Susumu Chikazawa

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.

Nonlinear Susceptibility of a Spin Glass Compound (Ti1-xVx)2O3: II

Linear (χ 0 ) and nonlinear (χ 2 ) susceptibilities of (Ti 1- x V x ) 2 O 3 have been studied in detail around the spin freezing temperature T g . The measurement on single crystal has revealed that both χ 0 and χ 2 are highly anisotropic. The linear susceptibility χ 0 shows a cusp at T g . The nonlinear susceptibility χ 2 , which is observed as the third higher harmonic of the applied ac field ( h = h 0 sin ω t ), shows the very sharp peak in the vicinity of T g . The frequency ν(=ω/2π) and amplitude h 0 dependence of χ 2 has been measured in detail to study the critical behavior around T g in the static limit ν→0 and in the infinitesimal field strength h 0 →0. |χ 2 | increases rapidly with decreasing ν and h 0 , which implies the divergence of χ 2 at T g in the limit ν→0 and h 0 →0.

Nonlinear Magnetization at the Spin Glass Phase Transition Temperature

Nonlinear magnetization, m=χ 0 h+χ 2 h 3 , was found in spin glass using a mutual inductance bridge. The susceptibility χ 0 showed a cusp at T g as usual, on the other hand χ 2 behaved like a divergence at T g .

A new ferromagnetic thiospinel CuCrZrS4 with re-entrant spin-glass behaviour

A new thiospinel CuCrZrS4 has been successfully synthesized by a solid-state chemical reaction. This CuCrZrS4 exhibits ferromagnetic properties below the Curie temperature at T c = 60 -2 K. The appearance of irreversible effect between field-cooled and zero-field-cooled magnetization is prominent below around 5 K in a magnetic field of less than 150 Oe. The ac susceptibility χAC shows a rapid decrease below about 10 K. This low magnetic-field behaviour indicates the existence of a re-entrant spin-glass phase below about 10 K. The dc magnetic susceptibility above 100 K shows Curie-Weiss behaviour with an effective magnetic moment of 3.61 μ B, which is a little less than the spin-only value of 3.87 μ B for the Cr3+ ion. The asymptotic Curie temperature θ P is approximately 65 K, which is a little higher than T c. The valence state is confirmed to be Cu+Cr3+Zr4+S4 2− on the basis of magnetic properties. The electrical resistivity ρ shows a semiconducting temperature dependence over the temperature range from 4.2 to 280 K with an activation energy of 6.84 210−3 eV in the higher temperature range from 50 to 283 K.

Linear and Non-Linear Susceptibilities in Canonical Spin Glass AuFe (1.5 at.%Fe)

Linear, χ 0 , and nonlinear, χ 2 and χ 4 , susceptibilities were measured in metallic spin glass AuFe. Nonlinear susceptibilities for AuFe (1.5 at.%Fe) diverge at T g showing the power law behavior and the critical behaviors around T g are interpreted by Suzukis theory based on Edwards and Anderson model. The critical exponents γ s , δ and β were obtained to be γ s =1.1±0.2, δ=2.0±0.2 and β∼0.9, respectively.

Linear and Nonlinear Susceptibilities in Cu97Co3 Alloy for Ferromagnetic Fine Particles in Metallic Matrix: Comparison with Spin Glass Au96Fe4 Alloy

The ac linear (χ 0 ) and nonlinear (χ 2 ) susceptibilities of ferromagnetic fine cobalt particles, which were precipitated in a Cu 97 Co 3 alloy, have been studied. The linear susceptibility χ 0 has a spin-glass-like maximum at low temperature. The nonlinear susceptibility χ 2 is negative and has a peak around the temperature where χ 0 exhibits the maximum. However, the negative peak in χ 2 is very broad compared with that of spin glasses. Furthermore, χ 2 is proportional to T -3 at high temperatures. This behavior of χ 2 in the Cu 97 Co 3 alloy is essentially different from the divergent behavior of χ 2 in spin glasses.

Critical behavior of linear and nonlinear susceptibilities near Curie temperature in Au82Fe18 alloy

The ac linear (χ 0 ) and nonlinear (χ 2 ) susceptibilities of Au 82 Fe 18 alloy have been studied around the Curie temperature T c , to make clear the feature of the magnetic phase transition. In very low field measurements, the nonlinear susceptibility χ 2 is observed only in the vicinity of T c and the temperature dependence of χ 2 is in good agreement with theoretical expectations in the transition from a ferromagnetic phase to a paramagnetic phase. Analysis of the critical behavior of χ 0 and χ 2 yields the susceptibility exponent γ=1.33 ±0.05 and the nonlinear susceptibility exponent γ 2 =4.82 ±0.50. These values are very close to theoretical predictions for 3d-Heisenberg pure ferromagnets.

Dynamic Susceptibility of a Spin Glass Compound (Ti0.9V0.1)2O3

The imaginary part of the susceptibility of (Ti 0.9 V 0.1 ) 2 O 3 was measured and the temperature dependence of the characteristic relaxation time, which showed a sharp peak at a spin freezing temperature T g , was obtained using a modified Debye formula proposed by Matsubara and Yoshimitsu.

Magnetic Properties and Magnetic Phase Diagram of bcc Cr-Fe-Mn Alloys

Magnetization measurements have been performed on bcc Cr 80- x Fe 20 Mn x (3.6≤ x ≤38.4) and Cr 75- x Fe 25 Mn x (5.0≤ x ≤29.1) alloys between 6 and 300 K using a SQUID magnetometer. The ferromagnetic and spin-glass transitions are observed in these alloys. The magnetic phase diagram of bcc Cr 80- x Fe 20 Mn x and Cr 75- x Fe 25 Mn x alloys are determined using the Curie temperature, T C and spin-glass temperature, T g decided in this work, and the Neel temperatures, T N reported on the same alloy system. The spin-glass phase spreads for a wide concentration region in the present Cr–Fe–Mn ternary alloy system. The magnetic moment of the ferromagnetic Cr–Fe–Mn alloys at T =0 is compared with Slater-Pauling curve.