Interfacial Spin Polarization Research Articles

The effects of electric and magnetic fields on the current spin polarization and magnetoresistance in a ferromagnetic/organic semiconductor/ferromagnetic (FM/OSC/FM) system

From experimental results of spin polarized injection and transport in organic semiconductors (OSCs), we theoretically study the current spin polarization and magnetoresistance under an electric and a magnetic field in a ferromagnetic/organic semiconductor/ferromagnetic (FM/OSC/FM) sandwich structure according to the spin drift-diffusion theory and Ohm's law. From the calculations, it is found that the interfacial current spin polarization is enhanced by several orders of magnitude through tuning the magnetic and electric fields by taking into account the specific characteristics of OSC. Furthermore, the effects of the electric and magnetic fields on the magnetoresistance are also discussed in the sandwich structure.

Large Magnetoresistance through a Single Molecule due to a Spin-Split Hybridized Orbital

Using organic materials in spintronic devices raises a lot of expectation for future applications due to their flexibility, low cost, long spin lifetime, and easy functionalization. However, the interfacial hybridization and spin polarization between the organic layer and the ferromagnetic electrodes still has to be understood at the molecular scale. Coupling state-of-the-art spin-polarized scanning tunneling spectroscopy and spin-resolved ab initio calculations, we give the first experimental evidence of the spin splitting of a molecular orbital on a single non magnetic C(60) molecule in contact with a magnetic material, namely, the Cr(001) surface. This hybridized molecular state is responsible for an inversion of sign of the tunneling magnetoresistance depending on energy. This result opens the way to spin filtering through molecular orbitals.

High spin polarization induced by the interface hybridization in Co/C composite films

Co/C composite films with 10at.% Co were fabricated by facing-target sputtering and postannealed in vacuum at 300–650°C for 1.5h. Co nanoparticles are dispersed in amorphous carbon matrix and Co–C compounds exist at the interface between Co nanoparticles and carbon matrix. Negative magnetoresistance was observed in the Co/C films and its maximum value of −19% was measured at 3.98×106A/m and 5K for the as-deposited film. Besides cotunneling effect, higher interfacial spin polarization, which is probably induced by the hybridization between Co 3d and C 2p electrons and consequently the preferred d-electron conduction and interfacial spin-filtering effect, also contributes to the large negative magnetoresistance. Postannealing treatment damages the interface hybridization, and thus leads to the disappearance of the higher interfacial spin polarization and large negative magnetoresistance.

Hybridization-induced oscillatory magnetic polarization of C60 orbitals at the C60/Fe(001) interface

We have studied the electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies of C60 monolayers on Fe(001) surfaces show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π∗-derived orbitals. The magnitude and also the sign of this polarization were found to depend markedly on the excitation energy. These observations underline the importance of tailoring the interfacial spin polarization at the Fermi level of ferromagnet/organic semiconductor interfaces for applications in organic spintronics.

Improved tunneling magnetoresistance at low temperature in manganite junctions grown by molecular beam epitaxy

We report resistance versus magnetic field measurements for a La0.65Sr0.35MnO3/SrTiO3/La0.65Sr0.35MnO3 tunnel junction grown by molecular-beam epitaxy, that show a large field window of extremely high tunneling magnetoresistance (TMR) at low temperature. Scanning the in-plane applied field orientation through 360°, the TMR shows fourfold symmetry, i.e., biaxial anisotropy, aligned with the crystalline axis but not the junction geometrical long axis. The TMR reaches ∼1900% at 4 K, corresponding to an interfacial spin polarization of >95% assuming identical interfaces. These results show that uniaxial anisotropy is not necessary for large TMR, and lay the groundwork for future improvements in TMR in manganite junctions.

Half-metallic Fe2CrSi and non-magnetic Cu2CrAl Heusler alloys for current-perpendicular-to-plane giant magneto-resistance: First principle and experimental study

We present results of our first principle and experimental study on new Heusler alloys candidates for half-metallic electrode and nonmagnetic spacer in current-perpendicular-to-plane giant magneto-resistance devices. The electronic structures of Fe2CrSi and Cu2CrAl are characterized by high density of states at the Fermi level in the majority spin band with a good match in their majority band structures near the Fermi level. The calculated interfacial spin polarization in Fe2CrSi/Cu2CrAl (100) superlattice is ∼80%. We have also epitaxially grown ferromagnetic A2 Fe-Cr-Si and nonmagnetic L21 Cu-Cr-Al (100) films on Cr-buffered MgO(100) substrates.

