Epitaxial Cr Research Articles

Growth and surface magnetism of ultrathin Cr(001) films

We investigate the growth of ultrathin Cr films on a Au(001) surface and observe that the growth of 1.5 nm thick Cr layers at 290 K, followed by post-annealing at 520 K, results in high-quality epitaxial Cr(001) films with atomically flat large terraces and distinct surface states. Subsequently, these optimized growth conditions are successfully applied to the growth of 1 nm and 3 nm thick Cr films. Magnetic imaging of 1 and 1.5 nm thick Cr(001) films prepared under the optimized growth conditions is performed using spin-polarized scanning tunneling microscopy. Distinct magnetic contrasts featuring a topological antiferromagnetic (TAF) order are observed in both films; however, spin frustration originating from the density of screw dislocations for both films shows a significant difference. The 1.0 nm thick Cr film, which exhibits a clear TAF order with the suppression of a large spin-frustrated area, is suitable for application to spin-electronic devices.

Electron spin resonance study of an annealed Cr,Mg:YAG epitaxial saturable absorber

We have demonstrated the use of electron spin resonance (ESR) spectroscopy to observe the influence of high-temperature annealing in an oxidizing atmosphere on Cr3+ ion concentration changes in an epitaxial Cr,Mg:YAG saturable absorber. For this purpose, a set of samples of epitaxial Cr,Mg:YAG/YAG structures differing in both the concentration of Cr and Mg ions and the concentrations ratio of these ions have been synthesized using liquid phase epitaxy. The X-ray diffraction analysis and optical transmission spectra measurements were performed to characterize the obtained layers. The usefulness of the ESR technique for monitoring certain high-temperature induced changes in epitaxial Cr,Mg:YAG absorber has been verified.

Antiferromagnetic domain formation and spin frustration induced by adjacent paired screw dislocations in 10 monolayer-thick Cr(001) films

We investigated the growth and surface morphology of 10 monolayer (ML)-thick Cr(001) films on clean Au(001) surfaces. High quality epitaxial Cr(001) films with large atomically flat terraces and distinct surface states were successfully fabricated through growth at 300 K and subsequent post-annealing at 520 K. At 300 K, spin-polarized scanning tunneling microscopy images of both the topological and magnetic structures of this Cr film were obtained. The magnetic images exhibited the following features: (1) The layered antiferromagnetic (AF) order appeared in adjacent terraces and one ML-depth shallow hole in the terraces; (2) significant spin frustrations induced by adjacent paired screw dislocations caused the AF domain formation with 90 degrees quantum axis rotation and a large spin frustration area, not always limited in the vicinity of screw dislocations. The feature (2) was qualitatively reproduced by the micromagnetic simulation. These findings may be essential for the further development of spin-electronics utilizing thin AF films.

Odd spin frustration in Cr(001) films thinner than three nanometers revealed by spin-polarized scanning tunneling microscopy

We have studied the surface structure and morphology of epitaxial Cr films on Au(001) substrate with Cr thickness (${d}_{\mathrm{Cr}}$) below 3 nm. We have characterized the films prepared under various growth conditions by using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction, and Auger electron spectroscopy. The good growth conditions of the Cr(001) films, which have atomically flat terraces without surface segregation of Au and with distinct surface states, are obtained by the following two methods. (I) ${d}_{\mathrm{Cr}}=3$ nm film realized by two-step growth, i.e., first 1.5 nm of Cr deposited at room temperature ($\ensuremath{\sim}290$ K) and an additional 1.5 nm Cr deposition at 570 K and (II) ${d}_{\mathrm{Cr}}=1.5$ nm film by room temperature growth and subsequently post-annealing at 470 K. The magnetic imaging of (I) ${d}_{\mathrm{Cr}}=3$ nm Cr(001) film was performed by means of spin-polarized STM/STS. The observed magnetic image indicates that the topological antiferromagnetic (AF) order appears in a series of adjacent terraces and two types of spin frustration caused by screw dislocations lead to modifications of the topological AF order. One type of spin frustration is accompanied by narrow domain walls between two screw dislocations. The other type is a complex spin frustration caused by a cluster of an odd number of screw dislocations. The difference of the two types of spin frustration is explained through a micromagnetic simulation. We have also identified a large spin-frustrated area, consisting of a cluster of multiple number of screw dislocations.

