[Show abstract][Hide abstract] ABSTRACT: We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based
magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate.
Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a
flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance
(TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on
a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching
was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a
switching layer. Using a thermal activation model, the intrinsic critical
current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower
than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett.
100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using
ferromagnetic resonance measurements for the polycrystalline CFA film was
~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0
for the polycrystalline MTJ was mainly attributed to the low damping of the CFA
layer compared with the value in the epitaxial one (~0.04).
[Show abstract][Hide abstract] ABSTRACT: An unusual crystallographic orientation of hexagonal Ru with a 4-fold
symmetry emerging in epitaxial MgO/Ru/Co2FeAl/MgO heterostructures is reported,
in which an approximately Ru(02-23) growth attributes to the lattice matching
among MgO, Ru, and Co2FeAl. Perpendicular magnetic anisotropy of the
Co2FeAl/MgO interface is substantially enhanced as compared with those with a
Cr(001) layer. The MTJs incorporating this structure gave rise to the largest
tunnel magnetoresistance for perpendicular MTJs using low damping Heusler
alloys. The 4-fold-symmetry hexagonal Ru arises from an epitaxial growth with
an unprecedentedly high crystal index, opening a unique pathway for the
development of perpendicular anisotropy films of cubic and tetragonal
[Show abstract][Hide abstract] ABSTRACT: We demonstrate monolithic integration of pseudo-spin-MOSFETs (PS-MOSFETs) using vendor-made MOSFETs fabricated in a low-cost multi-project wafer (MPW) product and lab-made magnetic tunnel junctions (MTJs) formed on the topmost passivation film of the MPW chip. The tunneling magnetoresistance (TMR) ratio of the fabricated MTJs strongly depends on the surface roughness of the passivation film. Nevertheless, after the chip surface was atomically flattened by SiO2 deposition on it and successive chemical–mechanical polish (CMP) process for the surface, the fabricated MTJs on the chip exhibits a sufficiently large TMR ratio (>140%) adaptable to the PS-MOSFET application. The implemented PS-MOSFETs show clear modulation of the output current controlled by the magnetization configuration of the MTJs, and a maximum magnetocurrent ratio of 90% is achieved. These magnetocurrent behaviour is quantitatively consistent with those predicted by HSPICE simulations. The developed integration technique using a MPW CMOS chip would also be applied to monolithic integration of CMOS devices/circuits and other various functional devices/materials, which would open the door for exploring CMOS-based new functional hybrid circuits.
[Show abstract][Hide abstract] ABSTRACT: We fabricated epitaxial Mg-Al-O(001) tunnel barriers using direct/indirect plasma oxidation and natural oxidation of an MgAl layer for use in Fe/Mg-Al-O/Fe magnetic tunnel junctions. All the oxidation processes formed epitaxial Mg-Al-O barriers, and a wide resistance area (RA) product range (101–106 Ω·μm2) and large tunnel magnetoresistance (TMR) ratios (185%–212%) at room temperature were achieved by optimizing the MgAl thickness (tMgAl). Near optimum oxidation conditions and tMgAl, small bias voltage dependence of the TMR ratio, and distinct local minima in the dI/dV spectra for the parallel magnetic configuration were observed, indicating that coherent tunneling transport was significant. This study showed that Mg-Al-O coherent tunnel barriers have excellent formability in a wide RA product range.
[Show abstract][Hide abstract] ABSTRACT: A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.
[Show abstract][Hide abstract] ABSTRACT: We investigated the temperature dependence of magnetoresistive (MR) outputs of current-perpendicular-to-plane pseudo spin valves with Co2FeAl1-xSix (0 ≤ x ≤ 1) Heusler alloy ferromagnetic layers and a Ag spacer layer to optimize x. The MR ratios at room temperature do not show x dependence, while the MR ratios measured at 5 K showed strong x dependence; the largest resistance change-area product ΔRA = 21 mΩ·μm2 was obtained for x = 1 and ΔRA decreased with decreasing x. The results apparently contradicts with the theoretical prediction that the highest spin polarization is attained for x = 0.5. The discrepancy is due to the structural origin that the L21 order is obtained only in the Co2FeSi alloy film.
Journal of Applied Physics 01/2013; 114(12):123910-123910-5. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Monocrystalline Mg-Al-O barriers for magnetic tunnel junctions (MTJs) were fabricated by natural oxidation of Mg-Al thin films. The naturally oxidized Mg-Al-O barrier had a cation-disorder MgAl2O4 structure, and Fe/Mg-Al-O/Fe(001) MTJs showed a tunnel magnetoresistance (TMR) ratio of over 180% at room temperature. In addition, the natural oxidation process resulted in tunnel barriers with a resistance area product (RA) of less than 5 Ω·μm2. We observed clear magnetization switching in a CoFe/Mg-Al-O/CoFe MTJ by spin-transfer torque. These results indicate that the natural oxidation of Mg-Al alloy is an effective method for processing cation-disorder MgAl2O4 barrier for coherent TMR with low RA.
