Growth-induced magnetic anisotropy and clustering in vapor-deposited Co-Pt alloy films (original) (raw)
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Journal of physics. Condensed matter : an Institute of Physics journal, 2010
The length scale of the local chemical anisotropy responsible for the growth-temperature-induced perpendicular magnetic anisotropy of face-centered cubic CoPt(3) alloy films was investigated using polarized extended x-ray absorption fine structure (EXAFS). These x-ray measurements were performed on a series of four (111) CoPt(3) films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preference for Co-Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, and random chemical order otherwise. Furthermore, atomic pair correlation anisotropy was evidenced only in the EXAFS signal from the next neighbors to the absorbing Co atoms and from multiple scattering paths focused through the next neighbors. This suggests that the Co clusters are no more than a few atoms in extent in the plane and one monolayer in extent out of the plane. Our EXAFS results confirm the correlation between perpendicular magnetic anisotropy and two-dimensional Co segreg...
Growth, structural, and magnetic properties of high coercivity Co/Pt multilayers
Journal of Applied Physics, 2001
Electron beam evaporated Co/Pt multilayers ͕͓Co(t Co nm͒/Pt͑1 nm͔͒ 10 , 0.2Ͻt Co Ͻ2 nm͖ with perpendicular magnetic anisotropy and room temperature coercivities H c ϭ2-15 kOe are studied as a function of growth temperature T G. Hysteresis loops and magnetic force microscopy ͑MFM͒ indicate changes in the magnetization reversal mechanism along with a sharp increase in coercivity for T G տ230-250°C. Films grown at T G Ͻ230°C (t Co ϭ0.2-0.4 nm͒ show micrometer size magnetic domains and rectangular hysteresis indicating magnetization reversal dominated by rapid domain wall motion following nucleation at H n ϳH c. Films grown at T G Ͼ250°C show fine-grained MFM features on the sub-100-nm length scale indicating reversal dominated by localized switching of small clusters. High-resolution cross-sectional transmission electron microscopy ͑TEM͒ with elemental analysis shows columnar grains extending throughout the multilayer stack. Co depletion and structural defects at the grain boundaries provide a mechanism for exchange decoupling of adjacent grains, which may result in the high coercivities observed.
Journal of Magnetism and Magnetic Materials, 2005
Co-rich, Co-Pt alloy films containing $20 at% Pt and small amounts of P, with thickness from 125 to 1000 nm have been galvanostatically (constant current) grown by electroplating onto Cu seed layers with strong (1 1 1) orientation. The influence of deposition current density (cd ¼ 10-50 mA/cm 2 ) and film thickness on their growth morphology, structural and magnetic properties have been investigated. When electrodeposited on Cu(1 1 1), Co-Pt(P) films develop a microstructure consisting of a disordered hexagonal-closed-packed (hcp) matrix with {0 0 0 1} preferential orientation. At low cd, a small amount of simple cubic L1 2 phase was detected, which disappears altogether by increasing cd to 50 mA/cm 2 . The plated films show saturation magnetization in the range 775-832 kA/m (778-832 emu/cm 3 ), large perpendicular magnetic anisotropy up to 1.02 MJ/m 3 (1.02 Â 10 7 erg/cm 3 ), and coercivity up to 486 kA/m (6.1 kOe) in the out-of-plane direction. The perpendicular anisotropy was found to originate predominantly from the high magnetocrystalline anisotropy of the hcp phase of Co-Pt(P) with c-axis perpendicular to the substrate. r
Physical Review B, 1999
We have simultaneously grown ͓Co(t Å)/Pt(10 Å)͔ 30 (tϭ2-10 Å) multilayers on Pt͑311͒ and Pt͑111͒ planes by molecular beam epitaxy. The Pt͑311͒ and Pt͑111͒ seeding layers were prepared on Mo͑211͒/Al 2 O 3 (1100) and Mo͑110͒/Al 2 O 3 (1120), respectively. New epitaxial relations between Pt and Mo layers in both cases are established by electron and x-ray diffraction. The Co/Pt multilayers on Pt͑311͒ were grown mainly as a ͑311͒ structure but mixed with a small amount of ͑211͒ phase. The Co/Pt͑311͒ multilayers display large perpendicular magnetic anisotropy with results comparable to those of the ͑111͒ samples. In addition, the Co/Pt͑311͒ multilayers exhibit stripelike magnetic domain structure with domain displacement growing along the ͓011͔ azimuth, in marked contrast to the ͑111͒ multilayers which show a very irregular domain structure. ͓S0163-1829͑99͒04401-X͔ PHYSICAL REVIEW B
Magnetic properties of thin films of Co and of (CoPt) superstructures on Pt(100) and Pt(111)
Physical Review B, 1999
The magnetic properties of Co n /Pt(100), Co n /Pt(111), (Co 0.5 Pt 0.5) n /Pt(100), (CoPt) n /Pt(100), and (CoPt) n /Pt(111), nр15, are investigated using the relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only the artificial superstructures (CoPt) n /Pt(100) and (CoPt) n /Pt(111) show a perpendicular magnetic anisotropy beyond nϭ10. For the free surfaces of Co on Pt in the case of the ͑100͒ orientation, a multiple reorientation transition below nϭ7 is found, while along ͑111͒ such a transition is predicted at about four layers of Co. For the homogeneous, statistically disordered alloy Co 0.5 Pt 0.5 on Pt͑100͒ the orientation of the magnetization remains in-plane for all n investigated. A comparison to experiment yields interesting insight into aspects of order and disorder, surface segregation, and phase separation frequently encountered in experimental studies of perpendicular magnetism in the Co/Pt system.
