Spin-tuned Conductivity in Antiferromagnetic Semiconductors

During this project I will investigate the structural and magnetotransport properties of semiconductor antiferromagnetic thin films. The ultimate goal is the demonstration of the spin-controlled conductivity of a semiconductor device near room temperature. I will join the groups of Vclav Hol (Charles University in Prague) and and Tomš Jungwirth (Institute of Physics ASCR in Prague) to pursue this goal. So far spintronics applications are mostly based on metallic ferromagnets. For example hard-disk read heads based on the tunneling magnetoresistance effect are using CoFe layers in a spin-valve structure whose electrical resistance depends on the relative orientation of the magnetic moments in the CoFe layers. A fundamentally different approach is to use antiferromagnets as the active component in spintronics devices. Devices based on antiferromagnets offer several advantages since they are not sensitive to weak external fields and do not produce stray fields due to their compensated magnetic moments. Just recently the groups of the project hosts have proven a spin-valve like signal in a tunnel junction based on an metallic antiferromagnet at T=4 K (Nat. Mater. 10 (2011) 347). We would like to extend this concept and investigate the vertical transport in metal/MnTe/ferromagnet stacks with the goal to to find a resistance depending on the applied magnetic field direction near room- temperature. We chose to investigate hexagonal MnTe since it is a II-VI semiconductor formed from Mn- ions with a large magnetic moment and critical temperature of 310 K above room-temperature. The group of Gunther Springholz at the University of Linz/Austria has grown such hexagonal MnTe layers using molecular beam epitaxy and will provide them for the purpose of this project. We will perform detailed analysis of the structural properties using x-ray diffraction and reflectometry to optimize the crystalline and surface properties. In a first step we will investigate the transport properties of bare MnTe layers in dependence of magnetic field and extend our studies towards exchange coupling at the interface of Fe and MnTe and the magnetoresistance in metal/MnTe/Fe devices. In a return phase to the Institute of Semiconductor and Solid State Physics at the University Linz I would like to investigate how the above devices depend on strain induced either via growth on different substrate or in- situ by strain tuning using an piezoelectric actuator driven by electrical voltage.

Antiferromagnets are usually assumed to be not manipulatable by magnetic fields and therefore it is also intrinsically hard to detect their magnetic state. In this project we showed both: The manipulation of a compensated antiferromagnet by a magnetic field when cooling through its magnetic ordering temperature and how to detect its state by simple electrical measurements. Furthermore we have shown that the state set by the mentioned cooling procedure is stable once the antiferromagnetic order is established. This means that these states can be used in magnetic memories. In contrast to common magnetic memories, which are only bistable, the antiferromagnetic state was found to possess multiple stable configurations. In agreement with the generally great rigidity of antiferromagnets to magnetic fields, the states possess significant stability once a temperature below the antiferromagnetic order temperature is maintained. All this was realized in the hexagonal antiferromagnetic MnTe grown in form of thin films by molecular beam epitaxy on common III-V material InP. Furthermore is MnTe a semiconductor whose electrical properties can be tuned by doping. Our results are also appealing for applications, since the multiple stable states are a route to increase the storage density of magnetic memories. So one storage unit could in future store more than only the states 1 or 0 but in fact combine multiple bits into one fundamental unit. Our memory states are detected by straightforward electrical resistance measurements. Due to the anisotropic magnetoresistance the electrical resistance depends on the relative orientation of the current direction with respect to the magnetic moments. By our thermally assisted setting of the states we change the average orientation of the magnetic moments and therefore influence the resistance of the material. Finally by using strong magnetic fields the anisotropic magnetoresistance effect in antiferromagnets was shown to possess the same symmetry as in ferromagnets.