The magnetic map of NiMn alloy thin films on Co(0 0 1) and Co(1 1 1)

Following the experimental work of Groudeva-Zotova et al. [S. Groudeva-Zotova, D. Elefant, R. Kaltofen, D. Tietjen, J. Thomas, V. Hoffmann, C.M. Schneider, J. Magn. Magn. Mater. 263 (2003) 57] where the magnetic and structural characteristics of a bi-layer NiMn-Co exchange biasing systems was investigated, density functional calculations with generalized gradient corrections were performed on (Mn_0.5Ni_0.5)_n ordered alloy on Co(0 0 1) and one Mn_1−xNi_x monolayer on Co(1 1 1). For the Mn_0.5Ni_0.5 monolayer on Co(0 0 1), magnetic moments per surface atom of 0.65 μ_B and 3.76 μ_B were obtained for Ni and Mn, respectively. Those magnetic moments are aligned parallel to the total moment of Co(0 0 1). A complex behavior of the Mn moment in dependence of the thickness “n” is obtained for (Mn_0.5Ni_0.5)_n on Co(0 0 1). Investigations on Mn_1−xNi_x monolayer on Co(1 1 1) have shown that the crystallographic orientation does not modify significantly neither the magnetic moments of Mn and Ni atoms nor their ferromagnetic coupling with the Co(1 1 1) substrate, except for x = 0.66. For x = 0.66 the Mn sub-lattice presents an antiferromagnetic coupling leading to a quenching of the Ni magnetic moment.     

Magnetic ordering of Vn/Mo(0 0 1) systems: Ab-initio calculations

A density-functional theory (DFT) study is performed using a full-potential linearized-augmented-plane-waves (FP-LAPW) method to investigate the magnetic structure of vanadium-molybdenum systems (V_n/Mo(0 0 1), n = 1, 2). The topmost V layers relax inward in both systems with a larger contraction in V₂/Mo(0 0 1) system. A p(1 × 1) in-plane ferromagnetic ordering with appreciable magnetic moments is obtained on V overlayers, which is found to be the ground state in both systems. The layers below the surface exhibit induced magnetism with antiferromagnetic interlayer coupling.     

Oscillatory ferromagnetic interlayer coupling in [Pt(5 Å)/Co(4 Å)]3/Cr(x Å)/[Co(4 Å)/Pt(12 Å)]3 multilayers with perpendicular anisotropy

Investigation has been performed on the interlayer coupling between two Co/Pt multilayers with perpendicular anisotropy separated by Cr spacers. As a function of the Cr spacer thickness, only ferromagnetic interlayer coupling has been observed between the two Co/Pt multilayers in contrast to the oscillatory interlayer coupling between ferromagnetic and antiferromagnetic observed in ferromagnetic layers with in-plane anisotropy separated by Cr spacers. It is the strength of the ferromagnetic interlayer coupling that has been observed to be oscillatory as a function of the Cr spacer thickness with a period of about 7 Å.     

From antiferromagnetic to ferromagnetic coupling for V adatoms on Co(001) substrates

We discuss the polarization of V atoms on Co(001) substrates within density functional calculations. For sub-monolayer coverage the coupling between V and Co is clearly of antiferromagnetic type whereas it changes to ferromagnetic coupling in the case of a full V monolayer on Co(001). The results obtained in the case of a sub-monolayer coverage are in agreement with recent X-ray magnetic circular dichroism by Huttel et al. [Phys. Rev. B 68, 174405 (2003)]. The transition from antiferromagnetic coupling (in the case of sub-monolayer coverage) to ferromagnetic coupling (for a full monolayer coverage) is discussed in terms of local coordination numbers and V-Co hybridization. Comparison with Cr and Mn coverages on Co(001) complicates the problem: i) submonolayer Cr coverage stabilizes the antiferromagnetic coupling between Cr and Co atoms (like for V on Co(001) whereas a Cr monolayer on Co presents in-plane antiferromagnetic coupling; ii) submonolayer Mn coverage stabilizes now the ferromagnetic coupling between Mn and Co whereas a Mn monolayer on Co(001) presents an in-plane antiferromagnetic coupling. Competition between Co induced magnetism and surface induced magnetism at V sites 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.     

