Magnetic Junctions Research Articles

Temperature and voltage effects in high performance magnetic junctions

AbstractBased on a quantum ballistic description of spindependent electronic transport in perfect magnetic junctions, the corrections are analyzed to the quantum resonances in their magnetoresistance (MR), due to finite temperature and to finite electric field bias. Considering them separately (varying temperature at fixed field or varying field at fixed temperature), we found monotonously decreasing MR with temperature but non‐monotonously with field. The latter behavior suggests new means for effective tuning and control of these devices and the temperature effect sets the criteria for reaching the quantum enhanced performance in real conditions (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of resonant spin-dependent tunneling in an Fe/Ag/MgO/Fe(001) junction

Calculation of the tunneling magnetoresistance (TMR) of an Fe/Ag/MgO/Fe(001) magnetic junction is reported. The magnetoresistance is determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure. It is shown that the calculated TMR oscillates as a function of Ag interlayer thickness between positive values in excess of 2000% and negative values of the order of ?100%. The oscillation period is determined by the spanning vector of the Ag Fermi surface. The large positive TMR and the changes in its sign are due to resonant enhancement of the tunneling conductance of majority-spin carriers in the ferromagnetic configuration and of the conductance of carriers tunneling in the antiferromagnetic configuration from the minority-spin channel in the Fe electrode adjacent to the Ag layer to the majority-spin channel in the other Fe electrode. The resonant enhancement occurs because the Ag interlayer creates potential steps for electrons in both the ferromagnetic and antiferromagnetic configurations of the junction. This mechanism, which results in a very large TMR, is quite different from the mechanism that causes large TMR in the standard Fe/MgO/Fe(001). It offers the possibility of tuning the magnitude and sign of the TMR by the choice of the interlayer thickness. A Lateral supercell method was also used to investigate the effect of interfacial roughness on the resonant tunneling in an Fe/Ag/MgO/Fe(001) junction. It is found that, in contrast to the Fe/MgO/Fe(001) junction whose TMR is reduced drastically by disorder, the junction with a silver interlayer is much less affected by interfacial roughness.

Noise of MgO-based magnetic tunnel junctions

Low-frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. We present here the noise for different degrees of crystallization and CoFeB/MgO interface quality depending on the annealing temperature. For optimized annealing temperature, an extremely low 1/ f noise, compared to magnetic junctions with Al 2O 3 barriers, has been observed. The origin of the low-frequency noise can be explained in terms of localized charge traps with the MgO barriers. Results for very thin CoFeB are presented in the second part as a function of temperature. Despite the absence of coercivity at room temperature for thinner free-layer structures, an important increase of H c appears under 180 K. Meanwhile, the random telegraph noise present at room temperature is suppressed due to magnetic domains freezing. These results are discussed in view of various sensors applications of MgO-MTJ, giving advantages and drawbacks in terms of signal-to-noise ratio with respect to the operating temperature.

Lab-On-Chip Fabrication of Atomic Scale Magnetic Junctions

We have demonstrated a unique and versatile approach for fabrication and measurement of magnetic nanocontacts on a single chip. Integration of microfluidics is shown to be a key advantage, enabling device miniaturisation as well as rapid and convenient changing of plating solutions. Initial tests are presented, showing that contacts made with thick deposits exhibit properties departing from anisotropic magnetoresistance, and mimicking early results of very large magnetoresistance values.

Alternating current driven instability in magnetic junctions

An effect is considered of alternating (high-frequency) current on the spin-valve-typemagnetic junction configuration. The stability with respect to small fluctuationsis investigated in the macrospin approximation. When the current frequency isclose to the eigenfrequency (precession frequency) of the free layer, parametricresonance occurs. Both collinear configurations, antiparallel and parallel, can becomeunstable under resonance conditions. The antiparallel configuration can also becomeunstable under non-resonant conditions. The threshold current density amplitude isof the order of the dc current density for switching of the magnetic junction.

