Continuous-wave, multi-frequency electron paramagnetic resonance (EPR) studies are reported for a series of single-crystal and powder samples containing different dilutions of a recently discovered mononuclear Ho(III) (4f(10)) single-molecule magnet (SMM) encapsulated in a highly symmetric polyoxometalate (POM) cage. The encapsulation offers the potential for applications in molecular spintronics devices, as it preserves the intrinsic properties of the nanomagnet outside of the crystal. A significant magnetic anisotropy arises due to a splitting of the Hund's coupled total angular momentum (J = L + S = 8) ground state in the POM ligand field. Thus, high-frequency (50.4 GHz) EPR studies reveal a highly anisotropic eight line spectrum corresponding to transitions within the lowest m(J) = ±4 doublet, split by a strong hyperfine interaction with the I = 7/2 Ho nucleus (100% natural abundance). X-band EPR studies reveal the presence of an appreciable tunneling gap between the m(J) = ±4 doublet states having the same nuclear spin projection, leading to a highly non-linear field-dependence of the spectrum at low-frequencies.

Low-energy excitations in the rare earth kagome spin system Pr 3 Ga 5 SiO 14 (PGS) have been investigated using high-field electron spin resonance (ESR) techniques. Previous work has shown that PGS does not exhibit long-range magnetic order at temperatures down to 30 mK. The present low-temperature (1.3-20 K) field-scan measurements, made on a single crystal sample in applied fields up to 30 T and for several microwave frequencies, give a series of temperature-dependent absorption peaks that are very different to conventional ESR spectra. The observed response is interpreted in terms of discrete spin-wave excitations in antiferromagnetically correlated spin clusters. We present a theoretical model which, in the limit of small magnetic frustration effects, qualitatively describes the microwave-induced transitions.

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