K. D. Sorge

Oriented ferromagnetic Fe-Pt alloy nanoparticles produced in Al2O3 by ion-beam synthesis

Oriented Fe1−xPtx nanoparticles have been formed in single-crystal Al2O3 host matrices by the sequential implantation of Fe and Pt ions followed by thermal annealing. For x in the range of ∼35–55 at. % Pt, these nanoparticles are in the chemically ordered tetragonal L10 structure of FePt and appear to be fully ordered. The nanoparticles are ferromagnetic, and the magnetic coercivity is a strong function of the alloy composition, reaching values in excess of 20 kOe for x∼45%. The crystallographic orientation and morphology of the nanoparticles are strongly dependent on the implantation conditions. Under certain implantation conditions, a buried amorphous layer can be formed in the Al2O3 matrix which crystallizes during annealing giving rise to the formation of an interconnected network of large FePt particles with a single orientation. Oriented nanoparticles of Fe3Pt and FePt3 were also synthesized. The Fe3Pt and FePt3 particles have the ordered, cubic L12 structure with an order parameter of 0.5–0.8; and ...

Vortex pinning and slow creep in high-Jc MgB2 thin films: a magnetic and transport study

We have investigated the pinning of vortices in high-Jc films of polycrystalline MgB2, by studying the dependence of current density J on electric field E using both magnetic and transport methods. Precursor films of amorphous boron, deposited on sapphire substrates, were converted to 0.6??m thick MgB2 by post-annealing in the presence of Mg vapour at 890??C for 1?h. In magnetic studies, a SQUID magnetometer was used conventionally to determine the induced current density by the Bean model. The decay of J with time t was determined unconventionally with the sample fixed in position, by monitoring the SQUID feedback voltage versus time. The logarithmic decay rate S = ?d ln(J)/d ln(t) was found to be very low in the H?T phase space away from the irreversibility line. Complementary four-probe transport studies of E(J) were analysed as a power law dependence of the form and used to obtain the corresponding creep rate S = 1/(n?1). Effective values for n approach and often significantly exceed 100. From these results, we estimate the effective energy U0 for vortex pinning, as a function of magnetizing field H.

Magnetic force microscopy of ferromagnetic nanoparticles formed in Al2O3 and SiO2 by ion implantation

Magnetic force microscopy (MFM) has been used to investigate the properties of ferromagnetic FePt nanoparticles produced by the implantation of Fe and Pt ions into single-crystal Al2O3 or fused SiO2 followed by thermal processing. The MFM results are compared to cross-section and plan view transmission electron microscopy images of the same samples. We demonstrate that MFM can detect magnetism in nanosized particles that are situated several hundred nm below the sample surface. MFM is shown to be a promising tool for studying the characteristics of magnetic nanoparticles produced by ion implantation.

Ferromagnetic FePt nanoparticles formed in Al2O3 by ion implantation

Abstract Oriented, ferromagnetic FePt nanoparticles have been produced in Al2O3 single crystals by ion implantation and annealing. FePt orientation and particle size depends strongly on the implantation conditions. Magnetic coercivities of these nanocomposites are extremely high, reaching values in excess of 20 kOe for Pt concentrations of ∼45% in the FePt alloy. The formation of ferromagnetic FePt nanoparticles in fused SiO2 is also demonstrated.

FePt nanoparticles formed in Al2O3 by ion beam synthesis: Annealing environment effects

The properties of FePt nanoparticles formed by the implantation of Fe+Pt into c-axis-oriented Al2O3 single crystals followed by thermal annealing are shown to be strongly dependent on the annealing environment. Annealing in a reducing environment (flowing Ar+4% H2, or ultrahigh vacuum) gives rise to ferromagnetic FePt nanoparticles with the L10 structure and very high magnetic coercivity (greater than 20 kOe). FePt alloy formation does not occur during annealing in an oxidizing environment. Instead, the implanted Pt precipitates out forming oriented Pt nanoparticles and the implanted Fe redistributes with ∼40% segregating to the surface where it forms epitaxial α-Fe2O3 precipitates at the surface; the remainder of the implanted Fe remains in the bulk, most likely in solid solution in the matrix. Results obtained by sequential annealing of Fe+Pt implanted samples in reducing (oxidizing) environments followed by annealing in an oxidizing (reducing) environment suggest that equilibrium, rather than kinetic, ...