Dynamics of exchange switching of magnetic junctions

The switching dynamics of a thin free layer of a spin-valve magnetic junction during domination of the injection mechanism of interaction of the spin-polarized current with the magnetic lattice is studied. It is shown that, in the absence of dissipation, there exists an integral of motion, even if current flows through the junction. The trajectory of the precessing magnetization vector below and above the instability threshold is found. A nonstationary solution describing the precession dynamics is obtained. The time of the junction switching in the limiting cases of weak and strong dissipation is estimated.

MAGNONS TRANSMISSION THROUGH AN ATOMIC WIRE CONNECTING TWO ULTRATHIN HEISENBERG FERROMAGNETS

The magnons transport properties of molecular wires connecting two Heisenberg ferromagnets are studied within the framework of the matching method and with use of a realistic atomic structure. The model system consists of two nanostructured ferromagnetic films on either side of the junction and the atomic wire consists of a linear molecule connecting two ultrathin solid ferromagnetic films. A theoretical model is presented for the study of the transmission and the reflection of spin waves at the atomic wire junction. The calculation was made at the atomic scale for two identical waveguides with ordered spins and coupled by Heisenberg exchange interaction between first neighbors. Our analysis yields a detailed understanding of the spin-wave coherent scattering at the linear molecular junction. We calculate, in particular, the coherent reflection R and transmission T coefficients, which constitute the elements of the scattering matrix in accordance with the Landauer–Büttiker scattering formalism, as well as the magnon transmittance of the atomic wire for spin-waves incident from the interior of the film on the junction. The most representative numerical results obtained for the system of two slabs made up of three Fe ferromagnetic atomic layers connected with an Fe or Gd atomic wire are presented as function of the dimensionless frequency Ω in the magnons energy band. The coherent reflection and transmission scattering cross sections show characteristic spectral features, depending on the length of the wire, on the cut-off frequencies for the propagating magnons, as well as on the magnons incidence angle. The results illustrate the occurrence of Fano resonances in the transmitted spectra due to the interaction of localized spin states on the atomic wire with the propagating spin waves of the waveguide. An interesting physical effect is observed for this magnetic atomic junction, namely the frequency selective conductance of the spin waves via Fano resonances, by an appropriate choice of the spin-wave incident angle.

Spin currents and magnetoresistance of graphene-based magnetic junctions

Using the tight-binding approximation and the nonequilibrium Green’s function approach, we investigate the coherent spin-dependent transport in planar magnetic junctions consisting of two ferromagnetic (FM) electrodes separated by a graphene flake (GF) with zigzag or armchair interfaces. It is found that the electron conduction strongly depends on the geometry of contact between the GF and the FM electrodes. In the case of zigzag interfaces, the junction demonstrates a spin-valve effect with high magnetoresistance (MR) ratios and shows negative differential resistance features for a single spin channel at positive gate voltage. In the case of armchair interfaces, the current-voltage characteristics behave linearly at low bias voltages and hence, both spin channels are in on state with low MR ratios.

Width of the0−πphase transition in diffusive magnetic Josephson junctions

We investigate the Josephson current between two superconductors (S) which are connected through a diffusive magnetic junction with a complex structure $({\text{F}}_{c})$. Using the quantum circuit theory, we obtain the phase diagram of 0 and $\ensuremath{\pi}$ Josephson couplings for ${\text{F}}_{c}$ being an insulator-ferromagnet-insulator (IFI) double barrier junction or an IFNFI structure (where N indicates a normal-metal layer). Compared to a simple SFS structure, we find that the width of the transition, defined by the interval of exchange fields in which a $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition is possible, is increased by insulating barriers at the interfaces and also by the presence of the additional N layer. The widest transition is found for symmetric ${\text{F}}_{c}$ structures. The symmetric SIFNFIS presents the most favorable condition to detect the temperature-induced $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition with a relative width, which is five times larger than that of the corresponding simple SFS structure.

