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Hiromi Yuasa Last modified date：2024.04.22

Professor / Electronic Devices / Department of Electronics / Faculty of Information Science and Electrical Engineering


Papers
1. C. Liu, Y. Kurokawa, N. Hashimoto, T. Tanaka, H. Yuasa, Biquadratic magnetic coupling effect in CoPt/Cr/Fe90Co10 orthogonal structures, Jpn. J. Appl. Phys., 63, 02SP32-02SP32, 2024.02.
2. Yuichiro Kurokawa, Keisuke Yamada, and Hiromi Yuasa, Inkjet-Printed Flexible Spin Seebeck Thermopile Device for Low-Cost Spintronics Device Fabrication, Adv. Eng. Mater., 2023, 2301069-2301069, 2024.01.
3. U. Kamihoki, Y. Kurokawa, M. Fujimoto, and H. Yuasa, Inversion symmetry breaking in spin–orbit torque-induced magnetization switching to improve the recording density of multi-level magnetoresistive random-access memory, J. Appl. Phys., 133, 143902-143902, 2023.04.
4. C. Liu, Y. Kurokawa, N. Hashimoto, T. Tanaka & H. Yuasa, High-frequency spin torque oscillation in orthogonal magnetization disks with strong biquadratic magnetic coupling, Scientific Reports, 12, 10849-10849, 2023.03.
5. Yuichiro Kurokawa, Hiromi Yuasa, Operating characteristics of domain walls in perpendicularly magnetized ferrimagnetic cylindrical nano-wires for three-dimensional magnetic memory, Jpn. J. Appl. Phys., 62, SC1070-SC1070, 2023.02.
6. Begoña Abad, Kirstin Alberi, Katherine E Ayers, Sushmee Badhulika, Chunmei Ban, Hélène Béa, Fanny Béron, Julie Cairney, Jane P Chang, Christine Charles, Mariadriana Creatore, Hui Dong, Jia Du, Renate Egan, Karin Everschor-Sitte, Cathy Foley, Anna Fontcuberta i Morral, Myung-Hwa Jung, Hyunjung Kim, Sarah Kurtz, Jieun Lee, Diana C Leitao, Kristina Lemmer, Amy C Marschilok, Bogdana Mitu, Bonna K Newman, Roisin Owens, Anna-Maria Pappa, Youngah Park, Michelle Peckham, Liane M Rossi, Sang-Hee Shim, Saima Afroz Siddiqui, Ji-Won Son, Sabina Spiga, Sedina Tsikata, Elisa Vianello, Karen Wilson, Hiromi Yuasa, Ilaria Zardo, Iryna Zenyuk, Yanfeng Zhang, Yudi Zhao, The 2022 applied physics by pioneering women: a roadmap, J. Phys. D: Appl. Phys, 56, 073001-073001, 2023.02.
7. Y. Kurokawa, Y. Tahara, Y. Hamada, M. Fujimoto, H. Yuasa, Scalable spin Seebeck thermoelectric generation using Fe-oxide nanoparticle assembled film on flexible substrate, Scalable spin Seebeck thermoelectric generation using Fe-oxide nanoparticle assembled film on flexible substrate, Scientific Reports, 12, 1, 1-10, 2022.10.
8. Y. Kurokawa, K. Yamada, T. Taniguchi, S. Horiike, T. Tanaka, H. Yuasa, Ultra-wide-band millimeter-wave generator using spin torque oscillator with strong interlayer exchange couplings, Scientific Reports, 12, 10849-10849, 2022.10.
9. Carmen Martín Valderrama, Mikel Quintana, Ane Martínez-de-Guerenu, Tomoki Yamauchi, Yuki Hamada, Yuichiro Kurokawa, Hiromi Yuasa, Andreas Berger, Sensitivity and reproducibility of transverse magneto-optical Kerr effect (T-MOKE) ellipsometry, J. Phys. D: Appl. Phys., 55, 435007-435007, 2022.08.
10. Y. Kurokawa, Y. Tahara, Y. Hamada, M. Fujimoto, H. Yuasa, Scalable spin Seebeck thermoelectric generation using Fe-oxide nanoparticle assembled film on flexible substrate, Scientific Reports, 12, 16605-16605, 2022.10.
11. T. Yamauchi, Y. Hamada, Y. Kurokawa, and H. Yuasa, Anomalous Nernst effect dependence on composition in Fe100−XRhX alloys, Jpn. J. Appl. Phys., 61, SC1019, 2022.02.
12. Yuichiro Kurokawa, Masahiro Itoh, Masakazu Wakae, Masahiro Fujimoto, Uraku Kamihoki, and Hiromi Yuasa, Inactivation of damping-like torque in Tb-Gd-Fe film on Ta layer, Jpn. J. Appl. Phys., 61, SC1025, 2022.02.
13. Yuki Hamada, Yuichiro Kurokawa, Tomoki Yamauchi, Hiroki Hanamoto, and Hiromi Yuasa, Anomalous Nernst effect in Fe–Si alloy films, Appl. Phys. Lett., 10.1063/5.0062637, 119, 152404, 2021.10.
14. C. Martín Valderrama, M. Quintana, A. Martínez-de-Guerenu, T. Yamauchi, Y. Hamada, Y. Kurokawa, H. Yuasa and A. Berger, Insertion layer magnetism detection and analysis using transverse magneto-optical Kerr effect (T-MOKE) ellipsometry, J. Phys. D: Appl. Phys., 10.1088/1361-6463/ac0d2a, 54, 435002, 2021.08.
