NiO-magnetite multilayers exhibit long range antiferromagnetic order with the magnetite ferrimagnetic correlations confined to a single layer due to stacking faults of the spinel structure at the interfaces[l, 2, 3, 4]. A systematic study of the field-dependence of the interlayer coupling in a series of NiO-magnetite multilayers has been made using neutron diffraction. Both NiO and magnetite single thin films were included in the measurements for comparison. In the magnetite film, intensity changes with magnetic field are consistent with domain reorientation of the net ferrimagnetic moments, while in the NiO film there are essentially no intensity changes. There is no significant field dependence of the magnetic correlation lengths in either film. For multilayers where the ratio of NiO to magnetite layer thickness is far from unity, the field dependence approximates that of the bulk films. However, for a Fe3O4(68Å)|NiO(34Å) multilayer the NiO antiferromagnetic intensity decreases with increasing field, and there is a broadening of the NiO peak on the order of 30 percent. Concomitantly, the magnetite spins rotate collinear with the field, as expected, due to the net 4.2 μB moment per unit-cell. The NiO moments appear to rotate into domains where the direction of propagation of the ferromagnetic sheets is closer to the field direction.

Superlattices of Fe3O4–NiO layers have been studied by high-resolution transmission electron microscopy (HRTEM). These superlattices are grown by oxygen-plasma-assisted molecular-beam epitaxy (MBE) on (001) oriented MgO substrates, and exhibit a high degree of ordering at the interfaces between the interlayers. The lack of misfit dislocations at the Fe3O4 –NiO interfaces suggests that lattice strain is largely accommodated by changes in the lattice spacing. By quantitative HRTEM analysis of Fe3O4 –NiO interfaces, possible atomic models are discussed, having implications in magnetic ordering and spin exchange mechanisms for such interlayer systems.

We report on magnetotransport measurements on magnetic junctions consisting of Co and half-metallic CrO2 as the electrodes. The insulating barrier in between is a CrOx-AlOx layer created via a chemical modification of the native CrO2 surface, followed by the deposition and oxidation of a thin Al layer. The junctions exhibit a hysteretic low-field magnetoresistance with switching closely matching that of the magnetization of the CrO2 and Co layers. The magnetoresistance is inverse in sign, with a maximum value of -24% at 5 K, implying a negative spin polarization for Co in such structures. The magnetoresistance shows strong temperature and bias dependence, diminishing quickly with increasing temperature and bias voltage.

Presently, the best epitaxial thin films of CrO2 are made by chemical-vapor deposition (CVD) in a two-zone furnace with oxygen flow from a CrO3 precursor. The growth mode has previously been described as CrO3 vaporizing in the first zone, and thermally decomposing at higher temperature in the second zone onto a substrate. In the more recent works, the focus has been on the properties of the obtained layers rather than on deposition mechanisms. In the present experimental work, we attack the epitaxial growth of CrO2 by two completely different methods, namely, molecular-beam epitaxy (MBE) and CVD. We focus on the CVD process itself, and show the importance of an intermediate compound, Cr8O21, for the growth of CrO2 films. We show that it is not necessary to start the CVD from CrO3; instead, one can prepare Cr8O21 ex situ, and use it directly for the growth of high-quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We discuss in parallel our failed attempts to deposit CrO2 from either CrO3 or Cr and oxygen plasma by MBE and our experiments with the CVD process, and conclude that CrO3 does not decompose directly to CrO2 and oxygen, as was expected. We propose a hypothesis that the role of Cr8O21 in the CVD process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4→CrO2+O2.

Magnetite (Fe3O4) films and multilayers were grown using plasma-assisted molecular beam epitaxy and result in single-phase films grown in registry with a MgO substrate. No evidence of interdiffusion is detected on as-grown films. Both structural and magnetic probes indicate behaviors expected for a magnetite thin film. A thermal stability study of these films was performed by annealing these films under ultrahigh vacuum conditions at temperatures below 900 K. Bulk techniques such as x-ray diffraction, superconducting quantum interference device magnetometry, and energy dispersive spectroscopy confirm that the magnesium interdiffuses throughout the entire film, and surface techniques such as x-ray photoelectron spectroscopy and scanning tunneling microscopy/ion scattering spectroscopy show changes in the surface structure and stoichiometry of the film caused by the magnesium intermixing.