Theoretical study of magnetism and electronic structure of Fe3/Crn(1 1 0) superlattices

The electronic structure and magnetism of Fe₃/Cr_n(1 1 0) (n=1, 3, 5) superlattices (SL) with varying layer thickness have been studied using the full-potential linearized augmented plane-wave (FLAPW) method within the first-principle formalism. The results show that the ferromagnetic state is the preferable phase in the ground state. The magnetic moments of the Fe layers are slightly modified by the presence of the Cr layers. The Cr magnetic moments alternate direction from layer to layer, and an antiferromagnetic coupling between Fe and Cr at the interfacial layer is seen. The magnetic moments of the Cr layers are suppressed because there is a strong hybridization between d-states of both Fe and Cr atoms. Only a small moment is found in the Cr layer. The Cr moment alignment is determined by a delicate balance between the different magnetic interaction.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Magnetism of Co overlayers and nanostructures on W(0 0 1): A first-principles study

The magnetic properties of Co nanostructures and a Co monolayer on W(0 0 1) have been studied in the framework of density functional theory. Different geometries such as planar and three-dimensional clusters have been considered, with cluster sizes varying between 2 and 13 atoms. The calculations were performed using the real-space linear muffin-tin orbital method (RS-LMTO-ASA). With respect to the stability of the magnetic state, we predict an antiferromagnetic (AFM) structure for the ground state of the planar Co clusters and a ferromagnetic (FM) state for the three-dimensional clusters. For the three-dimensional clusters, one of the AFM arrangements leads to frustration due to the competing FM and AFM exchange interactions between different atoms in the cluster, and gives rise to a non-collinear state with energy close to that of the FM ground state. The relative role of the Co–Co and Co–W exchange interactions is also investigated.     

First-principles study of the structural, magnetic, and electronic properties of LiMBO3 (M = Mn, Fe, Co)

We present a first-principles calculation for the structural, magnetic, and electronic properties of LiMBO₃ (M = Mn, Fe, Co). Along the [0 0 1] direction, transition metals shows antiferromagnetic coupling in LiMBO₃ of both hexagonal and monoclinic lattices. The calculated magnetic moment of 5μB per formula unit is close to the experimental value. These compounds are semiconductors with band gap of 0.4-2 eV, and with average intercalation voltages of 2-4.8 V.     

Magnetic and electronic structure in new iron-based layered SrFe1 − xCoxAsF systems

By using the first-principles calculation, we study the structural, magnetic, and electronic properties of the SrFeAsF compound and its Co-doped counterpart SrFe_1 − xCo_xAsF (x=0.125). It is shown that the competition of the nearest and next-nearest neighbor exchange coupling J₁ and J₂ between Fe ions gives rise to a frustrated striped antiferromagnetic order in SrFeAsF and an accompanied lattice distortion, while for the Co-doped case, both J₁ and J₂ decrease significantly as well as the lattice distortion, and thus the antiferromagnetic order is suppressed greatly. This is further confirmed by the electronic structure calculation that the density of states at the Fermi level increases with Co doping as well as the itinerancy of Fe d electron.     

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.     

The formation of sharp NiO(1 0 0)-cobalt interfaces

An atomically sharp interface between an antiferromagnetic oxide and a ferromagnetic metal may be obtained by the deposition of an epitaxial oxide buffer nanolayer in between. The buffer layer consists of the oxide of the ferromagnetic metal. The concept has been demonstrated on the NiO(1 0 0)-Co system, where the inclusion of a 1-2 ML CoO(1 0 0) interlayer inhibits the interfacial redox reaction which takes place between NiO and Co metal in the absence of the buffer layer.     