15. C. Liu, Y. Kurokawa, N. Hashimoto, S. Horiike, T. Tanaka, H. Yuasa, Spin Transfer Torque Oscillation in Orthogonal Magnetization Disks, IEEE Trans. Mag., 10.1109/TMAG.2021.3084608, 58, 4100305, 2021.05.
16. K. Yamada, S. Masaki, M. Yamamoto, K. Kondo, Y. Kurokawa, Y. Shiota, T. Moriyama, T. Ono, H. Yuasa, M. Shima, Change of longitudinal spin Seebeck voltage with annealing in Y3Fe5O12 films formed by densely packed nanocrystals, J. Magn. Magn. Mater., 535, 168093, 2021.06.
17. N. Hashimoto, S. Horiike, Y. Kurokawa, T. Tanaka, P. Chang, W. Lin, and H. Yuasa,, Direct observation of magnetic process in quasi-antiferromagnet by high-resolution Kerr microscopy, Jpn. J. Appl. Phys., 10.35848/1347-4065/abe5c2, 60, SBBI05-SBBI05, 2021.03.
18. T. Niimura, Y. Kurokawa, S. Horiike, H. Li, H. Hanamoto, R. Weber, A. Berger, and H. Yuasa, Influence of interface layer insertion on the spin Seebeck effect and the spin Hall magnetoresistance of Y3Fe5O12/Pt bilayer systems, Phys. Rev. B, 10.1103/PhysRevB.102.094411, 102, 094411-094411, 2020.09.
19. Houlin Li, Yuichiro Kurokawa, Takumi Niimura, Tomoki Yamauchi, and Hiromi Yuasa, Composition dependence of spin Seebeck voltage in YIG/Pt100−XRuX, Pt100−XCuX, and Pt100−X(Cu0.5Ru0.5)X, Jpn. J. Appl. Phys., 59, 73001, 2020.06.
20. Akihiro Mitsuda, Motoki Kaneda, Kanta Matsutomo, Takashi Kimura, Hiromi Yuasa, Pressure Effects on Magnetic and Transport Properties in CoFe-Based Spin Valve, MATERIALS TRANSACTIONS, https://doi.org/10.2320/matertrans.MT-MN2019040, 61, 1483, 2020.04.
21. Shuu Horiike, Gen Nagashima, Yuichiro Kurokawa, Yongshi Zhong, Kazuto Yamanoi, Terumitsu Tanaka, Kimihide Matsuyama, Hiromi Yuasa, Magnetic dynamics of quasi-Antiferromagnetic layer fabricated by 90° magnetic coupling, Japanese journal of applied physics, 10.7567/1347-4065/ab5d62, 59, SG, 2020.04, In this study, we fabricated quasi-Antiferromagnetic (AFM) layers and investigated the magnetic dynamics of quasi-AFM in both an experiment and simulation. Quasi-AFM has multi domains with alternating antiparallel magnetization, which can be realized by using 90° magnetic coupling between two ferromagnetic layers. In magnetic resonance measurement, a frequency and damping constant of CoFe-based quasi-AFM are higher than those of conventional CoFe. Likewise, in micromagnetic simulation, we obtained the spin torque oscillation in Quasi-AFM with high frequency. This suggests the possibility of a novel material with the high frequency wave generator without a stray field, like antiferromagnetic materials..
22. Hiromi Yuasa, Fabrication of spin torque devices by using quasi-antiferromagnetic materials, Impact, 2020, 1, 79-81, 2020.02.
23. Shuu Horiike, Gen Nagashima, Yuichiro Kurokawa, Yongshi Zhong, Kazuto Yamanoi, Terumitsu Tanaka, Kimihide Matsuyama and Hiromi Yuasa, Magnetic dynamics of Quasi-antiferromagnetic layer fabricated by 90 degrees magnetic coupling, Jpn J. Appl. Phys., 59, SGGI02-1-SGGI02-6, 2020.02.
24. Yongshi Zhong, Yuichiro Kurokawa, Gen Nagashima, Shu Horiike, Takayasu Hanashima, Daniel Schönke, Pascal Krautscheid, Robert M. Reeve, Mathias Kläui, and Hiromi Yuasa, Determination of fine magnetic structure of magnetic multilayer with quasi antiferromagnetic layer by using polarized neutron reflectivity analysis, AIP Advances, 10, 015323-1-015323-6, 2020.02.
25. Yongshi Zhong, Yuichiro Kurokawa, Gen Nagashima, Shu Horiike, Takayasu Hanashima, Daniel Schönke, Pascal Krautscheid, Robert M. Reeve, Mathias Kläui, Hiromi Yuasa, Determination of fine magnetic structure of magnetic multilayer with quasi antiferromagnetic layer by using polarized neutron reflectivity analysis, AIP Advances, 10.1063/1.5130445, 10, 1, 2020.01, We carried out polarized neutron reflectivity (PNR) analysis to determine the fine magnetic structure of magnetic multilayers with quasi-antiferromagnetic (quasi-AFM) layers realized by 90-deg coupling using two Co90Fe10 layers, and quantitatively evaluated the magnetization of quasi-AFM layers. Two types of samples with different buffer layers, Ru buffer and a NiFeCr buffer, were investigated and the average angles between the respective magnetization of the two Co90Fe10 layers were estimated to be +/- 39 degrees and +/- 53 degrees. In addition, less roughness was found in the NiFeCr buffer sample resulting stronger 90-deg coupling. A perfect quasi-AFM is expected to be realized by a flat interface of the magnetic multilayer..
