MAGNETIZATION IN CRYSTALS — DILLON 389 



and large melts. A run in which Nielsen [7] grew some of the large 

 samples took about 3 or 4 weeks to cool. The platinum crucible held 

 a melt weighing about 2 kg. Recently Dr. Lef ever of the Hughes Air- 

 craft Co. has pointed out that crystals grown so as to be free of traces 

 of silicon are markedly more transparent than any for which no 

 special precautions have been taken [8]. 



The preparation of samples suitable to the observation of domains 

 by transparent light is a delicate procedure. If possible, large sound 

 ci-ystals are chosen and slices of the desired orientation are sawn off 

 with a water-cooled circular diamond saw. These are, say, 0.015 to 

 0.020 inch in thickness. The slice is waxed down to a holder and 

 polished. The block is heated, the sample reversed, and the opposite 

 side is polished down to within the desired thickness of the first side. 

 If the crystal is to be viewed without further treatment, the best thick- 

 ness is about 0.001 inch thick. Sometimes a growth face has been 

 left as one side of the finished specimen. Samples which are to be 

 etched down to a convenient thickness after polishing are, of course, 

 considerably thicker. 



The procedure used to polish a sample has been, and will surely 

 continue to be, subject to improvements. It consists of grinding with 

 a rather fine abrasive, American Optical Co. 3031/^ emery, well past 

 the surface produced by any coarser abrasive. The polishing is done 

 with a slurry of Linde A alumina with a particle size of 0.3 micron. 

 About 0.002 inch of material is removed in this polishing past the 

 bottom of the pits left by the 3031^ emery. In some variations of 

 the procedure, diamond paste is used ; in some, great pains are taken 

 to reduce the weight on the surface of the sample. 



OPTICAL PROPERTIES 



We must examine two optical properties in order to understand the 

 visibility of the magnetic domains [9]. The first of these is the ab- 

 sorption. Light entering a medium is attenuated exponentially with 

 thickness. This is given by the Beer-Lambert law for the ratio of 

 the intensity / after traversing a thickness t to the incident intensity /« 



I ' 



The constant a is called the absorption coefficient and it varies with 

 wavelength. Figure 2 shows a in the visible portion of the spectrum. 

 It is smallest at the red end of the visible, rises to a peak in the yellow, 

 falls, then goes offscale upward in the green. A low value of a means 

 a high fraction of the light is transmitted, and a high a means a small 

 part is transmitted. For the thinnest samples we have been able to 

 make, it has not been possible to get a measurable transmission for 

 light much past the green shown on the plot. 