Magnetism of surface alloys of the type MxN1−x/Rh(1 1 1)

We present a density functional theory study on the magnetic properties of two-dimensional surface alloys of the type M_xN_1−x (M=Fe, Co and Ni; N=Pt, Au, Ag, Cd and Pb) on Rh(1 1 1) for x=0.0, 0.25, 0.33, 0.5, 0.67, 0.75 and 1.0, in two types of geometric arrangements—striped phases or linear-chain type, and non-striped phases or mixed checkerboard type. Many pairs among these are bulk-immiscible but show mixing on the surface. We find that the trend in the magnetic moment of surface alloys of N with a given M follows the number of valence electrons in N: the higher the number of valence electrons, the lower the magnetic moment. Overlayer atoms when put on hcp sites show higher moment compared to fcc sites. In general, for a given composition x, linear-chain type structures show a reduced magnetic moment compared to checkerboard type structures. We find that Pb, when alloyed with magnetic elements (Fe, Co and Ni), has a lowering effect on their magnetic moments.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

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.     

Magnetic and electrical properties of new magnetic phase in Si1−xMnx crystals

We have investigated the magnetic and electrical transport properties of Si_1−xMn_x single crystals grown by the vertical Bridgman method. The alloys with Mn concentrations up to x=0.64 have weak ferromagnetic ordering around T_C∼30 K. However, Si_0.25Mn_0.75 alloys show weak ferromagnetic ordering at 70 K and antiferromagnetic ordering at 104 K, which is confirmed by magnetization and electrical transport studies.     

Ab initio study of the fcc-WC(1 0 0) surface and its interaction with cobalt monolayers

The WC(1 0 0) surface has been studied by using ab initio methods of the density functional theory and pseudopotentials. Calculations have shown that surface and undersurface atoms move from their bulk positions. Namely, carbon atoms moved outward, while tungsten atoms moved inward. Five geometric cases for Co/WC(1 0 0) system were compared: (A) Co atoms are above C atoms; (B) Co atoms are above W atoms; (C) Co atoms are in the four-fold sites above WC pairs; (D and E) Co atoms are above the W-W-C and C-C-W three-fold sites, respectively - and the (A) case has been found to be energetically preferable. In all cases, Co layers have been found to be ferromagnetic. The densities of states for the bulk fcc-WC, the WC(1 0 0) surface, and the WC/Co system were compared.     

Interdiffusion of adsorbed Pb and Bi on Cu(1 0 0)

The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(1 0 0) has been studied, at T = 513 K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consist of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features lead to the conclusion that Pb atoms are more strongly bound at surface alloyed sites in Cu(1 0 0) than Bi atoms. Once the total coverage (Pb + Bi) on the substrate reaches about one monolayer, Pb and Bi are dealloyed from the substrate, and the interdiffusion profiles become essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of ∼10⁻¹³ m²/s. This is considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion is different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction, leading to the conclusion that binding of Bi atoms to the Cu(1 0 0) substrate is stronger than that of Pb atoms in the highest-coverage surface dealloyed layers.     

Metastable magnetic configurations of (V, Cr, Mn and Fe) on Ir(0 0 1) surfaces

We present a theoretical study of the electronic and magnetic structure of the 3d-transition metals (M = V, Cr, Mn and Fe) in several overlayer systems. The electronic as well as magnetic structures are investigated for pseudomorphic overlayers (M/Ir(0 0 1)), ordered alloyed overlayers of the type M_0.5Ir_0.5/Ir(0 0 1) and ordered binary surface alloys of V, Cr, Mn and Fe transition metals on Ir(0 0 1) substrates. The calculations are performed with a self-consistent tight-binding method using the unrestricted Hartree-Fock approximation within the Hubbard model. We obtained metastable c(2 × 2) configurations for V, Cr and Mn and a p(1 × 1) configuration for Fe pseudomorphic overlayers. However, ferrimagnetic configuration has been obtained for the ordered surface alloys M_0.5Ir_0.5 and the binary alloyed overlayers on Ir(0 0 1) surfaces.