It is now widely accepted that the magnetic transition in doped manganites that show large magnetoresistance is a type of percolation effect. This paper demonstrates that the transition should be viewed in the context of the Griffiths phase that arises when disorder suppresses a magnetic transition. This approach explains unusual aspects of susceptibility and heat capacity data from a single crystal of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$.

We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of $\text{250\hspace{0.05em}°}\mathrm{C}$ for less than 2 h significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material.

We report Curie temperatures up to 150 K in annealed ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ epilayers may be important in determining their physical properties.

The large, so-called colossal magnetoresistivity of doped ${\mathrm{LaMnO}}_3$ has attracted considerable attention, but is only one unusual feature of the ferromagnetic transition in these compounds. We examine in this paper the progression of magnetic and thermodynamic behavior as the transition temperature is made to vary from 360 K to 218 K by changing the divalent dopant. Single crystals of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3,$ as is well known, show modest magnetoresistivity and conventional critical behavior. ${\mathrm{La}}_{0.66}{\mathrm{Pb}}_{0.34}{\mathrm{MnO}}_3,$ and to an even greater extent, ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3,$ have unusual magnetic properties extending more than 100 K above the transition. We treat the properties of the latter samples in the context of a Griffiths phase in which the transition temperature is depressed from its maximum value $T_G$ by random bond-angle bending.