Physica C 235-240 (1994) 2699-2700 North-Holland PHYSICA Flux visualization in high-T superconductors using a high--resohlti(>u magneto-optical microscope R. J. Wijngaarden, M. R. Koblischka, and R. Griessen Free University, De Boelelaan 1081, Faculty of Physics and Astrouomy, 1081 HV Amsterdam, The Netherlands. We report on the first, observations of magnetic flux structures in high-To superconductors ushl~ a newly designed low temperature polarization microscope. Magnetic flux in the superconductor is visualized by detecting the rotation of the polarization vector of light (Faraday effect.) within a magneto-optically active EuSe layer which is evaporated onto the surface of the sample. The low temperature microscope as a whole is built as an inseri into a commercial cryostat equipped with a superconducting coil, enabling us to achieve magnetic fiekls up to 7 T. The optical system (lenses, polarizers and translation table) is cooled together wiih the sample in order to minimize the distance between ~he sample and the objective. Key properties are a large numerical aperlure, a high ext.inction ratio of the polarizers and a high sensitivity of the image-intensified camera systent. This should allow in the near future the visualization of individual vortices using the Faraday effeci in transmission in low magnetic fields. There is a great interest to obtain inforlnation on t.he niagnetic flux distribution in a supercon- ductor in the mixed state. Magnetometry can only provide global data (i. e. averaged ow_,r the whole sample), whereas a local technique en- ables orie to study the effect, of the geometry, the anisotropy and the interaction of flux wit li lhe niicrostructure of the sample. Various lo- cal i echniques have been used so far: magneto- optic microscopy, scanning tunneling microscopy, Bitter decoration, Lorentz microscopy, etc. Of these techniques magneto-optical maaging is fast. enough for real-time observation of the flux line lattice. It is well suited for visualization of flux flow, flux creep, flux avalanches et.c. [1-4]. Up to now, magneto-optical imaging setups consist typically of a commercial polarization microscope modified to carry an optical cryostat [.5,6]. Ill this way, all advantages of a commercial micro- scope are used. However, it. is thell impossible to use a superconducting coil due to space limita- liolis, and the niaxiumin field is thus lilliited to z0.3 T using a copper coil. Considering the low observer.ion temperatures required by the High Resolution Faraday effect (It RF) technique using thin layers of EuSe, this field region is not suffi- cient in the case of single crystals or bulk samples of high-T~ superconductors, since tl,e full pene- tration field is typically around 2 - 5 'F. Another 0921-4534/941507.00 O 1994- Elsevier Science B.V. All rights SSDI 0921-4534(94)01915-0 disadvantage is the long distance belwe~n the ob- jeclives and the sailiple. For high resolulioli, a large nulllerical apel'lllre is l'e<lllir~'<l. 'l'ogelller with i.he lliechaliical COlislrailiis o f 111~' lllaglltq t his necessitales a sliori <list :lil('l ,,I , V;t'<'II I lie .ieclive and l lw sallilih', lit Ollr llliCl'OSc~qw l lii~ is realiz~'d Iw c~,:)liii~: ilia' C>lilic~ ;til<l ltol:li'izers i~>- gellier w[lli ill~, saiiil~l~,. Ii IllliV lt~ ~ ili.,,,~ri~',l iill<t ~'illl~'l" ;tit ()xl',~r,I lli~ll'illii,.iil~ ,'rv~>.,l;il ,,,liiil~t>,,,I with a 1 T sut~ereonducling coil or iii cite with a 7 T coil. Boill are equilll)ed wilh a variable l einperal.ure iilserl (1..5...300 I(). Tile Sallillh' is in lteliunl c~xchange gas wliicli gllal'alllees a good thelnlal equilibriuin. The inicroscolw is I>uili for two operation inodcs, reflecl ion ali¢t I ralisinissioil. The ilnages are recorded ill boili ca~es IlSili~ all inlage-iiitonsified (.'(]11) caliiera syslelll. Ill i lie fu- lllre, ilie IraliSiliissioli lliOde sll(>uld alhav lit,' ~.>l>- ~.,rvation of indivi,luai vorlices. llere, we COllCeiilrale oi1 llie tirsi td~el'\'ali¢)liS l>erforllled Oll a l)yB;t._,('lia()._,, (i)vl~('()) .-il,~>l,' crvslal ill i'~,fl~wlioil IliCld<'. II i~ iilll,tflalil I,, !i<,l,, lilat the well characlerized [7] l)y lice ~iil~l,' crv.,,- l al lised in lhe experinielilS was irra<lial,.,l ,~ill~ lead iOliS alid lielice had a very liigh critical Cllr- rellt densily jc and al~o a very MrOllgj liililiilt~. The fi',s 1 (a) Io (e) show [lllx <tisirit>liii~:>ii~ ;li 2')" " three differeiil ieinlwrai~ires. "l'h~' salill;l~' w:l.', il~ every case zero-field ('o(>1~'~1 1o l li~' ('l!~)(~s,,li 1,.I'1- rcscrvcd.