M. Matsui

Development of half-metallic ultrathin Fe3O4 films for spin-transport devices

We attempted to fabricate a high-quality Fe3O4 film while satisfying both low-thermal preparation (≦573 K) and film thinness (≦500 A). X-ray diffractometry showed that our prepared Fe3O4 film was epitaxially grown onto a MgO (100) substrate. The saturation magnetization, resistivity, and Verwey point were, respectively, ∼438 emu/cm3, ∼10 000 μ Ω cm, and ∼110 K. These values were comparable to those of the Fe3O4 bulk. Our experimental results suggested that a high-quality Fe3O4 film could be obtained even under the crucial conditions of the deposition temperature being low (∼523 K) and the film being ultrathinned (∼100 A).

Giant magnetoresistance of a two‐dimensional ferromagnet La2−2xCa1+2xMn2O7

Bulk samples of La2−2xCa1+2xMn2O7 with the layered Sr3Ti2O7‐type perovskite structure have been successfully synthesized and investigated with respect to their magnetic and electrical properties. It is found that La2−2xCa1+2xMn2O7 (x=0.25) is a metallic ferromagnet with a magnetic transition temperature Tc of 215 K. The large magnetoresistance (MR) effect with Δρ/ρ0 of ∼60% at 1.8 T was observed in a wide temperature range below a cusp temperature in resistivity of 96 K, which is well below the magnetic Tc. This behavior is quite different from that of the well‐known double‐exchange ferromagnets such as La1−xCaxMnO3, where large MR effects are restricted to a narrow temperature range around the ferromagnetic transition. The present result could be interpreted by using the double‐exchange theory incorporating the anisotropy resulting from the two‐dimensional Mn‐O‐Mn networks in La2−2xCa1+2xMn2O7.

Further Investigations on Magnetic Properties of Co(SxSe1-x)2, (0≤x≤1)

Further investigations, succeeding to the works of J. Phys. Soc. Japan 26 (1969) 631 and 29 (1970) 323, on the magnetic properties of Co(S x Se 1- x ) 2 in the whole concentration ranges (0≤ x ≤1) were performed by means of high magnetic field, electrical resistivity and low temperature X-ray diffraction measurements. The first kind magnetic transition was found in the ferromagnetic phase except CoS 2 . The metamagnetic behaviors were observed in the ferromagnetic (0.89< x <1.0) as well as paramagnetic ( x <0.89) phases. The saturation moment per Co atom was obtained to be 0.855 µ B for CoS 2 and it decreased gradually with increasing Se content. The electrical resistivity in low temperatures showed a T 2 -dependence. It was concluded that the itinerant electron model base on the spin fluctuation theory is more adequate than models based on a localized spin scheme.

Specific Heat and Electrical Conductivity of Low temperature Phase of Magnetite

It was observed for Fe 3 O 4 that the presence of residual stresses lowers the so-called Verwey temperature, T v , considerably, broadens the temperature-width of the anomalous specific heat at T v , produces another additional peak and decreases the discontinuous conductivity change at T v . The entropy change at the phase transition was determined to be 5.4 J /mole·deg but when the effect of short range order is included it becomes 9.6 which is much larger than the value expected for electron hopping under the Andersons restriction. The anisotropic conductivity was measured below T v along the monoclinic axes a , b and c . The magnitude of conductivity was in the order of σ c >σ b >σ a in the range of T v > T >60 K, while it changed to σ b >σ a >σ c below 60 K. The conductivity along [\bar111], or the longest , was also measured by using a squeezed crystal in which a monoclinic single domain is realized. This direction was least conductive among four s in the whole temperature range.

Analysis of Anomalous Thermal Expansion Coefficient of Fe–Ni Invar Alloys

Thermal expansion coefficients of Fe–Ni alloys were analyzed by using the Gruneisen equation. The anomalous thermal expansion coefficient term, α a , was calculated by subtracting the lattice vibration term described by Gruneisen theory from the experimental data. The anomalous volume change obtained by integrating α a was analyzed in terms of the two-states model of iron, the energy separation of which is assumed to be a function of the exchange field and temperature. The temperature dependence of the energy splitting is in good agreement with that of the spontaneous magnetization.

Magnetization of Co(SxSe1-x)2 under High Magnetic Field Up to 500 kOe

Proceeding the previous paper (J. Phys. Soc. Jpn. 46 (1979) 1474) the high field magnetization process of Co(S x Se 1- x ) 2 system for 0.70 ≤ x ≤1.00 has been investigated by means of pulse fields up to 500 kOe at fixed temperatures. The metamagnetic behaviors were observed and the phase diagram between the transition field and temperature was given. The saturation moment seemed to disappear at x ≃0.6.

Interstitial hyperthermia of malignant brain tumors by implant heating system: Clinical experience

SummaryAn implant heating system using a ferromagnetic implant with low Curie temperature has been developed for treating human brain tumors. Safe and repeated hyperthermia was possible over periods averaging ten weeks in 23 out of 25 patients with malignant brain tumors without development of major side effects.Evaluation of the effects of this new treatment is still preliminary. Overall response rate was 34.8%. However, five of thirteen cases of malignant glioma and two of five cases of brain metastasis were responded well to interstitial hyperthermia given with or without irradiation. Pathological findings common to the treated tumors were circumscribed, ellipsoid shape of coagulation necrosis around the implant. Degeneration of tumor cells, hemorrhage, vascular stasis and thrombosis were found adjacent to the necrosis.In the future, a combination of interstitial hyperthermia with brachytherapy and chemotherapy may offer improved local control of brain tumor.

A New Low Temperature Phase (fct) of Fe-Pd Invar

A new fct phase of disordered Fe–Pd Invar alloys was found at low temperatures. The low temperature phase diagram was obtained from X-ray diffraction experiments (4.2 K to 300 K). The fct phase ranges from 28% to 33.5% Pd at 0 K and the temperature dependence of lattice constants was determined. A decrease of the X-ray Debye temperature associated with a lattice softening was observed for 34% Pd in the low temperature region.

Magnetic properties of fcc-Fe artificial superlattices

Abstract Fcc-Fe/Cu(M)(001) (M = Au,Ni) artificial superlattices were prepared. The interatomic distance of fcc-Fe was changed by the epitaxial growth of Cu(Au) or Cu(Ni) alloy layers. The magnetic moment of Fe is increased up to 2.7μB for only 1% increment of interatomic distance. Furthermore, non-ferromagnetic state of fcc-Fe was observed at 1.8% decrement.

Magnetic properties and thermal expansion of Fe-Pd invar alloys

Abstract Magnetic and electrical properties and thermal expansion of Fe-Pd Invar alloys were measured, especially at low temperatures. The results were compared with those of Fe-Ni Invar alloys in a standpoint of the two-spin states model.