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.

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

Neutron diffraction measurements reveal the nature of the magnetic structure in Fe3O4/NiO superlattices grown by molecular beam epitaxy. Taking advantage of differences between the Fe3O4 and NiO crystalline symmetries, we have determined independently the magnetic order parameters of the bilayer components. The NiO antiferromagnetic order propagates coherently through several superlattice bilayers, while the magnetic coherence of the ferrimagnetic Fe3O4 is restricted to a single interlayer due to the random stacking of the spinel unit cells at the interfaces. A model for the diffraction data, based upon a Hendricks–Teller description of the interfacial disorder, demonstrates that the observed broadening of selected reflections originates directly from these stacking faults.

The role of interfacial exchange coupling in the magnetic behavior of metal oxide materials has been investigated through the study of Fe3O4/NiO superlattices. We report results on a series of superlattices grown where one bilayer constituent was held to a fixed thickness while varying the other from single unit cell dimensions upward. High crystalline quality was confirmed by XRD, RHEED and neutron diffraction. Magnetization profiles show substantial deviations from bulklike iron oxide results, with an increase in domain rotation energies observed in the superlattices over that of bulk iron oxide (increasing with NiO layer thickness) indicating the strong nature of Fe3O4/NiO interfacial linkage. Neutron scattering at elevated temperatures shows that the NiO remains ordered above the 523 K bulk Néel temperature. This suggests that at least a portion of the NiO within a layer remains ordered well above the Néel temperature, with an increase in effective Néel transition temperature that approaches the Fe3O4 Curie temperature in the limit of very thin NiO layers. Although the exchange coupling dominates these effects, strain also plays an important role.