Neutron diffraction studies of Fe3O4/NiO superlattices reveal that the field dependence of domains in the antiferromagnetic NiO is correlated with the presence or absence of exchange biasing. Measurements of the full width at half maximum of the (111) NiO reflection show that after cooling in zero field, the antiferromagnetic domain size both parallel and perpendicular to the growth axis is sensitive to the strength of the applied field. In contrast, after cooling from room temperature in a field of 5 T, the domain size is generally smaller than the zero-field-cooled value and does not vary with field. These data suggest that exchange biasing originates from domain walls frozen into the antiferromagnet upon field cooling.

In order to understand the interplay between exchange coupling and magnetic structure, we have examined the magnetic ordering of a series of epitaxial Fe3O4/NiO superlattices using polarized neutron diffraction techniques. As expected, the net ferrimagnetic moment of the Fe3O4 layers aligns parallel to an applied magnetic field. The antiferromagnetic NiO spins order into alternating antiparallel 〈111〉 planes as in bulk, but the direction of the spins in the planes are determined by field preparation. The NiO moments tend to align perpendicular to the field. In addition, the relative population of the NiO domains varies as the field is raised. The changes in the antiferromagnetic spin order relative to bulk seem to result from magnetic coupling with the Fe3O4 moments.

The low frequency electrical noise has been studied in Fe3O4 films as a function of temperature and in a frequency range from 0.1 to 100 Hz. At room temperature, Hooge’s coefficient is around 80, at least three orders of magnitude greater than normally found for metals. A huge increase of the normalized noise is observed at the Verwey transition, whose kinetics cannot be described in the framework of the Dutta model. We show that the 1/f noise scales with the inverse of the number of carriers involved in the hopping conduction. The high noise level is analyzed in terms of slow electronic exchanges between a critical conduction network and traps or excitations widely distributed in energy near the Fermi level.

