A. Butera

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.

Nanostructured magnetic networks: a materials comparison

Abstract One method to achieve the enhanced coercivity necessary for the next generation of ultra high density recording media is to use a patterned substrate to nanostructure the magnetic material. By sputter-depositing a magnetic film onto the surface of a nanoporous substrate, unique magnetic properties result from the reduced dimension and topography of the film. The resultant “network” film has a coercivity nearly two orders of magnitude higher than a continuous thin film of the same thickness. This increase in coercivity has been attributed primarily to shape anisotropy due to the proportional relationship observed between the coercivity and the moment of the deposited network. To better understand the effect of the shape anisotropy, a number of different classes of magnetic materials of varying moment and magnetocrystalline anisotropy were deposited onto porous templates and compared. In general, these materials show a peak coercivity in the thickness range of 15–20xa0nm. In this thickness the range a linear relationship between the coercivity and moment is observed confirming the influence of shape anisotropy. A simple model assuming coherent reversal of a magnetized ellipsoid gives a ratio of 1.1 between the short and long axes. This value is significantly smaller than what it is observed by direct imaging. However, if a more realistic model of magnetization reversal is employed (such as fanning) the ratio obtained is in close agreement with the experiments.

Thickness dependence of the magnetic percolation threshold in as-deposited and annealed Fe–SiO2 granular thin films

The magnetic properties and microstructure of as-deposited and annealed Fe–SiO2 granular thin films were studied. As-deposited films have a maximum in coercivity at an Fe volume fraction (Fe volu2009%) ∼62% independent of film thickness. Iron grains in as-deposited films are well defined, nearly equiaxial and ∼5u2009nm in diameter. From 66 to 90 Fe volu2009%, some as-deposited films showed an unusual well defined in-plane uniaxial anisotropy. The magnetic percolation threshold, xp, as indicated by the maximum in the Hc vs Fe volu2009% curve, changed after the films were annealed. The percolation threshold (xp) of films annealed at 420u2009°C for 30 min shifted to ∼47u2009Fe volu2009% except for the 5 nm films, whose xp remained unchanged. After annealing at 510u2009°C for 3 h, a strong thickness dependence of the percolation threshold was revealed in films thinner than 40 nm, with values ranging from 78 Fe volu2009% to less than 44 Fe volu2009%. The shifts of xp in both 420 and 510u2009°C annealed films can be explained by the effects of reduced di...

Effect of annealing on nanostructured Fe Networks

Contiguous nanostructured Fe network structures sputter deposited on a porous template (nanochannel alumina) have been shown to have a high coercivity due to shape anisotropy. When these networks are vacuum annealed, even at modest temperatures, there is a significant improvement in their in-plane coercivities. The shape of the annealed coercivity vs. thickness plot is a function of the substrate geometry. The maximum coercivity obtained through annealing is /spl sim/1390 Oe and appears to be an absolute maximum. The topology of these networks when viewed by a scanning electron microscope undergoes morphological changes perpendicular to the plane of the network upon annealing. The annealed structures have the appearance of a modified chain of spheres, or a mesh of spheres.

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.

Shift in the magnetic percolation threshold of phase separated Co-rich CoAg very thin films due to reduced dimensionality

An investigation of the structural and magnetic properties of phase separated Co-rich CoAg very thin films (5 nm–50 nm) as a function of film thickness and Co concentration is presented. In the as-deposited state the films are very fine grained and the coercive field is relatively low (Hc<20u2009Oe). However, after annealing at 420u2009°C for half an hour in high vacuum, grain growth is promoted and a drastic change in the magnetic properties is observed. The coercivity now has a strong thickness dependence with a maximum at approximately 15 nm. The largest room temperature value of coercivity, Hc=850u2009Oe, was found for a CoAg 70:30 volume percent alloy. The concentration of maximum coercivity is usually associated with the percolation threshold, xp, of the ferromagnetic element and is much higher than the ∼50u2009volu2009% value normally observed in thick film and bulk granular ferromagnets. Values of xp≲0.65 were found for 30 nm thick films increasing up to xp∼0.75 for 10 nm films. This behavior is explained as a shift ...

Effects of oxidation and abnormal grain growth on the magnetic properties of thin Fe-SiO2 granular films

In the Fe–SiO2 granular system the effects of oxidation and abnormal grain growth on the magnetic behavior are significant. As film thickness is reduced a larger fraction of Fe grains intersect the free surface and are prone to oxidation even in good vacuum conditions. Films coated with protective SiO2 layers were found to be unaffected by oxidation. The presence of a surface during annealing also affects the microstructural evolution. Transmission electron microscopy observations revealed a bimodal distribution of Fe grain sizes in uncoated films due to grain merger assisted by surface diffusion. This bimodal distribution of Fe grain sizes is associated with a coercivity (Hc) vs vol.u2009% Fe curve containing two peaks. By contrast, SiO2 coated films exhibited a uniform microstructure without unusually large grains and a smoother Hc vs vol.u2009% Fe curve with a well defined maximum. Room temperature coercivities of ∼1000 Oe can be routinely achieved in Fe–SiO2 granular films as thin as 10 nm. X-ray photoelectro...

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.

Magnetic Interactions In Fe Films Sputtered On Nanochannel Alumina

We present in this work a study of the magnetic interactions observed in Fe films sputtered onto the surface of commercial porous nanochannel alumina. These films, which grow in a network-like structure, have been characterized by delta M plots, magnetic viscosity and activation volume as a function of film thickness (5 nm/spl les//spl phi//spl les/100 nm) and substrate pore size (20 nm, 100 nm and 200 nm average diameter). We have found a maximum in the remanent coercivity and a minimum in the interparticle interaction for a film thickness /spl phi//spl sim/20 nm for the three substrates. The behavior of the magnetic viscosity and the activation volume on the other hand is dependent on the pore size. We have interpreted our results in terms of the different porosity of the substrates, As the porosity is smaller for the substrates with smaller holes, the continuous film behavior is approached first because the pores fill in more rapidly.