Surface Magneto-optic Kerr Effect Research Articles

Low-temperature growth favours hcp structure, flatness and perpendicular magneticanisotropy of thin (1–5 nm) Co films on Pt(111)

The structural and magnetic properties of thin Co films grown onPt(111) were investigated in situ under UHV using simultaneous surfacex-ray diffraction, resonant magnetic surface x-ray diffraction at the PtLIII absorption edge and the surface magneto-optic Kerr effect. We focus on the difference betweenlow-temperature growth and growth at 300 K. Thin (1–5 nm) Co films grown at 120 K(low-T films) have the hcp structure and a small concentration of stacking faultscompared to films of the same thickness grown at 300 K (RT films), which containmany stacking faults and have a predominant fcc structure. In addition, thelow-T films are flatter. The RT films exhibit a clear reorientation transition of themagnetization, from perpendicular to parallel, at a thickness of 1 nm. In contrast, thelow-T films display intense polar Kerr signals up to the largest investigatedthickness (5 nm). In these films, the reorientation transition takes placeover a large thickness range, through a canted phase. Heating up alow-T film to 300 K induces a rotation of the average easy axis towards the film plane, still with ahigh polar Kerr signal at 300 K. This behaviour with temperature is mostly reversible.These results are discussed on the basis of the temperature dependence of the anisotropyconstants and magnetocrystalline anisotropy differences between the fcc and hcp phases.

Magnetic properties of ultrathin cobalt films on SiO2 substrates

Thickness dependent magnetism of the ultrathin Co films grown on SiO2 substrates was studied by surface magneto-optical Kerr effect in ultrahigh vacuum. The complicated thickness dependence of magnetization, coercive force and magnetic susceptibility was discussed in relation to surface morphology. Existence of 1 monolayer (ML) dead layer originated from partial oxidation at the Co/SiO2 interface. For the thinner films (∼2 ML), resistance to the magnetic reversal process was large because of larger surface roughness. For thicker films, reduction of the resistance was observed because of formation of continuous and defect less films.

Frozen Low-Spin Interface in Ultrathin Fe Films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.

Step-decorated Ferromagnetic Fe Nanostripes on Pt(997)

We investigate the step decoration growth and magnetic properties of Fe grown on a Pt(997) vicinal single crystal by means of reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and the surface magneto-optical Kerr effect (SMOKE). Pt(997) has 2 nm wide, (111)-oriented terraces separated by ordered, monoatomic steps along the $[1\overline{1}0]$ direction. The Fe is grown at room temperature as wedges with thickness ranging from 0 to 4 monolayers (ML). RHEED and STM show that Fe stripes form at low coverage due to step decoration. SMOKE data taken along a wedge indicate that the Fe is ferromagnetic above 0.2 ML with its magnetic easy axis canted from surface normal direction. As temperature is increased near the Curie temperature, there is an irreversible spin reorientation to the perpendicular out-of-plane direction. The canting can be understood as arising from a competition between an in-plane, step-induced magnetic anisotropy and a perpendicular surface anisotropy associated with extended terraces due to the polarization of the proximal Pt. Above 1.7 ML the perpendicular easy axis reorients to in-plane, along the step direction, due to the dominance of the shape anisotropy. The magnetic anisotropy energy is extracted by fitting the temperature dependence of the coercivity data.

Structure and magnetism of ultrathin Co film grown on Pt(100)

Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.Ultrathin Co films were deposited on Pt(100) at room temperature in ultrahigh vacuum, and investigated in situ by low-energy electron diffraction, scanning tunneling microscopy (STM), and surface magneto-optic Kerr effect. The Co film was grown into a wedge shape to provide a continuous change of the film thickness. We find that the Co film forms single-crystal ultrathin films at least up to 5 monolayers (ML). For as-grown films, we observe only in-plane magnetization. After annealing the film, the Co film develops a perpendicular magnetic anisotropy, leading to a spin reorientation transition at 2.7 ML Co thickness. STM measurements were performed at room temperature both before and after annealing the film. We found very different surface morphology and alloy formation after the film annealing, and attribute the perpendicular magnetic anisotropy to the formation of the Co-Pt alloy layer at the Co∕Pt(100) interface.

