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ABSTRACT: Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. Abstract—A two phase microstructure, consisting of nanocrys-tallites surrounded by an amorphous matrix, was produced by a melt spinning processing route. Alloys of this type have extrinsic properties that are dependent on the relative amounts of the amor-phous and nanocrystalline phases. One method for examination of the properties of the nanocrystalline and amorphous phases is by Mössbauer spectroscopy. This paper examines ribbons with the composition of Fe 44 Co 44 Zr 7 B 4 Cu 1 , both as-spun and after annealing at 650 C for 1 hour. Three Mössbauer techniques were used to examine these materials, including: transmission measurements, conversion electron Mössbauer spectroscopy, and radio frequency Mössbauer. The transmission spectrum for the annealed HITPERM alloy is composed of two superimposed sextets corresponding to the nanocrystalline FeCo and retained amorphous phases. The rf-Mössbauer results fail to show collapse of the sextet, indicating a large magnetocrystalline anisotropy of the nanocrystalline phase.
IEEE Transactions on Magnetics 01/2226; 37. · 1.36 Impact Factor
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ABSTRACT: The order parameter of FePt thin films plays an essential role in determining such diverse materials properties as magneto-crystalline anisotropy, magnetic coercivity and magnetic recording density. Typically, the order parameter for a bulk material is obtained by measuring the X-ray integrated intensity ratio of a super lattice peak to a fundamental peak and comparing this ratio to a theoretical value which has been calculated for a fully ordered sample. In this work, we present an analysis of the order parameter calculation in FePt L1<sub>0</sub> thin films taking into account the geometric features of the X-ray diffractometer, the crystallographic texture of FePt films, and the finite thickness of the films. The theoretical ratio of the (001) super lattice peak of FePt and the (002) fundamental peak of FePt is calculated as a function of the full width half maximum (FWHM) diffraction peaks from film and thickness for FePt thin films with perpendicular texture. A reliable order parameter calculation equation for Fe50Pt50 fiber textured perpendicular recording media is established.
IEEE Transactions on Magnetics 02/2012; · 1.36 Impact Factor
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ABSTRACT: Titanomagnetites offer a rich system to explore the role of fine microstructure on magnetic properties. They are important minerals in basalts, and are commonly found on the moon and Mars. Here magnetic measurements were used to monitor decomposition and phase evolution in the pseudo-binary Fe<sub>2</sub>TiO<sub>4</sub>-Fe<sub>3</sub>O<sub>4</sub> solid solution system. The phases appearing in the decomposition are a strongly magnetic magnetite and a weakly magnetic Ti-rich spinel. For the 40, 50, and 60 at% Fe<sub>2</sub>TiO<sub>4</sub> compounds (balance Fe<sub>3</sub>O<sub>4</sub>) explored here, a metastable solid solution is nonmagnetic at temperatures where decomposition kinetics can be monitored in reasonable experimental times. The magnetization of magnetite formed by the decomposition offers a direct measure of the volume fraction transformed. Time-dependent magnetization measurements were used to monitor the kinetics of decomposition and compared to models for spinodal decomposition and nucleation and growth kinetics for compositions outside the spinodes. The fine microstructure resulting from spinodal decomposition and exchange bias mechanisms for coupling, may be important in understanding the remnant state of these minerals on Mars.
IEEE Transactions on Magnetics 11/2011; · 1.36 Impact Factor
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ABSTRACT: M-type barium hexaferrite films have been grown by liquid phase epitaxy and examined by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and conventional and Lorentz-mode transmission electron microscopy (TEM). These films exhibit a diamond chevron shaped “brick wall” microstructure with c-axis oriented hexaferrite platelets. The films are oriented with their c axes in-plane, and parallel to the M-plane sapphire substrate, and exhibit a 30° rotation about the c axis with respect to the substrate. Rocking curves showed (20−20) and (22−40) FWHM values of 1.09° and 1.56°, respectively, for the thinner of two samples, and 0.31° and 0.50° for the thicker sample. The magnetic domain structures have been characterized by Lorentz-mode TEM and the domain walls were found to be pinned to small angle tilt boundaries. Using the measured rocking curve values, the effect of the overall crystalline misorientation on the dispersion of the magnetocrystalline anisotropy of the samples is estimated to be less than half a percent.
