Asymmetry reversal of thermomagnetic avalanches in Pb films with a ratchet pinning potential

We combine transport measurements and magneto-optical imaging in order to study vortex motion in Pb samples with a square lattice of asymmetric pinning potentials formed by two antidots of different sizes. Near the boundary with the normal phase, a vortex ratchet effect is detected when an ac-current is applied. As previously observed in similar systems, inversion of vortex rectification direction occurs as the magnetic field is changed. Further inside the superconducting phase, anisotropic flux penetration patterns are formed. An inversion of anisotropy is observed as the temperature is lowered. This effect can be understood in terms of a thermomagnetic avalanche model.

Figure 1

(Color online) AFM image of the sample and schematics of the current and voltage contact configuration. The lattice parameter of the array is $3\mu \mathrm{m}$. The big (small) square antidots have a lateral size of $1200\mathrm{nm}$ $\left(600\mathrm{nm}\right)$.

Figure 2

(Color online) (a) dc-voltage curves as a function of reduced field $H∕H_1$ for different reduced temperatures $t$. The applied ac-current has a magnitude of $I_{\mathit{ac}}=0.5\mathrm{mA}$. (b) Contour color plot of dc-voltage as a function of $H∕H_1$ and $I_{\mathit{ac}}$ at $t=0.99$. In both cases, the frequency of the ac-current is $1.1\mathrm{kHz}$.

Figure 3

Sequence of flux penetration patterns at [(a)–(c)] $t=0.93$ and [(d)–(f)] $t=0.28$ increasing $H$ after ZFC the sample. The corresponding applied fields are indicated below each image. The dotted lines indicate the border between patterned (lower part) and nonpatterned (upper part) regions. The scale bars correspond to $100\mu \mathrm{m}$ and the color bars indicate the values of $B$ in mT.

Figure 4

(Color online) Magnetic flux $\Phi$ that enters the sample from left and right edges of the sample as a function of applied field. (a) $t=0.93$ and (b) $t=0.28$.

Figure 5

Threshold field for the dendritic instability as a function of critical current density. The inset shows the behavior of $H_{\mathit{th}}$ over a wider range in $j_c$.