Exchange bias in (1 1 0)-orientated Bi0.9La0.1FeO3/La0.5Ca0.5MnO3 films

We performed a systematic study on the exchange bias in (1 1 0)-orientated Bi_0.9La_0.1FeO₃/La_0.5Ca_0.5MnO₃ (BLFO/LCMO) heterostructure with a fixed BLFO film thickness of 600 nm and different LCMO layers ranging from t=0 to 30 nm. The LCMO is found to be weakly ferromagnetic, with the Curie temperature descending from ∼225 K to 0 as the layer thickness decreases from 30 nm to 3 nm. The main magnetic contributions come from the BLFO film, and the areal magnetization ratio is 1:0.07 for t=5 nm and 1:0.82 for t=30 nm for BLFO to LCMO at the temperature of 5 K. Further experiments show the presence of significant exchange bias, and it is, at the temperature of 10 K, ∼40 Oe for t=0 and ∼260 Oe for t=30 nm. The exchange bias reduces dramatically upon warming and disappears above the blocking temperature of the spin-glasslike behavior observed in the samples. The possible origin for exchange bias is discussed.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

Spin-pumping-enhanced magnetic damping in ultrathin Cu(0 0 1)/Co/Cu and Cu(0 0 1)/Ni/Cu films

The influence of the Cu capping layer thickness on the spin pumping effect in ultrathin epitaxial Co and Ni films on Cu(0 0 1) was investigated by in situ ultrahigh vacuum ferromagnetic resonance. A pronounced increase in the linewidth is observed at the onset of spin pumping for capping layer thicknesses d_Cu larger than 5 ML, saturating at d_Cu = 20 ML for both systems. The spin mixing conductance for Co/Cu and Ni/Cu interfaces was evaluated.     

Combinatorial synthesis and characterization of a ternary epitaxial film of Co and Mn doped Ge (0 0 1)

We report combinatorial molecular beam epitaxy synthesis and properties of a ternary epitaxial film of Co and Mn co-doped Ge grown on Ge (0 0 1) substrate. Structural effects were examined in situ by reflection high-energy electron diffraction and ex situ by microbeam X-ray diffraction techniques, and magnetic properties were probed by using magnetooptic Kerr effect. Ternary epitaxial phase diagrams have been studied for total doping concentrations up to 30 at.%, where regions of coherent epitaxy and rough disordered growth and those of near room temperature ferromagnetic ordering have been identified.     

Spin excitations in exchange-dominated regime on (1 0 0) and (1 1 0) antiferromagnetic surfaces

Theoretical investigations in the context of Heisenberg model have been made for (1 0 0) and (1 1 0) magnetic surface dynamics for a semi-infinite antiferromagnet geometry. The calculations apply to the exchange dominated regime and are based on a spin-wave operator and matching technique within the framework of non-interacting spin-wave theory. The theoretical formalism developed here does not include either relaxation or reconstruction at the surface and no electronic effects have been considered. Dispersion curves of surface spin-waves are obtained within a single framework by matching the evanescent and travelling solutions, respectively, obtained from the secular equation and satisfying the boundary conditions brought about by the surface. The excitation spectrum of the surface spin-waves has been obtained and compared with that for bulk spin-waves. The quantized bulk modes of the same energy travelling to and away from the surface are related to one another by reflection coefficients, for which sum rules are derived. The numerical results for the evolution of acoustic and optical modes are presented for two different surface planes, namely (1 0 0) and (1 1 0). The findings reported here show that: (i) the reduced coordination number for atoms near the surface as well as the surface orientation play an obvious and crucial part in the surface spin-wave spectra; (ii) the evolutions of bulk as well as surface modes undergo significant changes as a function of the bulk-surface exchange integrals for a given direction of propagation of the spin-wave modes along the surface.     

