Sang-Koog Kim

Alloy-like Co environment in Co/Pd multilayer films having perpendicular magnetic anisotropy

Short-range atomic structures including the chemical environment and interatomic distance along the in- and out-of-plane directions in [Co (1 ML)/Pd(3 ML)]13 multilayer films (ML notes monolayer) are studied using polarized Co K-edge extended x-ray absorption fine structure. The chemical Co environment is found to be isotropic. The alloy-like character is dominant at interfaces in typical Co/Pd multilayers and yields perpendicular magnetic anisotropy (PMA) through the strain anisotropy of Co atoms. The broken symmetry, generally considered as of PMA origin, is not definitely necessary to result in PMA in Co/Pd multilayers.

In situ magnetoelastic coupling and stress-evolution studies of epitaxial Co35Pd65 alloy films in the monolayer regime

We report in situ measurements of magnetoelastic coupling, B2, and stress, σ, in Co35Pd65 alloy films epitaxially grown on a Cu/Si(001) substrate in a thickness range of 1–10 ML by means of a highly sensitive optical deflection-detecting system. It was found that the value of B2 increases from 0.72×107 J/m3 at 2 ML to 3.31×107 J/m3 at 10 ML. A second-order strain correction of B2=Bb+C1e+C2e2 rather than a first-order one of B2=Bb+C1e provides a better fit for the observed behavior of B2 versus film strain, e, where Bb is the bulk value. The relationship between B2 and e observed in the present study reveals that the second-order correction is crucial for understanding the dependence of B2 on e in an ultrathin regime.We report in situ measurements of magnetoelastic coupling, B2, and stress, σ, in Co35Pd65 alloy films epitaxially grown on a Cu/Si(001) substrate in a thickness range of 1–10 ML by means of a highly sensitive optical deflection-detecting system. It was found that the value of B2 increases from 0.72×107 J/m3 at 2 ML to 3.31×107 J/m3 at 10 ML. A second-order strain correction of B2=Bb+C1e+C2e2 rather than a first-order one of B2=Bb+C1e provides a better fit for the observed behavior of B2 versus film strain, e, where Bb is the bulk value. The relationship between B2 and e observed in the present study reveals that the second-order correction is crucial for understanding the dependence of B2 on e in an ultrathin regime.

Influence of substrate roughness on spin reorientation transition of ultrathin Co films on Pd(111)

We report a drastically different behavior in spin reorientation transition of ultrathin Co films grown on smooth versus rough Pd(111) single crystal substrates. The morphology and magnetic anisotropy of epitaxially grown Co films have been studied with in situ scanning tunneling microscopy (STM) and surface magneto-optical Kerr effects. On an atomically flat substrate, a smooth transition from perpendicular to in-plane magnetization occurs in a thickness interval of over 1.5 ML, beginning at ∼4.5 ML. In contrast, rough substrate causes the transition to begin quite earlier at ⩽3 ML and complete abruptly in less than 1 ML range. Morphology difference of the Co films obtained with STM suggests that on rough substrate, nonuniform coverage of Co leads to locally thicker regions triggering earlier spin reorientation transition.

Growth and magnetic properties of ultrathin Co films on Pd(111) investigated by ultrahigh vacuum in situ surface magneto-optical Kerr effect and scanning tunneling microscope

Growth and magnetic properties of Co ultrathin films on Pd(111) surface are studied with a ultrahigh vacuum scanning tunneling microscope (STM) and in situ surface magneto-optical Kerr effect measurement as a function of Co thickness. STM images of Co-covered surface reveal that the growth proceeds as a good layer-by-layer epitaxy up to a thickness of 2 ML. Image at a coverage of 1 ML shows almost complete monolayer coverage instead of disconnected islands. At higher coverages, the surface shows uniformly distributed three-dimensional (3D) clusters of ∼5 nm size manifesting 3D-like growth mode. Co films begin to show ferromagnetism at a coverage of 1.5 ML with a strong perpendicular magnetic anisotropy, which persists up to 5 ML. For the Co films thicker than 5 ML, reorientation transition from perpendicular to in-plane magnetic anisotropy is observed to occur over a thickness range of ∼1.5 ML. Interestingly, in this thickness range, Pd overlayer is found to drastically change the magnetic behavior.

