Spin Electronic Devices Research Articles

Heteroepitaxial growth of ferromagnetic rutile CoxTi1−xO2−δ on GaN (0001)

A rutile CoxTi1−xO2−δ (100) film was heteroepitaxially grown on GaN (0001) by pulsed laser deposition. Magneto-optical Kerr effect and cross-sectional transmission electron microscope (TEM) measurements revealed that Co0.03Ti0.97O2−δ films prepared at an oxygen partial pressure of 10−6–10−5Torr with a carrier density ne⩾3×1018cm−3 exhibit room-temperature ferromagnetism without any precipitates or secondary phase. High-resolution TEM observations confirmed that the interface between CoxTi1−xO2−δ and GaN is atomically smooth without intermixing. These results lead us to conclude that CoxTi1−xO2−δ is promising as a spin injector in GaN-based spin-electronic devices.

Magneto-Optical Effects in Superlattice Assemblies of Ferromagnetic Nanosheets

We have demonstrated a fabrication procedure for spin-electronic devices by using two-dimentional titania nanosheets. Magneto-optical Kerr measurements demonstrate that Co-substituted titania nanosheets (Ti0.8Co0.2O2) act as nanoscale ferromagnetic layers at room temperature, and their multilayer-assemblies exhibit robust magnetic circular dichroism (104–105 deg/cm) near the absorption edge at 260 nm, an operating wavelength being the shortest attained so far. The availability of ferromagnetic nanostructures and their molecule-level assembly allow the rational design and construction of high-efficiency magneto-optical devices by forming superlattices.

Spin Accumulation in Metallic Nanoparticles

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0-π oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×105 m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the ICRN product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.

Spin accumulation in metallic nanoparticles

The rapid development of spin electronics has emphasized the importance of spinaccumulation to transport properties and spin manipulation for quantum computing. Thelatter requires sufficiently long spin-relaxation times, that may be achieved in nanometricparticles. Spin accumulation is then expected to take place and strongly affect thetransport properties of the particles. The main transport phenomena occurring innanoparticles will be reviewed, as well as their interplay with spin accumulation.Recent experimental results confirming the theoretical predictions will also bepresented. They show clear evidence of the enhancement of the spin-relaxation time innanometric particles and are promising for the development of spin-electronics devices.

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (Hhard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (Jc) and the intrinsic critical current density (Jc0) estimated from the pulse duration dependence on Jc in CIMS are observed when applying Hhard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of Jc is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying Hhard. The Jc0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θf), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of Jc0 for the same value of Hhard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θf due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying Hhard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.

Magnetic and magneto-transport properties of double perovskiteSr2FeMoO6 with Co doping

The electrical, magnetic and magnetoresistance properties of polycrystallineSr2Fe1−xCoxMoO6 compoundswith x = 0.0, 0.05, 0.15 and 0.25 were investigated. With increasing Co content, both the degree of cationicorder and the saturation magnetic moment per formula unit of these compounds increase,while their resistivity decreases monotonically and exhibits an enhanced metallic behaviour forx≥0.15. The low-field magnetoresistance was greatly enhanced atx = 0.15, and the field dependence of magnetoresistance forx≥0.15 almost saturates above 1.5 T. These results can be well explained by the change of magnetic structureof Sr2FeMoO6 after replacing partial Fe ions by Co ions. The enhanced metallic behaviour and low-fieldmagnetoresistance also suggest a great application potential in spin electronic devices.

Preparation and characterization of ferromagnetic DO3-phase Fe3Si thin films on silicon-on-insulator substrates for Si-based spin-electronic device applications

The authors developed a preparation technique of stoichiometric and off-stoichiometric ferromagnetic Fe3+δSi1−δ thin films with a DO3 phase structure using silicon-on-insulator (SOI) substrates. Ferromagnetic Fe3+δSi1−δ films were formed by thermally activated silicidation reaction between an ultrathin SOI layer and a Fe layer deposited on it, induced by rapid thermal annealing. Their chemical composition was precisely controlled by the relative thickness ratio of the SOI and Fe layers, and highly oriented DO3-phase polycrystalline Fe3+δSi1−δ films were obtained with annealing temperature ranging from 620 to 780°C. The Fe3+δSi1−δ films exhibited ferromagnetic nature with the Curie temperature much higher than room temperature.

