G. V. H. Wilson

AC susceptibility and temperature modulation studies of gadolinium

Studies of the low field (Hrms<or approximately=14 A m-1) initial AC susceptibility chi i of unannealed and annealed gadolinium around the Curie temperature (Tc approximately 290K) are reported. The frequency dependence of chi i is found to be characteristic of an impurity diffusion magnetic after effect of mean relaxation time tau approximately 0.3 s. A comparison of thermal modulation experiments for annealed and unannealed samples shows that annealing greatly reduces both the observed temperature hysteresis of chi i and the thermal relaxation of domain properties in the temperature region of domain nucleation.

Low‐field magnetic hysteresis in gadolinium and terbium

The Fourier components of the ferromagnetic hysteresis of polycrystalline gadolinium and terbium have been measured in low ac magnetic fields (0.1–15 A m−1) using synchronous detection. The field dependences and magnitudes of the various harmonics cannot be described by the Rayleigh hysteresis law. It is shown that the dominant term in the absorptive component of the first harmonic is associated with a magnetic aftereffect. At the lowest fields (H≲1 A m−1) there are indications that magnetic hysteresis vanishes.

A.C. specific heat of terbium and dysprosium

Abstract The dependence of a.c. specific heat measurements upon the temperature modulation amplitude Δ T has confirmed the first-order nature of the antiferromagnetic-ferromagnetic transition in single crystal terbium ( T C ∼221K) and polycrystalline dysprosium ( T C ∼85K). No peak in a.c. specific heat is obtained for 2Δ T less than the temperature hysteresis at the transition. By comparison, the peaks expected in a.c. specific heat were observed at the higher-order antiferromagnetic-paramagnetic transition in both materials for Δ T amplitudes as low as∼40 mK.

Activation energies in Gd using transient enhancement of the AC permeability

Abstract Analysis of the time delay of the transiently enhanced AC permeability of ferromagnetic Gd over the temperature range 240–290 K reveals a series of discrete, temperature dependent, time constants. These each follow a simple activation energy equation from which activation energies 0.62, 0.65 and 0.69 eV are resolved. These components are attributed to impurities for which significantly different diffusion rates were observed.

New model for effects of domain wall pinning on ferromagnetic susceptibility

Abstract A simple mathematical model for the effects of pinning centres on ferromagnetic domain wall motion is outlined. Experimentally observed time-effects and field dependences are qualitatively described by the model as applied to the field dependence, diffusion aftereffect, and transient enhancement of low-field ac ferromagnetic susceptibility.

AC susceptibility and transient enhancement studies in the antiferromagnetic region of single crystal terbium

Abstract Measurements of the ac susceptibility χ and its transient enhancement are reported for a terbium single crystal. The sharp transition and temperature hysteresis in χ at the antiferromagnetic—ferromagnetic transition indicate that the transition is first order. The results also support the existence of spiral spin domains in the antiferrogmagnetic region.

Domain effects near the order-disorder and order-order ferromagnetic transitions in Gd and Dy

Abstract The technique of transient enhancement of the ac initial susceptibility χ has been used to compare domain effects in the ferromagnetic-paramagnetic transition of Gd and the ferromagnetic-antiferromagnetic transition of Dy. The temperature hysteresis in both χ and enhanced χ in the AF region of Dy indicates the existence of domains. The results for Dy support the suggestion that the transition from ferromagnetic to antiferromagnetic order upon warming in the vicinity of the Curie temperature takes place via an intermediate, moment-bearing domain structure.

Low-field magnetic hysteresis in iron, cobalt, nickel and steel

Abstract Spheroidal samples of high purity iron, cobalt and nickel and carbon steel have been examined in a low field (10 -3 to 10 3 Am -1 rms) ac magnetisation study of their hysteresis losses. Detailed comparisons of the field dependences of the fundamental and higher harmonic components show that only cobalt exhibits the features of the empirical Rayleigh-like behaviour for which both linear and quadratic dependences of magnetisation upon magnetic field are present.

Thermal modulation studies of the critical magnetic susceptibility of Gd

The technique of temperature modulation of the AC magnetic susceptibility is used for the first time to examine the critical region above a ferromagnetic-paramagnetic transition. The temperature modulation and low-field susceptibility data indicate a well defined critical exponent behaviour chi = chi 0 epsilon - gamma , where epsilon =(T-Tc)/Tc for polycrystalline Gd, and lead to the critical parameters gamma =1.24+or-0.03 and chi 0=0.001 for 9.9*10-3<or= epsilon <or=3.7*10-2.

A simple application of the AC specific heat method to bulk samples

An AC specific heat method based on AC resistance heating and phase sensitive detection of the thermal modulation signal has been applied to bulk samples. The problems of thermal skin effects are avoided by operation at low modulation frequencies of less than 1 Hz. The behaviour of the sample and environment is well represented by a simple cooling law. The relative advantages of detection at the first and second harmonic of the heater voltage signal are outlined and results presented for gadolinium using both techniques. In this case parallel accumulation of critical specific heat and magnetic data is possible.