Tiffany S. Santos

The phenomena of spin-filter tunnelling

The spin filtering phenomenon allows one to obtain highly spin-polarized charge carriers generated from nonmagnetic electrodes using magnetic tunnel barriers. The exponential dependence of tunnel current on the tunnel barrier height is operative here. The magnetic, semiconducting europium chalcogenide compounds have strikingly demonstrated this effect. The possibility of employing ferrites and other methods opens the potential for display of this phenomenon at room temperature, which can be expected to lead to huge progress in spin injection and detection in semiconductors. But first, extremely challenging material-related issues have to be addressed. This review covers the field.

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.

Frontiers in spin-polarized tunneling

Thanks to recent advances in materials research, magnetic tunnel junctions that control the flow of polarized electrons are poised to revolutionize information technology.

Effects of grain microstructure on magnetic properties in FePtAg-C media for heat assisted magnetic recording

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orient...

Observation of the triplet exciton in EuS-coated single-walled nanotubes.

Photon absorption by carbon nanotubes creates bound electron-hole pairs called excitons, which can exist in spin-polarized triplet or spin-unpolarized singlet configurations. Triplet excitons are optically inactive owing to the weak spin-orbit coupling in nanotubes. This prevents the optical injection of electron spin into nanotubes for spintronic applications and limits the efficiency of photocurrent generation. Here, we show that it is possible to optically excite the triplet exciton by using a ferromagnetic semiconductor as a spin filter to mix the singlet and triplet excitons. The triplet contribution to the photocurrent is detected, representing the first direct evidence of the triplet exciton in carbon nanotubes.

Controlled synthesis and characterization of Ag2S films with varied microstructures and its role as asymmetric barrier layer in trilayer junctions with dissimilar electrodes

In this study, we examine the possibility of electrode-barrier interactions in modifying the electrical characteristics and current switching behavior of a trilayer junction with silver sulfide as the barrier layer. A series of Al–Ag2S–Ag crossbar junction is fabricated by thermal evaporation technique varying the thickness (30–110A) of the sulfide layer. Current-voltage characteristics of the junctions are studied as a function of barrier layer thickness, which can suggest any role that electrode-barrier interaction may have in tuning their electrical behavior. To fully understand the performance of the barrier layer, structure and physical properties of Ag2S films are independently investigated. The microstructure of Ag2S films strongly depends on the deposition conditions that, in turn, affect their electrical and optical characteristics. The polarization of the lattice prevalent in Ag2S is shown to affect the charge carrier conduction in their films and dominates their electrical behavior and that of ...

Effect of electrode and EuO thickness on EuO-electrode interface in tunneling spin filter

The effect of electrode material and EuO film thickness on the interface between the two was studied. Of the electrodes examined, yttrium was found to decrease the formation of the nonmagnetic oxide Eu2O3. By decomposing the x-ray absorption spectroscopy (XAS) spectra of the samples with different electrodes against the reference EuO and Eu2O3 spectra, the relative fraction of these two species was quantified. Multilayers with silver electrodes had the highest amount of Eu2O3, about 41%, whereas aluminum had 28% compared to the less than 5% observed for the multilayers with yttrium electrodes. A slight decrease in the fraction of Eu2O3 with increasing EuO thickness was found. Angle dependent XAS measurements, done on 80A EuO film, indicated the presence of Eu2O3 at high grazing angles which then decreased drastically with decreasing grazing angle. This indicated that the Eu2O3 was localized at the EuO-electrode interface.

Relationship between tunnel magnetoresistance and magnetic layer structure in EuO-based tunnel junctions investigated using polarized neutron reflectivity

This paper presents a study of the depth-dependent magnetic structure of a EuO magnetic tunnel junction having a Gd electrode, Si∕Cu∕EuO∕Gd∕Al. Related samples that are patterned exhibit large tunneling magnetoresistance as high as 280%. Though Gd has a much higher coercivity than EuO in bulk, magnetometry reveals no “steps” in the hysteresis loop as expected for a true antiparallel alignment of the EuO and Gd layer magnetizations. Using polarized neutron reflectometry to measure the structural and field-dependent magnetic depth profile at 5K, we determine that the Gd and EuO layers have similar coercivities and that the Gd layer exhibits an anomalously small magnetization at all fields. Polarized neutron reflectometry results also suggest that the chemical density of the Gd layer is not that of bulk Gd. The differences of the structural and magnetic behavior of the Gd layer relative to bulk may be the key in optimizing the tunnel magnetoresistance in these samples.

Photoexcitation of the triplet exciton in single wall carbon nanotubes

The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), each with two-fold degeneracy. Because the triplet excitons are spin polarized with electron and hole spins both pointing in the same direction, they are optically inactive, and optical spectroscopy has revealed no evidence for their existence. Here, we show that by the interaction with a spin filter ferromagnetic semiconductor, photoexcitation of the carbon nanotube triplet exciton is possible, and its contribution to the photocurrent can be detected. The perturbation provided by the spin filter allows for inter-system mixing between the singlet and triplet excitonic states, and relaxes the spin selection rules. This supplies the first evidence for the existence of the triplet exciton, and provides an avenue for the optical excitation of spin polarized carriers in carbon nanotubes.