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The detailed BH loop measurements of an NiZn ferrite toroid have been studied at various temperatures below the Curie temperatures $T_{C}$ using a lock-in amplifier. The Jiles–Atherton (JA) analysis of the BH loops gives the power-law relations among the hysteresis parameters and the JA parameters. The Steinmetz analysis suggests the power-law relation proposed by Kobayashi et al. is valid in wide range of the temperature in low magnetic field.

A new method is proposed to measure the magnetic hysteresis as a function of temperature. It is based on the spectral decomposition of magnetization and measurement of different harmonics using the harmonic-detection function of a lock-in amplifier. Applying this method, a measuring system of the BH loop was designed and fabricated. This system gives spectra of magnetic flux density B driven by an ac-magnetic field h. The BH loops are reconstructed from the spectra of the B. The spectra and BH loops were measured for a NiZn ferrite toroid at various temperatures. When the temperature increases from the room temperature, the higher harmonics of the spectra increase. Approaching the Curie temperature TC from below, magnetic loss and nonlinearity of the BH loops increase, and the loop shows the saturation just below TC.

An extensive study of electrical resistivity, ac magnetic susceptibility, magnetization, specific heat, and NMR has been made on high purity samples of the spinel compounds ${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$ and ${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$. The superconducting transitions occur at 4.70 K in ${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$ and 3.48 K in ${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$. The magnetic susceptibilities show perfect diamagnetism in both compounds. Upper critical fields at T=0 are estimated to be 20.0 and 4.40 kOe, the lower critical fields at T=0 are 70 and 95 Oe, respectively. The thermodynamic critical fields at T=0 are 704 Oe (${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$) and 445 Oe (${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$). These values are obtained from the temperature dependence of the free energy. These compounds are type-II superconductors with Ginzburg-Landau parameters \ensuremath{\kappa}=21 (${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$) and \ensuremath{\kappa}=6.6 (${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$) at T=0. The electron-phonon coupling constants, ${\ensuremath{\lambda}}_{\mathrm{ep}}$=0.68 (${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$) and ${\ensuremath{\lambda}}_{\mathrm{ep}}$=0.64 (${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$), are obtained using the McMillan formula. The copper nuclear-spin-relaxation rate as a function of temperature shows a pronounced coherence peak just below ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$. Experimental results are analyzed on the basis of the BCS theory. These compounds exist between weak- and intermediate-coupling superconductors which are driven by electron-phonon interaction. The Debye temperatures FTHETA of these materials are 230 K (${\mathrm{CuRh}}_{2}$${\mathrm{S}}_{4}$) and 211 K (${\mathrm{CuRh}}_{2}$${\mathrm{Se}}_{4}$).

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 χ 0 has a spin-glass-like maximum and χ 2 has a very broad negative peak at a low temperature. At high temperatures, the in-phase linear susceptibility χ 0 ' obeys the Curie law and the in-phase nonlinear susceptibility χ 2 ' is proportional to T -3 . A quantitative analysis of χ 0 and χ 2 based on a simplified superparamagnetic blocking model has been performed. The temperature dependence of the in-phase and out-of-phase susceptibilities is well described quantitatively by the superparamagnetic blocking model. It is concluded the behavior of χ 2 can be explained by the summation of the nonlinear term of the Langevin function over particle volume distribution. The analysis also yields information about the particle volume distribution and a value of anisotropy energy.

The ac linear (χ 0 ) and nonlinear (χ 2 ) susceptibilities of a ferromagnetic Ni have been studied near the Curie temperature, to clarify the feature of the magnetic phase transition. The first observation for the critical behavior of χ 2 in a pure ferromagnetic system has been made at low frequency measurement. The observed χ 2 at higher frequencies is affected extremely by the contribution of the eddy currents. From analysis of the frequency dependence of χ 0 and χ 2 , the method of eddy-current correction for χ 2 is found. Analysis of the critical behavior of χ 0 and χ 2 yields the critical exponents γ=1.333±0.008 and γ 2 =4.69±0.30.

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.

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.