Magnetoresistance, a resistance induced by a magnetic field, is often associated with magnetic materials. This effect has been put into practice in geophysics, for example, in the production of sensors. Magnetoresistance research has many applications in computing and telecommunications. But it is above all with the discovery of giant magnetoresistance that the most important applications arrived and where an important application can be found in the read heads of computer disks. But recent studies have identified a new effect called unidirectional magnetoresistance (UMR) in non-magnetic materials such as germanium, a material commonly used in microelectronics.

UMR was first observed in 2017 in systems that were not inherently magnetic. Its effect is characterized by an increase or decrease of resistance depending on the intensity and direction of the applied electric current and magnetic field. It is the result of an intrinsic alignment of the spins of the electrons in a direction perpendicular to their momentum.
Germanium (Ge) in which the first transistor was made is commonly used in microelectronics with silicon. In this new study, the researchers of our institute [collaboration], highlight the existence of the UMR in Ge. While this effect has already been observed in two other non-magnetic materials of rather rare and exotic use, with germanium it is 100 times more intense!
To measure UMR in Ge, the researchers grew a layer of germanium in a particular crystal direction on a silicon substrate. In order to study the magnetoresistance of Ge, they passed a current through the germanium layer and applied an external magnetic field. They measured that the resistance was dependent on the current and the field and that the UMR was maximum when the current was perpendicular to the magnetic field.

The researchers propose a new theory to explain their results in which the UMR would find its origin in the two-dimensional electron gas that forms at the surface of the material. The spin of these electrons is aligned perpendicular to their momentum. The phenomenon, which has been satisfactorily modelled in collaboration with the CNRS-Thales Unit, would enable the UMR to be used in spintronic devices such as spin transistors.