The onset of a Griffiths-like phase has been observed in ${\mathrm{Tb}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ ($T_C=110\mathrm{K}$) by means of magnetic susceptibility and small-angle neutron scattering experiments. We show the growth of a ferromagnetic cluster system characterized by an inverse susceptibility exponent lower than unity at $T_C<T<T_G\approx 200\mathrm{K}$. We suggest that the Griffiths-like state is originated by local disorder within the crystallographic structure, stabilized and enhanced by competing intralayer and interlayer magnetic interactions. Both factors thus promote segregation of nanometric regions with ferromagnetic interactions.

Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers $\left[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}\mathrm{}\right(t)/{\mathrm{Al}}_2{\mathrm{O}}_3(3\mathrm{}\mathrm{}\mathrm{n}\mathrm{m}){]}_{10}$ with nominal thickness $1.1<~t<~1.3\mathrm{nm}$ by use of ac susceptometry and dc magnetometry. At $t=1.0\mathrm{}\mathrm{nm},$ superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.

Ferromagnetic single domain particles of CoFe in discontinuous magnetic multilayers $\left[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}\right(0.9\mathrm{}\mathrm{nm})/{\mathrm{Al}}_2{\mathrm{O}}_3(3\mathrm{}\mathrm{nm}){]}_{10}$ reveal spin-glass ordering below $T_g=43.6\mathrm{}\mathrm{K}$ as evidenced by the criticality of the nonlinear susceptibility. Dynamic critical scaling yields exponents $z\nu =8.0\mathrm{}$ and $\beta =1.0\mathrm{}$ in both zero and weak applied field, where an Almeida-Thouless line is encountered. In fields exceeding 2 mT a crossover into a chiral glass regime seems to occur.

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 Å) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 Å. At t13 Å was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in our system when the disordered granular structure is still well separated, as confirmed by the data of high resolution transmission electron microscopy. Hence such magnetic percolation is clearly distinct from usual electrical percolation in these discontinuous layers. At the same time, the highest MR (∼6.5% at room temperature) in this series is attained with decreasing t only at t=10 Å.

The ac susceptibility of a superferromagnetic discontinuous multilayer $[\mathrm{C}{\mathrm{o}}_{\mathrm{80}}\mathrm{F}{\mathrm{e}}_{\mathrm{20}}(1.4\mathrm{n}\mathrm{m})/\mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}(3\mathrm{n}\mathrm{m}){]}_{10}$ is measured as a function of temperature, frequency, and field amplitude and compared to static and dynamic hysteresis loops. Its properties are successfully mapped onto the predicted [T. Nattermann, V. Pokrovsky, and V. M. Vinokur, Phys. Rev. Lett. 87, 197005 (2001).] dynamical phase transitions, which link the relaxation, creep, sliding, and switching regimes of pinned domain walls.