Spin-wave theory predicts reduced thermal spin-wave excitations due to a magnetic anisotropy. Recent results show that a strong uniaxial in-plane anisotropy in ultrathin Fe∕GaAs(001) films indeed stabilizes the ferromagnetic order versus thermal spin excitations [Kipferl et al., J. Appl. Phys. 97, 10B313 (2005)]. In order to study whether a fourth-order in-plane anisotropy has a similar effect epitaxial Fe70Co30∕Au(001) samples with zero magnetocrystalline anisotropy were studied and compared to results for Fe∕Au(001). The temperature dependence of the spontaneous magnetization MS for T<0.5Tc can be well described by the Bloch [Z. Phys. 61, 206 (1930)] T3∕2 law in the entire thickness range. However, for each thickness B is reduced in Fe70Co30∕Au(001) compared to Fe∕Au(001) films despite the reduced anisotropy. This means that the effect of a weak in-plane anisotropy is overcompensated by the enhanced exchange interaction.

A magnetic tunnel transistor with spin-valve metallic base and epitaxial Schottky barrier is used to probe the temperature and energy dependence of the magnetocurrent. The magnetocurrent ratio (MCR) reaches values up to 900% in our ultrathin microstructured three-terminal devices. The effect of hot-electron energy has been studied in the range of 0.6 to 2.2 eV. We observe a clear maximum of the MCR at about 1.5 eV electron energy. With increasing temperature (5K<T<185K) the MC remains almost constant, but a rapidly growing spin-independent additional current is measured, due to increasing collector leakage.

Dot arrays with dots of well-defined circular shape have been patterned from epitaxial Fe films on GaAs(001). Magnetization reversal and temperature dependence of the spontaneous magnetization were studied by superconducting quantum interference device magnetometry, an alternating gradient magnetometer and the magneto-optical Kerr effect. It was observed that Bloch T3/2 law is valid for all dot diameters. The spin wave parameter B increases with a decrease in dot diameter by a factor of 4, which reduces the magnetization at room temperature by about 25% compared to its value at 10 K for the smallest dots. From this observation it is concluded that additional excitation modes are present in the spin system.

Thermal spin excitations in confined ferromagnetic structures become increasingly important, e.g., because they reduce tunnel magnetoresistance in highly integrated magnetic memories and the stability of stored information. Here, the effect of lateral confinement on the temperature dependence of magnetization in ultrathin films was studied. Epitaxial Fe films were grown on GaAs(001) by molecular beam epitaxy. Patterning into dot arrays with several million dots of well defined circular shape was accomplished by electron beam lithography, lift-off, and ion beam etching. The magnetic properties of the samples were investigated by superconducting quantum interference device magnetometry between 10 and 350 K. All films—in addition to the fourfold magnetocrystalline anisotropy—have an in-plane uniaxial magnetic anisotropy with the easy axis along [110], which is fully conserved during patterning. The temperature dependence of the spontaneous magnetization for T<0.5TC can be well described by Bloch’s law, MS(T)=M(0)(1−BT3/2), for all samples. For a dot diameter of 500 nm the spin wave parameter B is significantly increased compared to the extended 14 ML film, which in turn shows about twice the bulk value of BFe=5×10−6 K−3/2. The enhancement of spin wave excitations with decreasing film thickness and lateral dimension is discussed in comparison to existing theories and model simulations.

The thickness dependence of the fourfold in-plane magnetic anisotropy was first observed in epitaxial Fe(001) films and described by the volume anisotropy of bcc Fe with a positive anisotropy constant (K-1(vol)>0) superimposed by a negative interface term (K-1(int)<0). This results in a spin reorientation of the easy axis from [100] in thick films to [110] for thicknesses t(Fe)<t(crit)=6 ML. To clarify the origin of the interface term Fe and bcc Fe1-xCox alloy films (x<0.8) epitaxially grown on different substrates-Ag(001), Au(001), GaAs(001)-were studied. As a result, magnetoelastic interactions can be ruled out because exactly the same anisotropies are found for tensile strain [Fe on Ag(001) and Au(001)] and for compressive strain [Fe on GaAs(001)]. Also, K-1(int) is not affected by the particular interface material or overlayer, e.g., vacuum or Au. A universal correlation was found between the volume and interface anisotropy constants: -K-1(int)/K-1(vol)=t(crit)=6 ML. It is shown that this is indeed expected within Neel's phenomenological theory of magnetic anisotropy if contributions from nearest and next-nearest neighbors are taken into account. Electronic hybridization effects in this case seem to play a minor role compared to the local symmetry. It is suggested that observed effects of overlayers on perpendicular interface anisotropies might be related to surface relaxations which have not been sufficiently studied so far and which, on the other hand, would not affect the fourth-order in-plane anisotropy according to Neel's model in agreement with present experimental results. (C) 2003 American Institute of Physics.

