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ABSTRACT: Fe-B-P submicron particle chains and (Ni0.6Zn0.4)Fe2O4 nanoparticles were synthesized by chemical methods. (Ni0.6Zn0.4)Fe2O4 nanoparticles were used to fill in the space among Fe-B-P chains under a magnetic field. The synthetic composite of Fe-B-P chains filled with 20 vol.% (Ni0.6Zn0.4)Fe2O4 nanoparticles exhibits the highest intrinsic permeability(142). It is due to the enhanced performance of Fe-B-P particle chains and the effect of (Ni0.6Zn0.4)Fe2O4 nanoparticles, which both decrease the demagnetization field. The composite film also shows a better power loss feature on coplanar transmission line than pure Fe-B-P chains, and the resonance frequencies of the composite films on coplanar line shift to about 20-30 GHz, which is caused by a large demagnetizing field due to the narrow line width of coplanar line. (C) 2012 Elsevier B. V. All rights reserved.
Journal of Alloys and Compounds 01/2013; 554:414-418. · 2.29 Impact Factor
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ABSTRACT: Ni80Fe20 permalloy nanoparticles with narrow size distribution and homogeneous composition have been prepared by the polyol processing at 180 degrees C for 2 h and their particle sizes can be tunable in the size range of 20-440 nm by proper addition of K2PtCI4 agent. X-ray diffraction results show that the NiFe nanoparticles are of face centered cubic structure. The addition of K2PtCl4 does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe nanoparticles decrease with decreasing particle size. Annealed at 280 degrees C, however, the saturation magnetization of various sized NiFe nanoparticles increases drastically and approaches to the bulk for the -440 nm NiFe particles, and a maximum coercivity (-270 Oe) happens at a critical size of -50 nm. The magnetic property dependency of these NiFe nanoparticles on annealing has been discussed by considering the surface chemistry.
Journal of Nanoscience and Nanotechnology 12/2011; 11(12):10796-9. · 1.56 Impact Factor
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ABSTRACT: Co/Ni multilayers with a stack of Ta (2 nm)/[Co ( t <sub>Co</sub>)/Ni ( t <sub>Ni</sub>)] N /Ta (30 nm) were prepared by dc magnetron sputtering, and their magnetization dynamics were measured by time-resolved magneto-optical Kerr effect (TRMOKE). The total thickness of the multilayer and perpendicular anisotropy were varied by changing the bilayer period d = t <sub>Co</sub>+ t <sub>Ni</sub> and number of repeats N while t <sub>Ni</sub>/ t <sub>Co</sub> was kept at a constant of 2.5. The TRMOKE measurements show clear damped oscillation of the magnetization of Co/Ni multilayers after the pump pulse illumination, and the damping constant α of the Co/Ni multilayers was estimated from the TRMOKE waveform. The estimated α was found to be independent both on total thickness and anisotropy field of the multilayer and was estimated to be ~ 0.035 for all the multilayers. This means that the use of Ta capping and buffer layers is effective to evaluate intrinsic damping constant of the Co/Ni multilayer, and that independent control of α and perpendicular anisotropy are possible for the magnetic multilayers.
IEEE Transactions on Magnetics 11/2011; · 1.36 Impact Factor
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ABSTRACT: In this study, we have experimentally investigated the microwave assisted magnetization by switching (MAS) on the microstructured Co/Pt multilayer. The sample exhibits the typical magnetization curve peculiar to perpendicular anisotropy films, that is, a steep reversal initiated by nucleation of a reversed domain followed by its subsequent gradual expansion by the domain wall displacement. By applying microwaves with the frequency of GHz order, the nucleation field H n is significantly reduced at three frequencies. Taking into account the effective anisotropy field of our sample, the first dip of H n at the lowest frequency probably corresponds to the Kittel mode excitation, and the other two dips at higher frequencies correspond to unidentified excitation modes other than the Kittel mode. Among them, the last dip of H n at the highest frequency reaches about 1/3 of that without microwave application. These results suggest the existence of more effective excitation modes for MAS than the Kittel mode.
