Epitaxial Cr Research Articles

Cr-Fe surface alloy on Fe substrate: CPA-TB-LMTO and semi-empirical TB calculations

Abstract We present results of LSDA electronic structure calculations for the initial growth stage of epitaxial Cr overlayers on a ferromagnetic Fe(001) substrate in the presence of interdiffusion at the Cr/Fe interface. The interfacial interdiffusion is simulated by a two-dimensional substitutionally disordered alloy and the corresponding self-consistent electronic structure is obtained within the tight-binding linear muffin-tin orbital (TB-LMTO) method and the coherent-potential approximation (CPA). We analyse the total energies of different solutions of the LSDA-CPA problem and discuss several topics relevant to the Cr/Fc system, e.g. segregation tendencies for ultra-thin Cr films and a magnetic π phase shift for thicker Cr films. A comparison of the ab initio results to a semi-empirical tight-binding theory and experiment is presented as well.

Growth and structure of Cr on Ru(0001)-(2×2) and Cr/Ru multilayers

In-situ observations of reflection high-energy electron diffraction (RHEED) are used to study the structure and growth of epitaxial Cr films on hcp(0001) Ru buffer layers that are prepared on mica substrates under molecular beam conditions. During the growth of Cr at room temperature, we observed up to five intensity oscillations of the specular beam, indicating a layer-by-layer growth mode. Further growth results in the formation of three-dimensional islands. Simultaneously, the relaxation of the in-plane lattice parameter was recorded during growth, showing an enlarged value at 0.3 ML Cr coverage. At a coverage above 5 ML, the Cr overlayer adopts a bcc(110) structure; thereby, the Cr layers arrange into a Nishiyama–Wassermann-type epitaxy. Scanning tunneling microscopy (STM) investigations in UHV, allowing a comparison of real and reciprocal space techniques, confirm the growth behavior. A series of Cr/Ru multilayers were prepared and their magnetic properties examined by a Faraday balance. We found no resulting magnetic moment that could be expected in-between the resolution limit (>0.1 μ B per Cr atom) if the Cr layers were ferromagnetically ordered.

Analysis of the transport properties of epitaxial Fe and Cr films

Molecular beam epitaxy of Fe and Cr on single crystalline MgO(001) substrates produces films with very small defect concentrations. Accordingly, their low temperature transport properties are characterized by extremely low bulk resistivities. The temperature dependence of the resistivity of the Cr films shows anomalies due to magnetic ordering effects around the Néel temperature . We find values of K which is close to the bulk value. The anomalous Hall effect of the Fe films shows typical ferromagnetic behaviour. The spontaneous Hall coefficient is proportional to the square of the resistivity indicative of side jump scattering. Fe/Cr superlattices grown under similar conditions are particularly suitable to study the influence of interface scattering on the giant magnetoresistance amplitude.

Reorientation of Spin Density Waves in Cr(001) Films Induced by Fe(001) Cap Layers

Proximity effects of 20 \AA{} Fe layers on the spin density waves (SDWs) in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk Cr we observe a SDW with its wave vector $\mathbf{Q}$ pointing along only one ${100}$ direction which depends dramatically on the film thickness ${t}_{\mathrm{Cr}}$. For ${t}_{\mathrm{Cr}}<250\AA{}$ the SDW propagates out of plane with the spins in the film plane. For ${t}_{\mathrm{Cr}}>1000\AA{}$ the SDW propagates in the film plane with the spins out of plane perpendicular to the in-plane Fe moments. This reorientation transition is explained by frustration effects in the antiferromagnetic interaction between Fe and Cr across the Fe/Cr interface due to steps at the interface.

