Scanning Hall probe microscopy of superconductors and magnetic materials (original) (raw)

Real‐time scanning Hall probe microscopy

1996

We describe a low-noise scanning Hall probe microscope having unprecedented magnetic field sensitivity (ϳ2.9ϫ10 Ϫ8 T/ͱHz at 77 K͒, high spatial resolution, ͑ϳ0.85 m͒, and operating in real-time ͑ϳ1 frame/s͒ for studying flux profiles at surfaces. A submicron Hall probe manufactured in a GaAs/AlGaAs two-dimensional electron gas ͑2DEG͒ is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. Flux penetration into a high T c YBa 2 Cu 3 O 7Ϫ␦ 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, ⌽ 0 .

SCANNING HALL PROBE MICROSCOPY OF MAGNETIC VORTICES IN VERY UNDERDOPED YTTRIUM-BARIUM-COPPER-OXIDE

Since their discovery by , high-temperature cuprate superconductors have been the subject of intense experimental research and theoretical work. Despite this large-scale effort, agreement on the mechanism of high-T c has not been reached. Many theories make their strongest predictions for underdoped superconductors with very low superfluid density n s /m * . For this dissertation I implemented a scanning Hall probe microscope and used it to study magnetic vortices in newly available single crystals of very underdoped YBa 2 Cu 3 O 6+x . These studies have disproved a promising theory of spin-charge separation, measured the apparent vortex size (an upper bound on the penetration depth λ ab ), and revealed an intriguing phenomenon of "split" vortices.

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

Japanese Journal of Applied Physics, 2001

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 /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.

Cryogenic scanning Hall-probe microscope with centimeter scan range and submicron resolution

2005

We have constructed a scanning Hall-probe microscope that combines a 1 ϫ 4 cm scan range with 200 nm positioning resolution by coupling stepper motors to high-resolution drivers and reducing gears. The instrument is uniquely suited for efficient magnetic imaging of mesoscopic devices, media, and materials, operating from 4 K to room temperature with fast turn-around time. Its potential for studying dissipation in coated conductors-high-T c superconducting tapes-is demonstrated via model systems. We image an entire sample of YBa 2 Cu 3 O 7−␦ , then zoom in to individual fluxons. Flux penetration into a single artificial grain boundary is imaged with 4 ϫ 10 −3 G/ ͱ Hz field resolution and 25 s time resolution by averaging over cycles of ac driving current. Using the resulting magnetic movie, we map out ac power losses.

Application of the Hall-Probe Technique for Magnetization Measurements of Superconductors

A commercial Hall probe (supplied by Lake Shore Cryotronics, Inc.) is used as a sensor of the magnetic field at the surface of a magnetized sample. With the Linear Research resistance bridge LR-400, an excellent stability, linearity of response on field and an extremely small temperature dependence of the background signal is obtained, which is essential for this kind of applications of a Hall probe. At certain conditions, a resolution in field measurements of about 10 mG is reached. Examples of magnetization measurements as a function of temperature, magnetic field and time, performed on high-T c and heavy-fermion superconductors, are discussed.

An ultra-low temperature scanning Hall probe microscope for magnetic imaging below 40 mK

Review of Scientific Instruments

We describe the design of a low temperature scanning Hall probe microscope (SHPM) for a dilution refrigerator system. A detachable SHPM head with 25.4 mm OD and 200 mm length is integrated at the end of the mixing chamber base plate of the dilution refrigerator insert (Oxford Instruments, Kelvinox MX-400) by means of a dedicated docking station. It is also possible to use this detachable SHPM head with a variable temperature insert (VTI) for 2 K-300 K operations. A microfabricated 1μm size Hall sensor (GaAs/AlGaAs) with integrated scanning tunneling microscopy tip was used for magnetic imaging. The field sensitivity of the Hall sensor was better than 1 mG/√Hz at 1 kHz bandwidth at 4 K. Both the domain structure and topography of LiHoF4, which is a transverse-field Ising model ferromagnet which orders below TC = 1.53 K, were imaged simultaneously below 40 mK.

Scanning vector Hall probe microscopy

Journal of Magnetism and Magnetic Materials, 2004

We have developed a scanning vector Hall probe microscope for mapping magnetic field vector over magnetic samples. The microscope is based on a micromachined Hall sensor and the cryostat with scanning system. The vector Hall sensor active area is B5 Â 5 mm 2 . It is realized by patterning three Hall probes on the tilted faces of GaAs pyramids. Data from these 'tilted' Hall probes are used to reconstruct the full magnetic field vector. The scanning area of the microscope is 5 Â 5 mm 2 , space resolution 2.5 mm, field resolution B1 mT Hz À1/2 at temperatures 10-300 K. r

A milliKelvin scanning Hall probe microscope for high resolution magnetic imaging

Journal of Physics: Conference Series, 2009

The design and performance of a novel scanning Hall probe microscope for milliKelvin magnetic imaging with submicron lateral resolution is presented. The microscope head is housed in the vacuum chamber of a commercial 3 He-refrigerator and operates between room temperature and 300 mK in magnetic fields up to 10 T. Mapping of the local magnetic induction at the sample surface is performed by a micro-fabricated 2DEG Hall probe equipped with an integrated STM tip. The latter provides a reliable mechanism of surface tracking by sensing and controlling the tunnel currents. We discuss the results of tests of the system and illustrate its potential with images of suitable reference samples captured in different modes of operation.

Imaging of vortices in conventional superconductors by magnetic force microscopy

Physica C: Superconductivity, 2000

We have imaged vortices in the conventional superconductors NbSe2 (crystal) and Nb (thin film) with a low temperature magnetic force microscope (MFM). The MFM detection is based on commercially available piezoresistive cantilevers. A considerably improved sensitivity (0.2 pN/nm) at 4.3 K has been obtained by using a higher flexural mode of the cantilevers. The operation at higher (2nd or 3rd) mechanical resonances improves the signal-to-noise ratio by a factor of 5. The improved sensitivity allows us to reduce the heat dissipation down to 0.05 mW in the cantilevers without lowering their performance, which is highly desirable for MFM applications at liquid helium temperatures. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by oblique incidence molecular beam epitaxy. A magnetic field of 0.5–3 mT was used to induce the vortices under field-cooled conditions. In thin Nb films, we observed an irregular vortex arrangement and the imaged vortices are attached to individual pinning centers. On the cleaved surface of NbSe2 crystals, we observed an evolution from a disordered towards an ordered state of the vortex lattice. The possibility to image the Abrikosov vortex lattice in NbSe2 can be understood in terms of collective pinning effects.