Journal of Applied Physics 05/2011; · 2.17 Impact Factor
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ABSTRACT: Using ultrahigh vacuum sputter film deposition, we fabricated L10-type Fe50Pt50 ordered alloy perpendicular films on MgO(001) single-crystal substrates and 2.5 in. glass disks at low substrate temperatures of 200–350 °C. Then we examined the magnetic properties of the dot arrays made from these films. The uniaxial magnetic anisotropy Ku for L10-type FePt films (10 nm in thickness) deposited with a Pd underlayer on MgO(001) substrates reached about 2 × 107 erg/cm3 at the substrate temperature Ts of 200 °C, and 3 × 107 erg/cm3 at Ts = 250 °C. The order parameter S was about 0.46 at Ts = 300 °C. Moreover, Ku for L10-FePt films fabricated on glass disks using MgO/Cr underlayers shows 3.4 × 107 erg/cm3 at Ts = 300 °C, which was almost equal to that for FePt single-crystal films deposited on Pd/MgO(001). The switching field distribution σ/Hc for dot arrays made from L10-FePt film [5 nm in thickness, on Pd/MgO(001) at Ts = 250 °C] was small; σ/Hc = 0.11 for a dot diameter of 15 nm. This value was smaller than that of hcp-Co75Pt25 dot arrays (σ/Hc = 0.18). The difference was mainly attributable to the degree of the easy axis distribution. This result demonstrates the homogeneous formation of a L10-type ordered structure in the FePt layers.
Journal of Applied Physics 03/2011; 109(7):07B726-07B726-3. · 2.17 Impact Factor
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ABSTRACT: Magnetization switching experiments on a single Co/Pt multilayer dot of 300 nm in diameter have been carried out using pulse fields with the duration τp = 0.6–10.3 ns and the amplitude up to 3.2 kOe perpendicular to the film plane. The switching field increases from 3.4 kOe in a quasistatic field to 4.56 kOe in a pulse field for τp = 0.6 ns. From the analysis of the field duration dependence of the switching field based on the Néel-Arrhenius model, the energy barrier E0 = 214 kBT and the intrinsic switching field H0 = 5.0 kOe were obtained. Those two parameters well reproduce the experimentally observed dependence of switching probability on pulse field amplitude, indicating that the magnetization behavior of the dot can be described by the thermal fluctuation effect for a single barrier over the field duration ranging from subnanoseconds to quasistatic regime.
Journal of Applied Physics 03/2011; 109(7):07B904-07B904-3. · 2.17 Impact Factor
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ABSTRACT: Magnetization switching experiments on nanosized [Co(δCo = 0.3 − 0.8 nm)/Pd (2 nm)]3 multilayer dots with the diameter of D = 240 − 500 nm are carried out using pulse fields with the duration of 1.6 − 10 ns. The switching fields in the nanoseconds regime are significantly enhanced with respect to the values measured by conventional quasistatic magnetometry. The energy barriers are evaluated from the time-dependence of the switching field based on the Néel-Arrhenius law and are found to be proportional to the Co layer thickness.
Journal of Physics Conference Series 01/2011; 266(1):012083.
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ABSTRACT: Fe microparticle composites with different nanoparticles were prepared using Ni-Zn ferrite and Fe-B-P amorphous nanoparticles. Both additions into iron microparticles significantly increase magnetic permeability, whereas Fe-FeBP composite exhibits more advantages than Fe - (Ni(0.6)Zn(0.4))Fe(2)O(4) composite due to higher saturation magnetization ( Ms). The magnetic field range with high permeability is about 0-600 Oe for Fe-FeBP composite, whereas it decreases to 0-80 Oe for Fe - (Ni(0.6)Zn(0.4))Fe(2)O(4) composite. The permeability of Fe-FeBP composite remains high up to about 1 GHz and the resonance frequency is about 2 GHz, which is higher than that of Fe - (Ni(0.6)Zn(0.4))Fe(2)O(4) composite.
IEEE Transactions on Magnetics 01/2011; 47(10):3160-3162. · 1.36 Impact Factor
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ABSTRACT: Microwave assisted switching (MAS) of magnetization has been intensively studied as an alternative technique for ultrahigh density magnetic recording. In this paper, comparison between the Landau–Lifshitz–Gilbert simulation and the analytical model in the rotating frame {Bertotti etal, [Phys. Rev. Lett. 86, 724 (2001)]} reveals that the switching behaviors of MAS can be clearly divided into two groups, that is, stable and unstable switching regions, depending on the frequency and amplitude of the ac field. The stable switching exhibits small switching field and narrow switching field distribution, and perfectly coincides with by the analytical model. Furthermore, in this region, the Sharrock type thermal fluctuation formula can be applicable to the MAS at finite temperature. On the other hand, for the unstable switching, the switching field is rather large and the SDF becomes very broad. Obviously, the former is preferable for the practical application of MAS. The critical frequency of the ac field for the limit of stable switching is almost proportional to the ac field amplitude.
