Rob IJsselsteijn

High-T/sub c/ superconducting quantum interference filters for sensitive magnetometers

We present several kinds of Superconducting Quantum Interference Filters (SQIFs) which are all realized with high-T/sub c/ superconductors. All SQIFs use the same configuration of 30 loops of different size. The properties of these SQIF types - serial arrays, parallel arrays, and a combination of both - are discussed concerning their usefulness for magnetometry. These properties are the formation of the desired single voltage peak, its peak voltage and full width at half maximum, and the magnetic field noise. Concerning all parameters an improvement can be achieved with SQIFs of all types compared to a single SQUID.

Characteristics and performance of an intensity-modulated optically pumped magnetometer in comparison to the classical M x magnetometer

We compare the performance of two methods for the synchronization of the atomic spins in optically pumped magnetometers: intensity modulation of the pump light and the classical M(x) method using B(1) field modulation. Both techniques use the same set-up and measure the resulting features of the light after passing a micro-fabricated Cs cell. The intensity-modulated pumping shows several advantages: better noise-limited magnetic field sensitivity, misalignment between pumping and spin synchronization is excluded, and magnetometer arrays without any cross-talk can be easily set up.

Investigation of multiple SQUID arrangements in single layer high T/sub c/ magnetometers

Single layer high T/sub c/ dc SQUID sensors on bicrystal substrates were investigated. Special attention was paid to achieve a large flux-to-voltage transfer function in order to ensure stable operation of the SQUID electronics and to get low flux noise even in disturbed environment. Josephson junctions on 30/spl deg/ bicrystal were tested as well as sub-/spl mu/m junctions an 24/spl deg/ bicrystals. The steep edges achieved by sub-/spl mu/m patterning increased the resistance of the superconducting structures against large external fields. The design for large flux-to-voltage transfer functions focused on multiple de SQUID arrangements. For magnetometers with a directly coupled pickup-loop three junction SQUIDs show no advantage compared to standard two junction SQUIDs, but the series connection of two ordinary dc SQUIDs does. Although for magnetometers with inductively coupled pickup-loop twenty SQUIDs in series were used the effect on the flux-to-voltage transfer coefficient was poor, due to differences in the critical current and inductance of individual SQUIDs. The sensitivity of the whole magnetometer achieved with inductively coupled single layer pickup-loops is much smaller than with directly coupled ones. For the latter a pickup-loop formed as a conventional square washer shows better results compared to a slotted square.

Bicrystal submicrometer Josephson junctions and dc SQUIDs

Submicrometer Josephson junctions and dc SQUIDs with such junctions have been prepared on symmetrical 24/spl deg/ bicrystal substrates and were electrically characterized. The submicrometer structures are patterned using e-beam lithography and a C/Ti/e-beam resist mask system. Junctions with dimensions down to about 350 nm show no degradation of their superconducting properties at 77 K, when patterned at low temperatures. Series connections of directly coupled SQUIDs with large (110-160 pH) coupling-inductance, prepared with 0.5 /spl mu/m wide junctions show flux-to-voltage transfer function values up to 100 /spl mu/V//spl Phi//sub 0/. For these SQUIDs, typical white noise levels of 10 /spl mu//spl Phi//sub 0///spl radic/Hz are measured at 77 K.

Archaeometric prospection with high-T/sub c/ SQUID gradiometers

Mapping of the Earths magnetic field or its gradient is a widely used method in archaeological prospection. The use of SQUIDs promises to be advantageous for archaeometry, since they combine a high field resolution with a large bandwidth. Compared to conventional Cs vapor sensors SQUIDs can be used for much faster magnetic mapping, allowing, for the first time, the investigation of huge archaeological features in a reasonable time period. We have investigated several SQUID systems for their usability in archaeometry by measuring a Neolithic double ring ditch enclosure. We have used two electronic High Temperature Superconductor SQUID (HTS SQUID) gradiometers with base lengths of about 60 cm and a Low Temperature Superconductor SQUID (LTS SQUID) gradiometer with a base length of 4 cm. Their intrinsic magnetic field resolution was 6 pT/m//spl radic/Hz for the HTS SQUID gradiometers and about 0.1 pT/m//spl radic/Hz for the LTS SQUID gradiometer. In contrast to Cs vapor gradiometers, which measure the gradient of the total magnetic field, SQUID gradiometers measure one component of the gradient tensor. Since measurements have to be performed whilst movement in the background of the Earths magnetic field, balancing is the limiting factor for the magnetic gradient field resolution of vector gradiometers.