Two-Dimensional Intrinsic Ferromagnetism at Nitride-Boride Interfaces

We predict theoretically novel two-dimensional interface ferromagnetism at AlN/MgB(2)(0001) using first-principles calculations, where the interface is employed as an ordered structure of spin sites instead of point defects. Although N dangling bonds are apparently saturated, interfacial states exhibit spin polarization. Hund's coupling of the two N p(∥) orbitals as well as low density of states at the Fermi energy contribute to strong band ferromagnetism. Furthermore, first-principles electron transport calculations demonstrate that this interfacial spin polarization is responsible for quantum spin transport. The magnetization can be controlled by applied gate bias voltages.

Interfacial spin polarization and resistance in lateral spin-valves incorporating Bi and BiPb thin films

Electrical spin injection and detection have been investigated at cryogenic temperatures using a lateral spin-valve structure. Either Bi or BiPb was used as a nonmagnetic spin medium, while CoFe and NiFe were employed as the spin injector and spin detector, respectively. A large magnetoresistance signal corresponding to ΔR=1.2 mΩ was detected from the BiPb-based spin-valves. From this result, a large spin diffusion length of 230 μm and a high interfacial spin polarization of 10% were derived. From an independent calculation, it was found that the interfacial spin polarization and the associated spin accumulation are strongly correlated with the interfacial resistance.

Impact on Interface Spin Polarization of Molecular Bonding to Metallic Surfaces

We have studied the repercussion of the molecular adsorption mechanism on the electronic properties of the interface between model nonmagnetic or magnetic metallic surfaces and metallo-organic phthalocyanines molecules (Pcs). Our intertwined x-ray absorption spectroscopy experiments and computational studies reveal that manganese Pc (MnPc) is physisorbed onto a Cu(001) surface and retains the electronic properties of a free molecule. On the other hand, MnPc is chemisorbed onto Co(001), leading to a dominant direct exchange interaction between the Mn molecular site and the Co substrate. By promoting an interfacial spin-polarized conduction state on the molecule, these interactions reveal an important lever to tailor the spintronic properties of hybrid organic-metallic interfaces.

Signature of odd-frequency pairing correlations induced by a magnetic interface

We investigate the mutual proximity effect in a normal metal contacted to a superconductor through a magnetic interface. Analytical and self-consistent numerical results are presented, and we consider both the diffusive and ballistic regimes. We focus on the density of states in both the normal and superconducting region, and find that the presence of spin-dependent phase-shifts occurring at the interface qualitatively modifies the density of states. In particular, we find that the proximity-induced pairing amplitudes in the normal metal region undergo a conversion at the Fermi level from pure even-frequency to odd-frequency. Above a critical value of the interface spin-polarization (or, equivalently, for fixed interface spin-polarization, above a critical interface resistance), only odd frequency correlations remain. This is accompanied by the replacement of the familiar proximity minigap or pseudogap in the normal layer by an enhancement of the density of states above its normal state value for energies near the chemical potential. The robustness of this effect towards inelastic scattering, impurity scattering, and the depletion of the superconducting order parameter close to the interface is investigated. We also study the inverse proximity effect in the diffusive limit. We find that the above-mentioned conversion persists also for thin superconducting layers comparable in size to the superconducting coherence length $\xi_\text{S}$, as long as the inverse proximity effect is relatively weak. Concomitantly, we find a shift in the critical interface resistance where the pairing conversion occurs. Our findings suggest a robust and simple method for producing purely odd-frequency superconducting correlations, that can be tested experimentally.