Spin engineering in ultrathin Co0.35Pd0.65 alloy films

The easy axis of magnetization in CoxPd1−x alloy films with x=0.35 is controllably engineered by varying the thickness, tPd, of the Pd overlayers directly deposited on the alloy layers. In a Pd(50 A)/CoPd (20 A)/Pd (tPd) sample with a 10-A-height step-wedge Pd layer, the easy axis smoothly changes from in-plane orientation (tPd=0 A) through canted out of plane (0<tPd<30 A) to perpendicular (30⩽tPd⩽60 A). We also demonstrate that the spin switching is controllably reversible between in-plane and perpendicular orientations when the individual constituent layers of CoPd and Pd are alternately deposited. Smoothly continuous spin reorientation in a Pd (50 A)/CoPd (30 A)/Pd (tPd) film with increasing tPd in a broad range of 0–150 A convincingly evidences the magnetoelastic anisotropy origin for the observed spin switching.

Contrasting hysteresis behavior between the magneto-optical Kerr rotation and ellipticity in NiFe/FeMn/Co trilayer films

We report an experimental observation of strikingly contrasting hysteresis behaviors between the longitudinal magneto-optical Kerr rotation, θK, and ellipticity, eK, in NiFe/FeMn/Co films. For example, one is likely to reproduce a typical magnetic hysteresis loop obtainable with vibrating sample magnetometry while the other one does reproduce magnetoresistance (MR)-like curves in some cases. Interestingly, θK/eK loops form inverse MR curves. The contrasting hysteresis behavior could be explained by the discriminate sensitivity of θK and eK to the ferromagnetic NiFe as well as Co layers. Using a simple model, we determine the sensitivity ratios of the Co to NiFe layer for both signals, which are far from unity as much as θKCo/θKNiFe=−1.4 and eKCo/eKNiFe=8.5. These values may allow one to understand the anomaly of the remarkably contrasting loops observed in NiFe/FeMn/Co trilayer films.

In situ vectorial magnetization reversal study of ultrathin Co films on Pd (111) using magneto-optical Kerr effects

We have investigated magnetization reversal of ultrathin Co/Pd (111) films via in situ determination of three-dimensional magnetization orientations using magneto-optical Kerr effects of p and s waves. We find that with increasing the Co thickness the easy axis of magnetization is switching from normal-to-plane to in-plane through a stable canted phase and magnetization reversal under an applied magnetic field shows very contrastive behavior depending on the easy axis of magnetization. In particular, magnetization reversal via spiral motion is observed in the Co film of the canted phase which could be explained by an existence of in-plane anisotropy and an applied field slightly tilted from the film normal.

Magnetoelastic properties of epitaxially grown Co35Pd65 alloy films on Cu/Si(001)

Growth stress, magnetoelastic coupling, and growth structure of epitaxially grown Co35Pd65 alloy films on Cu/Si(001) are investigated in the monolayers regime via submonolayer sensitivity stress/magnetoelastic coupling measurements system and scanning tunneling microscopy (STM). In situ stress and STM study revealed that the formation of three-dimensional (3D) islands is a dominating stress relaxation mechanism in Co35Pd65 alloy films. The magnetoelastic coupling of Co35Pd65 alloy films, determined from the curvatures of a substrate, was found to be sensitively dependent on the alloy thickness in the thickness range of 2–10 ML: it is increased from 0.72×107 to 3.31×107 J/m3 with increasing Co35Pd65 layer thickness.

Reversible spin-reorientation transition in Co0.35 Pd0.65/Pd multilayer films

Abstract We report a reversible spin-reorientation transition between in-plane and out-of-plane orientation in Co 0.35 Pd 0.65 /Pd multilayer films, in situ measured with magneto-optical Kerr effects upon alternate depositions of the ultrathin CoPd and Pd layers. A misfit-strain modulation produced by the depositions of the alternating constituent layers might be one of promising candidates to controllably engineer spin orientations without applying an external magnetic field.