Ch. V. Mohan

Critical behaviour near the ferromagnetic–paramagnetic phase transition in La0.8Sr0.2MnO3

Abstract We have investigated the critical behaviour of a La 0.8 Sr 0.2 MnO 3 polycrystalline sample near its ferromagnetic – paramagnetic phase transition temperature by determining the critical exponents corresponding to various thermodynamical quantities. Such a study is very important and interesting because this magnetic phase transition is associated with a simultaneous electronic phase transition, i.e. a metal–insulator transition. In this paper, we report our results on this magnetic phase transition analysed through various techniques like the modified Arrott plots, Kouvel–Fisher method, scaling-equation-of-state analysis and the critical magnetisation isotherm. The values deduced for the critical exponents are in good agreement with those predicted, theoretically, for the mean field theory, indicating that the exchange interaction is of long-range type.

Growth, structure and magnetic properties of Co ultrathin films on Cu(111) by pulsed laser deposition

Deposition of Co on Cu(111) substrate by laser ablation changes the morphology, structure and magnetic properties of Co ultrathin films. The pulsed laser deposition method induces a good layer-by-layer growth of Co films and delays the face centred cubic to hexagonal close packed structural phase transition in the films. As a result, the pulsed laser deposited films retain an in-plane easy axis of magnetization at all thicknesses investigated (0-15 monolayers Co) while the thermally deposited films also exhibit a hysteresis loop in the perpendicular direction as soon as the hexagonal close packed structure is dominant in the film. In addition, the suppression of hexagonal close packed stacking in the pulsed laser deposited films leads to a complete antiferromagnetic coupling in Co/Cu/Co/Cu(111) trilayers for both in-plane and out-of-plane magnetization directions.

Influence of growth temperature on the spin reorientation of Ni/Cu(100) ultrathin films

Ni/Cu(100) films were prepared by thermal deposition at room temperature (RT) and 170 K low temperature (LT) separately to study the influence of substrate temperature on the spin reorientation. The critical thickness of the LT grown films is observed to be about 1 ML smaller than that of the RT films. Though both types of films show similar tetragonal distortion and chemical composition, their morphology differs dramatically: the island density of the LT films is significantly higher than that of the RT films. We use this to interpret the different magnetic behavior between the RT and LT films.

Suppression of the face-centered-cubic-hexagonal-close-packed stacking fault in Co/Cu(111) ultrathin films by pulsed laser deposition

The thermal deposition of Co onto Cu(111) results in three-dimensional island growth and a face-centered-cubic-hexagonal-close-packed stacking fault, which hinders a complete antiferromagnetic coupling in Co/Cu(111) superlattices. We report that Co/Cu(111) films can be grown with good layer-by-layer morphology and significantly less stacking faults by pulsed laser deposition. We show that a complete antiferromagnetic coupling can be achieved in the pulsed laser deposited Co/Cu trilayer.

Artificial FeCu(100) epitaxial ordered alloy films: Element-selective magnetic properties

Epitaxial artificial FeCu alloy thin films in the fcc L10 ordered phase were prepared by pulsed laser deposition on Cu(100). Magnetic circular dichroism in x-ray absorption at the Fe and Cu L2,3 edges was used to study the electronic and magnetic properties of the normally immiscible FeCu(100) alloy films. The Fe exhibits magnetic moments comparable to that of fcc Fe films, ruling out the occurrence of nonmagnetic fcc Fe. The ratio between orbital and spin contribution to the Fe d moments is significantly enhanced in FeCu with respect to Fe/Cu(100), and amounts to ≈0.12. An induced magnetic moment of the Cu atoms is observed, which carries about 7% of the total d-band moment of the FeCu film.

Detailed magnetostriction and magnetomechanical studies on aluminium substituted Tb0.27Dy0.73Fe2 alloy

Abstract X-ray diffraction studies carried out on the Tb 0.27 Dy 0.73 Fe 2− x Al x (0 ≤ X ≤ 0.20) alloy system revealed that all the compositions investigated retained the C-15 cubic Laves phase structure. The alloys were prepared by arc melting followed by zone melting using an induction furnace. The d.c. field dependence of the magnetostrictive strain has been measured at room temperature. The incremental relative permeability and the magnetomechanical coupling coefficient have been obtained as functions of the external static magnetic field at room temperature. A comparison of the magnetostrictive responses indicates that the saturation magnetostriction as well as the magnetomechanical coefficient decrease with increasing Al concentration. A slight lowering of the dynamic strain coefficient with Al content is attributed to a reduction in magnetoelastic coupling, also seen in a lower magnetomechanical coupling coefficient.

