H. Aruga

Reentrant Spin-Glass Transition and a Mixed Phase in an Ising System FexMn1-xTiO3

Neutron scattering demonstrates the clear reentrant spin-glass (SG) transition, for the first time, in the Ising system Fe x Mn 1- x TiO 3 for x =0.33, 0.38, 0.60, and 0.65. The samples with x =0.41, 0.50 and 0.55 exhibit paramagnetic to SG behavior upon lowering the temperature. The results confirm the SG and reentrant SG transitions which were suggested from the previous bulk susceptibility measurements by Ito et al . The reentrant SG phase is shown to be actually a mixed phase where the antiferromagnetic long range order coexists with the SG ordering. The behavior of the diffuse intensity as well as the inverse correlation length κ as a function of an Fe ion concentration x strongly suggests that there exists a phase boundary between the mixed phase and the pure SG phase. Thus the phase below the AT line in an Ising SG system should be strictly distinguished from the pure SG phase. By comparing the data of the x =0.58 sample to that of the x =0.57 sample, this phase boundary is parallel to the temperat...

Phase diagram and reentrant spin-glass behavior in a random mixture of two Ising-antiferromagnets FexMn1−xTiO3

Abstract Low-field DC susceptibility measurements have been made on a random-mixture of two Ising-antiferromagnets Fe x Mn 1−x TiO 3 . A spin-glass state appears in a wide range of intermediate concentration. In samples with certain x in the Fe-rich side or the Mn-rich side, the reentrant transition from the antiferromagnetic state or the random-antiferromagnetic state is found to occur. The reentrant behavior appears drastically, which we attribute to the strong frustrations existing in a magnetically non-diluted short-range system. The concentration versus temperature phase diagram is also shown.

Frequency-Dependent Phenomenon in a Short-Range Ising Spin-Glass, Fe0.5Mn0.5TiO3

The frequency dependence of paramagnetic-to-spin-glass transition temperature T g ( f ) for a short-range Ising spin-glass, Fe 0.5 Mn 0.5 TiO 3 , over a wide frequency range extending to ∼10 11 Hz has been examined by the ac-susceptibility measurement (2.0×10 Hz≦f≦6.65×10 4 Hz), the Mossbauer measurement (corresponding to f ∼6×10 6 Hz) and the neutron scattering measurement (corresponding to f ∼10 11 Hz). The frequency-dependent phenomenon is analyzed using the Fulcher law, the power law for transition at zero temperature and the power law for a finite transition temperature. All laws appear to account for the observed behavior equally well. By examining the values of the parameters obtained front the analysis of experimental data closely, we show that the power law for a finite transition temperature, f = f 0 [( T g ( f )- T * )/ T g ( f )] z v with f 0 =2.4×10 13 Hz, z v =11.5 and T * =20.5 K, is more appropriate than the other laws.

Behavior of the Transverse Spin Component in the Short-Range Ising Spin-Glass Fe0.5Mn0.5TiO3

The spin-glass Fe 0.5 Mn 0.5 TiO 3 ( T SG =21.5 K) has been studied by low-field dc-magnetization, neutron scattering and Mossbauer measurements. The results obtained by the magnetization and neutron scattering measurements indicate that only the longitudinal spin component freezes below T SG . This fact shows that Fe 0.5 Mn 0.5 TiO 3 behaves as a typical Ising spin-glass in both of the measurements. However, the hyperfine fields H hf at Fe nuclei observed by the Mossbauer measurement point to directions other than the hexagonal c -axis in the spin-glass state. This indicates that the transverse spin component contributes to H hf as does the longitudinal one. Combining the results obtained by the three methods, it is concluded that the transverse spin components fluctuate rather incoherently with one another with characteristic times longer than our Mossbauer time scale of ∼10 -7 s.

Field-Dependent Phenomena in Reentrant-Spin-Glass: FexMn1-xTiO3 with x=0.60, 0.65 and 0.75

Field-dependent phenomena in the exchange competing Ising system undergoing the reentrant transition from the antiferromagnetic state, Fe x Mn 1- x TiO 3 with x =0.60, 0.65 and 0.75, have been investigated by DC-magnetization measurements. It is observed that the temperature T M ( H ) giving the peak of magnetization decreases with the increase of magnetic field and it becomes ill-defined at high fields. On the other hand, the reentrant-spin-glass transition temperature T RSG ( H ) increases with magnetic field, and it begins to decrease after being coincident with T M ( H ). These kinds of field dependences of T M ( H ) and T RSG ( H ) are qualitatively consistent with the prediction given by the theoretical calculation using the mean-field approximation, and these are characteristic of systems undergoing the reentrant transition from the antiferromagnetic state.

Studies on spin-glass freezing and antiferromagnetic long-range order in an Ising spin-glass system Fe x Mn1−x TiO3

A Mössbauer time window has been positively used for studying the behavior of spins in FexMn1−xTiO3 withx=0.50, 0.60 and 0.65 which are of typical Ising spin-glass systems. Spin dynamics aboveTSG andTN (RSG) (definition is given in the text) are discussed. In the RSG samples, the existence of contributions of the transverse spin component to the hyperfine fields is pointed out.

Effect of magnetic fields on reentrant-spin-glass FexMn1−xTiO3 (x = 0.60, 0.65 and 0.75)

Abstract The effect of magnetic fields H on the reentrant transition temperature T RSG of Fe x Mn 1− x TiO 3 , with x = 0.60, 0.6 5 and 0.75, has been investigated. It is shown that T RSG changes with H in a rather complicated manner. However, in a high field region, T RSG can be described by a power function of H .

Mixed Phase in the Reentrant Ising System Fe x Mn 1-x TiO 3

Despite a number of experimental reports on the magnetic ion concentration x versus temperature phase diagram on spin glass systems,1) little attention has been paid to the difference between the pure spin glass phase and the reentrant spin glass phase. This is partly because the spin glass phase appears in a very narrow region on the concentration axis, and partly because almost all reentrant spin glass materials belong to the class of the ferromagnetic interaction dominated system, in other words, they reenter to the spin glass phase after establishing the ferromagnetic long range order. The former reason means that the concentration gradient in the sample is crucial in order to perform a reliable study of spin glass behavior as a function of the concentration x. The latter causes the technical difficulty in neutron scattering experiments.2) Since a ferromagnetic Bragg reflection always overlaps with a structual Bragg reflection, it is difficult to obtain definite information on the magnetic long range order. If the system exhibits the antiferromagnetic order, such a difficulty is obviously resolved. Recently A. Ito and coworkers discovered an antiferromagnetism-based spin glass system Fe0.5Mn0.5TiO3, and carried out intensive studies on relaxational phenomena.3)