Magnetic Junctions Research Articles

Sd-Exchange emission in ferromagnetic junctions

The spin-injection emission in a ferromagnetic junction at terahertz frequencies is theoretically analyzed. It is demonstrated that the efficiency of the emission caused by the sd-exchange interaction of the injected spin in which the electromagnetic field is involved strongly depends on the orientation angle of the magnetization of the active region relative to the magnetization direction of the injecting region. The fact that the calculated radiation frequency and power are close to the experimental results shows that the sd-exchange emission must be taken into account in the interpretation of the observed terahertz emission in magnetic junctions.

MAGNETORESISTANCE OF NARROW GAP MAGNETIC SEMICONDUCTOR HETEROJUNCTIONS

Narrow gap III–V semiconductors have been investigated for semiconductor spintronics. By alloying these semiconductors with manganese magnetic semiconductors result. Large magnetoresistance (MR) effects have been observed in narrow gap magnetic semiconductor p–n heterojunctions. The MR which is positive is attributed to spin selective carrier scattering. For an InMnAs / InAs heterojunction a diode MR of 2680% is observed at room temperature and high magnetic fields. This work indicates that highly spin-polarized magnetic semiconductor heterojunctions can be realized that operate at room temperature. Devices based on the MR include spin diodes and bipolar magnetic junction transistors. We utilize the diode MR states to create a binary logic family.

Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes

High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co2MnGe(13nm)/Al2O3(3nm)/Co(13nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates.

Magnetization Dynamics in Co<SUB>2</SUB>MnGe/Al<SUB>2</SUB>O<SUB>3</SUB>/Co Tunnel Junctions Grown on Different Substrates

We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions (TMJ), deposited on various single crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are compared to the magnetic properties of Co and of Co$_{2}$MnGe single films lying on sapphire substrates. X-rays diffraction always shows a (110) orientation of the Co$_{2}$MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the TMJ grown on sapphire. Our vibrating sample magnetometry measurements reveal in-plane anisotropy only in samples grown on a sapphire substrate. Depending on the substrate, the ferromagnetic resonance spectra of the TMJs, studied by the microstrip technique, show one or two pseudo-uniform modes. In the case of MgO and of Si substrates only one mode is observed: it is described by magnetic parameters (g-factor, effective magnetization, in-plane magnetic anisotropy) derived in the frame of a simple expression of the magnetic energy density; these parameters are practically identical to those obtained for the Co single film. With a sapphire substrate two modes are present: one of them does not appreciably differ from the observed mode in the Co single film while the other one is similar to the mode appearing in the Co$_{2}$MnGe single film: their magnetic parameters can thus be determined independently, using a classical model for the energy density in the absence of interlayer exchange coupling.

Conductance of ferro- and antiferro-magnetic single-atom contacts: A first-principles study

We present a first-principles study on the spin dependent conductance of five single-atom magnetic junctions consisting of a magnetic tip and an adatom adsorbed on a magnetic surface, i.e., the Co-Co/Co(001) and Ni-X/Ni(001) (X = Fe, Co, Ni, Cu) junctions. When their spin configuration changes from ferromagnetism to anti-ferromagnetism, the spin-up conductance increases while the spin-down one decreases. For the junctions with a magnetic adatom, there is nearly no spin valve effect as the decreased spin-down conductance counteracts the increased spin-up one. For the junction with a nonmagnetic adatom (Ni-Cu/Ni(001)), a spin valve effect is obtained with a variation of 22% in the total conductance. In addition, the change in spin configuration enhances the spin filter effect for the Ni-Fe/Ni(001) junction but suppresses it for the other junctions.

Inversion of magnetoresistance in La1−x Sr x MnO3/Nb-doped SrTiO3/CoFe junctions

Magnetoresistance (MR) devices using degenerated oxide semiconductors, Nb-doped SrTiO3, (Nb-STO) as intermediate layers between two ferromagnets were fabricated and their magnetic and transport properties were evaluated. Magnetic junctions using the trilayer films showed MR of ∼5% at 4.2 K, and the sign and the magnitude of the MR were changed depending on the thickness of Nb-STO layers. The origin of the inversion of the MR is not clear, however we consider spin injection from ferromagnetic electrodes to Nb-STO through Schottky tunnel barriers plays an important role.

