ALBERT

Notes: The temperature dependence of the unusual magnetic 90° coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. For specific values of applied field and temperature, in-plane anisotropy and the interlayer coupling drive abrupt first-order changes in the temperature-dependent moment of these trilayers. The occurrence of such a first-order transition can be used to determine the value of the interlayer coupling constant, JQ, at the transition temperature. We show that Fe/Al/Fe(001) trilayers also exhibit distinct second-order transitions in the temperature-dependent moment, and that the occurrence of a second-order transition can similarly be used to specify JQ(T). A comparison is made of the values of JQ(T) determined by these two approaches, and the dependence of JQ on T is analyzed.

Notes: The anisotropic magnetoresistance (AMR) has been measured at room temperature on a series of epitaxial iron films of various thicknesses. Seven of the films range in thickness from 5 to 20 nm, and one is 500 nm thick. The resistivity of the films was measured with current along photolithographically defined paths parallel to three directions of high symmetry in the single crystal films ([001], [110], and [111]). It was determined that the magnitude of the AMR depends upon the direction the current is applied and that this directional dependence increases with film thickness until saturating near 20 nm. The AMR is roughly 0.15% for all crystal directions in the thinnest films, while in the thickest film, the AMR is 0.08% with current along the [001] direction, 0.35% along the [110] direction, and 0.51% along the [111] direction. These values are to be compared with the AMR of bulk polycrystalline iron which is 0.2%; a weighted average over the different crystallographic directions.

Notes: Diluted magnetic semiconductors (DMS) offer the opportunity to significantly alter heterojunction band offsets via an applied magnetic field due to the large Zeeman splittings (enhanced g-factors) they exhibit. We have previously reported field-dependent spatial spin segregation of holes in DMS quantum well structures1,2 which resulted from the small band offset and large spin splitting exhibited by the valence band. We report here the growth of tailored Zn1−xFexSe/ZnSe quantum well structures in which both electrons and holes are spatially segregated according to their spin, resulting in spin-polarized carrier populations in the barriers and wells: the spin-down carriers are localized in the Zn1−xFexSe barriers, while the spin-up carriers are localized in the wells. Both single and multiple quantum well samples were grown by molecular-beam epitaxy on GaAs(001) substrates with 100 A(ring) barriers and wells. The heavy-hole excitonic transitions were studied with magnetoreflectivity at T=4.2 K and fields up to 8 T. The magnetoreflectivity data show that both (−3/2→−1/2) and (3/2→1/2) excitonic transitions are of equal intensity and spatially direct (type I), with the former showing the strong field dependence expected for localization in the magnetically active barriers.

Notes: Thin films of Fe with interleaved Cr have generated considerable interest because of the antiferromagnetic coupling between the Fe layers. This coupling has been examined using ferromagnetic resonance and other measurements. The results have been explained on the basis of the phenomenological expression for the magnetic free energy. We have determined the magnetic free energy for a single-crystal Fe/Cr/Fe(001) sandwich grown by molecular beam epitaxy using torque magnetometry. At fields below the anisotropy field, both the torque and free energy show a complex switching of the antiferromagnetically coupled Fe moments (Fig. 1). Preliminary calculations show that the exchange and the angle between the antiferromagnetically coupled Fe moments can be determined directly from these curves. We shall discuss our measurements and calculations and compare our results with the earlier ferromagnetic resonance results.

Notes: The stable hcp and the metastable bcc phases of cobalt have almost identical nearest-neighbor distances, but substantially different coordination numbers z (zbcc=8, zhcp=12). While z is central to magnetic behavior in insulating systems, these distinctive structural features of cobalt offer an opportunity to address the role of coordination in strong itinerant ferromagnets. We report a room-temperature Brillouin scattering study of surface and bulk spin waves in molecular-beam-epitaxially grown thin films of bcc cobalt. Fitting expressions for the surface and bulk magnon dispersion to the measured frequency shifts versus applied magnetic field provides values for the gyromagnetic ratio, saturation magnetization M, and the exchange stiffness constant D for bcc cobalt. Our results reveal that M is about 10% lower than that of the hcp phase, while D scales with z, a characteristic of an insulating ferromagnet.