Depth-dependent investigation of the distribution of the spin density waves in thin chromium films with surface X-ray and neutron scattering

For thin epitaxial Cr(0 0 1) films capped with a ferromagnetic Fe layer a transverse spin density wave (SDW) is expected which propagates in the out-of-plane direction with the Cr spins aligned parallel to the film plane in the direction of the Fe magnetization vector. Synchrotron and neutron scattering experiments show, however, that the SDW wave propagates parallel to the film plane with spins oriented out-of-plane. In addition, a commensurate antiferromagnetic phase is found. The re-orientation of the SDW is caused by a frustrated Fe–Cr exchange coupling introduced by monoatomic steps at the Fe–Cr interface. Complete re-orientation takes place over some distance close to the interface reducing severely the coherence length of the SDW structure. With the surface scattering method we have measured the coherence length of the SDW as a function of depth. Furthermore, we have investigated the role of the commensurate antiferromagnetic phase near the Fe–Cr interface. We find no scattering from a commensurate order, implying a layering of the two phases with the incommensurate phase on top.

Magnetism of thin epitaxial Cr films in sandwich structures and superlattices

The spin density wave (SDW) magnetism of thin epitaxial Cr films has recently become the focus of interest because of its mediating role in exchange coupled superlattices. While the incommensurate SDW and the Néel temperature, , are well established for bulk Cr, the question arises of how these properties are altered in thin films and superlattices with ferromagnetic boundary layers. We review here recent synchrotron and neutron scattering experiments to probe the commensurate and incommensurate SDWs in epitaxial Cr(100) layers and the phase diagrams which result from these measurements as a function of film thickness and temperature. Finally, we link the antiferromagnetic spin structure of Cr to the Fe layer magnetization vectors in exchange coupled superlattices, which provides a deeper insight for the cause of non-collinear magnetic ordering in these artificial superlattices.

Magnetic phase diagram for spin-density waves in thin epitaxial Cr(0 0 1) films

We have investigated the magnetic structure of thin [0 0 1] oriented Cr films using neutron and X-ray diffraction experiments to measure their spin density waves and the strain waves, respectively. For epitaxial Cr films with thicknesses between 1000 and 4000 Å grown on Nb films on sapphire substrates, we provide phase diagrams including incommensurate transverse and longitudinal spin-density waves (SDW) as well as commensurate antiferromagnetic spin structures. The results show that for Cr(0 0 1) on Nb a single domain SDW prevails with a wave vector Q perpendicular to the surface. At low temperatures the SDW is longitudinal and becomes mostly transverse between 150 and 250 K, higher than in bulk Cr where the spin–flip transition occurs at 123 K. Furthermore, the magnitude of Q is increased as compared to bulk Cr. These effects decrease with increasing film thickness. With neutron scattering we have also observed a commensurate antiferromagnetic phase with spins pointing out of the plane. The commensurate phase occurs at a temperature between 250 and 305 K and persists up to at least 340 K, far above the bulk Néel temperature of 311 K for the incommensurate phase.

Electric transport properties of epitaxial Fe and Cr films with very low intralayer scattering

The low-temperature transport properties of epitaxial Fe and Cr films grown on MgO(1 0 0) substrates by molecular beam epitaxy are characterised by extremely low intralayer resistivities, indicating a very small concentration of defects. This makes such films particularly suitable for studies of the influence of the interface scattering on the transport properties of Fe/Cr superlattices.

Oxidation induced roughening during Cr 2O 3(0001) growth on Cr(110)

X-ray reflectivity measurements on the oxidation of epitaxial Cr(110) films are reported, providing the roughness for the oxide surface and metal/oxide interface as a function of the oxidation temperature. Up to the temperature of T R∼725 K the oxide film thickness in quasi-equilibrium increases linearly with temperature and the oxide surface and metal/oxide interface remain smooth on the order of 0.2–0.4 nm. At higher temperatures the oxide surface roughens and cavities are formed at the metal/oxide interface. The transition from a layer-by-layer like to a three-dimensional island-like growth mode can be explained in terms of epitaxial growth models. At the roughening temperature the Mott-field aided diffusion current of the metal ions through the oxide layer exceeds the surface diffusion current, resulting in a rough oxide surface.

Spin density waves in epitaxial Fe/Cr films

We have investigated proximity effects of Fe layers on the spin density waves (SDW) in thin epitaxial Cr[0 0 1] films with neutron scattering. Unlike in bulk Cr we observe a strong anisotropic occupation of the three possible SDWs. In pure Cr[0 0 1] films the SDWs exhibit an almost complete out-of-plane propagation with spins parallel to Q. With only 20 A Fe on top of these Cr films, the SDW becomes completely re-oriented with Q now propagating in the plane but the Cr-spins still point out of the plane. This implies that the Fe and Cr moments are ooriented perpendicular to each other.

