·Table of Contents
·Methods and Instrumentation
Eddy Current Measurements with a high spatial resolution SQUID systemFriederike Gruhl, Marc von Kreutzbruck, Michael Mueck and Christoph Heiden
Institute of Applied Physics, University of Giessen, Germany
In a scanning SQUID microscope the sample is scanned by a SQUID, which measures the magnetic field above the surface of the sample. At present there are several different approaches to design a scanning SQUID microscope. Kirtley et al.  and Vu et al.  use a SQUID made from niobium, a so-called low temperature superconductor (LTS). In their microscope, the SQUID and the sample are at 4.2 K, and the stand-off distance between SQUID and sample is on the order of 10 µm . In another approach, a cryostat with a thin window is used to scan a sample at room temperature. This allows for quick sample changes, but the thickness of the window limits the minimum stand-off distance between SQUID and sample -and thus the spatial resolution- to about 100 m m. One can use a SQUID made from so-called high temperature superconductor (HTS), such as YBa2Cu3O7, cooled to liquid nitrogen temperature in order to reduce costs for the coolant [4,5], or a LTS SQUID offering a higher magnetic flux resolution . By cooling the SQUID to 4.2 K and having the sample at room temperature, a spatial resolution of better than 100 µm can be achieved. In this paper we describe such a scanning SQUID microscope using a niobium dc SQUID, and also some applications in the field of nondestructive evaluation.
A. SQUID Sensor and Read-Out Electronics
The microscope is built around a thin film DC-SQUID with Nb-Al-AlOx-Nb tunnel junctions and integrated aluminium shunt. Such a SQUID can be produced in a relatively simple and reproducible process. The SQUID sensor has an intrinsic flux noise of about 10-6F0/ÖHz(1 F 0» 2´ 10-15 Vs) and an effective sensor area of 45 µm ´ 45 µm. The corresponding magnetic field resolution is on the order of 1pT/ÖHz . The SQUID is read out with conventional DC-SQUID electronics in a flux locked loop. A modulation frequency of 70 kHz is used. The bandwidth is limited to 1kHz which corresponds to the maximum scanning speed of the sample stage. The analog output signal is able to cover a dynamic range of maximal 180 dB/ÖHz.
|Fig 1: Bottom cap of the cryostat with SQUID, sapphire window and the mechanism to adjust the distance between SQUID and sample.|
The separation between the sapphire window and the SQUID can be adjusted by rotating a sleeve nut (Fig. 1, part 5). In order to achieve the smallest separation possible, by rotating the sleeve nut we first move the sapphire window closer to the SQUID until it touches the SQUID. Due to the high heat input, the temperature of the SQUID is raised to well above 9 K where the SQUID stops working. We then slightly increase the separation between SQUID and sample, until the SQUID works again. In this way, a minimum separation between SQUID and window on the order of 30 µm can be achieved.
Atmospheric pressure on the sapphire window leads to bowing of the window, which increases the minimum stand-off distance between SQUID and sample. We have investigated several other window materials, such as iridium and tungsten, which, due to their larger elastic modulus, may offer a smaller bowing. The best results were obtained with a 50 µm thick iridium foil. With such a foil it should be possible to reduce the stand-off distance between SQUID and sample further.
C. Sample Stage and Computer Control
The sample is mounted on a x-y-stage moved by two perpendicular linear drives below the cryostat with a maximum speed of 1 mm / s. The spatial resolution of the sample stage is 1 µm, the movement range is about 15 cm for each axis. The two linear drives are controlled by a personal computer using a micro controller. The computer also reads out the digitzed SQUID signal and displays the image in real time during the scanning of a sample.
A. System Characteristics
|Fig 2: Graph showing a line scan across a small feature printed on an one dollar bill. The width of the main peak is resolved to about 50 µm.|
In our geometry a rms flux noise caused by the thermal fluctuations of about is coupled to the SQUID.
B. Magnetic Signatures
Fig 3: Magnetic image of the serial number of a 100.000 Rubel bill printed with ferromagnetic ink.
Fig 4: Magnetic structure on the surface of a single density floppy disk. Marked: Track with a FM encoded bit pattern
|Fig 5: Field distribution around a strong magnetic dipole (upper left) on the surface of a thin geological sample of Prehnite.|
C. Nondestructive Evaluation
Local damage such as scratches and dents as well as fatigue and mechanical stress can generate a remanent magnetisation in certain stainless steels. This remanent magnetisation can easily be detected with a SQUID .
