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This work investigates nanosecond superparamagnetic switching in 50 nmdiameter in-plane magnetized magnetic tunnel junctions (MTJs). Due to the smallin-plane uniaxial anisotropy, dwell times below 10 ns and auto-correlationtimes down to 5 ns are measured for circular superparamagnetic tunnel junctions(SMTJs). SMTJs exhibit probabilistic switching of the magnetic free layer,which can be used for the generation of true random numbers. The quality ofrandom bitstreams, generated by our SMTJ, is evaluated with a statistical testsuite (NIST STS, sp 800-22) and shows true randomness after three XORoperations of four random SMTJ bitstreams. A low footprint CMOS circuit isproposed for fast and energy-efficient random number generation. We demonstratethat the probability of a 1 or 0 can be tuned by spin-transfer-torque (STT),while the average bit generation rate is mainly affected by the current densityvia Joule heating. Although both effects are always present in MTJs, Jouleheating most often is neglected. However, with a resistance area (RA) productof 15 $\Omega \mu$m$^2$ and current densities of the order of 1 MA/cm$^2$, anincreasing temperature at the tunneling site results in a significant increasein the switching rate. As Joule heating and STT scale differently with currentdensity, device design can be optimized based on our findings.

Nanosecond True Random Number Generation with Superparamagnetic Tunnel Junctions -- Identification of Joule Heating and Spin-Transfer-Torque effects