Effect of Fe cap layers on the spin density waves in epitaxial Cr(001) films

We have investigated proximity effects of Fe layers on the spin density waves (SDW) and the concomitant charge density waves or strain waves in thin epitaxial Cr[001] films using synchrotron and neutron scattering. Unlike in bulk Cr we observe a strong anisotropic occupation of the three possible SDWs with their wave vectors Q pointing along the {001} directions. In a pure 3000 Å thick Cr[001] film, the SDW exhibits an almost complete out-of-plane orientation, whereas in a Cr film of the same thickness capped by a 20 Å Fe layer the SDW becomes completely reoriented with Q now propagating in the plane. This SDW is preserved over the entire temperature range from 10 K up to the Néel temperature of about 311 K.

Magnetic anisotropy, coupling, and transport in epitaxial Co/Cr superlattices on MgO(100) and (110) substrates

Superlattices of Co/Cr have been epitaxially sputtered onto MgO(100) and (110) substrates coated with epitaxial Cr(100) and (211) buffer layers. The Co thickness is fixed at 20 Å and the Cr thickness varied from 7 to 22 Å. On the MgO(110)/Cr(211) substrates, coherent hcp-Co(11̄00)/bcc-Cr(211) superlattice structures are formed. On MgO(100)/Cr(100), x-ray-diffraction results suggest strained hcp-Co(112̄)/bcc-Cr(100) superlattices. Magnetization measurements show fourfold magnetic in-plane anisotropy for the MgO(100) orientation and twofold for the MgO(110). By utilizing a simple model based upon perpendicular uniaxial anisotropies, we have concluded that the fourfold anisotropy has its origin in the second-order uniaxial Co anisotropy energy. The antiferromagnetic interlayer coupling strength exhibits a maximum value of 0.15 erg/cm2 at a Cr thickness of 13 Å in the MgO(110) orientation. The MgO(100) orientation exhibits its strongest coupling of 0.55 erg/cm3 at 10 Å Cr thickness. Modest giant magnetoresistance values no larger than 3% are observed and we find no evidence of enhanced anisotropic magnetoresistance effects recently reported for Co(11̄00)/Cr(211) superlattices.

K-Space Origin of the Long-Period Oscillation in Fe/Cr Multilayers: A Photoemission Study of Epitaxial Cr Grown on an Fe(100) Whisker

The k-space origin of the long-period (18 \AA{}) oscillation in the interlayer magnetic coupling of Fe/Cr(100) multilayers is investigated by means of angle-resolved photoemission by probing quantum well (QW) states at the Fermi energy for epitaxial Cr grown on an Fe(100) whisker at 300 \ifmmode^\circ\else\textdegree\fi{}C. The periodicity of the intensity oscillations of the Cr QW states in the vicinity of the d-derived ``lens'' feature of the Fermi surface is $17\ifmmode\pm\else\textpm\fi{}2\AA{}$. Thus the lens is identified as a prime candidate for the origin of the long period.

Temperature-dependent study of ion-channeling in [formula omitted] superlattices

Giant- Magneto- Resistance (GMR), as large as 150% at 4 K, occurs in Fe Cr superlattices as a result of antiferromagnetic interlayer coupling. We have successfully grown epitaxial single-crystal Fe Cr multilayers using magnetron sputtering. Ion channeling was employed to study the structural and vibrational properties of the sputter-deposited Fe Cr superlattices, and of a Cr thin film, between temperatures of 100 and 330 K. Channeling in the latter specimen was used to investigate the importance of depositing a Cr-buffer layer in order to obtain superlattices with large GMR values. Once the buffer layer exceeded a critical thickness, a high quality Cr film was observed. The epitaxial quality of the superlattices grown on such buffer layers by sputtering was found to be excellent. Minimum yields nearly equal to theoretical predictions were found for channeling along the 〈001〉 growth direction; slightly higher values were found along the 〈111〉 axis. Because of the high structural quality of the sputter-deposited films, it was possible to investigate changes in thermal vibration amplitudes, even though their magnitude is only of the order of a few pm (10 −12 m). No unusual structural changes of this magnitude were observed in angular channeling scans obtained while cooling the Fe Cr superlattice from 330 down to 100 K.