Journal of Applied Physics 04/2011; 109(7):07E535-07E535-3. · 2.17 Impact Factor
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ABSTRACT: Thin film alternating gradient field magnetometers (AGFM) have potential for measuring magnetic moments of minerals in extraterrestrial soil samples. AGFM sensors offer increased spatial resolution required to detect magnetic nanoparticles for biosensing applications. We have fabricated a patterned thin film with the properties necessary for use in a small AGFM system. Hexagonal-close-packed CoCrPt thin films of 20 and 500 nm were sputtered (nominal composition of Co66Cr15Pt19), showing a high magnetic moment and large out-of-plane anisotropy. The films showed a Δθ50 of better than 3° for the (002) CoCrPt peak for all films, which improves with thickness. The texture is partly due to the NiW and Ru underlayers. The films showed an out-of-plane easy axis, indicating a strong uniaxial anisotropy that exceeds the shape demagnetization energy. This is due to the addition of Cr, which decreases the magnetic moment of the films; magnetoelastic coupling and film stresses may also aid in achieving a perpendicular anisotropy. The first-order uniaxial anisotropy constants were calculated as a function of temperature, ranging from 3.7 × 106 ergs/cm3 at room temperature to 6.8 × 105 ergs/cm3 at 500 °C, and the T dependence agrees with Akulov’s theory for uniaxial materials. The thickest film was etched with a checkerboard pattern to decrease the demagnetization effects, which are seen more influentially in the thicker films. This opened up the hysteresis loop, and decreased the amount of field necessary to overcome the thin film geometry.
Journal of Applied Physics 03/2011; 109(7):07E512-07E512-3. · 2.17 Impact Factor
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ABSTRACT: We present an experimental approach for obtaining highly ordered L1<sub>0</sub> FePt-oxide thin film media with small grains by using a RuAl layer as a grain size defining underlayer. In most previous studies, the FePt grain size was controlled by tuning the oxide volume fraction of the film. By introducing the RuAl grain size defining layer, 6 nm of FePt grains can be obtained at 520°C with 9% SiO<sub>2</sub> in the film. A 5 nm thin barrier layer was introduced between FePt and RuAl to prevent the inter-diffusion between RuAl and FePt. The influence of different barrier layers was studied. With a thin Ag sacrificial layer inserted between the barrier layer and the FePt magnetic layer, a smaller grain size can be achieved, the ordering temperature was lowered, and the (001) texture of FePt was enhanced.
IEEE Transactions on Magnetics 02/2011; · 1.36 Impact Factor
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ABSTRACT: In this paper, we present a set of systematic experimental investigations on possible noise mechanisms for current perpendicular thin film media of small grain sizes. In particular, we focus on intergranular exchange coupling and grain boundary surface anisotropy in the granular layer of the present continuous-granular-composite film structure. Micromagnetic modeling studies are conducted to study the impact of the observed experimental phenomenon. Modeled experiments show that significant intergranular exchange coupling may occur when oxide grain boundary thickness becomes less than 1 nm. If the grain boundary thickness has significant distribution below this critical value, the exponential dependence of the coupling strength on the oxide thickness would yield significant degradation of the medium signal-to-noise ratio. Carefully designed experiments have also been conducted to study possible grain boundary interfacial anisotropy. Co/Cr, CoPt/Cr, Co/SiO<sub>2</sub>, Co/Cr<sub>2</sub>O<sub>3</sub>, and Co/TiO <sub>2</sub> interfaces are investigated and the corresponding interfacial anisotropy strengths are quantitatively measured. Although Co/SiO<sub>2</sub> interfacial anisotropy appears to be the weakest among them, the measured interfacial anisotropy energy strengths for all of them are significant fractions of the crystalline perpendicular anisotropy of the grains at present grain sizes. Finally, we investigated the impact of stacking faults in hcp Co-alloy grains. It is found that when the anisotropy strength of a small segment of a grain substantially reduces due to the existence of stacking faults, it will yield a switching field reduction disproportional to the volume ratio of the segment.
IEEE Transactions on Magnetics 02/2011; · 1.36 Impact Factor
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ABSTRACT: Classical steady-state nucleation theory is applied to Co-rich Fe,Co-based alloys to provide a rationale for experimental observations during the nanocrystallization of Co-rich (Co,Fe) 89 Zr 7 B 4 and (Co,Fe) 88 Zr 7 B 4 Cu 1 amorphous precursors. The amorphous precursor free energy is estimated using density functional theory. This simple theory suggests: (i) strain or interface energy effects could explain a ten-dency for a body-centered cubic (bcc) phase to form during crystallization. Dissolved glass formers (Zr,B) in crystalline phases may also contribute; (ii) similar face-centered cubic (fcc) and hexagonal close-packed (hcp) free energies could explain the presence of some hcp phase after crystallization even though fcc is stable at the crystallization temperature; (iii) nanocrystal compositions vary monotonically with the Co:Fe ratio of the amorphous precursor even when multiple phases are nucleating because nucleation is not dictated by the common tangency condition governing bulk phase equilibria; and (iv) Fe-enrichment of the bcc phase can be attributed to a relatively small free energy difference between the amorphous and bcc phases for high Co-containing alloys.