Study and optimization of epitaxial films of Cr and Pt/Cr bilayers on MgO

We report a systematic study of the fabrication and chemical and structural characterization of epitaxial Cr films and Pt/Cr bilayers on MgO substrates. Cr films of different thicknesses were grown by molecular beam epitaxy at 373 K and then post-annealed at different temperatures. These films are chemically stable, present a well-ordered tetragonal structure, with only a low degree of oxidation distributed throughout the films. The lattice parameter can be finely tuned by varying the film thickness and the post-annealing temperature, as the elastic energy is progressively relaxed by edge dislocations, so that the film can approach the structural and electrical properties of bulk chromium. Moreover, the chemical and structural properties of Cr are not affected by a Pt capping layer, which grows in a well-ordered Pt[1 1 0]||Cr[1 0 0] orientation, holding potential for applications in recently proposed Pt/Cr-based antiferromagnet spintronics and for the realization of epitaxial Pt films on MgO by using a thin buffer layer of chromium.

Tailoring low energy electron absorption via surface nano-engineering of cesiated chromium films

In this letter, we demonstrate that improved low energy electron absorption is achieved by suppressing the crystallinity of chromium thin-films grown on W[110], which points to a promising route for achieving highly efficient thermionic energy converters. Using low energy electron microscopy (LEEM) and in situ film growth, we show that substrate temperature control permits well-controlled fabrication of either epitaxial Cr[110] films or nanocrystalline Cr layers. We show that the work function of cesium saturated nanocrystalline Cr thin-films is ∼0.20 eV lower than that of epitaxial Cr[110] films. Our LEEM measurements of absorbed and reflected currents as a function of electron energy demonstrate that nanocrystallinity of cesiated chromium films results in 96% electron absorption in the range up to 1 eV above the work function, compared to just 79% absorption in cesiated crystalline Cr[110] films. These results point to metal films with suppressed crystallinity as an economical and scalable means to synthesize nanoengineered surfaces with optimized properties for next generation anode materials in high performance thermionic energy converters.

Electrical and structural properties of epitaxially deposited chromium thin films

We studied the electrical resistance and crystal structure of epitaxial chromium (Cr) films. The lattice constant of the Cr films was larger than that of the bulk Cr because of MgO substrate on which Cr was epitaxially deposited. A chromium oxide layer having a thickness of 1 nm was found on all films from the result of X-ray reflectivity measurements. The electrical resistivity ρ(T) shows metallic behavior for all epitaxial Cr films in contrast with polycrystalline one. However, the magnitude of ρ tends to increase and the antiferromagnetic interaction is suppressed as decreasing thickness of film.

Giant planar Hall effect in reactive sputtered epitaxial CrxFe3–xO4 films

The epitaxial CrxFe3-xO4 films were fabricated by dc reactive sputtering on MgO (001) substrates. The longitudinal resistivity ρxx was enhanced 3 orders of magnitude with the increase of Cr content x from 0 to 0.87. The tunneling magnetoresistance like planar Hall effect in the CrxFe3-xO4 films was observed while the angle between ⟨100⟩ in (001) oriented films and the direction of magnetic field is 45° and 135°, respectively. Both the phase relation of angular dependent planar Hall resistivity ρxy and the numeral relation of ρxy with longitudinal resistivity difference ρ//−ρ⊥ cannot be understood by the planar Hall effect expression in isotropic magnetic medium. The largest planar Hall resistivity was ∼105 μΩ cm for x = 0.71, which is one, two, and six orders of magnitude larger than that in Fe3O4, GaMnAs, and ferromagnetic metals, respectively. The giant planar Hall resistivity ρxy is weak saturated at high fields and increases with the magnetic field. This giant planar Hall effect in the highly resistive CrxFe3-xO4 films is closely correlated to the longitudinal resistivity and antiphase boundaries.

Diagnostics of the atomic structure of multilayer metallic nanoheterostructures from reflectometry data: A new approach to low-contrast systems

A new method for determining the concentration profiles of chemical elements in multilayer metallic nanoheterostructures from X-ray reflectometry data has been developed as applied to low-contrast systems. The method is based on the solution of the Fredholm integral equation of the first kind, which relates the reflection coefficient and the concentration profile of the chemical elements involved in the composition of the sample. The ill-posed inverse problem of the determination of the concentration profile is solved by the regularization method. The efficiency of the proposed method is confirmed by model calculations performed for a four-layer Cr/Gd/Fe/Cr//Si structure with high-contrast Cr/Gd pairs and low-contrast Fe/Cr pairs. The experimental results on the determination of the concentration profile of the surface layer of the epitaxial thin Cr films and three-layer Cr/Fe/Cr structure deposited on the Al2O3 substrate are presented.

