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Notes: It has been empirically established that the net anisotropy exhibited by even a simple metal epitaxial film is strongly effected by the interactions at the metal/semiconductor interface, although the mechanisms have not been systematically addressed and remain an open issue. An Fe(001) film, for example, is expected to exhibit a four fold symmetry in the in-plane magnetic anisotropy, consistent with the crystalline surface symmetry. Such films grown on a bulk GaAs(001) surface, however, exhibit a significant uniaxial anisotropy (twofold symmetry). It has been speculated that this uniaxial anisotropy arises from the strongly directional bonds of the substrate surface, based on a model that assumes a bulk-terminated, unreconstructed zinc-blende (001) surface. This model fails to explain, however, the nearly ideal fourfold symmetry observed for similar Fe(001) films on the ZnSe(001) surface, which has an identical crystalline structure as GaAs and differs in lattice constant by only 0.25%. We propose here a conceptual model to account for these pronounced differences, which describes how contributions to the net magnetic anisotropy arise from the formation of the interface and early stages of the metal film growth. We suggest that these contributions originate in the initial metal adsorption sites and subsequent bond or site filling, and are strongly dependent on the semiconductor surface reconstruction. We compare this model with results for Fe growth on several surface reconstructions.

Notes: A new system, exhibiting unusually large interlayer biquadratic exchange coupling, has been discovered, consisting of single crystal FeCo/Mn/FeCo sandwiches epitaxially grown on GaAs substrates. Normalized m(H) curves yield a remanent moment of approximately 0.5 for all cases studied, with a number of examples having saturation fields as large as several Tesla. This implies a biquadratic coupling constant as large or larger than many known bilinear coupling constants. None of the normalized m(H) data exhibits a remanence of less than 0.5, indicating the absence of comparable contributions from bilinear coupling. Angular dependent FMR at 9 GHz implies a fourfold anisotropy of opposite sign to that measured for single FeCo layers, whereas FMR at 35 GHz agrees in sign with the single layer anisotropy. Detailed analysis of this contradiction shows that this is an apparent anisotropy reversal which emerges within the theory of FMR in the presence of large biquadratic coupling, requiring values of the coupling constant in excess of −j2=2 ergs/cm2, where the coupling term in the energy is written J2[(m1⋅m2)/m1m2]2. This large biquadratic coupling, together with the absence of bilinear coupling, appears to contradict existing theories of interlayer exchange.

Notes: Ferromagnetic resonance, SQUID magnetometry, and x-ray diffraction have been used to characterize a set of [Fe3O4(68 A(ring))/NiO(17 A(ring))]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5-μm-thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in-plane anisotropy is much less than the 330-Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in-plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5-GHz data showing significant hysteresis associated with the sample magnetization. X-ray diffraction indicates that both the film and SL's are nearly cubic single-crystalline structures with long-range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL's, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in-plane anisotropy with its easy axis along the field direction. Decreased microwave loss also occurs abruptly at the film TV.

Notes: Trilayers of CoFe/Mn/CoFe(001) have been prepared by molecular beam epitaxy and their magnetic properties measured by magnetometry and ferromagnetic resonance. Very strong near-90° coupling between the CoFe layers, with no evidence for 180° coupling, was found in all but the thickest Mn-layer samples. The coupling energy has the form suggested recently for the case when the interlayer itself is antiferromagnetic. An analysis of the ferromagnetic resonance data indicates that the magnitude of the coupling oscillates with the Mn thickness. © 1996 American Institute of Physics.

Notes: We have prepared single crystal face centered cubic (fcc) magnetic transition metal films (Co,Ni,Fe) on (001) diamonds, the thickness of the films varying between a few tenths of a nanometer to over 100 nm. The crystalline quality and fourfold symmetry of these layers was monitored in situ during the film growth with RHEED and a chemical analysis was performed using Auger spectroscopy. In addition, the structure of the samples was investigated ex situ using X-ray diffraction and EXAFS, demonstrating the single crystal, fcc (001) structure throughout each entire film. The magnetic characterization was performed with Ferromagnetic Resonance (FMR) and Superconducting Quantum Interference Device (SQUID) hysteresis loops. The saturation magnetization of the Co films is only slightly lower than the literature values. The coercive fields are very small (∼25 Oe) and the magnetization reversal very sharp. The FMR yielded a fourfold anisotropy comparable to literature values. The observation of the first standing spinwaves underlines the good quality. The Ni films are tetragonally distorted due to the 1.2% mismatch. The FMR data indicate a significant perpendicular anisotropy slightly smaller than 4πM, the fourfold in-plane anisotropy being comparable to the bulk value. The saturation magnetization is reduced by 30% compared to bulk values, probably due to nickel-carbide which was observed in the XRD data from some samples. Neither the in-plane 〈110〉 nor the in-plane 〈100〉 axis show an easy axis behavior and both require fields in excess of 6000 Oe to saturate, the coercive fields being about 200 Oe. The RHEED patterns of the Fe films show single crystal growth with a lattice constant comparable to diamond, indicating an fcc structure. A similar structural and magnetic characterization of these films will also be presented. © 1996 American Institute of Physics.

