SPINTRONICS & NANOMAGNETICS GROUP

HOME           RESEARCH           FACILITIES           PUBLICATIONS           TEACHING           CONTACTS    

Research Areas

Illustration of the DOS for majority and minority spin states, with various band gaps.

Magnetic moment of V₃Al at  2T showing a very low moment and a peak at the Neel temperature T_N=600 K. (inset) m(H) showing linear in H dependence and temperature independence. Jamer PRB 91, (2015).

Curie temperature for V-doped and Cr-doped Sb₂Te₃ topological insulator. (inset) Extreme quantization of the QAHE.

Making Topological Insulators Magnetic

A goal of research in topological insulators (TI) is to imbue them with magnetism. Doing this in the right way leads to a semiconductor that is nearly superconducting, through the quantum anomalous Hall effect (QAHE). The figure shows extreme quantization of the QAHE conductivity reaching σ_yx = 0.9998 e²/h. The zero-field longitudinal resistance is only ρ_xx = 0.00013 h/e. [C.-Z. Chang Nature Matl  2015]

Although we have observed the QAHE at very low temperatures, our recent paper in Nature [F. Katmis 2016] reports on the discovery of a method to increase the operating temperature of the TI Bi₂Se₃. Instead of doping the TI with magnetic atoms as shown above, a magnetic exchange field is induced in bilayer Bi₂Se₃/EuS. In another study, magnetic exchange was also observed in a similar bilayer structure of the topological "crystalline" insulator SnTe/EuS. [B.A. Assaf  PRB 2015]

 

Gapless, Low-moment and Spin Filter Semiconductors for Spintronics

The efficient production of spin-polarized currents at room temperature is fundamental to the advancement of spintronics. Spin-polarized currents can be generated several ways: by passing an unpolarized current through a ferromagnetic contact, using the spin-Hall effect, by ballistic and hot electron injection, by using spin-polarized materials such as half-metals, or using a spin filter material. Half metals and spin filter (SF) materials are especially suitable for spin injectors that are based on magnetic tunnel junctions (MTJs). However, a spin injector using a spin filter material is simpler, as it does not require magnetic electrodes.

Spin gapless semiconductors (SGS) and spin filter materials (SFM) uniquely merge the properties of diversely gapped semiconductors and high Curie temperature magnetism. This remarkable combination of properties in Heusler compounds could have advantages for spin-transport in electronic/magnetic devices and quantum information processing. New functionalities are poised to take advantage of several novel and valuable properties, especially at room temperature. Some of the unique properties are:

 (1) half-metallic high spin polarization (100 %);

(2) carrier flexibility allowing for generation of spin-polarized holes;

(3) voltage-tunable spin polarization with n-type/p-type switching;

(4) spin-polarized ferrimagnets with zero moment;

(5) spin filters.

A number of these fascinating Heusler materials have been grown in bulk form, but the synthesis of high-quality thin films necessary for devices is difficult.