ELECTRON MIRROR MICROSCOPY 



visual obsen^ation of the magnetic stray 

 fields on grain boundaries in magnetic mate- 

 rials is also made possible by electron mirror 

 microscopy (9). This possibility might be of 

 significance for the still controversial subject 

 of grain size effects on the magnetic proper- 

 ties of such important magnetic materials as 

 silicon-iron and in studies concerned with the 

 basic physics of magnetism in such narrow 

 regions of distorted order. 



The wide field of thin magnetic films which 

 is becoming more and more important might 

 also profit from the capabilities of electron 

 mirror microscopy. For instance, magnetic 

 domain patterns and their movements with 

 applied magnetic fields can be observed in 

 films of the "Permalloy" type by electron 

 mirror microscopy. Another application in 

 the area of thin magnetic films is the depic- 

 tion of magnetic patterns recorded on ferro- 

 magnetic thin films such as AInBi films. For 

 example, Figure 7 shows an electron mirror 

 micrograph of a sine wave south pole trace 

 in north pole surroundings recorded on MnBi 

 film with an electron beam (10) by utilizing 

 the method of Curie point writing (11). 



Image and contrast forming in electron 

 mirror microscopy is not of the Gaussian 

 dioptrics type, but is based on a point-to-di- 

 rection correlation rather than on a point-to- 

 point correlation. Optically speaking, elec- 

 tron mirror microscopy resembles the 

 shadow-schlieren method (12) in which the 

 specimen is also used as a mirror and wherein 

 inhomogeneities in the equipotentials in 

 front of the mirror represent the electron op- 

 tical "schlieren." Formation of the image 

 contrast therefore requires the deflection of 

 the electron pencils by the object points 

 which are to be depicted against their back- 

 ground. Contrast formation is thus based on 

 a kind of deflection modulation of the elec- 

 tron paths by that component of the electric 

 field which is parallel to the mirror-specimen. 

 The force causing the deflection is propor- 

 tional to and in the direction of the electric 

 field. This force is independent of electron 



•^ 



** 



V. 



i 

 i 



1 



Fig. 7. South pole trace recorded onto MnBi 

 film by writing with an electron beam as revealed 

 bj^ electron mirror microscopj'. Magnification ap- 

 prox. 60X 



velocity and thus independent of the direc- 

 tion of the velocity. The force exerted by the 

 electric field providing the deflection retains, 

 therefore, the same direction for the elec- 

 trons when they approach the mirror-speci- 

 men as when they recede from it. In the elec- 

 trical case, and also, of course, in the case of 

 the depiction of a surface relief structure, 

 the resulting contrast-formmg deflection is 

 therefore the sum of both deflections. 



If one desires, however, to use the in- 

 homogeneities of a magnetic field in front of 

 a specimen as electron optical schlieren to 

 obtain a pictorial representation of the dis- 

 tribution of magnetic fields on a specimen, 

 the case is quite different. The force on an 

 electron caused by a magnetic field (Lorentz 

 force) is more complex and velocity-depend- 

 ent. There is in the magnetic case, therefore, 

 a deflection canceling trend, because for the 

 electron's velocity component normal to the 

 plane of the mirror the direction of the force 

 on the electrons approaching the mirror- 

 specimen is opposite to the direction of the 

 force exerted on the receding electrons. Yet 

 there remains the possibihty of utiUzing the 

 radial component of electron velocity, small 

 as this may be, for image and contrast forma- 

 tion in the magnetic case. This radial ve- 

 locity does not reverse its sign, and thus the 



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