Atsuko Fukaya

Soft X-ray absorption spectra of ilmenite family

We have carried out soft x-ray absorption spectroscopy to study the electronic structure of ilmenite family, such as MnTiO3, FeTiO3, and CoTiO3 at the soft x-ray beamline, BL23SU, at the SPring-8. The Ti and M L2,3 absorption spectra of MTiO3 (M=Mn, Fe, and Co) show spectra of Ti4+ and M2+ electron configurations, respectively. Except the Fe L2,3 spectrum, those spectra were understood within the O(h) symmetry around the transition metal ions. The Fe L3-edge spectrum clearly shows a doublet peak at the L3 edge, which is attributed to Fe2+ state, moreover the very high-resolution the L-edge spectra of transition metals show fine structures. The spectra of those ilmenites are compared.

Anomalous magnetic field dependence of muon spin relaxation in double-chain system KCuCl3 and TlCuCl3

Abstract In μ+SR studies on the S= 1 2 double-chain systems KCuCl3 and TlCuCl3, which are thought to have a spin singlet ground state with finite spin gaps, we found that the muon spin becomes strongly depolarized below 20 and 10 K, respectively. We observed an increase in the relaxation rate with increasing longitudinal magnetic fields at 4.8 K in KCuCl3. At 1.8 K, we observed no decrease of the relaxation rate with increasing longitudinal fields. However, the shapes of these spectra cannot be represented by using usual formulae. Such an anomalous behavior was not seen in TlCuCl3. We discuss the origin of the muon spin relaxation and difference between the properties of KCuCl3 and TlCuCl3.

Concentration—temperature phase diagram of the diluted antiferromagnet MnXMg1-XTiO3

Abstract The temperature variation of low-field dc magnetization of the diluted antiferromagnet Mn X Mg 1- X TiO 3 has been measured. It has been found that the crossover phenomenon from random-field to spin-glass behavior occurs near the percolation concentration X P = 0.57, which is experimentally estimated. The origin of the appearance of the spin-glass-like behavior is discussed in relation to spin structures near the percolation concentration.

Effect of Random Fields on the Phase Diagram of the Diluted Antiferromagnet Mn0.80Mg0.20TiO3 in the Field-Temperature Plane

The diluted antiferromagnet Mn 0.80 Mg 0.20 TiO 3 has been studied by measuring magnetization processes and temperature variations of magnetization, and the magnetic phase diagram in the field-temperature ( H - T ) plane has been determined. It is demonstrated that the existence of random fields strongly affects the phase transitions. The most noteworthy result is that the H - T phase diagram consists of four phases instead of the generally known three phases: a paramagnetic, an antiferromagnetic and a spin-flop phase. We propose that the fourth phase corresponds to the region in the H - T phase diagram of Mn 0.75 Zn 0.25 F 2 where the behavior of the spin system is suggested to be anomalous by Cowley et al . (Z. Phys. B 75 (1989) 303).

Electric Properties of Mixed Compound [(CH3)4N][NixPd1-x(dmit)2]2

Single crystals of the mixed system [(CH 3 ) 4 N][Ni x Pd 1- x (dmit) 2 ] 2 with the space group C 2/ c have been prepared, and their crystal structure and electric properties have been studied. Crystals with x ≤0.31 and x ≥0.91 were found. In the Pd-rich region ( x ≤0.31), interestingly, the resistivity at low temperatures decreases with doping. On the other hand, in the Ni-rich region ( x ≥0.91), the resistivity increases with doping, and the crossover temperature from the metallic state to the semiconducting state increases with doping.

Radio-frequency muon spin resonance of Haldane-gap systems, Ni(C2H8N2)2NO2(ClO4) and NiC2O42·(2-methylimidazole)

In quantum spin systems with a singlet ground state, an enhancement of the muon spin relaxation was frequently observed at low temperatures, though the ground state is non-magnetic. In order to clarify the origin of this enhancement, we have measured the muon spin resonance with radio-frequency field (RF-μSR) for Haldane-gap systems, Ni(C2H8N2)2NO2(ClO4) (NENP) and NiC2O42·(2-methylimidazole). Possible origins of an enhancement of the muon spin relaxation at low temperatures are discussed.

study of a spin-ladder system -diazacycloheptane

Abstract We measured the muon spin relaxation ( μSR ) for Zn doped spin-ladder compounds ( Cu 1−x Zn x ) 2 (1,4 -diazacycloheptane ) 2 Cl 4 with x=0 and 0.1 in the temperature region from 2 to 150 K . In both of x=0 and 0.1 samples, only a slight enhancement of the relaxation is observed at low temperatures. This result indicates that both samples exhibit no magnetic order. Besides, we have not observed significant difference of the μSR spectra in both samples. This fact indicates that the Cu 2+ spins induced by the impurity fluctuate fast.

Magnetic phase diagram of the diluted antiferromagnet Mn0.81Mg0.19TiO3 in the field–temperature plane

Abstract In the H – T phase diagram of the diluted antiferromagnet Mn x Mg 1− x TiO 3 with x >0.6, which shows the random field effects under fields parallel to the spin easy axis, a “new phase” was found in a narrow region surrounded by the antiferromagnetic (AF) phase, spin-flop (SF) phase and paramagnetic (P) phase. To examine the microscopic properties and magnetic structure, we performed neutron scattering measurements for Mn 0.81 Mg 0.19 TiO 3 under fields. We obtained the results suggesting that only short-range correlation among the spins exists in the “new phase”.

μSR study of a mixed compound NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x

We examined the muon spin relaxation (μSR) of mixed compounds NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x with x=1.0 and 0.49. Although the macroscopic magnetic properties are obviously different from each other, both systems exhibit similar behavior in the muon spin relaxation. In addition, in the x=0.49 (SG) sample, a critical slowing down of spin dynamics was not observed in this μSR measurement, though the spin-glass like freezing was observed in the susceptibility measurements. Qualitative explanation of these anomalous observations is given.

Spin dynamics of diluted antiferromagnets MnxMg1-xTiO3 by μSR method

The diluted antiferromagnet MnxMg1-xTiO3 transforms from the antiferromagnet (AF) to the spin-glass (SG) system at x ∼ 0.6. In order to study the microscopic nature of spin dynamics and ordering, we performed muon spin relaxation measurements for the powder samples of x=1.0∼ 0.55 and examined the concentration dependence. From these results, a drastic change in the spin dynamics was found to occur at x ∼ 0.6.