Pseudo spin valves based on L10 (111)-oriented FePt and FePtCu fixed layer with tilted anisotropy

We demonstrate a series of pseudo-spin-valve structures based on L10 (111)-oriented FePt and FePtCu with titled magnetocrystalline anisotropy. Highly ordered (111)-oriented L10 FePtCu with large anisotropy is achieved by optimizing the Cu content. Magnetoresistance (MR) up to 5% has been obtained by i) optimizing the fixed-layer growth using different underlayers, ii) enhancing the interface spin polarization using thin CoFe at the Cu interfaces, and iii) adjusting the Cu spacer thickness. The substantial MR realized with tilted fixed layer magnetization is an important prerequisite for the realization of tilted polarizer Spin Torque Oscillators (STO) or Spin-Transfer Torque Magnetoresistive Random Access Memories (STT-MRAM).

Study of Pseudo Spin Valves Based on $L1_{0}$ (111)-Oriented FePt and FePtCu Fixed Layer With Tilted Magnetocrystalline Anisotropy

In this paper, we demonstrate a series of pseudo-spin-valve structures based on L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (111)-oriented FePt and FePtCu with titled magnetocrystalline anisotropy. Highly ordered (111)-oriented L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePtCu with large anisotropy is achieved by optimizing the Cu content. Magnetoresistance (MR) up to 5% has been obtained by 1) optimizing the FePtCu growth using different underlayers, 2) enhancing the interface spin polarization using thin CoFe at the Cu interfaces, and 3) adjusting the Cu spacer thickness. The substantial MR realized with tilted fixed layer magnetization is an important prerequisite for the realization of tilted polarizer spin torque oscillators (STOs) or spin-transfer torque magnetoresistive random access memories (STT-MRAMs).

Spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions with a Heusler alloy Co2MnGe thin film and a MgO barrier

We investigated the spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Heusler alloy Co2MnGe (CMG) lower electrode, a MgO barrier and a Co50Fe50 upper electrode, which were fabricated as a function of Ta, where Ta is the temperature at which the MTJ trilayer was in situ annealed right after deposition of the upper electrode. We found that the tunnel magnetoresistance (TMR) ratio increased discontinuously and significantly from 92% at room temperature (RT) (244% at 4.2 K) to 160% at RT (376% at 4.2 K) when Ta was increased from 475 to 500 °C. We also found that the dI/dV versus V characteristics of fabricated MTJs for the parallel (P) and antiparallel (AP) magnetization configurations changed discontinuously and markedly with increasing Ta from 475 °C or less to 500 °C or higher; i.e. the dI/dV versus V characteristics of the MTJs with Ta of 475 °C or less exhibited distinct peak structures at V ∼ 0.22 V for P and at V ∼ −0.38 and 0.27 V for AP, where the bias voltage (V) was defined with respect to the CMG lower electrode. On the other hand, these structures were not observed in the dI/dV versus V characteristics of the MTJs when Ta was 500 °C or higher. We ascribe the peak structures in the dI/dV versus V characteristics to the existence of peak structures in the interfacial density of states at the CMG electrode–MgO barrier interface arising from possible thermodynamically unstable interface bonding in CMG/MgO/Co50Fe50 MTJs with Ta of 475 °C or less. We attribute the discontinuous and complete disappearance of these peaks in the dI/dV versus V characteristics to the change in the interface bonding from thermodynamically unstable bonding for Ta of 475 °C or less to stable bonding for Ta of 500 °C or higher. The significant increase in the TMR ratio with increasing Ta from 475 to 500 °C is attributed to the increase in the interfacial spin polarization at the Fermi level associated with the change in the spin-dependent interfacial density of states.

Pairing Symmetry Conversion by Spin-Active Interfaces in Magnetic Normal-Metal–Superconductor Junctions

We study the proximity-induced superconducting correlations in a normal metal connected to a superconductor when the interface between them is spin active and the normal metal is ballistic or diffusive. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional even-frequency proximity component vanishes completely at the chemical potential, while the odd-frequency component remains finite. We propose a way to unambiguously observe the odd-frequency component.

Measurements of interfacial spin-polarization of ferromagnetic materials with the use of epitaxial NbN junctions

Measurements of interfacial spin-polarization of ferromagnetic materials with the use of epitaxial NbN junctions

Spin-transfer torque in magnetic tunnel junctions: Scattering theory

We study the bias-dependent spin-transfer torque in magnetic tunnel junctions in the Stoner model by scattering theory. We show that the in-plane (Slonczewski type) torque vanishes and subsequently reverses its direction when the bias voltage becomes larger or the barrier wider than material and device-dependent critical values. We are able to reproduce the magnitude and the bias dependence of measured in-plane and out-of-plane torques using realistic parameters. The condition for the vanishing torque is summarized by a phase diagram depending on the applied bias and barrier width, which is explained in terms of an interface spin polarization and the electron focusing by the barrier. Quantum size effects in the spin-transfer torque are predicted as a function of the thickness of a normal-metal layer inserted between the ferromagnet and tunnel barrier.

