P. Lubitz

CoFe2O4 thin films grown on (100) MgO substrates using pulsed laser deposition

Thin films (≊0.4 μm) of cobalt ferrite (CoFe2O4) have been grown on single‐crystal (100) MgO substrates using pulsed laser deposition (PLD). The phase, orientation, and microstructure of the as‐deposited films were investigated as a function of substrate temperature (i.e., 200–800 °C) at a constant oxygen deposition pressure of 30 mTorr. The as‐deposited films were found to be single phase, well oriented, and approximately matching the stoichiometry of the target, but the cubic lattice constant of the films depended on the substrate temperature indicating that the films were strained. The greatest effect of the substrate temperature was on the magnetic properties of the as‐deposited films. At 800 °C, 4πMs was measured to be 5370 G which is approximately the accepted bulk value for cobalt ferrite. In addition, PLD cobalt ferrite films grown at substrate temperatures of 600 and 800 °C exhibited a uniaxial magnetic anisotropy with an easy direction normal to the film plane. Films grown at 200 and 400 °C also...

The magnetic and structural properties of pulsed laser deposited epitaxial MnZn–ferrite films

The magnetic and structural properties of pulsed laser deposited MnZn–ferrite films have been examined. The results show that the uniaxial anisotropy, ferromagnetic resonance linewidth and coercive force are strongly influenced by the microstructure of the films, and the saturation magnetization and first‐order magnetocrystalline anisotropy constant depend on intrinsic properties such as composition and cation site occupation. A comparison of bulk and film magnetic properties shows that the magnetic properties of the films are comparable to the bulk, which makes pulsed laser deposition ferrite films a prime candidate for thin film high‐frequency microwave device applications.

Pulsed laser deposition of epitaxial BaFe12O19 thin films

Epitaxial thin films of barium hexaferrite (BaFe12O19) have been fabricated by the pulsed laser deposition technique on basal plane sapphire. Structural studies reveal the films to be predominantly single phase and crystalline, with the c axis oriented perpendicular to the film plane. The magnetic parameters deduced from vibrating sample magnetometer and ferromagnetic resonance (FMR) measurements are close to the parameters associated with bulk materials. Post annealing of the films reduced the FMR linewidth by more than a factor of 3 so that it compares reasonably well with single‐crystal films. The derivative FMR linewidth was measured to be 66 Oe at 58 GHz and 54 Oe at 86 GHz. Spin‐wave‐like modes have been observed for the first time in barium ferrite films. The deduced exchange stiffness constant of 0.5×10−6 ergs/cm is in reasonable agreement with recent calculations.

Magnetostriction of amorphous TbxFe1−x thin films

A systematic investigation was made of magnetostriction and magnetization in amorphous thin films (∼1 micron) of TbxFe1−x with x=0 to 0.5 prepared by electron‐beam co‐evaporation. The amorphous or crystalline character of the films was determined using x‐ray and Mossbauer techniques. Net magnetization, coercivity, and the initial in‐plane and out‐of‐plane distribution of magnetization was determined using a vibrating sample magnetometer. Both the sign and magnitude of the magnetostriction was measured as a function of magnetic field using a capacitance method. Over most of the concentration range the amorphous samples have a positive magnetostriction with a maximum of Δl/l=285×10−6 at x=0.4 and going to zero at the compensation point (x=0.22) and near x=0.5 where the amorphous alloys are no longer magnetically ordered at room temperature. In the region x=0.1 to 0.2 the magnetostriction is more complex, being negative for fields up to 5 kOe and positive at higher fields. All the films studied with x?0.1 we...

Ferromagnetic resonance studies of very thin epitaxial single crystals of iron

Ferromagnetic resonance (FMR) has been used to study a wide variety of very thin single crystals of Fe grown on (110) GaAs substrates by molecular beam epitaxy. Data were taken at room and liquid nitrogen temperatures for films with thicknesses L in the range 18–200 A. Due to surface anisotropy, the easy axis of the magnetization switches from [100] to [110] when L≤50 A, independent of whether the the film surface is passivated by an Al‐overcoat or has a thin Fe oxide surface layer. We suggest that this is an effective surface anisotropy arising in part from a depth dependent strain near the film‐substrate interface. The changes in the parameters describing the angular dependence of the FMR spectra upon cooling to 77 K can be explained as due to magnetostriction arising from thermally induced strains plus the temperature dependence of the cubic volume anisotropy. The FMR linewidth is shown to be a linear function of frequency in the range 5–40 GHz.

