B. N. Das

Growth and magnetic properties of single crystal Co/sub 2/MnX (X=Si,Ge) Heusler alloys

Half-metallic (HM) ferromagnetic materials have recently drawn intense interests due to their potential use in magnetoelectronic devices. Co-based Heusler alloys of the type CO/sub 2/MnX (X=Si,Ge), predicted to be HM by first principles band structure calculations, are of particular interest since they alone possess Curie temperatures in excess of 900 K. Since the spin polarization (P) is believed to be sensitive to antisite defects that are likely to occur in vapor-quenched thin film synthesis, single crystals of Co/sub 2/MnX (X=Si, Ge) were prepared using the tri-arc Czochralski method. X-ray diffraction, including Laue backscattering, was employed to determine the high crystalline quality of these crystals. SQUID magnetometry measured a magnetic moment per formular unit that is close to the calculated value indicating that these alloys may in fact be HM. However, point contact Andreev reflection, a technique that has reliably measured high P in CrO/sub 2/, measures P values for these crystals of 50-60%, well below their theoretical values. The reduced spin polarization may be due to the effects of crystal symmetry breaking at the surface or the presence of anti-site defects, or that these materials are not truly half-metallic.

Structure of melt-spun Fe-Ga based magnetostrictive alloys

Fe/sub 1-x/Ga/sub x/ (x=0.17, 0.21, 0.25 and 0.3) alloys were rapidly quenched by the melt-spinning technique to determine the maximum Ga concentration that retains the disordered bcc (/spl alpha/-Fe) phase at room temperature. The texture of the ribbons as a function of melt-spinning parameters and annealing was also extensively studied. All of the as-spun samples were found to be crystalline. For x=0.17, only the disordered bcc phase was found and for x=0.25 and 0.3, primarily DO/sub 3/ phase was found, while for x=0.21, the phase is bcc at high wheel speed and DO/sub 3/ at low speed. For the free side of the as-spun ribbons, the easy growth axis of /spl alpha/-Fe or of DO/sub 3/ tilts about 10/spl deg/ to 20/spl deg/ from the ribbon normal and is along the ribbon length. This texture does not change significantly with wheel speed. The texture on the wheel side of the ribbon is close to random at high speed but increases with slower speed. For Fe/sub 83/Ga/sub 17/, 900/spl deg/C annealing reduces the tilt angle while 1100/spl deg/C annealing recrystallizes the grains and produces strong out of plane texture but randomizes the directionality in the plane. Annealing with sulfur addition was found to enhance the {100} texture but did not produce the desired along the longitudinal direction of the ribbon. The saturation magnetostriction, /spl lambda//sub s/, of as-spun Fe/sub 79/Ga/sub 21/ and Fe/sub 83/Ga/sub 17/ both having a disordered bcc structure and strong {100} texture was measured as 98 and 130 ppm, respectively, along the ribbon length.

Determination of the structural anisotropy in amorphous Tb-Fe films

The authors report the measurement and description of an anisotropy between the in-plane and out-of-plane atomic structure in as-deposited amorphous Tb-Fe alloy films. This anisotropy, measured using a conversion-electron extended X-ray absorption fine structure (EXAFS) technique, correlates closely with measured values of the perpendicular magnetic anisotropy energy as a function of both film composition and annealing temperature. Modeling of the EXAFS data using theoretical codes and structural standards suggests that the Tb-Tb and Fe-Fe pair correlations are greater parallel to the film plane and that the Tb-Fe pair correlations are greater perpendicular to the film plane. Upon annealing at 300 degrees C the structural anisotropy is eliminated, and the magnetic anisotropy is lowered to a level consistent with magnetoelastic interactions between the film and substrate. >

Growth and magnetic properties of single crystal Fe1−xCoxS2 (x=0.35–1)

The pyrite Fe1−xCoxS2 (0.25⩽x⩽0.9) was predicted to be half metal, insensitive to Fe–Co disorder. To verify its half metallicity, we have grown single crystals of Fe1−xCoxS2 (x=0.35, 0.5, 0.75, 0.9, and 1). Crystals of up to a few mm in size by temperature gradient (923–873 K) solution growth from Te melts, which showed high crystalline quality by scanning electron microscopy, x-ray diffraction, and magnetic measurements. A half-metallic magnetization of 1±0.03μB per Co atom was found for x=0.35–0.9. Point contact Andreev reflection measurements, however, showed spin polarization of 47%–61% with the maximum around x=0.5, well below predictions. Ferromagnetic resonance measurements give g∼2.08 with large intrinsic damping, which indicates a reduced half-metallicity. Inductively coupled plasma and energy dispersive x-ray spectroscopy results confirm a sulfur deficiency of ∼1.5%–10% [(Fe,Co)S2−y,0.03⩽y⩽0.2] and small amounts of Te. Electronic structure calculations show the extra electron provided by Co subs...

