Surface Magneto-optic Kerr Effect Measurements Research Articles

Absence of spontaneous time-reversal symmetry breaking and ferromagnetism in superconducting NiBi3 single crystal

Recent experiments have pointed to chiral $p$-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that, in nonepitaxial Bi/Ni bilayers, superconductivity arises due to the formation of $\mathrm{Ni}{\mathrm{Bi}}_{3}$, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high-resolution surface magneto-optic Kerr effect measurements using a Sagnac interferometer on single-crystal $\mathrm{Ni}{\mathrm{Bi}}_{3}$ and find no sign of any spontaneous Kerr signal except for contributions from trapped vortices. This strongly indicates the absence of TRSB in $\mathrm{Ni}{\mathrm{Bi}}_{3}$, whether due to TRSB in the superconducting state or any coexisting ferromagnetism, and we conclude that the superconductivity found in nonepitaxial Bi/Ni is distinctively different from that in epitaxial Bi/Ni.

Variation of blocking temperatures for exchange biased CoO/Co/Ge(100) films

Variations of the blocking temperature and related structures for CoO/Co/Ge(100) films are investigated by employing reflection high energy electron diffraction, Auger electron spectroscopy, and surface magneto-optic Kerr effect measurements. By increasing the CoO thickness, the blocking temperature is smaller than the Neel temperature of CoO. The monotonous increase of the blocking temperature is mainly attributed to the increasing thermal stability of the antiferromagnetic grains by way of increasing the antiferromagnetic thickness. The deviation of the blocking temperature from the linear relation and the full widths at half maximum of the diffraction spots show a similar trend. The minimums appear around 25 monolayer of CoO and are related to the formation of larger grains.

Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)

We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) $\ensuremath{\gamma}$ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) $\ensuremath{\alpha}$ phase. A structural transformation of these films from fcc to bcc can be induced by ${\text{Ar}}^{+}$ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing ${\text{Ar}}^{+}$ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.

Interface-driven manipulation of the magnetic anisotropy of ultrathin Co films on Pt(111): Substrate deposition of hydrogen and model calculations

The magnetic anisotropy energy (MAE) and the resulting spin-reorientation transitions of Co/Pt(111) ultrathin films are investigated by manipulating the Co/Pt interface with controlled adsorption of hydrogen prior to Co deposition. In situ low-energy electron-diffraction and surface magneto-optical Kerr-effect measurements are performed on Co films grown at low temperatures. The results show that interface H deposition leads to a remarkable change in the magnetization direction from perpendicular to in-plane, even for the thinnest Co films, which is accompanied by an important increase in the coercive force. Layer-resolved self-consistent electronic calculations of the MAE are performed in order to identify the interface contributions responsible for perpendicular magnetic anisotropy (PMA) and to quantify how the MAE depends on various possible adsorbate-induced modifications in the local magnetic moments. The results show that the PMA is quite insensitive to changes in the local magnetic moments at the Co film surface even if they are relatively large. However, the PMA depends crucially on the Co-interface moments, on the Pt-interface moments induced by the proximity to Co, and on the resulting spin-orbit interactions at the Pt atoms. The observed suppression of PMA by interface H adsorption is interpreted as the consequence of the reduction in the interface-Pt moments which, originates either a reduction in the Co-interface moments or at a decoupling of the Pt substrate from the magnetic film. Experiment and theory thus prove the dominant role of the Co-Pt interface MAE on the development of the relative stability of perpendicular and in-plane magnetization directions. The magnetic properties of ultrathin films may thus be tailored to a large extent by adsorbates trapped at $3d\text{\ensuremath{-}}4d$ or $3d\text{\ensuremath{-}}5d$ film-substrate interfaces.

Co nanocrystallites on an icosahedral Al-Pd-Mn quasicrystal

Recently, we reported that submonolayer deposits of Co onto the pentagonal surface of i-Al-Pd-Mn form a stable intermetallic compound of five CsCl-type AlCo domains rotated by 72° with respect to each other. For multilayer deposition, Co was found to grow epitaxially on the AlCo domains in a body-centred cubic structure. Here we discuss this conclusion in more detail and report the formation of atomic rows at the surface for deposits larger than three monolayers. The details of the growth resulting in ordered nanostructures are extracted from low-energy electron diffraction investigations. Surface magneto-optical Kerr effect measurements confirm the decrease in the domain size for thicker films.

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.

Thickness dependence of the magnetic properties in ultrathin Co/Ge(1 1 1) films

The existence of a low-Curie-temperature phase for Co/Ge(1 1 1) films is proofed by performing surface magneto-optic Kerr effect measurements at 150 K. The coercive force of the films increases slowly between 7.5 and 15 monolayers, and then increases promptly at larger thicknesses. The different increasing rates of the coercive force can be explained by the different interfacial roughness that influences the pinning of the domain walls.