Magnetization processes in hard Co-rich Co–Pt films with perpendicular anisotropy
Journal of Applied Physics, 2006
We present a study of the magnetic properties and magnetization processes in hard Co-Pt ͑Pt ϳ 20 at. % ͒ films. Co-rich Co-Pt films, with thickness t ranging from 5 nm up to 2 m, were prepared by electrodeposition on ͑0001͒-oriented Ru underlayers. All samples displayed strong perpendicular magnetic anisotropy and high coercivity. Virgin magnetic domain structures for varying thickness were investigated by magnetic force microscopy ͑MFM͒. The observed increase of domain width with film thickness is well understood by full two-dimensional micromagnetic computations with no adjustable parameters. The easy-axis magnetization process, as observed by measuring virgin curves by magnetometry and imaging the corresponding magnetization configurations by MFM in variable field, consists of two stages separated by a well-defined critical field, marking the onset of domain wall propagation. A thorough analysis of the out-of-plane angular dependence of the switching field points out that unpinning of domain walls is the dominant magnetization process.
Magnetic and structural properties of Co/Pt multilayers
Journal of Magnetism and Magnetic Materials, 1991
The magnetic properties of Co/Pt multilayers, in particular anisotropy and coercivity, are very sensitive to the Co layer thickness and less dependent on the Pt layer thickness. Such dependence is illustrated and discussed for e-beam evaporated Co/Pt multilayers of various Co and Pt thicknesses. It is shown that the largest perpendicular anisotropy occurs for Co layer thickness of 1–2 monolayers. The magnetic anisotropy of Co/Pt multilayers also strongly depends on the crystallographic orientation. The largest perpendicular magnetic anisotropy was obtained for Co/Pt multilayers with Pt(111) parallel to the film plane. This was demonstrated in both evaporated and sputtered polycrystalline multilayers, as well as in molecular beam epitaxy (MBE) grown Co/Pt(111) superlattices. By contrast, MBE grown 3.7 Å Co/16.8 Å Pt(100) multilayers show in-plane anisotropy, and 3.7 Å Co/16.8 Å Pt(110) multilayers exhibit very strong anisotropy within the film plane. These observations suggest that the magneto-crystalline anisotropy plays a key role in the magnetic anisotropy of these structures. Significantly enhanced effective magnetization was observed for Co/Pt multilayers with ultrathin Co layer.
Journal of Applied Physics, 2008
The effects of growth rate ͑G r ͒, deposition temperature ͑T d ͒, film thickness ͑t F ͒, and substrate-induced strain ͑⑀͒ on morphological, crystallographic, and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with pulsed laser deposition are investigated. The ͑001͒-oriented single-crystal substrates of MgO, SrTiO 3 , and LaAlO 3 provide different degrees of epitaxial strain for growth of the disordered face-centered cubic ͑fcc͒ and ordered face-centered tetragonal ͑L1 0 ͒ phases of CoPt. The films deposited at T d Ϸ 600°C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width Ϸ200 Oe. The L1 0 phase, stabilized only at T d Ն 700°C, becomes predominantly c-axis oriented as T d is increased to 800°C. While the crystallographic structure of the films depends solely on the T d , their microstructure and magnetization characteristics are decided by the growth rate. At the higher G r ͑Ϸ1 Å/ s͒ the L1 0 films have a maze-like structure which converts to a continuous film as the t F is increased from 20 to 50 nm. The magnetic coercivity of these films increases as the L1 0 phase fraction grows with T d and its orientation becomes out of the film plane. The evolution of microstructure with T d is remarkably different at a lower growth rate ͑Ϸ0.4 Å / s͒. Here, the structure changes from a self-similar fractal pattern to a disordered assembly of nanodots as the T d is raised from 700 to 800°C, and is understood in terms of the imbalance between strain and interfacial energies. Magnetic force microscopy of such films reveals no distinct domain walls within the nanoislands, while a clear contrast is seen between the islands of reversed magnetization. Magnetic relaxation measurements on these assemblies of single-domain islands show a negligible decay of magnetization unless a reverse field close to the coercive field ͑H c Ϸ 30 kOe͒ is applied. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.
physica status solidi (a), 2010
Co-rich Co-Pt alloy films have been grown by electrodeposition on Au buffer layer. Detailed texture measurements have been performed in order to understand the dependence of magnetic properties on phase composition and texture. X-ray analysis in Bragg-Brentano geometry shows that films seem to consist of hexagonal closed packed (hcp) or fcc phase. Due to similar lattice spacing of the densely packed planes of both possible phases, the (111) fcc and (002) hcp planes of Co-Pt and also the higher order reflections coincide. As a result, the information supplied by X-ray diffraction was of limited value. Hence pole figure measurements are used which determine the directional distribution of planes with a selected spacing. Here we present an approach to use pole figure measurements for phase identification. We show that this method allow a direct correlation with the magnetic properties, in particular with the magnetocrystalline anisotropy and magnetic texture.