189 DT-04. TRANSPORT AND MAGNETIZATION PROPERTIES OF EPITAXIAL Fe30, FILMS GROWN ON SrTi03 (100) AND SAPPHIRE (OOOI). S. B. Ogale, K. Ghosh, S. P. Pai, R. P. Sharma, R. L. Greene, R. Ramesh, and T. Venkatesan (NSF-MRSEC on oxides and surfaces and Ctr. for Superconductivity Res., Dept. of Phys., Univ. of Maryland, College Park, MD 20742) Epitaxial thin films of Fe,04 (Magnetite) have been grown on SrTiO, (100) and Sapphire (oo01) substrates by pulsed laser deposition. The target used was a-FezO, and all depositions were carried out in vacuum better than 1 X Torr. The substrate temperature was varied between 500 C to 700 C. The films on SrTiO, (100) grow with (100) orientation normal to the film plane while those on Sapphire(oo01) grow with (1 11) orientation. Only the film deposited at substrate temperature below 600 C show clear Venvey transition at 120 K. The room temperature coercivity is a function of growth temperature and it varies from 200 Oe to 400 Oe as the temperature is decreased from 700 C to 500 C. The coercivity and the nature of Venvey transition has also been studied as a function of film thickness. Data at low field (0.2 Oe) and high field (8 Tesla) magnetization and magnetoresistance as a function of temperature is also obtained and will be presented. Magnetic anisotropy issues will be discussed in the light of these observations. DT-05. MAGNESIUM INTERDIFFUSION STUDIES OF MAGNETITE THIN FILMS AND MAGNETITUBUNSENITW MAGNETITE TRILAYERS GROWN ON MAGNESIUM OXIDE (001) SUBSTRATES. K. A. Shaw, E. Lochner, and D. M. Lind* (Dept. of Phys., Florida State Univ., Tallahassee, FL 32306-3016) The synthesis and thermal stability of magnetite, and magnetitehunsenite composite films grown on magnesium oxide (001) substrates via molecular beam epitaxy is examined. The characterization of these films by X-ray diffraction and SQUID magnetometry is discussed. Structural changes in the magnetite films as a function of anneal temperature are also discussed. X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED), and low energy electron diffraction (LEED) are utilized to monitor the change in surface and bulk structural characteristics of the film as the magnetitdmagnesium oxide sample is progressively annealed. Scanning tunneling microscopy (STM) and LEED indicate a change from a surface reconstruction to a (1 X4) cell in magnetite after annealing. Magnetic changes in the thin films are discussed. Large changes in the hysteresis curves of the magnetite films were observed by comparison of the annealed samples to their original behavior. Reductions in saturation moment, remanence, and coercive field are significant, and indicate a definitive change in the whole thin film due to the anneal. The observed Venvey transition disappears in the annealed magnetite films. Magnesium is found to interdiffuse to the surface of micron thick films of magnetite at 700K. X-ray photoelectron spectroscopy is performed on samples of magnetite, bunsenite, and composite trilayers grown on MgO as a function of anneal time. Significant magnesium interdiffusion is only seen for pure magnetite films. Depth profiling of annealed trilayers shows some interdiffusion occurs through the nickel oxide layer, resulting from formation of grain boundaries in the films as a lattice mismatch relief mechanism. X-ray diffraction while heating magnetite samples confirms the presence of grain boundaries. *Research supported by NSF and ONR grants. DT-06. MAGNETIC PROPERTIES OF NiZn AND MnZn FERRITE FILMS DEPOSITED BY LASER ABLATION. M. Amado, M. S. Rogalski, L. Guimkaes, J. B. Sousa (IFIMUP and Phys. Dept., Univ. of Porto, 4150 Porto, Portugal), I. Bibicu (Inst. of Atomic Phys., P.O. Box MG-06, Bucharest, Romania), R. G. Welch, and S. B. Palmer (Univ. of Warwick, Coventry CV4 7AL, U.K.) Microstructure and the magnetic properties of polycrystalline films of NiZn and MnZn ferrites with various stoichiometries have been studied by conversion electron Mossbauer spectroscopy (CEMS) and vibrating sample magnetometer (VSM) measurements. The films have been ablated as stoichiometric phases (N&,3sZ~.6sFe20,, Mn, , y ~ n , M e $ e ~ + Y 0 4 where Me=Ti,Nb, x=0.2, y=0.12, S=0.02) onto high silicon content glass substrates, at temperatures of 8O0C-85O"C. VSM measurements show