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Tuesday, 30 July 2013

Measuring spin-orbit torques in ferromagnetic heterostructures

Reporting in Nature Nanotechnology, team from ICN2, SPINTEC, FZJ, ICREA and ETH Zurich describes measurement of two different spin-orbit torques in heavy metal/ferromagnetic layers

The researchers measured the amplitude and symmetry of the spin-orbit torques induced by the injection of an electric current in ultrathin ferromagnetic films. Such torques can replace an external magnetic field to change the orientation of small magnets.

A team of researchers led by ICREA Professor Pietro Gambardella, of ICN2 and ETH Zurich, has just published an article in Nature Nanotechnology in which they report measuring the amplitude and direction of spin-orbit torques as a function of magnetisation direction in heavy metal/ferromagnetic trilayers. The article, "Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures", was co-authored by Prof Gambardella's colleagues at ICN2, SPINTEC (Grenoble, France) and the Peter Grünberg Institute (Jülich, Germany). Their findings have important implications for the design of new spintronics devices.In spintronics devices, such as magnetic random access memories (MRAMs), data are carried, stored and processed by manipulating the spin of electrons. This entails generating, maintaining, transferring, and reversing (switching) spins using electric currents, which can then influence the magnetisation state of magnetic materials, or vice versa.As standard electric currents do not have any net electron spin (because they are comprised by even amounts of spin-up and spin-down electrons), they must be spin-polarised—that is, forced to have a net spin. This is typically achieved by passing the current through a sufficiently strong magnet, which causes all the electrons in the current to spin in the same direction.Net spins can also accumulate through alternative mechanisms (the spin Hall effect, the Rashba effect, etc.), which rely on the interplay between electron spin and orbital motion, which in turn generates a torque (a spin-orbit torque) that influences that magnetisation state of magnetic components.Thus, researchers are endeavouring to understand the mechanisms behind the generation and control of net spins, and the interplay between electron spin and magnetisation state, in order to develop advanced spintronics devices.Professor Gambardella and colleagues sought to characterise the spin-orbit torques in heavy metal/ferromagnetic/oxide trilayers (AlOx/Co/Pt and MgO/CoFeB/Ta). Such systems are of interest because they represent the storage layer of an MRAM. Firstly, they devised mathematical expressions to quantify the respective contributions of different types of spin-orbit torque in these systems. They then devised a method for three-dimensional vector measurement of spin-orbit torques. Lastly, they demonstrated the action of two distinct spin-orbit torques in these systems, whose magnitudes are highly dependent on interfacial interactions and therefore, on the material growth and annealing processes.The team's results are important, as they underscore the potential of spin-orbit torques to perform different functions —namely switching of nanomagnets or inducing high-frequency oscillations of the magnetisation, with a notable gain in terms of miniaturisation and power consumption relative to external magnetic fields.

To read the article “Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures”, click here.