Kunihide Okada

Difference between the giant magnetoresistance of Fe/Cu and Co/Cu magnetic multilayers under high pressure

Abstract The giant magnetoresistance (GMR) of T/Cu (T  Fe, Co) magnetic multilayers (MML) has been investigated under high pressure at 4.2, 77 K and room temperature. It is found that the magnitudes of GMR at 77 K and at room temperature for both MMLs are almost independent of pressure up to 0.8 GPa, but 9.2% decrease in GMR is observed at 4.2 K under the pressure of 2 GPa. On the basis of these results, the difference between GMR for Fe/Cu and Co/Cu MMLs under high pressure is discussed briefly.

Br NMR in triangular-lattice XY antiferromagnetic CsMnBr3 in a high external field

Abstract The Br NMR in a triangular-lattice antiferromagnetic CsMnBr 3 has been studied by Monte Carlo simulation and by an NMR experiment at 4.2 K in an external field. The result of Monte Carlo simulation shows the spin reorientation is accompanied by a second-order phase transition in relatively small external field. Using the simulation results and the NMR parameters that have been determined in zero external field, we calculated external field dependence of resonance intensity. The result showing 36 resonance peaks agrees with experimental data.

Effect of pressure on the giant magnetoresistance of Fe/Cu magnetic multilayer

Abstract Giant magnetoresistance (GMR) of a [Fe 10.1 A/Cu 13.7 A]15 magnetic multilayer (Fe/Cu MML) has been measured at room temperature, 77 and 4.2 K under high pressure. It is found that the magnitude of GMR of Fe/Cu MML is almost independent of pressure at room temperature and 77 K, while it decreases with increasing pressure at 4.2 K. The result of Fe/Cu MML is compared with that of Co/Cu MML.

The disordered magnetic structure in the triangular-lattice XY-antiferromagnet derived from the zero-field spectrum of Br nuclear magnetic resonance

The Br NMR in the triangular-lattice antiferromagnetic is investigated in the ordered state at low temperatures. Two distinct peaks of the Br NMR spin-echo spectrum were observed at MHz and MHz accompanied by a broad signal ranging from 34 to 55 MHz in zero field. The peak frequencies are independent of temperature and the ratio of the frequencies is the same as that of the quadrupole moments of and . This suggests that the quadrupole resonance of Br is perturbed by the distribution of the internal magnetic field. Theoretical results obtained under the assumption that the Mn spins have disordered structure in the c-plane due to the frustration effect agree with the experimental data.

A phase transition in at 2.7 K

Specific heat and magnetic susceptibility of the triangular-lattice XY antiferromagnetic have been measured. Results of our experiments show that a phase transition occurs at around 2.7 K in addition to one at 8.31 K. Findings of neutron diffraction experiments suggest that Mn spins still lie on the basal plane with angles between the neighbouring spins.

Anomaly of triangular-lattice antiferromagnetic CsMnBr3 at 2.7K

Temperature dependence of specific heat (B=0, 0.35K<T<20K), Br-NMR (B=0, 1.7K<T<4.2K) and magnetization (B<5T, 1.8K<T<20K) of CsMnBr3 have been measured at low temperatures. The data of specific heat in zero field shows that there are small bumps at 2.7K and 1.6K in addition to the peak at the Neel temperature 8.3K. The spin-echo spectrum of81Br-NMR at 1.7K is different from that at 4.2K and the intensity has minimum at 2.7K. The magnetization under the external field applied in perpendicular direction to the spin easy axes of Mn2+ ions shows the small shifts at 2.7K simillar to that at 8.3K. All of these experimental results can be concluded that the successive transitions occures at 2.7K and 8.3K up to 1.7K. The lower critical temperature is independent of the field applied along the crystallographic c-axis.

NMR study of formate dihydrate at millikelvin temperatures

Abstract We have observed 59 Co spin echo signals in Co(HCOO)2·2H2O by pulsed NMR without external magnetic field at 90 and 75 mK using a dilution refrigerator. Co(HCOO)2·2H2O has a magnetic sandwich structure in which the Co2+ spins on one plane are antiferromagnetically ordered while the Co2+ spins on another plane remain paramagnetic below TN (5.1 K ) . There are two peaks in NMR spectra below 90 mK . It is concluded that the higher-frequency peak corresponds to the signal from antiferromagnetically ordered planes and the lower-frequency peak from paramagnetic planes.

Partially disordered states of the three-dimensional ANNNI model

Abstract We analyze the three-dimensional axial next-nearest-neighbor Ising model composed of two kinds of alternately stacked ferromagnetic layers with different intralayer interactions by using Monte Carlo (MC) simulation. The detailed analyses of the magnetization and the spin correlations perpendicular to the stacked direction, assure and clarify the existence and the nature of the paramagnetic layers in some modulated phases with wave numbers, 1 4 , 1 8 , 3 16 . This is the first MC confirmation of the partially disordered states which have been predicted by previous molecular field calculation and/or observed in materials such as CsCoCl3, CuFeO2 and CeSb.

NMR study of two-dimensional ferromagnet K2CuF4 below 1 K

Abstract Two-dimensional ferromagnet K2CuF4 is an ideal compound to study two-dimensional magnetic systems because the interplane exchange coupling constant is much smaller than the intraplane exchange coupling constant. We measured the temperature dependence of the NMR frequencies of the 1 2 ↔ 3 2 transition for 63 Cu and 65 Cu nuclei in K2CuF4 single crystal far below T c =6.25 K . The NMR measurements were made by spin-echo method at zero magnetic field. The experimental results are discussed in the framework of the two-dimensional spin wave theory at millikelvin temperatures.

Study of triangular-lattice XY antiferromagnetic CsMnBr3 at low temperatures

Br-NMR in the triangular-lattice antiferromagnetic CsMnBr3 has been investigated in the ordered state at low temperatures. We have observed two distinct peaks at 36.0±0.5 MHz and 43.0±0.5 MHz accompanied by a broad signal ranging from 34 to 55 MHz in zero field at 4.2K. We suggest that the Mn spins have a disordered structure in the c-plane due to the frustration effect. The calculated results on the suggested model are in good agreement with the experimental data. We assume that, when the temperature tends to zero Kelvin, the frustration vanishes, and, as a result, the material takes a uniform ground state. The result of an experiment (81Br-NMR below 1 K) supports our assumption