26. G. Nagashima, Y. Kurokawa, Y. Zhong, S. Horiike, D. Schönke, P. Krautscheid, R. Reeve, M. Kläui, Y. Inagaki, T. Kawae, T. Tanaka, K. Matsuyama, K. Ohnishi, T. Kimura, and H. Yuasa, Quasi-antiferromagnetic multilayer stacks with 90 degree coupling mediated by thin Fe oxide spacers, J. Appl. Phys., 126, 093901-1-093901-8, 2019.10.
27. G. Nagashima, Y. Kurokawa, Y. Zhong, S. Horiike, D. Schönke, P. Krautscheid, R. Reeve, M. Kläui, Y. Inagaki, T. Kawae, T. Tanaka, K. Matsuyama, K. Ohnishi, T. Kimura, H. Yuasa, Quasi-antiferromagnetic multilayer stacks with 90 degree coupling mediated by thin Fe oxide spacers, Journal of Applied Physics, 10.1063/1.5117869, 126, 9, 2019.09, We fabricated quasiantiferromagnetic (quasi-AFM) layers with alternating antiparallel magnetization in the neighboring domains via 90° magnetic coupling through an Fe-O layer. We investigated the magnetic properties and the relationship between the magnetic domain size and the 90° magnetic coupling via experiments and calculations. Two types of samples with a Ru buffer and a (Ni80Fe20)Cr40 buffer were prepared, and we found that with the NiFeCr buffer, the sample has a flatter Fe-O layer, leading to stronger 90° magnetic coupling and a smaller domain size compared with the Ru buffer sample. This trend is well explained by the bilinear and biquadratic coupling coefficients, A12 and B12, in Landau-Lifshitz-Gilbert simulations, suggesting the possibility of using both AFM and FM properties by controlling the quasi-AFM domain size..
28. K. Yamada, Y. Kurokawa, K. Kogiso, H. Yuasa, M. Shima, Observation of Longitudinal Spin Seebeck Voltage in YIG Films Chemically Prepared by Co-Precipitation and Spin Coating, IEEE Trans. Mag, 55, 451004, 2019.03.
29. M. Wakae, Y. Kurokawa, H. Yuasa, Observation of spin–orbit torque-induced magnetization switching in Gd-Fe perpendicular magnetized wire with in-plane exchange bias field, M. Wakae, Y. Kurokawa, H. Yuasa, Jpn J. Appl. Phys., 58, SBBI04-1-SBBI04-4, 2019.03.
30. F. Nakata, T. Niimura, Y. Kurokawa, H. Yuasa, Spin Seebeck voltage enhancement by Mn system metals insertion at the interface between YIG and nonmagnetic layer, , Jpn J. Appl. Phys., 2019.03.
31. Y. Kurokawa, M. Wakae, S. Sumi, H. Awano, K. Ohnishi, H. Yuasa,, Spin orbit torque driven current-induced domain wall motion in Gd–Fe magnetic wires, Jpn J. Appl. Phys., 58, 30905-1-30905-4, 2019.03.
32. Yuichiro Kurokawa, Masakazu Wakae, Satoshi Sumi, Hiroyuki Awano, Kohei Ohnishi, Hiromi Yuasa, Spin-orbit torque-driven current-induced domain wall motion in Gd-Fe magnetic wires, Japanese Journal of Applied Physics, 10.7567/1347-4065/aafa91, 58, 3, 2019.03, We fabricated Gd-Fe/Ta/sub. and W/Gd-Fe/Ta/sub. perpendicular magnetized magnetic wires and investigated current-induced domain wall motion (CIDWM) in them. The propagation field of the Gd-Fe wires is low (∼3 mT), and the wires were found to have a low threshold current density (∼3.1 ×1010 A m-2) of CIDWM. The directions of the CIDWM in Gd-Fe/Ta and W/Gd-Fe/Ta wires are opposite to each other, indicating that the CIDWM in both wires is driven by spin-orbit torque because the spin Hall torque generated by the Ta underlayer and W cap layer oppose each other..
33. Keisuke Yamada, Yuichiro Kurokawa, Kazuma Kogiso, Hiromi Yuasa, Mutsuhiro Shima, Observation of longitudinal spin seebeck voltage in YIG films chemically prepared by co-precipitation and spin coating, IEEE Transactions on Magnetics, 10.1109/TMAG.2018.2865199, 55, 2, 2019.02, Longitudinal spin Seebeck effect (LSSE) of Y3Fe5O12 (YIG) thin films chemically prepared on a Si substrate with a SiO2 surface layer has been investigated with a particular focus on its dependence on the metal (Pt or W) for the capping layer used to carry out the measurements. The YIG films are prepared by a combination of co-precipitation, spin coating, and annealing processes. The structural characterization by X-ray diffraction proves that the films are polycrystalline in a garnet phase with an average crystallite size of 34 nm. The microstructural analyses of the film surface by scanning electron microscopy and scanning probe microscopy show that the roughness Ra of the film is 31 nm. The magnetic easy axis of the YIG films is in the direction parallel to the films. We have found that the polarity of the LSSE voltage VLSSE measured with a Pt capping layer apparently differs from that observed when a W capping layer is used. The value of VLSSE increases almost linearly with increasing temperature difference Δ T independent of the choice of metal for the capping layer. The observed LSSE for the polycrystalline YIG films is one order of magnitude smaller than that reported for YIG and Bi:YIG single-crystal samples grown by metal-organic decomposition..