DETERMINATION OF LOCAL ORDER IN THE AMORPHOUS PRECURSOR TO BA-HEXAFERRITE THIN-FILM RECORDING MEDIA

Ba‐hexaferrite thin films for recording media applications are often fabricated by a two‐step process: sputter deposition of an amorphous precursor, followed by annealing to crystallize the BaFe12O19 phase. The magnetic anisotropy of the crystalline films can be either in‐plane or perpendicular, depending on the sputtering process used in the first step. However, conventional structural characterization techniques have not been able to distinguish between different as‐sputtered films. Using polarization‐dependent extended x‐ray absorption fine structure (PD‐EXAFS), we have observed anisotropic local structure around both Ba and Fe atoms in the amorphous precursor films. Comparison of the results suggests that the amorphousfilms consist of networks of Fe atoms surrounded by their O nearest neighbors, with Ba atoms fitting into in‐between spaces as network modifiers (there might also be some minor Fe network modifying contribution). The local structuralanisotropy of the amorphousfilms appears to determine the orientation of the fast‐growing basal plane directions during annealing, and thus the directions of the c axes and the magnetic anisotropy.

Reduced‐temperature crystallization of thin amorphous Fe80B20 films studied via empirical modeling of extended x‐ray absorption fine structure

The evolution of the local atomic environment around Fe atoms in very thin (15 nm), amorphous, partially crystallized and fully crystallized films of Fe80B20 was studied using extended x‐ray absorption fine structure (EXAFS) measurements. The relative atomic fraction of each crystalline phase present in the annealed samples was extracted from the Fe EXAFS data by a least‐squares fitting procedure, using data collected from t‐Fe3B, t‐Fe2B, and α‐Fe standards. The type and relative fraction of the crystallization products follows the trends previously measured in Fe80B20 melt‐spun ribbons, except for the fact that crystallization temperatures are ≊200 K lower than those measured in bulk equivalents. This greatly reduced crystallization temperature may arise from the dominant role of surface nucleation sites in the crystallization of very thin amorphous films.

Structural evolution and magnetoresistance properties of heat treated Cu/sub 80/Co/sub 15/Fe/sub 5/ ribbons

Heat treated Cu/sub 80/C/sub 15/Fe/sub 5/ melt-spun ribbons are analyzed for their structural and magnetoresistance (MR) properties. Magnetoresistance values are found to be a maximum for the sample annealed at 500 degrees C. Structural characterization using X-ray diffraction and absorption measurements and transmission electron microscopy show this sample to have two precipitate phases within a Cu-rich fcc matrix, i.e., a Co-rich fcc phase and a CoFe bcc phase. Heat treatments at higher temperatures bring about a decrease in MR. Dark field transmission electron microscope (TEM) images illustrate the clustering of the precipitate particles concurrent with the decrease in MR, suggesting that a clustering mechanism may contribute to the MR effect in these materials. >

Structure and magnetism of multiphase Sm0.080Co0.645Fe0.276 powders

A Sm-poor mixture of Fe-substituted Sm2Co17, having the nominal stoichiometry of Sm0.080Co0.645Fe0.276, was ball-milled to explore the possibility of enhancing its remanence through direct microstructural refinement. With milling, the Sm2(Co0.7Fe0.3)17 compound disassociates to a body-centered-cubic supersaturated SmCoFe solid solution and a residual SmCoFe amorphous phase. Correspondingly, the coercive field values first increase, peaking at 0.83 kOe after 180 min of milling, then decrease with continued milling to <0.1 kOe after 1200 min. The remanence, as Mr/Ms, is measured to track closely the coercive field behavior, experiencing modest increases to 0.26 after just 120 min of milling. Magnetization values are found to increase first with the dissociation of the 2:17 phase, and again with the ejection of Sm from the bcc component.

Correlation of x‐ray diffraction and Mössbauer effect measurements with magnetic properties of heat‐treated Cu80Co15Fe5 ribbons

X‐ray diffraction (XRD) and Mossbauer Effect (ME) measurements were performed on heat‐treated Cu80Co15Fe5 melt‐spun ribbons in an attempt to understand the trends in magnetic properties with heat treatment. ME measurements indicate that the majority of Fe atoms (86%) occupy sites in ferromagnetic FCC CoFe clusters after the initial quench. A heat treatment at 900u2009°C acts to complete the chemical separation of Fe from the Cu matrix. The presence of Co in the Cu matrix, even after high temperature anneals, provides a paramagnetic component that prohibits saturation even at high fields.

Properties of melt‐spun Co15Fe5Cu80, a bulk granular magnetic solid (abstract)

We have investigated the magnetic and transport properties of melt‐spun Co15Fe5Cu80 as a function of heat treatment. Recently Xiao, Jiang, and Chien1 reported clustering and giant magnetoresistance in thin films of related alloys prepared by sputtering. We report qualitatively similar phenomena in these rapidly quenched ribbons. The unannealed samples exhibited saturation magnetoresistance of 0.2%. Annealing at 500u2009°C for 15 min in hydrogen increases the room temperature for magnetoresistance to 3%, and lowering the temperature to 77 K increased the magnetoresistance to 12.6%. Ferromagnetic resonance measurements at 35 GHz were also made. At room temperature the unannealed sample displayed a very broad resonance centered at 10 kG, consistent with a broad distribution of single domain particles of roughly spherical shape and an overall global magnetization 4πM=3.5 kOe. As the annealing temperature increases, the single domain particles with an inhomogeneous linewidth of ∼2 kOe grow into multidomain particl...