M. Maret

Structural order related to the magnetic anisotropy in epitaxial (111) CoPt3 alloy films

Abstract (111) Co x Pt 1− x epitaxial films (0.25 ≤ x ≤ 0.33) grown on a (0001) Ru buffer at temperatures ranging from 500 to 800 K were investigated. The dependence of the perpendicular magnetic anisotropy on the growth temperature is clearly related to the structural state of the alloy films. The largest perpendicular anisotropy found in a film grown at 690 K is correlated with the existence of a compressive strain resulting from preferential heteroatomic correlations oriented along the growth direction [111], driven by both Pt surface segregation and associated dominant surface diffusion. By contrast, the observation of L1 2 long-range chemical ordering in a film grown at 800 K, favoured by an increase of volume diffusion, is accompanied by the disappearance of perpendicular anisotropy.

FePtCu alloy thin films: Morphology, L10 chemical ordering, and perpendicular magnetic anisotropy

Rapid thermal annealing was applied to transform sputter-deposited Fe51Pt49/Cu bilayers into L10 chemically ordered ternary (Fe51Pt49)100−xCux alloys with (001) texture on amorphous SiO2/Si substrates. It was found that for thin film samples, which were processed at 600 °C for 30 s, the addition of Cu strongly favors the L10 ordering and (001) texture formation. Furthermore, it could be revealed by transmission electron microscopy and electron backscatter diffraction that the observed reduction of the ordering temperature with Cu content is accompanied by an increased amount of nucleation sites forming L10 ordered grains. The change of the structural properties with Cu content and annealing temperature is closely related to the magnetic properties. While an annealing temperature of 800 °C induces strong perpendicular magnetic anisotropy (PMA) in binary Fe51Pt49 films, the addition of Cu systematically reduces the PMA. However, due to the enhancement of both the A1-L10 phase transformation and the developm...

Control of magnetic anisotropy and magnetic patterning of perpendicular Co∕Pt multilayers by laser irradiation

We report an approach to altering the magnetic properties of (111) textured Co∕Pt multilayer films grown on sapphire (0001) substrates in a controlled way using single-pulse laser irradiation. The as-grown films reveal a strong perpendicular magnetic anisotropy induced by interfacial anisotropy. We show that laser irradiation can chemically mix the multilayer structure particularly at the interfaces, hence reducing the perpendicular magnetic anisotropy and coercivity in a controlled manner depending on laser fluence. As a result, perpendicular films can also be magnetically patterned into hard and soft magnetic regions using a regular two-dimensional lattice of polystyrene particles acting as an array of microlenses.

Nanopatterned CoPt alloys with perpendicular magnetic anisotropy

CoPt alloy films with perpendicular magnetic anisotropy were grown on SiO2 nanoparticle arrays with particle sizes as small as 10 nm. In order to induce perpendicular magnetic anisotropy in the CoPt film, a MgO seed layer was sputter deposited. Despite the fact that neighboring CoPt film caps are interconnected, individual caps appear as single domain and for most of them their magnetization orientation can be reversed individually. This behavior might be caused by domain wall nucleation and pinning preferentially at the rim of each cap. Thus, arrays of magnetic caps with defined pinning sites can be considered as a percolated perpendicular medium.

Perpendicular magnetic anisotropy in CoPt3(111) films grown on a low energy surface at room temperature

Epitaxial (111)-oriented CoPt3 films were deposited on WSe2(0001) substrates at room temperature using molecular beam epitaxy. We observed strong growth induced uniaxial perpendicular magnetic anisotropy which has a maximum of 3.2×106 erg/cm3 and coercivity of about 200 Oe for films with thicknesses <6 nm. At a thickness larger than 6 nm the easy magnetization axis progressively rotates into the plane of the film as the film thickness is increased. The magnetic domain structure in films with perpendicular magnetic anisotropy was investigated by photoemission electron microscopy revealing a characteristic thickness dependence near the reorientation transition.

FePt films on self-assembled SiO2 particle arrays

Chemically L10 ordered (001) textured FePt thin films with perpendicular magnetic anisotropy can be grown on amorphous planar SiO2 substrate with an underlayer stack of [Pt (3nm)∕Cr (50nm)] adopting a [002] orientation when deposited at 350°C. This knowledge of optimum layer stack was transferred to self-assembled SiO2 particle arrays. While 330nm SiO2 particle arrays reveal perpendicular magnetic anisotropy with a remanence of almost one and a coercivity of 370mT, on 160nm particles, the FePt caps show a (111) texturing, leading to the random orientation of the easy axis of the magnetization.

