Electric-field-assisted switching in magnetic tunnel junctions

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.     

Tunneling current-induced butterfly-shaped domains and magnetization switching in DBMTJs

Double barrier magnetic tunnel junctions (DBMTJs) with the layer architecture of Ta (5 nm)/Cu (20 nm)/Ni/sub 79/Fe/sub 21/(10 nm)/Ir/sub 22/Mn/sub 78/ (12 nm)/Co/sub 75/Fe/sub 25/ (4 nm)/Al (0.9 nm)-oxide/Ni/sub 79/Fe/sub 21/(3 nm)/Al (0.9 nm)-oxide/Co/sub 75/Fe/sub 25/(4 nm)/Ir/sub 22/Mn/sub 78/ (12 nm)/Py(10 nm)/Cu(30 nm)/Ta(5 nm) were mircofabricated. At room temperature, TMR ratio of 18.7% and 28.4%, resistance-area products(RS) of around 10.3 and 12.7 k/spl Omega//spl mu/m/sup 2/ and coercivity H/sub C/ of 17.5 and 2.0 Oe, were obtained for the DBMTJ at the as-deposited state and the after annealing state respectively. The micromagnetics simulations show that the dynamic butterfly-shaped domains and magnetization switching may occur in the free layer when a tunneling current of the order of 1 to 20 mA passes though the DBMTJ. It decreases the magnetization in the free layer, which may be the one of the reasons of the low TMR ratio observed in the DBMTJ.     

Seebeck effect in magnetic tunnel junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K?1 from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K?1. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.     

Nanoscopic processes of current-induced switching in thin tunnel junctions

In magnetic nanostructures, one usually uses a magnetic field to commute between two resistance (R) states. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current, preferably of low intensity. Such current-induced switching (CIS) was recently observed in thin magnetic tunnel junctions and attributed to electromigration of atoms into/out of the insulator. Here, we study the CIS, electrical resistance, and magnetoresistance (MR) of thin MnIr/CoFe/AlO/sub x//CoFe tunnel junctions. The CIS effect at room temperature amounts to 6.9% R-change between the high and low states and is attributed to nanostructural rearrangements of metallic ions in the electrode/barrier interfaces. After switching to the low R-state, some electromigrated ions return to their initial sites through two different energy channels. A low (high) energy barrier of /spl sim/0.13 eV (/spl sim/0.85 eV) was estimated. Ionic electromigration then occurs through two microscopic processes associated with different types of ions sites/defects. Measurements under an external magnetic field showed an additional intermediate R-state due to the simultaneous conjugation of the MR (magnetic) and CIS (structural) effects.     

磁性隧道结中自旋相关的隧穿输运

全面回顾和总结了磁性隧道结中自旋相关的隧穿这一研究领域的理论和实验方面的最新研究进展。讨论了影响磁性隧道结的自旋极化和隧穿磁电阻的各种因素及反映铁磁层和铁磁／绝缘层界面电子结构在隧穿中重要作用的理论模型和近期实验，同时也讨论了绝缘势垒和铁磁／绝缘层界面中的无序性在隧穿过程中对自旋极化与磁电阻效应的影响。     

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.     

Magnetoresistance in magnetic tunnel junctions with amorphous electrodes

Magnetic tunnel junctions (MTJs) with amorphous CoFeB and Co/sub 2/MnSi electrodes were fabricated and examined. In the case of [Co/sub 90/Fe/sub 10/]/sub 100-x/B/sub x/, the x=32% boron addition reduces the magnetization by 30% compared to Co/sub 90/Fe/sub 10/, yet the reduction of the tunnel magnetoresistance (TMR) is over 95%. On the contrary, in the case of Co/sub (100-x-y)/Mn/sub x/Si/sub y/, although net magnetization is very small at room temperature, the TMR can be as large as 7%. The character of each metalloid (boron and silicon) could be responsible for the peculiar behavior to each system.     

