386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



-{M3}-[Fe2] (Fe04)3, where M stands for any one of the following ele- 

 ments: yttrium, samarimn, europium, gadolinium, terbium, dyspro- 

 sium, liolmium, erbium, thulium, ytterbium, or lutetimn. They are 

 called yttrium (or samarium, etc.) iron garnet. In addition, there are 

 a number of other elements which can be substituted in to some degree. 

 Except for yttriimi, the metals just listed belong to a group Imown as 

 the rare earths (or more explicitly as the 4f rare earths). These ele- 

 ments possess remarkably similar chemical properties, and are in fact 

 difficult to separate from each other chemically. 



STRUCTURE OF THE MAGNETIC GARNETS 



The manner in which we have chosen to write the chemical formula 

 [4a] of the ferrimagnetic garnets ^MsJ-CFeo] (Fe04)3 is intended to 

 convey some structural information. There are two kinds of sites on 

 which iron ions are located in this structure. They may be at a posi- 

 tion in which there are four nearest neighbors, oxygen ions. The 

 centers of these oxygens define the vertices of a slightly distorted 

 tetrahedron. These particular iron ions (in parentheses in the 

 formula) are said to be on tetrahedral sites. For every three iron 

 ions on tetrahedral sites, there are two on octahedral sites. These (in 

 square brackets) are surromided by six nearest neighbor oxygen ions 

 defining the vertices of a slightly distorted octahedron. Finally, the 

 yttrium or rare earth ions (in braces) have eight nearest neighbors; 

 these sites are called dodecahedral. 



All the iron ions in these crystals have a valence of 3, i.e., iron is 

 present as Fe+ + +, and each has associated with it a magnetic moment. 

 We may think of this as a small permanent magnet. Very powerful 

 electrostatic forces called "exchange" forces act to aline the magnetic 

 moments of these iron ions. At temperatures near absolute zero, all 

 the moments of the tetrahedral ions are parallel ; all the moments of 

 octahedral ions are parallel to each other but antiparallel to the 

 tetrahedral ions. Since for every formula unit there are three tet- 

 rahedral and two octahedral ions, the net magnetic moment of the 

 ions in one formula is that of a single iron ion. The moment of a 

 single trivalent iron ion is five times that of a single electron. These 

 units are called Bohr magnetons. Thus, the magnetic moment of an 

 Fe+ + + ion is 5 Bohr magnetons. Yttrium iron garnet at absolute 

 zero has a magnetic moment of 5 Bohr magnetons for each formula 

 as given above. However, the rare earth ions (except Lu) have a 

 magnetic moment. The moments of the rare earth ions are alined so 

 as to be approximately parallel to each other but antiparallel to the 

 net moment of the iron ions. In several of the rare earth iron garnets 

 at absolute zero, the rare earth magnetic moment is greater than that 

 of the iron sublattice. 