Oriented barium hexaferrite thick films grown on c-plane and m-plane sapphire substrates

The magnetic and structural properties of thick pulsed laser deposited (PLD) barium hexaferrite (BaM) films grown on c-plane and m-plane sapphire substrates were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), vibrating sample magnetometry (VSM), and ferrimagnetic resonance (FMR). Previously, BaM thin films (-0.5 /spl mu/m) grown on c-plane (0001) sapphire substrates exhibited magnetic properties closely approaching those of single crystal spheres. These films are potentially useful for thin film millimeter-wave devices such as circulators, isolators, and phase shifters, provided that thick films (e.g., 20 to 100 /spl mu/m) with suitable magnetic and dielectric properties can be grown. In general, it was found that an increase in film thickness leads to the growth of either textured polycrystalline BaM, which can be loosely adherent, or delamination of the films. However, well oriented thick BaM films were grown up to 15 and 20 /spl mu/m on the c-plane and m-plane sapphire substrates, respectively, before delamination occurred. The FMR linewidth, /spl Delta/H, was 200 Oe at 85 GHz for an annealed 15 /spl mu/m thick PLD BaM film on c-plane sapphire with 4/spl pi/M=4200 G, H/sub A/=16000 Oe and an XRD /spl omega/-scan of 0.51/spl deg/ FWHM about the (008) BaM plane. The FMR linewidth of PLD BaM films grown on m-plane (11_00) sapphire substrates were greater than 450 Oe with 4/spl pi/M-4000 G H/sub A/=16000 Oe. the m-plane films were magnetically well oriented in the film plane with M/sub r//M/sub s/ along the easy axis greater than 90% for all PLD m-plane BaM films. >

Observation of magnetic resonance modes of Fe layers coupled via intervening Cr (invited)

Multiple‐frequency (2–14 GHz) ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes of a large set of single‐crystal Fe/Cr/Fe(001) sandwiches grown by molecular‐beam epitaxy. The FMR data reveal two resonance modes which have complicated frequency dependencies for those samples that show antialigned Fe layers in zero applied field. Such alignment is only found for samples with 12 A<t(Cr)<25 A. These samples also have dramatic discontinuities in their M‐vs‐H curves and magnetoresistance curves. Angle‐dependent 35‐GHz FMR was used to determine the anisotropy and effective magnetization of these samples when they were magnetically aligned. We show that the detailed FMR and M‐vs‐H behavior can be quantitatively explained by an AF coupling term of the form J M1⋅M2 and find that J has a t(Cr) dependence peaked about 16 A.

Growth, structure, and magnetic properties of γ-Fe2O3 epitaxial films on MgO

Single-crystal epitaxial thin films of γ-Fe2O3(001) have been grown on MgO(001) using oxygen-plasma-assisted molecular beam epitaxy. The structure and magnetic properties of these films have been characterized by a variety of techniques, including reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy and x-ray photoelectron/Auger electron diffraction (XPD/AED), vibrating sample magnetometry, and ferromagnetic resonance. Real-time RHEED reveals that the film growth occurs in a layer-by-layer fashion. The γ-Fe2O3(001) film surface exhibits a (1×1) LEED pattern. The growth of γ-Fe2O3 films at 450 °C is accompanied by significant Mg outdiffusion. AED of Mg KLL Auger emission reveals that Mg substitutionally incorporates in the γ-Fe2O3 lattice, occupying the octahedral sites. Magnetic moments are ∼2300 G and ∼4500 G for γ-Fe2O3 films grown at 250 °C and 450 °C, respectively. The high magnetic moment for the films grown at 450 °C could be a...

Properties of electrodeposited Co‐Cu multilayer structures

Alternate layers of Co and Cu of individual layer thicknesses from 1.5 to 8.0 nm and total thickness of about 100 layers and of Co bilayers separated by Cu have been electroplated from an electrolyte having a low concentration of Cu and a high concentration of Co atoms. The properties of the resulting structures have been analyzed using SEM, x‐ray diffraction, VSM, and FMR methods. Comparable structures have also been fabricated by e‐beam evaporation in high vacuum for comparison purposes. X‐ray diffraction patterns indicate only the fcc structure. Assuming the Co thickness to be that deduced from the plating charge, we infer 4πM values in the range 6.0–14.5 kG from VSM and FMR. Some uniaxial anisotropy is apparent in this system. These results are similar to those of earlier work on thin fcc Co layers in the same range of thicknesses, where the room‐temperature moment was reduced but depended only slightly on layer thickness, and where the uniaxial anisotropy was observed to be small.

Low‐field spin‐valve magnetoresistance in Fe‐Cu‐Co sandwiches

Fe‐Cu‐Co sandwiches have been grown on glass and silicon substrates using electron beam evaporation. Because the typical coercive field of a thin Co film is higher than that of a typical Fe film, the magnetization curves of these sandwiches show clearly the separate, sequential reversal of the Fe and Co moments as a function of applied field. In the applied field region where the moments of the two magnetic layers are antialigned, the spin‐valve magnetoresistance shows a peak of over 3% in amplitude at room temperature.