Teruo Bitoh

Metal-insulator transition in thiospinel CuIr2S4

The metal-insulator (M - I) transition in the thiospinel CuIr2S4 and the investigations related to this M - I transition have been extensively made in the last decade. This paper will present a brief survey of the M - I transition in CuIr2S4 and the other compounds with some chemical modifications of CuIr2S4. The spinel CuIr2S4 goes through a temperature-induced M - I transition at TM-I = 230 K with the structural transformation, at which a simultaneous bond-dimerization with spin-singlet state and charge-ordering transition takes place. The magnetic susceptibility exhibits a step-like anomaly at TM-I. Most noticeable characteristic is the absence of localized magnetic moment below TM-I. The monovalent state of Cu^+ ion at A-site has been verified, then the ionic state Cu^+Ir^+3Ir^+4S^2− 4 is realized in T ＜ TM-I, where Ir ions at B-site have mixed valence states of Ir+3 and Ir+4, where Ir^+3 is non-magnetic state with t2g^6 occupation. Recent exploration has revealed extensive evidence for a bond-dimerization. This bond-dimerization of Ir^+4 – Ir^+4 having S = 0 could be induced due to the Peierls instability mediated through the lattice. These results indicate that CuIr2S4 is a rare and unique example displaying a spin-dimerization transition although a three-dimensional compound.

Origin of low coercivity of (Fe0.75B0.15Si0.10)100-xNbx (x=1-4) glassy alloys

The density and the magnetization process of the melt-spun (Fe0.75B0.15Si0.10)100−xNbx (x=1–4) glassy alloys have been investigated to clarify the origin of low coercivity (Hc). Both Hc and the difference of the densities between the crystalline and glassy phases, which corresponds to the free volume in the glassy phase, decrease with increasing Nb content. An analysis of the magnetization process based on the law of approach to ferromagnetic saturation reveals that quasidislocation dipole (QDD)-type defects are the main sources of elastic stress. The results also suggest that the pinning force for magnetic domain walls generated by one QDD-type defect is independent of the Nb content, but the number density of QDDs decreases with increasing the Nb content. Therefore, it concluded that the origin of low Hc of the glassy alloys is the low number density of QDDs which corresponds to the low number density of the domain-wall pinning sites.

Field-cooled and zero-field-cooled magnetization of superparamagnetic fine particles in Cu97Co3 alloy: comparison with spin-glass Au96Fe4 alloy

The field-cooled (FCM) and zero-field-cooled (ZFCM) magnetization of ferromagnetic fine cobalt particles in a Cu 97 Co 3 alloy has been studied. The spin-glass-like temperature dependence of the magnetization has been observed; ZFCM exhibits a spin-glass-like maximum (at T p ) and FCM is larger than ZFCM at low temperatures. However, the difference between FCM and ZFCM obviously exists far above T p . Furthermore, FCM increases monotonically with decreasing temperature even below T p while that of typical spin glasses is nearly independent of temperature. The analysis of the magnetization shows that the temperature dependence of FCM and ZFCM of Cu 97 Co 3 is well described on the basis of the superparamagnetic blocking model with no interaction between the particles, whereas that of a typical spin-glass Au 96 Fe 4 cannot be explained by the blocking model.

Comparative study of linear and nonlinear susceptibilities of fine-particle and spin-glass systems: quantitative analysis based on the superparamagnetic blocking model

Abstract The ac linear ( χ 0 ) and nonlinear ( χ 2 ) susceptibilities of ferromagnetic cobalt fine particles (which were precipitated in a Cu 97 Co 3 alloy) and of a spin-glass Au 96 Fe 4 alloy have been measured and analyzed based on the Wohlfarth superparamagnetic blocking model with no interaction between the particles. The linear susceptibility χ 0 of Cu 97 Co 3 exhibits the spin-glass-like behavior, and χ 2 has a broad negative peak at low temperature and is proportional to T −3 at high temperatures. The blocking model reproduced the features of observed χ 0 and χ 2 in Cu 97 Co 3 . On the other hand, χ 2 of Au 96 Fe 4 has a sharp negative peak at the transition temperature. The behavior of χ 2 in Au 96 Fe 4 cannot be explained by the blocking model. It is concluded that χ 2 gives the key to clarify the difference between the spin-glass transition and the progressive freezing of the particle moments.

Nb-Poor Fe-Nb-B nanocrystalline soft magnetic alloys with small amount of P and Cu prepared by melt-spinning in air

Abstract The effect of the addition of P and Cu to Fe85Nb6B9 alloy on an as-quenched structure and soft magnetic properties in a nanocrystallized state has been investigated. The Fe85Nb6B9 alloy melt-spun in air has an as-quenched structure of an amorphous phase and α-Fe grains with 20–45 nm in size. The coarse grains should still remain in the nanocrystallized structure, which deteriorates the soft magnetic properties. The simultaneous addition of 1 at.% P and 0.1 at.% Cu to the Fe85Nb6B9 alloy decreases the α-Fe grain size to nanoscale in an as-quenched state, and realizes a uniform crystallized structure with high saturation induction of 1.61 T as well as high permeability of 41,000.

