Excited state EPR and exchange interactions in paramagnetic singlet-ground-state systems

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Abstract
We have performed EPR in excited crystal‐field states of singlet‐ground‐state systems, and relate these new results, and earlier results in TmN and TmP, to the expected behavior including anisotropic exchange effects. The Tm3+ excited Γ5 EPR in TmAs is qualitatively similar to that in TmP. The g‐factor approaches from below the predominantly fourth‐order crystal‐field‐only value of approximately 2.2 at high temperatures; while below 100°K the g‐factor shows anomalous behavior like that in TmP. For Tm0.5La0.5As the g‐factor between 200°K and 500°K shows no temperature dependence within experimental uncertainties. However, the experimental uncertainty in the g‐factor for Tm0.5La0.5As is very substantial, ∠0.4; and the fact that the lattice constant differs from that of pure TmAs makes it likely that effects besides simple dilution of exchange are going on. For TmSb, the g‐factor measured between 100 and 300°K smoothly approaches a value of about 2.1 from below. In Tb0.1La0.9P, above about 30°K the resonance g‐factor for the excited Γ5 state is near the fourth‐order crystal‐field value, while below 30°K the g value increases smoothly to a value 2 or 3% greater at 4.2°K. (TbP is a singlet‐ground‐state antiferromagnet; while below the critical value of x the behavior of TbxLa1−xP is purely paramagnetic even at 0°K. So Tb0.1La0.9P is a singlet‐ground‐state paramagnet). We have extended our theory for the temperature dependence of the excited state g‐factor to include anisotropic exchange effects, and relate this to the anomalous behavior in TmN, TmP, and TmAs. From the high temperature g‐factor behavior giving the shift from the crystal‐field‐only asymptotic limit, we deduce the Tm‐Tm exchange, J(0), for the thulium monopnictides. These values of J(0) and their overall variation going from the nitride to the antimonide compare well with the behavior for the same quantity found by comparison of the g‐shift of Gd in the thulium monopnictides with the temperature dependence of the susceptibility for the pure host.

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