Low-temperature studies of magnetic phases of the interfacial layers for Co∕Ge(100) and Co∕Ge(111) films

Magnetic phases of the interfacial layers were comparatively investigated for Co∕Ge(100) and Co∕Ge(111) films thinner than 13 ML using surface magneto-optic Kerr effect technique. Co∕Ge(100) films are nonferromagnetic up to 12 ML as deposited and measured at 300K. A transition from nonferromagnetism to ferromagnetism occurs upon cryogenic treatments. Systematic investigations of the magnetic properties for Co∕Ge(100) reveal a magnetic phase diagram. The boundary between nonferromagnetic and ferromagnetic phases was experimentally determined to be from below 150K to above 300K as the cobalt thickness increases from 9 to 13 ML. This behavior is consistent with the thickness-dependent scaling law of the Curie temperature for a thin film system. Because of the difference of the densities of surface atoms, the different thickness of the formed interfacial compounds could explain the shift of the phase boundary to higher Co thickness side as compared to the Co∕Ge(111) system. In addition, the critical exponent β in the power law relationship of magnetization for Co∕Ge(100) films lies close to the value expected by the three-dimensional Heisenberg model, while the critical exponent of Co∕Ge(111) is close to the value of the two-dimensional XY model. The different behaviors of the magnetic transition are attributed to the different interfacial structures.

Structures and magnetism of two types ofc(2×2)−Mn∕Pd(001)surface alloys

$\mathrm{Mn}∕\mathrm{Pd}(001)$ surface alloy was investigated by a tensor low-energy electron diffraction (LEED) analysis. After deposition of Mn on Pd(001) at room temperature, the surface was annealed at $570--620\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which produced two types of $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface alloys, according to the Mn coverage. At a low-Mn coverage, we obtained a Pd-capped $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface, in which the first layer was composed of a $(1\ifmmode\times\else\texttimes\fi{}1)\text{\ensuremath{-}}\mathrm{Pd}$ layer, and the second layer was a $c(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}\mathrm{Mn}\mathrm{Pd}$ mixed layer $[\ensuremath{\alpha}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)]$. The deposition of greater amounts of Mn followed by annealing resulted in another $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface, in which Mn atoms existed in the substitutional sites of the first and third layers $[\ensuremath{\beta}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)]$. The first layer consisted of a $c(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}\mathrm{Mn}\mathrm{Pd}$ mixed layer, the second layer was a $(1\ifmmode\times\else\texttimes\fi{}1)\text{\ensuremath{-}}\mathrm{Pd}$ layer, and the third layer was another $c(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}\mathrm{Mn}\mathrm{Pd}$ mixed layer. The structure of the $\ensuremath{\beta}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)$ surface qualitatively agreed with the one previously investigated by LEED. These two types of surface alloys, $\ensuremath{\alpha}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)$ and $\ensuremath{\beta}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)$, may be considered as being precursors to the formation of the bulk $\mathrm{Mn}{\mathrm{Pd}}_{3}$ alloy. We also investigated the magnetic properties of the $\ensuremath{\alpha}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)$ and $\ensuremath{\beta}\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}2)$ surfaces by using surface magneto-optic Kerr effect (MOKE) and self-consistent, total-energy calculations. The MOKE measurements for both surface alloys show no hysterisis loop, even at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The total-energy calculation shows that Mn atoms have a local-spin moment of $3.9--4.1\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ and that they are antiferromagnetically ordered in the ground state.

