Exchange Interactions in the Samarium Monochalcogenides

In the insulating state of the samarium monochalcogenides SmS, SmSe, and Sm Te, the Sm²⁺(4f⁶) ion has a nonmagnetic ⁷F₀ ground state exhibiting Van Vleck paramagnetism at low temperatures. From an analysis of the susceptibility, information about the Sm-Sm exchange interactions is obtained. The deduced variation of this exchange with lattice parameter is shown to be consistent with the pressure dependence of the susceptibilities. Electron paramagnetic resonance of Eu²⁺, Gd³⁺, and Mn²⁺ present as dilute impurities has also been observed. The resonance fields are subject to large shifts (nearly 50% for Mn²⁺ in SmS) due to the impurity-host lattice interaction; this enables one to deduce the Sm-impurity exchange interactions. It is found that the Eu-Sm and Sm-Sm interactions are nearly identical in each lattice and furthermore that they follow the same systematic trend as the Eu-Eu exchange in the corresponding europium chalcogenides. The magnitude however is much larger in the Sm lattice, directly reflecting the closer proximity of the conduction band. The Gd-Sm exchange interaction is roughly twice as large as that found for Eu-Sm in each host lattice. This increase in exchange in going from the larger Eu²⁺ ion to the smaller isoelectronic Gd³⁺ ion implies that the extra electron accompanying the Gd³⁺ ion plays an active role in the exchange process. The Mn-Sm interactions are found to be of opposite sign (antiferromagnetic) and approximately four times larger than those for Eu-Sm.