron spin system. A drastic increase of the nuclear spin-lattice relaxation rate of 133 Cs in CsFeCl 3 observed at 4.2 K around the level crossing field at 7.5 T is well explained by this model. The magnetization as a function of external magnetic field is also discussed and compared with the result of CsFeCl 3 up to 12 T.

Chaotic Dynamics During Slow Relaxation Process in Magnon Systems

Experimental evidence of chaotic dynamics during a slow relaxation process with very low frequency magnetization auto-oscillations in magnon systems is presented. A ferromagnetic resonance experiment using a sphere of pure yttrium iron garnet (YIG) at the X-band pumping frequency within the coincidence regime reveals a slow relaxation process with a long tail after a stepwise drop in external pumping power. The relaxation process is found to follow a power-law with time as a transient process involving a transition of local dynamics from a quasi-periodic state to a chaotic state as the collapse of torus.

Successive Phase Transitions in CsNiBr 3 Observed by 133 Cs NMR Including Critical Examination of the Local Field

The ordering process in the Ising-like Heisenberg antiferromagnet on the hexagonal lattice has been studied through the 133 Cs NMR in CsNiBr 3 . The observed local field at 133 Cs was 3.5 times as large as the calculated field from the point dipoles at the Ni 2+ sites. The limited applicability of the conventional point dipole model has been overcome by treating the symmetry property at the 133 Cs site. Under the applied field of 4.6 T, we have found that the Ni 2+ spin component parallel to the c -axis orders below T N1 =15.0 K, whereas the perpendicular component orders below T N2 =12.5 K to form the triangular three-sublattices in the c-plane: one spin is parallel to the c -axis and the other two are canted ± 38° from the c -axis. The comparison is made with the existing data of CsNiCl 3 with an emphasis on the dynamical behavior of the triangular spin arrangement.

Anomalous Spectrum of 133Cs NMR in CsCuCl3 around the Applied Field 11 T

133 Cs NMR experiment has been performed on CsCuCl 3 at 4.2 K under the applied magnetic field parallel to the crystal c -axis. An anomalous spectrum has been observed at the critical field of 11.19 T, where the magnetization jump was detected previously. At the critical field, the spectrum is abruptly cut off with no hysteresis for the sweep direction of the magnetic field. The critical field is independent of the operating frequency of the NMR experiment. The result can be explained by the theoretical model of the field-induced transition due to the quantum spin fluctuation in the one-dimensional ferromagnet with S =1/2.

Nuclear Spin-Lattice Relaxation Due to Domain Wall Solitons in CsCoCl3

Nuclear spin-lattice relaxation of 133 Cs has been studied in the frustrated antiferromagnet, CsCoCl 3 . A pronounced anomaly in the relaxation rate was observed at the lower transition temperature, T N2 =10 K, whereas a weak change in the relaxation rate was observed at the upper transition temperature, T N1 =21 K. The results are discussed in terms of the characteristic dynamics of domain wall solitons which propagate in disordered magnetic chains which feel no molecular field from the neighboring chains.

ESR study on metamagnetic transition in CsFeCl3 up to 40 T

Abstract In the S =1 fictitious spin system CsFeCl 3 an anomalous magnetization has been observed under the magnetic field around 33 T , being much higher than the saturation field. It cannot be explained in terms of the fictitious spin S =1. In order to clarify the nature of the metamagnetic transition, an ESR experiment was performed under magnetic fields up to 40 T . The mechanism of the metamagnetic transition is discussed under the basis of the ESR frequency-field diagram.