Magnetic Imaging and Domain Nucleation in CrSBr Down to the 2D Limit.

Recent advancements in 2D materials have revealed the potential of van der Waals magnets, and specifically of their magnetic anisotropy that allows applications down to the 2D limit. Among these materials, CrSBr has emerged as a promising candidate, because its intriguing magnetic and electronic properties have appeal for both fundamental and applied research in spintronics or magnonics. In this work, nano-SQUID-on-tip (SOT) microscopy is used to obtain direct magnetic imaging of CrSBr flakes with thicknesses ranging from monolayer (N = 1) to few-layer (N = 5). The ferromagnetic order is preserved down to the monolayer, while the antiferromagnetic coupling of the layers starts from the bilayer case. For odd layers, at zero applied magnetic field, the stray field resulting from the uncompensated layer is directly imaged. The progressive spin reorientation along the out-of-plane direction (hard axis) is also measured with a finite applied magnetic field, allowing evaluation of the anisotropy constant, which remains stable down to the monolayer and is close to the bulk value. Finally, by selecting the applied magnetic field protocol, the formation of Néel magnetic domain walls is observed down to the single-layer limit.

Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices.

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.

Direct observation of a superconducting vortex diode.

The interplay between magnetism and superconductivity can lead to unconventional proximity and Josephson effects. A related phenomenon that has recently attracted considerable attention is the superconducting diode effect, in which a nonreciprocal critical current emerges. Although superconducting diodes based on superconductor/ferromagnet (S/F) bilayers were demonstrated more than a decade ago, the precise underlying mechanism remains unclear. While not formally linked to this effect, the Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state is a plausible mechanism due to the twofold rotational symmetry breaking caused by the finite center-of-mass-momentum of the Cooper pairs. Here, we directly observe asymmetric vortex dynamics that uncover the mechanism behind the superconducting vortex diode effect in Nb/EuS (S/F) bilayers. Based on our nanoscale SQUID-on-tip (SOT) microscope and supported by in-situ transport measurements, we propose a theoretical model that captures our key results. The key conclusion of our model is that screening currents induced by the stray fields from the F layer are responsible for the measured nonreciprocal critical current. Thus, we determine the origin of the vortex diode effect, which builds a foundation for new device concepts.

Chip-Integrated Vortex Manipulation.

The positions of Abrikosov vortices have long been considered as means to encode classical information. Although it is possible to move individual vortices using local probes, the challenge of scalable on-chip vortex-control remains outstanding, especially when considering the demands of controlling multiple vortices. Realization of vortex logic requires means to shuttle vortices reliably between engineered pinning potentials, while concomitantly keeping all other vortices fixed. We demonstrate such capabilities using Nb loops patterned below a NbSe2 layer. SQUID-on-Tip (SOT) microscopy reveals that the loops localize vortices in designated sites to a precision better than 100 nm; they realize "push" and "pull" operations of vortices as far as 3 µm. Successive application of such operations shuttles a vortex between adjacent loops. Our results may be used as means to integrate vortices in future quantum circuitry. Strikingly, we demonstrate a winding operation, paving the way for future topological quantum computing and simulations.

Interior and Edge Magnetization in Thin Exfoliated CrGeTe3 Films.

CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism down to a two-layer thick sample. Although CGT is one of the leading candidates for spintronics devices, a comprehensive analysis of CGT thickness dependent magnetization is currently lacking. In this work, we employ scanning SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with SOT images, we present the magnetic texture and hysteretic magnetism of CGT, thereby matching the global behavior of CGT to the domain structure extracted from local SOT magnetic imaging. Using this method, we provide a thickness dependent magnetization state diagram of bare CGT films. No zero-field magnetic memory was found for films thicker than 10 nm, and hard ferromagnetism was found below that critical thickness. Using scanning SOT microscopy, we identify a unique edge magnetism, contrasting the results attained in the CGT interior.

Electrically Tunable Multiterminal SQUID-on-Tip.

We present a new nanoscale superconducting quantum interference device (SQUID) whose interference pattern can be shifted electrically in situ. The device consists of a nanoscale four-terminal-four-junction SQUID fabricated at the apex of a sharp pipet using a self-aligned three-step deposition of Pb. In contrast to conventional two-terminal-two-junction SQUIDs that display optimal sensitivity when flux biased to about a quarter of the flux quantum, the additional terminals and junctions allow optimal sensitivity at arbitrary applied flux, thus eliminating the magnetic field "blind spots". We demonstrate spin sensitivity of 5 to 8 µB/Hz1/2 over a continuous field range of 0 to 0.5 T with promising applications for nanoscale scanning magnetic imaging.

