J. I. Arnaudas

Magnetocaloric effect in random magnetic anisotropy materials

In this letter we report the results of entropy variations in random anisotropy magnets composed of TbxY1−xAl2, with x=0.15, 0.20, 0.25, 0.35, 0.40, and 0.50. We discovered large entropy variation associated with the spin glass to paramagnetic transition. Both temperature transition and entropy changes were studied at different temperatures and with different compositions. Our conclusion is that these materials are suitable candidates for use as magnetic refrigerants in a temperature range below 40 K.

Crystal field effects on polycrystalline (rare earth)Ni2 intermetallic compounds

Abstract Magnetization measurements have been performed between 3.8 K and above the respective Curie temperatures, with magnetic fields up to 20.5 kOe, in polycrystalline samples of the cubic RENi 2 intermetallic compounds (RE = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm). The compounds show ferromagnetic order, except PrNi 2 and TmNi 2 which do not order down to 4 K. Crystal electric field (CEF) calculations for the thermal variation of the susceptibility in PrNi 2 agree well with experiment. Also the calculated thermal variation of magnetization in the polycrystals, by the use of the CEF theory, agrees quite well with experiment for the compounds TbNi 2 , DyNi 2 , HoNi 2 and ErNi 2 . Polycrystalline magnetization and susceptibility are therefore quite valuable to study CEF effects in cubic rare-earth intermetallic compounds.

Magnetometer for anisotropy measurement using perpendicular magnetization

Abstract An instrument for magnetic anisotropy determination based on perpendicular magnetization measurement is described. The principal advantages compared with the usual torsion magnetometers are a much simpler construction and operation. In addition it is possible to measure high anisotrpies, the overall sensitivity in torque is ≈ 10 dyne cm. Some experimental torque curves obtained with the apparatus are also given.

Flux-pinning and crystal-field magnetostrictions in REBa2Cu3O7−x high-Tc superconductors

Abstract High-field (up to 12T) magnetostriction measurements have been performed on single crystals of REBa 2 Cu 3 O 7− x (REDy, Ho and Er) at the paramagnetic and superconducting phase, between 3.8 and 100 K. Low-temperature contributions to the shape magnetostriction, related to the vortex flux pinning, are found evidence for, through the strong hysteresis shown by the magnetostriction isotherms. This flux-pinning magnetostriction shows a scaling behavior which has been analyzed in terms of Kramers flux-lattice shear stress model, and represents a new method to study pinning mechanisms using magnetostriction measurements. We have also isolated the single-ion crystal electric field (CEF) origin basal plane symmetry breaking magnetostrictive mode, ϵ γ (≌−300 × 10 −6 at 10 K and 12 T, for Dy and Ho compounds), obtaining the ratio M γ / C 0 γ (=1.3 × 10 −4 , for the Ho compound and ≈5×10 −4 , for the Dy compound) where M γ is the single-ion CEF irreducible magnetoelastic coupling parameter and C 0 γ the paramagnetic symmetry elastic constant. The obtained quadrupolar coupling parameter G γ for the Ho compounds is − 35 mK/ion.

Intrinsic coercive field in pseudobinary cubic intermetallic compounds. I: DyxY1−xAl2 and Dy1−δAl2

Abstract The thermal variation of the coercive field in pseudodobinary Laves phase compounds DyxY1−xAl2 (0.10 ⪯ x ⪯ 1.10) and Dy1−δAl2 (δ = 0.025 and 0.048) has been measured from 3.7 K up to the Curie temperatures. It turns out the coercive field has two contributions: one from the intrinsic pinning of narrow domain walls in the Peierls crystal potential wells of the anisotropy energy; this contribution dominates at low temperatures and decays exponentially with the narrow domain wall width. The other comes from the pinning of the domain walls by deflecs, being explained by the Kersten theory of critical domain wall bowing. The domain walls seem very narrow in these materials, the widths estimated from the coercive field being between two and three atomic spacings.

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.

