Browsing by Author "Suzuki, Takao"
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Item Exploring the magnetic phases in dysprosium by neutron scattering techniques(University of Alabama Libraries, 2014) Yu, Jian; Mankey, Gary J.; University of Alabama TuscaloosaWith one of the highest intrinsic magnetic moments (10.6 μ_B) among the heavy rare-earth elements, dysprosium (Dy) exhibits a rich magnetic phase diagram, including a few modulated magnetic phases. Aided by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the magnetic modulations propagate coherently over a long range, even with intervening non-magnetic layers. Neutron diffraction experiments were performed to determine the microscopic magnetic origin of the field induced phases in bulk Dy as a function of temperature, covering regions of the well-known ferromagnetic, helical antiferromagnetic, fan phases and several possible new phases suggested by previous studies. A short range ordered (SRO) fan phase was identified as the intermediate state between ferromagnetism and long range ordered (LRO) fan. The temperature range of a coexisting helix/fan phase was also determined. The magnetic phase diagram of Dy was thus refined to include the newly determined magnetic structures and the associated phase boundaries. Based on the period of the magnetic modulation and the average magnetization, the evolution of the spin arrangement upon heating was derived quantitatively for the modulated magnetic phases. To gauge the effect of nanostructuring on the magnetic phases with nonmagnetic Y layers, epitaxial Dy/Y superlattices of various layer thicknesses and repeats were fabricated by magnetron sputtering under carefully controlled conditions. X-ray characterizations confirmed that the crystallographic and interfacial qualities of the superlattices are comparable to those grown by MBE in previous studies. The macroscopic magnetization was characterized by magnetometry, whereas the microscopic magnetic structures were extracted from neutron diffraction and polarized neutron reflectometry (PNR) measurements. The ordering of helical modulation is sensitive to the interfacial roughness of the multilayer as well as the cooling histories. Off-specular PNR was applied the first time to characterize the helical domain structures in Dy/Y multilayers. The lateral correlation length in the helical magnetic structure was in the order of 100 nm.Item The growth temperature and measurement temperature dependences of soft magnetic properties and effective damping parameter of (FeCo)-Al alloy thin films(American Institute of Physics, 2018-01-05) Ariake, Yusuke; Wu, Shuang; Kanada, Isao; Mewes, Tim; Tanaka, Yoshitomo; Mankey, Gary; Mewes, Claudia; Suzuki, Takao; University of Alabama Tuscaloosa; TDK CorporationThe soft magnetic properties and effective damping parameters of Fe73Co25Al2 alloy thin films are discussed. The effective damping parameter alpha(eff) measured by ferromagnetic resonance for the 10 nm-thick sample is nearly constant (approximate to 0.004 +/- 0.0008) for a growth temperature T-s from ambient to 200 degrees C, and then tends to decrease for higher temperatures and alpha(eff) is 0.002 +/- 0.0004 at T-s = 300 degrees C. For the 80 nm-thick sample, the alpha(eff) seems to increase with T-s from alpha(eff) = 0.001 +/- 0.0002 at T-s = ambient to alpha(eff) = 0.002 +/- 0.0004. The alpha(eff) is found nearly constant (alpha(eff) = 0.004 +/- 0.0008) over a temperature range from 10 to 300 K for the 10 nm films with the different T-s (ambient, 100 and 200 degrees C). Together with an increasing non-linearity of the frequency dependence of the linewidth at low Ts, extrinsic contributions such as two-magnon scattering dominate the observed temperature dependence of effective damping and linewidth. (C) 2018 Author(s).Item The Growth Temperature and Measurement Temperature Dependences of Soft Magnetic Properties and Effective Damping Parameter of (FeCo)-Al Alloy Thin Films(2018) Ariake, Yusuke; Wu, Shuang; Kanada, Isao; Mewes, Tim; Tanaka, Yoshitomo; Mankey, Gary; Mewes, Claudia; Suzuki, Takao; University of Alabama TuscaloosaItem Magnetic properties and structure of low temperature phase MnBi with island structure(American Institute of Physics, 2017-02-22) Ito, Masashi; Tanaka, Yoshitomo; Satoh, Takuya; Mankey, Gary; Schad, Rainer; Suzuki, Takao; University of Alabama Tuscaloosa; TDK CorporationThe magnetic properties of the lowtemperature phase (LTP) MnBi thin films of islands structure are discussed. The LTP MnBi islands are formed onto silica substrates after the multilayers {Bi(3.2nm)/Mn(2nm)}x N are deposited and then annealed at 450C for 0.5hr, where N is the number of the repetition of a pair of Mn and Bi layer. Those islands are found to be of the LTP MnBi, with the c-axis orientation along the normal to the sample plane for N= 10 similar to 40. Their size vary from place to place, but are averagely of about a few hundred nm in height and a few mu m in width for N from 10 to 40. For N= 200, the elongated islands are formed densely, with the length of about a few