The dynamic magnetic properties of the distorted perovskite system ${\mathrm{La}}_{0.7-x}{\mathrm{Y}}_x{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ $(0\mathrm{}<~x<~0.15)$ have been investigated by ac susceptibility and dc magnetization measurements, including relaxation and aging studies. All investigated samples display a metal-insulator transition. As yttrium is added in the compounds the overall results show evidence for the gradual appearance of a cluster glass behavior. For the $x=0.15\mathrm{}$ sample, magnetization measurements as a function of time at various temperatures show that the magnetic relaxation is maximum at a given temperature, well below the ferromagnetic transition. This maximum coincides in temperature with a frequency-dependent feature in the imaginary part of the ac susceptibility, associated with a freezing process. This is interpreted as due to ferromagnetic clusters, which grow with decreasing temperature down to a temperature at which they freeze due to severe intercluster frustration.

A systematic study of phase separation effects in polycrystalline ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ obtained under different thermal treatments is reported. Samples with average grain size ranging from 200 to 1300 nm were studied. Magnetic and electrical measurements show quantitative differences among samples in their low-temperature behavior, indicating that the fraction of the ferromagnetic (FM) phase gradually decreases as the grain size increases. Percolation of the FM phase in samples with a small fraction of this phase suggests that grain boundaries play a distinctive role in the spatial distribution of coexisting phases. The defective structure at the grain surface could explain the local inhibition of the antiferromagnetic charge ordered phase, an effect that is gradually removed with increasing grain size. Qualitative agreement of the data with this description is found. This effect is also found to be highly dependent on the oxygen content of the samples and its spatial distribution.

The third law of thermodynamics dictates that the entropy of a system in thermal equilibrium goes to zero as its temperature approaches absolute zero. In ice, however, a ‘zero point’ or residual entropy can be measured—attributable to a high degeneracy in the energetically preferred positions of hydrogen ions associated with the so-called ‘ice rules’^1,2. Remarkably, the spins in certain magnetic materials with the pyrochlore structure of corner-sharing tetrahedra, called ‘spin ice’, have an equivalent degeneracy of energetically preferred states, and also have a zero-point entropy^3,4,5,6,7. Here, we chemically alter Ho₂Ti₂O₇ spin ice by ‘stuffing’ extra Ho magnetic moments into otherwise non-magnetic Ti sites surrounding the Ho tetrahedra. The resulting series, Ho₂(Ti_2−xHo_x)O_7−x/2, provides a unique opportunity to study the effects of increased connectivity between spins on a frustrated lattice. Surprisingly, the zero-point entropy per spin measured appears unchanged by these excess spins. The results suggest a chemical approach for studying ice-like frustration and other properties of the broad family of geometrically frustrated magnets based on the pyrochlore structure.

We have studied the low-frequency dielectric properties of the phase-separated manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in this material with the field-induced suppression of the polaron activation energy.