Showing papers by "Laura H. Lewis published in 2004"

Stoichiometry–anisotropy connections in epitaxial L10 FePt(001) films

Abstract: The order parameters and anisotropy constants of a series of epitaxial, L1{sub o} FePt films with compositions in the range of 45-55 at% Fe and nominal thicknesses of 50 nm have been characterized. The films were made by cosputtering the elements onto single crystal MgO(001) substrates. The substrates were coated with 1 nm Pt/1 nm Fe bilayer seeds prior to alloy deposition. Both the bilayer seed and the alloy film were deposited at 620 C. Lattice and order parameters were obtained by x-ray diffraction. Film compositions and thicknesses were determined by Rutherford Backscattering Spectrometry, and room-temperature magnetocrystalline anisotropies were determined with a torque magnetometer. It is found that the order parameter has a maximum for the film composition closest to the equiatomic composition, whereas the magnetocrystalline anisotropy increases as the Fe content is increased from below to slightly above the equiatomic composition. These results imply that non-stoichiometric FePt compositions, with a slight excess of Fe, may in fact be preferred for applications that require high anisotropy.

Magnetization Processes in melt-spun Sm-Co-based alloys with the TbCu/sub 7/-type structure

Abstract: The initial magnetization processes in melt-spun Sm-Co alloys with remanent ratios above 0.7 have been studied. Alloys of (Sm/sub 1/(6+x)/Co/sub (5+x)/(6+x)/)/sub 94/Nb/sub 3/C/sub 3/ (x=0.67, 1.7, 2.3 and 3) alloys modified with Nb and C were melt spun at wheel speeds of 20 and 40 m/s, and the compositions span the range between the Sm/sub 2/Co/sub 17/ and SmCo/sub 5/ stoichiometries. Structurally, all alloys formed in the TbCu/sub 7/-type structure, although a small amount of Sm/sub 2/Co/sub 7/ was observed in Sm-rich samples, and a small amount of fcc Co formed in Co-rich samples. From transmission electron microscopy analysis, grain sizes typically ranged from 100 to 500 nm, and the grains were randomly oriented. The as-spun ribbons had remanence ratios of 0.7 and coercivity values ranging from 3 to 18 kOe. The initial magnetization curves showed a steep linear response to the applied field for all samples, suggesting that nucleation-controlled processes dominate the magnetization process. In addition, the initial susceptibility decreased with increasing Sm content, reflecting the anisotropy differences between SmCo/sub 5/ and Sm/sub 2/Co/sub 17/ compounds. The initial susceptibility also increased with increasing wheel speed, suggesting that the microstructure has important ramifications on the magnetization process as well.

Magnetic entropy in Ni2MnGa single crystals

Abstract: A first-order phase transition in mechanically soft, nonstoichiometric Ni2MnGa oriented single crystals was achieved by the application of a magnetic field lower than 1 T on a sample with a forward martensitic transformation temperature TM of 314 K. The observed change in magnetization corresponds to a temperature decrease of 1 K. The entropy variation, corresponding to this partial phase rearrangement, is of the order of −1.5 J/kg K at 1 T, and reaches a value of −18 J/kg K at 5 T.

Interphase exchange effects in CoPt/Co bilayer thin films

Abstract: The condition of the interphase interface in model exchange-spring CoPt/Co bilayers was correlated with the magnetic response. Polycrystalline bilayer samples of 16.7-nm Co and 25-nm L10-type CoPt were fabricated and subsequently annealed to vary the condition of the interphase interface. Although the thickness of the soft phase component exceeded that for ideal (i.e. completely reversible) interlayer exchange coupling, this thickness allowed correlations between the interfacial condition and the magnetic quality of the exchange spring components to be observed and quantified via magnetic recoil measurements. Annealing the bilayers at 300°C for 20 min produced a 63.0% increase in the coercivity of the soft component, while at the same time the coercivity of the hard component was decreased by 18.5%.

Exchange bias in plasma-sprayed MnZn ferrite

Abstract: The antiferromagnetic phase FeO (wustite) forms in plasma-sprayed MnZn ferrites from pure spinel phase powder. An exchange bias is observed in hysteresis loops of both ferrite coatings and single splats; the exchange bias decreases and disappears with annealing. X-ray diffraction indicates that the wustite FeO changes to hematite Fe/sub 2/O/sub 3/ upon annealing. Annealing -induced cation ordering and diffusion influence the ferrite magnetic properties by increasing the saturation magnetization and decreasing the coercivity.

Manipulation of the metamagnetic transition and entropy change in Gd5(Si,Ge)4

Abstract: 100 nm coatings of Fe and of Al were found to alter the magnetic attributes, such as the initial susceptibility and metamagnetic transition field, of the giant magnetocaloric material Gd5(Ge,Si)4 and thereby increase the entropy change for a given applied magnetic field, relative to uncoated samples. The experimental data strongly suggest that these coatings confer a strain that propagates through the material and enhances the effect of the magnetic field in the metamagnetic phase transition range. A simplified Landau theory of coupled magnetic and structural transitions was developed for Gd5(Si1−xGex)4 and used to analyze the thermodynamics and phase transitions of this material under the influence of magnetic fields and elastic strain fields.