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Register a new userMagnetoelectric coupling in a frustrated spinel studied using high-field scanning probe microscopy
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Below its Neel temperature, the frustrated magnet CdCr$_2$O$_4$ exhibits an antiferromagnetic spin-spiral ground state. Such states can give rise to a sizable magnetoelectric coupling. In this report, we measure the electric polarization induced in single-crystalline CdCr$_2$O$_4$ by a large applied magnetic field. Because the detection of a macroscopic polarization is hindered by the structural domains in the tetragonal spin-spiral phase, we have pioneered an alternative method of measuring polarization induced by high magnetic fields, using electrostatic force microscopy. This method enables us to measure polarization from nanometer sized areas of the sample surface, as well as imaging how to charge inhomogeneities change with magnetic field.
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Moire superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena when stacking 2D materials with small twist angles. Here, we reveal local surface potentials associated with stacking domains in twisted double bilayer graphene (TDBG) moire superlattices. Using a combination of both lateral Piezoresponse Force Microscopy (LPFM) and Scanning Kelvin Probe Microscopy (SKPM), we distinguish between Bernal (ABAB) and rhombohedral (ABCA) stacked graphene and directly correlate these stacking configurations with local surface potential. We find that the surface potential of the ABCA domains is ~15 mV higher (smaller work function) than that of the ABAB domains. First-principles calculations based on density functional theory further show that the different work functions between ABCA and ABAB domains arise from the stacking dependent electronic structure. We show that, while the moire superlattice visualized by LPFM can change with time, imaging the surface potential distribution via SKPM appears more stable, enabling the mapping of ABAB and ABCA domains without tip-sample contact-induced effects. Our results provide a new means to visualize and probe local domain stacking in moire superlattices along with its impact on electronic properties.
We report on state-of-the-art scanning probe microscopy measurements performed in a pulse tube based top-loading closed-cycle cryostat with a base temperature of 4 K and a 9 T magnet. We decoupled the sample space from the mechanical and acoustic noise from the cryocooling system to enable scanning probe experiments. The extremely low vibration amplitudes in our system enabled successful imaging of 0.39 nm lattice steps on single crystalline SrTiO$_{3}$ as well as magnetic vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ superconductor. Fine control over sample temperature and applied magnetic field further enabled us to probe the helimagnetic and the skyrmion-lattice phases in Fe$_{0.5}$Co$_{0.5}$Si with unprecedented signal-to-noise ratio of 20:1. Finally, we demonstrate for the first time quartz-crystal tuning fork shear-force microscopy in a closed-cycle cryostat.
The graphene moire structures on metals, as they demonstrate both long (moire) and short (atomic) scale ordered structures, are the ideal systems for the application of scanning probe methods. Here we present the complex studies of the graphene/Ir(111) system by means of 3D scanning tunnelling and atomic force microscopy/spectroscopy as well as Kelvin-probe force microscopy. All results clearly demonstrate a variation of the moire and atomic scale contrasts as a function of the bias voltage as well as the distance between the scanning probe and the sample, allowing to discriminate between topographic and electronic contributions in the imaging of a graphene layer on metals. The presented results are accompanied by the state-of-the-art density functional theory calculations demonstrating the excellent agreement between theoretical and experimental data.
Time-resolved scanning Kerr microscopy (TRSKM) is a powerful technique for the investigation of picosecond magnetization dynamics at sub-micron length scales by means of the magneto-optical Kerr effect (MOKE). The spatial resolution of conventional (focused) Kerr microscopy using a microscope objective lens is determined by the optical diffraction limit so that the nanoscale character of the magnetization dynamics is lost. Here we present a platform to overcome this limitation by means of a near-field TRSKM that incorporates an atomic force microscope (AFM) with optical access to a metallic AFM probe with a nanoscale aperture at its tip. We demonstrate the near-field capability of the instrument through the comparison of time-resolved polar Kerr images of magnetization dynamics within a microscale NiFe rectangle acquired using both near-field and focused TRSKM techniques at a wavelength of 800 nm. The flux-closure domain state of the in-plane equilibrium magnetization provided the maximum possible dynamic polar Kerr contrast across the central domain wall, and enabled an assessment of the magneto-optical spatial resolution of each technique. Line profiles extracted from the Kerr images demonstrate that the near-field spatial resolution was enhanced with respect to that of the focused Kerr images. Furthermore, the near-field polar Kerr signal (~1 mdeg) was more than half that of the focused Kerr signal, despite the potential loss of probe light due to internal reflections within the AFM tip. We have confirmed the near-field operation by exploring the influence of the tip-sample separation, and have determined the spatial resolution to be ~550 nm for an aperture with a sub-wavelength diameter of 400 nm. The spatial resolution of the near-field TRSKM was in good agreement with finite element modelling of the aperture...
Thermodynamics of tip-induced nanodomain formation in scanning probe microscopy of ferroelectric films and crystals is studied using the Landau-Ginzburg-Devonshire phenomenological approach. The local redistribution of polarization induced by the biased probe apex is analyzed including the effects of polarization gradients, field dependence of dielectric properties, intrinsic domain wall width, and film thickness. The polarization distribution inside subcritical nucleus of the domain preceding the nucleation event is very smooth and localized below the probe, and the electrostatic field distribution is dominated by the tip. In contrast, polarization distribution inside the stable domain is rectangular-like, and the associated electrostatic fields clearly illustrate the presence of tip-induced and depolarization field components. The calculated coercive biases of domain formation are in a good agreement with available experimental results for typical ferroelectric materials. The microscopic origin of the observed domain tip elongation in the region where the probe electric field is much smaller than the intrinsic coercive field is the positive depolarization field in front of the moving counter domain wall. For infinitely thin domain walls local domain breakdown through the sample depth appears. The results obtained here are complementary to the Landauer-Molotskii energetic approach.
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