Tatsuichi Hamasaki

NMR Study of Spin State in Geometrically Frustrated Compound Co2Cl(OH)3

NMR studies of a geometrically frustrated system, Co 2 Cl(OH) 3 , are carried out at low temperature. The NMR results of 59 Co and 1 H nuclei show that long-range ordered ferromagnetic moments on the triangular lattice plane coexist microscopically with disordered moments on the kagome lattice plane. The moment direction is not ordered. Although the disordered moments freeze within the NMR observation time (10 -4 s) at low temperature, the frozen moments begin to fluctuate with increasing temperature. Up to 4.2 K (0.4 T C ), the magnitude of the ferromagnetically ordered moments is independent of temperature.

Proton NMR in a random mixture with competing anisotropies: CsMn1-xCoxCl3·2H2O

Abstract The NMR intensities and relaxation rates of proton nuclei suggest that the phase transition induced by an external field from the antiferromagnetic to intermediate phase is confined to a very small part of the total spins and that the other spins are hardly affected.

NMR Study of Spin State of Mn2+ Impurity in Antiferromagnet CoCl2·2H2O

The spin state of the isolated Mn 2+ impurity in the antiferromagnetic phase of CoCl 2 ·2H 2 O is studied by proton NMR. The observed two satellite lines at 1.7 K are revealed to be due to the nearest neighboring protons to Mn 2+ impurity having the moment of 5 µ B and 3µ B . On the time scale of proton NMR, Mn 2+ moments are observed to take not the thermal averaged value but each S z value because of very long relaxation times of Mn 2+ spin.

NMR Study of Covalency in Ferromagnet Rb2CrCl4

NMR measurements of Cl nuclei were carried out to clarify the covalency between Cr 2+ and Cl - ions. By analyzing the NMR spectra, the unpaired spin densities of s-and p-orbitals of Cl - ion are estimated to be f s =0.21% and f σ - f π =-1.1%. Contrary to what was anticipated, the π-bonding is stronger than the σ-bonding.

Proton NMR Study of Atacamite Cu2Cl(OH)3

The proton NMR spectrum of Cu2Cl(OH)3 at low temperature in zero applied field has two sharp peaks. This result means that the majority of magnetic moments order antiferromagnetically at low temperature. The magnetic structure determined by comparing the observed proton NMR spectrum with the numerical calculated ones in several magnetic models shows that the magnetic moments order “all-in all-out” orienting to body center. This magnetic structure is similar to antiferromagnetic pyrochlore with geometric frustration, and different from the one of atacamite Mn2Cl(OH)3, though having the same crystal structure. The magnitude of magnetic moments of Cu(I) is 1.12 \(\mu_{\text{B}}\) and Cu(II) 0.25 \(\mu_{\text{B}}\). In increasing temperature, the number of moments deviated from “all-in all-out” directions becomes large. However, the clear evidence of the disordering or spin-glass moments is not detected from observed NMR spectrum.

Proton NMR Study of Pyrochlore-Like Atacamite Mn2Cl(OH)3

The proton NMR spectrum of Mn 2 Cl(OH) 3 with zero applied field has rather sharp peaks, and a glassy property is not observed. The temperature dependence of NMR frequency shows that antiferromagnetic transition occurs at T N1 (=3.4 K), and that no anomaly appears at T N2 (=2.7 K). However, the change in frequency is considerably larger than the normal change caused by thermal fluctuation. The magnetic structure of Mn 2 Cl(OH) 3 determined by comparison with the observed proton NMR spectrum and the numerical calculated ones of various magnetic models shows that magnetic moments do not order in an “all-in all-out” orientation relative to the body center like antiferromagnetic pyrochlore, and not parallel to the local crystal field. The coexistence of antiferromagnetic and spin-glass moments does not reproduce the observed NMR spectrum. Our result suggests that the magnetic structure of atacamite Mn 2 Cl(OH) 3 is antiferromagnetic, but different from that of pyrochlore, although it is similar to the crystal...

Specific heat measurements of pyrochlore-type R2Mo2O7 (R=Nd–Ho)

Abstract Specific heat measurements have been performed on pyrochlore-type R2Mo2O7 (R=Nd–Ho) single crystals. For the ferromagnetic compounds with R=Nd–Gd (TC≈50– 90 K ) , Schottky-like specific heat anomalies corresponding to the level splitting for the 4f electrons of R3+ are observed at low temperatures, in contrast to the specimens with R=Tb–Ho which show spin-glass-like order below Tg≈20– 25 K . Low-temperature specific heat data of Sm2Mo2O7 under a magnetic field of 10 T are presented.

Chlorine NMR in the Re-Entrant Spin-Glass State of the Quasi-Two-Dimensional XY System Rb2Mn1-xCrxCl4

Cl 35 NMR measurements are carried out at 1.7 K on a random mixture of quasi-two-dimensional ferro- and antiferromagnets Rb 2 Mn 1- x Cr x Cl 4 ( x =1.0, 0.9, 0.8). NMR spectra of x =1.0 and 0.9 in zero applied field show that these compounds are ferromagnetic. In the re-entrant spin-glass state of x =0.8 a satellite line is observed together with the main lines. It is pointed out from the analysis of the results that, in the re-entrant spin-glass state, the ferromagnetic moments coexist with the moments whose directions homogeneously distribute in the c -plane, such as vortices.

Ga NMR study of GdGa2-xAlx

69Ga and 71Ga NMR signals in GdGa2-xAlx (x = 0.1, 0.25, 0.5, 0.75,1) have been observed at a low temperature in a zero magnetic field. Gd NMR signals were observed only for the samples with x ≤ 0.5 in the frequency range less than 110MHz, but Ga NMR signals have been observed for all samples in the frequency range higher than Gd NMR frequencies. The features of the observed Ga NMR spectra of the samples with low Al concentration are quite different from those of highly Al concentrated samples. This difference is attributed to the change of the magnetic structure of these compounds. NMR parameters calculated from the Ga NMR results are reported, too. The magnetic structures of these compounds are discussed on the basis of the Ga NMR results.

Magnetic phase diagram of GdGa1.75Al0.25

Abstract DC and AC magnetization of GdGa1.75Al0.25 have been measured in the magnetic field up to 5 T on polycrystalline material. A specific heat measurement has shown an antiferromagnetic transition at T N =24.0 K and a second transition at 22.9 K , immediately below TN. A third transition has been observed at 7.7 K , which may be due to the change of the easy axis. Every temperature below TN we have observed metamagnetic transitions and obtained complex H–T phase diagram. The magnetic structure of GdGa1.75Al0.25 is similar to that of GdGa2.