S. J. Bending

Real‐time scanning Hall probe microscopy

We describe a low‐noise scanning Hall probe microscope having unprecedented magnetic field sensitivity (∼2.9×10−8 T/√Hz at 77 K), high spatial resolution, (∼0.85 μm), and operating in real‐time (∼1 frame/s) for studying flux profiles at surfaces. A submicron Hall probe manufactured in a GaAs/AlGaAs two‐dimensional electron gas (2DEG) is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. Flux penetration into a high Tc YBa2Cu3O7−δ thin film has been observed in real time at 85 K with single vortex resolution. Flux is seen to enter the film in the form of vortex bundles as well as single flux quanta, Φ0.

Raman Spectra of Monolayer, Few-Layer, and Bulk ReSe2: An Anisotropic Layered Semiconductor

Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.

Scanning Hall probe microscopy of superconductors and magnetic materials

We describe results from a scanning Hall probe microscope operating in a broad temperature range, 4–300 K. A submicron Hall probe manufactured in a GaAs/AlGaAs two‐dimensional electron gas is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. The magnetic field structure of the sample together with the topography can be obtained simultaneously. The technique is noninvasive with an extremely low self‐field of <10−2 G and yields a quantitative measurement of the surface magnetic field in contrast to magnetic force microscopy. In addition the microscope has an outstanding magnetic field resolution (∼1.1×10−3 G/√Hz at 77 K) and high spatial resolution, ∼0.85 μm. Images of individual vortices in a high‐Tc Y1Ba2Cu3O7−δ thin film at low temperatures and magnetic domains in an Fe‐garnet crystal at room temperature are presented.

Hall effect in a highly inhomogeneous magnetic field distribution

Two-dimensional Hall probes are becoming increasingly popular as “local” magnetometers for ferromagnetic and superconducting materials. In many applications, the magnetic field at a sample surface varies on a length scale much smaller than the Hall probe dimensions, and data interpretation requires a quantitative model of the Hall voltage in this situation. We present here a classical numerical model of the Hall effect in a strongly inhomogeneous magnetic field and show how a response function can be defined to calculate the Hall voltage for an arbitrary magnetic-field distribution. The results are successfully applied to recent scanning Hall probe microscopy data on superconducting vortices.

Superconductivity in two-dimensional NbSe2 field effect transistors

We describe investigations of superconductivity in few molecular layer NbSe2 field effect transistors. While devices fabricated from NbSe2 flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset Tc that was only slightly depressed from the bulk value for 2H-NbSe2 (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, Hc2(0), and coherence length, ξ(0), to be independently estimated. Our results are analyzed in terms of available theories for these phenomena.

Integrated piezoresistive sensors for atomic force-guided scanning Hall probe microscopy

We report the development of an advanced sensor for atomic force-guided scanning Hall probe microscopy whereby both a high mobility heterostructure Hall effect magnetic sensor and an n-Al0.4Ga0.6As piezoresistive displacement sensor have been integrated in a single III–V semiconductor cantilever. This allows simple operation in high-vacuum/variable-temperature environments and enables very high magnetic and topographic resolution to be achieved simultaneously. Scans of magnetic induction and topography of a number of samples are presented to illustrate the sensor performance at 300 and 77 K.

Micromachined III–V cantilevers for AFM-tracking scanning Hall probe microscopy

In this paper we report the development of a new III–V cantilever-based atomic force sensor with piezoresistive detection and an integrated Hall probe for scanning Hall probe microscopy. We give detailed descriptions of the fabrication process and characterization of the new integrated sensor, which will allow the investigation of magnetic samples with no sample preparation at both room and cryogenic temperatures. We also introduce a novel piezoresistive material based on the ternary alloy n+-Al0.4Ga0.6As which allows us to achieve a cantilever deflection sensitivity ΔR/(R Δz) = 2 × 10−6 A−1 at room temperature.

Room Temperature Sub-Micron Magnetic Imaging by Scanning Hall Probe Microscopy

An ultra-high sensitive room temperature scanning Hall probe microscope (RT-SHPM) system incorporating a GaAs/AlGaAs micro-Hall probe was used for the direct magnetic imaging of localized magnetic field fluctuations in very close proximity to the surface of ferromagnetic materials. The active area, Hall coefficient and field sensitivity of the Hall probe were 0.8 µm×0.8 µm, 0.3 Ω/G and 0.04 G/√Hz, respectively. The use of a semiconducting Hall probe sensor enabled measurements in the presence of externally applied magnetic fields. Samples studied included magnetic recording media, demagnetized strontium ferrite permanent magnets, and low coercivity perpendicular garnet thin films. The RT-SHPM offers a simple means for quantitatively monitoring sub-micron magnetic domain structures at room temperature.

Controlled suppression of superconductivity by the generation of polarized Cooper pairs in spin valve structures

(Received 10 April 2014; revised manuscript received 13 January 2015; published 2 February 2015) Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.

Artificial domain structures realized by local gallium focused Ion-beam modification of Pt∕Co∕Pt trilayer transport structure

We demonstrate that a high-resolution Ga focused ion beam can be used to introduce artificial domain structures in Pt(1.6nm)∕Co(0.5nm)∕Pt(3.5nm) trilayer transport structures. We have used thin SiO2 overlayers to control the effective energy and dose of Ga ions at the Pt∕Co interface. The extraordinary Hall effect (EHE) was used to characterize the magnetic properties of the patterned films. Using 30keV Ga ions and SiO2 overlayer thicknesses in the range of 0–24nm, we achieve complete control of the coercive field of our Pt∕Co∕Pt trilayer structures. The magnetization reversal mechanism for an artificial domain of size of 3×0.5μm2 is investigated using EHE.