Spintronics

Key personnel: Dr Volodymyr Kruglyak, Prof Rob Hicken

The sources used to fabricate magnetic tunnel junctions.

The substrate platter used to fabricate magnetic tunnel junctions.

The term Spintronics refers to the exploitation of electron spin as an additional degree of freedom with which to control the motion of an electron.  Many observers believe that Spintronics will deliver future generations of low-power electronics, while providing the most feasible route to the realization of a quantum computer.  Many device concepts have been proposed within semiconductor spintronics, yet fundamental problems remain in the development of room temperature ferromagnetic semiconductors and reliable transfer of spin-polarized electrons to and from other ferromagnetic materials.  In contrast, room temperature metal-based spintronics is now well established, with Albert Fert and Peter Grünberg receiving the 2007 Nobel Prize in Physics for the discovery of the Giant Magnetoresistance effect, which has proved to be of enormous economic significance with its application to disk drive sensor technology.  More recently the discovery of spin transfer torque (STT) has presented a new challenge within spintronics.  By controlling non-local spin populations it seems feasible to develop an entirely new generation of electronics in which the transfer of spin angular momentum associated with an electrical current is used ot control the state of ferromagnetic components within the circuit.

Exeter is a founding member of the Spin@RT (http://www.spinart.org/) consortium that brought together 7 institutions to give the UK critical mass within metals based spintronics research.  Within Exeter we are able to fabricate thin films and magnetic tunnel junctions (MTJ) by magnetron sputtering system using an in-situ shadow mask carousel, while we are also studying devices fabricated within the consortium and at Seagate Technology.

Photoconductance apparatus

Apparatus used for measurement of spin transfer torque induced microwave emission. High frequency probes deliver the dc bias current and relay the microwave signal to the spectrum anlayzer. A quadrupole electromagnet is used to apply a magnetic field in any direction within the plane of the substrate

We characterize spintronic devices and materials by conventional magnetotransport and magneto-optical Kerr effect (MOKE) magnetometry techniques, and also by more novel approaches such as photoconductance, which may be used to characterize the height of the tunnel barrier within a MTJ, and X-ray magnetic circular dichroism (XMCD) and Magnetic Second Harmonic Generation (MSHG) that are able to probe the magnetic state of buried interfaces within the MTJ. Additional XMCD studies are being performed to detect local moments induced by spin current.

The main thrust of our spintronics research is directed towards measurement and understanding of high frequency processes. Besides time resolved optical techniques we also perform high frequency transport measurements. When a sufficiently large dc current bias is applied to a MTJ with nanoscale lateral dimensions, the STT may cause the magnetization within the free layer of the MTJ to either switch or precess continuously. When coupled with the Tunnel Magnetoresistance (TMR) of the MTJ, the motion of the free layer causes an oscillatory microwave frequency voltage to develop across the MTJ. The microwave signal from this spin transfer oscillator (STO) may be detected with a spectrum analyzer, providing a simple means of detecting STT induced dynamics. The STO is said to be agile since its frequency of oscillation may be tuned rapidly by varying the drive current.

Due to its nanoscopic size, the power output of an individual STO is very small. However the power output of an array of N phase-locked oscillators is expected to vary as N². We are exploring potential phase locking mechanisms with the EU FP7 programme MASTER.

Emission spectra from a STO as the bias magnetic field is swept up and down. Peaks due to both STT and thermal magnetic fluctuations are in general observed