tens of mu m. The coverage of those islands increases with N. The temperature dependence of saturation magnetization Ms is qualitatively similar to that for bulk, though the absolute values for Ms are smaller by 20%. The magnetic anisotropy constants of K-u1 and K-u2 are evaluated for the samples with N= 10 similar to 40, where K-u1 and K-u2 are the magnetic anisotropy constants corresponding to the second and fourth power term in the uniaxial magnetic anisotropy energy expression. It is found that the K-u1 increases with T monotonously, reaching to about 1x10(7) erg/cc at 400K. On the other hand, the K-u2 remains nearly zero for temperatures below 300K, and then becomes negative, reaching to about 7 x 10(6) erg/cc at 400K. This is the first to report of the temperature dependence of K-u1 and K-u2 in the LTP MnBi of an island structure. It is also noted that the decrease of K-u for a temperature range beyond around 450 K is possibly due to the decrease of the K-u2 component, as demonstrated in the present study. (C) 2017 Author(s).Item Magnetic properties and structure of Mn-x alloy thin films(University of Alabama Libraries, 2018) Zhao, Siqian; Suzuki, Takao; University of Alabama TuscaloosaAmong the rare earth free/less permanent magnet candidates, the L10 MnAl, L10 MnGa and D022 Mn3-δGa (δ=0~1) exhibit high magnetic anisotropy energy, which is of the order of 10^7 erg/cm3, despite the spin orbit coupling (SOC) of Mn atom is small. The present project aims to understand the magnetic anisotropy Ku and coercivity Hc mechanism of Mn-X alloy thin films in conjunction with structure. The polycrystalline samples of L10 Mn52Al48, L10 Mn52Ga48 and D022 Mn71Ga29 were sputter-deposited onto silica glass substrates, and the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx (x=0~6) of island structure were deposited onto MgO (100) and SrTiO3 (100). The initial curves and “apparent Hc” were measured, and the relation between Hc(T)/Ms(T) and 2Ku(T)/Ms^2(T) were fitted linearly to Kronmuller’s empirical equation to estimate the pinning size r0 and effective demagnetizing factor Neff, in order to understand the Hc mechanism. The Hc in out-of-plane directions for polycrystalline L10 Mn52Al48, L10 Mn52Ga48, and both in-plane and out-of-plane directions for polycrystalline D022 Mn71Ga29 and epitaxial L10 Mn53Ga47 are governed by domain wall pinning mechanism, while the in-plane directions for polycrystalline L10 Mn52Al48 and L10 Mn52Ga48 by a mixture of domain wall pinning and nucleation modes. The Neff for polycrystalline L10 Mn52Al48, polycrystalline L10 Mn52Ga48, and epitaxial L10 Mn53Ga47 are 0.1, -2, and 12, respectively, while the r0 are 1.1~1.9 (non-magnetic), 2.2~3.3 (non-magnetic), and 2.0~2.8 nm (magnetic), respectively. The normalized Ku(T) as a function of Ms(T) was fitted to Ku(T) ≈ Ms(T)^n. The power law exponent n for the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx is found to be n ≈ 1.6~3.9, which is temperature dependent and larger at lower temperatures. The results show discrepancy with the single-ion (n=3) or two-ion mechanism (n=2) predictions for Ku1 based on a localized model. At the current stage, the theory for the temperature dependence of SOC and Ku is not well established, and the Ku for transition metal systems is based on itinerant models. Therefore, it is not understood which model (either single- or two-ion mechanism) is responsible for the magnetic anisotropy. A more detailed theory of Ku(T) for transition metal systems is needed.Item Magnetic properties of nano-composite particles(University of Alabama Libraries, 2015) Xu, Xia; Lane, Alan M.; University of Alabama TuscaloosaChemical synthesis routes for hollow spherical BaFe12O19, hollow mesoporous spherical BaFe12O19, worm-shape BaFe12O19 and FeCo particles were developed. These structured particles have great potentials for the applications including magnetic recording medium, catalyst support, and energy storage. Magnetically exchange coupled hard/soft SrFe12O19/FeCo and MnBi/FeCo composites were synthesized through a newly proposed process of magnetic self-assembly. These exchange coupled composites can be potentially used as rare-earth free permanent magnets. Hollow spherical BaFe12O19 particles (shell thickness ~5 nm) were synthesized from eth-ylene glycol assisted spray pyrolysis. Hollow mesoporous spherical BaFe12O19 particles (shell thickness ~100 nm) were synthesized from ethanol assisted spray pyrolysis, followed by alkaline ethylene glycol etching at 185 oC. An α-Fe2O3 and BaCO3 nanoparticle mixture was synthesized with reverse microemulsion, followed by annealing at 900 oC for 2 hours to get worm-shape BaFe12O19 particles, which consisted of 3-7 stacked hexagonal plates. FeCo nanoparticles were synthesized by reducing FeCl2 and CoCl2 in diphenyl ether with n-butyllithium at 200 oC in an inert gas environment. The surfactant of oleic acid was used in the synthesis to make particles well dispersed in nonpolar solvents (such as hexane). SrFe12O19/FeCo core/shell particles were prepared through a magnetic self-assembly process. The