Magnetic phenomena are at the foundation of modern-day information storage and processing technologies. In this dissertation,we study three phenomena that hold promise for the next generation of information technology: excitons in two-dimensional semiconductors, two-dimensional magnets and magnetic excitations. First, we introduce the reader to magnetism, its role in society (Chapter 1) and how magnetic phenomena can be imaged with high resolution using the nitrogen vacancy (NV) center in diamond (Chapter 2). Before presenting research results, we introduce the aforementioned phenomena, along with two experimental setups that we constructed ground-up to probe them using spectroscopy and cryogenic NV magnetometry (Chapter 2).

Valleytronics aims to encode information using valleys in the band structure of crystalline solids. In group-IV transition metal dichalcogenides such as WS2, MoS2, WSe2 and MoSe2, optical selection rules enable selective valley-excitation of excitons (electron-hole pairs), making them attractive for valleytronic applications. We use polarization sensitive spectroscopy to demonstrate that chemical doping ofWS2 using anisole changes the balance between neutral and charged excitons, and thereby the degree of valley polarization under optical excitation. We capture this behaviour by developing a rate equation model and show that optical quenching can increase valley polarization at the cost of exciton lifetimes (Chapter 3). These experiments are an important step towards NV-based detection of the magnetic moment associated with the valley index (Chapter 6).

A central challenge in NV magnetometry is minimizing the distance between sample and the sensor spins. We address this challenge by fabricating small 50 × 50 µm² diamond membranes that facilitate direct contact between sample and sensor (Chapter 2). We demonstrate their sensing potential by imaging uncompensatedmonolayer stray fields of Van der Waals interlayer antiferromagnet CrSBr (Chapter 4). Using the quantitative nature of NV measurements, we extract the CrSBr monolayer magnetization and Néel temperature. These results are an important stepping stone towards detecting spin waves, oscillations of the magnetic order, in Van derWaals magnets (Chapter 6).

Spin waves and their quanta, magnons, are promising signal carriers for next generation information devices that exploit their wave nature, non-reciprocal transport properties, and low intrinsic damping. An outstanding challenge is the efficient gating of spin-wave transport, which might be achieved by shaping their magnetic environment using diamagnetism. We employ diamond membranes to image spin waves in an yttrium iron garnet thin film as they travel underneath an optically opaque superconductor, to reveal hybridization of Meissner currents and spin wave modes (Chapter 5). We show that the superconductor modulates the spin-wave dispersion, enabling tuning of spin-wave transport using temperature, magnetic field and lasers. Finally, we suggest the use of superconductors to create magnonic cavities, crystals and spin-wave optics (Chapter 6). @en

Magnetic imaging using nitrogen-vacancy (NV) spins in diamonds is a powerful technique for acquiring quantitative information about sub-micron scale magnetic order. A major challenge for its application in the research on two-dimensional (2D) magnets is the positioning of the NV centers at a well-defined, nanoscale distance to the target material required for detecting the small magnetic fields generated by magnetic monolayers. Here, we develop a diamond “dry-transfer” technique akin to the state-of-the-art 2D-materials assembly methods and use it to place a diamond micro-membrane in direct contact with the 2D interlayer antiferromagnet CrSBr. We harness the resulting NV-sample proximity to spatially resolve the magnetic stray fields generated by the CrSBr, present only where the CrSBr thickness changes by an odd number of layers. From the magnetic stray field of a single uncompensated ferromagnetic layer in the CrSBr, we extract a monolayer magnetization of M _CSB = 0.46(2) T, without the need for exfoliation of monolayer crystals or applying large external magnetic fields. The ability to deterministically place NV-ensemble sensors into contact with target materials and detect ferromagnetic monolayer magnetizations paves the way for quantitative analysis of a wide range of 2D magnets assembled on arbitrary target substrates.

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Superconductors are materials with zero electrical resistivity and the ability to expel magnetic fields, which is known as the Meissner effect. Their dissipationless diamagnetic response is central to magnetic levitation and circuits such as quantum interference devices. In this work, we used superconducting diamagnetism to shape the magnetic environment governing the transport of spin waves-collective spin excitations in magnets that are promising on-chip signal carriers-in a thin-film magnet. Using diamond-based magnetic imaging, we observed hybridized spin-wave-Meissner-current transport modes with strongly altered, temperature-tunable wavelengths and then demonstrated local control of spin-wave refraction using a focused laser. Our results demonstrate the versatility of superconductor-manipulated spin-wave transport and have potential applications in spin-wave gratings, filters, crystals, and cavities.

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Nitrogen-vacancy (NV) magnetometry is a new technique for imaging spin waves in magnetic materials. It detects spin waves by their microwave magnetic stray fields, which decay evanescently on the scale of the spin-wavelength. Here, we use nanoscale control of a single-NV sensor as a wavelength filter to characterize frequency-degenerate spin waves excited by a microstrip in a thin-film magnetic insulator. With the NV probe in contact with the magnet, we observe an incoherent mixture of thermal and microwave-driven spin waves. By retracting the tip, we progressively suppress the small-wavelength modes until a single coherent mode emerges from the mixture. In-contact scans at low drive power surprisingly show occupation of the entire isofrequency contour of the two-dimensional spin-wave dispersion despite our one-dimensional microstrip geometry. Our distance-tunable filter sheds light on the spin-wave band occupation under microwave excitation and opens opportunities for imaging magnon condensates and other coherent spin-wave modes.

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Transition metal dichalcogenide (TMD) monolayers are two-dimensional semiconductors with two valleys in their band structure that can be selectively addressed using circularly polarized light. Their photoluminescence spectrum is characterized by neutral and charged excitons (trions) that form a chemical equilibrium governed by the net charge density. Here, we use chemical doping to drive the conversion of excitons into trions in WS ₂ monolayers at room temperature, and study the resulting valley polarization via photoluminescence measurements under valley-selective optical excitation. We show that the doping causes the emission to become dominated by trions with a strong valley polarization associated with rapid non-radiative recombination. Simultaneously, the doping results in strongly quenched but highly valley-polarized exciton emission due to the enhanced conversion into trions. A rate equation model explains the observed valley polarization in terms of the doping-controlled exciton-trion equilibrium. Our results shed light on the important role of exciton-trion conversion on valley polarization in monolayer TMDs.

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