Nanoscale dislocation patterning in Bi(1 1 1)/Si(0 0 1) heteroepitaxy

Continuous, atomically flat, and epitaxial Bi(1 1 1) films could be grown on Si(0 0 1). The inherent strain of 2.3% between the Bi(1 1 1) and Si(0 0 1) lattices is relieved by the formation of a grating like one-dimensional misfit dislocation array at the heterointerface. The lattice distortions around each dislocation give rise to a pronounced height depression Δh = 0.12 nm of the surface, which results in a spot splitting in low-energy electron diffraction and a height contrast in scanning tunneling microscopy (STM). Using STM surface profiles across these depressions, the Burgers vector of the underlying isolated non-interacting dislocations is estimated to be 0.377 nm. For thicker Bi films the ordering of the dislocation network is increased. This reflects an increase of repulsive interaction between neighboring dislocations.     

Surface order dependent magnetic thin film growth: Fe on GaN(0 0 0 1)

X-ray photoemission spectroscopy and X-ray magnetic circular dichroism have been used to study the growth process, chemical composition and magnetic character of iron deposited on ordered and disordered GaN(0 0 0 1) surfaces. On the (1 × 1) ordered surface the Fe grows uniformly but with disruption to the substrate surface, subsequently nitrogen desorbs from the surface, some of which diffuses into the Fe overlayer. The film is magnetically fractured, with high magnetic coercivity and broad switching fields. Conversely, the gallium rich disordered surface protects the underlying substrate from any disruption and initially induces non-uniform growth, the Fe clusters coalesce at ∼12 Å, to produce a uniform film with desirable magnetic characteristics. Films beyond this point (>12 Å) indicate sharp hysteresis loops with low coercivities. For the resultant film (36 Å) we measure a magnetic moment of 2.02 μ_B, in agreement with bulk bcc iron (2.068 μ_B.)     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

Density-functional study of the CO adsorption on ferromagnetic Co(0 0 0 1) and Co(1 1 1) surfaces

The regular CO overlayers at coverage θ = 1/3 adsorbed on the (0 0 0 1) surface of hcp Co and (1 1 1) surface of fcc Co are studied by first-principles density-functional theory with the exchange-correlation component in the PBE form. Adsorption in atop, bridge, and three-fold hcp or fcc position are considered. The adsorption energies, CO stretching frequencies, geometry, work function, and local magnetic moments are studied, and, when possible, compared with experimental or theoretical data. Particularly, we show that the recently proposed correction to adsorption energy of CO prefers correctly the atop adsorption site, whereas the remaining sites are almost degenerate in energy. The CO molecule lowers magnetization on neighbouring Co atoms, and the effect decreases with the adsorption site coordination. We show, however, that this trend is not the result of the different C-Co separation at different adsorption sites. A very small magnetic moment appears on CO that couples antiferromagnetically to Co. Most results are very similar for the Co(0 0 0 1) and Co(1 1 1) surfaces.     

Adsorbate influence on the coercive field of ultrathin Co/Cu{1 1 0}

We report on experimental coercive field measurements for ultrathin films of Co on Cu{1 1 0}, revealing a complex dependence upon the surface coverage of oxygen and hydrogen adatoms. With reference to first-principles calculations, we rationalise this behaviour in terms of initial reaction with surface contaminants and/or adsorption at defects, followed by (in the case of oxygen) subsequent surface ordering and eventual formation of surface oxide.     

Effect of crystallinity of Co layer on perpendicular exchange bias in Au-capped ultrathin Co film on Cr2O3(0 0 0 1) thin film

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr₂O₃ thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr₂O₃(0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr₂O₃(0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr₂O₃(0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr₂O₃. The valence state of the Cr₂O₃(0 0 0 1) layer is similar to that of bulk Cr₂O₃. The ultrathin Co film directly grown on the α-Cr₂O₃(0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Néel temperature of Cr₂O₃ for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr³⁺ in Cr₂O₃(0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Epitaxial growth of bcc-FexCo100-x thin films on MgO(1 1 0) single-crystal substrates

Fe_xCo_100-x (x=100, 65, 50 at%) epitaxial thin films were prepared on MgO(1 1 0) single-crystal substrates heated at 300 °C by ultra-high vacuum molecular beam epitaxy. The film structure and the growth mechanism are discussed. FeCo(2 1 1) films with bcc structure grow epitaxially on MgO(1 1 0) substrates with two types of variants whose orientations are rotated around the film normal by 180° each other for all compositions. Fe_xCo_100-x film growth follows the Volmer Weber mode. X-ray diffraction analysis indicates the out-of-plane and the in-plane lattice spacings are in agreement with the values of respective bulk Fe_xCo_100-x crystals with very small errors less than ±0.4%, suggesting the strains in the films are very small. High-resolution cross-sectional transmission electron microscopy shows that periodical misfit dislocations are preferentially introduced in the film at the Fe₅₀Co₅₀/MgO interface along the MgO[1 1¯ 0] direction. The presence of such periodical dislocations decreases the large lattice mismatch of about −17% existing at the FeCo/MgO interface along the MgO[1 1¯ 0] direction.     