Novel magnetic tips developed for the switching magnetization magnetic force microscopy

Using micromagnetic calculations we search for optimal magnetic properties of novel magnetic tips to be used for a Switching Magnetization Magnetic Force Microscopy (SM-MFM), a novel technique based on two-pass scanning with reversed tip magnetization. Within the technique the sum of two scans images local atomic forces and their difference maps the local magnetic forces. The tip magnetization is switched during the scanning by a small magnetic field. The technology of novel low-coercitive magnetic tips is proposed. For best performance the tips must exhibit low magnetic moment, low switching field, and single-domain state at remanence. Such tips are equipped with Permalloy objects of a precise shape that are defined on their tilted sides. We calculate switching fields of such tips by solving the micromagnetic problem to find the optimum shape and dimensions of the Permalloy objects located on the tips. Among them, hexagon was found as the best shape for the tips.     

Ab initio simulation of magnetic tunnel junctions

In this paper, we present the mathematical and implementation details of an ab initio method for calculating spin-polarized quantum transport properties of atomic scale spintronic devices under external bias potential. The method is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Green's function (NEGF) formalism to calculate the self-consistent spin densities. We apply this method to investigate nonlinear and non-equilibrium spin-polarized transport in a Fe/MgO/Fe trilayer structure as a function of external bias voltage.     

Surface morphology of epitaxial magnetic tunnel junctions

The surface morphology of epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions, which show the giant tunneling magnetoresistance effect, was investigated by in situ scanning tunneling microscopy. It was observed that an epitaxial MgO barrier layer forms flat surface structures. The surface was flatter with distinct steps and terraces after annealing, which would lead to an increase of the tunneling magnetoresistance ratio. Examination of the local electronic structures of 1.05-nm-thick MgO barrier layers by scanning tunneling spectroscopy revealed no pinholes in the layers, so they would be perfect barriers in magnetic tunnel junctions.     

Graphene as a tunnel barrier: graphene-based magnetic tunnel junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic.     

Annular Josephson tunnel junctions in an external magnetic field

We have investigated the static and dynamic configurations of the phase inside an annular Josephson tunnel junction in the presence of an externally applied magnetic field. We report here a detailed study of the dependence on the magnetic field of the critical current and of the Fiske singularities. The behaviour is investigated perturbatively and numerically.     

Current-induced torques in magnetic materials

The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.     

Magnetic tunnel junctions for magnetic random access memory applications

Magnetic Tunnel Junctions (MTJ) are in the center of a large number of studies in spin polarized transport, because of their potential use in Magnetic Random Access Memory (MRAM) applications. The most commonly used tunnel junction is composed of an aluminium-oxide as a tunnel barrier and presents very interesting tunnel magnetoresistance (TMR) of about 40%. Nevertheless, this barrier needs an oxidation step and has a large resistance, which is not suitable for electronic devices. Thus a MTJ with an alternative ZnS barrier grown by sputtering on Si(111) substrate at room temperature is presented here with the following structure: Fe_{6 nm}Cu_{30 nm}(CoFe)_{1.8 nm}Ru_{0.8 nm}(CoFe)_{3 nm}ZnS_x(CoFe)_{1 nm}Fe_{4 nm}Cu_{10 nm}Ru_{3 nm}. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. This alternative barrier is most suitable for electronic devices and we will discuss its possible application in MRAM.     

Nanofabrication of magnetic tunnel junctions by using side-edge thin film deposition

Nanostructured double ferromagnetic tunnel junctions (MTJs) are indispensable for investigation of spin-dependent single-electron transport at low temperature. A new fabrication process that enables us to reduce the size of MTJs down to nanometer scale by using the side edge of a patterned film were developed. The multilayers of MTJ partially replaced by thick Al₂O₃/Cu double layer were prepared by using electron beam lithography and lift-off, then Pt film was vacuum-evaporated onto the side edge of Al₂O₃/Cu film, which masked MTJ during following Ar ion milling. As a result, the double MTJs with the dimension of 10 nm £ 10 mm were formed beneath the Pt film. The large tunnel magnetoresistive ratio of 35% and symmetrical I–V characteristics were obtained at room temperature.     