07/2010;
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ABSTRACT: In this work, we present an experimental technique for obtaining highly ordered L1<sub>0</sub> FePt-oxide thin film media at moderate deposition temperatures. In most previous studies, a FePt-Oxide layer is directly deposited on a (001) textured MgO layer. By introducing a buffer layer in between the FePt-oxide layer and the MgO underlayer, we are able to substantially enhance the L1<sub>0</sub> ordering of the FePt-oxide layer while lowering the deposition temperature to 400 . The buffer layer also yields a significantly enhanced (001) texture of the formed L1<sub>0</sub> FePt structure. With the order parameter near unity, the coercivity of the resulting granular L1<sub>0</sub> FePt-oxide film exceeds 20 kOe with an average grain size about 8 nm. With the buffer layer technique, 18kOe coercivity has also been achieved for L1<sub>0</sub> FePt-oxide film at a grain size of about 4.5 nm. In this work, the detailed material composition choice of the buffer layers and the corresponding results are presented.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: We report on continued measurements using an experimental model system to quantify intergranular exchange coupling in CoCrPt-oxide perpendicular magnetic recording media. A thin film multi-layered structure comprising a high coercivity CoPt unicrystal layer separated from a low coercivity CoPt layer by a thin oxide interlayer is used to model the vertically aligned grains separated by oxide boundaries in CoCrPt-oxide media. Exchange coupling is measured by field shifts of the minor loop from the low coercivity layer. Results on coupling energy as a function of interlayer thickness are presented for several different oxides. Additional measurements are presented to understand the possible role of the added alloying elements, Cr and Mn, to the magnetic grains in terms of intergranular exchange coupling.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: The current perpendicular magnetic recording (PMR) media uses a non-magnetic dual Ru intermediate layer. Replacing the bottom Ru layer with a Co-7at% Ir soft magnetic crystalline interlayer is expected to enhance both the magnetic field strength and gradient. Fabrication processing parameters were varied to develop the CoIr layer with desired properties (soft magnetic properties, texture, and dome morphology) to be utilized in practical PMR media design. First, the oxide was added to create a magnetically grain-to-grain isolated structure and the well segregated microstructure was obtained. Secondly, CoIr without the oxide was deposited on amorphous Ta and confirmed to have the hcp (00.2) texture and 5 nm Ru on top develops the hcp (00.2) texture epitaxially. The recording layer properties were not deteriorated with the replacement of Ru bottom layer with CoIr. The feasibility of CoIr application in the current PMR media will be discussed based on the experimental results.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: Magnetic nanoparticles (MNPs) possessing low Curie temperatures ( T <sub> C </sub> ’s) offer the possibility for self-regulated heating of cancer cells, where the T <sub> C </sub> acts as an upper limit to heating to prevent damage to neighboring healthy tissue. We report the synthesis of monodisperse metastable fcc γ -phase Fe–Ni MNPs possessing tunable T <sub> C </sub> ’s, whose stoichiometries have been predicted from metastable extensions to an equilibrium Fe–Ni phase diagram. Metastable alloys have been predicted within the constructs of the T <sub>0</sub> construction in the Fe–Ni eutectoid phase diagram. Fe–Ni MNPs have been synthesized via chemical reduction in Fe- and Ni-precursors with stoichiometries ranging from Fe <sub>90</sub> Ni <sub>10</sub> to Fe <sub>70</sub> Ni <sub>30</sub> . Mn-precursors have been added to further reduce the alloy’s T <sub> C </sub> . MNP morphology and structure have been confirmed by x-ray diffraction and transmission electron microscopy while magnetic properties were investigated using vibrating sample magnetometry. Fe <sub>70</sub> Ni <sub>30</sub> MNPs were found to have a T <sub> C </sub> of 82 ° C and specific magnetization of 66 emu/g. Addition of 1 wt % Mn to Fe <sub>75</sub> Ni <sub>25</sub> reduced the T <sub> C </sub> to 78 ° C , which is the lowest reported -