[Show abstract][Hide abstract] ABSTRACT: Perpendicular magnetic anisotropy (PMA) in Heusler alloy Co2FeAl thin films sharing an interface with a MgO layer is investigated by angular-dependent x-ray magnetic circular dichroism. Orbital and spin magnetic moments are deduced separately for Fe and Co 3d electrons. In addition, the PMA energies are estimated using the orbital magnetic moments parallel and perpendicular to the film surfaces. We found that PMA in Co2FeAl is determined mainly by the contribution of Fe atoms with large orbital magnetic moments, which are enhanced at the interface between Co2FeAl and MgO. Furthermore, element specific magnetization curves of Fe and Co are found to be similar, suggesting the existence of ferromagnetic coupling between Fe and Co PMA directions.
[Show abstract][Hide abstract] ABSTRACT: We report enhanced tunnel magnetoresistance (TMR) ratios of 188% (308%) at room temperature and 328% (479%) at 15 K for cation-site-disordered MgAl2O4-barrier magnetic tunnel junctions (MTJs) with Fe (Fe0.5Co0.5 alloy) electrodes, which exceed the TMR ratios theoretically calculated and experimentally observed for ordered spinel barriers. The enhancement of TMR ratios is attributed to the suppression of the so-called band-folding effect in ordered spinel MTJs [ Phys. Rev. B 86 024426 (2012)]. First-principles calculations describe a dominant role of the oxygen sublattice for spin-dependent coherent tunneling, suggesting a mechanism of coherent tunneling occurring even in the disordered systems.
[Show abstract][Hide abstract] ABSTRACT: We report on a systematic study of the structural, magnetic properties and
the anomalous Hall effect, in the Heusler alloy Co2FeAl (CFA) epitaxial films
on MgO(001), as a function of film thickness. It was found that the epitaxial
CFA films show a highly ordered B2 structure with an in-plane uniaxial magnetic
anisotropy. An analysis of the electrical transport properties reveals that the
lattice and magnon scattering contributions to the longitudinal resistivity.
Independent on the thickness of films, the anomalous Hall resistivity of CFA
films is found to be dominated by skew scattering only. Moreover, the anomalous
Hall resistivity shows weakly temperature dependent behavior, and its absolute
value increases as the thickness decreases. We attribute this temperature
insensitivity in the anomalous Hall resistivity to the weak temperature
dependent of tunneling spin-polarization in the CFA films, while the thickness
dependence behavior is likely due to the increasing significance of interface
or free surface electronic states.
[Show abstract][Hide abstract] ABSTRACT: We fabricated perpendicularly magnetized magnetic tunnel junctions
(p-MTJs) with an ultrathin Co2FeAl (CFA) full-Heusler alloy
electrode having large interface magnetic anisotropy of CFA/MgO. An
out-of-plane tunnel magnetoresistance (TMR) ratio of 53% at room
temperature was observed in
CFA/MgO/Co20Fe60B20 p-MTJs. By
inserting a 0.1-nm-thick Fe (Co50Fe50) layer
between the MgO and Co20Fe60B20 layers,
The TMR ratio was significantly enhanced to 91% (82%) due to the
improved interface. The bias voltage dependence of differential
conductance did not clearly show coherent tunneling characteristics for
ultrathin CFA-MTJs, suggesting that a higher TMR ratio may be achieved
by improving the B2 ordering of CFA and/or interface structure.
[Show abstract][Hide abstract] ABSTRACT: We demonstrated spin-transfer magnetization switching using magnetic tunnel junctions (MTJs) with a full-Heusler alloy Co2FeAl (CFA). We prepared CFA (1.5 nm)/MgO/CoFe (4 nm) (“CFA-free”) and CFA (30 nm)/MgO/CoFeB (2 nm) (“CFA-reference”) MTJs on a Cr(001) layer. The intrinsic critical current density (Jc0) of the CFA-free (CFA-reference) MTJ was 29 MA/cm2 (7.1 MA/cm2). The larger Jc0 of the CFA-free MTJ is attributed to the significant enhancement of the Gilbert damping factor (∼0.04) of the CFA due to the Cr layer. The Jc0 of the CFA-reference is as small as that reported for typical CoFeB/MgO/CoFeB MTJs.
[Show abstract][Hide abstract] ABSTRACT: A systematic 59Co NMR study has been carried out at 4.2 K in a series of quaternary Co2FeAl1−xSix polycrystalline bulk Heusler alloys (x = 0, 0.3, 0.5, 0.7, 1). It was shown that the effect of Si substitution consists in a significant modification of 59Co hyperfine field and that this modification is mainly due to the contribution from s valence electron polarization, suggesting a shift of the Fermi energy level inside the half-metallic gap. This observation supports the theoretical predictions that the Fermi-level position in Co2FeZ Heusler alloys can be effectively tuned by varying the composition of the Z sublattice.
