16 



B. V. BORRIES t, G. LANGNER AND W. SCHEFFELS 



ijjjj' 



"S*. 



J 



/ = 3250 Amp., (J = 60 kV. Continuous variation of magnifi- 

 cation between the three stage diffraction patterns and 

 Amax = 28 000. 



Fig. 3. Cross section of a permanent magnetic microscope 

 system. 



three-Stage diffraction patterns with different magni- 

 fications. 



It is also possible to use the whole system as a 

 diffraction condensor, and also to use it as an elec- 

 tron shadow microscope. If crystalline matter is put 

 into the caustic, dynamic diffraction patterns can 

 be obtained which resemble Kikuchi lines. Such 

 patterns from MgO crystals are shown in fig. 4. 



Another problem in the field of permanent mag- 

 netic lenses which we have investigated is the 

 so-called "reflexion microscopy" which is really a 

 dark field microscopy of surfaces hit by the electron 

 beam in grazing incidence. In order to avoid immer- 

 sion the focal length must be considerably greater 

 than half the gap width in order to give space 

 for the axial movement of the specimen. On the 

 other hand the chromatic aberration constant C< 

 should be as small as possible because the mostly 

 inelastically scattered electrons which are used to 

 image have suffered considerable energy losses AE. 

 If we put for a moment C- «^/ (/- focal length), 

 and d be the diameter of the aperture stop, we obtain 

 for the resolving power 



d> 



/ 



C 



eU* 



cU* 



Fig. 4. Kikuchi lines of MgO crystals obtained in the caustic 

 of a strong permanent magnetic lens. 



when using permanent magnetic lenses with the rela- 

 tivistic accelerating voltage U*. The image brightness 

 goes with the 2nd power of a, so using half the focal 

 length one should get four times the brightness in 

 the image at the same resolution and magnification. 



The requirement of non-immersion objective lenses 

 sets a limit to the lens excitation. We have tried 

 an objective lens with h = 1 mm bore, .v = 0.8 mm 

 gap-width,/ 1.45 mm, C = 1-35 mm which needs 

 / - 1450 Amp at 60 kV. In a two-stage permanent 

 magnetic system this demands the same low excita- 

 tion also for the projector which then is likely to 

 cause distortion. This is a difficulty which arises 

 when using permanent magnetic microscopes for 

 "reflexion microscopy". This difficulty can be over- 

 come by using two-gap objective lenses, over which 

 the effective ampere turns of the magnet are dis- 

 tributed so that the objective lens gets a suitable low 

 excitation and the series gap takes the rest and is 

 used as an intermediate lens. The latter, however, 

 may also cause some distortion. 



We leave now the permanent magnetic lens systems 

 with rotational symmetric lenses, and turn to ele- 

 ments with less symmetry. For "reflexion micros- 

 copy" normally the condensor is tilted. It is also 

 possible to deflect the beam so that its angle of in- 

 cidence is suitable for "reflexion microscopy". K. 

 Ito and T. Ito (5) have used deflection coils. We have 

 tried to use small permanent magnets. For small 

 deflection angles [i the following relation holds 



