C. H. Sowers

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Structure and magnetic properties of exchange-spring Sm--Co/Co superlattices

We present structural and magnetic properties of epitaxial Sm–Co/Co superlattice films prepared via magnetron sputtering. X-ray diffraction and cross-sectional transmission electron microscopy show that the films are structurally coherent. The oriented nature of the interleaved ferromagnetically “hard” and “soft” layers comprising the superlattice provides a realization of the ideal nanostructure of exchange-spring magnets as well as a model system to study layer thickness dependences of the magnetic properties. The superlattice films have an effective fourfold, in-plane magnetic anisotropy. Room-temperature hysteresis loops are relatively square and the demagnetization of the Co is reversible, as expected of exchange-spring magnets with aligned hard magnet layers.

High coercivity, epitaxial Sm–Co films with uniaxial in-plane anisotropy

Epitaxial Sm–Co(1100) and (1120) films have been grown by magnetron sputtering onto Cr(211) and (100) buffer layers, respectively. The Sm–Co(1100) films exhibit uniaxial in-plane anisotropies of ≈20–25 T and room-temperature coercive fields that increase to 4.1 T as the film thickness decreases to 75 A. The 3 T coercivities of the (1120) films are independent of thickness.

Non-oscillatory antiferromagnetic coupling in sputtered Fe/Si superlattices

A series of sputtered Fe(30{Angstrom})/Si(x) superlattices were grown for x=10--40{Angstrom}. Magnetization and Kerr hysteresis loops, and neutron-reflectively measurements identify antiferromagnetic (AF) coupling of the Fe layers at room temperature for x=15{Angstrom} nominal thickness, with switching fields of 6kOe. X-ray structural analysis indicate that the spacer medium is crystalline for x<20{Angstrom}, while sputtered Si is amorphous (a). Failure to detect oscillations in the AF coupling for thicker Si layers is due to the formation of a-Si, as opposed to the crystalline silicide responsible for the coupling.

150% magnetoresistance in sputtered Fe/Cr(100) superlattices

We report the epitaxial growth of Fe/Cr(100) superlattices onto MgO(100) single‐crystal substrates by magnetron sputtering. Superlattices that are epitaxially oriented within 1° both in‐plane and out‐of‐plane with the MgO substrate are achieved by initial growth of a Cr base layer at high temperature. Multiple superlattice diffraction peaks are observed in the low‐ and high‐angle x‐ray diffraction spectra. Three peaks are observed in the magnetoresistance associated with the oscillatory antiferromagnetic interlayer magnetic coupling as a function of Cr thickness. A maximum magnetoresistance of 150% at 4.2 K (28% at room temperature) is observed for a Cr(100 A)/[Fe(14 A)/Cr(8 A)]50 superlattice.

Oscillatory interlayer magnetic coupling of sputtered Fe/Mo superlattices

Sputtered Fe/Mo superlattices grown on sapphire exhibit an oscillatory magnetic coupling as a function of Mo thickness. Ferromagnetic Fe layers 25 A thick couple across the nonmagnetic Mo layers ferro‐ or antiferromagnetically with a period of ∼11 A Mo. Ferromagnetically aligned films show the anticipated trend in longitudinal Kerr‐rotation values based on calculation, while the antiferromagnetically coupled films in zero field yield only weak Kerr signals due to their characteristically ‘‘pinched’’ hysteresis loops. The films are well ordered and exhibit up to seven low‐angle, x‐ray diffraction peaks, but have negative magnetoresistive anomalies that are only ≲2% at 4.2 K.

EXCHANGE-SPRING SYSTEMS : COUPLING OF HARD AND SOFT FERROMAGNETS AS MEASURED BY MAGNETIZATION AND BRILLOUIN LIGHT SCATTERING (INVITED)

An experimental and theoretical study is presented of the normal magnetic modes in spiral ferromagnetic structures. The bilayer system studied consists of Fe layers (25, 50, 100, and 200 A thick) that are exchange coupled to 200 A thick SmCo films that have ≈200 kOe anisotropies. The Fe spiral—induced by an external magnetic field that is applied opposite to the direction of the magnetized film—results in a structure similar to that encountered in a Bloch domain wall. The magnetization and the field dependence of the magnons in various Fe films are explained by the theoretical model.

A GENERAL APPROACH TO THE EPITAXIAL GROWTH OF RARE-EARTH-TRANSITION-METAL FILMS

The growth of epitaxial rare‐earth‐transition‐metal thin films is reported by magnetron sputtering on single‐crystal MgO substrates. The use of epitaxial W buffer layers demonstrates a general approach to control the phase and orientation of the films. Structure and magnetism results for SmFe12(001) on W(100) and magnetically hard Sm2Co7 (110) and (001) on W(100) and (110), respectively, are highlighted to illustrate the utility of the approach.

Magnetic decoupling in sputtered Fe/Si superlattices and multilayers

Sputtered Fe/Si superlattices were grown to study the magnetic coupling between ferromagnetic Fe layers (30 A thick) for Si spacer‐layer thicknesses (tSi) between 10 and 40 A. The material is ferromagnetical for tSi≤13 A and antiferromagnetically coupled for 13 A≤tSi≤17 A. For tSi≥17 A the Fe layers are uncoupled. X‐ray analysis indicates that the system is well layered, but that the crystal structure remains coherent only for tSi≤17 A. These results, along with our Mossbauer investigation, strongly suggest that the Si layer is crystalline for tSi≤17 A, and is silicide in nature. For thicker spacers, Si becomes amorphous. We propose a model of the layering that is consistent with the known properties of Fe silicide.

Spring magnet films

The properties of exchange-spring-coupled bilayer and superlattice films are highlighted for Sm-Co hard magnet (nominally Sm/sub 2/Co/sub 7/) and Fe or Co soft magnet layers. The hexagonal Sm-Co is grown via magnetron sputtering in a- and b-axis epitaxial orientations. In both cases the c-axis, in the film plane, is the easy axis of magnetization. Trends in coercivity with film thickness are established and related to the respective microstructures of the two orientations. The magnetization reversal process for the bilayers is examined by magnetometry and magneto-optical imaging, as well as by simulations that utilize a one-dimensional model to provide the spin configuration for each atomic layer. The Fe magnetization is pinned to that of the Sm-Co at the interface, and reversal proceeds via a progressive twisting of the Fe magnetization. The Fe demagnetization curves are reversible as expected for a spring magnet. Comparison of experiment and simulations indicates that the spring magnet behavior can be understood from the intrinsic properties of the hard and soft layers. Estimates are made of the ultimate gain in performance that can potentially be realized in this system.