Fabrication of sub-50nm (La,Ba)MnO3 ferromagnetic nanochannels by atomic force microscopy lithography and their electrical properties

The technique of atomic force microscope nanolithography was used to construct robust nano-structures with atomically flat surface of ferromagnetic perovskite manganites down to 30nm in size over 5μm long. The resulting (La,Ba)MnO3 nanochannel possessed no lithographic damage and exhibited bulklike room temperature ferromagnetism down to 30nm beyond electrical phase separation picture. High-quality ferromagnetic oxide nanostructures allow to instigate intrinsic nanoscale electrical transport properties of these materials and to construct high-performance and novel spin electronics devices.

Ab initio estimates of the Curie temperature for magnetic compounds

The local spin-density functional approximation is increasingly being used for materialsapplications such as predicting structural and magnetic properties. The recently aquiredability to model low-lying magnetic excitations like magnons is used for derivinguseful formulae to estimate the Curie (Néel) temperatures of magnetic compounds.Underlying this is Moriya’s unified approach, appropriately formulated for densityfunctional use. The theory is applied to the classical compounds FeNi, CoNi,FeNi3,CoNi3,and RhMn3 to establish a level of confidence, and to the Heusler compounds NiMnSb,Mn2VAl,Co2FeSi,Mn3Al,and Mn3Ga, which are presently studied with possible applications in mind for advanced spinelectronic devices.

Graphene Spin Valve Devices

Graphene-a single atomic layer of graphite-is a recently found two-dimensional (2-D) form of carbon, which exhibits high crystal quality and ballistic electron transport at room temperature. Soft magnetic NiFe electrodes have been used to inject polarized spins into graphene, and a 10% change in resistance has been observed as the electrodes switch from the parallel to the antiparallel state. This coupled with the fact that a field-effect electrode can modulate the conductivity of these graphene films makes them exciting potential candidates for spin electronic devices

Theoretical study of dilute GaN–4d transition metal alloys

Electronic calculations were carried out for the dilute ordered alloysGa0.94 (TM)0.06N (TM = Y,Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag) in the zinc-blende structure. The theoreticalframework used was the density functional theory, using the local spin densityapproximation, as implemented in the full-potential linearized augmented plane wavemethod. We examine energy band structures, densities of states, charge distributions, andlocal magnetic moments and anticipate the properties of these promising systems forapplications in spin electronic devices.

Schottky barrier height of ferromagnet/Si(001) junctions

The Schottky barrier height of ferromagnetic metal/semiconductor junctions is one of the most important parameters for the performance of semiconductor-based spin-electronic devices. The authors investigated the Schottky barrier height ϕB of ferromagnet/Si(001) junctions with various ferromagnetic metals (CoFe, CoFeB, and MnAs), and found that ϕB of epitaxial MnAs∕Si(001) junctions fabricated by molecular beam epitaxy was 0.16eV for electrons, which is much lower than ϕB (∼0.7eV) of CoFe∕Si(001) and CoFeB∕Si(001) junctions. This implies that MnAs is a promising ferromagnetic material for Si-based spin-electronic devices, especially for the source and drain of spin metal-oxide-semiconductor field effect transistors.

Bias voltage and temperature dependence of magneto-electric properties in double-barrier magnetic tunnel junction with amorphous Co-Fe-B electrodes

Bias voltage and temperature dependence of magneto-electric properties in double-barrier magnetic tunnel junctions(DBMTJs) with a structure of [IrMn/CoFe/Ru/CoFeB]/Al-O/CoFeB/Al-O/[CoFeB/Ru/CoFe/IrMn], have been investigated. The DBMTJs show a large tunnel magnetoresistance (TMR) ratio of up to 57.6%, a high V1/2 value of 1.26 V and small switching field Hc of 9.5 Oe at room temperature (RT). The TMR reaches the maximum at 30 K, about 89.0%, and decreases slightly from 30 to 4.2 K. A novel zero-bias anomaly (ZBA) in the P state is found and is temperature dependent, more sharply at low temperature, whereas a normal ZBA exists in the AP state. These effects are ascribed to magnon-, phonon- and impurity-assisted tunneling, and variation of density of states. The DBMTJ with a large TMR ratio, a high V1/2, and small switching field Hc is promising for developing the future spin electronic devices.