Journal of Applied Physics 07/2010; · 2.17 Impact Factor
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ABSTRACT: Microwave assisted switching (MAS) of magnetization has attracted much attention as an alternative technique for future ultrahigh density magnetic recording. In this study, to elucidate the effect of magnetostatic interparticle interaction on MAS, we have calculated the switching behavior for a row of three uniaxial magnetic particles as a simplest case. When a dc field is applied only to one particle of the row in an ac field, MAS selectively occurs in the particle and not in the neighboring particles. This behavior may be very effective to realize narrow track recording without adjacent track erasure problem by downsizing a dc field source below an ac source. It has been also found that when both ac and dc fields are applied to all the particles, coherent magnetization precession is cooperatively induced in all the particles due to dipole-dipole interaction among neighboring particles, resulting in significant reduction of the switching field. This coherent magnetization precession is not only effective to reduce the switching field but also to suppress the effect of magnetic anisotropy dispersion, suggesting the effectiveness of MAS in granular recording media.
Journal of Applied Physics 03/2010; · 2.17 Impact Factor
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ABSTRACT: We substituted 3d transition elements M (M=Ni, Fe, Cr, and Mn) for some of the Co, or Pd for some of the Pt, in L11-type Co50Pt50 ordered alloy perpendicular films, and studied magnetic and structural properties. Films were fabricated at a substrate temperature of 360oC using ultra-high vacuum (UHV) sputter film deposition. The order parameter S was almost a constant of about 0.5 when Ni was substituted for some of the Co. The uniaxial magnetic anisotropy Ku and the saturation magnetization, Ms, decreased as the Ni content increased, however, Ku maintained a relatively large value of 1.8x107 erg/cm3 (Ms=570 emu/cm3) even for a Co20Ni30Pt50 composition. In the substitutions of Fe for some of the Co, or Pd for some of the Pt, no significant variation in Ms was observed, however, the rate of decrease of Ku on increasing the Fe or Pd content was larger than when Ni was substituted. The substitution of Cr or Mn for some Co resulted in significant reductions in S, Ms and Ku. Experimental results showed the potential of Co-Ni-Pt ordered films for use in data storage applications, due to their very high Ku potential, relatively low Ms, the relatively low fabrication temperature, and good controllability of the grain orientation.
Journal of Physics Conference Series 02/2010; 200(10):102002.
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ABSTRACT: Magnetic and structural properties of L11 type (Co0.5Pt0.5)100-X-CX ordered alloy perpendicular films, fabricated on 2.5 inch size glass disks by sputter deposition, were examined as a function of C content, X. L11 type Co0.5Pt0.5-C polycrystalline films (10 nm thickness), with <111> axis (the easy axis) perpendicular to the film plane, were successfully fabricated even for a 30 vol% C content. Structural analysis indicated the segregation of C to the grain boundaries. Uniaxial magnetic anisotropy, Ku, of Co0.5Pt0.5 films without C addition was relatively low, about 1.5x107 erg/cm3 under the present deposition conditions. However, the addition of 5 vol.% C to Co-Pt films enhanced the ordering, resulting in an increase in Ku to around 2.5 x107 erg/cm3. A further increase in C content reduced Ku; however, Ku maintained a relatively large value of about 1.8x107 erg/cm3 even for a 20vol% C content, without degrading the easy axis orientation perpendicular to the film plane. Experimental results demonstrated the potential of the L11 type Co0.5Pt0.5-C films for use in granular media applications, due to their very high Ku, the relatively low fabrication temperature, and good controllability of the grain orientation.
Journal of Physics Conference Series 02/2010; 200(10):102008.