34. Masakazu Wakae, Yuichiro Kurokawa, Hiromi Yuasa, Observation of spin-orbit torque-induced magnetization switching in Gd-Fe perpendicular magnetized wire with in-plane exchange bias field, Japanese Journal of Applied Physics, 10.7567/1347-4065/aaf877, 58, 2019.01, We fabricated a Ta/Gd-Fe/Ta and a Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire and investigated spin-orbit torque current-induced magnetization switching in these wires. Magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire can be observed by an electric current even if the external in-plane magnetic field is not applied at all. Moreover, we successfully observed the periodical magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire in zero magnetic field. This indicates that the present wire is a promising material to realize magnetic random access memory with low power consumption..
35. Fumiya Nakata, Takumi Niimura, Yuichiro Kurokawa, Hiromi Yuasa, Spin Seebeck voltage enhancement by Mn system metals insertion at the interface between YIG and nonmagnetic layer, Japanese Journal of Applied Physics, 10.7567/1347-4065/aafb5c, 58, 2019.01, The spin Seebeck voltage was successfully increased by IrMn or Mn insertion into the interface Y3Fe5O12 and nonmagnetic Pt and TaW with positive and negative spin Hall angle, respectively. The magnitude of the spin Seebeck coefficient |S| of the samples with IrMn and Mn were higher than that with Ru insertion, which indicates that the existence of the magnetic moment is important for the spin Seebeck coefficient. Furthermore, |S| of the samples with IrMn and Mn were higher than oxide antiferromagnetic NiO, which suggests the metallic layer insertion is effective for enlarging |S| owing to the relatively long spin diffusion length. Since the inserted IrMn and NiO take the paramagnetic states as well as Mn, it is supposed that the spin fluctuation in the paramagnetic insertion layers play an important role in enhancing the spin current..
36. Hiromi Yuasa, Fumiya Nakata, Ryohei Nakamura and Yuichiro Kurokawa, Spin Seebeck coefficient enhancement by using Ta50W50 alloy and YIG/Ru interface, J. Phys. D: Appl. Phys., 51, 134002-1-134002-5, 2018.03.
37. Hiromi Yuasa, Fumiya Nakata, Ryohei Nakamura, Yuichiro Kurokawa, Spin Seebeck coefficient enhancement by using Ta50W50 alloy and YIG/Ru interface, Journal of Physics D: Applied Physics, 10.1088/1361-6463/aaaf89, 51, 13, 2018.03, This paper reports that Ta50W50 alloy has a higher spin Hall angle than that of β-W by measuring the spin Seebeck coefficients for Fe5Y3O12 (YIG)/Ru/Ta50W50 and YIG/Ru/W systems. The spin Seebeck coefficient increased by inserting Ru, and the YIG/Ru/Ta50W50 system achieved 2.4 times the magnitude of spin Seebeck coefficient of the conventional YIG/Pt system although Ru and Ta50W50 have spin Hall angles with opposite signs. Ru seems to recover the spin mixing conductance that is decreased by Ta-W oxidation at its interface with YIG. To enhance the spin Seebeck coefficient, materials having a high spin Hall angle can be combined with materials having a high spin mixing conductance..
38. Hiromi Yuasa, Kouki Tamae, Norimasa Onizuka, Spin mixing conductance enhancement by increasing magnetic density, AIP Advances, 10.1063/1.4977496, 7, 5, 2017.05, We obtained the spin Seebeck effect enhancement in Y3Fe5O12 (YIG)/Pt system by inserting a very thin Ni80Fe20 at the interface of YIG/Pt. One of important parameters determining the spin Seebeck coefficient is the spin mixing conductance at the interface between magnetic oxide and nonmagnetic metal, and it was theoretically reported that the spin mixing conductance depends on the magnetic density at the interface. However, a ferrimagnetic oxide such as YIG has a small magnetic moment density. In this report, we fabricated YIG (1 mm)/Ni80Fe20 (t nm)/Pt (5 nm) and the spin Seebeck coefficient of t=0.6 nm becomes 2.1 times larger than that of t=0. It is considered owing to improvement of the magnetic moment density and the spin mixing conductance..
39. Hiromi Yuasa, Kouki Tamae, and Norimasa Onizuka, Spin mixing conductance enhancement by increasing magnetic density, AIP Advances, 7, 5, 055928-1-055928-5, 2017.02.
40. H. Yuasa, M. Hara, Y. Fuji and H. Fukuzawa, Fabrication of nanoscale metal paths in oxide thin layers by noble-gas ion beams, Europhysics Lett., 101, 47005-p1-47005-p4, 2013.02.
41. Hiromi Yuasa, Michiko Hara, Yoshihiko Fuji, Hideaki Fukuzawa, Fabrication of nanoscale metal paths in oxide thin layers by noble-gas ion beams, Europhysics Letters, 10.1209/0295-5075/101/47005, 101, 4, 2013.02, We investigated the fabrication process of a nanostructure with alumina insulator layer and nanoscale Cu paths punching through an alumina layer inserted between magnetic multilayers. Ion-beam-assisted oxidation was applied to an AlCu layer, where ion beams with three kinds of noble gases of different mass, Ne, Ar and Xe, were compared. The heavy gas overcame the trade-off between the increasing purity of nanoscale Cu paths and the decreasing oxygen defects of the alumina. It is considered that the high mobility of surface atoms in the AlCu layer brought about by the heavy-gas ion beam promotes segregation of alumina and Cu..