Phase-dependent heat transport through magnetic Josephson tunnel junctions

We present an exhaustive study of the coherent heat transport through superconductor-ferromagnet(S-F) Josephson junctions including a spin-filter (I$_{sf}$) tunneling barrier. By using the quasiclassical Keldysh Green's function technique we derive a general expression for the heat current flowing through a S/F/I$_{sf}$/F/S junction and analyze the dependence of the thermal conductance on the spin-filter efficiency, the phase difference between the superconductors and the magnetization direction of the ferromagnetic layers. In the case of non-collinear magnetizations we show explicitly the contributions to the heat current stemming from the singlet and triplet components of the superconducting condensate. We also demonstrate that the magnetothermal resistance ratio of a S/F/I$_{sf}$/F/S heat valve can be increased by the spin-filter effect under suitable conditions.

Domain wall trajectory determined by its fractional topological edge defects

A domain wall (DW) in a ferromagnetic nanowire is composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively, whose relative spatial arrangement determines the chirality of the DW. Here we show how we can understand and control the trajectory of DWs in magnetic branched networks, composed of connected nanowires, by considering their fractional elementary topological defects and how they interact with those innate to the network. We first develop a highly reliable mechanism for the injection of a DW of a given chirality into a nanowire and show that its chirality determines which branch the DW follows at a symmetric Y-shaped magnetic junction—the fundamental building block of the network. Using these concepts, we unravel the origin of the one-dimensional nature of magnetization reversal of connected artificial spin ice systems that have been observed in the form of Dirac strings. When a domain wall of a given chirality is injected into a magnetic nanowire, its trajectory through a branched network of Y-shaped nanowire junctions—such as a honeycomb lattice, for instance—can be pre-determined. This property has implications for data storage and processing.

Experimental investigations of SiO2 based ferrite magnetic tunnel junction

• CoFe 2 O 4 /SiO 2 /Co–NiFe 2 O 4 magnetic junction was fabricated using RF/DC sputtering. • Spin transport through nanostructure silicon oxide with ferrite as free and pinned layer is our first report. • Magnetization studies were done to justify the free layer and pinned layer for our multilayer. • Magnetoresistance behavior shows a sharp discriminating between parallel and antiparallel alignment with TMR value of 16%. We report experimental results of ferrite based magnetic tunnel junction. Ferrite junction and spin transport through SiO 2 were interesting since they can readily replace the conventional electronics. We fabricated a cobalt ferrite/SiO 2 /cobalt nickel ferrite based magnetic tunnel junction over a copper coated n-silicon substrate using a RF/DC magnetron sputtering. The tunneling magnetoresistance shows a very good response to applied field and we achieved a TMR of about 16%. Although theoretically it was predicted infinite TMR for half metallic ferromagnetic junction, the deviation was explained on the basis of incoherent scattering along the interfaces.

Enhanced temperature dependent junction magnetoresistance in La0.7Sr0.3MnO3/Zn(Fe, Al)O carrier induced dilute magnetic semiconductor junctions

Metal-semiconductor type junction based on p-type La0.7Sr0.3MnO3 and n-type ZnO, Zn(Fe)O or Zn(Fe,Al)O with different Fe concentrations have been fabricated on (0001) sapphire substrate using pulsed laser deposition technique. While the metal-semiconductor type junction with La0.7Sr0.3MnO3 (LSMO) and ZnO or Zn(Fe)O show very good rectifying behavior, the heterojunction with La0.7Sr0.3MnO3 and Zn(Fe,Al)O fails to show such rectifying characteristics which most likely due to the much higher carrier concentration and thin depletion width. These metal-semiconductor type junctions show reasonably high temperature dependent junction magnetoresistive behavior. At 77 and 300 K all the junctions show negative junction magnetoresistance, whereas they show positive junction magnetoresistance at certain temperature range (150–280 K). The junction MR attains a peak with high positive value ∼76%, 38%, and 25% for LSMO/Zn(Fe,Al)O, LSMO/Zn(Fe)O, and LSMO/ZnO junctions, respectively, near 250 K, where La0.7Sr0.3MnO3 shows highest spin relaxation near 250 K. The junction magnetoresistive properties have been explained using the standard spin injection mechanism through the magnetic p-n junction. Junction magnetoresistance dies out with the increase of doping concentrations in all the three type of metal-semiconductor type junctions due to the less non equilibrium population of polarized electrons.