Effect of the shape anisotropy on the magnetic configuration of (Ga,Mn)As and its evolution with temperature

We study the effect of the shape anisotropy on the magnetic domain configurations of a ferromagnetic semiconductor (Ga,Mn)As∕GaAs(001) epitaxial wire as a function of temperature. Using magnetoresistance measurements, we deduce the magnetic configurations and estimate the relative strength of the shape anisotropy compared with the intrinsic anisotropies. Since the intrinsic anisotropy is found to show a stronger temperature dependence than the shape anisotropy, the effect of the shape anisotropy on the magnetic domain configuration is relatively enhanced with increasing temperature. This information about the shape anisotropy provides a practical means of designing nanostructured spin electronic devices using (Ga,Mn)As.

Controllableπjunction with magnetic nanostructures

We propose a novel Josephson device in which 0 and $\pi$ states are controlled by an electrical current. In this system, the $\pi$ state appears in a superconductor/normal metal/superconductor junction due to the non-local spin accumulation in the normal metal which is induced by spin injection from a ferromagnetic electrode. Our proposal offers not only new possibilities for application of superconducting spin-electronic devices but also the in-depth understanding of the spin-dependent phenomena in magnetic nanostructures.

Magnetoresistance and performance of amorphous-CoNbZr∕Co∕Cu∕Co magnetic multilayers

A pseudo spin-valve (PSV) using amorphous CoNbZr alloy as a soft magnetic layer and buffer layer was proposed. The giant magnetoresistance (GMR), thermal annealing effect, and the performance were investigated. Antiparallel magnetization alignments were observed in the samples with 2–4nm CoNbZr layer and a maximum GMR of 6.5% was obtained. Furthermore, sandwich with relatively thick CoNbZr layer has a superior thermal stability to 400°C and a GMR enhancement to about 9%. After patterning to 6μm×1μm elliptic stripe, a single domain forms and the dynamic MR behavior indicates the stripe has a linear and stable GMR response. Therefore, it is believed that the PSVs with amorphous CoNbZr layers have good potentials for spin-electronic devices.

Spin-electronic devices with half-metallic Heusler alloys

We have integrated Co 2MnSi as a representative of the full-Heusler compound family as one magnetic electrode into technological relevant magnetic tunnel junctions. The resulting tunnel magnetoresistance at 20 K currently achieved is 108% associated with a Co 2MnSi spin polarization of 70% clearly proving that Co 2MnSi is already superior to 3d-based magnetic elements or their alloys. The corresponding room temperature value of the tunnel magnetoresistance is 42%. The presence of a step like tunnel barrier which is already created during plasma oxidation, while preparing the AlO x tunnel barrier, has been identified as the current limitation to achieve larger tunnel magnetoresistance and hence larger spin polarization and is a direct consequence of the oxygen affinity of the Co 2MnSi-Heusler element Mn. In addition preliminarily results on Co 2FeSi as a new full-Heusler compound integrated as magnetic electrode into technological relevant magnetic tunnel junctions are shown and discussed.

High magnetoresistance in Co–Fe–B-based double barrier magnetic tunnel junction

The magnetic transport properties of the double barrier magnetic tunnel junctions (DBMTJs) with amorphous Co–Fe–B electrodes, deposited by magnetron sputtering and patterned using optical lithography combined with lift-off technique, were investigated. A high tunnel magnetoresistance (TMR) ratio of 90.3% and 59.4%, resistance-area product of about 6.2 and 5.5 kΩ μm 2 at 4.2 K and room temperature, respectively, and a high bias voltage at which the TMR signal decreased to half of its maximum value, V 1/2 =1.14 V at negative bias voltage were measured experimentally for the DBMTJs. These results show that the DBMTJs are promising candidates for spin electronic devices.

高スピン分極フルホイスラー合金薄膜とスピンエレクトロニクスデバイス

高スピン分極フルホイスラー合金薄膜とスピンエレクトロニクスデバイス