M. Kisielewski

Spin reorientation transitions in Pt/Co/Pt films under low dose Ga+ ion irradiation

An elegant route for tuning the magnetic anisotropy of ultrathin Co films by Ga+ ion irradiation is presented. The magnetic anisotropy of a Pt/Co(2.6 nm)/Pt film is first changed from in-plane to out-of-plane by uniform low dose Ga+ ion irradiation at 30 keV. When increasing the dose, a second spin reorientation transition toward the sample plane is also evidenced. This could be a way to design magnetic nanowires with perpendicular anisotropy, embedded in an in-plane magnetized environment, either by irradiation through a mask or focused ion beam. Tentative explanations on the origin of these two successive spin reorientations are proposed.

Drastic changes of the domain size in an ultrathin magnetic film

A general framework for the domain size in any ultrathin film with perpendicular magnetic anisotropy is here discussed. The domain structure is analyzed by using the classical theory taking into consideration the demagnetization field contribution to the domain wall energy. A sinusoidal model is considered to describe the domain structure while approaching, in two different cases, the monodomain state with in-plane magnetization. The first case is realized applying a large enough in-plane magnetic field. The second one is obtained by decreasing the perpendicular magnetic anisotropy, which is connected in many ultrathin systems with the increase of film thickness. A change in the domain size of several orders of magnitude is obtained while approaching the magnetization reorientation region. The minimal stripe domain period p=8πlex2/d is calculated from the sinusoidal model, where lex is the exchange length and d is the thickness of the film. The range of possible domain size changes in ultrathin films is p...

Magnetic ordering in ultrathin cobalt film covered by an overlayer of noble metals

The spatial distribution of magnetic properties studied by polar Kerr-effect-based magnetometry are carried out on an ultrathin cobalt wedge covered with a silver wedge whose slopes are perpendicular to each other and subsequently capped with a gold layer. The coercivity field was studied as a function of both cobalt thickness d and silver thickness h. A few monolayers of Ag were found to have a substantial influence on the coercivity field. A similar strong influence of silver coverage on the magnetic anisotropy field was observed. Domain structure evolution during magnetization reversal is investigated using an optical microscope. With an increase of the magnetic field, a magnetization reversal process occurs: first by domain nucleation and next by a domain wall movement towards the higher coercivity region of the sample. Changes of the fractal dimension of the domain wall as well as nucleation center density are investigated. A rapid increase of these parameters is observed while d increases towards the value of spin reorientation transition, which goes from easy-axis to easy-plane magnetization. Similar strong changes were observed while decreasing d to the lowest thickness available for observation. The problem discussed here, of tuning magnetic properties by silver–gold structure, is important from both a general physics and an application point of view, especially because of the possibilities for magnetic film patterning.

The effect of magnetostatic coupling on spin configurations in ultrathin multilayers

(Co/Au)N ultrathin multilayers with perpendicular and in-plane magnetic anisotropy were studied experimentally using a combination of ferromagnetic resonance, magneto-optical magnetometry and microscopy (with both in-plane and out-of-plane magnetization-sensitive longitudinal and polar Kerr effects), and magnetic force microscopy (MFM). Three-dimensional magnetization distributions were reconstructed from micromagnetic simulations complemented by the measured magnetic parameters of the multilayers and observations by Kerr microscopy and MFM. It is shown that, in the reorientation phase transition (RPT) zone – the range of anisotropy characterized by 0 < Q < 1 (the ratio of the anisotropy energy to be gained by magnetization along the easy axis perpendicular to the sample surface, and the magnetostatic energy of a uniformly magnetized layer along the surface normal) – the three-dimensional magnetization distributions consist of alternating pairs of vortices and half-antivortices, both with in-plane magneti...

Analysis of Magnetic Domain Sizes in Ultrathin Ferromagnetic Films

Ultrathin magnetic films are considered where the thickness-induced second order phase transition occurs due to surface magnetic anisotropy changes. By increasing the thickness, the easy magnetization axis state passes through the easy cone state (for film thickness t: t 1 < t < t 2 ) into the easy magnetization plane state. The characteristic material length l c (defined as the ratio of the domain wall energy density to the demagnetization energy) was calculated up to thickness t 2 . The exact determination of stripe domain period, p, was proposed, applying the transcendent Lerch functions to the classical Kooy-Enz model. Near t 2 where l c vanishes, the critical period, p c , was also discussed. Considering real sets of material parameters taken for ultrathin cobalt films, it was found that by increasing the thickness, the equilibrium domain period drastically decreases from practically infinity (many kilometers for t = 0.5 nm) into the nanometer scale near t 2 .

