Exchange Interactions and Curie Temperatures of 3D- and 2D-Ferromagnets

Effective exchange interactions in bulk ferromagnets as well as in magnetic overlayers on Cu(001) covered by a Cu-cap layer of varying thickness were determined from first principles by mapping of corresponding total energies onto the effective Heisenberg model in the framework of the adiabatic approximation and magnetic force theorem. The effective Heisenberg model is then used to determine spin-wave stiffness constants and Curie temperatures evaluated in the framework of the random-phase approximation. Calculations are in a fair agreement with available experimental data for bulk ferromagnets and reproduce an oscillatory Curie temperature of magnetic overlayers as a function of Cu-cap thickness in a qualitative agreement with recent experiments.     

Magnetic properties of thin epitaxial films investigated by spin-polarized photoemission

The surface sensitivity of the spin-polarized photoemission experiment was exploited to study two-dimensional magnetism. The magnetization of thin films of Fe, Co, and V in the monolayer (ML) range, grown on Cu(001) and Ag(001) single crystals, was measured as a function of perpendicularly applied field and temperature. Bcc Fe films and fcc Fe and Co films exhibit ferromagnetism down to the single monolayer range, while no evidence for ferromagnetism is found for V on Ag(001). All Co films are magnetized in plane and have a Curie temperature far above room temperature. A thickness dependence of the anisotropy and Curie temperature is observed for the two phases of Fe. Remanent magnetization perpendicular to the surface is found at 30 K for fcc Fe films thicker than 2 ML and for bcc Fe between 3 and 4 ML. The magnetic effects caused by coating and by interdiffusion are discussed in the light of measurements of Cu/Fe/Cu sandwiches and of overlayers obtained by simultaneous evaporation of Fe and Cu. The fcc Fe films are shown to be suitable for thermomagnetic writing.     

Finite-temperature magnetism of thin Ni films in sandwiches of Cu

The finite-temperature magnetism of thin Ni films sandwiched in Cu(001) layers, is discussed with the use of the itinerant-electron theory including the effect of spin fluctuations by means of the functional-integral method within the static and single-site approximations. The spatial- and temperature-dependent magnetization and the Curie temperature are computed by varying the Ni-film thickness from one to ten layers. The calculated results are discussed with reference to the recent experiments on Cu/Ni/Cu and Au/Ni/Au sandwiches and on compositionally modulated Ni-Cu superlattices.     

Cu cap layer on Ni8/Cu(001): reorientation and TC-shift

Adding a non-magnetic Cu overlayer on Ni₈/Cu(001) is known to induce the reorientation of the magnetic easy axis from in-plane to out-of-plane and to reduce the Curie temperature. In this paper both effects are described on the same footing using an effective Heisenberg trilayer. The model takes into account thermal fluctuations of the magnetization and allows to separate explicitly between two possible mechanisms behind the experimental observations, namely between a reduction of the magnetic moment by hybridization and a lattice relaxation at the Ni/Cu interface. Ferromagnetic resonance data for the reorientation and the decreased Curie temperature due to the Cu cap layer are best reproduced by assuming a reduction of the magnetic moment at the interface by ≈1/3.     

Ultrathin Co/Cu(110) film as a lattice of ferromagnetic grains with dipolar interaction

The well-known fact of magnetic ordering in ultrathin Co films (with an effective thickness of several monoatomic layers) on a single-crystal Cu(110) substrate is described quantitatively using the model according to which the thin film is a lattice of three-dimensional ferromagnetic grains with dipole-dipole interactions. The critical film thickness corresponding to the ferromagnetic transition and the corresponding Curie temperature were calculated.     

Long-range chemical interaction in solid-state synthesis: The formation of a CuAu alloy in Au/β-Co(001)/Cu(001) epitaxial film structures

The effect of an inert Co layer (0, 210, 480 nm) on the chemical interaction between Cu and Au in Au/β-Co(001)/Cu(001) epitaxial films has been investigated by X-ray diffraction, nuclear magnetic resonance, photoelectron spectroscopy, and magnetic structure measurements. Mixing at interfaces in Cu/β-Co(001) and Au/β-Co(001) bilayer films has not been revealed up to a temperature of 600°C. The solid-state synthesis of ordered CuAu| and CuAu∥ phases occurs through the Co inert buffer layer in Au/β-Co(001)/Cu(001) trilayer film systems with an increase in the annealing temperature. The initiation temperatures of the CuAu| and CuAu∥ phases increase only slightly with the thickness of the Co buffer layer. The assumption of the long range of the chemical interaction between Cu and Au through the chemically inert Co layer is justified using the performed investigations.     