Spin injection-driven switching in a magnetic junction

We investigate a perpendicular electric current passing through a “ferromagnetic nanojunction”, that is through some layered nanosized structure of spin-valve type, containing two ferromagnetic metallic layers. Spacer may be used between the metallic layers to prevent the rotation of the moving spin phases. Such an arrangement is typical for spin valves: one of the metallic layers has strongly pinned magnetic lattice and the other one has free magnetic lattice and free mobile spins. Further the conditions are derived to provide a very high nonequilibrium spin injection level. It appears that the so-called spin resistances of the constitutive layers should be in definite relations to each other. These relations lead to the situation where the spin injection becomes dominant and significantly suppresses the “ordinary” spin-transfer torque. As a result, the threshold current becomes lowered down to 2–3 and even more orders of magnitude.

Coherent Effects for Electronic Equilibrium and Transport in Perfect Magnetic Junctions

The analytical treatment of electron transport processes in spintronics layered nanostructures is used to consider the specifics of electronic distribution function at short distances from the magnetic tunnel junction. It is shown that already in absence of external electrical field, an essential perturbation of the equilibrium Fermi distribution appears for the antiparallel magnetic configuration of electrodes, leading to reduced occupation of one of the spin subbands (minority for negative or majority for positive band dispersion). This effect can locally influence (respectively, enhance or reduce) the polarization of charge carriers and subsequently the magnetoresistance of the device.

Pumped spin and charge currents from applying a microwave field to a quantum dot between two magnetic leads

The evolution-operator approach is applied to studying photon-electron-pumping effects on a quantum dot connected to two magnetic leads in the presence of both via-dot and over-dot tunneling channels. It is found that a microwave field applied to the quantum dot may give rise to charge and spin pumpings at zero-bias voltage for asymmetric magnetic junctions. More interestingly, a pure spin current can be pumped for symmetric magnetic junctions in the antiparallel magnetization configuration, providing an idea for the design of spin batteries.

Optical orientation in bipolar spintronic devices

Optical orientation is a highly efficient tool for the generation of nonequilibrium spin polarization in semiconductors. Combined with spin-polarized transport it offers new functionalities for conventional electronic devices, such as pn junction bipolar diodes or transistors. In nominally nonmagnetic junctions optical orientation can provide a source for spin capacitance—the bias-dependent nonequilibrium spin accumulation—or for spin-polarized current in bipolar spin-polarized solar cells. In magnetic junctions, the nonequilibrium spin polarization generated by spin orientation in the proximity of an equilibrium magnetization gives rise to the spin-voltaic effect (a realization of the Silsbee–Johnson coupling), enabling efficient control of electrical properties such as the I–V characteristics of the junctions by magnetic and optical fields. This paper reviews the main results of investigations of spin-polarized and magnetic pn junctions, from spin capacitance to the spin-voltaic effect.

Coherent transport in high performance double-spacer magnetic junctions

Abstract Spin-resonant tunneling mediated by quantized interlayer states has been studied using the tight-binding model for a composite magnetic junction with a thin non-magnetic metallic spacer, coherently inserted between the tunnel barrier and one of the ferromagnetic electrodes (F/I/N/F). An explicit analytical formula for the quantum transmission coefficient is derived and used for calculation of tunnel magnetoresistance (TMR) ratio in the Landauer formalism. The existence of strong magnetoresistance peaks is shown either at varying the on-site atomic energy and the width (number of atomic planes) of the N-spacer. The strongest TMR enhancement in the shallow band regime suggests a search for new spacer materials to optimize the device performance.

Current-induced magnetization dynamics in single and double layer magnetic nanopillars grown by molecular beam epitaxy

Molecular beam epitaxy is used to fabricate magnetic single and double layer junctions, which are deposited in prefabricated nanostencil masks. For all Co | Cu | Co double layer junctions we observe a stable intermediate resistance state, which can be reached by current starting from the parallel configuration of the respective ferromagnetic layers. The generation of spin waves is investigated at room temperature in the frequency domain by spectrum analysis, demonstrating both in-plane and out-of-plane precessions of the magnetization of the free magnetic layer. Current-induced magnetization dynamics in magnetic single layer junctions of Cu | Cu | Cu has been investigated in magnetic fields, which are applied perpendicular to the magnetic layer. We find a hysteretic switching in the current sweeps with resistance changes significantly larger than the anisotropic magnetoresistance effect.

