Magneto-optical Kerr spectroscopy of a new chemically ordered alloy:Co3Pt

Abstract

The sensitivity of magneto-optical spectra to alloy chemical ordering is established. This is demonstrated for a new ordered CosPt phase, which was synthesized in thin film form using molecular beam epitaxial growth. This compound consists of alternating planes of Co and CoPt atoms with a hexagonal ABAB... layer sequence. The chemical ordering is accompanied by a substantial electronic structure change as reflected in a new peak in magneto-optical spectra at ~ 3.2 eV photon energy PACS numbers: 78.20.Ls, 61.66.Dk, 81.30.Bx Magneto-optical effects have attracted a lot of attention recently in research of magnetic thin films and multilayers. Strong resonance effects due to reduced optical constants [1,2], quantum confinement effects in ultrathin Fe layers [3], oscillations of the Kerr rotation with magnetic layer thickness due to quantum well states [4,5] and strong correlations between the magneto-optical Kerr effect (MOKE), and magnetic anisotropies [6] are among the most exciting recent discoveries in this field. While magnetostructural correlations have been known for a long time [7], the possibility of chemical ordering-related effects in the magneto-optical spectra has been suggested only recently [8,9]. With the advent of improved bandstructure-based magneto-optical calculations [10], it is possible to understand magneto-optical transitions from first principles and predict the effects of material structure upon Kerr rotation. In particular, the effect of chemical ordering on magneto-optical properties provides an important test of current theory. Co-Pt alloys provide an attractive candidate material for such studies since the Co-Pt bulk phase diagram reveals a continuous series of solid solutions with ferromagnetic order up to 90 at. % Pt at room temperature and the existence of chemically ordered CoPt and CoPt3 phases [11]. No chemically ordered compound is known for the composition CoaPt [11], although an LI2 cubic phase has been predicted in a recent calculation by Sanchez et at [12]. In this Letter we report the observation of a chemically ordered Co3Pt structure in films grown by molecular beam epitaxy (MBE). More importantly, we report the first unambiguous correlation between chemical ordering and magneto-optical Kerr rotation. 1000 A thick Co77Pt23 alloy films were deposited by MBE at various temperatures onto 10 A Pt buffer layers which were grown at 875 K on sapphire (0001) substrates. This results in high quality (111) [or (0001)] oriented alloy films [13]. The deposition chamber was maintained at a pressure < 4 x 10~1 0 mbar throughout growth. Electron gun sources were used for both Co and Pt, with deposition rates in the range 0.05-0.2 A/s. All compositions were checked with x-ray fluorescence. The spectral dependences of the polar Kerr effect were measured in saturating fields of ±20 kOe in the photon energy range 0.8-5.3 eV [14]. The saturation magnetization was determined using vibrating sample magnetometry. All magnetic and magneto-optic measurements were made at room temperature.

Modification of Magnetic Properties of Pt/Co/Pt Films by Ga+ Ion Irradiation: Focused versus Uniform Irradiation

“…For metallic compounds, it was known that order/disorder transitions can occur under irradiation, but for long range disorder/order transition additional thermal annealing during or after irradiation is necessary [50,2]. Recent spectroscopic magneto-optical investigations [18] revealed a negative PKR peak at 3.2 eV in a comparable weakly irradiated (in branch 1) Pt/Co(2.6 nm)/Pt sputtered film (with F = 10 14 Ga + ions/cm 2 ) that can be the signature of an hcp Co 0.75 Pt 0.25 ordered alloy phase with high magnetic anisotropy, as already reported by several authors [51][52][53][54]. According to the simulation results [24] the amount of local chemical ordering phase is not significant but together with induced tensile in-plane strain [17,24] can explain the restored PMA in the "branch 1" region of the (d Co , F ) phase diagram [22].…”

“…In the future, more direct experiments would be necessary to make structural confirmation this ordered alloy and its precise spatial profile in the film structure at a nanometer scale. Such an experimental task is rather difficult even for high resolution transmission electron microscopy analysis of film cross-section due to the small ratio of in-plane correlation length hcp Co 0.75 Pt 0.25 structure -0.8 nm [51] to the analyzed slice thickness.…”

“…For irradiation at low fluence, magneto-optical spectroscopy results allowed to identify the presence of additional Co-Pt alloy, similar to hcp Co 0.75 Pt 0.25 ordered alloy phase [51][52][53][54] with huge PMA that can be initiated in the vicinity of Co/Pt interfaces. In the future, more direct experiments would be necessary to make structural confirmation this ordered alloy and its precise spatial profile in the film structure at a nanometer scale.…”

Development of Media Nanostructure for Perpendicular Magnetic Recording

“…Figure 15 shows the typical crystal structures of high Ku magnetic materials. Table 2 lists the structural and magnetic properties of high Ku magnetic materials [127][128][129][151][152][153][154][155][156] . The degree of atomic ordering in a magnetic material, where a higher order degree corresponds to a higher Ku value, depends on the processing condition, in particular, the temperature.…”

Structure analysis of CoPt alloy film with metastable ordered phases ofL11andBhformed on Ru(0001) underlayer

“…There is thus a possibility that an hcp-based ordered phase of B h shown in figure 1(b) is formed in CoPt films. However, there are few reports on the structural analysis of films including ordered phases by taking into account the stacking sequence of close-packed plane [7,8,10]. The stacking sequence of Co-based alloy film is influenced by the substrate or underlayer material [11].…”