42. Y. Fuji, M. Hara, H. Yuasa, S. Murakami, and H. Fukuzawa, Enhancement of magnetoresistance by ultra-thin Zn wüstite layer, Appl. Phys. Lett., 99, 13, 132103-1-132103-3, 2011.09.
43. Yoshihiko Fuji, Michiko Hara, Hiromi Yuasa, Shuichi Murakami, Hideaki Fukuzawa, Enhancement of magnetoresistance by ultra-thin Zn wstite layer, Applied Physics Letters, 10.1063/1.3644470, 99, 13, 2011.09, We have investigated a hybrid magnetic material composed of ultra-thin oxide spin-filtering layer/ferromagnetic layer (FML) for magnetoresistance (MR) enhancement. Using Zn-Fe oxide/Fe50Co50 as a free layer in current-perpendicular-to-plane giant magnetoresistive films, the MR ratio and ΔRA (the change of resistance area product) were enhanced to 26.0% and 52 mΩ μm2 at a small RA (resistance area product) of about 0.2 Ω μm2, respectively. Structural analysis revealed that the Zn wstite structure is responsible for the enhancement of the MR ratio, and spin-polarization of wstite/FML is higher than that of spinel ferrite/FML..
44. A. D. Giddings, J. G. Keizer, M. Hara, G. J. Hamhuis, H. Yuasa, H. Fukuzawa, and P. M. Koenraad, Composition profiling of InAs quantum dots and wetting layers by atom probe tomography and cross-sectional scanning tunneling microscopy, Phys. Rev. B, 83, 20, 205308, 2011.05.
45. A. D. Giddings, J. G. Keizer, M. Hara, G. J. Hamhuis, H. Yuasa, H. Fukuzawa, P. M. Koenraad, Composition profiling of InAs quantum dots and wetting layers by atom probe tomography and cross-sectional scanning tunneling microscopy, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.83.205308, 83, 20, 2011.05, This study compares cross-sectional scanning tunneling microscopy and atom probe tomography. We use epitaxially grown self-assembled InAs quantum dots (QDs) in GaAs as an exemplary material with which to compare these two nanostructural analysis techniques. We studied the composition of the wetting layer and the QDs, and performed quantitative comparisons of the indium concentration profiles measured by each method. We show that computational models of the wetting layer and the QDs, based on experimental data, are consistent with both analytical approaches. This establishes a link between the two techniques and shows their complimentary behavior, an advantage which we exploit in order to highlight unique features of the examined QD material..
46. H. Yuasa, M. Hara, S. Murakami, Y. Fuji, H. Fukuzawa, K. Zhang, M. Li, E. Schreck, P. Wang, and M. Chen, Enhancement of magnetoresistance by hydrogen ion treatment for current-perpendicular-to-plane giant magnetoresistive films with a current-confined-path nano-oxide layer, Appl. Phys. Lett., 97, 11, 112501-1-112501-3, 2010.09.
47. Hiromi Yuasa, M. Hara, S. Murakami, Y. Fuji, H. Fukuzawa, K. Zhang, M. Li, E. Schreck, P. Wang, M. Chen, Enhancement of magnetoresistance by hydrogen ion treatment for current-perpendicular-to-plane giant magnetoresistive films with a current-confined-path nano-oxide layer, Applied Physics Letters, 10.1063/1.3486117, 97, 11, 2010.09, We have enhanced magnetoresistance (MR) for current-perpendicular-to-plane giant-magnetoresistive (CPP-GMR) films with a current-confined-path nano-oxide layer (CCP-NOL). In order to realize higher purity in Cu for CCPs, hydrogen ion treatment (HIT) was applied as the CuO
x
reduction process. By applying the HIT process, an MR ratio was increased to 27.4% even in the case of using conventional FeCo magnetic layer, from 13.0% for a reference without the HIT process. Atom probe tomography data confirmed oxygen reduction by the HIT process in the CCP-NOL. The relationship between oxygen counts and MR ratio indicates that further oxygen reduction would realize an MR ratio greater than 50%..
48. Y. Fuji, H. Yuasa, and H. Fukuzawa, Formation of a nanobarrel structure in CPP-GMR spin-valve films, J. Magn. Magn. Mater., 322, 12, 1449-1451, 2010.06.
49. Yoshihiko Fuji, Hiromi Yuasa, Hideaki Fukuzawa, Formation of a nanobarrel structure in CPP-GMR spin-valve films, Journal of Magnetism and Magnetic Materials, 10.1016/j.jmmm.2009.04.071, 322, 9-12, 1449-1451, 2010.05, As a new approach for current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) spin-valve films, we have proposed a new nanostructure to cause mesoscopic phenomena. For this purpose, we have successfully formed a nanobarrel structure composed of Cu granular-like shape surrounded by Al2O3 insulator in which Cu nanoconducting channel punches through the Al2O3 insulator both at the top and at the bottom. This nanobarrel structure was confirmed by high-resolution transmission electron microscope (HR-TEM) analysis. Both the resistance-voltage characteristics and the resistance-temperature characteristics showed the metallic conductance property, which is in good agreement with the nanostructure identified by HR-TEM analysis. The nanobarrel structure offers features compatible with both nano-ordered electron-confined structure by insulator and low resistance with metallic conductance..
