Physica B 194-196 (1994) 379-380 North-Holland PHYSICA

Principles of scanning photomagnetic microscopy using SQUID magnetometers

X.-F. He, A. Kotlicki, G. Meagher, Y.-F. Lu, and B.G. Turrell Department of Physics, University of British Columbia, Vancouver, B.C., V6T 1Z1, Canada

We are investigating a scanning imaging technique using integrated dc SQUID magnetometers for studying magnetic microstructures. The technique utilizes the SQUID to probe the flux produced by the photomagnetic source near the surface, and imaging is achieved by scanning the sample close to the pick-up loop of the SQUID. This scheme is demonstrated numerically for a planar array of superconducting spheres. In combination with the technique of scanning laser microscopy, the sensitivity, resolution as well as potential applications are discussed.

1. INTRODUCTION

Scanning imaging technology has important applications in physical and biological sciences [1]. In this method, imaging is achieved by scanning the sample while monitoring specific physical quantities. Here, we propose a technique of scanning microscopy for studying photomagnetic effects. The technique utilizes a superconducting quantum interference device (SQUID) to probe the flux produced by the light-induced magnetic source on and/or near the surface of the material under study. The imaging is achieved by scanning the sample in two dimensions close to a pick-up loop of the SQUID. To analyse the sensitivity, this scheme is examined numerically for a planar array of superconducting spheres in a magnetic field. This system was chosen not only because the calculations are relatively simple but also because arrays can be fabricated in our laboratory [2] and will be used in experimental tests of the method.

2. SQUID MAGNETOMETRY

The operation of the scanning microscope is demonstrated using a planar array of super- conducting spheres in an external magnetic field H o, set below the critical field value of the super- conductor. In this case, each superconducting sphere behaves as a dipole (Meissner effect),

p = -2 rc a 3 H o ( 1 )

where a is the radius of the sphere. Assuming all the spheres are the same, the system can simply be described by a planar array of dipoles, which give an additional magnetic induction,

B=--d-d2v p. (2)

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where r i is the vector from the centre of i-th sphere pointing to the point concerned, and /,t o is the magnetic permeability of free space.

Signals are picked up by a gradiometric coil consisting of two counter-wound square loops, and it is a good approximation to assume that the net flux through the coil is due to the field generated by the superconducting spheres close to either loop. The threading flux can then be calculated using Eqs. (1) and (2). The SQUID output voltage is proportional to the flux change in the pick-up loop.

Figure 1 shows two typical results of magnetic flux (in terms of the flux quantum ~o=2.07x10 -15 Wb) through a square pick-up loop of side 10 ~m parallel to the plane of the array at a distance of 10 ~tm. The array consists of 100 x 100 super- conducting spheres with 1 lxm radius separated by 50 tam in a magnetic field of 10 mT. Figure 1 illustrates the image of four superconducting spheres of the array. In Fig.l(a), the field is perpendicular to the plane of the array, while for Fig. 1 (b) it lies in the plane. Clearly, the different spatial orientation of magnetization causes the different image morphology.

For a given size of pick-up loop, the difference between the maximum and minimum flux signals when scanning over an area larger than the periodicity of the array are found to decay approximately exponentially with the distance of the loop from the array (Fig.2). Practically, the pick-up loop should therefore be placed as close as possible to the surface of the sample to optimize the SQUID output. In fact, the gradiometer is particularly suitable for reading out these exponentially decaying signals since it is sensitive in nearby signals while eliminates background fields.

A superconducting sphere with 1 ~tm radius in a field of 10 mT is equivalent to a dipole of N5xl0 -14 Am 2. This yields a signal of ~ ~o in a loop at a distance of 10 lam. Using an integrated dc SQUID with flux sensitivity - 10 -6 ~o Hz-1/2 [3], the

1 J m'croscope could determine about 10 polarized electron spins in such configuration.

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Figure 1. hnages of the superconducting spheres in a field (a) parallel and (b) perpendicular to the plane.

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Figure 2, Flux signal difference versus distance of the pick-up loop.

3. F I B E R O P T I C S

The spatial resolution of a magnetic microscope using a SQUID is limited by the dimensions of the pick-up loop. A smaller loop would improve the lateral resolution, and a pick-up loop as small as a few microns with a good matching coefficient with the input coil of the SQUID could be fabricated using integrated circuit technology. Such integrated dc SQUIDs could be utilized very well for the microscope and a resolution of ~~tm would be expected in the photomagnetic measurements, A further improvement could then be achieved by combining the technique of scanning laser microscopy. Submicron resolution could be achieved using a focused laser beam from ,an optical fiber and, in this case, the SQUID probes the magnetic responses of an area selected by the laser. This would be an ultrasensitive diagnostic tool of potential use in studying photomagnetic materials, such as diluted magnetic semiconductors, spin glasses, high-temperature superconductors, and impurities and defects in solids.

Scanning photomagnetic microscopy has certain advantages over other microscopies since it is non- destructive and a high vacuum is not needed. Only the appropriate temperature need to be maintained for the operation of the SQUID system.

4. CONCLUSIONS

We have proposed a technique of scanning photomagnetic microscopy using SQUID magnetometers. Our calculations show that it provides an ultrasensitive imaging method for studying magnetic microstructures. Both the spatial orientation and the magnitude of the magnetization can be determined. In combination with the technique of scanning laser microscopy, this method could be a unique tool for photomagnetic investigations.

R E F E R E N C E S

1. See, for example, E.C. Teague (ed.), Scanning Microscopy Technologies and Applications, SPIE, Bellingham, 1988.

2. M. Le Gros, A. Da Silva, B.G. Turrell, A. Kotlichi, and A.K. Drukier, Appl. Phys. Lett., 56 (1990) 2234.

3. See, for example, J. Clarke, Physica, 126 B (1984) 441.