as-synthesized soft FeCo nanoparticles were magnetically attracted by hard SrFe12O19 parti-cles, forming a SrFe12O19/FeCo core/shell structure. The magnetic self-assembly mechanism was confirmed by applying alternating-current demagnetization to the core/shell particles, which re-sulted in a separation of SrFe12O19 and FeCo particles. MnBi/FeCo composites were synthesized, and the exchange coupling between MnBi and FeCo phases was demonstrated by smooth magnetic hysteresis loop of MnBi/FeCo composites. The thermal stability of MnBi/FeCo composites was investigated by annealing at 250 oC for 2 hours in N2 environment. The results showed that FeCo nanoparticles were sintered and agglom-erated during the annealing, and exchange coupling between MnBi and FeCo was destroyed. Future work was proposed in three aspects: chemical synthesis of MnBi particles; decreas-ing the particle size of MnBi particles and maintaining their stability; improving the thermal sta-bility of MnBi/FeCo composites.Item New soft magnetic materials for high frequency applications(University of Alabama Libraries, 2019) Wu, Shuang; Mewes, Tim; University of Alabama TuscaloosaSoft magnetic materials are widely used in devices such as inductors, transformers, antennas, magnetic hard drives, etc. Some of those devices will benefit greatly from operating at high frequencies. Thus fundamental study on finding the materials that have better soft magnetic properties is essential for improving the performance of those devices. Fe alloys have been proved to be promising candidates for high frequency applications. In this dissertation, an extensive study of magnetic properties of FeAl, (FeCo)-Al and (FeCo)-Si alloy thin films and their dependence on the film thickness and growth temperature has been presented. These films have body-centered cubic structure and columnar growth morphology. It is shown that the thickness of the film, which has an influence on the stress inside the film, may affect the coercivity through the magnetic-elastic coupling. The same mechanism is observed in the growth temperature dependence study, where reduced stress caused by increased growth temperature leads to a decrease in coercivity. The effective damping parameter shows a huge increase at small thickness due to the spin pumping effect. In-plane rotation ferromagnetic resonance measurements unveil the existence of four-fold anisotropy in (FeCo)-Si films. In addition, a four-fold symmetry is observed in the FMR linewidth vs. in-plane angle plot, which indicates anisotropic damping caused by the two-magnon scattering contribution. The film thickness dependence of FMR linewidth caused by the two-magnon scattering suggests that the origin of the two-magnon scattering is not pure interfacial.Item Soft magnetic properties and damping parameter of (FeCo)-Al alloy thin films(American Institute of Physics, 2017-02-06) Kanada, Isao; Cruce, Alex; Mewes, Tim; Wu, Shuang; Mewes, Claudia; Mankey, Gary; Suzuki, Takao; University of Alabama Tuscaloosa; TDK CorporationFor high frequency device applications, a systematic study of the soft magnetic properties and magnetization dynamics of (FeCo)-Al alloy thin films has been carried out. A low effective damping parameter alpha(eff) of 0.002 and a high saturation magnetization of about 1,800 emu/cc are obtained at y=0.2 similar to 0.3 for (Fe1-yCoy)(98)Al-2 alloy thin films deposited onto fused silica and MgO(100) at an ambient temperature during deposition. Those films are of the bcc structure with the <110> orientation normal to the film plane. They possess a columnar structure, grown along the film normal. The column width is found to be about 20 nm for y=0.25. It is concluded that the (FeCo)-Al thin films with a damping parameter as low as 0.002 and high saturation magnetization of about 1,800 emu/cc have been successfully fabricated, and that they are potential for future high frequency device applications. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Item Soft Magnetic Properties and Damping Parameter of (FeCo)-Al Alloy Thin Films(2017) Kanada, Isao; Cruce, Alex; Mewes, Tim; Wu, Shuang; Mewes, Claudia; Mankey, Gary; Suzuki, Takao; University of Alabama TuscaloosaFor high frequency device applications, a systematic study of the soft magnetic properties and magnetization dynamics of (FeCo)-Al alloy thin films has been carried out. A low effective damping parameter αeff of 0.002 and a high saturation magnetization of about 1,800 emu/cc are obtained at y=0.2∼0.3 for (Fe1-yCoy)98Al2 alloy thin films deposited onto fused silica and MgO(100) at an ambient temperature during deposition. Those films are of the bcc structure with the <110> orientation normal to the film plane. They possess a columnar structure, grown along the film normal. The column width is found to be about 20 nm for y=0.25. It is concluded that the (FeCo)-Al thin films with a damping parameter as low as 0.002 and high saturation magnetization of about 1,800 emu/cc have been successfully fabricated, and that they are potential for future high frequency device applications.