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for fcc Fe–Ni alloys. We also report a model for radio frequency self-regulated heating, in which the maximum achievable temperature of water-MNP suspensions ( T <sub> C </sub><100 ° C ) or octyl ether-MNP suspensions ( T <sub> C </sub>>100 ° C ) is approximately the T <sub> C </sub> of the MNPs suspended in the solution.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: The temperature stability of field induced uniaxial anisotropy (K<sub>U</sub>) was investigated by thermomagnetic treatments of ( Co <sub>1-x</sub> Fe <sub>x</sub>)<sub>89</sub> Zr <sub>7</sub> B <sub>4</sub> amorphous ribbons after field annealing below and above the crystallization temperature. We conclude: (1) Field annealing treatments are necessary to properly investigate the temperature stability of K<sub>U</sub> , (2) K<sub>U</sub> of field crystallized alloys exhibit improved temperature stability relative to alloys remaining amorphous after field annealing, and (3) larger K<sub>U</sub> is obtained for field crystallization treatments as compared to zero-field crystallization followed by field reannealing. Field crystallization may be required for elevated temperature applications when field induced anisotropy is critical for performance.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: We report the oxidation products and qualitative rates for polydisperse FeCo magnetic nanoparticles (MNPs) synthesized using an induction plasma torch. X-ray diffraction (XRD) and TEM showed MNPs to have a thin ferrite shell. Nanopowders were isochronally annealed to promote oxidation and XRD was used to follow the evolution of the FeCo core and the Fe <sub>3</sub> O <sub>4</sub> and FeO oxide shells. Isothermal anneals were used to follow oxidation kinetics at 350 and 500 ° C . High resolution transmission electron microscopy (HRTEM) revealed faceted morphologies terminated at (100) and (110) FeCo faces with (110)<sub> Fe Co </sub>||(111)<sub> oxide </sub> and (100)<sub> Fe Co </sub>||(100)<sub> oxide </sub> , and [010]<sub> Fe Co </sub>||[011]<sub> oxide </sub> orientation relationships between the FeCo core and oxide shell. We show HRTEM images of MNP chaining and compare the rf heating of samples of aqueous ferrofluids similarly loaded with as synthesized and oxidized MNPs.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: In this paper, we present a combined transmission electron microscopy study and micromagnetic modeling investigation of current granular-continuous-composite (CGC) perpendicular thin-film media. Our transmission electron microscopy-EELS elemental analysis on CoCrPt-TiO<sub>x</sub> layers found evident Cr segregation to the grain boundaries as well as in the middle of a grain where Co and Pt appears to be deficient. Similar localized Co and Pt deficiency is also found in CoCrPt-SiO<sub>x</sub> media, however, Cr segregation is much less evident. STEM-HAADF analysis reveals that the apparent non-uniform composition is the result of a ldquocavity-likerdquo topography of the magnetic grains. It is argued that the ldquocavityrdquo grain topography significantly enhances the grain boundary surface area. Micromagnetic modeling investigation was conducted to study the impact of possible surface anisotropy to the corresponding grain switching field. A systematic numerical analysis is presented.
IEEE Transactions on Magnetics 03/2009; · 1.36 Impact Factor
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ABSTRACT: The composition dependence of field induced anisotropy K<sub>U</sub> of field annealed soft ferromagnetic ( Co <sub>1-x</sub> Fe <sub>x</sub>)<sub>89</sub> Zr <sub>7</sub> B <sub>4</sub> and ( Co <sub>1-x</sub> Fe <sub>x</sub>)<sub>88</sub> Zr <sub>7</sub> B <sub>4</sub> Cu <sub>1</sub> amorphous and amorphous/nanocrystalline “nanocomposite” melt spun ribbons is investigated. With the exception of the highest Co-containing alloys (x≪∼0.10) , the observations are discussed in terms of a superposition of directional pair ordering of Fe,Co atoms and an additional contribution presumably due to the presence of Zr and B in both the field crystallized and field annealed amorphous ribbons. The highest Co-containing alloys (x≪∼0.10) contain multiple nanocrystalline phases (bcc, fcc, and hcp) for which a peak in K<sub>U</sub> is observed (K<sub>U</sub>∼2000–2500 J / m <sup>3</sup>) . In this framework, asymmetry in the compositional dependence of K<sub>U</sub> resulting in larger values for Co-rich alloys relative to Fe-rich alloys for both the field crystallized and field annealed amorphous alloys is explained in terms of a strong dependence of the Curie temperature of the amorphous phase on the Co content.
