J. L. Weston

The growth of nanoscale structured iron films by glancing angle deposition

Chiral nanostructured thin films can be produced through precise control of the angle of incidence of a vapor flux concurrent with substrate rotation. The technique has been employed to create unique porous iron thin film structures on Si(100) with a columnar microstructure. Scanning electron microscopy images illustrate columnar iron films produced with azimuthal rotation during sample growth with the incident flux at an angle of 75° with respect to the surface normal. The columns were found to be well isolated with a narrow distribution of diameters, resulting in aspect ratios of approximately 8 to 1. Hysteresis loops reveal the columnar growth induced a large magnetic shape anisotropy relative to that observed for an iron film grown with normal incidence. The evolution of the columnar microstructure was followed from simple oblique deposition (no substrate rotation), giving a fibrous slanted microstructure, to high-speed rotation where a broad size distribution of highly faceted columnar structures was observed. The measured microstructure is related to the observed magnetic properties.Chiral nanostructured thin films can be produced through precise control of the angle of incidence of a vapor flux concurrent with substrate rotation. The technique has been employed to create unique porous iron thin film structures on Si(100) with a columnar microstructure. Scanning electron microscopy images illustrate columnar iron films produced with azimuthal rotation during sample growth with the incident flux at an angle of 75° with respect to the surface normal. The columns were found to be well isolated with a narrow distribution of diameters, resulting in aspect ratios of approximately 8 to 1. Hysteresis loops reveal the columnar growth induced a large magnetic shape anisotropy relative to that observed for an iron film grown with normal incidence. The evolution of the columnar microstructure was followed from simple oblique deposition (no substrate rotation), giving a fibrous slanted microstructure, to high-speed rotation where a broad size distribution of highly faceted columnar structures was...

Nanostructured magnetic networks

Nanostructured contiguous networks of Fe well suited to fundamental studies of the effects of confinement are described. The Fe networks are prepared by sputter deposition onto the surface of nanochannel alumina. One of the most obvious consequences of confining the magnetic material to a nanostructured network is a dramatic enhancement in the coercivity when compared to a continuous Fe film of the same thickness.

Growth and magnetic characterization of epitaxial Fe81Ga19∕MgO (100) thin films

Using magnetization and ferromagnetic resonance techniques, we have characterized Fe81Ga19 (100) thin films (90 nm thick) grown on MgO (100). We have observed that for low sputtering powers (<35W) it is possible to grow films with cubic magnetic symmetry, and that larger powers induce an in-plane magnetic easy axis. Films with cubic symmetry were further characterized using ferromagnetic resonance at frequencies of 34 and 9.7 GHz. From the angular variation of the resonance field we have obtained the cubic magnetocrystalline anisotropy constant, K1=2×105erg∕cm3, and the saturation magnetization, M∼1460G. The magnitude and the angular variation of the linewidth suggest an important contribution of the two-magnon scattering mechanism to the relaxation of the magnetic excitations.

Fabrication and characterization of Fe/sub 81/Ga/sub 19/ thin films

Summary form only given. Certain substitutional additions to Fe can increase both its resistivity and magnetostriction /spl lambda/. Recently Fe-Ga has been investigated as a bulk magnetostrictive material, with maximum /spl lambda/ localized along the . Further, it has been discovered that /spl lambda/ along increased from /spl sim/300 to /spl sim/350 ppm when the samples were quenched, rather than furnace-cooled. Fe-Ga, due to its low anisotropy and high magnetostriction is an attractive candidate as a high magnetostrictive susceptibility MEMS material. In order to investigate the thin film properties of Fe-Ga materials epitaxial fabrication is necessary, in order to locate the in specific orientations.

High coercivity nanostructured networks

Films of Fe, Co, and Co35Fe65 alloy have been sputter deposited onto the surface of porous nanochannel alumina substrates producing nanostructured contiguous magnetic “networks.” Large room temperature coercivities have been measured which approximately scale with the bulk saturation magnetization of the material used. In the as-deposited state the largest coercivities (e.g., >1000 Oe for Co35Fe65) are observed in ∼15-nm-thick networks deposited on the smallest commercially available pore size substrates (those having an average pore diameter of ∼20 nm and a wall thickness of ∼15 nm). Preliminary studies of the effects of annealing indicate that coercivities can be substantially increased (by as much as ∼50%) with an appropriate post-deposition thermal treatment.