Notes: Sensitivity to magnetic atoms and low intrinsic absorption characterize the interaction of neutrons with matter. Consequently, polarized neutron reflectivity provides a unique means of performing depth-resolved vector magnetometry. We have used this technique to determine the magnetization depth profiles of Fe/Cr superlattices. Superlattices of bilayer composition [55 A(ring) Fe/17 A(ring) Cr], grown at 523 K, exhibit biquadratic coupling with large saturation fields (∼3 kOe), while those grown at 293 K are ferromagnetically ordered. We have directly measured the evolution of the coupling angle between adjacent Fe layers as a function of applied field and will discuss how bilinear, biquadratic, and external field terms produce the observed order. The weaker coupling found in the Fe/Al system makes possible the investigation of a range of spin configurations at temperatures that do not endanger the sample. We have mapped the phase diagram of a [42 A(ring) Fe/12 A(ring) Al/39 A(ring) Fe] (100) trilayer and find evidence of biquadratic coupling at low temperatures and fields (e.g., when H=180 Oe, the Fe layer spins relax away from ferromagnetic alignment below T≈170 K). Our measurements agree qualitatively with energy minimization calculations and the results of bulk magnetometry.

Notes: Using micron-level photolithography, we have prepared a set of patterned multilayer samples which have the form [Co/Cu/NiFeCo/Cu]×10 and which permit perpendicular current transport through the structure. Based on vibrating sample magnetometry, the multilayer stacks show a loss of ≈2 A(ring) of magnetic material at each interface. The magnetoresistance was measured as a function of temperature, magnetic layer thickness, and element size and the ΔR/R values range up to 8% at 300 K and 18% at 10 K. The thickness dependent data are modeled using an extension of existing methods and can be fit using comparable resistivity, interface resistance, and spin asymmetry parameters. The magnetic fringe fields of the elements have an important effect on the observed R vs H curves. © 1996 American Institute of Physics.

Notes: We have examined the initial interface formation and subsequent film growth for Fe films on the As-terminated (2×4) reconstructed surface of GaAs(001) in an effort to correlate the initial adsorption sites and film growth with the magnetic anisotropy. The growth and surface studies were performed in a four chamber ultrahigh vacuum system which incorporates UHV sample transfer between the chambers for III–V semiconductor growth, metal growth, photoelectron diffraction (PED), and scanning tunneling microscopy (STM). After coating with a thin film of Au (40 A(ring)) at room temperature, the samples were removed from the system for postgrowth characterization, which included ferromagnetic resonance and magnetometry. The GaAs(001) surface was prepared via homoepitaxial growth by MBE, with the growth terminated in a manner which resulted in a well-ordered As-dimer terminated (2×4) reconstructed surface as revealed by reflection high energy electron diffraction and STM. Fe deposition was performed in a second MBE chamber for coverages which ranged from 0.1 ML to several tens of monolayers at a substrate temperature of 175 °C.The sample was then transferred under UHV to either the PED or STM chamber to determine the initial adsorption sites, growth mode, and evolution of film structure with increasing coverage. For the lowest coverages studied (0.1 ML), we obtain evidence for preferential adsorption at As-dimer sites. At a coverage of 1 ML, two-dimensional islands form approximately 20 A(ring) in size, but elongated along the missing dimer rows with an aspect ratio of 2:1. At high coverages ((approximately-greater-than)40 A(ring)), we observe three-dimensional mounds or clusters with an average in-plane diameter of approximately 100 A(ring) and a narrow size distribution. Even at this coverage, the monolayer terraces of the GaAs substrate are clearly visible in the STM image, with the Fe film uniformly growing on each terrace. Photoelectron diffraction in the forward scattering regime (high electron kinetic energy) is utilized to determined the growth mode. At low kinetic energy, information on the adsorption site is obtained from Auger electron diffraction and compared with the results of the real space STM images.

Notes: We have made spin-valve structures of Permalloy/Cu/Co by sputtering or electron-beam deposition onto the antiferromagnetic oxide NiO. The oxides were made either by deposition of the metals and subsequent oxidation or by growing them in situ using reactive sputtering. The magnetic properties of the giant magnetoresistance structures were studied by magnetoresistance, vibrating sample magnetometry, and ferromagnetic resonance methods. The oxides were characterized by x-ray diffraction and atomic force microscopy. We studied surface roughness and structure as functions of thickness and oxidation temperature and correlated the oxide properties with the magnetic performance. We found that the metal layer roughened during the postdeposition oxidation process and that the resulting oxide layers were very effective in pinning the direction of the magnetic moment of adjacent metal films. Coercive fields over 500 Oe were obtained for Co overlayers on NiO films but the exchange bias field was generally less than 100 Oe and was not strongly dependent on the roughness. The beneficial effects of this strong pinning were offset to some degree by higher switching fields required in spin-valve structures deposited over the Co. We also made reactively sputtered oxide antiferromagnetic films which had smoother surfaces than those made by postdeposition oxidation.

Notes: We report the first growth and characterization of fcc Co epitaxial films (t=10–130 nm) on diamond. Growth was carried out under UHV conditions in a commercial MBE machine and monitored during growth using RHEED which showed single crystal growth oriented with 〈100〉Co(parallel)〈100〉C. X-ray diffraction studies of the 130 nm sample demonstrated the single crystal fcc growth throughout the entire sample. Separate studies to determine growth mode and structure were carried out using angle-resolved Auger forward scattering. Vibrating sample and SQUID magnetometry yields a magnetic moment of (1.05±0.1) 103 emu/cm3. Ferromagnetic resonance measurements carried out at 35 GHz yield a large cubic anisotropy K1/MS=(480±30)Oe and linewidth of only ΔH=100 Oe. Spin waves were observed in the thicker samples and the exchange constant determined to be ACo=1.09×10−6 erg/cm. This work has been supported by the Office of Naval Research.