A TEM structural study of thermal stability of magnetic tunnel junctions integrated with CMOS devices

We present a TEM/STEM study of the structure and thermal stability of Co/AlOx/Si, CoFe(B)/MgO/Si and FeNi/SiOx/Si magnetic junctions deposited directly on patterned Si. In all three types of junctions the films are uniform and continuous. Annealing at 300 at 550 °C does not change their structure. CoFe and CoFeB layers remained amorphous after the annealing process.

Spin-polarized tunneling through randomly transparent magnetic junctions: Reentrant magnetoresistance approaching the Jullière limit

Electron conductance in planar magnetic tunnel junctions with long-range barrier disorder is studied within Glauber-eikonal approximation enabling exact disorder ensemble averaging by means of the Holtsmark--Markov method. This allows us to address a hitherto unexplored regime of the tunneling magnetoresistance effect characterized by the crossover from ${\mathbit{k}}_{\ensuremath{\parallel}}$ conserving to random tunneling (${\mathbit{k}}_{\ensuremath{\parallel}}$ is the in-plane wave vector) as a function of the defect concentration. We demonstrate that such a crossover results in a reentrant magnetoresistance: It goes through a pronounced minimum before reaching disorder- and geometry-independent Julli\`ere's value at high defect concentrations.

Direct Observation of Tunnelling through 100-nm-Wide All Metal Magnetic Junction into Si

Nanoscaled spin-dependent tunnelling lines were patterned on doped Si and studied for tunnelling from the SDT ferromagnetic layer through an insulating barrier into Si. The injection contacts have the form of long strips with width and separation, ranging from 100 nm to 2 μm, and are patterned using e-beam lithography. The measured I-V characteristics versus temperature (80 to 300K) on the 100nm scaled devices between the layered-magnetic metals and the semiconductor clearly showed ballistic tunnelling, with weak dependence on the temperature. This is qualitatively different, at elevated temperatures, from 2-μm-wide scaled-up spin-dependent tunnelling structures, where thermal-ionic emission was observed to dominate carrier transport.

Fabrication of Magnetic Barrier NbN Tunnel Junctions with the Gap Voltage over 5 mV

Superconductor–insulator–superconductor (SIS) tunnel junctions including magnetic barriers, with structures NbN/AlNiN/NbN or NbN/MgO/NiO/MgO/NbN, were fabricated. The junctions with upper NbN layers of good crystallinity showed a gap voltage (Vg) higher than 5 mV. In an NbN/Al0.942Ni0.058N/NbN junction, its Josephson current Ic was suppressed to approximately a third of a theoretical value under no magnetic field. Therefore, it would be possible to avoid Josephson noise in magnetic barrier junctions with a much smaller magnetic field than junctions with conventional barriers. The Ni ion area density dependence of Ic suppression ratio was identical for both types of junction, suggesting that the area density of magnetic ions is the factor that determines the Ic suppression ratio. On the other hand, the extents of degradation of Vg and increase in leakage current caused by the increase in Ni ion area density were smaller in MgO/NiO/MgO barrier junctions than in AlNiN barrier junctions. This suggests that the proximity effect of magnetic ions was weakened owing to the separation of Ni ions and NbN layers by the inserted thin MgO layers.

Real-space pseudopotential calculations of spin-dependent electron transport in magnetic molecular junctions

A real-space pseudopotential approach is developed to calculate the spin-dependent transport in nanoscale junctions. Our method is based on self-consistent solution of the Kohn-Sham equation of density functional theory with asymptotic boundary conditions. This method is applied to a simple magnetic molecule, the Sc dimer, bridging nonmagnetic, planar jellium electrodes for a series of molecule-lead spacing. We find that the spin-dependent conductance within this formalism is rather robust over a wide range of electronic coupling parameters. The minority channel of parallel-aligned ${\mathrm{Sc}}_{2}$ produces a fairly stable conductance of roughly half of a quantum unit $({e}^{2}∕h)$. Other systems show sensitive dependence on the coupling strength. Atomic origins of the dependence are discussed.