Contrast enhancement of data measured with area detectors: a way to generalize the use of neutron diffraction for thin-film studies

A denoising method is reported for the treatment of neutron scattering data obtained with position-sensitive detectors, which enhances the information obtained from weak and very weak Bragg peaks. The core element of the method is the application of a Laplacian of Gaussian filter calculated using the parameters of the resolution of the instrument. This adaptation of well established image-processing techniques offers a very efficient way to denoise the data, as shown through the application of the reported method to a study of the magnetic Bragg peaks of a 300 nm-thick epitaxial Cr film. The procedure enhances the contrast by a factor of more than 35 and thus allows precise determination of the position of the integration mask. The large contrast enhancement also lowers the detection threshold of standard elastic neutron diffractometers down to the level usually available solely on optimized triple-axis spectrometers.

Barkhausen-type noise in the resistance of antiferromagnetic Cr thin films

We present an experimental study of the changes generated on the electrical resistance R(T) of epitaxial Cr thin films by the transformation of quantized spin density wave domains as the temperature is changed. A characteristic resistance noise appears only within the same temperature region where a cooling-warming cycle in R(T) displays hysteretic behavior. We propose an analysis based on an analogy with the Barkhausen noise seen in ferromagnets. There fluctuations in the magnetization M(H) occur when the magnetic field H is swept. By mapping and , where Ψ0 corresponds to the order parameter of the spin density wave, we generalize the Preisach model in terms of a random distribution of resistive hysterons to explain our results. These hysterons are related to distributions of quantized spin density wave domains with different sizes, local energies and number of nodes.

Chromium nanostructures formed by dewetting of heteroepitaxial films on W(100)

In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer on W(100). We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands' shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of \ensuremath{\sim}1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets \ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} off of the substrate normal bound by (210) planes at \ensuremath{\sim}26\ifmmode^\circ\else\textdegree\fi{}. The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal. We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology.

Effects of spin density wave quantization on the electrical transport in epitaxial Cr thin films

We present measurements of the electrical resistivity $\ensuremath{\rho}$ in epitaxial Cr films of different thicknesses grown on MgO (100) substrates, as a function of temperature $T$. The $\ensuremath{\rho}(T)$ curves display hysteretic behavior in a certain temperature range, which is film thickness dependent. The hysteresis are related to the confinement of quantized incommensurate spin density waves (ISDW) in the film thickness. Our important finding is to experimentally show that the temperature ${T}_{\text{mid}}$ where the ISDW changes from $N$ to $N+1$ nodes decreases as the film thickness increases. Identifying ${T}_{\text{mid}}$ with a first-order transition between ISDW states with $N$ and $N+1$ nodes, and using a Landau approach to the free energy of the ISDW together with Monte Carlo simulations, we show that the system at high temperatures explores all available modes for the ISDW, freezing out in one particular mode at a transition temperature that indeed decreases with film thickness $L$. The detailed dependence of ${T}_{\text{mid}}(L)$ seems to depend rather strongly on the boundary conditions at the Cr film interfaces.

Epitaxial Cr on n-SrTiO3(001)—An ideal Ohmic contact

Epitaxial Cr metallizations grown on n-SrTiO3(001) by molecular beam epitaxy are shown to result in an ordered interface with Cr bound to O in the terminal TiO2 layer, no reduction of the SrTiO3, and a near-perfect Ohmic contact. Cr/n-SrTiO3(001) thus constitutes an ideal interface between a pure metal and wide gap oxide in which interface redox chemistry does not occur, and the Fermi level remains unpinned.

Plastic response of the native oxide on Cr and Al thin films from in situ conductive nanoindentation

Abstract

Correlation between symmetry-selective transport and spin-dependent resonant tunneling in fully epitaxial Cr/ultrathin-Fe/MgO/Fe(001) magnetic tunnel junctions

Spin-dependent resonant tunneling was investigated in fully epitaxial Cr(001)/ultrathin Fe(001)/MgO(001)/Fe(001) pseudo double-barrier magnetic tunnel junctions (MTJs) by varying structural coherence of the MgO barrier through postdeposition annealing and Mg layer insertion. It was clearly demonstrated that not only the flatness of the Fe quantum-well layer, but also the structural coherence of the MgO barrier caused the appearance of sharp resonant conductance peaks which were not obtained for spin-dependent resonant tunneling in amorphous AlO-barrier MTJs. The symmetry-selective transport via MgO barrier plays a crucial role for the resonant tunneling as well as large tunneling magnetoresistance.

