ELECtrical Tuning of the antifeRromagnetic ordering of nAnostructures – ELECTRA

The magnetic properties of nanostructures have been intensely investigated in the last few years since it offers the opportunity to unfold new physical phenomena and design novel devices and applications all at once. An example of such simultaneous progress of fundamental understanding and practical developments can be found in the recent trend consisting in the electrical manipulation of magnetic properties. This opens the way to the design of spintronics devices in which the application of some magnetic field is no longer necessary. Up to now, the research on this topic has essentially focused on manipulating the magnetization of ferromagnetic nanostructures, yet some recent theoretical results suggest that it is also possible to control the magnetic ordering in antiferromagnets (AF) with an electric field or a current, in a more efficient way than for ferromagnets. Antiferromagnets would then play an active role, and not merely act as complementary layers in complex stacking as they do in present devices.
The aim of the ELECTR-AF project is to explore the physical mechanisms underlying the electrical control of AF ordering. To unravel the intrinsic phenomena, we choose to focus on model systems. We will focus on heterostructures build around chromium epitaxial thin films, since the AF ordering of bulk Cr is both well known and easy to manipulate. Indeed, high quality chromium samples exhibit a spin density wave (SDW) ordering, the period of the modulated structure being incommensurate with the crystalline lattice. These model AF layers will be included in model heterostructures: we will grow epitaxial bcc metal/MgO/bcc metal trilayers (Cr being either the top or bottom metallic layer). This class of system has played a crucial role in the detailed understanding of spin-dependent tunnelling, and we will thus be able to build on the accumulated knowledge to explore the physics of spin polarized transport in antiferromagnets.
We will first carry out thorough studies of the magnetic properties of Cr thin films and of the Cr/MgO interface, in order to obtain a detailed knowledge of our system. We will follow two distinct strategies to manipulate the magnetic ordering of Cr layers:
- we will apply a voltage across an MgO layer in order to accumulate charges at the Cr/MgO interface. Given the large sensitivity of Cr to doping, we expect to modify the SDW period.
- we will flow a spin polarized current through a Cr layer. We expect to observe spin transfer torque effects, and thus induce switching or precession of Cr ordering parameter.
To observe the evolution of Cr magnetic ordering with the external perturbation, we will combine diffraction and magnetotransport measurements. One challenge of this project is to obtain information on the elusive magnetic ordering of Cr. Neutron diffraction is the ideal tool to do so, since it gave direct access to the properties of the SDW (direction of propagation, period, polarization). This project will give us the impetus to push the limits of the technique. We will also use synchrotron-based techniques and benefit from the latest developments in terms of electronic microscopy. The experimental aspects of this project are thus highly ambitious, but we are plainly confident these challenging experiments can be done, in the light of feasibility tests we have run and recent developments in the different techniques.