micro-Hall Probe Research Articles

Determination of the magnetic penetration depth with measurements of the vortex-penetration field for type-II superconductors

Using a known distribution of the Meissner currents over the surface of an infinitely long superconducting slab with a rectangular cross section, we find an applied magnetic field at which vortices begin to penetrate into the superconductor. This vortex-penetration field is determined by an interplay of the geometrical and Bean-Livingston barriers. The obtained results enable one to find the lower critical field and the London penetration depth from measurements of the magnetic induction on the surface of the superconducting slab, using, e.g., the micro-Hall probes. Published by the American Physical Society 2024

Vortex Dynamics in Ferromagnetic Nanoelements Observed by Micro-Hall Probes

In this work we measure the nucleation and annihilation of magnetic vortices in Pacman-like (PL) micromagnets prepared from Permalloy (Ni81Fe19, Py) at 77 K. Lateral dimensions of explored objects are ≤ 1 μm with thickness of about 40 nm. The micromagnets are located directly on the high-sensitive micro-Hall probe based on GaAs/AlGaAs heterostructure by lift-o process. Experiments show good agreement of the magnetization reversal with the micromagnetic simulation. Other shapes of micromagnets are also considered to obtain more precise picture of the vortex dynamics.

Single layer graphene Hall sensors for scanning Hall probe microscopy (SHPM) in 3–300K temperature range

• Graphene Hall probes (GHPs) were fabricated for a scanning Hall probe microscope (SHPM). • Minimum detectable magnetic field of GHPs was found to be comparable with conventional Hall probe materials. • The magnetic domains in NdFeB demagnetized magnet were successfully observed at 300 K, 126 K and 3 K for the first time by using GHP. Graphene micro-Hall probes were developed for a scanning Hall probe microscope system and used for the direct magnetic imaging domains of demagnetized NdFeB permanent magnet for the first time. The Hall coefficient and minimum magnetic field resolution of graphene Hall probes at 1 kHz were found to be 0.18 Ω/G and 0.20 G / Hz for a drive current of 3 μA at room temperature in vacuum. The magnetic domains in NdFeB demagnetized magnet were observed at 300 K, 126 K and 3 K successfully.

Characteristics of a sensitive micro-Hall probe fabricated on chemical vapor deposited graphene over the temperature range from liquid-helium to room temperature

We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ∼0.031-0.12 Ω/G, while the field resolution could reach ∼0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

In this article, we review our recent studies of microwave penetration depth, lower critical fields, and quasiparticle conductivity in the superconducting state of Fe-arsenide superconductors. High-sensitivity microwave surface impedance measurements of the in-plane penetration depth λ ab in single crystals of electron-doped PrFeAsO 1 - y ( y ∼ 0.1 ) and hole-doped Ba 1 - x K x Fe 2 As 2 ( x ≈ 0.55 ) are presented. In clean crystals of Ba 1 - x K x Fe 2 As 2 , as well as in PrFeAsO 1 - y crystals, the penetration depth shows flat temperature dependence at low temperatures, indicating that the superconducting gap opens all over the Fermi surface. The temperature dependence of superfluid density λ ab 2 ( 0 ) / λ ab 2 ( T ) in both systems is most consistent with the existence of two different gaps. In Ba 1 - x K x Fe 2 As 2 , we find that the superfluid density is sensitive to degrees of disorder inherent in the crystals, implying unconventional impurity effect. We also determine the lower critical field H c 1 in PrFeAsO 1 - y by using an array of micro-Hall probes. The temperature dependence of H c 1 saturates at low temperatures, fully consistent with the superfluid density determined by microwave measurements. The anisotropy of H c 1 has a weak temperature dependence with smaller values than the anisotropy of upper critical fields at low temperatures, which further supports the multi-gap superconductivity in Fe-arsenide systems. The quasiparticle conductivity shows an enhancement in the superconducting state, which suggests the reduction of quasiparticle scattering rate due to the gap formation below T c . From these results, we discuss the structure of the superconducting gap in these Fe-arsenides, in comparison with the high- T c cuprate superconductors.