We examined the surface of several 1 mm thick stainless steel slabs with artificial scratches and dents. Figure 6 shows a photograph of a stainless steel sample with various surface damages: six indentations caused by strikes with various power and several shallow scratches. The corresponding magnetic field distributions are shown as plots A and B. We found that the remanent magnetisation is proportional to the strength of the mechanical stress: one can recognize the correlation between the strength with which the indentations were created and the magnetic field amplitude above the indentations. In this measurements, a spatial resolution of about 100 µm was obtained. Although it is possible to meassure such samples with conventional sensors by magnetizising the damaged stainless steel slabs, one will loose the information about the force which with the defects were created in this case.
|Fig 6: Nondestructive evaluation of a stainless steel sample: Scan images of artificial dents (A) and scratches (B) with a spatial resolution of 40 µm|
|Fig 7: Sketch of an Airbus-300 fuselage skin|
Figure 8 shows the cross section of the investigated samples, which consist of three or four layers of aluminium sheets. The top layer has a thickness of 1.4 to 2 mm, the second layer one of only 0.6 to 0.8 mm and the third layer is 1.8 to 2.5 mm thick. Due to the specific cyclic stress loading, the defects occur in the third layer and the fatigue cracks are most likely to develop along the direction of the fastener row. The rivets have a diameter of 4.8 mm and a distance between each other of 22.5 mm.
|Fig.8: left - cross-section AA. Two by two rivet rows in the sections AFT and FWD. The cracks are located in the third layer of the rivet row 1 FWD and AFT. right - cross-section BB. 1st rivet row in the FWD-Sektion. The cracks are located in the third layer (No. 3) of the rivet row 1 FWD and AFT.|
For the eddy current measurements, we used a single wire positioned perpendicular to the rivet rows to excite eddy currents in the sample. The wire was positioned below the SQUID such that the excitation field was minimized at the location of the SQUID, making an additional electronic compensation unnecessary. The excitation frequency was 2000 Hz and the separation between sample and SQUID was 600 µm
The evaluation of the measurement data was done in the following way:
Figure 9 shows a linescan along the AFT rivet row. The defects can be clearly recognized. The data were raised to the third power. Then the amplitudes of the signals are nearly proportional to the crack length.
Fig 9: Eddy current measurement of an Airbus-300 sample with a SQUID microscope.Measurement was performed with wire excitation at 2 kHz.|
S1: threshold for possible defects,
S2: threshold for definite defects
Fig.10: Eddy current measurement of an Airbus-300 sample FWD 3-14 with a SQUID microscope.
Measurement via wire excitation at 2 kHz and a DC LTc-SQUID.|
S1: threshold of possible defects,
S2: threshold of definitive defects
By exciting with a wire the signal of a flawless rivet shows an increase in magnetic field on one side and a decrease on the other side. This is caused by induced currents which flow in clockwise direction on one side and counter-clockwise on the other. A defect induces an additional current which leads to an increase or decrease of the z-component of the magnetic field, depending on the position of the defect. The grey shaded areas in Fig. 9 show the chosen threshold amplitudes S1 for a possible defect and S2 for a definite defect. A 2.9 mm long crack for example is obviously above S2, while a 1.2 mm long defect produces a signal just above S1. Defects in the FWD row produce smaller signals becuase of the presence of a fourth layer of aluminium. Here a 1.6 mm long crack produces a signal just above the S1, whereas a 1.0 long crack is inside the margins for a possible defect. It is also remarkable that a 11.6 mm long crack causes a relatively small but spatially extended signal, in contrast to a conventional system with lower spatial resolution, which measures the integral over the entire flaw area.
We compared the measurements performed with the high spatial resolution SQUID microscope to measurements done with an HTS SQUID-based eddy-current NDE system we developed some time ago in cooperation with Lufthansa, DASA, ESR-Rohmann and the FZ Jülich [9,10]. This NDE system was optimised for eddy-current measurements on multi-layered aircraft structures, but had only a spatial resolution of about 8 mm. In order to show the advantages of a better spatial resolution, we plot in Fig. 11 the results of a measurement on a AFT rivet row with the microscope (solid line) and the HTS SQUID system (dotted line). In the HTS system a small ring coil (3-5 mm diameter) was used for excitation. The stand-off distance between SQUID and sample was about 8 mm. The much higher spatial resolution in the case of the SQUID microscope can easily be seen. The difficulty in case of the HTS system to locate the position of a crack which is shorter than 5 mm can completly be overcome if the spatial resolution is only high enough.
Fig 11: Comparison of a LTc-SQUID microscope with a conentional HTS-system.|
Eddy current measurement of the sample A-300,FWD section(BB) by
1.a SQUID microscope with wire excitation at 2 KHz and a DC LTc-SQUID
2. a rf HTc SQUID in a glas cryostat with circular coil excitation at 2KHz.
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