I will present a platform for functional sub-50 nm room temperature magnetic skyrmions. Multilayers of Ir/Fe(x)/Co(y)/Pt enable us tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of 2 and 10 respectively. For a systematic investigation of the magnetization dynamics, we determined the damping parameter characterizing the magnetization response, and identified a gyrotropic skyrmion excitation that persists over a wide range of temperatures and across varying sample compositions. Importantly, the skyrmion excitation spectrum is demonstrably renormalized by the interplay of interlayer and interfacial magnetic interactions. To tailor the phenomenology of nanoscale skyrmions, including topological stability and malleability we also studied the formation and evolution of skyrmions at zero field through confinement effects. Notably, confinement-induced stabilization of room temperature skyrmions at zero field in nanodots is found to occur over a wide range of magnetic and geometric parameters. The zero-field skyrmion size can be as small as 50 nm, and varies by a factor of 4 with dot size and magnetic parameters. Crucially, skyrmions with varying thermodynamic stability exhibit markedly different confinement phenomenologies. Through these studies, I will discuss quantifiable insights towards understanding skyrmion stability and dynamics in multilayers, and immediate directions for exploiting their properties in nanoscale devices.
Studies of low-frequency noise in the c-axis resistance of lightly doped La<sub>2-x</sub> Sr<sub>x</sub>CuO<sub>4</sub> (x = 0.03) have revealed
distinct switching fluctuations at low temperatures and in magnetic fields <i>B</i> of up to 9 T parallel to the c-axis of
the crystal. The switching noise is modulated by some slower events and becomes less prominent with increasing
temperature <i>T</i>. Our results demonstrate the existence of multiple metastable states in the presence of <i>B</i>. The
overall behavior of the noise is consistent with the picture of microscopic segregation of doped holes into hole-rich
regions separated by undoped domains in CuO<sub>2</sub> planes. It also strongly suggests that interactions should be
included in possible theoretical models to describe the data.
The spin glass transition temperature of the non-superconducting series La<sub>2-x</sub>Sr<sub>x</sub>Cu<sub>0.95</sub>Zn<sub>0.05</sub>O<sub>4</sub> (x=0.10-0.22) is found to decrease with increasing x and vanish at a critical doping x<sub>c</sub>=0.19. This value of x<sub>c</sub> coincides with the doping at which the superfluid density and critical current density peak and normal state pseudogap extrapolates to zero in the superconducting analogues of this high-T<sub>c</sub> family. The presence and disappearance of T<sub>g</sub> for x<0.19 and at x=0.19, are discussed in terms of a competing order and a quantum glass transition, respectively.