Imaging capability of pseudomorphic high electron mobility transistors, AlGaN∕GaN, and Si micro-Hall probes for scanning Hall probe microscopy between 25 and 125°C

The authors present a comparative study on imaging capabilities of three different micro-Hall probe sensors fabricated from narrow and wide band gap semiconductors for scanning hall probe microscopy at variable temperatures. A novel method of quartz tuning fork atomic force microscopy feedback has been used which provides extremely simple operation in atmospheric pressures, high-vacuum, and variable-temperature environments and enables very high magnetic and reasonable topographic resolution to be achieved simultaneously. Micro-Hall probes were produced using optical lithography and reactive ion etching process. The active area of all different types of Hall probes were 1×1μm2. Electrical and magnetic characteristics show Hall coefficient, carrier concentration, and series resistance of the hall sensors to be 10mΩ∕G, 6.3×1012cm−2, and 12kΩ at 25°C and 7mΩ∕G, 8.9×1012cm−2 and 24kΩ at 125°C for AlGaN∕GaN two-dimensional electron gas (2DEG), 0.281mΩ∕G, 2.2×1014cm−2, and 139kΩ at 25°C and 0.418mΩ∕G, 1.5×1014cm−2 and 155kΩ at 100°C for Si and 5–10mΩ∕G, 6.25×1012cm−2, and 12kΩ at 25°C for pseudomorphic high electron mobility transistors (PHEMT) 2DEG Hall probe. Scan of magnetic field and topography of hard disc sample at variable temperatures using all three kinds of probes are presented. The best low noise image was achieved at temperatures of 25, 100, and 125°C for PHEMT, Si, and AlGaN∕GaN Hall probes, respectively. This upper limit on the working temperature can be associated with their band gaps and noise associated with thermal activation of carriers at high temperatures.

Novel vortex distribution in the β-pyrochlore superconductor KOs2O6

By utilizing the micro-Hall probe array, we study the superconducting and the vortex states in a single crystal of KOS2O6 near the first-order transition at Tp ∼ 8 K, which is associated with a change in the rattling motion of K ions. We show that below the first-order transition, the lower critical field exhibits a distinct decrease, suggesting that the electron-phonon coupling is weakened below the transition. At high magnetic fields, the local magnetization shows an unexpectedly large jump at Tp with a sign change near the sample edges. Our results demonstrate a novel redistribution of vortices whose energy is reduced abruptly below the first-order transition at Tp.

A silicon metal-oxide-semiconductor field-effect transistor Hall bar for scanning Hall probe microscopy

We demonstrate successful operation of a scanning Hall probe microscope with a few micron-size resolution by using a silicon metal-oxide semiconductor field-effect transistor (Si-MOSFET) Hall bar, which is designed to improve not only the mechanical strength but also the temperature stability. The Si-MOSFET micro-Hall probe is cheaper than the current micro-Hall probes and is found to be as sensitive as a micro-Hall probe with GaAs/AlGaAs heterostructure or an epitaxial InSb two-dimensional electron gas. This was used to magnetically image the surface of a Sm(2)Co(17) permanent magnet during the magnetization reversal process as a function of an external magnetic field below 1.5 T. This revealed firm evidence of the presence of the inverse magnetic seed as theoretically predicted earlier. Magnetically pinned centers, with a typical size 80 mum, are observed to persist even under a high magnetic field, clearly indicating the robustness of the Si Hall probe against the field application as well as the repetition of the measurement.

Local magnetization study of the first-order superconducting transition in CeCoIn 5

We report local magnetization measurements on the heavy fermion superconductor CeCoIn 5 using a micro-Hall probe. Below a critical point T 0 ( ∼ 0.8 K ), the local magnetization shows a clear jump at the superconducting transition temperatures for both H ∥ a and H ∥ c , indicating that the phase transition at the upper critical field H c 2 becomes a first-order phase transition. In addition, we observed an undershoot behavior of magnetization jump above μ 0 H ∼ 4.75 T ( H ∥ c ), which suggests a rapid change of textured superconducting structure with vortices and the nodal planes expected in the Fulde–Ferrell–Larkin–Ovchinnikov state.

Superconducting Double Transition in PrOs4Sb12Probed by Local Magnetization Measurements and Magneto-Optical Imaging

Double superconducting transition in PrOs 4 Sb 12 has been studied carefully by performing the local magnetization measurements using micro-Hall probe and the magneto-optical imaging. Local magnetization curves for each element of the Hall probe reveal that there exists a significant difference in the transition temperature at each point; two phases which give different T c 's are stabilized in the crystal. Magneto-optical imaging at T < T c2 and T c2 < T < T c1 also reveals a non-uniform superconducting nature. The characteristics of superconducting double transition in PrOs 4 Sb 12 are discussed based on the local field inductions at multi-points.

