## God may not play dice, but spin sure does

It’s all aboard the Applied Physics Express this week with “Spin dice: A truly scalable random number generator based on spintronics” by Fukushima et al.  Random number generators are important for cryptography applications, but true random number generators don’t scale well.  This paper reports on a novel approach to a true random number generator that utilises the stochastic nature of spin transfer torque (STT) switching in magnetic tunnel junctions (MTJs).

Thermal fluctuations in the probability of relaxing to a parallel or antiparallel state randomly generate a ‘0’ or ‘1’

The device that is the subject of the paper is dubbed a “spin dice”, and consists of an array of 8 MTJs, where the free layer is FeB, the tunnel layer is MgO, and the fixed layer is CoPt.  Applying a current pulse through the MTJ can lead to switching of the free layer due to STT.  At the critical point of switching of the free layer, thermal fluctuations allow the layer to relax into an either parallel (P) or anti-parallel (AP) state.  The associated probability of this relaxation depends upon factors that include the current pulse magnitude and width, as well as the sample temperature.  By tuning the pulse amplitude and width at a given temperature, the authors attempt to obtain a switching probability of 0.5 (in contrast, STT-MRAM requires a switching probability of 1).

The sequence of 8 bits can generate a true random number

The state of the free layer of the MTJ can be read out by the tunnel magnetoresistance effect, and these high and low resistance states for AP and P configurations correspond to ‘0’ and ‘1’ binary random number bits.  By using the array of 8 devices, the sequence of random binary numbers can generate a true random number.  To reset the device, a large current pulse can be applied with a switching probability of 1.

Two devices are tested that give switching probabilities of 0.513 and 0.487 which stray from the ideal 0.5 ratio due thermal and current fluctuations.  Nevertheless, the spin dice shows potential as a random number generator as the scalability of STT switching devices allows a large number of spin dice to be fabricated on a single chip.

http://iopscience.iop.org/1882-0786/7/8/083001/article