Spin Filter Tunnel Research Articles

The impact of barrier height distributions in tunnel junctions

We demonstrate that including continuous and discrete tunnel barrier height distributions in otherwise traditional tunneling formalisms enables straightforward modeling of several phenomena important to tunneling. Random barrier height inhomogeneities significantly impact the tunneling conductance, as evidenced by ideal tunneling models extracting faulty barrier parameters, with the incurred errors strongly dependent on the variance. Thermal smearing is addressed by transferring the energy distribution from the electrons to the barrier potential energy, thereby enabling zero-temperature tunneling models to model temperature dependent tunneling. For discrete tunneling channels, a secondary, impuritylike channel is shown to dominate the net conductance at surprisingly low impurity levels, implying that the observation of intrinsically large barrier heights is highly unlikely with transport measurements. Finally, spin-filter tunneling is modeled with independent tunneling channels whose barrier heights are linked to a temperature-dependent exchange splitting.

The critical role of the barrier thickness in spin filter tunneling

Spin filter tunneling is considered in the low bias limit as functions of the temperature dependent barrier parameters. We demonstrate the generation of spin polarized tunneling currents in relation to the magnetic order parameter, and discuss how an interfacially suppressed order parameter leads to a temperature dependent tunneling current asymmetry. Analyzing the full parameter space reveals that the often overlooked barrier thickness plays a critical role in spin filter tunneling. With all else fixed, thicker barriers yield higher spin polarization, and allow a given polarization to be achieved at higher temperatures. This insight may open the door for new materials to serve as spin filter barriers.

Multiterminal semiconductor/ferromagnet probes for spin-filter scanning tunneling microscopy

We describe the fabrication of multiterminal semiconductor/ferromagnet probes for a new technique to study magnetic nanostructures: spin-filter scanning tunneling microscopy. We describe the principle of the technique, which is based on spin-polarized tunneling and subsequent analysis of the spin polarization using spin-dependent transmission in the probe tip. We report the fabrication of the probes having a submicron semiconductor/ferromagnet heterostructure at the end of the tip.

Magnetoresistance in Double Spin Filter Tunnel Junctions with Nonmagnetic Electrodes and its Unconventional Bias Dependence

Spin filtering happens due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetic barrier and can result in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to large magnetoresistance without the necessity of magnetic electrodes. We demonstrate the realization of such unconventional tunnel junctions using double EuS spin filter barriers with Al electrodes. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations.

Double or nothing for spin filters

Using a double spin-filter tunnel junction consisting of two magnetic insulating layers, researchers have observed a sizeable magnetoresistance without using magnetic electrodes, thus tuning the tunneling via the magnetic state of the insulating layers and by application of an electric voltage.

Electronic transport properties of the multilayer structure double spin-filter tunnel junction

Based on the transfer matrix method and the quantum coherent transport theory of Mireles and Kirczenow, the effects of the Rashba spin-orbit coupling and the spin-filter on the electronic transport properties in the NM/FS1/I/FS2/NM（NM represents the nonmagnetic metal layer, I represents the nonmagnetic insulator layer, and FS represents the magnetic semiconductor layer）double spin-filter tunnel junction are investigated. The influence of thickness of insulator layer and magnetic semiconductor layer on the tunnel magnetoresistance （TMR） and conductance are studied for different Rashba spin-orbit coupling strength and the different angle θ between the two magnetic moments of the left and right magnetic semiconductor layer. The results indicate that: in the presence of the spin-filter effect and the Rashba spin-orbit coupling interaction in the magnetic semiconductor layer, large TMR can be obtained in this double spin-filter junction. With the strength of Rashba spin-orbit coupling increasing, the tunnel magnetoresistance and conductance exhibit rapidly oscillating behavior and the oscillation period decreases gradually.

Determining Exchange Splitting in a Magnetic Semiconductor by Spin-Filter Tunneling

A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].

Spin Transport in Spin Filtering Magnetic Tunneling Junctions

Taking into account spin-orbit coupling and s-d interaction, we investigate spin transport properties of the magnetic tunneling junctions with spin filtering barrier using Landauer-Büttiker formalism implemented with the recursive algorithm to calculate the real-space Green function. We predict completely different bias dependence of negative tunnel magnetoresistance (TMR) between the systems composed of nonmagnetic electrode (NM)/ferromagnetic barrier (FB)/ferromagnet (FM) and NM/FB/FM/NM spin filtering tunnel junctions (SFTJs). Analyses of the results provide us possible ways of designing the systems which modulate the TMR in the negative magnetoresistance regime.

Enhanced Magnetotransport at High Bias in Quasimagnetic Tunnel Junctions with EuS Spin-Filter Barriers

In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.

Spin-filter tunneling magnetoresistance in a magnetic tunnel junction

A systematic study is carried out on the spin-filter (SF) tunneling magnetoresistance (TMR) occurring in a ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal (FM/FI/FM) tunnel junction. The theoretical investigation gives a unified and compact description on the SF and TMR effects in this structure, and qualitatively explains the relevant experiments in this area. Specifically, due to the strong SF effect, the TMR can be separately controlled by the extended Slonczewski's polarization factors, leading to both the barrier-height and bias-voltage induced sign-change behavior. It is also proved that this structure can provide a positively or negatively large and stable TMR, which does not vary appreciably with increasing the bias. These features are very prominent compared with an FM/I/FM conventional magnetic tunnel junction and are believed to be of practical use in designing spintronic devices.

Spin-filter magnetoresistance in magnetic barrier junctions

The tunnel current and magnetoresistance (TMR) are investigated in magnetic tunnel junctions consisting of a spin-filter tunnel barrier, sandwiched between a ferromagnetic (FM) electrode and a nonmagnetic electrode. The investigations are based on the transfer matrix method and the free-electron approximation. The numerical results show that the spin transport depends on the relative magnetization orientation of the FM electrode and the spin-filter barrier, such that the tunnel current reaches its maximum when the magnetic moments of the FM electrode and the magnetic barrier are parallel. It is also found that the TMR increases with increasing the applied voltage.

Magnetoresistive double spin filter tunnel junction

We propose a magnetoresistive tunnel device that takes advantage of the spin filter effect. Two magnetic tunnel barriers are contacted by normal metal electrodes. The resistance of the device is lower (higher) when the magnetic moments of the two barriers are parallel (antiparallel). We present a theoretical calculation of the magnetoresistance. This device has the potential to work above room temperature, in very small fields, and to give a sensitivity orders of magnitude larger than what is possible with standard magnetic tunnel junctions.