Bulk and Local Magnetic Properties of Iron-Based Oxypnictide Superconductor SmFeAsO1-xFx

We have characterized bulk and local magnetic properties of iron-based oxypnictide superconductor SmFeAsO 0.7 F 0.3 polycrystalline sample ( T c ∼53 K) synthesized at ambient pressure and NdFeAsO 0.7 ( T c ∼52 K) synthesized at high pressure by using SQUID magnetometer, micro-Hall probe, and magneto-optical imaging technique. Intragranular critical current densities evaluated from bulk magnetization and local magnetic profile give values of about 1×10 5 A/cm 2 at 10 K in both compounds. Magnetic field profiles of both samples are far from Bean-like, suggesting that intergrain connection is not strong. Low-field characteristics including flux-penetration field are measured by using a micro-Hall probe. Obtained results are compared with data on high temperature superconductors.

Local magnetization measurements of the first-order transition of CeCoIn 5

We measured the local magnetization in CeCoIn 5 with a micro-Hall probe for magnetic fields H along the [0 0 1] and [1 0 0] directions. We observed a sharp jump in the local magnetization at T c ( H ) below 0.77 K for H ∥ [0 0 1] and 0.79 K for H ∥ [1 0 0], indicating the first-order phase transitions at upper critical fields. We also observed a dip structure in the local magnetization at the first-order phase transitions for H ∥ [0 0 1], below which the FFLO state is realized. We infer that the dip structure is consistent with the development of planar nodes in the FFLO state.

High temperature scanning Hall probe microscopy using AlGaN∕GaN two dimensional electron gas micro-Hall probes

Hall sensors fabricated using conventional narrow band gap semiconductors such as InSb and GaAs are unstable above room temperature due to the onset of intrinsic conduction and physical degradation of the semiconductor materials. Gallium nitride (GaN) based wide band gap semiconductors are stable at elevated temperatures and show potential for fabrication of high temperature electronic devices. Here, we incorporated high sensitivity AlGaN∕GaN two dimensional electron gas heterostructure micro-Hall probes (HPs) into a high temperature scanning Hall probe microscope and for magnetic imaging of domains in crystalline iron garnet thin films from room temperature to 100°C. The active area and Hall coefficient the HPs were 2×2μm2 and 0.01Ω∕G at 100°C, respectively. The evolution of the size of magnetic domains with increasing temperature under external magnetic fields is described.

Ratchet without spatial asymmetry: Controlling the motion of magnetic flux quanta using time-asymmetric drives

We report on vortex manipulation experiments which exploit the attractive interaction between ‘crossing’ pancake vortex (PV) and Josephson vortex (JV) lattices in Bi 2Sr 2CaCu 2O 8+ δ (BSCCO) single crystals under tilted magnetic fields. Deformations of the JV lattice allow us to indirectly move mutually interacting PVs. In dc-driven mode the in-plane field is varied slowly (adiabatically) while the PV density is monitored at different points in our single crystal BSCCO sample using a micro-Hall probe array. Variations in PV density as large as 40% have been demonstrated in this way and both enhancement and depletion are observed, depending on the location in the crystal, temperature and PV density. In ac-driven experiments, manipulation is achieved by applying trains of time-asymmetric (sawtooth) in-plane field pulses. We show how both flux focusing and defocussing can be readily achieved merely by varying the time-asymmetry of the drive. Our experimental results are well described by molecular dynamics simulations which consider the dragging of PVs by JVs.

Variable temperature scanning Hall probe microscopy of ferromagnetic garnet thin films

Variable temperature scanning Hall probe microscope system (VT-SHPM) was constructed for magnetic imaging of domains over large areas at cryogenic temperatures using InGaAs/AlGaAs heterostructure micro-Hall probes. The VT-SHPM system was used to measure the temperature dependence of magnetic domains in 50 μm thick (BiTbHo) 3Fe 5O 12 epitaxially grown garnets. The domain width increased with decreasing temperature with a rapid increase occurring at the compensation point of the garnets. Correlation of the stray magnetic field measured by the VT-SHPM and conventional magnetization measured is discussed.