Theoretical calculation of magnetic properties of ultrathin Fe films on Cu(100)

The temperature dependence of the magnetization in fcc Fe on Cu(100) is calculated using a self-consistent local mean-field theory. The model reproduces an experimental magnetization oscillation as a function of film thickness and supports a picture where the top two layers are ferromagnetically coupled, and the remaining layers are antiferromagnetically coupled. The origin of the puzzling linear temperature dependence in oscillation amplitude is understood as a "surface phenomena" of the antiferromagnetic layer at the Fe/Cu interface. Proximity effects between a thin antiferromagnet with a low Neel temperature and a neighboring ferromagnet with a higher Curie temperature are discussed.     

Curie temperature oscillations and critical phenomena in Co/Cu multilayer films

A study is made of Co/Cu multilayer structures with ultrathin Co layers. Oscillations of the saturation magnetization and Curie temperature as a function of the thickness of the intermediate Cu conducting layer are observed. The behavior of the magnetization near the second-order phase transition is studied, and the critical exponents of the magnetization are determined. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 341–345 (10 September 1996)     

Solid-phase synthesis of solid solutions in Cu/Ni(001) epitaxial nanofilms

Solid-phase synthesis of solid solutions in the epitaxial Cu/Ni(001) bilayer film systems of compositions 3Cu: 1Ni, 1Cu: 1Ni, and 1Cu: 3Ni has been studied using the X-ray diffraction methods. The saturation magnetization and the magnetic crystallographic anisotropy constant on nickel vary in accordance with the solid solution formation. The initiation temperature of the solid solutions is about 350 °C and is independent of the Ni: Cu layer thickness ratio. The solid-phase synthesis of the solid solutions is presumably attributed to the transport of the Cu atoms to the epitaxial Ni(001) layer. It is found that the solid-phase synthesis in the Cu/Ni bilayer nanofilms and multilayers is determined by the spinodal decomposition in the Cu-Ni system.     

Cu-Au体系非对称异质外延行为的分子动力学研究

利用分子动力学模拟方法研究了Cu/Au(001)和Au/Cu(001)异质外延岛的演化行为. 研究结果显示：Cu-Au体系的相互外延行为呈现出明显的非对称性. Cu在Au(001)基体表面可以形成完整的外延结构，而Au在Cu(001)基体表面外延将导致失配位错的出现. 导致非对称外延生长行为的根本原因是外延岛的应变状态的差异和外延岛自身性质的不同. 随着外延岛的长大，Cu外延岛与Au(001)基体的微观失配度由最初的接近宏观失配度的9%左右迅速单调下降，并最终趋于晶格匹配；而Au在Cu(001)基体表面外延的微观失配度则呈现出振荡增加趋势. Cu/Au(001)体系的基体形变主要发生在外延岛的边缘，而Au/Cu(001)体系的基体形变主要发生在外延岛内部所对应的区域.     

Evidence for an anisotropy-induced non-collinear spin state in exchange-coupled Co/Cu(001)

The magnetization behaviour of a Co/Cu/Co(001) sandwich has been studied by magneto-optical Kerr effect measurements. The sample was grown by molecular beam epitaxy onto a sapphire (1 .2) substrate with a Cu/Cr/Nb(001) buffer system. The copper layer had the form of a wedge with the thickness range chosen to be around the second region of antiferromagnetic exchange coupling. The hysteresis loops in the regime of weak antiferromagnetic coupling show characteristic steps, which can be explained by an anisotropy-induced non-collinear spin state. Indication for a similar behaviour is also found in the regime of strong antiferromagnetic coupling. This behaviour is explained by taking into account the competition between anisotropy, interlayer exchange coupling and external field energy. The nature of this metastable non-collinear magnetization state is in marked contrast to the biquadratic (90°) exchange coupling which was discovered in Fe/Cr(001).     

