Nanoscale true random bit generator based on magnetic state transitions in magnetic tunnel junctions

Abstract

We present an investigation into the in-plane field induced free layer state transitions found in magnetic tunnel junctions. By applying an ac current into an integrated field antenna, the magnetisation of the free layer can be switched between the magnetic vortex state and the quasi-uniform anti-parallel state. When in the magnetic vortex state, the vortex core gyrates a discrete number of times, and at certain frequencies there is a 50% chance of the core gyrating n or n - 1 times, leading to the proposal of a novel nanoscale continuous digital true random bit generator.

Figure 1

(a) micrograph of device with the MTJ contacted via a top and bottom contact (TC and BC) with an antenna integrated above. The schematic shows MTJ and antenna, the dashed line on the MTJ represents the trajectory of the vortex core calculated from micromagnetic simulations. (b) voltage measured at ${\mathit{I}}_{\mathit{MTJ}}^{\mathit{dc}}$ = −8 mA and (c) magnetisation component along the y-axis for an excitation frequency of 70 MHz, with examples of the magnetisation configuration of (i) the QUAP state and (ii) the vortex state.

Figure 2

(a) number of gyrations, n, versus frequency and (b) the measured voltage as a function of time for five different excitation frequencies showing the discrete number of vortex core gyrations before expulsion, n. The trajectories of the vortex core on the right hand side are calculated from micromagnetic simulations.

Figure 3

The measured voltage as a function of time at a given frequency of (a) 32 MHz, (b) 43 MHz and (c) 59 MHz. The shaded regions show time when free layer is in the QUAP state.

Figure 4

(a) number gyrations and (b) # number of NIST tests passed as a function of the excitation frequency.