Control of pancake vortex distribution by in-plane fields in Bi2Sr2CaCu2O8+y

The moderately strong attractive interactions between pancake vortices and Josephson vortices in the crossing-lattices state of Bi2Sr2CaCu2O8+y are utilized to change the distribution of pancake vortices by Josephson vortices. We have confirmed such a controllability by demonstrating the changes in the number of pancake vortices by using a micro-Hall probe while changing only the in-plane field. Increase in the in-plane field results in an increase in the density of pancake vortices near the center of the sample, demonstrating the lensing effect of vortices. We have also succeeded in visualizing the lensing effect by the differential magneto-optical method. Effects of the history, the density of pancake vortices, and the temperature on the lensing effect are studied.

Visualization and control of vortex chains in highly anisotropic superconductors

We report the evolution of the vortex state under tilted fields in anisotropic superconductors. Both in highly anisotropic Bi2Sr2CaCu2O8+y and moderately anisotropic underdoped YBa2Cu3O7−δ, vortex chains coexisting with vortex lattices are observed under tilted fields. Despite the similarity of the vortex states, we believe that the state realized in Bi2Sr2CaCu2O8+y and underdoped YBa2Cu3O7−δ are different considering the recent theoretical studies. We visualize the vortex chains in Bi2Sr2CaCu2O8+y using the magneto-optical technique, and demonstrate their controllability by the in-plane field. We also demonstrate the lensing effect of vortices in Bi2Sr2CaCu2O8+y by micro-Hall probe magnetometry and by direct visualization using magneto-optical imaging.

An integrated micro-Hall probe for scanning magnetic microscopy

A new microsystem with the ability to detect magnetic microstructures based on the non-invasive Hall principle is presented. The micro-Hall plate embedded in the microsystem and scaled down to the physical limits of the employed CMOS technology has an active area of only 2.4 μm × 2.4 μm. The microsystem exhibits an output sensitivity of 7.5 V/T in perpendicular direction to the chip surface and a magnetic field resolution of 300 nT/√Hz at 1 Hz. A two-dimensional magnetic scanner was developed to demonstrate the performances of the developed microsystem.

Anisotropic pancake vortex transport in the crossing lattices regime ofBi2Sr2CaCu2O8+δsingle crystals

We have used micro-Hall probe magnetometry to investigate how the out-of-plane ``local'' magnetization of a superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal depends on the strength and direction of an in-plane magnetic field, ${H}_{\ensuremath{\Vert}}$, in the crossing vortex lattices regime. The ${H}_{z}=0$ remanent magnetization exhibits a pronounced anisotropy, being largest with ${H}_{\ensuremath{\Vert}}$ parallel to the crystalline $a$-axis, and smallest when it is parallel to the orthogonal $b$-axis. We attribute this to the presence of strongly pinning linear defects (LDs), which are known to lie close to the $a$-axis in these crystals. With ${H}_{\ensuremath{\Vert}}$ parallel to the $a$-axis Josephson vortex (JV) stacks become indirectly pinned along LDs, and channel pancake vortices (PVs) into these regions of high disorder, increasing the measured irreversibility. With ${H}_{\ensuremath{\Vert}}$ along the $b$-axis PVs are efficiently channeled into the sample centre, readily cutting across LDs, and the irreversibility is low. At high temperatures the remanent magnetisation as a function of the in-plane field angle exhibits a pronounced shoulder which appears to be related to the critical accommodation angle for the indirect trapping of JVs by LDs, above which JVs snap free. At low temperatures random bulk pinning is increasingly in competition with LDs, and the measured anisotropy becomes much weaker.

Peculiar superconductivity in PrOs 4Sb 12 probed by local magnetization

We performed local magnetization measurements in the superconducting state of PrOs 4Sb 12 using micro-Hall probe. For a magnetic field applied along one of the principal axes, a remarkable dip structure in the magnetization hysteresis loop was observed close to zero local magnetic induction. The anomaly is most pronounced when the magnetic field is applied only in a certain axis of the crystal, and is not observed along other two axes. We believe that the anomaly close to zero induction is universal to the spin-triplet superconductors with broken time-reversal symmetry.