T.J. McKenna

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.

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.

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 data logging system for AC magnetic measurements (4.2-300K)

A semi-automated apparatus for the simultaneous measurement of various AC magnetic properties in low applied fields (<or approximately=1 kA m-1) over the temperature range 4.2-300K is described. The system is based on a variable temperature cryostat and a high resolution data logger capable of measuring ten signals to <or approximately=1 mu V resolution. The capability of the apparatus is demonstrated by the simultaneous measurement of AC susceptibility chi and its temperature modulation signal S for a single crystal of terbium. The design of a current amplifier used for generating the magnetic field and with the ability to sum AC and DC inputs is also described. A method of assessing the effects of an average demagnetising factor on chi and S measurements on irregularly shaped samples is presented.

Time dependent effects in the ferromagnetic susceptibility of Tb - II

Abstract Measurements of the low field ( H p = 1–2000 Am -1 ; f p = 100–1000 Hz) ac magnetic susceptibility, transient enhancement of the susceptibility and temperature modulation of the susceptibility on single crystal and polycrystalline terbium are reported. For the ferromagnetic region ( T C ≈ 221 K), temperature hysteresis and time dependence in susceptibility as well as the detailed response of the susceptibility to temperature changes are shown to be due to the temperature dependent domain structures and the motion of the mobile domain wall pinning centres. The temperature dependence of the diffusion times for these centres is consistent with an activation energy ≈ 0.5 eV, similar to those for impurities and dislocations.

Examination of domains in Tb and Dy by thermal modulation studies

Abstract Initial ac susceptibility χ( H rms “≲” 650 Am -1 ) and thermal modulation measurements on Dy over the range 4.2–200 K are reported. Temperature cycling through the ferromagnetic and antiferromagnetic regions reveals hysteresis effects due to spiral spin antiferromagnetic domains and the intermediate domain structure which links ferromagnetic with antiferromagnetic domains.

Ferromagnetic domain size of Tb

Abstract The temperature dependence of ferromagnetic domain size for Tb is calculated over the range 0 K to the ferromagnetic ordering temperature T c ∼221 K and compared with earlier calculations and experimental data. The present results show a decrease in domain size with increasing temperature in broad agreement with the earlier calculations. Discrepancies between sets of calculations and experiments at high temperatures ( T ⪆ 180 K) may be due to domain nucleation and/or thermal history effects.