Glassy Vortex Dynamics Induced by a Random Array of Magnetic Particles above a Superconductor

The magnetic relaxation of a Nb film covered with a random array of permalloy particles has been studied using various procedures. When the sample undergoes a field-cooled process, the magnetic relaxation becomes logarithmic in time. The relaxation rate is nearly temperature independent at low temperature and characteristic glassy dynamics—aging and memory effects—are observed. These results are interpreted as the consequence of pinning by the statistical variation of the number of nanoparticles within the area of a vortex core.

Figure 1

(a) A TEM image of the magnetic particles, captured on a grid during deposition on Nb. The circle approximates the area of a vortex core. (b) $M\mathrm{\text{−}}H$ curves of the sample at 7 and 9 K (inset).

Figure 2

Magnetic relaxation of the sample at 6 K for both the ZFC and FC processes. The FC relaxation is logarithmic in time while the ZFC relaxation is much faster. The upper inset illustrates the difference between ZFC and FC magnetization for a typical magnetic nanoparticle system. The lower inset shows the magnetic relaxation of a Nb film without magnetic particles.

Figure 3

Temperature dependence of the normalized magnetic relaxation rate $S=(1/M_0)(dM/dt)$ and $R=d\ln M/d\ln t$ for the FC process. At low temperatures, both $S$ and $R$ are nearly temperature independent.

Figure 4

Aging and memory effects at (a) $T=6\mathrm{K}$, and (b) $T=1.8\mathrm{K}$. The arrows indicate when the measuring time equals the waiting time. The solid lines are linear fits.