Abstract
Antiferromagnetic (AF) materials are currently attracting considerable excitement for low dissipative and ultrafast spintronic devices. However, most of conventional real-space magnetic microscopy techniques cannot probe the AF order at the nanoscale because magnetic moments are mostly compensated, resulting in very low magnetic signals. This is a major obstacle to the fundamental understanding of nanoscale AF order and its response to external stimuli, such as spin polarized currents or electric fields. To release the full potential of antiferromagnets for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be extended to AF materials. In this thesis we prove that scanning magnetometry based on a single nitrogen–vacancy (NV) defect in diamond is ideally suited for imaging complex AF orders at the nanoscale, even under ambient conditions.A promising platform for AF spintronics is BiFeO3 (BFO), a prototypical room-temperature multiferroic material in which the AF order is intimately linked to the ferroelectric one via magnetoelectric coupling. Scanning NV magnetometry here demonstrates its ability to image the AF cycloidal order in BFO by mapping the magnetic stray field it produces. It also permits inferring interesting quantities such as the uncompensated magnetic moment of the spin density wave, and the real-space visualization of the intimate link between the ferroelectric and the AF orders. In order for BFO to make its way into device applications, the thin film form must be employed. The effect of epitaxial constraint on the behaviour of the AF order in strained BFO thin films is investigated. NV magnetometry proved that strain-tuning and electric-field switching, can stabilize a wide variety of complex antiferromagnetic spin textures in BFO thin films.Beyond imaging the static stray field, we demonstrate a new approach for imaging AF textures. It consists of mapping the magnetic noise they locally produce rather than their static magnetic fields. This technique exploits the strong dependence of the NV defect photoluminescence on magnetic fluctuations at the NV spin resonance frequency. As a proof of principle of the efficiency of the technique, the high tunability of synthetic antiferromagnets that host spin waves, is exploited to stabilize different spin textures. These AF textures ranging from domain walls, to exotic spin-spirals and AF skyrmions are imaged through this novel relaxometry technique.