Lundi 24 Octobre 2022 à 14h00, Mafalda Jotta Garcia défendra a thèse intitulée : "Spin transfer dynamics in nano-oscillators : from detection to energy harvesting of radio-frequency signals"
Abstract :
Spin-torque nano-oscillators (STNOs) are spintronic devices in which the coupling between a spin current and magnetisation is used to excite dynamical magnetic modes. Using a magnetic vortex configuration for the STNO, this thesis focuses on optimising these devices for sensing applications, namely radio-frequency (rf) signal detection and energy harvesting. The characterisation of the 1/f low-frequency noise of the studied devices is made in the different existing magnetic configurations as a function of the external magnetic field. The main result shows that the higher vortex noise, associated with the pinning of its core into material defects, can be suppressed through the excitation of its dynamics, either through the spin-transfer torque or an rf signal. As for the rf detection capabilities, they hinge on the so-called spin-torque diode effect, a rectification effect that occurs when an STNO is submitted to an rf current with a frequency corresponding to one of the magnetic resonance modes of the system. Different strategies are studied to achieve better detection sensitivity, a crucial figure of merit for STNO-based rf detectors and harvesters. Firstly, we demonstrate an increase in the sensitivity as the STNO diameter decreases, down to 300 nm. Secondly, we show an improvement of the sensitivity values can be also achieved by connecting two STNOs in series, compared to a single one, for a large range of rf injection powers. Thirdly, we investigate the simultaneous excitation of the spin transfer dynamics in vortex-based STNOs by two separate rf signals - an rf current and an rf magnetic field. By splitting the input signal between these two paths, a gain in sensitivity is achieved. Furthermore, based on the latter, a novel functionality of STNOs as phase detectors is demonstrated. The rf energy harvesting capabilities of vortex-STNOs are demonstrated for an rf signal near its resonance frequency in a proof-of-concept circuit. The STNO is used to power a magnetic field TMR-sensor. With this demonstrator, we show that vortex-based STNOs are promising candidates for energy harvesting ubiquitous rf signals such as WiFi or GSM.