Spin-dependent Fabry-Pérot interference from a Cu thin film grown on fcc Co(001)

Spin-dependent electron reflection from a Cu thin film grown on Co/Cu(001) was investigated using spin-polarized low-energy electron microscopy (SPLEEM). Fabry-Pe rot type interference was observed and is explained using the phase accumulation model. SPLEEM images of the Cu overlayer reveal magnetic domains in the Co underlayer, with the domain contrast oscillating with electron energy and Cu film thickness. This behavior is attributed to the spin-dependent electron reflectivity at the Cu/Co interface which leads to spin-dependent Fabry-Pe rot electron interference in the Cu film.     

Crystallographic structure of ultrathin Fe films on Cu(100)

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.     

Quantum-size-induced oscillations of the electron-spin motion in Cu films on Co(001)

We report on spin-polarized electron reflection experiments in which the electron-spin motion is studied in spin-dependent quantum well structures. Oscillations of the electron-spin motion due to quantum interference are observed in the model system Cu/Co(001) both as a function of electron energy and Cu overlayer thickness. The reflectivity as well as the spin-motion data can be well interpreted in terms of a Fabry-Pérot interferometer model. In particular, this opens the possibility of studying the spin-dependent reflection properties of the buried Cu/Co interface.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Spin polarized low energy electron microscopy investigations of magnetic transitions in Fe/Cu(1 0 0)

Magnetic transitions in ultrathin Fe films on the Cu(1 0 0) surface have been studied with spin polarized low energy electron microscopy. By monitoring averaged image intensity oscillations and the evolution of magnetization and magnetic domain structure simultaneously and continuously during growth, magnetism and film thickness are correlated with unprecedented precision. The thickness range over which ferromagnetism exists in films grown at room temperature generally increases as the deposition rate is decreased. This trend is attributed to the influence of residual hydrogen. The two-dimensional Ising model with finite size scaling of the Curie temperature accurately describes the evolution of magnetization with increasing film thickness.     

Temperature driven reversible breakdown of pseudomorphism in ultrathin Fe/Cu3Au films

We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.     

Bias dependence of apparent layer thickness and Moiré pattern on NaCl/Cu(001)

Bias-dependent features of the insulating NaCl layer grown on Cu(001) have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The apparent layer thickness of the NaCl film is variable at bias voltages ranging from 2.8 to 3.2 V as well as from 4.0 to 5.0 V, and the Moiré pattern induced by NaCl–Cu lattice mismatch also shows bias dependence. The z–V (dz/dV–V) curves and dI/dV mapping measurements reveal that the resonant tunneling between the image potential states (IPSs) on Cu(001) and the Fermi level of the STM tip leads to drastic variations of these features.     

Intrinsic stress of ultrathin epitaxial films

The present article focuses on the stress developing during the deposition of ultrathin epitaxial films in the thickness range of a few atomic layers. The studied systems exhibit the three well-known modes of film growth: Stranski–Krastanow mode [Ge/Si(001), Ge/Si(111), Ag/Si(111)], Frank–Van der Merwe mode [Fe/MgO(001)] and Volmer–Weber mode [Ag/mica(001), Cu/mica(001)]. The experimental results demonstrate the important role of the misfit strain as well as the contribution of surface stress effects as mechanisms for the stress in single atomic layers. Received: 26 April 1999 / Accepted: 25 June 1999 / Published online: 6 October 1999     

Wetting and structural transition induced by segregation at grain boundaries: a monte carlo study.

Wetting of the Sigma = 5 (310) <001> symmetrical tilt grain boundary (GB) close to the solubility limit in the Cu(Ag) solid solution has been observed by means of Monte Carlo simulations at T = 600 K. More precisely, a finite thickness film almost pure in Ag, separating the two initial Cu(Ag) grains, can be obtained from a critical intergranular germ induced by the strong segregation of Ag in the GB. As this film is actually a single crystal, this implies a complete rearrangement of the GB core structure. Thus the initial GB is replaced by two Cu(Ag)/Ag(Cu) interfaces. Evidence is presented for the increase of the film thickness when approaching the solubility limit, as expected in wetting phenomena.