M. Ohta

Spin-glass behaviour, thermal expansion anomaly and spin fluctuations in Y20(Mn1-xFex)80 amorphous alloys

The spin-glass behaviour and the thermal expansion anomaly in annealed Y20(Mn1-xFex)80 amorphous alloys have been investigated. The spin freezing temperature Tg shows a minimum at around x = 0.50, consistent with the theoretical investigation. The sign of the paramagnetic Curie temperature p indicates that the average exchange interaction is antiferromagnetic in the Mn-rich concentration region and gradually becomes ferromagnetic with increasing x. The minimum of Tg occurs at the concentration where the sign of p changes. The spontaneous volume magnetostriction is negligibly small in the Mn-rich concentration region and becomes larger with increasing x. These results are explained by the change in the spin fluctuation characteristics. In the Fe-rich concentration region, the thermal expansion anomaly takes place even in the paramagnetic temperature range due to the single-site spin fluctuation.

Disappearance of perpendicular magnetic anisotropy in amorphous Tb–Fe alloys

Abstract After annealing, the disappearance of the perpendicular magnetic anisotropy in the amorphous Tb–Fe alloys without the Cu substrate was confirmed. By inducing thermal strains, the anisotropy was revealed. Therefore, it is concluded that strong strains induced due to the difference between the thermal expansion of the specimen and that of the substrate bring about the perpendicular magnetic anisotropy.

Kadowaki–Woods plot of exchange-enhanced Pauli paramagnetic Laves phase quasi-binary compounds Lu(Co1−xMx)2

The electrical resistivity and low-temperature specific heat of exchange-enhanced Pauli paramagnetic Laves phase quasi-binary compounds Lu(Co1−xMx)2 (M = Al, Ga and Si) have been investigated in order to discuss the spin fluctuation characteristics and the Kadowaki–Woods plot. In these three kinds of quasi-binary system, both the electrical resistivity coefficient A and the specific heat coefficient γtot at low temperatures decrease at first and then significantly increase with increasing x. The concentration dependence of magnetic susceptibility at 0 K, χ(0), is also similar to that of A and γtot. The Wilson ratio becomes very large because of the enhancement of magnetic susceptibility. The Kadowaki–Woods plot is fitted to the same universal line of heavy-fermion compounds, A15-type Nb3Sn and V3Si compounds because of significant spin fluctuations in the present quasi-binary compound systems.

Thermal expansion anomaly and pressure effect on the Curie temperature of Lu6(Mn1?xFex)23 compounds

The thermal expansion and specific heat measurements under ambient pressure and the thermomagnetization measurement under applied pressure for new Lu6(Mn1−xFex)23 compounds have been carried out in order to discuss the relationship between the thermal expansion anomaly and the spin fluctuations. The thermal expansion coefficient α significantly increases above the Curie temperature TC. The electronic specific heat coefficient increases with increasing x, accompanied by the decrease of the spin fluctuation spectral width. The value of the pressure dependence of TC, ∂TC/∂P, is large at about −50 K GPa−1. The anti-invar effect and the pressure effect of TC are closely related to a significant thermal variation of the amplitude of spin fluctuations.

Anti-invar behaviour due to spin fluctuations in Y6(Mn1-xFex)23 compounds

The thermal expansion and low-temperature specific heat measurements for Y6(Mn1-xFex)23 compounds have been carried out in order to discuss the relationship between the thermal expansion anomaly and the spin fluctuations. The thermal expansion coefficient α significantly increases above the Curie temperature TC. This behaviour, the so-called anti-invar characteristic, is attributed to a remarkable increase of the amplitude of spin fluctuations in paramagnetic temperature regions. The anti-invar characteristic becomes weaker with increasing x. The temperature dependence of the thermal expansion coefficient of the magnetic term αmag is convex upward in the paramagnetic temperature range, which is explained within the framework of spin fluctuations.

Spin fluctuations in Y6(Mn1−xMx)23(M=Al,Fe) systems

Abstract The formation of Y 6 (Mn 1− x Al x ) 23 compounds has been confirmed below x =0.05, and a significant increase of the thermal expansion coefficient α above the Curie temperature, i.e. anti-invar behavior is observed. The difference between the anti-invar behavior caused by spin fluctuations in Y 6 (Mn 1− x Al x ) 23 and Y 6 (Mn 1− x Fe x ) 23 is attributed to 3d itinerant-electron perturbing the band structure.

Spin–glass transition in annealed Y6(Mn1−xFex)23 compounds

Abstract AC and dc magnetic susceptibility measurements have been carried out after annealing under an appropriate condition for Y 6 (Mn 1− x Fe x ) 23 compounds with x =0.50, 0.55 and 0.60. At low temperatures, the onset of a spin–glass state has been confirmed in contrast with the existing data. It is concluded that the blocking of antiferromagnetic clusters claimed in previous studies is attributed to compositional inhomogeneity.

Magnetic phase diagrams of NbxFe100−x and TaxFe100−x amorphous alloy systems

Abstract The magnetic phase diagrams of Nb x Fe 100− x and Ta x Fe 100− x amorphous alloy systems have been investigated. The phase transition from the paramagnetic state to the spin-glass state was observed above about x =34% for Nb x Fe 100− x and x =36% for Ta x Fe 100− x amorphous alloys. The re-entrant spin-glass behavior was confirmed below these concentrations. The saturation magnetic moment M s of both amorphous alloy systems decreases with increasing x . The value of M s at the concentration where the spin-glass state takes place is small of about 0.4 μ B per Fe atom due to the short average Fe–Fe interatomic distance, compared with that of other Fe-based amorphous alloy systems such as La–Fe, Ce–Fe and Lu–Fe. The magnetic phase diagrams of the Nb–Fe and Ta–Fe amorphous alloy systems are similar to those of other Fe-based amorphous alloy systems, although the Curie temperature and the magnetic moment of the former two systems are much lower than those of the latter alloys systems.