50. H. Fukuzawa, M. Hara, H. Yuasa and Y. Fuji:, Analysis of the current-confined-paths in the film plane for CPP-GMR films, IEEE Trans. Mag., 44, 11, 3580-3583, 2008.11.
51. Hideaki Fukuzawa, Michiko Hara, Hiromi Yuasa, Yoshihiko Fuji, Analysis of the current-confined-paths in the film plane for CPP-GMR films, IEEE Transactions on Magnetics, 10.1109/TMAG.2008.2002511, 44, 11 PART 2, 3580-3583, 2008.11, Nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) films with a current-confined-path nano-oxide layer (CCP-NOL) is analyzed by high-resolution transmission electron microscope (HRTEM) and three-dimensional atom probe (3DAP). By HRTEM analysis, it is found that the CCP is formed on the center of the grain of a pinned layer. By 3DAP analysis, the grain size of a free layer can be visually detected and its size is about 5-10 nm. The grain of a free layer is well aligned with a CCP, suggesting that the decrease of the grain size of a pinned layer is effective for increasing the number of the CCPs per unit area. Moreover, by using a 3DAP image, the density of the CCPs per unit area can be derived. As a result, it is clearly shown that the density of the CCPs per unit area increases with decreasing resistance area product (RA), which cannot be obtained from HRTEM analysis. By combining HRTEM result and 3DAP result, the nanostructure of the CCP can be clarified, and the decrease of the grain size of a pinned layer and the decrease of RA are preferable for an extremely small element size for a higher-density recording head..
52. Hiromi Yuasa, Michiko Hara, Hideaki Fukuzawa, Relation between magnetoresistance and nanostructure of current-perpendicular-to-plane giant-magnetoresistance film with current-confined-path nano-oxide layer, Applied Physics Letters, 10.1063/1.2952773, 92, 26, 2008.07, Nanostructure of a current-perpendicular-to-plane giant-magnetoresistance film with a current-confined-path (CCP) nano-oxide layer was analyzed by high-resolution transmission electron microscopy and three-dimensional atom probe. It was found that the CCP of a film with a higher magnetoresistance (MR) ratio has better crystalline orientation and higher purity than the CCP of a film with a smaller MR ratio. Moreover, the free layer on the CCP of a film with a high MR ratio is well crystallized. Both the CCP with the good crystalline orientation and high purity and the well-crystallized free layer diminish the diffusive electron scattering, which improves an MR ratio..
53. H. Yuasa, M. Hara and H. Fukuzawa, Relation between magnetoresistance and nanostructure of current-perpendicular to-plane giant-magnetoresistance film with current-confined-path nano-oxide layer, Appl. Phys. Lett, 92, 26, 262509-1-262509-3, 2008.06.
54. Hideaki Fukuzawa, Hiromi Yuasa, Hitoshi Iwasaki, CPP-GMR films with a current-confined-path nano-oxide layer (CCP-NOL), Journal of Physics D: Applied Physics, 10.1088/0022-3727/40/5/S01, 40, 5, 1213-1220, 2007.03, We investigated the film performance and nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve film with a current-confined-path nano-oxide layer (CCP-NOL). By applying ion-assisted oxidation (IAO) for the CCP-NOL formation, we enhanced the MR ratio to 5.4% at a small RA value of 500 mΩ νm2 for conventional Co90Fe10 layers. Furthermore, the use of bcc-Fe 50Co50 also increased the MR ratio to 8.2% at a small RA value of 580 mΩ νm2. A modified Valet-Fert model for the CCP-NOL showed that the MR enhancement by the IAO is due to the improvement in resistivity of the CCP, and that by Fe50Co50 is due to a larger spin-dependent interface scattering effect. Analysis by cross-sectional TEM and three-dimensional atom probe confirmed the formation of the CCP-NOL structure. A reliability test for test element devices showed almost no change even under acceleration stress. The CPP-GMR spin-valve film with the CCP-NOL is extendable to future high-density recording heads due to its potential for a higher MR ratio at a small value of RA..
55. H. Fukuzawa, H. Yuasa and H. Iwasaki, CPP-GMR films with a current-confined-path nano-oxide layer (CCP-NOL), J. Phys. D.: Appl. Phys., 40, 1213-1220, 2007.02.
56. H. Iwasaki, H. Fukuzawa, H. Yuasa, K. Kubo, K. Koi, T. Funayama, M. Takagishi, Y. Tanaka, MR enhancement of CPP-GMR by CCP-NOL spacer and Fe50Co 50 mgnetic layers, INTERMAG ASIA 2005: Digests of the IEEE International Magnetics Conference INTERMAG ASIA 2005 Digests of the IEEE International Magnetics Conference, 2005.12.
57. H. Yuasa, H. Fukuzawa, M. Takagishi, H. Iwasaki, Y. Tanaka, Spin dependent scattering effect in CPP-GMR with current confined path, INTERMAG ASIA 2005: Digests of the IEEE International Magnetics Conference INTERMAG ASIA 2005 Digests of the IEEE International Magnetics Conference, 2005.12.