Magnetocrystalline anisotropy in a (110)?(Tb0.27Dy0.73)Fe2 thin-film

Magnetic anisotropy measurements performed in a (110) (Tb0.27Dy0.73)Fe2 (Terfenol-D) film epitaxially grown on a sapphire substrate are presented. The magnetic torque curves have been determined by using a vectorial vibrating sample magnetometer, which allows us to measure the angular dependence of magnetization components parallel, , and perpendicular, , to the applied field up to 2 T. The fourfold symmetry associated with the cubic structure within the (110) plane is clearly observed. The analysis of the experimental torque has been carried out considering magnetocrystalline anisotropy up to sixth order and magnetoelastic energy up to second order; so, the magnetocrystalline anisotropy constants in the (110) plane of the film, K1 and K2, have been obtained. This allows us to determine the direction of the magnetization easy axis for (110) Terfenol-D thin-film: it is at RT, passes through at 140 K and then changes to at 40 K. It was completely impossible to explain the angular dependence of the experimental magnetic torque without including shear and tetragonal magnetoelastic stress parameters, b2 and b1, respectively. This confirms the paramount role of the strain in the determination of the magnetic properties in this kind of Terfenol-D thin film.

A.C. initial magnetic susceptibility and spin reorientation transitions in (ErxR1−x)2Fe14B magnets (R=Nd and Dy)

Abstract A.C. initial magnetic susceptibility has been measured in the series (Er x Nd 1−x ) 2 Fe 14 B and (Er x Dy 1−x ) 2 Fe 14 B in the range of temperatures between 3.8 and 350 K. The results clearly show that these measurements constitute a very useful tool in order to detect spin reorientation phenomena. The interplay of different kinds of anisotropies: axial, conical and planar in these systems produces a dependence of the spin reorientation transition (SRT) on concentration, x. In the case of Er-Nd compounds SRT processes were found for all the concentrations in the range of temperatures measured, and for intermediate concentrations two peak anomalies were observed indicating the existence of two SRTs. In the case of the Er-Dy systems, no SRT was observed for concentrations x

Antiferromagnetic Spin Coupling between Rare Earth Adatoms and Iron Islands Probed by Spin-Polarized Tunneling

High-density magnetic storage or quantum computing could be achieved using small magnets with large magnetic anisotropy, a requirement that rare-earth iron alloys fulfill in bulk. This compelling property demands a thorough investigation of the magnetism in low dimensional rare-earth iron structures. Here, we report on the magnetic coupling between 4f single atoms and a 3d magnetic nanoisland. Thulium and lutetium adatoms deposited on iron monolayer islands pseudomorphically grown on W(110) have been investigated at low temperature with scanning tunneling microscopy and spectroscopy. The spin-polarized current indicates that both kind of adatoms have in-plane magnetic moments, which couple antiferromagnetically with their underlying iron islands. Our first-principles calculations explain the observed behavior, predicting an antiparallel coupling of the induced 5d electrons magnetic moment of the lanthanides with the 3d magnetic moment of iron, as well as their in-plane orientation, and pointing to a non-contribution of 4f electrons to the spin-polarized tunneling processes in rare earths.

Magnetic properties of cubic RxY1−xAl2 (R=Dy, Tb) intermetallic random anisotropy magnets (invited)

A short review is made of the key magnetic properties of dilute cubic RxY1−xAl2 (R=Tb, Dy) intermetallics, in order to show their main magnetic features. Dilution by Y introduces a weak random magnetic anisotropy (RMA). The rich magnetic phase diagram is described, including paramagnetic (P), spin glass (SG), correlated spin glass (CSG), random ferromagnetic (RFM), and ferromagnetic (F) phases, with a triple point and two multicritical ones. The paper deals with the induced macroscopic magnetic anisotropy cooling in a magnetic field below TSG or TCSG transition temperatures, which can be either unidirectional or uniaxial or both. High‐field (3 T) magnetostriction in Tb series shows, for x=0.48, 0.59, 0.87, a decrease of the Callen α exponent (λt∼mα, m=reduced magnetization) below 3. The Sompolinsky irreversibility parameter Δ has been determined for the Tb series in the SG regime, and a replica model is presented to explain the Δ(T) dependence. The character of the P→SG or P→CSG transitions is addressed, ...