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ABSTRACT: The magnetization reversal process for dot arrays is likely to start from a nucleation followed by propagation process. In this study, we estimated the nucleation diameter Dn for dot arrays made from thin hcp-CoPt perpendicular films (thickness δ=3 nm) and Co/Pt multilayered films (δ=9 nm), respectively. The dot diameter, D, was varied from 30 to 200 nm for CoPt dot arrays, and from 40 to 80 nm for Co/Pt dot arrays. The remanence coercivity was measured at measurement times t' = 103 s and 10-5 s (pulse field), and defined as Hr and HrP. The energy barrier ΔE was evaluated by fitting Hr and HrP to Sharrock's equation. The value of Dn was estimated from ΔE, δ and the effective magnetic anisotropy of dot arrays including the demagnetizing energy due to the dot shape Kueff. Dn was independent of Din both series of dot arrays, and about 17 nm for CoPt dot arrays and about 11 nm for Co/Pt dot arrays. These values were close to both the grain size and the exchange length of these films.
Journal of Physics Conference Series 02/2010; 200(10):102003.
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ABSTRACT: Ni(80)Fe(20) permalloy nanoparticles (NPs) with narrow size distribution and homogeneous composition have been prepared by the polyol processing at 180 degrees C for 2h and their particle sizes can be tunable in the size range of 20 similar to 440 rim by proper addition of K(2)PtCl(4) agent. X-ray diffraction results show that the NiFe NPs are of face centered cubic (FCC) structure. The addition of K(2)PtCl(4) does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe NPs decrease with decreasing particle size. Annealed at 280 degrees C, however, the saturation magnetization of various size NiFe nanoparticles increases drastically and approaches to the bulk for the similar to 440nm NiFe particles, however, a maximum coercivity (similar to 270 Oe) happens at a critical size of similar to 50 nm. The magnetic property dependency of these NiFe NPs on annealing has been discussed by considering the surface chemistry.
01/2010: pages 1116-1117;
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ABSTRACT: Submicron-size amorphous particles were produced chemically from aqueous solutions. The particles in the as-made state exhibit a high saturation magnetization and magnetic softness almost equivalent to that of much larger amorphous particles produced by water atmizing. Platinum ions work effectively to control the particle size without losing magnetic softness. Magnetic softness observed for the as-made particles becomes degraded slightly by annealing. A calculation of eddy current within spheres predicts no eddy current loss even at 100 MHz for 0.3-mum particles, whereas initial permeability measurements exhibit slight indication of magnetic loss due to the broad spin resonance profile extending from 100 MHz to 10 GHz.
IEEE Transactions on Magnetics 11/2009; · 1.36 Impact Factor
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ABSTRACT: For future advancement in magnetic and spin electronic devices, it is essential to understand the magnetic behaviour and spin dynamics of a single nanomagnet. However, no effective method has been available for detection of such extremely small magnetic moment. To overcome this difficulty, we have newly developed a highly sensitive magnetic detection technique utilizing the anomalous Hall effect (AHE). This technique has systematically revealed the magnetic behaviors of various magnetic materials in the nanoscale regime. In this article, we demonstrate how single nanomagnets of L10 FePt and Co/Pt multilayers behave depending on their size, especially focusing on the magnetization reversal process and the bistability condition necessary for digital memory use.
Journal of Physics Conference Series 06/2009; 165(1):012029.
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ABSTRACT: We have performed a systematic study on the correlation between magnetic anisotropy energy (MAE) and crystal structures, such as lattice parameters, stacking fault densities, lattice strain, and so on, for epitaxially grown Co, Co-Pt, and Co-Pd alloy thin films, and have found that the MAE strongly depends on the axial ratio c/a of the hcp crystal lattice. As the c/a of hcp Co decreases down to ∼1.61 which is smaller than 1.622 for bulk Co, the MAE becomes significantly enhanced up to ∼10(6) J m(-3). Similar trends have also been verified for hcp Co-Pt and -Pd. These results, which are qualitatively consistent with the classic single-ion anisotropy model and the recent first principles calculation, suggest a new effective way to control the MAE of magnetic thin films.