Notes: We have measured the transmission Mossbauer spectra of Fe/Ag(100) single crystal multilayers with large thicknesses (∼100 ML) of intervening Ag as a function of both temperature and magnetic field applied perpendicular to the surface. In contrast to earlier work on samples with less silver between layers, ordering temperatures are much reduced and the initial relaxation effects observed on cooling down appear to set in with a very small or zero order parameter.1,2 Measurements with a magnetic field applied perpendicular to the film surface showed a single well defined order parameter, which rules out superparamagnetizatism as the source of the observed relaxation effects. As an example, a sample measured earlier with 2.4 ML of Fe and 4 ML of Ag exhibited a well defined order parameter in zero field with a transition temperature near 200 K. In the present case a 3.0 ML Fe sample with ∼100 ML of intervening Ag exhibited relaxation effects setting in near 100 K, with a nonzero order parameter below ∼60–70 K. Measurements in a magnetic field showed clear evidence for long range spin–spin correlations even at temperatures above 100 K. Our interpretation is that for the films with relatively thin (∼4 ML) Ag layers between the Fe, there is sufficient coupling between layers to drive an essentially 3-d magnetic transition. With a very thick Ag interlayer separation, the characteristics of the film become much more like an ideal 2-d Heisenberg model, which builds up very long range spin–spin correlations at low temperatures, but whose actual magnetic ordering transition is depressed to T=0 (finite T with anisotropy).3 Presumably the transition which we estimate to be at 60–70 K is driven by the fact that the perpendicular anisotropy is not zero, but in fact is larger than the demagnetizing field.〈ks〉

Notes: Resistivity measurements have been performed as a function of temperature (1–300 K) and magnetic field (0–10 T) on antiferromagnetically coupled Fe-Cr-Fe sandwiches. Two types of samples were studied: MBE-grown sandwiches deposited epitaxially on the ZnSe (100) surface, and evaporated polycrystalline sandwiches deposited on glass substrates. The magnetic saturation field Hs determined from the resistivity ρ(H,T) is linear with temperature throughout the full temperature range for all samples. In the polycrystalline sandwiches, where the observed in-plane magnetic anisotropy is small, the linearity of Hs(T) implies that the antiferromagnetic interlayer exchange coupling A12 is also linear with temperature. The magnetoresistance of the sandwiches is constant at low temperature, and decreases linearly with increasing temperature above about 70 K.

Notes: The incorporation of magnetic layers in semiconductor heterostructures is an increasingly active area of study. We are interested in the growth of ferromagnetic layers on diluted magnetic semiconductor (DMS) substrates as a means of magnetically polarizing the DMS medium, a necessary condition for these materials to exhibit the extraordinary magneto-optic and transport properties for which they are known. Since interfacial effects are expected to play an important role in thin-film heterostructures, we have examined the growth of Fe and Co films on ZnSe(001) to determine the mode of film growth, the formation of the interface, and the structure of the overlayer at the 1–10 monolayer level. The results obtained should apply to the growth of such overlayers on the ZnSe-based DMS compounds for modest magnetic ion concentrations as well, such as (Zn,Mn)Se, (Zn,Fe)Se, and (Zn,Co)Se. The ZnSe(001) samples were grown by molecular-beam epitaxy on GaAs(001) substrates, passivated with a Se coating in a manner analagous to the As passivation of GaAs, and transferred to a surface analysis system.The Se coating was thermally desorbed in UHV at 150–200 °C just prior to metal deposition. The metal films were deposited from miniature electron-beam sources and characterized in situ with Auger electron diffraction, RHEED, and x-ray photoelectron spectroscopy. The coverages were determined by x-ray flourescence measurements. The coverage dependence of the forward scattering peaks in the Auger electron diffraction scans due to the occupation of second and third monolayer sites provides information on the initial mode of film growth. The angular position of these peaks provides a measure of the out-of-plane lattice spacing, and hence the structure of the overlayer. We find that the growth of Fe(001) on a ZnSe(001) epilayer is predominantly 2D in nature at a substrate temperature of 175 °C, while growth directly on the oxide-desorbed GaAs(001) bulk substrate shows more significant 3D clustering. While the growth of Co on GaAs(001) results in a nearly ideal bcc single-crystal structure, growth on ZnSe(001) epilayers yields a multicrystalline structure. Little interaction with the ZnSe is observed with XPS.

Notes: Magnetization and magnetoreflectivity studies of two Zn1−xCoxSe epilayers (x=0.0076 and 0.0104) were carried out. These experiments yield a value for the exchange parameter difference N0(α−β)=2420±40 meV, which is significantly higher than the values observed in other diluted magnetic semiconductors. The study verified that ZnCoSe is a Brillouin paramagnet.

Notes: Ferromagnetic alloy films of bcc FexCo1−x(001) were epitaxially grown on ZnSe-epilayered GaAs substrates spanning both the thermodynamically allowed bcc regime (0.25〈x≤1) and the epitaxy-extended metastable bcc regime (0≤x≤0.25). Conversion-electron extended x-ray absorption fine-structure (EXAFS) and reflection high-energy electron-diffraction (RHEED) measurements verify that metastable bcc FexCo1−x films with 0≤x≤0.25 are stabilized by epitaxy on ZnSe. Additionally, the composition-dependent magnetic properties of these films were characterized by ferromagnetic resonance and vibrating sample magnetometry.