Journal of Applied Physics 01/2009; · 2.17 Impact Factor
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ABSTRACT: The body-centered cubic (bcc) phase tends to preferentially nucleate during solidification of highly undercooled liquid droplets of binary alloy systems, including Fe–Co, Fe–Ni and Fe–Cr–Ni. We investigate a similar tendency during the partial devitrification of Co-rich amorphous precursors of composition (Co 1Àx Fe x) 88 Zr 7 B 4 Cu 1 by identifying the structure and composition of the nanocrystal-line grains. The Co:Fe ratio of the bcc nanocrystals varies linearly with the Co:Fe ratio of the amorphous precursor, and can lie well within the single-phase face-centered cubic (fcc) region of the Fe–Co phase diagram at the crystallization temperature. Classical nucle-ation theory therefore suggests several potential explanations for the preferential nucleation of bcc phase from an amorphous precursor, including: (i) a reduced amorphous/bcc interface energy as compared to the close-packed phases; (ii) a lower strain of precipitation for bcc nuclei as compared to close-packed fcc and hexagonal close-packed nuclei; and (iii) stabilization of the bcc phase by dissolved glass-formers such as Zr and B.
11/2008;
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ABSTRACT: A new nanocrystalline Fe-based soft magnetic alloy is discussed here. Ingots of nanocrystalline alloys ( Fe Cu )<sub>80</sub> Zr <sub>x</sub> Si <sub>20-x</sub> ( x=5 , 6, and 8) were prepared by arc melting. The ingots were remelted and cast into 25–30 μ m thick ribbons by a single roller melt spinning method. X-ray diffraction (XRD) revealed the as-spun ribbons to be amorphous. The structural evolution of these samples was studied by XRD and transmission electron microscopy (TEM) after annealing at 450, 480, and 550 ° C . XRD shows the primary nanocrystallization product to be the α- Fe ( Si ) phase. The grain size was observed by TEM to be ∼10 nm after annealing at 480 ° C for 1 h and 14 nm after 550 ° C for 1 h . ac soft magnetic properties were measured using a Walker AMH 401 ac permeameter. The core loss at an exciting frequency f=100 kHz and maximum induction B<sub>m</sub>=1 kG was determined to be less than 19 W / kg .
Journal of Applied Physics 05/2008; · 2.17 Impact Factor
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ABSTRACT: First order reverse curves have been used to investigate the Preisach distribution and electron holography to observe the spatial variation of the magnetization in Fe <sub>40</sub> Co <sub>40</sub> Nb <sub>4</sub> B <sub>13</sub> Ge <sub>2</sub> Cu <sub>1</sub> nanocomposite alloys after annealing at 500, 550, 610, and 960 ° C for 1 h . Grain sizes observed for these annealing temperatures varied from 10 to 200 nm . The Preisach distribution reveals that the magnetization process in the sample annealed at 500 ° C for 1 h was dominated by reversible processes, consistent with a small coercivity and mobile domain walls in an external field. At higher annealing temperatures, the irreversible magnetization processes became dominant with increasing grain size. Electron holography observations of the domains show that, for the sample annealed at 500 ° C , the magnetic flux distribution was uniform with few pinning barriers to domain wall motion consistent with reversible magnetization. The sample annealed at 610 ° C exhibited irregularity in the shape of magnetic flux lines attributed to the inhomogeneous magnetization distribution due to the α- Fe Co and ( Fe Co Nb )<sub>23</sub> B <sub>6</sub> phases present after secondary crystallization.
Journal of Applied Physics 05/2008; · 2.17 Impact Factor
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ABSTRACT: We investigated bias-sputtered Co alloy-films (CoCrPt- and CoPt-) for high density magnetic tape media. We used bias sputtering to achieve desirable properties (good in-plane orientation and low noise) in these media. The CoCrPt-sputtered tape media showed a flat frequency response with a of 160 kfci and a corresponding signal-to-noise ratio of 21 dB. The overwrite performance was better than 40 dB for an overwriting frequency of 12 MHz (90 kfci) and overwritten frequency ranging from 1 to 10 MHz (8-75 kfci). An overcoat (carbon nitride) with a thickness of 6 nm resulted in a 4-5 dB spacing loss in the roll-off curve at high recording densities, and a 3-4 dB lower overwrite ratio. In addition, the CoPt-media with no Cr present produced a maximum output 4-5 dB higher, but also higher noise power compared to the CoCrPt-media.
IEEE Transactions on Magnetics 09/2007; · 1.36 Impact Factor