Journal of Physics Condensed Matter 05/2009; 21(18):185008. · 2.55 Impact Factor
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ABSTRACT: Magnetic properties of hard/soft stacked dot arrays consisting of thin Co/Pt superlattice hard layers (3.6 nm in thickness, uniaxial magnetic anisotropy, K<sub>u</sub>=1.3×10<sup>7</sup> erg / cm <sup>3</sup> ) and Co soft layers, with dot diameters of 30–40 nm, were investigated as functions of Co soft layer thickness and the interfacial exchange coupling between the hard and soft layers. Pt was used as the control layer of the interfacial exchange coupling, and Co soft layers were sandwiched with Pt layers to induce surface anisotropy on the Co soft layers. The remanence coercivity, H<sub>r</sub> , was 2.7 kOe for Co/Pt(3.6 nm)/Co(4 nm) stacked dot arrays and 3.2 kOe for Co/Pt(3.6 nm)/Pt(1.2 nm)/Co(3 nm) stacked dot arrays, and these values were less than half that of single hard layer dot arrays (7.1 kOe). H<sub>r</sub> was nearly constant in the Φ range from 0° to about 45° ( Φ is the applied field angle from the easy axis), and increased significantly as Φ increased further, as theoretically predicted. Hard/soft dot arrays maintained a relatively large K<sub>u</sub> due to the surface anisotropy of the Co soft layers. It was suggested that the ratio of magnetic energy to the thermal energy, K<sub>u</sub><sup> eff </sup> V / kT , for Co/Pt(3.6 nm)/Pt(1.2 nm)/Co(3 nm) hard/soft dot arrays was 1.5 times larger than that for Co/Pt(3.6 nm) single dot arrays because of the relatively large K<sub>u</sub> .
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: L1<sub>1</sub> -type (Co–Ni)–Pt ordered alloy perpendicular films were successfully fabricated on MgO(111) single crystal substrates using ultrahigh vacuum sputter film deposition; the addition of Ni to Co–Pt was effective to reduce saturation magnetization, M<sub>s</sub> , maintaining a large K<sub>u</sub> of the order of 10<sup>7</sup> erg / cm <sup>3</sup> . L1<sub>1</sub> -type ordered structures, with the <111> direction (easy axis of magnetization) perpendicular to the films, were successfully fabricated at a substrate temperature of 360 ° C in a wide composition range with Co content less than 60 at . % . The order parameter, S , was almost a constant of about 0.5 in the stoichiometric composition of ( Co <sub>1-X</sub> Ni <sub>X</sub>)<sub>50</sub> Pt <sub>50</sub> , independent of Ni content, X . L1<sub>1</sub> -type Co–Ni–Pt perpendicular films having a large K<sub>u</sub> of (1–2.5)×10<sup>7</sup> erg / cm <sup>3</sup> and a relatively low M<sub>s</sub> of 400–700 emu / cm <sup>3</sup> were successfully fabricated in the composition range of 10–35 at . % Co, 20–55 at . % Ni, and bal. Pt. K<sub>u</sub> should increase further with enhanced ordering. Experimental results demonstrated the potential of these Co–Ni–Pt ordered films for use in data st-
orage applications due to very high K<sub>u</sub> potential comparable to L1<sub>0</sub> -type Fe <sub>50</sub> Pt <sub>50</sub> films, relatively low M<sub>s</sub> , the relatively low fabrication temperature, and good controllability of the grain orientation.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: Magnetic properties of dot arrays of L1<sub>1</sub> type Co–Pt ordered alloy perpendicular films were studied. L1<sub>1</sub> -Co – Pt films with a large uniaxial magnetic anisotropy K<sub>u</sub> of the order of 10<sup>7</sup> erg / cm <sup>3</sup> were fabricated at a substrate temperature of 360 ° C using ultrahigh vacuum sputter film deposition. Dot patterns with dot diameters of 70–200 nm were made using high resolution e-beam lithography and reactive ion etching (RIE). The values of K<sub>u</sub> were measured by the GST method using the Anomalous Hall Effect; we observed the averaged signals of 6000 dots. The values of K<sub>u</sub> for dot arrays of 10-nm-thick L1<sub>1</sub> -Co <sub>50</sub> Pt <sub>50</sub> films deposited on MgO(111) substrates (single crystal films) and glass disks (polycrystalline films) were nearly the same as those of the original films independent of D , indicating no significant etching damage by the RIE process. Magnetic force microscopy images revealed that all dots were single domains in the present D region. The coercivity H<sub>c</sub> of the dot arrays was 25.0 kOe [MgO(111) substrate, D=70 nm ] and 14.3 kOe (glass disks, D=80 nm ). The switching field distribution σ/H<sub>c</sub> was relatively small, σ/H<sub>c</sub>=0.15 , even for dot arrays fabricated on glass disks, indicating the homogeneous formation of a L1<sub>1</sub> type ordered structure in the Co <sub>50</sub> Pt <sub>50</sub> -
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Journal of Applied Physics 05/2009; · 2.17 Impact Factor
