Spin polarized currents in magnetic and superconducting structures

An electrical current with high spin polarization is desirable for the performance of novel spintronics devices, such as magnetic tunnel junction and giant magnetoresistance devices. The generation of spin polarized current can be from ferromagnetic materials or triplet superconductors.

Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.

Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.

Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.

Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.

Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.

Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.