Abstract
Bulk magnetic properties of the related antiferromagnets KNi and Ni are analyzed by use of both high- and low-temperature theoretical methods. A crystal-field theory for the ion is outlined, using the strong-field coupling scheme, and is used to construct relevant spin Hamiltonians for the two systems. KNi is found to be a simple-cubic antiferromagnet for which all except nearest-neighbor interactions are negligible. Writing an exchange interaction between nearest-neighbor spins and , the experimental observations are quantitatively fitted for °K with a suggestion of a small temperature dependence of . Reasons are put forward to suggest that Ni is, to a very good approximation, a two-dimensional quadratic-layer antiferromagnet with a magnetic state which, over a large temperature region, shows a long-range order in two dimensions but not in the third. A good theoretical interpretation of most of the published experimental data can be obtained using such a picture. Ni is made up of Ni magnetic planes, which are essentially the same as those found in KNi, but with these magnetic layers separated in the third dimension by two nonmagnetic KF layers. We find an intraplane exchange °K and an interplane exchange which is at least an order of magnitude smaller and probably ferromagnetic in sign. A number of major differences between typical two- and three-dimensional magnetic structures are discussed, and some experiments are outlined for which the use of two- dimensional systems like Ni could provide new and interesting information concerning magnetic cooperative phenomena. Finally, the possible origins of the difference in exchange values between KNi and Ni are discussed, and the relevance of our findings in connection with the supposedly anomalous properties of another series containing two-dimensional antiferromagnets, namely, the calcium manganites, is assessed.
- Received 25 May 1967
DOI:https://doi.org/10.1103/PhysRev.164.736
©1967 American Physical Society