coercive field values of about 80 Oe, which are larger than those found for the targets, in direct relation to the crystalline shape anisotropy originating in the film microstructure, while saturation magnetization values are comparable with the bulk, namely 660 emdcm' and 220-280 emdcm' for NiZn and MnZn films respectively. Room temperature CEMS spectra indicate that the films exhibit the single phase pattern of the bulk material. On the grounds of the collective magnetic excitations associated with the spectral line broadening, the grain size distribution of crystallites was derived, with average values in the range of 300-600 nm. The major effect of the smaller grain size found in the film, as compared with the target, is to increase the number of pinning sites for domain walls, which are considered responsible for the higher coercive field values. The magnetic behavior of NiZn and MnZn ferrite films is consistent with the CEMS data on local structure, with the saturation magnetization depending more on intrinsic properties such as stoichiometry, and the coercive field being directly influenced by extrinsic properties such as grain size. DT-07. MAGNETIC PROPERTIES OF SOL-GEL DERIVED Ni-Zn FERRITE THIN FILMS. S. Y. Bae and Y. J. Oh (Thin Film Technol. Res. Ctr., Korea Inst. of Sci. and Technol.. Seoul 131-159, Korea) A new sol-gel processing to deposit soft ferrite thin films was developed, and the magnetic properties of the films were investigated. The stock solution was prepared from Ni(C2H302)2*4H20. Zn(N03)2 *6H20 and Fe(N03)3*9H20 in prescribed molar ratio (Ni:Zn:Fe=0.5:0.5:2) dissolved in absolute ethanol. The films were deposited by spin casting the stock solution on Si( 100) wafer with thermally oxides Si02 layer (3000 8,). Thin films with the thickness of loo0 8, were obtained by annealing the as-deposited films at 500-800°C in air or 02. X-ray diffraction pattern of the film in both symmetric and glazing angle geometry showed that homogeneous spinnel ferrite phase was obtained in the films annealed in 0 2 , while an unknown peak was detected in the film annealed in air at higher temperature ( >7OO0C). Atomic force micrograph showed that the films were composed of spherical grains of 100-loo0 8, in size and 50100 8, in surface roughness (rms). The magnetic properties of the films were self-consistent, showing Ms=140-300 emdcc and Hc= 150 -200 Oe. The films annealed in 0 2 showed better magnetic properties (higher Ms and lower Hc). While the films annealed in 0 2 showed decreasing Hc with increasing grain size, the Hc of the films annealed in air increased with increasing grain size, which is often found in noninteracting nano-paniculated magnetic thin films.' This may be attributed to non-magnetic secondary phase, such as a-Fe203, segregated at grain boundaries, which suppress the magnetic interaction between the grains. RBS spectra and AES depth profile spectra showed that in the films annealed fairly high temperature (>700"C), the evaporation of Zn and the diffusion of Si into the films were significant. The optimized magnetic properties were attained in the film annealed 600°C in 0 2 where Ms=300 emdcc and Hc= 170 Oe. 'E. F. Kneller and F. E. Luborsky, J. Appl. Phys. 34, 656 (1963). DT-08. STRUCTURE AND SOFT MAGNETIC PROPERTIES OF SPUTI'ER DEPOSITED MnZn FERRITE FILMS. M. F. Gillies', R. Coehoom' , J. van a n ' , and D. Alders' ( I Philips Res., Prof. Holstlaan 4, 5656AA Eindhoven, The Netherlands) ( 2Eindhoven Univ. of Technol., Dept. of Phys., P.O. Box 513, 5600MB Eindhoven, The Netherlands) Although bulk ferrites have been extensively studied for a considerable time it is only recently that applications involving thin films of the material have been suggested. The primary motivation for this study is to use ferrite as a fluxguide material in thin film yoke-type heads. The study is therefore focused on samples prepared on polycrystalline substrates (thermally oxidised Si). The MnZn-films, which were approximately 1 mm in thickness, were prepared using R.F. diode sputtering, in an Ar sputter gas, from a bulk MnZn ferrite target with a permeability of 3000. The magnetic properties were investigated using a SQUID-magnetometer and a doublecoil ax. permeability measurement system. The structural properties of the

