K. Myrtle

Magnetic anisotropies in single and multilayered structures (invited)

Ultrathin magnetic metallic structures provide a variety of systems in which unique magnetic properties can be engineered. The investigation of magnetic anisotropies in ultrathin structures has brought exciting results to the basic studies of magnetism in systems with reduced dimensionality and to the engineering of new magnetic materials. Molecular‐beam epitaxy (MBE) techniques turned out to be particularly useful in the formation of new stable and metastable structures whose sharply defined interfaces, lattice relaxations, and lattice reconstructions have resulted in a wide range of interesting magnetic properties. In this presentation the present authors’ recent studies of single and multilayered structures composed of bcc Fe(001), bcc Cu(001), lattice‐expanded Pd(001), fcc Co(001), and fcc Cu(001) will be summarized. Besides presenting their interesting magnetic properties the following techniques will be highlighted: reflection high‐energy electron diffraction (RHEED) and ferromagnetic resonance (FMR...

Interface alloying at Fe/Cr interfaces and its role in exchange coupling, angular resolved Auger electron, magneto‐optic Kerr effect, and Brillouin light scattering studies (invited)

Angular resolved Auger electron studies were carried out for Fe whisker/Cr(001) interfaces which were prepared at 100, 180, 246, and 296 °C. The Cr atoms penetrate progressively into the second (counting from the surface) atomic layer at 100, 180, and 246 °C. At 296 °C the Cr atoms enter the third atomic layer. No noticeable fraction of the Cr atoms was found in the fourth atomic layer. The exchange coupling was studied in Fe whisker/Cr/Fe(001) films which were grown in a nearly perfect layer by layer mode. Magneto‐optic Kerr effect and Brillouin light scattering measurements showed that the measured change in the phase of the short wavelength oscillations, the presence of a slowly varying exchange coupling bias, and the small measured values of exchange coupling are caused by the same mechanism: interface alloying. The exchange coupling in Fe whisker/Cr/nFe specimens, for n=10, 20, 30, and 40 ML, showed no obvious dependence on the Fe layer thickness.

Ferromagnetic resonance in ultrahigh vacuum of bcc Fe(001) films grown on Ag(001)

Ferromagnetic resonance studies carried out in ultrahigh vacuum at 16.88 GHz on bcc Fe (001) films 5–14.2 monolayers (ML) thick grown on Ag (001) substrates indicate that an ultrathin Fe film 5 ML thick should be magnetized perpendicular to the specimen plane at room temperature. Covering the bare Fe specimens with Ag causes a substantial reduction in the uniaxial surface anisotropy for all Fe film thicknesses and would put the moment of a 5‐ML film back into the plane. For a given Fe film thickness, the maximum obtainable uniaxial surface anisotropy depends on both the amount of oxygen contamination in the film and on the surface roughness.

In-Situ Techniques for Studying Epitaxially Grown Layers and Determining their Magnetic Properties

Ultrathin films of bcc Fe (001) on Ag (001) and Fe/Ni (001) bilayers on Ag were grown by molecular beam epitaxy. A wide range of surface science tools (RHEED, REELFS, AES, and XPS) were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with the easy axis perpendicular to the film surface were observed in all ultrathin structures studied. In sufficiently thin samples the saturation magnetization was oriented perpendicular to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. Fe/Ni bilayers were also investigated. Ni grows in the pure bcc structure for the first 3–6ML and then transforms to a new structure which exhibits unique magnetic properties. Transformed ultrathin bilayers possesses large in-plane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

The results of microwave transmission and FMR absorption studies in iron‐rich Metglas Fe82B12Si6 at 9, 24, and 73 GHz

We have carried out microwave measurements on an iron‐rich Metglas at 9, 24, and 73 GHz. Two different techniques were used. FMR absorption measurements were made at 9, 24, and 73 GHz which allowed us to study the magnetic properties within a thin surface layer. FMAR transmission measurements at 73 GHz were also made which revealed some of the bulk properties of this material. It was found that the values of magnetic damping obtained from the FMR absorption measurements were significantly different from those obtained by FMAR transmission. We believe this difference is due to the insensitivity of the FMAR transmission measurements to microscopic inhomogeneities. The role of specimen treatment (annealing, polishing) was also investigated.

The exchange splitting of phonon assisted microwave transmission at 9.5 GHz

We have studied the transmission of 9‐GHz microwave power through Ni single crystals cut in the (110) plane with the applied magnetic field along the [100] direction. Our studies were carried out in the FMR field region. The transmission of microwave power was observed to be greatly enhanced by the propagation of ultrasonic waves generated at the front surface of the sample by magnetoelastic coupling. The observed transmission peak has an envelope whose shape is similar to the FMR absorption line but which has a sharp dip at the field corresponding to the magnon‐phonon dispersion curve crossover for the applied frequency ω.

Role of interface alloying in Fe whisker/Cr/Fe(001) structures, angular-resolved Auger electron and MOKE studies

Abstract Angular-resolved Auger electron spectroscopy (ARAES) studies were carried out at Fe whisker/Cr(001) interfaces. The interfaces were prepared at 100, 180, 246 and 296°C. Interface alloying increases progressively with increasing substrate temperature. The MOKE measurements on Fe whisker/Cr/Fe(001) showed a strong dependence of the bilinear exchange coupling on interface alloying.

Probing interfaces of ultra thin films on magnetic substrates using ferromagnetic resonance at 73.55 GHz

Abstract Fe single crystals cut in the (100) orientation are used as substrates for the deposition of ferromagnetic and non-ferromagnetic overlayers. The properties of the overlayer are investigated by measuring the field dependence of the absorption of microwave power, including the ferromagnetic resonance (FMR). Measurements of FMR of (100)Fe with and without an epitaxial body centered cubic (bbc) Ni overlayer are consistent with calculations for a ferromagnetic overlayer with 0.4 μ B per atom of Ni. Epitaxial growth is demonstrated for Ni ( bcc ) on Au and Au on Fe. An epitaxial multilayer has been formed of Au on Ni ( bcc ) on Au on Fe.

Temperature and frequency dependence of microwave magnetic properties of amorphous Fe100−xBx

Ferromagnetic resonance linewidths, ΔH, have been measured at 9.5, 24, 38, and 73 GHz for a series of amorphous iron‐boron ribbons with compositions, Fe100−xBx (x=14 to 22). Transmission measurements at 73 GHz have also been carried out to establish the contribution of the bulk intrinsic damping to the frequency dependence of the FMR linewidth. All measurements were carried out over a temperature range of 20–350 °C. The linewidths of the present specimens could be approximately represented by an expression of the form ΔH=ΔH0+(slope)×f; this observation is in agreement with the results reported on the commercial ribbons. The slope was found to be in agreement with the contribution from the intrinsic Landau–Lifshitz damping as measured by transmission. The Landau–Lifshitz damping was found to increase with decreasing boron concentration.

Anisotropic ferromagnetic resonance linewidth in nickel at low temperatures

We have measured the ferromagnetic resonance linewidth ΔH at 24 GHz in (110) nickel disks at 4 K and from 60 K to room temperature. Samples had a nominal purity of 99.99% and a residual resisitivy ratio of 40. The applied field was in the plane of the sample and measurements were made with the field along each of the three principal axes [100], [110], and [111]. We find ΔH[110] >ΔH[111] and ΔH[100] for temperatures below 200 K. At 4 K we found ΔH[100] =1600±50 Oe, ΔH[111] =1800±50 Oe, and ΔH110] =2000±50 Oe.