para-Sexiphenyl thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O surfaces

We present a reflectance difference spectroscopy (RDS) study of para-sexiphenyl (p-6P) thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O substrates. The RDS spectra show pronounced anisotropies for p-6P films formed on both substrates at room temperature, demonstrating that the molecules are uniaxially aligned within the films. Based on the RD spectra and the evolution of the optical transitions with p-6P coverage the growth mode on both substrates could be identified. From the dominating RDS feature, assigned to the lowest energy HOMO-LUMO transition, the orientation of the molecular chain can be determined. On Cu(1 1 0), the p-6P molecular chains align in the direction, i.e., along the Cu atomic rows, whereas on the Cu(1 1 0)-(2 × 1)O surface, the molecules are oriented in the orthogonal [0 0 1] direction, i.e., along the “added” Cu-O rows of the Cu(1 1 0)-(2 × 1)O surface. The energetic position and line shape of the main RDS feature differs for the two substrates and varies with p-6P coverage. This fine structure is discussed in terms of different molecular conformations, adlayer structure and vibronic replicas.     

Magnetization of epitaxial MnGe alloys on Ge(1 1 1) substrates

The structural and magnetic properties of an Mn rich solid phase epitaxy Mn_xGe_1−x alloy grown on a clean 2 × 8-Ge(1 1 1) surface, with a Curie temperature of about 300 K are investigated. Magneto-optical  Kerr effect infers the existence of in-plane easy magnetization direction. We describe the epitaxial registry condition, the room temperature—zero field magnetic structure observed by magnetic force microscopy and the magneto-optical properties. The observations are consistent with the formation of epitaxial Mn₅Ge₃ alloy, with a modulated magnetic structure characterized by asymmetric 180° Bloch walls consisting of a vortex-like configuration of the local magnetization.     

Epitaxial growth of titanium oxycarbide on MgO (0 0 1) substrates by pulsed laser deposition

Epitaxial TiC_xO_y thin films were grown on MgO (0 0 1) substrates by using pulsed laser deposition method. High-resolution X-ray diffraction and transmission electron microscopy were used to examine crystallinity and microstructure of epitaxial TiC_xO_y film on MgO. The chemical composition of the film is determined to be x ∼ 0.47 and y ∼ 0.69 by X-ray photoelectron spectroscopy. Atomic force microscopy revealed that the surface of TiC_xO_y film is very smooth with roughness of 0.18 nm. The resistivity of the TiC_xO_y film measured by four-point-probe method was about 137 μ Ω cm.     

Surface investigation of Si(1 0 0), Cu, Cu on Si(1 0 0), and Au on Cu with positron annihilation induced Auger-electron spectroscopy

The surfaces of polycrystalline Cu, Au-coated Cu, Si(1 0 0) and of Si(1 0 0) coated with 1.5 monolayer Cu were investigated with positron annihilation induced Auger-electron spectroscopy (PAES). Since the electron background has been reduced considerably we observed the Cu M_2,3VV-Auger transition on a copper surface within only three hours which is the shortest acquisition time reported so far for PAES. In order to demonstrate PAES’ high potential the Auger-yield, the signal-to-background ratio as well as the surface selectivity were compared with accompanying EAES-measurements quantitatively. A more efficient electron energy analyzer for the present PAES setup would lead to an additional efficiency gain of more than two orders of magnitude. The presented measurements were performed at the low-energy positron beam of high intensity NEPOMUC at the research reactor FRM II.     

Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(0 0 1) via ion beam triangulation and spin-polarized electron capture

The structure and magnetism of thin epitaxial Fe layers grown on Cu(0 0 1) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation with protons. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during scattering of He atoms. For the formation of bcc(1 1 0)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.