Magnetic moments in thin epitaxial Cr films on Fe(100).

The absolute magnetic moments of thin Cr overlayers on Fe(100) are directly determined by energy-resolved spin-polarized secondary-electron emission. Spin-dependent attenuation of low-energy secondary electrons is quantitatively treated, following a model by Siegmann, to extract magnetic depth profiles in the adlayer. The first monolayer of Cr couples antiferromagnetically to the Fe substrate and exhibits a maximum magnetic moment of 1.8\ifmmode\pm\else\textpm\fi{}0.2${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per atom for a submonolayer coverage. Subsequent Cr layers show a positive magnetization. \textcopyright{} 1996 The American Physical Society.

Relation between structural and physical properties in magnetic and superconducting superlattices

The physical properties of superlattices are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. In this paper the relationship between structure and properties will be illustrated by the giant magnetoresistance (GMR) effect, as well as the dimensional transitions and pinning mechanisms in respectively magnetic and superconducting superlattices. In the first example we will analyse the effect of the sample structure and interface quality on the GMR of poly crystalline and epitaxial Fe Cr superlattices grown by molecular beam epitaxy. Secondly, the effect of thermal annealing and ion irradiation on the electrical and magnetic properties of Ag Fe multilayers is discussed. In a third example, the influence of a very thin Fe interlayer on the coupling phenomena in Fe Nb multilayers is analysed. Finally, it will be shown that an additional lateral modulation (lattice of submicron holes) in Pb Ge multilayers substantially changes the critical superconducting parameters.

Charge-density waves and strain waves in thin epitaxial Cr(001) films on Nb.

We have investigated the magnetic structure of thin epitaxial (001)-oriented Cr films grown on a Nb buffer layer on sapphire. By means of x-ray diffraction measurements the charge density waves (CDW) and strain waves (SW) in Cr film with thicknesses between 500 and 3000 A have been studied. The results show that there exists an orientational pinning effect at both the Cr surface and the interface between Cr and the Nb buffer layer which causes an enlargement of the CDW-SW period, and a single {bold q} domain mode having a {bold q} vector pointing perpendicular to the surface. This pinning behavior relaxes wth increasing film thickness.

Magnetic polarization of epitaxial Cr and Pd by a Co(0001) substrate

The magnetic polarization induced in Cr or in Pd by contact with Co(0001) consists of a local contribution in the very interface layer superimposed by a polarization tail which extends over several atomic layers.

Charge density waves and strain waves in thin epitaxial Cr films

We present the observation of charge density waves and strain waves in thin epitaxial Cr(001) films by means of scattering with synchrotron radiation. The Cr films were grown by molecular beam epitaxial techniques on MgO(001) substrates and on Al2O3 (11¯02) substrates with a Nb(001) buffer layer. The ratio between the amplitudes of both modulations can be derived from the measured intensities. From the data a substrate induced change of this ratio and of the modulation wavelength as compared to the bulk value can be infered.

Magnetoresistance in epitaxial FeCr(001) superlattices grown on MgO(001) and sapphire( [formula omitted]) substrates

To clarify the effect of the substrate on the giant magnetoresistance of epitaxial FeCr(001) superlattices, Cr-thickness dependence of both magnetoresistance (MR) and magnetization are studied using the same superlattice structure grown on different substrates, MgO(001) or Al 2O 3( 1 102 ). With varying thickness of Cr layer, the MR ratio undergoes an oscillatory behavior. At the first peak (∼ 1 nm Cr), the superlattice on sapphire shows a larger MR ratio and a larger saturation field than that on MgO. At the second peak (∼ 1.7 nm Cr), however, a larger MR ratio is observed for the superlattice on MgO.