E.W. Lee

The reversible susceptibilities of dysprosium and terbium

The initial susceptibilities of Dy and Tb have been measured in the ferromagnetic and antiferromagnetic phases. In the ferromagnetic phase it is concluded that the susceptibility arises from domain wall displacements and that these are limited by an intrinsic restoring force resulting from the very narrow domain wall and not by local impedances due to imperfections and impurities. From measurements in the antiferromagnetic phase it is concluded that the often quoted two constant expression for the exchange coupling between sublattices cannot provide a quantitative explanation for the observed behaviour but that the susceptibility of Dy and the temperature variation is quantitatively consistent with the seven exchange constants and the turn angles determined from neutron scattering measurements. The reversible susceptibility measured in the presence of external fields shows a complicated behaviour and does not appear to be consistent with the predictions of the theory of Nagamiya et al. (1962).

Magnetic anisotropy in the paramagnetic phase of TbAl2

Abstract The magnetic anisotropy of a single crystal of TbAl2 has been measured by torque magnetometry from below the Curie point up to 170 K, well into the paramagnetic phase. Within a (110) plane the torque can be described by the expression L(θ) = {P sin 2θ} H2 + {Q sin 2θ + S sin 4θ} H4 + {T sin 4θ} H6, where θ is the an gle formed by the magnetization vector with a [001] axis. The first term (in H2) is interpreted as produced by arrays of defects with axial symmetry. The second (in H4) and third (in H6) terms arise from anisotropic fourth and sixth rank tensor paramagnetic susceptibilities. On the other hand if the anisotropy is described in terms of effective conventional anisotropy constants K1 and K2 within the temperature range 90–170 K it is found that both constants change continuously across the Curie temperature and furthermore the [111] direction remains the easy direction in the paramagnetic range. Anisotropy measurements reveal themselves as a sensitive indicator of the level of macroscopic defects in magnetic crystals.

The determination of anisotropy constants from torque curves — II

Abstract This is an extension of a previous paper which examined the methods available for the determination of the anisotropy constant from torque curves when the anisotropy is governed by a single constant i.e. E = K 1 sin 2 φ. The two methods singled out in that paper as being of special merit are briefly re-examined when the anisotropy energy takes the form E = K 1 sin 2 φ + K 2 sin 4 φ.

Magnetic anisotropy and spin-reorientation in HoAl2

Abstract Torque measurements have been performed at several temperatures and magnetic fields, on a single crystal of HoAl2 in the (1 1 0) plane. The easy magnetization direction changes from the [1 0 0] to [1 1 0] near TR = 20 K in magnetic fields between 0.2 and 1.6 T. At temperatures below TR a new non-major cubic symmetry easy direction appears which rotates with temperature in the plane of the sample. This behaviour and that of the complete torque curves have been interpreted on the basis of a single ion model.

Exchange force explanation for the antiferromagnet-ferromagnet transition in dysprosium

An explanation based on exchange forces is proposed for the antiferromagnet-ferromagnet transition in Dy which is in good agreement with the available experimental results. Contrary to the usual idea that the driving mechanism is the magnetostriction and anisotropy in the basal plane of the HCP structure it is shown that the exchange forces are mainly responsible for the transition. Magnetoelastic, anisotropy and applied magnetic field energies only provide the delicate balance of energy necessary to assist the exchange forces for the transition. These energies only amount to a relatively small fraction (<20%) of the total driving energy.

Magnetoelastic coupling in DyAl2

Abstract The magnetostriction of a single-crystal of DyAl 2 has been measured in the ordered and paramagnetic phases along [100] easy axis. Volume thermal expansion and forced magnetostriction have been measured simultaneously along three orthogonal directions on the same sample. We report a set of magnetoelastic CEF parameters using a single-ion model, incorporating high order CEF magnetoelastic contributions.

Magnetic moment and magnetostriction of ternary TbxGd1-xAl2 intermetallic compounds

The magnetostriction and magnetic moment of the pseudo-binary compounds TbxGd1-xAl2 (0.05<or=x<or=1) have been measured throughout the ordered state, on single crystals and polycrystals. A pair of crystal-field (CEF) parameters suffice to explain the behaviour of the whole series. The magnetoelastic constants M2gamma and M2E have been determined, and they remain practically constant across the series, and in reasonable agreement with the point charge model. The Tb3+ magnetic moment remains constant with Tb content and unquenched. The overall consequence is that the single-ion CEF model applies remarkably well to this series of compounds, in fact as well as does the mean-field approximation for random pseudo-binary alloys including Tb-Gd and Tb-Tb exchange interactions.

The magnetic field dependence of the magnetisation of nickel

Abstract The magnetisation of a single crystal has been measured parallel to the easy direction as a function of field within the temperature range 4.2 to 300 K. The results are analysed in terms of spin-wave and single particle excitations and reveal an additional excitation of unknown origin.

Anomalous thermal expansion and magnetostriction in HoAl2

Abstract Thermal expansion and magnetisation measurements have been performed on a single crystal of HoAl 2 between 4 and 300K. A sharp spontaneous volume anomaly has been observed at the spin re-orientation temperature where the easy direction of magnetisation changes from [101] to [010]. A model incorporating single-ion crystal field, exchange and magnetoelastic contributions has been found which adequately explains the temperature dependence of the anomalous volume distortion. From a fit to the experimental data, the magnetoelastic coupling constants M α 4 and M α 6 have been determined.

Thermal expansion and forced volume magnetostriction in DyAl2

Abstract Thermal expansion of a DyAl2 single crystal has been simultaneously measured along the main three symmetry directions [0 0 1], [1 1 0] and [ 1 1 0]. Below the Curie point (Tc=61 K) the three strains become different, indicating that the crystal is macroscopically orthorhombic. The thermal expansion at the paramagnetic-regime can be scaled from the YAl2 one and a Debye temperature θD(DyAl2)=434°K is derived. The forced volume magnetostriction is produced by the strain dependence of the crystal field (CF) and exchange magnetoslastic interactions, and values for the coupling parameters are derived.