Nanofabrication of 30 nm devices incorporating low resistance magnetic tunnel junctions

In this paper the electron-beam lithography conditions and the nanofabrication process are described for current-perpendicular-to-plane (CPP) pillar devices with 30 nm critical dimensions. This work combines a RAITH-150 tool with a negative e-beam resist (AR-7520) so that dense nanopillar arrays are patterned fast into large area samples. The resist dilution and coating conditions are optimized, aiming at its thickness reduction down to 80 nm. The exposure parameters are tuned for different geometries and dimensions, so that features down to 30 nm are exposed with good accuracy (+/- 1.9 nm) and reproducibility. The complete integration of these nanoelements into CPP devices involved electron beam lithography, ion milling for pattern transfer and chemical-mechanical polishing (CMP). Results on devices incorporating very low resistance-area (R x A) MTJ films deposited by Ion beam assisted deposition are shown, for MTJ stacks with R x A down to 0.8 omega x microm2. Device characterization includes electrical measurement of the pillar resistance and the transfer curves under dc magnetic fields (TMR up to 40%).     

Growth of magnetic tunnel junctions on Si(001) substrates

We present in this work the growth of magnetic tunnel junctions on Si(001) substrates using a template layer technique and the implementation of the layer-by-layer method to form the oxide barrier layer. By using a Co₂Si template layer formed by deposition of Co on Si at a temperature of ∼ 300 °C, we show that it is possible to considerably reduce the reaction between transition metals with Si substrate. We have also investigated the growth of alumina (Al₂O₃) barrier layer by an alternative layer-by-layer deposition method, which consists of successive cycles of molecular-beam deposition of an Al monolayer and oxidation under an O₂ flux at room temperature. Numerous Co(Fe)/AlO_x/NiFe tunnel junctions have been fabricated on Si(001) substrates. The oxidation kinetics, the surface morphology as well as the interface roughness and abruptness are studied by means of Auger profilometry, transmission electron microscopy and atomic force microscopy. We show that it is possible to realize a uniform and homogeneous nanometer-thick AlO_x layer with smooth and sharp interfaces. Current-voltage and Kerr effect measurements are also used to investigate the electric and magnetic properties of these junctions.     

Vortex structures in exponentially shaped Josephson junctions

We report the numerical calculations of the static vortex structure and critical curves in exponentially shaped long Josephson junctions for in-line and overlap geometries. Stability of the static solutions is investigated by checking the sign of the smallest eigenvalue of the associated Sturm-Liouville problem. The change in the junction width leads to the renormalization of the magnetic flux in comparison with the case of a linear one-dimensional model. We study the influence of the model's parameters, and particularly, the shape parameter on the stability of the states of the magnetic flux. We compare the vortex structure and critical curves for the in-line and overlap geometries. Our numerically constructed critical curve of the Josephson junction matches well with the experimental one.     

Structural and tunneling properties of magnetic tunnel junctions with Al-O and MgO barrier

Two types of magnetic tunnel junctions (MTJs) with the configuration: substrate Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 12 nm/CoFe 2.5 nm/Al-O 1.5 nm/NiFe 3 nm/Ta 5 nm and Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 10 nm/CoFeB 3 nm/MgO 2 nm/CoFeB 4 nm/Ta 5 nm were prepared by the sputtering technique with two different buffers: A-Cu 25 nm and B-Ta 5 nm/Cu 25 nm. The B buffer caused a high texture of MTJs whereas in the case of the A buffer junctions texture was weak. Crystallites in the textured layers grew in a columnar like shape that induced interfacial roughness. High textured buffer B caused high interfacial roughness that reduced the resistance-area (RA) product due to a barrier thickness fluctuation. RA also changed substantially depending on the type of a barrier. The highest RA product ∼15 MΩ μm² was achieved for a low textured junction with Al-O barrier whereas in the high textured MgO sample RA product was ∼100 kΩ μm². Tunnel magnetoresistance (TMR) measured at room temperature was about 45% for the samples with Al-O barrier, whereas for the samples with MgO barrier TMR was about three times higher and achieved 140%.