Magneto-optical properties of Co∕Ge(100) with ultrathin Ag buffer layers

Magnetic properties of Co films (<2nm) with Ag buffer layers (<0.7nm) grown on Ge(100) at room temperature and 200K were studied by surface magneto-optical Kerr effect. Without the buffer, the films reveal in-plane magnetic anisotropy even Co and Ge forms nonmagnetic interfacial alloys. The hysteresis due to intercalation of Ag can be detected at thinner Co thicknesses. The buffer can effectively cutoff the intermixing of Co and Ge. As the thickness of Ag is reduced, out-of-plane magnetic anisotropy due to the interface interactions between Co∕Ag and Co∕Ge was discovered and was only at 200K.

Spin reorientation transition in ultrathin Co film on InP(2×4) reconstructed surface

We have investigated magnetic properties of monolayer (ML)-thickness Co film deposited on InP(2×4) reconstructed surface using in situ surface magneto-optical Kerr effects (SMOKE) measurement system. InP(2×4) reconstructed surface, obtained by several cycles of sputtering-and-annealing process, was confirmed by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) measurements. Co film grown on InP(2×4) reconstructed surface shows three distinguishable thickness regions which have different magnetic properties, depending on Co film thickness. In the Co film thickness region smaller than 7 ML, no SMOKE signal was detected. In the thickness region between 8 ML and 15 ML, both longitudinal and polar Kerr hysteresis loops were observed. In the film thickness larger than 16 ML, only longitudinal SMOKE signal without polar signal was detected.

Developments in surface magneto-optical Kerr effect setup for ultrahigh vacuum analysis of magnetic ultrathin films

We describe a surface magneto-optical Kerr effect (SMOKE) setup designed to study the magnetic properties of ultrathin films in ultrahigh vacuum (UHV). As in earlier systems, a crossed-poles electromagnet is used, which allows measurements in the longitudinal or polar configurations to be carried out without displacing the sample or changing the optical path. The poles are mounted in-vacuum, while the coils and yoke are mounted externally to obtain a higher magnetic field (∼2.8kOe). The width of the gap between the poles can be adjusted depending on the size of the sample. The performance of the instrument is demonstrated by measurements of the paramagnetic-ferromagnetic phase transition in a 2 ML Fe film grown on the Mo(110) surface.

Influence of exchange bias coupling on the single-crystalline FeMn ultrathin film

Polarization dependent x-ray photoemission electron microscopy was used to investigate the influence of the exchange bias coupling on the disordered ultrathin single-crystalline fcc Fe50Mn50. We find that the critical thickness of the FeMn film, where the antiferromagnetic (AF) order is formed, varies with changing the magnetization direction of the ferromagnetic (FM) layer from out-of-plane to in-plane. Surface magneto-optical Kerr effect measurements (SMOKE) further manifest the shift of the critical thickness with alternating the exchange bias coupling. It indicates that the spin structure of the FeMn layer near the FM layer is modified by the presence of exchange bias coupling and the properties of the coupling. Our results provide direct experimental evidence that the AF spin structure at the interface between the FM and AF layers is strongly influenced by the exchange bias coupling.

Magnetic Properties of Ferromagnetic Nanostructures at Surface Studied by Surface Magneto-optical Kerr Effect

Formation process of ferromagnetic nanodot arrays on a periodically nanopatterned Cu(001) surface with nitrogen atoms are overviewed, and their magnetic properties studied by surface magneto-optical Kerr effect (SMOKE) in ultra high vacuum are described. The square lattice nanopattern induced by the strain due to nitrogen adsorption is formed by the strain-relief mechanism of the Cu lattice near the surface. On top of that, the diffusion and the adsorption of the deposited atoms depend on the substrate lattice strain, and the ferromagnetic metal island preferentially grow at the less strained clean Cu surface. As a result, a square nanodot array of 7 nm × 7 nm is formed on the surface. The hysteresis loops of the arrays were in-situ measured by SMOKE between 400 K and 95 K. The results indicate long range ferromagnetic order is established among the superparamagnetic Co dots of two-monoatomic-layers high by the magnetic interaction through the monolayer Co strips.

