Tom George

Unusual spin correlations in a nanomagnet

We show how atomic-scale exchange phenomena can be controlled and exploited in nanoscale itinerant magnets to substantially improve magnetic properties. Cluster-deposition experiments, first-principle simulations, and analytical calculations are used to demonstrate the effect in Co2Si nanoclusters, which have average sizes varying from about 0.6 to 29.5 nm. The cluster-deposited nanoparticles exhibit average magnetic moments of up to 0.70 μB/Co at 10 K and 0.49 μB/Co at 300 K with appreciable magnetocrystalline anisotropies, in sharp contrast to the nearly vanishing bulk magnetization. The underlying spin correlations and associated cluster-size dependence of the magnetization are explained by a surface induced ferromagnetic spin polarization with a decay length of the order of 1 nm, much larger than the nearest-neighbor interatomic distance in the alloy.

Nanostructure and magnetic properties of L10 FePt:X films

Nonepitaxial FePt:X films (X=Cu,Au,CuAu) with tunable magnetic properties are fabricated and investigated. Emphasis is on controlling and adjusting the magnetic properties of high-density perpendicular recording media through exchange decoupling and anisotropy. The films are initially deposited as multilayers with the structure [FePt∕X]n and have individual thicknesses from about 0.06to1.1nm. To create an (001)-oriented granular L10 structure, the films are then annealed at temperatures of 600°C for 5min and 550°C for 10min. The data indicate that Cu enters the L10 lattice whereas Au segregates at the grain boundaries and reduces the intergranular exchange coupling. For X=CuAu, we obtain coercivities Hc below 10kOe, and slopes α=(dM∕dH)Hc of about 1. For X=Cu, we find a favorable reduction in Curie temperature and Hc.

Effects of total thickness on (001) texture, surface morphology, and magnetic properties of [Fe/Pt]n multilayer films by monatomic layer deposition

Atomic-scale [Fe/Pt]n multilayer films with different total thickness were prepared on thermally oxidized Si (100) substrates at room temperature by monatomic layer deposition using dc-magnetron and rf-magnetron sputtering. Effects of the total thickness on (001) texture, surface morphology, and magnetic properties of the postannealed films have been investigated. It is found that the particlelike structure films with perfect (001) texture and perpendicular magnetic anisotropy are obtained with a thickness of less than or equal to 6.5 nm. After 500 °C annealing, the films with thickness of 6.5 and 11.9 nm show very smooth surface. In addition, with increasing total thickness of the films, (001) texture and perpendicular magnetic anisotropy of the annealed films deteriorate, and the films become continuous in structure. The total thickness of the films also affects the exchange-coupling interaction among FePt magnetic grains and the magnetization reversal process.

Structural Studies of Mono-Atomic Layer-Deposited (FePt) 1-x Cu x Thin Films and FePt Thin Films with Pt Top-Layer Deposition

Due to large magnetocrystalline anisotropy, L10 FePt and related films are investigated as potential candidates for high density magnetic recording media [1]. Great progress has been made in the synthesis of the L10 FePt and related films with controlled nanostructures and enhanced magnetic properties. Nevertheless, further studies are needed to achieve the final goal. Two important areas of focus remain in reducing the L10 phase transformation temperature and improving the practical methods to produce high degree of (001) texture in the L10 FePt and related films.

Control of Coercivity in Exchange-Coupled Graded (001) Nanocomposite Films

We fabricated exchange-coupled composite films with perpendicular anisotropy using nanograms of L10 FePt as a magnetically hard base layer and FePt : SiO2 as a relatively soft phase. The degree of anisotropy and nanostructure in the soft layer was controlled by varying the deposition temperature. Growth of the low-anisotropy layer followed the particulate structure of the base layer at deposition temperatures of 390°C or greater. Perpendicular coercivities were strongly dependent on both the anisotropy and thickness of the softer layer.