SQUID gradiometers for archaeometry

The mapping of the Earths magnetic field or field gradient is a proven method in surface exploration and archaeometry. Caesium vapour magnetometers show the best magnetic field resolution of commercial devices, but their sampling frequency is limited to 10 Hz. Using SQUIDs it is possible to achieve the same or even better magnetic field resolution with a sampling frequency as high as 100 Hz or more. This allows significantly shorter acquisition times, which is essential for the mapping of large objects. In this paper we check the performance of our developed systems on a neolithic double-ring ditch enclosure near Weimar, Germany. We compare mappings of this area using an electronic caesium gradiometer, an electronic HTS SQUID gradiometer and an integrated planar LTS SQUID gradiometer. With all three systems the magnetic pattern of the ditch is visible; however, the electronic HTS gradiometer shows disturbances of the same order of magnitude as the gradient signal of the ditch, due to an insufficient common mode rejection whilst being moved. The planar LTS SQUID gradiometer shows superior performance. Its mapping shows a much better contrast and features that are not visible in the mapping of the caesium gradiometer.

Highly balanced single-layer high-temperature superconductor SQUID gradiometer freely movable within the Earth's magnetic field

We developed a gradiometer system based on a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID), which can be freely moved within the Earths magnetic field during measurement. The problem of circumferential shielding currents in the parallel gradiometer pick-up loop is solved by the use of an appropriately designed magnetometer SQUID integrated on the gradiometer chip. The magnetometers feedback coil of the flux-locked loop is laid out as a small Helmholtz coil pair, thus keeping the homogeneous magnetic field constant for both the magnetometer and the gradiometer. Therefore, the balance of the directly coupled gradiometer SQUID is enhanced from 100 up to 3800. The noise limited magnetic field gradient resolution of 45 pT m−1 Hz−1/2 is preserved down to frequencies of several Hz even after strong motion in the Earths magnetic field.

Integrated SQUID gradiometers for measurement in disturbed environments

Planar dc SQUID gradiometers with integrated antenna were designed and investigated. With low-T/sub c/ superconductors they were realized as washer type and as transformer type SQUID gradiometers. With high-T/sub c/ superconductors we tested galvanometer SQUIDs. Field gradient sensitivities of 32 fT/(cm/spl radic/(Hz)) could be achieved with low-T/sub c/ SQUIDs and of 520 fT/(cm/spl radic/(Hz)) with high-T/sub c/ SQUIDs, respectively. All types of gradiometers could work stable in disturbed environments. The influence of the SQUID gradiometer layout and the construction of the sample holder on the degree of disturbance suppression is shown.

Light-shift suppression in a miniaturized Mx optically pumped Cs magnetometer array with enhanced resonance signal using off-resonant laser pumping.

The performance of an optically pumped Mx magnetometer with miniaturized Cs cell at earths magnetic field strength (50 μT) is investigated. Operation using detuned high intensity laser light is shown to be superior to the conventional resonant operation in terms of the projected shot-noise-limited ( 50 fT/√Hz) and the actual noise-limited sensitivity using a noise compensation method. The Zeeman light shift effect, emerging due to the off-resonant circularly polarized laser radiation and leading to a strong orientational dependence of the measurement, is suppressed by averaging two identical magnetometer configurations pumped with oppositely circularly polarized light. A residual heading error within the range of 14 nT, limited by the present experimental characterization setup, was achieved.

Low-noise Y-Ba-Cu-O flip-chip dc SQUID magnetometers

We have prepared low-noise flip-chip SQUID magnetometers by using dc SQUIDs with large flux-to-voltage transfer functions and superconducting flux transformers. The dc SQUIDs have a square washer geometry and are prepared on a SrTiO/sub 3/ bicrystal substrate with a misorientation angle of 30/spl deg/. The maximum peak-to-peak flux modulated voltage varies typically between 20 and 60 /spl mu/V for a SQUID inductance of 80 pH. The YBa/sub 2/Cu/sub 3/O/sub 7-x/ flux transformers are prepared on a SrTiO/sub 3/ single crystal polished on both sides. The preparation process of the flux transformers involves the deposition of YBCO and SrTiO/sub 3/ layers by pulsed laser ablation and pattern definition by ion beam etching. The flip-chip magnetometers are encapsulated and robust to temperature cycling. We operate the magnetometers in magnetically shielded and in unshielded environment by using a direct-coupled read-out electronics. The field sensitivity of the magnetometers is equal to 2.2 nT//spl Phi//sub 0/. The field resolution of the magnetometers is typically <40 fT/Hz/sup 1/2/ at 1 kHz and about 100 fT/Hz/sup 1/2/ at 1 Hz inside shielding. The suitability of these devices for geophysical applications is shown by Transient Electromagnetics measurements.