Magnetic properties of some paramagnetic crystals at low temperatures

Abstract

The principal magnetic susceptibilities and anisotropies of a number of salts of the iron group have been measured from room temperature down to about 80° K. The results are discussed in the light of the crystalline field theory developed by Van Vleck, and Schlapp & Penney. 1. Both the Mn ++ and Fe +++ ions being in the S -state, the anisotropies are extremely feeble, and the temperature variation of the susceptibility follows almost exactly the Curie law. 2. In the Stark pattern for Cr +++ the lowest level is a singlet, with a (2 s + l)-fold spin degeneracy, which is far removed from the upper levels, and so the ion behaves as if it were in the S -state. 3. The spin-orbit coupling of Ni ++ is much greater than that of Cr +++ . This is responsible for an appreciable anisotropy and deviation from the Curie law. These deviations are utilized to calculate the crystal field constants. It is found that both the cubic and rhombic parts of the field are nearly of the same magnitude in all the nickel salts, and they are practically independent of temperature. 4. The ground level of Co ++ and Fe ++ is a triplet, and hence their magnetic properties are much more sensitive to the rhombic part of the field than in nickel salts. An estimate of the field is made. 5. The copper salts fall into three distinct classes as judged by their magnetic properties. In the first group, to which the Tutton salts belong, the principal moments are different for different directions, but all of them are practically independent of temperature. In the second class the moments are nearly of the same order as before, but they decrease with the lowering of temperature. The double chloride of copper and ammonium is a typical example of this class. To the third class belongs cupric acetate monohydrate, in which the moments are all very low even at room temperature, and they decrease rapidly as the temperature is lowered, the three classes correspond to the non-cubic part of the field being very different, lowest in the first class and highest in the third. 6. In the nickel and ferrous salts there is an axis of magnetic symmetry, though their crystal structures do not lead to it. 7. The magnetic axes of a few crystals change their directions in the temperature range covered. 8. The influence of the covalent binding on the electric field and its effect on the magnetic properties of the ion is discussed.