Structural effect of junction interface on magnetic properties in a Co/MgO/Co system: First-principles calculations

The structural and magnetic properties of Co(001)/MgO(001)/Co(001) magnetic tunnel junctions with two types of junction interface, on an O-site adsorption case (Case I) and on a Mg-O twofold hollow site adsorption case (Case II), were investigated using density functional theory. Interfacial spin polarization values of electrodes were calculated to be −81.3% for Case I and −90.5% for Case II, resulting from the adsorption site dependency of electrode atoms on the MgO(001) surface. Interestingly, Mg and O atoms also showed significantly induced spin polarization values due to the 2p−3d hybridization between the Mg/O and Co-interface atoms. The magnetic moments of the electrode atoms were calculated to be 1.67 μB and 1.72 μB on average without significant variation across the electrode, except for the surface layer.

Magnetotransport Properties of Fe/Pentacene/Co:TiO2 Junctions with Fe Top Contact Electrodes Prepared by Thermal Evaporation and Pulsed Laser Deposition

We fabricated heterojunctions using organic semiconductor pentacene sandwiched between two different magnetic electrodes: Co-doped TiO2 (Co:TiO2) and Fe. The conductance of the junctions was measured as a function of magnetic field. We found that the characteristics of the junctions are strongly dependent on the method used to deposit the Fe top electrode on pentacene. The magnetotransport properties can be explained by the effect of the interface morphology and spin polarization of interface traps.

Negative spin polarization in (La,Sr)CoO3 probed by a magnetic tunnel junction with (La,Sr)MnO3

By using perovskite (La,Sr)MnO3 (LSMO)-based magnetic tunnel junctions with LaAlO3 barrier, we have experimentally revealed the spin state of (La,Sr)CoO3 (LSCO) that was employed as the counterelectrode of the junctions. Inverse tunnel magnetoresistance (TMR) up to −14% was observed at 10K in low-bias measurement. This indicates that t2g↓ electrons are the majority at the Fermi level in the LSCO band structure, taking into account the half-metallic character of LSMO. The temperature dependence of the interfacial spin polarization of LSCO as deduced from the TMR ratio indicates the reduction of the Curie temperature at the interface by 70K. The TMR ratio exhibits an asymmetric bias dependence in qualitative agreement with the calculated spin band structure of LSCO.

Spin response in organic spin valves based onLa2∕3Sr1∕3MnO3electrodes

We fabricated spin-valve devices made of organic semiconductor thin films sandwiched between ferromagnetic half-metal ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) and cobalt electrodes, using three different organic molecules. Subsequently, we studied the spin injection and transport properties by measuring the device magnetoresistance (MR) response at various biasing voltages $V$ and temperatures $T$. We found that the spin-valve MR response in all devices monotonically decreases with $V$ and is asymmetric with respect to the voltage polarity. We also found a steep MR decrease with $T$, where it vanishes at $T\ensuremath{\sim}220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, similar to other MR responses in inorganic tunneling junction devices based on LSMO and Co ferromagnetic electrodes. In contrast, the spin-$\frac{1}{2}$ photoluminescence detected magnetic resonance of the organic interlayer, which directly depends on the spin-lattice relaxation rate of polarons in the organic semiconductor, was found to be temperature independent. We thus conclude that the steep MR dependence on $T$ is due to the temperature dependence of the interfacial spin polarization of the LSMO electrode, which also drastically decreases up to $T\ensuremath{\sim}220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We thus conclude that (i) the spin-lattice relaxation time in organic semiconductors should not be the limiting factor in fabricating room temperature organic spin valves, and (ii) in order to achieve room temperature spin-valve operation with substantial MR value, spin-injection electrodes other than LSMO need to be involved, having large and less temperature dependent spin polarization.