Operational and structural optimizations of spin-torque microwave detectors

Abstract

The enormous business opportunities offered by Internet of Things have dramatically increased the demand for microwave detectors with elevated sensitivity, extended bandwidth, wide dynamic range, and high thermal stability. Spin-torque microwave detectors (STMD) are novel microwave power meters that utilize the magnetoresistance effect of magnetic thin films and the spin-transfer torque effect of microwave current. Compared with currently used Schottky diode microwave detectors, STMDs offer benefits such as high sensitivity, a small footprint, and low power consumption. However, the widespread application of STMD remains constrained by their relatively narrow bandwidth, small dynamic range, fabrication complexity, and poor thermal stability. This thesis addresses the preceding constraints through several operational and structural optimizations. First, a macrospin model based on a magnetic tunnel junction was established to explore the influence of the tilt angle of reference-layer (RL) magnetization, the magnetic field angle, and working temperature on STMD performance. The simulated field angle dependence of the DC voltage (VDC) and resonant frequency (fR) was further verified experimentally. Increased sensitivity (19.9 mV/mW) and extended bandwidth (9.8 GHz) has been achieved under an optimized out-of-plane magnetic field. Second, STMD based on giant magnetoresistance (GMR) was developed with high-power compatibility and low device complexity. The VDC and fR are freely tunable by the magnetic field angle and DC bias current. Incorporation of a NiFe-based synthetic ferrimagnetic (SyF) free layer (FL) in STMD minimizes the frequency drift under elevated temperatures and alleviates the reduction in sensitivity after thermal cycling. The increased working power (>3 mW) and thermal stability (low temperature coefficient of 0.26 MHz/K) of GMR-based STMD suggest their potential for future applications. Finally, structural modifications were made to multilayers for potential application as FLs and RLs to further enhance STMD performance. Ion-beam bombardment on the Co and Fe layers alters the coercivity (Hc), suggesting that the application of such treatment to the FL may modify the bandwidth of STMD. A NiFeCuMo-based SyF nanostructure was developed to show considerably reduced saturation magnetization and Hc, demonstrating its potential to operate as the FL of highly sensitive STMD. Ion-beam bombardment and field cooling effectively enhances the exchange bias (Hex) of the NiCo/(Ni,Co)O bilayers. The irreversibility temperature (Tirr) can be further modified by antiferromagnetic (AF)-AF coupling in the NiFe/CoO/Fe2O3 trilayer. The Hex of nanopatterned CoFeB/IrMn is also influenced by ferromagnetic (FM) layer thickness and annealing temperature. The introduction of exchange-coupled multilayers with modified Hex and Tirr may allow greater thermal stability in STMD. This thesis has demonstrated that the sensitivity, bandwidth, dynamic range, and thermal stability of STMD can be improved by tuning the operational conditions, including temperature, magnetic field, and DC voltage, and engaging novel structures such as GMR microstripe and SyF FL. Structural modifications such as conducting post-treatment, incorporating different FM and AF materials, and alternating the layer thickness were conducted on magnetic multilayers as possible approaches to further improving STMD performance. The work reported herein deepens the physical understanding of spin-torque diode effect and accelerates the practical applications of STMD.