Magnetic properties of Fe nanocubes with magnetostatic interactions

Recent experiments indicate that the magnetic properties of single domain Fe nanoparticles (dispersed in an insulating matrix) may be dominated by magnetostatic interactions at packing fractions as low as 10%, where the separation between particles is of the same order as the particle size [Sorge et al., IEEE Trans. Magn. 37, 2197 (2001)]. We use extensive Monte Carlo simulations to calculate the temperature dependence of the remnant magnetization as a direct test of this hypothesis [Sorge et al., IEEE Trans. Magn. 37, 2197 (2001)]. The particle distribution is constructed with a computer model that imitates the experimental system for which data were obtained from transmission electron microscopy images, and the Fe particles are modeled as point dipoles with cubic anisotropy. Using bulk values for the anisotropy and the Fe magnetization, our simulations reproduce very well the experimental remnant magnetization. Furthermore, we find that the magnetic properties are dominated by the effects of dipole–dipole interactions and that the experimental results cannot be reproduced with noninteracting particles.

Annealing-environment effects on the properties of CoPt nanoparticles formed in single-crystal Al2O3 by ion implantation

The ion implantation of nearly equal doses of Co and Pt into a single-crystal Al2O3 host followed by thermal annealing leads to the formation of nanoparticles whose phase, structure, and physical properties are strongly dependent on the annealing environment. Annealing in 96%Ar+4%H2 gives rise to ferromagnetic, chemically ordered CoPt nanoparticles with the L10 structure and a magnetic coercivity that can exceed 10kOe at 5K. Annealing in O2 (or in Ar) does not result in the formation of a CoPt alloy. Instead, the implanted Pt precipitates to form oriented elemental Pt nanoparticles, and the implanted Co combines with oxygen to form Co3O4 oxide nanoparticles. Annealing in ultrahigh-vacuum conditions results in a mixture of phases including CoPt3 and Co3O4 and possibly Co. The results obtained for Co+Pt-implanted Al2O3 are compared with the previous results for Fe+Pt-implanted Al2O3 annealed in similar environments.

Controlling the Microstructure and Magnetic Properties of Ferromagnetic Nanocrystals Produced by Ion Implantation

Abstract : Ion implantation coupled with annealing is a versatile and flexible approach to creating ferromagnetic near-surface nanocomposites that represent a wide range of particle/host combinations. We have used ion implantation and thermal processing to create a layer of Co nanoparticles in a sapphire host that was subsequently irradiated with Xe, Pt, or Pb in order to systematically modify the magnetic properties of the composite. Transmission electron microscopy (reported in an accompanying paper in this volume) was used to carry out a detailed characterization of the microstructure of the resulting near-surface composites whose magnetic properties were determined using SQUID magnetometry or magnetic circular dichroism. These composites exhibit magnetic hysteresis with coercivities ranging from near zero (i.e., superparamagnetism) up to 1.2 kG depending on the composition and microstructure. We also present the results of preliminary experiments in which we attempt to control the spatial distribution of magnetic elements within ion-implanted ferromagnetic nanocomposites. The results demonstrate methods for tailoring the magnetic properties of nanocomposites produced by ion implantation for specific applications.

Diminished equilibrium magnetization in Hg-1223 and Tl-2212 superconductors with fission-generated columnar defects

When randomly oriented columnar defects (CDs) are added to Hg-1223 and Tl-2212 superconductors, their vortex state equilibrium magnetization Meq decreases substantially. Meq progressively deviates from the usual London ln(B) dependence and the curves become S-shaped. Vortex-defect interactions quantitatively account for this behavior.