Theoretical model of superconducting spintronic SIsFS devices

Motivated by recent progress in the development of cryogenic memory compatible with single flux quantum (SFQ) circuits, we have performed a theoretical study of magnetic SIsFS Josephson junctions, where “S” is a bulk superconductor, “s” is a thin superconducting film, “F” is a metallic ferromagnet, and “I” is an insulator. We calculate the Josephson current as a function of s and F layers thickness, temperature, and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have large product of a critical current IC and a normal-state resistance RN in the π state, comparable to that in superconductor–insulator–superconductor tunnel junctions commonly used in SFQ circuits. We develop a model describing switching of the Josephson critical current in these devices by external magnetic field. The results are in good agreement with the experimental data for Nb-Al/AlOx-Nb-Pd0.99Fe0.01-Nb junctions.

Field modulation of the critical current in magnetic Josephson junctions

The dependence of the critical current of a simple Josephson junction on the applied magnetic field is well known and, for a rectangular junction, gives rise to the classic ‘Fraunhofer’ modulation with periodic zeros at the fields that introduce a flux quantum into the junction region. Much recent work has been performed on Josephson junctions that contain magnetic layers. The magnetization of such layers introduces additional flux into the junction and, for large junction areas or strong magnetic materials, can significantly distort the modulation of the critical current and strongly suppress the maximum critical current. The growing interest in junctions that induce odd-frequency triplet pairing in a ferromagnet, and the need to make quantitative comparisons with theory, mean that a full understanding of the role of magnetic barriers in controlling the critical current is necessary. This paper analyses the effect of magnetism and various magnetic configurations on Josephson critical currents; the overall treatment applies to junctions of general shape, but the specific cases of square and rectangular junctions are considered.

Tunneling characteristics of a double-barrier magnetic junction

The spin-polarized current through a planar double-barrier magnetic tunnel junction has been calculated using the quasi-classical model. The coefficients of electron transmission through the barriers have been calculated in terms of the quantum theory. The dependences of the transmission coefficients, spinpolarized currents, and tunneling magnetoresistance on the applied voltage under resonant conditions have been shown. Under non-resonant conditions, the tunneling magnetoresistance has been compared with the experimental data.

MAGNETORESISTANCE AND SPIN-FILTERING EFFICIENCY OF DNA-SANDWICHED FERROMAGNETIC NANOSTRUCTURES

We report magnetoresistance and spin-filtering efficiency of DNA-sandwiched ferromagnetic nanostructures. Commercially available FeCo target was sputtered over a gold-coated silicon wafer coated with mercaptosuccinic acid and DNA. This was made to contact with another FeCo sputtered gold-coated silicon wafer coated with mercaptosuccinic acid forming FM/DNA/FM based magnetic junction. DNA forms a covalent bonding with FeCo nanoparticles through mercaptosuccinic acid. Magnetoresistance for this was measured in the room temperature and was found to be 18%. Spin-filtering efficiency was calculated as 30%, which is half the value of experimental observation of spin-filtering of electron on pure gold surface coated with DNA.

Magneto-Josephson effects in junctions with Majorana bound states

We investigate 1D quantum systems that support Majorana bound states at interfaces between topologically distinct regions. In particular, we show that there exists a duality between particle-hole and spin degrees of freedom in certain spin-orbit-coupled 1D platforms such as topological insulator edges. This duality results in a spin analogue of previously explored `fractional Josephson effects'---that is, the spin current flowing across a magnetic junction exhibits 4\pi-periodicity in the relative magnetic field angle across the junction. Furthermore, the interplay between the particle-hole and spin degrees of freedom results in unconventional magneto-Josephson effects, such that the Josephson current is a function of the magnetic field orientation with periodicity 4\pi.