Nonepitaxially grown nanopatterned Co–Pt alloys with out-of-plane magnetic anisotropy

A study on the structural and magnetic properties of 5-nm-thick Co–Pt alloy films grown on thermally oxidized SiO2/Si(100) substrates as well as on self-assemblies of spherical SiO2 particles with sizes down to 10 nm is presented. An out-of-plane easy axis of magnetization was stabilized at deposition temperatures as low as 250 °C in a broad composition range between 40 and 70 at. % of Pt. Owing to the low deposition temperatures, no chemical long-range order is found. Thus, the strong out-of-plane magnetic anisotropy is expected to be caused by anisotropic short-range order effects. The magnetic behavior of CoPt alloys with an equiatomic composition grown on arrays of SiO2 particles was found to be similar to those on planar substrates. Structural investigations using high-resolution transmission electron microscopy revealed that a continuous CoPt layer has been formed, covering the particle tops and connecting them. The magnetic CoPt caps exhibit an out-of-plane easy axis for all particle sizes; however...

Structural and magnetic properties of CrPt3(111) films grown on WSe2(0001)

We report on the growth-induced properties of 30 A thick CrPt3(111) deposits on surfaces of WSe2(0001) with a van der Waals interaction. Assemblies of (111)-oriented fcc grains are obtained with average lateral sizes ranging from 5 to 15 nm for growth temperatures increasing from 50 °C to 550 °C. The grains develop a partial L12-type chemical long-range order starting at a deposition temperature as low as 50 °C, much lower than reported for films grown on other suitable substrate surfaces. An ordering parameter of about 0.3 was found over a wide temperature range between 200 °C and 550 °C, for a deposition rate of 0.1 A/s. Lowering the deposition rate by a factor of 5 results in a higher ordering parameter of about 0.4. Above 400 °C, ordering is accompanied by a pronounced segregation of Se atoms toward the free surface, forming a (4×4) superstructure. All samples exhibit superparamagnetic behavior with blocking temperatures that scale with the activation volume of the ferrimagnetic grains. This work show...

Morphology of epitaxial magnetic alloy nanostructures grown on WSe2(0001) studied by grazing-incidence small-angle X-ray scattering

The morphology of epitaxial alloy nanostructures grown on a van der Waals-type WSe2(0001) surface was studied using grazing-incidence small-angle X-ray scattering (GISAXS). Assemblies of 111-oriented islands of (Co,Cr)Pt3 and (Co,Fe)Pt alloys were grown at different deposition temperatures, with nominal thicknesses from 0.1 to 3 nm, resulting in various island densities. Evaluation of the GISAXS patterns indicates that for similar growth conditions CrPt3 islands are flatter than CoPt or FePt islands and exhibit larger island volumes. These features are correlated with the better wetting behaviour and more negative formation enthalpy of the CrPt3 alloy. For dense arrays of self-assembled CoPt islands, much smaller island volumes are extracted from GISAXS experiments than are observed by scanning tunnelling microscope imaging, which indicates that only the upper parts of the islands contribute to the GISAXS signal. Another aspect that needs to be taken into account for interpreting GISAXS patterns is the sensitivity of GISAXS to facetting and thus its capacity to extract the island shape. The latter is strongly dependent on the island size. For islands with an average volume smaller than ∼20 nm3, the shape cannot be determined unequivocally. Furthermore, for dense island assemblies with some size dispersity, the identification of steep side-wall facets from the GISAXS patterns is not straightforward as observed for truncated tetrahedron-shaped CoPt3 islands.

The Cr local structure in epitaxial CrPt3(111) films probed using polarized x-ray absorption fine structure

The local atomic structure of CrPt3(111) thin films grown by molecular beam epitaxy at different temperatures (400 and 850 °C) has been investigated by polarized x-ray absorption spectroscopy (XAS) at the chromium K edge. Previous x-ray diffraction and magnetic studies have revealed that films grown at 850 °C present the L12 chemically long range ordered structure and are ferrimagnetic with the occurrence of perpendicular magnetic anisotropy (PMA). In contrast, films grown at 400 °C, which are non-magnetic, do not show any indication of L12-type long range order. From the polarization dependence of XAS, different chemical orders in the film plane and perpendicular to it are reported for the non-magnetic film: Cr absorbers are surrounded by only Pt atoms in the film plane while out of the film plane Cr nearest neighbours are observed. The higher deposition temperature of the ferrimagnetic film (850 °C) leads to a more isotropic structure reducing the Cr–Cr out-of-plane correlations, as expected from the L12 ordered structure. These findings show that ferrimagnetism in CrPt3 films is strongly sensitive to the Cr–Cr correlations. Furthermore, the remaining in-plane tensile deformation found in the ferrimagnetic film is too small for yielding the observed PMA. It is suggested that the strong PMA originates from the ferromagnetic coupling between Cr second-nearest neighbours lined up along the directions and which develop an orbital moment parallel to the spin moment as found by relativistic energy band calculations.