Pr Patrick LeClair

Spin-dependent tunnelling in magnetic tunnel junctions

The phenomenon of electron tunnelling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT) in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunnelling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random-access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. In this review article we present an overview of this field of research. We discuss various factors that control the spin polarization and magnetoresistance in MTJs. Starting from early experiments on SDT and their interpretation, we consider thereafter recent experiments and models which highlight the role of the electronic structure of the ferromagnets, the insulating layer, and the ferromagnet/insulator interfaces. We also discuss the role of disorder in the barrier and in the ferromagnetic electrodes and their influence on TMR.

Geometrically enhanced magnetoresistance in ferromagnet–insulator–ferromagnet tunnel junctions

Ferromagnetic–insulator–ferromagnetic trilayer tunnel junctions show magnetoresistance (JMR) effects of about 14% or greater at room temperature. Much larger values of the JMR (100% or more) are observed when the actual tunneling resistance (RT) is comparable to electrode film resistance (RL) over the junction area. This latter apparent JMR is an artifact of the nonuniform current flow over the junction in the cross geometry of the electrodes. The ferromagnetic films were CoFe and Co or Ni0.8Fe0.2, and the tunnel barrier was AlN or Al2O3. These junctions show nonvolatile memory effects. The geometrically enhanced large JMR in junctions can be effectively used as magnetic sensors and memory elements.

Large magnetoresistance using hybrid spin filter devices

A magnetic “spin filter” tunnel barrier, sandwiched between a nonmagnetic metal and a magnetic metal, is used to create a magnetoresistive tunnel device, somewhat analogous to an optical polarizer-analyzer configuration. The resistance of these trilayer structures depends on the relative magnetization orientation of the spin filter and the ferromagnetic electrode. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%.

Band Structure and Density of States Effects in Co-Based Magnetic Tunnel Junctions

Utilizing Co/Al(2)O(3)/Co magnetic tunnel junctions with Co electrodes of different crystalline phases, a clear relationship between electrode crystal structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (polyphase and polydirectional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated band structures (density of states) of fcc-Co and hcp-Co.

Titanium nitride thin films obtained by a modified physical vapor deposition process

The Enhanced Activated Reactive Evaporation (EARE) and Electron Shower (ES) methods are two closely related hybrid physical vapor deposition processes which have proven extremely flexible for thin film growth. Activation of metal vapor and gas phases greatly enhances component reactivity and allows synthesis of a variety of compounds with low formation energies. The present work focuses on modifications to the ES and EARE processes to further enhance reactivity and increase flexibility for growing titanium nitride thin films with improved optical properties and greater adhesion. Preparing films that are highly adhesive to glass with good optical and mechanical properties has proven difficult in previous studies. Optical reflection, X-ray diffraction, atomic force microscopy, electrical resistivity, and corrosion resistance measurements were performed. Highly adhesive films on glass with infrared reflection as high as 70% were produced in this study.

Spin-injection device based on EuS magnetic tunnel barriers

We propose a spin-valve device consisting of a nonmagnetic semiconductor quantum well, sandwiched between ferromagnetic semiconductor layers that act as barriers. The total conductance through such a trilayer depends on the relative magnetization of the two ferromagnetic-barrier layers which act as “spin filters.” With respect to practical realization, EuS/PbS heterostructures may be a suitable candidate. The magnetoresistance should exceed 100% for a wide range of the thicknesses of both the quantum well and the ferromagnetic barriers. From a fundamental physics point of view, the device may not only give insight into the spin lifetimes of the nonmagnetic layer, but the strong spin accumulation taking place in the quantum well may lead to novel optical and nuclear magnetic resonance properties.

Optical and in situ characterization of plasma oxidized Al for magnetic tunnel junctions

An optical polarization modulation technique was adapted to provide a simple, fast, and flexible method for studying the kinetics and growth characteristics of thin oxide layers, using Al2O3 as an example. The optical technique allows precise determination of the amount of remaining metallic Al as a function of the initial Al thickness, while scanning a laser spot across the wedge. Optical data suggest that the oxide growth rate for the ultrathin layers may be dependent on the specific microstructure. In situ x-ray photoelectron spectroscopy performed on homogenous samples confirmed the interpretation of the optical results.

Tunnel conductance as a probe of spin polarization decay in Cu dusted Co/Al2O3/Co tunnel junctions

Tunneling magnetoresistance (TMR), dynamic resistance and bias dependence measurements were performed on Co/Al2O3/Co magnetic tunnel junctions with a thin Cu layer inserted at either the Co/Al2O3 (“bottom”) or Al2O3/Co (“top”) interfaces. Careful comparative analysis allows detailed growth characteristics to be elucidated, as well as providing information on the underlying mechanisms behind spin polarized transport in these structures. Conductance for top dusted junctions is indicative of parallel Co/Al2O3/Co and Co/Al2O3/Cu junctions, consistent with three-dimensional growth of Co and Cu on Al2O3, while conductance for bottom dusted junctions show novel behavior dissimilar to either type of junction. The bias dependence of the TMR, surprisingly, is unaffected by either type of dusting.

Ferromagnetic‐ferromagnetic tunneling and the spin filter effect

Tunneling characteristics of a ferromagnetic‐antiferromagnetic‐ferromagnetic (FM‐AFM‐FM) thin film tunnel junction were studied in high magnetic fields with a view to investigate magnetic coupling by the tunneling process. Gd2O3, a stable oxide which undergoes antiferromagnetic ordering below about 3.9 K, was chosen as the tunnel barrier between the ferromagnetic electrodes Gd and permalloy. Tunnel characteristics showed as much as 32% decrease in junction resistance in an applied field of 20 T, below 4.2 K. The resistance behavior as a function of H can be explained by two different effects: firstly, the change in tunnel conductance due to change in the relative magnetization of the two FM electrodes in low H; secondly, the spin filter effect in high fields, due to the exchange splitting of the Gd2O3 conduction band.

Inherent temperature effects in magnetic tunnel junctions

Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR.