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Abstract We review two magnetic tunnel junction (MTJ) approaches for compact, low-power, CMOS-integrated true random number generation (TRNG). The first employs passive-read, easy-plane superparamagnetic MTJs (sMTJs) that generate thermal-fluctuation-driven bitstreams at 0.5–1 Gb s⁻¹per device. The second uses MTJs with magnetically stable free layers, operated with stochastic write pulses to achieve switching probabilities of about 0.5 (i.e. write error rates of $\simeq 0.5$), achieving $\gtrsim 0.1$ Gb s⁻¹per device; we refer to these as stochastic-write MTJs (SW-MTJs). Randomness from both approaches has been validated using the NIST SP 800-22r1a test suites. sMTJ approach uses a read-only cell with low power and can be compatible with most advanced CMOS nodes, while SW-MTJs leverage standard CMOS MTJ process flows, enabling co-integration with embedded spin-transfer torque magnetic random access memory. Both approaches can achieve deep sub-0.01 µm²MTJ footprints and offer orders-of-magnitude better energy efficiency than CPU/GPU-based generators, enabling placement near logic for high-throughput random bitstreams for probabilistic computing, statistical modeling, and cryptography. In terms of performance, sMTJs generally suit applications requiring very high data-rate random bits near logic processors, such as probabilistic computing or large-scale statistical modeling. Whereas SW-MTJs are attractive option for edge-oriented microcontrollers, providing entropy sources for computing or cryptographic enhancement. We highlight the strengths, limitations, and integration challenges of each approach, emphasizing the need to reduce device-to-device variability in sMTJs—particularly by mitigating magnetostriction-induced in-plane anisotropy—and to improve temporal stability in SW-MTJs for robust, large-scale deployment. 

Abstract Ultra-thin films of low damping ferromagnetic insulators with perpendicular magnetic anisotropy have been identified as critical to advancing spin-based electronics by significantly reducing the threshold for current-induced magnetization switching while enabling new types of hybrid structures or devices. Here, we have developed a new class of ultra-thin spinel structure Li_0.5Al_1.0Fe_1.5O₄(LAFO) films on MgGa₂O₄(MGO) substrates with: 1) perpendicular magnetic anisotropy; 2) low magnetic damping and 3) the absence of degraded or magnetic dead layers. These films have been integrated with epitaxial Pt spin source layers to demonstrate record low magnetization switching currents and high spin-orbit torque efficiencies. These LAFO films on MGO thus combine all of the desirable properties of ferromagnetic insulators with perpendicular magnetic anisotropy, opening new possibilities for spin based electronics. 

We demonstrate efficient spin transfer across a disordered interfacial layer that forms in low damping ferrimagnetic insulator lithium aluminum ferrite (LAFO) and tantalum bilayers. Despite the interfacial disorder, confirmed by transmission electron microscopy, we find a room temperature interfacial spin mixing conductance on the order of 1014 Ω−1m−2 similar to other LAFO-based bilayers with epitaxial interfaces. Broadband ferromagnetic resonance measurements confirm a linewidth broadening in LAFO following the addition of a Ta layer, consistent with the effects of spin pumping. Furthermore, the presence of spin current generated in the Ta layer by spin pumping is confirmed with inverse spin Hall effect measurements. Measurements of the Ta thickness dependence of the spin Hall magnetoresistance and the Gilbert damping enhancement indicate that the Ta spin diffusion length is on the order of 1 nm. This work not only provides a surprising example of efficient spin transport across a disordered interface but also demonstrates the potential for low damping spinel ferrites as a robust system for efficient spin wave spintronics. 

Spin-Hall nano-oscillators (SHNOs) are nanoscale spintronic devices that generate high-frequency (GHz) microwave signals useful for various applications, such as neuromorphic computing and creating Ising systems. Recent research demonstrated that hybrid SHNOs consisting of a ferromagnetic metal (permalloy) and lithium ferrite-based (LAFO) insulating ferrimagnetic thin films have advantages in having lower auto-oscillation threshold currents (Ith) and generating larger microwave output power, making this hybrid structure an attractive candidate for spintronic applications. It is essential to understand how the tunable material properties of LAFO, e.g., its thickness, perpendicular magnetic anisotropy (Ku,LAFO), and saturation magnetization (Ms,LAFO), affect magnetic dynamics in hybrid SHNOs. We investigate the change in Ith and the output power of the device as the LAFO parameters vary. We find the Ith does not depend strongly on these parameters, but the output power has a highly nonlinear dependence on Ms,LAFO and Ku,LAFO. We further investigate the nature of the excited spin-wave modes as a function of Ku,LAFO and determine a critical value of Ku,LAFO above which propagating spin-waves are excited. Our simulation results provide a roadmap for designing hybrid SHNOs to achieve targeted spin excitation characteristics. 

In a non-collinear antiferromagnet, elementary spins rotate with opposite handedness with respect to the collective octupole magnetic moment when stirred by spin currents. 

Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material's coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM.