Syohei Miyahara

Magnetic Properties of FeS2 and CoS2

Magnetic properties of FeS2 and CoS2 having pyrites structure are studied. Magnetic susceptibility of a good natural pyrites (FeS2) crystal is almost temperature‐independent down to 4.2°K. The constant susceptibility is explained by the Van Vleck paramagnetic term and diamagnetic one of the core. This suggests iron in pyrites is in divalent and low‐spin state contrary to the assumption of L. R. Walker et al. that iron in pyrites is in a trivalent state from the Mossbauer isomer‐shift data. CoS2 is a ferromagnet of a Curie temperature of 130°K. The spontaneous magnetization σ and the Curie constant Cmol are measured on a synthetic crystal of CoS2. We obtained the values, σ=44.4 emu/g and Cmol=0.37 emu·deg. The values are compatible when we assume Co is in divalent and low‐spin state. This fact accords with the result of magnetic measurement of FeS2.

Magnetic Properties of Several Metasilicates and Metagermanates with Pyroxene Structure

Manganese metasilicate and manganese, ferrous and cobalt metager-manates with pyroxene structure were prepared by usual ceramic techniques. Ferrous and cobalt metasilicates were synthesized under high pressure. The magnetic susceptibilities of the eight pyroxenes were measured in the temperature range from 4.2 to 300°K. All of these pyroxenes are antiferromagnetic at low temperatures. The Neel temperatures of MnSiO 3 , MnGeO 3 , CoSiO 3 , and CoGeO 3 are respectively 7°K, 10°K, 50°K and 41°K. The susceptibility-temperature curves for FeSiO 3 and FeGeO 3 show two peaks. They are discussed on the basis of a simple molecular field approximation.

Magnetic properties of pyrite type NiS2

Abstract Neutron diffraction and magnetic measurements were performed on the same powder sample of NiS 2 . It was found that T c (= 30°K) of the weak ferromagnetism of this sample coincides well with T N of antiferromagnetic M2-mode determined by ND. No anomaly in the magnetic susceptibility was observed above T c .

The magnetic susceptibility of MnO associated with the first-order phase transition

Abstract A discontinuous change in the magnetic susceptibility of MnO was found at the Neel point. This is attributed to the exchange striction and ϵ 1 (= -( r J 1 0 )( ∂J 1 ∂r )) is estimated to be 27 ± 5. Comparisons with other data are given.

Magnetic Properties of Zn_ Ni_xCr_2S_4(0≤x≤0.40) with Spinel Structure

Magnetization and susceptibility measurements were made on polycrystalline samples of Zn 1- x Ni x Cr 2 S 4 from 4.2 K to 800 K in magnetic fields up to 16.5 kOe. The inverse susceptibilities except for ZnCr 2 S 4 show a non-linear behavior characteristic of a ferrimagnet. This indicates that the system has a magnetic moment on Ni 2+ in contrast with NiRh 2 S 4 and the sign of Ni 2+ –Cr 3+ exchange interaction is negative. The saturation magnetization of the sample (x=0.40) is equal to 3.0±0.1 µ B /molecule at 4.2 K. Electrical resistivity of the sample (x=0.40) was measured between 77 K and 300 K, and it is semiconducting (3×10 3 \(\varOmega\) cm at R. T.).

Magnetic resonance in CoS2 powder

Abstract At and above Curie temperature ( T c ), a g -value of 2.06 was obtained. Above T c , the line width increases very rapidly with increasing temperature. Below T c , the ferromagnetic resonance was observed.

Effect of pressure on the electrical resistivity of some vanadium spinels

Abstract The vanadium spinels MV2O4, where M = Mn, Fe, Zn and Co, were prepared. The effect of pressure on the resistivity of the four samples up to 320 kbars and the lattice constant of CoV2O4 up to 50 kbars were measured by using the high pressure cells of Drickamer type. In general, the resistivity is found to decrease with increasing pressure. The temperature coefficient of resistivity for CoV2O4 increases with increasing pressure. The V3+ -V3+ distance decreases by 2.5 per cent at 50 kbars. The critical cation-cation separation for localized vs. collective electron is smaller than the value 2.97 A given by Goodenough et al .

Magnetic Susceptibility and Thermal Expansion of MnSe 2 with Pyrite Structure

The magnetic susceptibility of MnSe 2 with pyrite structure was measured from 4.2 K to 473 K. The susceptibility changes discontinuously at the Neel point T N (47.4 K) and its change δχ is -0.016 in terms of the maximum. The linear thermal expansion of the polycrystalline sample was measured from 20 K to 280 K by a strain gauge method. MnSe 2 contracts discontinuously at T N as it is cooled, which indicates a first order phase transition. The change Δ L ( T N )/ L 273 is -6.5×10 -5 . On the basis of the assumptions that exchange interactions vary as - n th power of inter-ionic distance and that the discontinuous change of the susceptibility is due only to the isotropic volume contraction, n is calculated to be 246 from δχ and Δ L ( T N )/ L 273 by molecular field theory. From this extraordinary large value of n , it is suggested that an anisotropic lattice distortion occurs below T N in MnSe 2