Critical phenomena in amorphous thin- and ultrathin-film multilayers

In order to investigate the magnetic phase transition that occurs at the Curie temperature in magnetic thin- and ultrathin-film multilayers, we have carried out extensive magnetization measurements in the temperature range embracing the critical region on amorphous (Tb 0.27 Dy 0.73 ) 0.32 Fe 0.68 thin- and ultrathin-film multilayers separated by magnetic (Cr) or non-magnetic (Nb) interlayers. These data are analyzed by using the conventional methods like the modified Arrott plots, Kouvel—Fisher method, scaling-equation-of-state analysis and the magnetization isotherm at the Curie point and the critical exponents b, c and d which describe the phase transition based on the power laws of the spontaneous magnetization, the initial susceptibility and the critical magnetization isotherm, respectively, have been deduced. Depending on the type of the sample, we could see two types of transitions, either characterized by three-dimensional Heisenberg-like critical exponents or two-dimensional Ising-like. The reasons for this are discussed in detail. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 75.40.Cx; 75.30.Kz; 75.50Kj

The magnetic phase transition in Fe17Dy2 and Fe23Er6 single crystals

Abstract The magnetic phase transition in Fe 17 Dy 2 and Fe 23 Er 6 single crystals is investigated from detailed magnetisation measurements performed in the critical region. Two sets of measurements were performed while keeping the direction of the applied magnetic field along the easy plane as well as hard directions. Values of the Curie temperature ( T C ) and the critical exponents corresponding to the spontaneous magnetisation (β), initial susceptibility (γ′, γ) and the magnetisation isotherm at the Curie point (δ) are deduced from the analysis based on several methods: modified Arrott plots, the Kouvel-Fisher method, scaling plots and ln J versus ln( μ 0 H ) plots. Very good agreement has been observed between the exponent values obtained by the various methods. The values of β, γ and δ for Fe 17 Dy 2 are close to those predicted theoretically for a three-dimensional Heisenberg ferromagnet, while those for Fe 23 Er 6 do not belong to any known universality class. The exponents deduced from the measurements in the easy plane and hard directions vary only within our error limits.

Magnetic phase transition in amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Abstract We have obtained the critical exponents, the corresponding amplitudes and the universal amplitude ratios for the amorphous Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy from detailed bulk magnetization measurements performed in the critical region near the magnetic phase transition temperature. The critical exponents determined are β, γ, γ′ and δ, among which β describes the temperature dependence of the spontaneous magnetization, γ ( T ≥ T C ) and γ ′ ( T ≤ T C ) describe the temperature dependence of the zero-field susceptibility, and δ describes the field dependence of the magnetization at the Curie temperature, T C . The values of T C obtained in the present investigation match very well those reported in the literature. The critical exponent values are very close to those theoretically predicted for the three-dimensional Heisenberg spin system.

On the asymptotic critical behavior of Fe17Dy2, Fe17Ho2 and Fe17Er2 single crystals

The asymptotic critical behavior of Fe 17 Dy 2 , Fe 17 Ho 2 , and Fe 17 Er 2 single crystals is investigated by deducing the asymptotic and correction-to-scaling critical exponents from detailed magnetic polarization measurements performed in the civinity of the Curie temperature, where these systems undergo a magnetic phase transition from a ferrimagnetic to a paramagnetic state. these intermetallic compounds are characterized by a planar anisotropy for which, so far, measurements of critical exponents are not available. Well-defined phase transitions are found to exist in all these three syhstems with values of the asymptotic critical exponents very close to those predicted by the renormalization group calculations for a three-dimensional isotropic short-range Heisenberg ferromagnet. The nonanalytic corrections to the singular behavior at the Curie point, which originate from the nonlinear irrelevant scaling fields were found to have a significant role in deducing the true asymptotic critical amplitudes. Since the main interest of the present study lies only in the critical region, we have, for the time being, not investigated about these correction terms outside the critical region.