We report the structural and magnetic characterization of La-substituted Gd 1−x La x RhIn 5 (x ≤ 0.50) antiferromagnetic (AFM) compounds. The magnetic responses of pure GdRhIn 5 are well described by a S = 7/2 Heisenberg model. When Gd 3+ ions are substituted by La 3+ , the maximum of the susceptibility and the inflection point of the magnetic specific heat are systematically shifted to lower temperatures accompanied by a broadening of the transition. The data is qualitatively explained by a phenomenological model which incorporates a distribution of magnetic regions with different transition temperatures (T N ). The universal behaviour of the low temperature specific heat is found for La (vacancies) concentrations below x = 0.40 which is consistent with spin wave excitations. For x = 0.5 this universal behaviour is lost. The sharp second order transition of GdRhIn 5 is destroyed, as seen in the specific heat data, contrary to what is expected for a Heisenberg model. The results are discussed in the context of the magnetic behavior observed for the La-substituted (Ce,Tb,Nd)RhIn 5 compounds.

We report a structural/magnetic investigation by X-ray absorption spectroscopy (XAS), neutron diffraction, dc-susceptibility (χ dc ) and electron spin resonance (ESR) of the 12R-type perovskite BaTi 1/2 Mn 1/2 O3. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn 4+ and Ti 4+ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and X-band (9.4 GHz) electron spin resonance, both in the temperature interval of 2 ≤ T ≤ 1000 K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants Ja/kB = 200(2) K and J b /kB = 130(10) K, and orphan spins. Thus, BaTi 1/2 Mn 1/2 O3 is proposed as a rare case of an intrinsically disordered S = 3/2 spin gap system with a frustrated ground state.

Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite BaTi 1/2 Mn 1/2 O3, Mn 4+ cations, with S = 3/2 spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature (T ), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-T magnetic properties of the remaining magnetic degrees of freedom of BaTi 1/2 Mn 1/2 O3 are investigated. Heat-capacity data and magnetic susceptibility display no evidence for a phase transition to a magnetically ordered phase but indicate the formation of a correlated spin state. The low-temperature spin dynamics of this state is then explored by µSR experiments. The zero field µ + relaxation rate data show no static magnetism down to T = 19 mK and longitudinal field experiments support as well that dynamic magnetism persists at low T . Our results are interpreted in terms of a spin glass state which stems from a disordered lattice of orphans spins and trimers. A spin liquid state in BaTi 1/2 Mn 1/2 O3, however, is not excluded and is also discussed.

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