Perpendicular anisotropy in electrodeposited, Co-rich Co–Pt films by use of Ru underlayers

Co-rich Co–Pt alloys were electrodeposited from an amino-citrate-based solution on a Si (011)∕Cu(111)∕Ru(0001) seed layer. Such a template provides an appropriate interface structure for the growth of a (0002)-oriented hexagonal phase, stabilized at larger thickness by the electrolyte chemistry, providing for a columnar structure with no detectable grain coarsening. This results in as-deposited Co–Pt films exhibiting perpendicular anisotropy (anisotropy constant up to 1.2MJ∕m3) and hard magnetic properties (coercivity up to 358.2kA∕m) in a wide thickness range, from 5to2000nm. Crystalline, magnetoelastic, and interface effects are discussed as possible origins of the observed perpendicular anisotropy.

Soft/hard exchange-coupled layered structures with modulated exchange coupling

Magnetically soft/hard exchange-coupled Ni80Fe20/Si3N4/Sm40Fe60 and Ni80Fe20/mixture/Sm40Fe60 layered structures with induced in-plane uniaxial anisotropy were deposited by sputtering on Si (100) substrates. The interfacial exchange coupling strength between the soft and hard layers was tailored by inserting a thin nonmagnetic insulating Si3N4 layer or by varying interfacial mixture of NiFe and SmFe. It was found that the reduction in the exchange coupling greatly reduces the nucleation field HN of the soft layer and increases the irreversible switching field Hirr of the hard layer. The simple formula used before to describe the nucleation field of the soft layer does not work in the case of the reduced interfacial exchange coupling.

Activation volumes and interparticle interaction effects in nanostructured Fe networks

We have studied the film thickness dependence of the remanent magnetization (delta M plots), the magnetic viscosity, and the activation volumes of Fe thin films sputtered on commercial nanochannel alumina of 20 nm average pore diameter. The films have the form of a contiguous interconnected network. A critical film thickness of φ∼20 nm defines two regions with very different magnetic behavior. For this critical value of φ we have found a maximum in the remanent coercivity, a minimum in the interparticle interaction parameter α, and a rapid increase in the magnetic viscosity, the irreversible susceptibility and the activation volume. Our results are interpreted in terms of the pinning effects caused by nonmagnetic inclusions. The functional relationship between the remanent coercivity and the activation volume is similar to the one found in long cylinders that reverse magnetization by a curling mechanism.

Magnetic properties of Co-rich Co–Pt thin films electrodeposited on a Ru underlayer

Co-rich Co–Pt films were grown by electrodeposition from an amino-citrate based electrolyte on Si(011)∕Cu(111)∕Ru(0001) templates. The Ru(0001) surface provides an interface which induces the growth of an oriented hexagonal phase, thereby yielding hard magnetic properties and perpendicular anisotropy in films as thin as 10nm. Increase in film thickness only marginally compromises the hard magnetic properties probably due to the limited coarsening of the microstructure, oriented columnar growth, and magnetic grain isolation. The deposition current density plays an important role in optimizing microstructure and hard magnetic properties. Perpendicular squareness close to 1 and coercivities as high as 4.5kOe were obtained in thin as-deposited Co–Pt films.

Temperature dependence of the coercivity of Fe films sputtered on nanochannel alumina

The morphology, magnetic microstructure, and temperature dependence of the magnetic coercivity of Fe films sputter deposited on commercial nanoporous substrates has been examined. Images obtained using Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) showed that the films grow in the form of a contiguous ferro-magnetic network of small interconnected grains on the walls that separate the nanopores. Magnetic Force Microscopy (MFM) showed a very complex domain structured with magnetic clusters much larger than the grain size. This complexity in the magnetic structure is not unexpected if the network-like shape of the films is considered. The coercivity of all films increases as the temperature is decreased, the largest variation occurring for a film 5 nm thick. Maximum coercivities H/sub c//spl sim/1800 Oe at 30 K were obtained in as-deposited 5 mn films. The temperature variation of H/sub c/ for the thinner films was found to be qualitatively similar to that found in Fe particles covered with an oxide shell.