Large bulk soft magnetic [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 glassy alloy prepared by B2O3 flux melting and water quenching

The large bulk soft magnetic glassy [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 alloy specimens with the diameters up to 7.7mm have been prepared by water quenching the melt immersed in the molten flux of B2O3. The maximum diameter of the obtained specimens is approximately 1.5 times as large as the previous result for copper mold casting. The bulk specimen with 7.7mm in diameter exhibits the saturation magnetization of 1.13T, the coercivity lower than 20A∕m at room temperature, and the Curie temperature of 732K. This bulk specimen is the thickest of any soft magnetic glassy alloys formed until now.

Nanocrystalline Fe–M–B–Cu (M=Zr,Nb) alloys with improved soft magnetic properties

The soft magnetic properties of the nanocrystalline Fe–M–B (M=Zr,Nb) alloys, which exhibit a high saturation flux density (Bs) above 1.5 T as well as a high effective permeability (μe) above 30 000 at 1 kHz, were found to be improved by adding small amounts of Cu and by optimizing the chemical composition. The addition of Cu to the alloys decreases the bcc grain size. The excellent soft magnetic properties (a high μe of 100 000 at 1 kHz combined with a high Bs of 1.53 T) can be achieved in the region where small grain size, as well as nearly zero-magnetostriction are obtained, which is attained in the compositional range around Fe84Nb3.5Zr3.5B8Cu1. The soft magnetic properties can be further improved by low temperature annealing before the crystallization treatment, probably as a result of a decreased grain size distribution in the crystallized state. Consequently, the μe reaches the maximum value of 120 000 for the nanocrystalline Fe84Nb3.5Zr3.5B8Cu1 alloy.

Microstructure and properties of nanocrystalline Fe–Zr–Nb–B soft magnetic alloys with low magnetostriction

We have investigated the microstructure–property relationship of nanocrystalline Fe85Zr1.2Nb5.8B8 and Fe85.5Zr2Nb4B8.5 soft magnetic alloys in order to understand the origin of drastic change in the permeability regardless of the zero magnetostriction in these two alloy compositions. Plan-view and cross-section transmission electron microscopy (TEM) observations showed strongly textured α-Fe particles on the free surface of the Fe85Zr1.2Nb5.8B8 alloy ribbon, while uniform nanocrystalline microstructure was observed in the Fe85.5Zr2Nb4B8.5 alloy ribbon. The high Zr content of the latter improves the glass forming ability, thereby suppressing the surface crystallization, resulting in higher permeability. By adding Cu in the Fe–Zr–Nb–B alloy, uniform nanocrystalline microstructure was obtained, from which superior soft magnetic properties with zero magnetostriction was achieved.

High coercivity of melt-spun (Fe0.55Pt0.45)78Zr2–4B18–20 nanocrystalline alloys with L10 structure

The structure and the coercivity (Hc) of rapidly quenched (Fe0.55Pt0.45)bal.Zr0–8B0–24 alloys prepared by the melt-spinning technique have been investigated. Ordered L10 Fe–Pt phase of 20–100 nm was obtained by rapidly quenching the melt for (Fe0.55Pt0.45)78Zr2B20 and (Fe0.55Pt0.45)78Zr4B18 alloys with high Hc of 341 and 649 kA/m in an as-quenched state, respectively. On the other hand, the (Fe0.55Pt0.45)78Zr4B18 alloy produced by Cu mold casting at a lower cooling rate than melt spinning is found to be composed of a mixed structure of Fe–Pt L10, ZrB12, PtZr and Fe3B phases and the alloy has much lower Hc of 74 kA/m than that of the melt-spun (Fe0.55Pt0.45)78Zr4B18 alloy. The lattice parameters (a and c) of the L10 phase in the melt-spun alloys suggest that Zr and B elements are contained in the L10 phase for the melt-spun alloys, which is possibly related to direct formation of the L10 structure by rapidly quenching the melt.

Improvement of soft magnetic properties by simultaneous addition of P and Cu for nanocrystalline FeNbB alloys

The additional effect of P and/or Cu on the structure and the magnetic properties of Fe–Nb–B nanocrystalline soft magnetic alloy tapes with a wide width of 5mm produced in air was investigated. Although Cu or P addition changes the magnetic softness a little, only the simultaneous addition of 1at.% P and 0.1at.% Cu significantly improves the soft magnetic properties of the crystallized alloys. The best magnetic properties (higher permeability than 45×103, lower coercivity than 5A∕m, and saturation magnetic flux density of 1.50–1.54T) were obtained in the compositional ranges of 6.6–6.8at.% Nb and 8.4–8.8at.% B with 1% P and 0.1% Cu, and these values are superior to the typical nanocrystallized Fe84Nb7B9 alloy prepared in a vacuum. The improvement probably originates from the decrease in the distribution of the α-Fe grain size in the crystallized structure by the simultaneous addition.