Grain alignment due to magnetic-field annealing in MnBi:Bi nanocomposites

High-anisotropy arc-melted and field-annealed Mn100−x Bi x (x = 50–65) alloys have been investigated. Samples predominantly consist of low-temperature phase (LTP) MnBi and elemental Bi, where the LTP volume fraction and sample magnetization decrease with increasing x. All samples are hard ferromagnetic at room temperature and demonstrate a structural and magnetic phase transition at 630 K. For Mn35Bi65, the highest values for coercivity (4.6 kOe), magnetization remanence ratio (0.97), and energy product (4.9 MGOe) are measured at 540 K, near the melting point of bismuth. We explain the trend in magnetic properties as a consequence of c-axis alignment of MnBi grains facilitated by the molten Bi during magnetic field annealing.

Magnetism of FePt nanoclusters in polyimide

FePt nanoclusters have been implanted onto polyimide films and subjected to thermal annealing in order to obtain a special magnetic phase (L10) dispersed within the polymer. SQUID measurements quantified the magnetic features of the as-prepared and annealed hybrid films. As-implanted FePt nanoparticles in polyimide films exhibited a blocking temperature of 70 ± 5K. Thermal annealing in zero and 10 kOe applied magnetic field increased the magnetic anisotropy and coercivity of the samples. Wide Angle X-Ray Scattering confirmed the presence of FePt and L10 phase. All samples (as deposited and annealed) exhibited electron spin resonance spectra consisting of two overlapping lines. The broad line was a ferromagnetic resonance originating from FePt nanoparticles. Its angular dependence indicated the magnetic anisotropy of FePt nanoparticles. SEM micrographs suggest a negligible coalescence of FePt nanoparticles, supporting that the enhancement of the magnetic properties is a consequence of the improvement of the L10 structure. The narrow ESR line was assigned to nonmagnetic (paramagnetic) impurities within the samples consistent with graphite-like structures generated by the local degradation of the polymer during implantation and annealing. Raman spectroscopy confirmed the formation of graphitic structures in annealed KHN and in KHN-FePt.

Magnetism of Rapidly Quenched Sm

The effect of Zr addition on nanostructure and magnetic properties in nanocrystalline Sm1-xZrxCo5 (x = 0-0.6) has been investigated. (Sm, Zr)Co5 with the CaCu5 structure was synthesized by melt spinning. The lattice parameters a and b decrease with x, whereas c increases. Thus, the unit cell volume of (Sm, Zr)Co5 shrinks because the smaller Zr atoms occupy the sites of the larger Sm atoms. Zr addition decreases the grain size and induces the formation of planar defects. The coercivity decreases with x, due to weakening of magnetocrystalline anisotropy energy and effective intergrain exchange coupling. A very high coercivity of 39 kOe and energy product of 13.9 MGOe are obtained for x = 0. The remanence of (Sm, Zr)Co5 increases with x. For x ≤ 0.4, the energy product slightly decreases with x. The results show that 40% of the Sm can be replaced by the less expensive Zr, with an energy-product reduction of only 10%. In addition, the planar defects are responsible for the change of coercivity mechanism from the nucleation-type of reverse domain for the x = 0 to the pinning-type of domain wall for the x = 0.4.

_{1 - {\rm x}}

We fabricated exchange-coupled composite films with perpendicular anisotropy using nanograms of L10 FePt as a magnetically hard base layer and FePt : SiO2 as a relatively soft phase. The degree of anisotropy and nanostructure in the soft layer was controlled by varying the deposition temperature. Growth of the low-anisotropy layer followed the particulate structure of the base layer at deposition temperatures of 390°C or greater. Perpendicular coercivities were strongly dependent on both the anisotropy and thickness of the softer layer.

Zr

We fabricated exchange-coupled composite films with perpendicular anisotropy using nanograms of L10 FePt as a magnetically hard base layer and FePt : SiO2 as a relatively soft phase. The degree of anisotropy and nanostructure in the soft layer was controlled by varying the deposition temperature. Growth of the low-anisotropy layer followed the particulate structure of the base layer at deposition temperatures of 390°C or greater. Perpendicular coercivities were strongly dependent on both the anisotropy and thickness of the softer layer.