Structural and magnetic properties of bcc Co films on Pt(001) studied by magnetic resonant surface x-ray diffraction, STM, and magneto-optical Kerr effect

The structure of Co epitaxial films deposited on Pt(001) has been investigated with scanning tunnelling microscopy and surface x-ray diffraction. The Co films grow in a layer-by-layer mode up to the highest examined thickness $(\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. Their crystalline structure is a tetragonal distortion of the bcc structure in pseudomorphic epitaxy with the substrate. It consists of a $\ensuremath{-}1.8%$ in-plane compressive strain and 5.1% out-of-plane tensile strain. A similar pseudomorphic structure is also observed in $\mathrm{Pt}∕\mathrm{Co}∕\mathrm{Pt}(001)$ trilayers. The magnetic properties of the films have been investigated by transverse magneto-optical Kerr effect and magnetic-resonant surface x-ray diffraction. The films exhibit in-plane magnetic anisotropy easy axes within the thickness range of $5--30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The magnetization curves of Co and interfacial Pt are similar, indicating ferromagnetic coupling.

Effect of Mn overlayer on spin reorientation transition at Ni/Cu(0 0 1)

Effect of Mn overlayer on the spin reorientation transition (SRT) of Ni/Cu(001) is investigated in situ by multi-functional techniques such as surface magneto–optic Kerr effect (SMOKE) measurement. It is found that the SRT thickness of Ni/Cu(100) shifts to thinner Ni thickness by ∼0.5ML after covering the Ni film with 0.5ML Mn.

Spin Reorientation and Structural Relaxation of Atomic Layers: Pushing the Limits of Accuracy

The correlation between an ad-layer-induced spin reorientation transition (SRT) and the ad-layer-induced structural relaxation is investigated by combined in situ surface x-ray diffraction and magneto-optical Kerr-effect experiments on Ni/Fe/Ni(111) layers on W(110). The Fe-induced SRT from in-plane to out-of-plane, and the SRT back to in-plane upon subsequent coverage by Ni, are each accompanied by a small lattice relaxation of at most 0.002 A. Such a small strain variation excludes a magnetoelasticity driven SRT, and we suggest the interface anisotropy as a possible driving force.

Extremely surface-sensitive hysteresis loop measurement with a spin-polarized metastable helium atom beam

The sample current induced by the deexcitation of He* atoms on a ferromagnetic surface was found to depend on the He* spin direction. This spin dependence was used for measuring the hysteresis loop of an Fe film on Cu(100). The hysteresis loop is extremely surface specific because the spin dependence is due to the electron emission via the He* deexcitation that occurs on the vacuum side of the topmost surface layer. The hysteresis loop for a 2.2ML Fe film on Cu(100) has been found to agree well with that measured with the surface magneto-optic Kerr effect.

In situ study of in-plane magnetic anisotropy of ultrathin Fe films on Mo(1 1 0)

In-vacuum surface magneto-optical Kerr effect was used in conjunction with scanning tunneling microscopy and low-energy electron diffraction to study the in-plane magnetic anisotropy of ultra-thin Fe films on a vicinal Mo(110) substrate. A uniaxial anisotropy with the easy axis of magnetization along the [001] direction was found for this system down to nearly 2ML coverage, contradicting the results of recent theoretical calculations. The easy axis of a step-induced anisotropy was found to coincide with the surface step edges.

Magnetic properties of Co nano-dot arrays on vicinal Cu(001)–c(2×2)N surfaces

On a Cu (001) surface vicinal to a [100] direction, single rows of square patches c(2×2)N in the direction of [010] can be formed on narrow terraces. We deposited Co on the vicinal Cu(001)–c(2×2)N surface, and investigated the magnetic properties of the Co nano-dot array at a temperature range of between 90 and 300 K, using an in situ surface magneto optical Kerr effect. When an average of 1.4 ML of Co was deposited on the surface, rectangular shape hysteresis loops of in-plane magnetization, parallel to the direction of [010], were observed below 130 K. This indicates that the easy magnetization axis is along the step edge.

