Ferromagnetic semiconductors are materials that are capable of exhibiting both ferromagnetic and semiconducting behavior. Because of the contradictory crystal structure requirements for these two behaviors, the existence of these materials was not theorized until recently. It was not until the 1960s that the coexistence of semiconducting and magnetic properties was first observed in europium based chalcogenides and chromium based spinels. Unfortunately, both of these materials are natural ferromagnetic semiconductors with low Curie temperatures and poor reproducibility, making them poor candidates for any practical applications.
The main reason for interest in finding better ferromagnetic semiconductors lies in the fact that the spin of these materials can be used to manipulate the response of these materials to electrical and magnetic fields. Ferromagnetic semiconductors also exhibit magnetoresistance (the magnetic properties split the valence band into two bands, as shown above, which can be thought of as forming a parallel circuit) and magneto-optic effects (making the usage of ferromagnetic semiconductors in optical isolators and optical data storage devices feasible).
As a consequence of these properties ferromagnetic semiconductors have wide ranging applications, most notably in the field of spintronics. Many applications are still in the research and development phase but hold great promise. Prototype spin MOSFETs and bipolar transistors have already been developed, where the direction of spin affects the transistor characteristics (antiparallel spin yields high conductance whereas parallel spin yields low conductance). These materials can also be used to manufacture MRAM’s which are three times as dense as currently used RAMs, and have potential applications in quantum computing and neuromorphic computing.
The quest to obtain ferromagnetic semiconductors with Curie temperatures approaching room temperature has been extremely rewarding as the commercial viability of devices based on these materials is constantly increasing, but also because the search for such materials gives key insights into fundamental material behavior.
Written by: @deepesttheoristlady; Edited by this blog.