Session J1: Invited Session: Buckley Prize Session

Oliver E. Buckley Condensed Matter Prize Lecture: Transfer of spin momentum between magnets: its genesis and prospect

Consider two nanoscopic monodomain magnets connected by a spacer that is composed of a non-magnetic metal or a tunnel barrier. Any externally applied electric current flowing through these three layers contributes tiny pseudo-torques to both magnetic moments ($J. S.$ 1989). Such a weak spin-transfer torque (STT) may counteract and overcome a comparably small torque caused by viscous dissipation (\textit{L. Berger }1996; $J. S.$ 1996). Any initial motion (e. g. excited by ambient temperature) of one moment (or both), may grow in amplitude and culminate in steady precession or a transient switch to a new direction of static equilibrium. In a memory element, the STT effect writes 0 or 1 in a magnetic-tunnel junction. Indeed, world-wide developments of memory arrays and radio-frequency oscillators utilizing current-driven STT today enjoy a nine-digit dollar commitment. But the fact that transfer of each half-unit of spin momentum $h$/4$\pi $ through a barrier requires the transfer of at least one unit of electric charge limits its efficiency. Arguably, STT should also arise from the flow of external heat, in either direction, between an insulating magnet, of ferrite or garnet (e. g. YIG) composition, and a metallic spacer ($J. S.$ 2010). Whenever s-d exchange annihilates a hot magnon at the insulator/metal-spacer interface, it transfers one unit $h$/2$\pi $ of spin momentum to the spacer. Conduction electrons within the spacer will transport this spin momentum to the second magnet without requiring an electric current. Such a \textit{thermagnonic }method, modestly powered by a Joule-effect heater, can substantially increase the efficiency of STT. Support for this prediction comes from (1) an estimate of the sd-exchange coefficient from data on spin relaxation in magnetically dilute (Cu,Ag,Au):Mn alloys; (2) a DFT computation (\textit{J. Xiao et al} 2010); and (3) most persuasively, data from spin pumping driven across a YIG/Au interface by ferromagnetic resonance (\textit{B. Heinrich} \textit{et al} 2011; \textit{C. Burrowes} \textit{et al} 2012).   

Oliver E. Buckley Condensed Matter Prize Lecture: S-d Exchange, Spin Accumulation, And The Roots Of Spintronics

The success of spintronics in metals such as nickel, cobalt, Ni-Fe and Ni-Co is based on the existence of high-mobility spin-up 4s electrons at the Fermi level, which carry most of the current. The spin-up Fermi level is located above the top of the 3d band. This basic fact, first recognized by Mott in 1936, was confirmed by the Hall-effect measurements of Pugh et al. (1950-1965), and by data of deviation from Matthiessen's rule by Campbell, Fert and Jaoul (1967-1977). In order to explain giant magnetoresistance and the existence of the spin-transfer torque, an interaction is needed which couples 4s conduction electrons to magnetic 3d electrons. This is the s-d exchange interaction, introduced by Vonsovskii in 1946 and Zener in 1951. Theories of Gilbert damping, based on s-d exchange, were soon developed (Turov (1955), Mitchell (1957)). But a serious problem was caused by the existence of a momentum gap between spin-up and spin-down Fermi surfaces, which prevents spin switching from happening at low T. The problem can be solved if local defects exist which act as extra sources of momentum. One such source is spin-flip scattering (Turov (1961), Heinrich, Freitova and Kambersky (1967)). A second one is the presence of an interface (Slonczewski (1996), Berger (1996)). Spin accumulation is another concept of importance to spintronics. It represents an imbalance between spin-up and spin-down Fermi levels. Introduced by Aronov in 1976, it was developed by Johnson and Silsbee (1985-1993) and by Valet and Fert (1993). It is the hidden agent through which the current ``pumps'' energy into many spintronics devices. In semiconductor lasers, the same role is played by the difference between conduction-band and valence-band Fermi levels. A momentum gap problem also exists in lasers made of indirect-gap semiconductors, and it is solved similarly.   

From point contacts to spin-transfer torque

Point contacts - nanoscale electrical contacts between conductors - have been around for decades and proved to be unique experimental tools for studying the electronic transport properties of metals. Following the theoretical prediction of spin-transfer torque (STT) by John Slonczewski [1] and Luc Berger [2], point contacts were instrumental for the first experimental demonstration of STT in spin-valve multilayers [3], thanks to extremely high current densities routinely produced in such contacts. In this talk I will briefly review the point-contact technique and its contributions to the field of current-induced control over magnetic nanostructures. \\[4pt] [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [2] L. Berger, Phys. Rev. B 54, 9353 (1996).\\[0pt] [3] M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998).   

The Spin Torque Lego - from spin torque nano-devices to advanced computing architectures

Spin transfer torque (STT), predicted in 1996 [1], and first observed around 2000, brought spintronic devices to the realm of active elements. A whole class of new devices, based on the combined effects of STT for writing and Giant Magneto-Resistance or Tunnel Magneto-Resistance for reading has emerged. The second generation of MRAMs, based on spin torque writing : the STT-RAM, is under industrial development and should be out on the market in three years. But spin torque devices are not limited to binary memories. We will rapidly present how the spin torque effect also allows to implement non-linear nano-oscillators, spin-wave emitters, controlled stochastic devices and microwave nano-detectors. What is extremely interesting is that all these functionalities can be obtained using the same materials, the exact same stack, simply by changing the device geometry and its bias conditions. So these different devices can be seen as Lego bricks, each brick with its own functionality. During this talk, I will show how spin torque can be engineered to build new bricks, such as the Spintronic Memristor, an artificial magnetic nano-synapse. I will then give hints on how to assemble these bricks in order to build novel types of computing architectures, with a special focus on neuromorphic circuits. \\[4pt] [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, 1 (1996) \& L. Berger, Phys. Rev. B 54, 9353 (1996)