Magnetic phases of Co/Ge(1 1 1) studied using surface magneto-optic Kerr effect

Co/Ge(1 1 1) films are nonferromagnetic up to 9.5 monolayers as deposited at 300 K. Upon cryogenic treatments, magnetic phases are experimentally resolved for Co/Ge(1 1 1) films thinner than 12 monolayers using the surface magneto-optic Kerr effect technique. Systematic investigations of the magnetic properties for Co/Ge(1 1 1) films revealed a transition of nonferromagnetism to ferromagnetism at low temperatures. The transition temperature increases from below 125 K to above 300 K as the cobalt coverage increases from 6.3 to 12 monolayers. The critical exponent β in the power law relationship of magnetization is about 0.23 that is close to the value expected by a two-dimensional XY model. Auger electron spectroscopy measurements suggest the co-existence of a pure cobalt and a Co–Ge compound layers in the nonferromagnetic Co/Ge(1 1 1) films.

InP(001)(2×4)재구성된 표면 위에 원자층 단위로 증착된 Co 박막의 자성 특성

본 연구에서는 InP(2<TEX>${\times}$</TEX>4) 재구성된 표면 위에 원자층 단위로 증착된 Co 박막의 특성을 표면 자기광 커 효과(surface magneto-optical Kerr effect, SMOKE) 시스템, 반사 고에너지 전자 회절(reflection high energy electron diffraction), 주사 터널링 현미경(scanning tunneling microscope)이 장착된 초고진공 챔버를 이용하여 조사하였다. 실시간(in situ) SMOKE 연구 결과, Co 박막이 InP(2<TEX>${\times}$</TEX>4) 재구성된 표면 위에 성장할 때, Co박막의 두께에 따라 자성 특성이 대조적으로 구분되는 세 가지 두께 영역이 존재함을 확인할 수 있었다. 즉, Co 박막 두께가 7 단층(monolayer, ML)보다 작은 두께 영역에서는 가로 방향(longitudinal)과 수직방향(polar) 측정에서 모두 SMOKE신호를 관찰할 수 없었다. 8<TEX>$m\ell$</TEX>에서 15<TEX>$m\ell$</TEX>까지의 Co두께 영역에서는 수평 자기 이방성과 수직 자기 이방성이 공존하는 준안정상(metastable phase)을 관찰할 수 있었다. 그리고 마지막 영역은 16<TEX>$m\ell$</TEX>이상의 두께를 갖는 영역으로 수평 자기 이방 강자성을 확인할 수 있었다. We have investigated magnetic properties of monolayer (ML)-thickness Co film deposited on InP(2<TEX>${\times}$</TEX>4) reconstruction surface using in situ Surface Magneto-Optical Kerr Effects (SMOKE) measurement system. InP(2<TEX>${\times}$</TEX>4) reconstruction surface, obtained by repeated sputtering and annealing, was confirmed by reflection hish energy electron diffraction (RHEED) and scanning tunneling microscope (STM) measurements. From both longitudinal and polar SMOKE measurements, we have observed three distinguishable regions showing different magnetic properties depending on the Co thickness. In the Co film thickness smaller than 7 <TEX>$m\ell$</TEX>, no SMOKE signal was detected. In the following thickness between 8 <TEX>$m\ell$</TEX> and 15 <TEX>$m\ell$</TEX>, both longitudinal and polar Kerr hysteresis loops were observed, which implies a metastable phase coexisted of in-plane and perpendicular anisotropies. In the film thickness larger than 16 <TEX>$m\ell$</TEX>, only longitudinal MOKE signal without polar signal was detected, which implies existence of in-plane anisotropy in this thickness region.