J.M. Alameda

Effects of the initial stages of film growth on the magnetic anisotropy of obliquely-deposited cobalt thin films

Abstract We have studied the in-plane magnetic anisotropy in polycrystalline cobalt films, which were obliquely deposited by e-beam evaporation in UHV at T s = 300 K. Three film series were studied, with average film thicknesses t = 15, 45 and 100 nm. For all cases, the incidence angle of the vapor beam with respect to the surface normal was α = 0, 30, 40, 50, 60 and 70°. At normal incidence, the anisotropy and the coercive fields were found to be H K = 15–25 Oe, and H c ⋍ 25 Oe , independent of the film thickness. In all series it has been observed that, as α increases, the easy axis of in-plane anisotropy switches from perpendicular to parallel with respect to the incidence plane of atoms during film deposition. For t = 15 nm, such a transition occurs at α t ⋍ 70° , whereas for larger values of t we found that α t = 60°. At constant film thicknesses, H K and H c increase with increasing α; the lower the film thickness, the sharper this effect, e.g. for α = 40°, and t = 100, 45 and 15 nm, H K ⋍ 15 , 30 and 150 Oe, respectively. We analyzed two possible contributions to this effect: the shape anisotropy and the magnetocrystalline anisotropy (texture). At low film thicknesses the former becomes dominant. In order to understand the effect of oblique incidence on the in-plane easy axis location we studied the polar plot of reduced remanence m r as a function of both the angle α (between the surface normal and the vapor beam) and the angle γ (between the in-plane applied field and the normal to the incidence plane of atoms during deposition). The behavior of m r ( α , γ ) for α = 10° and 70° is reminiscent of that predicted by the coherent-rotation Stoner and Wohlfarth model, although fallback processes are observed. We found that the films tend to be magnetically isotropic for α = 60°, whereas they are highly anisotropic for α = 10°.

On the physical origin of in-plane anisotropy axis switch in oblique-deposited thin films

Abstract We propose a simple physical model, based on the occurrence of mixed anisotropies (rod and plate-like shape anisotropies), justified from experimental structural evidences, to explain the behaviour of magnetic anisotropy in oblique-deposited thin films. From this model the common features observed in polycrystalline thin films for high incidence angles, may be qualitatively explained. In particular the switch from positive to negative in-plane component of magnetic anisotropy at α ≈ 70° is fully established. Moreover, the in-plane and off-plane components of anisotropy may be related through a phenomenological packing parameter closely connected with structural observations.

Magnetic properties of sandwich films based on RCo amorphous alloys (R = rare-earth, Y or Zr)

Abstract The magnetic properties of sandwich films, consisting of a soft amorphous CoZr layer placed between two SmCo layers with different coercivities, have been studied. Magnetization reversal in the different layers occurs under different field values and domain walls are then created (or annihilated) at the interface, due to the exchange interactions existing between the Co layers. A peculiar magnetization state has been stabilized in which the magnetization in the two SmCo layers are of opposite directions and a domain wall divides the CoZr central layer into two regions. In this state the magnetization processes are almost completely reversible, in contrast to the irreversible process observed in a single CoZr layer. The magnetization reversal mechanisms in this type of systems have been studied in YCo/GdCo bilayer, using transverse Kerr effect measurements. Nucleation/propagation has been found to occur in all instances.

Surface induced magnetic and magneto-optical properties in amorphous CoxY1−x thin films

Abstract Room temperature magneto-optical spectroscopy (transverse Kerr effect) (MOKE) and transverse biased initial susceptibility (TBIS) measurements have been carried out in amorphous Co x Y 1 − x thin films (thickness: 1000 A). The chemical composition range studied was 0.50 ⩽ x ⩽0.80. The MOKE, TBIS and coercive force show a marked dependence on x . For x > 0.65 the MOKE parameter δ k is positive and TBIS behaviour leads to a magnetization ripple contribution to the effective field. When x k is negative and TBIS behaviour may be explained by a cluster model of non-interacting magnetically ordered regions and random distribution of associated anisotropies. At x = 0.65, two magnetic phases become apparent from MOKE a TBIS behaviour. These facts are explained by the occurence of sublayer Co-enriched regions induced by surface segregation of Y. Optical effects in protective Y-oxide layers may be at the origin of the change in Kerr effect polarity.

Co anisotropy in amorphous Y1-xCox films

Abstract Transverse biased initial susceptibility measurements have been performed in amorphous Y1-xCox films. In the framework of the random anisotropy model for amorphous magnetism, a local anisotropy constant for Co of the order 107 erg/cm3 may be inferred from the results. This agrees with the low symmetry of environments in the amorphous state.

Direct observation of interlayer exchange coupling by MOKE in amorphous RE-TM sandwiches

Abstract Here we report transverse bias initial susceptibility (TBIS) measurements on amorphous GdCo 2 /YCo 2 /GdCo 2 (Gd/Y/Gd) and YCo 2 /GdCo 2 /YCo 2 (Y/Gd/Y) sandwiches (1000 A/1000 A/1000 A) in order to study the interlayer exchange coupling. TBIS was measured by using the magneto-optic Kerr effect. The sandwiches layer of the Gd/Y/Gd and Y/Gd/Y system reverses its magnetization at an applied magnetic field determined by the compensation between the gain in Zeeman energy and the loss in wall energy. It is observed that the magnetization reversal of the inner layer cannot occur by a coherent-rotation process.

Analysis of magnetization rotational processes in amorphous and crystallized Fe-Si thin films

Abstract Analysis of magnetization rotational processes under the simultaneous application of crossed alternating and steady dc fields has been made. Amorphous Fe0.65Si0.35 and related thin films crystallized in vacuum are studied. Thickness was varied between 300 and 3000 A. Both ripple and stripe domain contributions are discussed.

Magnetic coupling in YCo2/(Mo)/GdCo2 amorphous sandwiches evidenced by Kerr effect

Abstract Magnetic properties of YCo 2 /(Mo)/GdCo 2 amorphous sandwiches, in which the thickness of individual YCo and GdCo layers was 100 nm, were studied. Three cases were considered, viz., YCo/GdCo bilayers and YCo/Mo/GdCo sandwiches where the thickness of Mo was either 50 nm or close to 2 nm. Magnetic coupling effects were analyzed by transverse biased initial susceptibility measurements.

Magnetic properties of amorphous NdFeB thin films

Abstract Transverse biased initial susceptibility (TBIS) measurements have been performed in as-prepared and in UHV annealed NdFeB and FeB amorphous films. The dependence of magnetic anisotropies (local and macroscopic) on substrate temperature during deposition ( T s = RT and 77 K) and subsequent annealing treatments is discussed.

Magnetic and magneto-optical properties of NdFeB/Mo and NdFeB/FeB amorphous bilayers

Abstract Magnetic and magneto-optical properties of NdFeB/Mo and NdFeB/FeB amorphous bilayers, grown by dc triode sputtering on cooled glass substrates, were studied. Transverse Kerr effect, transverse biased initial susceptibility and SQUID magnetometry measurements were performed. Glass/film interface effects, regarding magnetization and anisotropies (dead layers, isotropic sublayers), were found in NdFeB films.