Seeing the Magnetization in Transparent 

 Magnetic Crystals 



By J. F. Dillon, Jr. 



Bell Telephone Laboratories, Inc. 

 Murray Hill, NJ. 



[With 8 plates] 



The metallic magnetic materials iron, nickel, cobalt, and their alloys 

 are within the everyday experience of us all. However, magnetic ma- 

 terials that are not metallic have been the subject of a great deal of 

 study in the past 15 or 20 years. Those which have been most widely 

 studied and used are mixed oxides ; i.e., oxides of two or more metals. 

 The technological interest in these compounds arises from the fact that 

 they are exceedingly poor conductors of electricity. Alternating mag- 

 netic fields can penetrate them easily. This contrasts sharply with 

 the magnetic metals in which alternating magnetic fields cause eddy 

 currents to flow near the surfaces, and thus shield the body of the metal 

 from the field. This effect becomes more pronounced as the alternat- 

 ing frequency increases, and quite effectively prevents the use of the 

 magnetic properties of the solid metals at ordinary radiofrequencies 

 and above. We will concern ourselves in this paper with a particular 

 class of mixed oxides, the ferrimagnetic garnets, which will transmit 

 radiation up to frequencies in the optical range [1].^ This specializa- 

 tion should not obscure the fact that there are a number of other trans- 

 parent magnetic compounds [2]. The term "transparent" should be 

 qualified. These crystals are transparent in that sections 0.005 inch 

 thick or less transmit enough light for microscopic examination. The 

 most satisfactory specimens are about 0.001 inch thick. 



The ferrimagnetic garnets were discovered only a few years ago 

 [3, 4]. They have the same crystal structure as the minerals known 

 as garnets; that is to say, they contain oxygen atoms and metal 

 atoms in the same spatial arrangement. However, it should be clearly 

 understood that compounds with the particular combinations of metal 

 atoms with which we are concerned do not occur in nature. The 

 chemical formula [5] of these compomids may be written 



1 Numbers In brackets Indicate references at end of text. 



385 