58. H. Yuasa, H. Fukuzawa, H. Iwasaki and M. Sahashi:, The number of Cu lamination effect on CPP-GMR of spin valves with Fe50Co50 alloy, J. Appl. Phys., 97, 11, 113907-1-113907-4, 2005.09.
59. H. Fukuzawa, H. Yuasa, S. Hashimoto, H. Iwasaki and Y. Tanaka, Large magnetoresistance ratio of 10% by Fe50Co50 layers for current-confined-path current-perpendicular-to-plane giant magnetoresistance spin-valve films, Appl. Phys. Lett., 87, 8, 092507-1-092507-1, 2005.08.
60. Hideaki Fukuzawa, Hiromi Yuasa, Susumu Hashimoto, Hitoshi Iwasaki, Yoichiro Tanaka, Large magnetoresistance ratio of 10% by Fe 50 Co 50 layers for current-confined-path current-perpendicular-to-plane giant magnetoresistance spin-valve films, Applied Physics Letters, 10.1063/1.2009837, 87, 8, 2005.08, We have realized a large magnetoresistance (MR) ratio of 10.2% by current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve films having current-confined-path (CCP) structure formed by AlCu-NOL (nano-oxide-layer). CPP-GMR with conventional Co90 Fe10 pinned and free layers showed an MR ratio and a ΔRA (the change of resistance area product) were 4% and 20 mΩ μ m2, respectively, at a small RA (resistance area product) of 500 mΩ μ m2. By replacing the Co90 Fe10 layers by Fe50 Co50 layers both for pinned and free layers, we have successfully realized a MR ratio and a ΔRA of 7.5% and 37.5 mΩ μ m2, respectively, at a small RA of 500 mΩ μ m2. Moreover, a large MR ratio of 10.2% and a large ΔRA of 418 mΩ μ m2 were realized at a relatively large RA of 4100 mΩ μ m2. This large MR ratio by using Fe50 Co50 layers was due to a larger spin-dependent interface scattering factor γ of 0.72 for the interface between Fe50 Co50 and Cu, which was improved from a γ of 0.62 for the interface between Co90 Fe10 and Cu..
61. Hiromi Yuasa, H. Fukuzawa, H. Iwasaki, M. Sahashi, The number of Cu lamination effect on current-perpendicular-to-plane giant-magnetoresistance of spin valves with Fe 50Co 50 alloy, Journal of Applied Physics, 10.1063/1.1925333, 97, 11, 2005.06, The current-perpendicular-to-plane (CPP) giant-magnetoresistance (GMR) of spin valves with Fe50 Co50 alloy was investigated. It has been reported that the Cu inserted in Fe50 Co50 is effective for enhancing CPP-GMR. In this paper, we investigated the number of Cu lamination effect on CPP-GMR and clarified that higher CPP-GMR is obtained by decreasing the number of Cu layers. In order to ascertain the reason for this tendency, the crystalline structure and magnetization were examined. As a result, it was found that the d spacing of Fe50 Co50 increases when the number of Cu layers is increased, although saturation magnetization does not change..
62. H. Fukuzawa, H. Yuasa, K. Koi, H. Iwasaki, Y. Tanaka, Y. K. Takahashi and K. Hono, Nanoconstricted structure for current-confined-path in current-perpendicular-to-plane spin valves with high magnetoresistance, J. Appl. Phys., 97, 10, 10C509-1-10C509-3, 2005.05.
63. H. Fukuzawa, H. Yuasa, K. Koi, H. Iwasaki, Y. Tanaka, Y. K. Takahashi, K. Hono, Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance, Journal of Applied Physics, 10.1063/1.1851673, 97, 10, 2005.05, We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA= 380 mΩ μ m2, ΔRA=16 mΩ μ m2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous oxide region, indicating that the metallic channel is made of Cu and the oxide is made of Al2 O3. The nanoconstricted structure with good segregation between the metallic channel and the oxide layer enables us to realize a large MR ratio in CCP-CPP spin valves..
64. H. Yuasa, H. Fukuzawa, H. Iwasaki, CPP-GMR of spin valves with Co xFe 1-x alloy, Journal of Magnetism and Magnetic Materials, 10.1016/j.jmmm.2004.09.045, 286, SPEC. ISS., 95-98, 2005.02, Spin valves with free and pinned layers constructed from Co 50Fe 50 exhibit higher current-perpendicular-to-plane (CPP) giant-magnetoresistance (GMR) than the conventional Co 90Fe 10 alloy. In order to investigate this enhancement mechanism, we measured CPP-GMR and the crystalline structure of spin valves with Co 90Fe 10, Co 70Fe 30, Co 50Fe l50 and Co 20Fe 80 for pinned and free layers. The spin valves with Co 50Fe 50 and Co 20Fe 80 show the same CPP-GMR and these are higher than those with Co 90Fe 10 and Co 70Fe 30. These results do not agree with the report about the Fe-Ni and Co-Ni systems that calculated that concludes that the stoichiometric concentration has the highest CPP-GMR. In the Co-Fe system, the crystalline structure and the strain also contribute to CPP-GMR..
65. H. Yuasa, H. Fukuzawa and H. Iwasaki, CPP-GMR of spin valves with FexCo1-x alloy, J. Magn. Magn. Mater., 286, 95-98, 2004.10.