In situ vectorial magnetization study of ultrathin magnetic films using a surface magneto-optical Kerr effect measurement system

The three-configuration in situ surface magneto-optical Kerr effect (SMOKE) system, which enables full vectorial analysis of magnetization for ultrathin magnetic films in a UHV chamber, has been developed. The system has been applied to in situ measurements of the three components of magnetization vector of Co films in an applied magnetic field at the thickness range of 5/spl sim/6 ML, where the spin reorientation was found to occur. We observed contrasting vectorial magnetization reversal behavior dependent on the Co thickness: In particular, for 5.25 ML of Co, a slightly canted magnetization vector from the film normal rotates in the film plane under an applied field normal to the film plane.

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.

In situ investigation of ultrathin Fe/Cu(110) films by Brillouin light scattering

A compact ultrahigh-vacuum chamber, specially designed to allow in situ Brillouin light-scattering (BLS) and surface magneto-optical Kerr-effect measurements has been set up. Experimental results relative to both the structure and the magnetic properties of ultrathin Fe films grown by e-beam evaporation on a Cu(110) single crystal are presented. Low-energy electron diffraction patterns account for epitaxial growth of fcc Fe(110) up to 4–6 Å, followed by a transition to a more disordered state, consisting of bcc Fe domains. The BLS analysis enabled us to study the dependence of the spin-wave frequency on the angle of incidence of light, on the intensity of the magnetic field, and on its direction on the surface plane.

Three-configurational surface magneto-optical Kerr effect measurement system for an ultrahigh vacuum in situ study of ultrathin magnetic films

We have constructed a three-configurational surface magneto-optical Kerr effect system, which provides the simultaneous measurements of the “polar,” “longitudinal,” and “transverse” Kerr hysteresis loops at the position where deposition is carried out in an ultrahigh vacuum growth chamber. The present system enables in situ three-dimensional vectorial studies of ultrathin film magnetism with a submonolayer sensitivity. We present three-configurational hysteresis loops measured during the growth of Co films on Pd(111), glass, and Pd/glass substrates.

Reduced coercivity in ferromagnetic Co–Cu coevaporated epitaxial films on Cu(111)

Epitaxial films grown by coevaporation of Co and Cu on Cu(111) were investigated by low-energy electron diffraction and surface magneto-optical Kerr-effect measurements. The films are dominantly face-centered-cubic stacked up to high Co concentrations and exhibit ferromagnetism. Their coercivity is significantly reduced compared to pure Co films produced by thermal Co deposition on Cu(111) independent of the use of Pb as a surfactant.

Investigation of face-centered-cubic Fe thin films using wedged samples

The structural and magnetic properties of metastable face-centered-cubic (fcc) Fe thin films on fcc Co(100) substrate were studied using wedged samples. fcc Co(100) was chosen for the substrate because it is structurally very similar to Cu(100) but is ferromagnetic at room temperature. Reflection high energy electron diffraction and low energy electron diffraction characterizations confirm that epitaxially grown (MBE) Fe on Co(100) is structurally very similar to Fe on Cu(100): face-centered-tetragonal (fct) for dFe<6 ML, fcc for 6 ML<dFe<11 ML, bcc for dFe≳11 ML. In situ surface magneto-optic Kerr effect measurements show that at room temperature the fct and bcc regions are ferromagnetic, while the fcc region is nonferromagnetic with some magnetic live layers. All magnetizations are in-plane. Oxygen absorption experiments suggest that these live layers are at the Fe/Co interface. Low temperature growth Fe/Co(100) shows a Kerr signal that increases linearly with dFe and suggests that the magnetic moments for fcc Fe and bcc Fe are the same. To further study the magnetic properties of the nonferromagnetic ‘‘fcc’’ phase, we used metastable fcc Fe as a spacer layer between two Co layers. The Co/fcc Fe/Co on Cu(100) sandwiches exhibit ferromagnetic coupling, strong antiferromagnetic coupling (AFC) and weak AFC. An oscillation in the strong AFC was found by artificially lengthening the thickness range of the nonferromagnetic fcc phase.

Finite-size scaling behavior of ferromagnetic thin films

We have used molecular-beam epitaxy to grow high-quality pseudomorphic Ni and Co1Ni9 films on Cu(001). From temperature-dependent surface magneto-optic Kerr effect measurements of these films, we have determined the finite-size scaling behavior of the Curie temperature of ultrathin films for a thickness range of n=2.5–16 monolayers (ML). The film thickness dependent Curie temperature for each of these ferromagnetic thin-film systems, TC(n), is described by a finite-size scaling formula: [TC(∞) − TC(n)]/TC(n) = [(n − n′)/n0]−1/ν, where TC(∞) is the bulk Curie temperature, n0=2.5±0.5 ML for Co films and 3.5±0.4 ML for Ni and Co1Ni9 films is the microscopic length scale, and ν=0.76±0.08 is the bulk correlation length exponent. An interesting result is that TC(n) extrapolates to zero in the single mononolayer limit, n′=1.

Magnetism and growth of ultrathin Co films grown epitaxially on Ru(0001)

Low-energy electron diffraction and Auger spectroscopy have been used to determine that ultrathin films of Co grow pseudomorphically on Ru(0001) at room temperature in an island structure. Co thicknesses of 1–14 MLE (monolayer equivalents) were explored. The expanded Co lattice relaxes to its bulk spacing after a 500°C anneal. In-situ surface magneto-optic Kerr-effect measurements show ferromagnetic hysteresis loops with in-plane easy axes of magnetization for film thicknesses > 4 MLE. Thinner films are presumably affected by superparamagnetism. Annealing or Au coating the thicker films induces an out-of-plane easy axis of magnetization.