Magnetic states and magnetization reversal in magnetostatically coupled multilayers with low perpendicular anisotropy

Multilayers of (F/NF)N composition, where F means a ferromagnetic layer, NF a nonferromagnetic one, and N the number of repetitions, are studied by simulations and theoretically for different magnetic anisotropy characterized by the quality factor, Q (the ratio of the anisotropy energy to be gained by a magnetization along the easy axis perpendicular to the sample surface and the magnetostatic energy of a uniformly magnetized layer along the surface normal). It is shown that the range of the existence of out-of-plane magnetization states could be extended to Q<1 by proper choice of N and the layers thicknesses. The role of magnetostatic interlayer coupling in the formation of vortexlike and sinusoidal-like distributions of the magnetization is revealed. Different magnetization states are mapped onto a (Q,N)-phase diagram. We demonstrate that a (F/NF)N multilayer is a soft magnetic system in which the domain size could be changed by a few orders of magnitude by small variations of Q, N, and/or the NF space...

The growth modes of epitaxial Au/Co/Au sandwiches

Optimum growth conditions of epitaxial Au/Co/Au sandwiches with a strong perpendicular magnetic anisotropy have been investigated. The thermally induced evolution of the sandwich morphology, which determines its magnetic properties, was studied by means of reflection high-energy electron diffraction and Auger electron spectroscopy. The roughness of Au and Co surfaces, affected by the sample annealing, was evaluated from the length-dependent variance of topography acquired by atomic force microscopy.

Correlation of morphology and magnetic properties in ultrathin epitaxial Co films on Au(111)

Structural and magnetic properties of ultrathin Co films were studied by scanning tunneling microscopy and magneto-optics. Hundred nm Au(1 1 1) films grown epitaxially on mica were used as buffer layers. The initial Co nucleation at the elbows of the ‘‘herringbone’’ reconstruction determined the film morphology, which displayed long chains resulting from the coalescence of the self-organized Co nuclei. A height amplitude of the chains changed considerably upon film annealing. For the ex situ magnetic measurements, wedge samples were prepared, protected with a 5 nm Au layer. Using Kerr magnetometry it was possible to observe the spin reorientation transition. It turned out that the critical thickness decreases for annealed films. The magnetic surface anisotropy was determined and correlated with the film microstructure. 2002 Elsevier Science B.V. All rights reserved.

Magnetic Domains and Anisotropy in Ultrathin Au/Co/Au Wedges Deposited on Mica Substrates

Ultrathin cobalt films and wedges in gold envelope Au/Co(d Co 0) were determined for samples with different thicknesses. Annealing induced in the sample: (i) flattening of the cobalt surface; (ii) a decrease of the coercivity field and magnetic anisotropy caused a decrease of thickness of reorientation from easy axis state to easy plane one; (iii) an increase of the Barkhausen volume which contains many cobalt islands creating the chains. Approaching the reorientation region, the character of the magnetization reversal drastically changes from that with the preference of domain wall propagation into the one connected to the domain wall nucleation.

Magnetic coercivity of focused ion beam irradiated lines in a Pt/Co(1.4 nm)/Pt film

Regular sets of ultranarrow lines have been patterned on a Pt/Co(1.4 nm)/Pt ultrathin magnetic film by focusing a Ga+ ion beam (FIB) at different fluences. Ga+ ion irradiation is known to soften magnetic properties in such films. Even at low Ga+ ion fluence, and as proved by atomic force microscopy, the transverse perturbed region in the lines is much wider than calculated by Stopping and Range of Ions in Matter (SRIM) simulations, demonstrating that a swelling effect superimposes to more localized irradiation damages. Field-induced magnetization reversal and static or dynamic coercivity of these FIB lines separated by nonirradiated ferromagnetic tracks have been investigated by polar MOKE microscopy. The nucleation of small reversed domains is much easier in FIB lines irradiated at high fluence, and a remnant domain structure can be stabilized in that case. The coercive field of the FIB irradiated lines is calculated using a modified one-dimensional-droplet model.