[Show abstract][Hide abstract] ABSTRACT: The design and performance of pseudo-spin-MOSFETs (PS-MOSFETs) using nano-CMOS devices were computationally investigated. The operations of a PS-MOSFET with current-induced magnetization switching were also experimentally demonstrated by the hybrid integration of a vendor-made MOSFET and our-developed spin-transfer-torque magnetic tunnel junction. The nonvolatile SRAM and delay flip-flop applications of PS-MOSFETs were also examined.
Electron Devices Meeting (IEDM), 2012 IEEE International; 01/2012
[Show abstract][Hide abstract] ABSTRACT: The spin-resolved electronic structure of buried magnetic layers is
studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin
polarimeter in combination with a high-energy hemispherical electron
analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan).
Spin-resolved photoelectron spectra are analyzed in comparison with the
results of magnetic linear and circular dichroism in photoelectron
emission in the case of buried
Co2FeAl0.5Si0.5 layers. The relatively
large inelastic mean free path (up to 20 nm) of fast photoelectrons
enables us to extend the HAXPES technique with electron-spin polarimetry
and to develop spin analysis techniques for buried magnetic multilayers
Japanese Journal of Applied Physics 12/2011; 51:016602. · 1.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed a new microbeam scanning system for efficient local area analyses and also for micro fabrication. The system accomplished a pattern-to-pattern spacing procedure, in which the microbeam was just scanned with a corresponding pattern of the analyzing sample area or of the fabrication form. Between the patterns, the beam was moved with the fastest responsive scanning speed. In order to set patterns of the analyzing sample in the scanning system, an image of the sample was previously obtained with a scanning transmission ion microscopy (STIM). As the result, analyzing time was greatly reduced for the cell sample in which cells are not distributed all over. To demonstrate the scanning system for micro fabrication, such as proton beam writing (PBW). Patterns which were translated from bitmap data including color scale were inscribed on thin films.
[Show abstract][Hide abstract] ABSTRACT: An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate arsenic ions of different oxidation states in water samples. Arsenate ions are separated from arsenite ions by co-precipitating into 10 ppm indium hydroxide colloids that are generated at pH 4.0 in a 25 ml solution containing 1 ppm phosphate ions and 25 ppm sulfate ions. Arsenite ions are oxidized to the pentavalent state with permanganate ions and adsorbed by indium hydroxide colloids generated afterwards in solution. The standard procedure for collecting the colloids adsorbing arsenic ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved arsenic ions and the obtained standard calibration curve covers the concentration range from 1 to 100 ppb for arsenic ions. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beam (0.7-4 nA beam currents). The lower detection limit of arsenic in a 25 ml aquatic sample is 0.3 ppb for the arsenic-precipitated targets based on the 3σ error of background counts integrated over the FWHM of arsenic peak in the PIXE spectrum. This sample preparation technique was then applied to analyze concentrations and oxidation states of arsenic in a river basin where hot springs are located upstream being possible sources for releasing arsenic in the river.
[Show abstract][Hide abstract] ABSTRACT: A microbeam analysis system at Tohoku University has been improved in detection efficiency for application to single cell analysis. The system is applicable to STIM analysis and to simultaneous PIXE and RBS analysis. Sample preparation methods suitable for non-adhesive single cell analysis were developed and first results with the improved analysis system are shown.
[Show abstract][Hide abstract] ABSTRACT: An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate chromium ions of different valence in water samples. Cr3+ ions are selectively adsorbed by ferric hydroxide colloids generated in the solution, and a PIXE target for analyzing the total concentration of chromium of hexavalent and trivalent states is prepared by depositing 0.15 ml of sample solution on a user-made thin polycarbonate film. PIXE analyses for the two kinds of targets reveal the fractions of chromium of different oxidation state. The standard method for collecting the colloids adsorbing Cr3+ ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved Cr3+ ions and the calibration curve is measured. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beams (0.7-5 nA beam currents). The lower detection limit of chromium in a 25 ml aquatic sample is 1 ppb for the Cr3+-precipitated targets and 3.4 ppb for the CrO42--deposit targets based on the 3σ error of background counts integrated over the FWHM of chromium peak in the PIXE spectrum. The applicability of PIXE using these sample-preparation techniques for determining chromium oxidation states was confirmed using river water samples to which Cr3+ and CrO42- ions were added in 50 ppb concentrations.
[Show abstract][Hide abstract] ABSTRACT: A microbeam system has been developed for the analysis of single aerosol particles. Combination of PIXE, RBS and off-axis STIM methods enabled simultaneous analysis for hydrogen to metal elements. Aerosol particles were collected on thin polycarbonate film (~0.3 μm) resulting in good signal-to-noise ratio. Quantitative elemental correlation was measured for single aerosol particles. A total of 270 particles were analyzed and clustered into 4 groups. The analysis system reveals the chemical composition of aerosol particles and is a powerful tool for source identification.