Visualization of superparamagnetic dynamics in magnetic topological insulators.

Quantized Hall conductance is a generic feature of two-dimensional electronic systems with broken time reversal symmetry. In the quantum anomalous Hall state recently discovered in magnetic topological insulators, time reversal symmetry is believed to be broken by long-range ferromagnetic order, with quantized resistance observed even at zero external magnetic field. We use scanning nanoSQUID (nano-superconducting quantum interference device) magnetic imaging to provide a direct visualization of the dynamics of the quantum phase transition between the two anomalous Hall plateaus in a Cr-doped (Bi,Sb)2Te3 thin film. Contrary to naive expectations based on macroscopic magnetometry, our measurements reveal a superparamagnetic state formed by weakly interacting magnetic domains with a characteristic size of a few tens of nanometers. The magnetic phase transition occurs through random reversals of these local moments, which drive the electronic Hall plateau transition. Surprisingly, we find that the electronic system can, in turn, drive the dynamics of the magnetic system, revealing a subtle interplay between the two coupled quantum phase transitions.

Three-junction SQUID-on-tip with tunable in-plane and out-of-plane magnetic field sensitivity.

Nanoscale superconducting quantum interference devices (SQUIDs) demonstrate record sensitivities to small magnetic moments but are typically sensitive only to the field component that is normal to the plane of the SQUID and out-of-plane with respect to the scanned surface. We report on a nanoscale three-junction Pb SQUID, which is fabricated on the apex of a sharp tip. Because of its three-dimensional structure, it exhibits a unique tunable sensitivity to both in-plane and out-of-plane fields. We analyze the two-dimensional interference pattern from both numerical and experimental points of view. This device is integrated into a scanning microscope, and its ability to independently measure the different components of the magnetic field with outstanding spin sensitivity better than 5 µB/Hz(1/2) is demonstrated. This highlights its potential as a local probe of nanoscale magnetic structures.

A scanning superconducting quantum interference device with single electron spin sensitivity.

Superconducting quantum interference devices (SQUIDs) can be used to detect weak magnetic fields and have traditionally been the most sensitive magnetometers available. However, because of their relatively large effective size (on the order of 1 µm), the devices have so far been unable to achieve the level of sensitivity required to detect the field generated by the spin magnetic moment (µB) of a single electron. Here we show that nanoscale SQUIDs with diameters as small as 46 nm can be fabricated on the apex of a sharp tip. The nano-SQUIDs have an extremely low flux noise of 50 nΦ0 Hz(-1/2) and a spin sensitivity of down to 0.38 µB Hz(-1/2), which is almost two orders of magnitude better than previous devices. They can also operate over a wide range of magnetic fields, providing a sensitivity of 0.6 µB Hz(-1/2) at 1 T. The unique geometry of our nano-SQUIDs makes them well suited to scanning probe microscopy, and we use the devices to image vortices in a type II superconductor, spaced 120 nm apart, and to record magnetic fields due to alternating currents down to 50 nT.

Replenish and relax: explaining logarithmic annealing in ion-implanted c-Si.

We study ion-damaged crystalline silicon by combining nanocalorimetric experiments with an off-lattice kinetic Monte Carlo simulation to identify the atomistic mechanisms responsible for the structural relaxation over long time scales. We relate the logarithmic relaxation, observed in a number of disordered systems, with heat-release measurements. The microscopic mechanism associated with this logarithmic relaxation can be described as a two-step replenish and relax process. As the system relaxes, it reaches deeper energy states with logarithmically growing barriers that need to be unlocked to replenish the heat-releasing events leading to lower-energy configurations.

Synthesis and characterization of single-layer silver-decanethiolate lamellar crystals.

We report the synthesis of silver-decanethiolate (AgSC10) lamellar crystals. Nanometer-sized Ag clusters grown on inert substrates react with decanethiol vapor to form multilayer AgSC10 lamellar crystals with both layer-by-layer and in-plane ordering. The crystals have strong (010) texture with the layers parallel to the substrates. The synthesis method allows for a precise control of the number of layers. The thickness of the lamellae can be manipulated and systematically reduced to a single layer by decreasing the amount of Ag and lowering the annealing temperature. The single-layer AgSC10 lamellae are two-dimensional crystals and have uniform thickness and in-plane ordering. These samples were characterized with nanocalorimetry, atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), and Rutherford backscattering spectroscopy (RBS).