Epitaxial growth and electrical transport properties of Cr2GeC thin films

Cr${}_{2}$GeC thin films were grown by magnetron sputtering from elemental targets. Phase-pure Cr${}_{2}$GeC was grown directly onto Al${}_{2}$O${}_{3}$(0001) at temperatures of 700--800 \ifmmode^\circ\else\textdegree\fi{}C. These films have an epitaxial component with the well-known epitaxial relationship Cr${}_{2}$GeC(0001)//Al${}_{2}$O${}_{3}$(0001) and ${\mathrm{Cr}}_{2}\mathrm{GeC}(11\overline{2}0)//{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(1\overline{1}00)$ or ${\mathrm{Cr}}_{2}\mathrm{GeC}(11\overline{2}0)//{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(\overline{1}2\overline{1}0).$ There is also a large secondary grain population with $(10\overline{1}3)$ orientation. Deposition onto Al${}_{2}$O${}_{3}$(0001) with a TiN(111) seed layer and onto MgO(111) yielded growth of globally epitaxial Cr${}_{2}$GeC(0001) with a virtually negligible $(10\overline{1}3)$ contribution. In contrast to the films deposited at 700--800 \ifmmode^\circ\else\textdegree\fi{}C, the ones grown at 500--600 \ifmmode^\circ\else\textdegree\fi{}C are polycrystalline Cr${}_{2}$GeC with $(10\overline{1}0)$-dominated orientation; they also exhibit surface segregations of Ge as a consequence of fast Ge diffusion rates along the basal planes. The room-temperature resistivity of our samples is 53--66 \ensuremath{\mu}\ensuremath{\Omega}cm. Temperature-dependent resistivity measurements from 15--295 K show that electron-phonon coupling is important and likely anisotropic, which emphasizes that the electrical transport properties cannot be understood in terms of ground state electronic structure calculations only.

Textured growth of the high moment material Gd(0 0 0 1)/Cr(0 0 1)/Fe(0 0 1)

By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of 57Fe-CEMS (conversion electron Mössbauer spectroscopy) in combination with the 57Fe tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)∥Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory.

Structural and magnetic phase diagrams of epitaxial Cr–Mn alloy thin films

We report here a systematic study of Cr–Mn alloy films that have been epitaxially stabilized on GaSb (100) using molecular beam epitaxy. The crystal structural transition between the α-Cr-type and the α-Mn-type for the Cr–Mn alloy films is observed along with changes in growth temperature, film thickness, and the ratio of Cr to Mn. Ferrimagnetism is observed in the Cr–Mn films containing the α-Mn-type phase based on the magnetic field-dependent anomalous Hall effect hysteresis and is corroborated by the magnetization hysteresis. The α-Mn-type CrMn phase at the expanded lattice parameter induces the observed ferrimagnetic ordering. The magnetic moments of Cr–Mn films can be tuned by adjusting the growth temperature, film thickness, and the ratio of Cr to Mn. Eventually, new structural and magnetic phase diagrams of the epitaxial Cr–Mn alloy films are established. The results of this study can prove helpful in both forming a comprehensive understanding of Cr–Mn alloys and in finding new applications for it in spintronic devices.

ChemInform Abstract: Magnetism of Chromium at Surfaces, at Interfaces, and in Thin Films

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 magnetism and the Neel temperature are well established for bulk Cr, the question arises of how these properties are altered in thin films and superlattices either due to dimensionality effects or due to proximity with the ferromagnetic or paramagnetic boundary layers. After a brief introduction to the basic properties of bulk Cr, this review provides an overview of the SDW magnetism in thin Cr films, starting with surface properties and continuing with the discussion of Cr films of various thickness. The emphasis is more on SDW order in different confined environments than on exchange coupling. The scaling of the Neel temperature with thickness, the critical thickness for the onset of SDW order, the orientation of the SDW wave vector in different environments and the enhancement of SDW order due to proximity effects are extensively discussed. Most important is the role of the interface roughness in case of contact with a ferromagnetic layer. Conflicting results obtained with different experimental techniques are critically reviewed and an interpretation of the SDW order depending on interface quality is proposed.