Shwetha G. Bhat

Room temperature electrical spin injection into GaAs by an oxide spin injector

Spin injection, manipulation and detection are the integral parts of spintronics devices and have attracted tremendous attention in the last decade. It is necessary to judiciously choose the right combination of materials to have compatibility with the existing semiconductor technology. Conventional metallic magnets were the first choice for injecting spins into semiconductors in the past. So far there is no success in using a magnetic oxide material for spin injection, which is very important for the development of oxide based spintronics devices. Here we demonstrate the electrical spin injection from an oxide magnetic material Fe3O4, into GaAs with the help of tunnel barrier MgO at room temperature using 3-terminal Hanle measurement technique. A spin relaxation time τ ~ 0.9 ns for n-GaAs at 300 K is observed along with expected temperature dependence of τ. Spin injection using Fe3O4/MgO system is further established by injecting spins into p-GaAs and a τ of ~0.32 ns is obtained at 300 K. Enhancement of spin injection efficiency is seen with barrier thickness. In the field of spin injection and detection, our work using an oxide magnetic material establishes a good platform for the development of room temperature oxide based spintronics devices.

Demonstration of efficient spin injection and detection in various systems using Fe3O4 based spin injectors

Half-metal based spin injector devices for spin injection and detection application have proven to be efficient owing to their enhanced injection and detection efficiency. In this study, we extend the all-electrical spin injection and detection studies into different systems like Si and GaAs using half-metal Fe3O4 as a spin injector in the presence and absence of tunnel barrier MgO. Injection into GaAs is verified using conventional Fe/MgO/GaAs devices. Room temperature spin injection into both p-type and n-type Si is achieved and the spin injection could be observed down to 100K. Obtained spin relaxation time for these n-type and p-type Si at different temperatures agree well with the existing reports. Further, the temperature dependent spin injection and detection is also successfully achieved in Fe3O4/GaAs (n-type) Schottkydevices, and a comparison study of the results with control experiment using Fe/MgO/GaAs (n-type) devices confirm the relaxation to be similar in the GaAs substrate, as expected. Hence, even Fe3O4material can be effectively used as an efficient spin injector as well as detector, making it an attractive candidate for the room temperaturespintronics device applications.

Investigation on the origin of exchange bias in epitaxial, oriented and polycrystalline Fe3O4 thin films

We observe exchange bias (EB) in a single magnetic film Fe3O4 at temperature T < 200 K. Irrespective of crystallographic orientations of grown Fe3O4; they exhibit similar nature of EB for (100) epitaxial, (111) oriented and polycrystalline Fe3O4 thin films. Growth induced defects such as anti-phase boundaries (APBs) in epitaxial Fe3O4 thin film is known to have an influence on the magnetic interaction. But, it is noticed that according to the common consensus of APBs alone cannot explain the origin of EB. If majority of APBs end up with mainly anti-ferromagnetic interactions across these boundaries together with the internal ordering modification in Fe3O4, then EB can emerge at low temperatures. Hence, we propose the idea of directional anti-ferromagnetic APB induced EB in Fe3O4 triggered by internal ordering for T ≤ 200 K. Similar arguments are extended to (111) oriented as well as polycrystalline Fe3O4 films where the grain boundaries can impart same consequence as that of APBs.

Structural and magnetic properties of polycrystalline La2NiMnO6 thin films

Polycrystalline thin films of La2NiMnO6 (LNMO) are deposited successfully on SiO2/Si substrates using pulsed laser deposition technique. Structural characterization using X-ray diffraction confirms the formation of a single phase with P21/n space group. Cross-sectional FE-SEM shows the film thickness ∼195 nm. The deposition temperature and the oxygen pressure played a crucial role determining the crystallization behavior and the magnetic transition temperatures. The ferromagnetic transition temperature is achieved to be ∼277 K by optimizing the deposition conditions.

Tuning the growth of IrO2 on SrTiO3 (100) for spin-hall effect based oxide devices

In this study, growth of (100) oriented IrO2 using pulsed laser ablation technique is explored on (100) oriented SrTiO3 by varying different deposition conditions. We have observed that the texture formation of IrO2 can be tuned by controlling the deposition temperature, oxygen pressure and the repetition rate of the deposition using excimer laser. The best quality of the textured IrO2 thin films were evident from the temperature dependent resistivity exhibiting a typical metallic behavior, with a lowest ρ of ∼ 60 μΩ.cm at 300 K against a ρbulk of ∼ 34 μΩ.cm. Subsequently, the island growth of IrO2 was confirmed from the thickness variation of IrO2 thin films which showed the cross-over of metallic to insulating behavior for thin films of thickness; such as 25 nm.

Controlled tuning of thin film deposition of IrO2 on Si using pulsed laser ablation technique

We have successfully grown a stable phase of polycrystalline IrO2 on Si (100) substrate. We have found that the phase of IrO2 can be controllably tuned to obtain either Ir or IrO2, using pulsed laser ablation technique. O-2, conditions during the deposition influences the phase directly and drastically whereas annealing conditions do not show any variation in the phase of thin film. X-ray diffraction and X-ray photoetnission experiments confirm both Ir and IrO2 can be successively grown on Si using IrO2, target. Also, the morphology is found to be influenced by the O-2 conditions.