The in-plane magnetic anisotropy of Fe films epitaxially grown on GaAs(001), in addition to a thickness-dependent four-fold contribution has a uniaxial component originating from the Fe/GaAs interface. This has been observed in several previous investigations. The orientation of the uniaxial easy axis (e.a.), however, was found to be along the [110] direction in most studies, but also an e.a. parallel to [−110] was reported in a few cases. It has been suggested that different reconstructions of the GaAs surface prior to Fe deposition could be responsible for this discrepancy. In the present contribution, it is shown that in Fe(001) films grown by molecular-beam epitaxy on Ga-rich GaAs(001) surfaces at room temperature the uniaxial anisotropy always has its easy axis along [110] with practically the same magnitude. In particular, the surface reconstruction of the GaAs substrate — either (4×2) or (2×6) — has no effect on the resulting uniaxial magnetic anisotropy. This [together with recent results related to the phase transition of Fe/GaAs(001)] suggests that the same atomic configuration is formed at the Fe/GaAs(001) interface in both cases connected with the segregation of a certain amount of As (and Ga) to the surface.

Epitaxial Fe1−xCox alloy films with x=0.22 and x=0.33 were grown on GaAs(001) by molecular-beam epitaxy in a thickness range of 3 to 80 monolayers (ML). The magnetic properties were investigated by magneto-optic Kerr effect. Ferromagnetic order at room temperature was observed for thicknesses above 4 ML. The in-plane magnetic anisotropy of all films is a superposition of a uniaxial component with the easy axis along [110], which is not discussed here, and a four-fold contribution. The effective uniaxial and four-fold anisotropy constants, KUeff and K1eff, were determined by fits to the hard axis magnetization loops. K1eff contains an interface and a volume term which lead to a linear variation of K1eff with the inverse film thickness. It turns out that the surface and volume anisotropy constants are of opposite sign for all alloy compositions causing a sign reversal at a critical thickness, tcrit. This critical thickness seems to be a universal value, which is caused by a general proportionality between the volume and the interface anisotropy constants with the same negative constant of proportionality for Fe1−xCox/GaAs(001), Fe/Au(001), and Fe/Ag(001). This behavior of the four-fold anisotropy constants is consistently explained within Néel’s pair energy model for a body-centered-cubic ferromagnet.

Fe was epitaxially grown on an atomically flat GaAs(00 I) surface with a (2 x 6) or a (4 x 2) surface reconstruction. This films are ferromagnetic at room temperature with 100% remanence along the easy direction for a nominal Fe coverage of 3.6 ML and above, Mossbauer and superconducting-quantum-interference-device (SQUID) measurements have shown, that even the first monolayers have the full bulk moment of Fe. In order to study the onset of ferromagnetism the Curie temperature T-C was measured for film thicknesses between 2.8 and 3.6ML, using ac Magneto-optical Kerr effect (ac MOKE). Tc decreases with decreasing film thickness and vanishes at about 2.5ML, which can be interpreted as a percolation effect. The in-plane anisotropy of Fe and FeCo alloys on GaAs(00 1) consists of an uniaxial (K-U) and a fourfold anisotropy (K-1). The uniaxial anisotropy is in both cases a pure interface effect, while the effective fourfold anisotropy constant can be separated into an interface (K-1(int)) and a volume term (K-1(vol)). Both K-1(int) and K-1(vol) of FeCo have the opposite sign of K-1(int) and K-1(vol) of Fe. Upon increasing film thickness, this leads to a change of sign of the effective fourfold anisotropy constant at about 6ML from negative to positive for Fe and from positive to negative for FeCo. The strong uniaxial anisotropy of Fe on GaAs(001) is preserved in patterned Fe dots with 200nm diameter, 1.4nm height and 500nm period, which leads to a remanence of 100% along the easy direction.

For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching current pulses into micro-coils and micro-striplines 1 , 2 , 3 , 4 , 5 , 6 and by using high-energy electron beams 7 . Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements 8 , 9 , and experiments seem to confirm this 10 , 11 , 12 , 13 . Spin injection has also been observed in hybrid ferromagnetic–semiconductor structures 14 , 15 . Here we introduce a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of our approach is to optically pump a Schottky diode with a focused, ∼150-fs laser pulse. The laser pulse generates a current across the semiconductor–metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques 11 , 12 , 13 , 16 with the speed of current generation at a Schottky barrier. Specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements and the ability to tune the magnitude of the field with the diode bias voltage.

Epitaxial Fe34Co66 films in a thickness range from 3 to 100 monolayers (MLs) were grown by molecular beam epitaxy on GaAs(001) at room temperature. The growth was characterized by reflection high energy electron diffraction and x-ray diffraction. The magnetic properties were investigated by alternating gradient magnetometry magneto-optic Kerr effect, and superconducting quantum interference device magnetometry. The films show a strong interface-induced uniaxial in-plane anisotropy with the easy axis along [110]. In addition, the fourfold anisotropy coefficient changes sign around 6 ML i.e., the easy axis of the fourfold anisotropy switches from 〈110〉 to 〈100〉 with decreasing thickness.