We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1fum-thick Fe 3 O 4 films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction (XRD), and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, single-crystalline Fe 3 O 4 films. For the surface studies, we have combined Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). Initial and final surface characterization employed X-ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spec-troscopy (ISS) respectively. The surfaces of the MBE-grown samples are flat and show a (√2×√2)R45° reconstruction with respect to the Fe 3 O 4 surface unit cell. We observe the onset of Mg segregation to the surface at around 700 K, with long, narrow extensions of terraces being observed growing along the [110] and [110] directions. Upon prolonged heating at 800 K, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1x4 surface reconstruction, and results in extremely long (≈ 1000 Å ), wide terraces.

Neutron diffraction studies indicate that synthetic single crystalline [001] and [110] Fe3O4/NiO superlattices, prepared by oxygen-plasma-assisted molecular-beam epitaxy, exhibit long range antiferromagnetic ordering through the NiO layers despite the contrasting symmetries of the NiO rocksalt and Fe3O4 spinel unit cells, and that this ordering extends through multiple superlattice repeats. The structural and magnetic coherence in the Fe3O4 layers is confined to individual bilayers (<10 nm) due to interfacial stacking faults and this reduction gives rise to broadening of the selected neutron reflections. The coherence lengths for the antiferromagnetic and ferrimagnetic layers also differ in-plane, suggestive of magnetic frustration through the interfaces. Strong magnetic coupling at the Fe3O4/NiO interfaces, which leads to exchange biasing and to an enhancement of the NiO Neél temperature above the bulk value of 520 K (and approaching the TCurie=858 K for Fe3O4), has been observed. In contrast to results observed for a 1 μm NiO film, polarized neutron diffraction measurements of the [Fe3O4(6.8 nm)|NiO(3.4 nm)] superlattice found no preferred direction for NiO spins within a single 111 domain in a small field, but did show a tendency to reorient under application of a 1.5 T magnetic field. This seems consistent with the picture of an exchanged coupled but frustrated interface, since we have observed that the magnetic spins follow the field direction.

A next-generation experimental end station for surface and interface science studies has been developed with the aim of making full utilization of the capabilities of a third generation synchrotron radiation facility such as the Advanced Light Source. This system, the Advanced Photoelectron Spectrometer/Diffractrometer (APSD), has initially been installed on one half of the rotating platform on the bending magnet beamline 9.3.2 and is planned to be moved to the Elliptical Polarized Undulator (EPU) beamline 4.0.1 in the latter part of 1997. The first phase of the APSD system is completely commissioned and it has been routinely used by both the PRT and independent investigators since November, 1994, for numerous studies, as discussed in a number of other reports in this compendium. The following is a brief description of the important characteristics of this experimental end station as it now stands, together with the various improvements and additions for enhancing its capabilities that are either underway or planned.

Scanning tunneling microscopy (STM) studies of 1 μm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 Å), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 Å) in order to accommodate the Mg that has migrated to the surface. Superconducting quantum interference device magnetometry studies reveal a significant change in the magnetic behavior of the film and large decreases in the saturation moment, remanence, and coercive field. The Verwey transition is greatly altered in these films after the annealing sequence. X-ray photoelectron spectroscopy studies of the films confirm the presence of magnesium in the uppermost layers of the film, and indicate a concentration gradient, with the highest concentrations of magnesium in the surface layer. X-ray fluorescence in scanning electron microscopy qualitatively indicate the presence of magnesium throughout the film, consistent with migration of the magnesium from the substrate. These results are compared with those on an unannealed Fe3O4 film of the same thickness and growth parameters, which shows no magnesium migration into the film during growth up to substrate temperatures of 300 °C.

The magnetic behavior of a set of [110]-oriented single crystalline Fe3O4/NiO superlattices was studied by SQUID magnetometry and by magneto-optical Kerr-effect (MOKE) measurements. Samples for this study were grown on MgO substrates by means of plasma-assisted molecular beam epitaxy with the NiO layer thickness kept constant, while the Fe3O4 layer thickness and the number of bilayers were varied in such way as to preserve the total Fe3O4 thickness throughout the entire set. The structural ordering was studied by x-ray diffraction and reflection high energy electron diffraction. The growth mode was single crystalline, but with rough interfaces. The hysteresis profiles vary with the Fe3O4/NiO thickness ratio; with thinner Fe3O4 layers the degree of influence of the high-anisotropy antiferromagnet (NiO) spin system over that of the ferrimagnet (Fe3O4) increased. The optical constants of the substrate and the component layers at 632.8 nm wavelength were obtained in two steps by unconstrained and constrained optimization algorithms based on the Fresnel equations and ex situ ellipsometric measurements. These optical constants were found to differ by a small but distinct amount from the available standard values. The magneto-optical constant (also at 632.8 nm) of the Fe3O4 layer was determined from longitudinal and polar Kerr effect measurements using one of the existing matrix methods and was found to be a direction-dependent function of the Fe3O4 layer thickness.