66. H. Fukuzawa, H. Yuasa, S. Hashimoto, K. Koi, H. Iwasaki, M. Takagishi, Y. Tanaka and M. Sahashi:, MR ratio enhancement by NOL Current-confined-path structure in CPP spin valves, IEEE Trans. Magn. Mater., 40, 4, 2236.0-2239.0, 2004.07.
67. Hideaki Fukuzawa, Hiromi Yuasa, Susumu Hashimoto, Katsuhiko Koi, Hitoshi Iwasaki, Masayuki Takagishi, Yoichiro Tanaka, Masashi Sahashi, MR ratio enhancement by NOL current-confined-path structures in CPP spin valves, IEEE Transactions on Magnetics, 10.1109/TMAG.2004.829185, 40, 4 II, 2236-2238, 2004.07, We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 mωμm2, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 mωμm2. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 mωμm2. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording..
68. Hiromi Yuasa, Y. Kamiguchi, M. Sahashi, Dual spin valves with nano-oxide layers, Journal of Magnetism and Magnetic Materials, 10.1016/S0304-8853(03)00304-4, 267, 1, 53-59, 2003.11, A novel method is proposed for increasing the giant magnetoresistance (GMR) of dual spin valves of the CoFe/Cu/CoFe/Cu/CoFe type by inserting nano-oxide layers (NOLs) into the pinned layers. Using this method, MR ratio of 23.5% was obtained, a value equal to those of specular spin valves with antiferromagnetic oxide, e.g., NiO. This method allows the selection of metallic materials for the antiferromagnetic layer. In addition, we obtained the specularity factor of upper NOLs, 0.8, and that of lower NOLs, 0.7, by calculating Boltzmann equations taking into account the roughness of each interlayer. This model shows that the MR ratio would be 27.5% for dual spin valves with ideal NOLs..
69. H. Yuasa, Y. Kamiguchi and M. Sahashi, Dual spin valves with nano-oxide layers, J. Magn. Magn. Mater., 276, 53-59, 2003.03.
70. H. Yuasa, H. Fukuzawa, H. Iwasaki, M. Yoshikawa, M. Takagishi and M. Sahashi:, Effect of inserted Cu on current-perpendicular-to-plane -giant-magnetoresistance of Fe50Co50 spin valves, J. Appl. Phys., 93, 10, 7915-7917, 2003.05.
71. H. Yuasa, H. Fukuzawa, H. Iwasaki, M. Yoshikawa, M. Takagishi, M. Sahashi, Effect of inserted Cu on current-perpendicular-to-plane-giant magnetoresistance of Fe50Co50 spin valves, Journal of Applied Physics, 10.1063/1.1558075, 93, 10 3, 7915-7917, 2003.05, The effect of inserted Cu was studied on current-perpendicular-to-plane-giant magnetoresistance of Fe50Co50 spin valves. The investigation of structural state of the inserted Cu was done by extended x-ray absorption structure (EXAFS) spectra to clarify its role in specific resistance behavior. It was found that solid solute Cu participates in the specific resistance increase in Fe50Co50 spin valves..
72. H. Yuasa, M. Yoshikawa, Y. Kamiguchi, K. Koi, H. Iwasaki and M. Sahashi, Output Enhancement of Spin-Valve GMR in CPP geometry, J. Appl. Phys., 92, 5, 2646-2650, 2002.09.
73. H. Yuasa, M. Yoshikawa, Y. Kamiguchi, K. Koi, H. Iwasaki, M. Takagishi, M. Sahashi, Output enhancement of spin-valve giant magnetoresistance in current-perpendicular-to-plane geometry, Journal of Applied Physics, 10.1063/1.1499744, 92, 5, 2646-2650, 2002.09, In this work, we present a suitable material for metal-based spin-valve in current-perpendicular-to-plane (CPP) geometry. The AΔR (A is the element size and ΔR is the change in resistance) was investigated for three kinds of free and pinned layers material, that is, Co 90Fe 10, Fe 50Co 50, and Fe 50Co 50 with half-atomic Cu layers. When the free and pinned layers are 5 nm, AΔR is 1.0 mμm 2, 1.6 mμm 2, and 2.9 mμm 2, respectively. Moreover, the dual-type spin valve having Fe 50Co 50 with half-atomic Cu layers achieved 5.2 mμm 2. According to the free and pinned layers thickness dependence of AΔR, the spin-dependent resistance at the interfaces between ferromagnetic layers and spacer Cu is enhanced mainly by changing from Co 90Fe 10 to Fe 50Co 50, and the spin-dependent bulk resistance in free and pinned layers is enlarged by inserting half-atomic Cu layers..
74. Y. Kamiguchi, Hiromi Yuasa, H. Fukuzawa, K. Koui, H. Iwasaki, M. Sahashi, CoFe specular spin valves with a nano oxide layer, Proceedings of the 1999 IEEE International Magnetics Conference 'Digest of Intermag 99' Digest of INTERMAG, DB-01, 1999.12, The CoFe specular spin valve with pinned layer containing nano oxide layer is described. This valve satisfies large magnetoresistance (MR) ratio, large exchange bias and soft magnetic property of a free layer with an ultrathin CoFe free layer..
75. H. Yuasa, K. Takizawa, H. Miyajima, S. Hane, H. Mitamura and T. Goto, Field-Induced Magnetic Phase Transition of FeRh1-xPdx Alloy with Body-Centered Tetragonal Structure, J. Magn. Soc. Japan, 23, 424-426, 1999.09.


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