Pulsed Laser Impact on Ferrimagnetic Nanostructures

We have studied the mechanisms of a pulsed laser impact on the magnetization conFigureuration in ferrimagnetic multilayered magnetic nanostructures, specifically, tunneling magnetic junctions. The mechanism of such the laser-induced impact is a complex process of laser-induced thermal demagnetization of magnetic sublattices with subsequent biasing by internal magnetic fields of different nature. Depending on an intensity of laser pulses it can be effective internal magnetic fields of laser irradiation or internal magnetic fields connected with different rates of the heat demagnetization of ferrimagnetic sublattices. It is shown that investigated ferrimagnetic nanostructure are characterized very small times of the laser-induced remagnetization, which can attain subpicosecond scales.

単一ナノ分子・磁気抵抗素子

Giant magnetoresistance (GMR) through a single phthalocyanine molecule (H2Pc) was studied by means of a spin-polarized scanning tunneling microscopy (STM) technique. Single molecular magnetic junction was fabricated by contacting a ferromagnetic STM tip to a single molecule on nano magnets. We obtained GMR of +60% and −50% through [Co/H2Pc/Co(111)] and [Fe/H2Pc/Mn(001)] junctions, respectively, in ultra-high vacuum at 4.6 K. Single organic molecules can be a new material for spintronics.

ON LASER-INDUCED MAGNETORESISTANCE EFFECT IN MAGNETIC JUNCTIONS

We have studied the laser-induced high-speed remagnetization and tunneling magnetoresistance (TMR) effects in tunnel magnetic junctions (TMJ). Using magneto-optical investigations, all-optical pump and probe technique, we have studied mechanisms of the laser-induced remagnetization and shown feasibility of the high-speed laser-controlled remagnetization and resistance switching in the TMJ. It is shown that the laser-induced remagnetization is caused by the internal effective magnetic fields of the inverse magneto-optical Faraday effect and the effective magnetic field of s–d-exchange interaction between the laser-induced spin-polarized current and a lattice magnetization together with a laser-induced thermal demagnetization.

Conductance of single-atom magnetic junctions: A first-principles study

We present a first-principles investigation to show that the contact conductance of a half conductance quantum (G0/2) found previously does not generally hold for single-atom magnetic junctions composed of a tip and an adatom adsorbed on a surface. The contact conductance of the Ni-Co/Co(111) junction is approximately G0/2, while for the Co-Co/Co(111), Ni-Ni/Ni(111), and Ni-Ni/Ni(001) junctions the contact conductances are 0.80G0, 1.55G0, and 1.77G0, respectively. The deviation from G0/2 is mainly caused by the variation of the spin-down conductance largely determined by the minority d orbitals, as the spin-up one changes little for different junctions.

Spin-dependent electron transport in manganite bicrystal junctions

Magnetic bicrystal films and junctions of magnetic La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) films epitaxially grown on NdGaO3 substrates with the (110) planes of their two parts misoriented (tilted) at angles of 12A degrees, 22A degrees, 28A degrees, and 38A degrees are investigated. For comparison, bicrystal boundaries with a 90A degrees misorientation of the axes of the NdGaO3 (110) planes were fabricated. The directions of the axes and the magnetic anisotropy constants of the films on both sides of the boundary are determined by two independent techniques of magnetic resonance spectroscopy. The magnetic misorientation of the axes in the substrate plane has been found to be much smaller than the crystallographic misorientation for tilted bicrystal boundaries, while the crystallographic and magnetic misorientation angles coincide for boundaries with rotation of the axes. An increase in the magnetoresistance and characteristic resistance of bicrystal junctions with increasing misorientation angle was observed experimentally. The magnetoresistance of bicrystal junctions has been calculated by taking into account the uniaxial anisotropy, which has allowed the contributions from the tunneling and anisotropic magnetoresistances to be separated. The largest tunneling magnetoresistance was observed on LCMO bicrystal junctions, in which the characteristic resistance of the boundary is higher than that in LSMO boundaries.