Magnetic properties of ultrathin Co/Ag(001) films

We have used polarised neutron reflection to investigate the saturated in-plane magnetisation of ultrathin single Co films incorporated into Ag/Co/Ag(001) epitaxial sandwich structures. For Co thicknesses of 1 and 2 monolayers (ML) we find ferromagnetic order at 5 K with moments per Co atom close to 2.0 μ B, significantly exceeding the bulk hcp phase value of 1.7 μ B. For a Co thickness of 3ML we find a ferromagnetic moment close to 1.7 μ B at 5 K with the moment falling significantly beneath this value for a 5ML film. The collapse of the effective magnetic moment is attributed to the strain-induced structural disorder of the film observed during growth beyond 3ML. Surface magneto-optic Kerr effect measurements reveal large in-plane magnetic anisotropies and high Curie temperatures for Co thicknesses in the 1–3ML range. A weak temperature-dependence of the magnetisation M( T) is observed for a 2ML film.

Magnetic properties of ultrathin epitaxial films of iron

Ultrathin Fe films epitaxially grown on Cu(100), Au(100), Pd(100) and Ru(0001) substrates are used to study monolayer magnetism, surface magnetic anisotropy and two-dimensional (2D) critical behavior, by means of in-situ surface magneto-optical Kerr-effect measurements. Epitaxial structures and growth modes are characterized by auxiliary low-energy electron diffraction and Auger electron studies. The magnetic properties of the films depend on the growth temperature. For 100 K growth, all films exhibit a universal behavior of perpendicular spin orientations in the monolayer regime, while for 300 K growth, Fe/Pd(100) and Fe/Au(100) have in-plane easy-axis of magnetization for all thickness. Fe/Ru(0001) lacks a ferromagnetic response for thickness less than two monolayers (ML). Fe/Pd(100) has remarkable thermal stability in the monolayer and submonolayer range. Reversible magnetization data are obtained for 0.6 to 3.0 ML thick films, which show second-order phase transitions at thickness-dependent Curie temperatures ( T c ). An effective magnetization exponent β eff of 0.127 ± 0.004 is obtained for a 1.2 ML Fe/Pd (100) film, in agreement with the theoretical value of β c = 0.125 for an ideal 2D Ising model. The T c trend with film thickness for Fe/Pd(100) is also in agreement with Monte Carlo calculations for Ising films.

Absolute value of the magnetic moment per atom in Ag/Fe/Ag(001) and Ag/Co/Ag(001) epitaxial sandwich structures

We present the results of an investigation of epitaxial Ag/bcc Fe/Ag(001) and Ag/fcc Co/Ag(001) sandwich structures in which we use polarized neutron reflection (PNR) as a direct probe of the absolute value of the magnetic moment per atom in order to test recent theoretical calculations for these systems. An 8-ML bcc 57Fe and a 2-ML fcc Co film were found to display planar anisotropy at 300 K from surface magneto-optic Kerr effect measurements, and were selected for PNR measurements in the temperature range 4–300 K with the films magnetically saturated in-plane. The flipping ratio observed for the 8-ML 57Fe film is consistent with a ferromagnetic moment in the range 0.9–1.1 Bohr magnetons, which is significantly reduced from the bulk value, in contrast with recent theoretical predictions of a strongly enhanced ferromagnetic moment for this epitaxial system. No temperature dependence of the magnetization is detected, confirming the anticipated bulklike behavior. For the 2-ML fcc Co film, the observed flipping ratio is consistent with a ferromagnetic moment in the range 1.8–2.2 Bohr magnetons, which is significantly enhanced from the bulk hcp value, in agreement with recent theoretical calculations. No temperature dependence of the Co magnetization was detected.

Two-dimensional magnetic phase transition of ultrathin iron films on Pd(100)

Epitaxial Fe films grown on Pd(100) are used to study monolayer magnetism, critical behavior, and surface magnetic anisotropy, by means of in situ surface magneto-optical Kerr-effect measurements. Auxiliary LEED-Auger observations in 10−11 Torr vacuum are used to characterize the (1×1) epitaxy and the layer-by-layer film-growth mode. Ferromagnetic hysteresis loops were detected for all Fe thicknesses from 0.6–4 monolayers (ML) with the TC monotonically increasing with thickness, independent of the easy-axis orientation. The easy axis is perpendicular to the film plane below a critical thickness of 2.5 ML for 100-K film growth, and reorients in-plane above this thickness, and for all thicknesses for films grown at 300 K. The temperature dependence of the magnetization was obtained from the height of the Kerr loops in the remanent state and used to extract an effective magnetization exponent β for different film thicknesses and spin orientations. A value of β=0.127±0.004 is reported for a 1.2-ML Fe film with perpendicular spin orientation from a log-log plot for T>0.9TC, in agreement with the value of the theoretical two-dimensional Ising critical exponent βc=1/8=0.125.