Insights from Emerging Leaders: Spin transport in antiferromagnets made simple

By Tom Sharp on March 17, 2017

Journal of Physics: Condensed Matter (JPCM) has invited some of the best early-career researchers in condensed matter physics to contribute to a special issue. Called ‘Emerging Leaders’, this special issue will be part of the Journal of Physics (JPhys) series’ 50th anniversary celebrations in 2017, recognising the talents of exceptional, upcoming researchers. Here we catch up with Aurelien Manchon from King Abdullah University of Science and Technology (KAUST), whose recent Emerging Leaders paper sheds new light on spin transport in antiferromagnets. Read on to find out more from Aurelien himself:

Antiferromagnetic materials are attracting a massive amount of attention in the spintronics community. After the pioneering work of Nunez et al., we know that antiferromagnets can actually be used for spintronics applications, as beautifully demonstrated by Wadley et al. My group at KAUST explores the interplay between spin transport and magnetization dynamics in spintronic devices, magnetic textures, topological materials, and more recently, antiferromagnets and frustrated magnets. We develop phenomenological models and microscopic descriptions with the objective of proposing efficient ways to control and detect magnetic order parameters.

Following the predictions of Nunez et al. and other groups, our first project was to understand spin transport in antiferromagnetic spin-valves using a tight-binding model. Such a minimalistic approach is useful but lacks transparency: you can’t get to the bottom of the physics involved unless you perform a full numerical calculation, which becomes problematic when it comes to considering disorder and incoherent transport. Indeed, disorder is ubiquitous in mesoscale devices and drives the transition from quantum-coherent to diffusive transport regimes. As a result, certain effects observed within tight-binding models are simply nonexistent in real devices.

In the present paper, my intention was to develop a semiclassical theory that would be easy to comprehend, namely a “Valet-Fert theory for antiferromagnets”. Indeed, semiclassical Valet-Fert theory has been extremely successful for modelling effects such as magnetoresistance and spin transfer torque, but is limited to devices composed of ferromagnets and normal metals. I hoped that deriving such equations for antiferromagnets would uncover what is going on in these materials. The questions that I asked myself were: “How is an antiferromagnet different from a normal metal?” ; “What are the essential variables to describe spin transport in antiferromagnet?”; “What is going to be left after disorder averaging?”

The equations I obtained show that in the case of a bipartite antiferromagnet (most complex systems are pretty challenging to treat with this method) a practical way to understand spin transport is to parse the spin density at the level of magnetic unit cell into two contributions: a uniform and a so-called staggered component. The former is the spin density averaged over the magnetic unit cell, while the latter reflects the imbalance of the spin density present on the two sublattices. It turns out that the uniform spin density behaves as if the antiferromagnet were an anisotropic normal metal. On the other hand the staggered spin density, which is crucial to obtain spin torque, can be viewed as a correction of the uniform spin density due to precession around the local moments.

What remains an important question in my view is “how is spin transport modified in a non-collinear antiferromagnet?” These materials are quite exciting because they may display topological transport, giving rise to spin or even anomalous Hall effects. Deriving a handy transport equation in this context could be quite fascinating.

Aurelien Manchon is an Associate Professor of Materials Science and Engineering at the King Abdullah University of Science and Technology, in Saudi Arabia. He joined KAUST in 2009 after a postdoctoral fellowship at University of Missouri-Columbia and University of Arizona-Tuscon. He graduated from the Ecole Polytechnique, Palaiseau, France in 2004 and earned his PhD in Physics in 2007 from University Joseph Fourier and CEA/SPINTEC laboratory in France. Manchon’s research focuses on spintronics, which aims at utilizing the quantum spin degree of freedom of the carrier to generate disruptive solutions for electronics. His research interest spans from spin-orbit coupled transport to chiral magnetism, antiferromagnets and ultrafast spin dynamics. Manchon has engaged in a number of collaborations with both experimentalists and theorists in his field around the world, including researchers at National University of Singapore, CEA/CNRS Grenoble in France, University of Mainz, and Vienna University of Technology. He has authored over 85 publications in esteemed journals including Nature, Nature Materials, Nature Physics, Physical Review Letters, Nano Letters, Physical Review B, and Applied Physics Letters.