Research


I am an experimentalist working in condensed matter physics. My research primarily focuses on ground states and driven dynamics of disordered magnetic and superconducting systems. In our group, we use Scanning Hall Microscopy and other imaging techniques to investigate meso and nano-scale magnetic properties of superconducting and magnetic materials.

In the recent years I studied disorder-driven phase transitions in the vortex lattice of type-II superconductors and continue to work on this topic. My current research interests include stochastic fluxon dynamics in periodic superconducting networks and arrays, local susceptibility of domain walls in ferromagnets, topologically-frustrated superconducting and magnetic states and physics of hybrid superconductor/ferromagnet systems.

Laboratory and experimental techniques

 

Vortices in superconductors


Magnetic flux lines in type-II superconductors exibit complex behavior determined by their mutual interactions, pinning and thermal disorder and driving forces. We use Scanning Hall Susceptometer to study pinning and microscopic dynamics of vortices.

We do it by applying a small "shaking" a.c. magnetic field to a superconducting sample and measuring the resulting motion of the vortex lines. With this technique the microscoipic distribution of flux pinning barriers in the sample can be mapped with micron-scale resolution.

 

The "peak effect" phenomenon in the vortex lattice is characterized by a sudden increase of the pinning strength that reaches a sharp maximum just before vanishing at the superconducting ttansition.

It was recently discovered that the peak effect phenomenon is characterized by the two distinct phases with different pinning strenght that coexist in the superconductor at the macroscopic scale. This observation is a hallmark of a disorder-driven phase transition in the vortex lattice and is responsible for many peculiar transport properties of type-II superconductors in the "peak effect" regime, such as transient voltage responce and memory effects. We continue to investigate the local dynamics of the two-phase mixture.

Relevant publications:

"Two coexisting vortex phases in the peak effect regime in a superconductor"
  M. Marchevsky, M.J. Higgins and S. Bhattacharya, Nature 409, 591 (2001).

"Driven dynamics of the vortex-phase mixture near the peak effect: The "vortex capacitor",
  M. Marchevsky, M.J. Higgins and S. Bhattacharya, Phys. Rev. Lett., 88, 087002 (2002).

 

Topology and dynamics of magnetic domains

In soft magnetic materials with perpendicular anisotropy such as yttrium iron garnets, domains form a labyrinthine structure in the demagnetized state. This structure is also observed in other physical systems, such as ferrofluids, Langmur films, oscillatory chemical reactions, polymers, etc. It appears that local mobility of domain walls is closely related to domain topology. We do imaging studies of the domain wall responce to a combination of periodic and non-periodic (noise) magnetic drive and investigate depinning and creep phenomena in the labyrinthine domain structures.

"Localized ac responce and stochastic amplification in the labyrinthine magnetic domain structure"
M.P. DeFeo and M. Marchevsky, preprint
 
 

Flux dynamics in periodic superconducting networks

Periodic superconducting networks and arrays show a rich variety of magnetic flux configurations. Close to the superconducting transition, collective interactions between the quatized flux quanta drive the network into a spatially ordered state. That occurs at the matching field corresponding to an integer number of flux quanta number per unit cell of the network. As temperature is lowered, pinning barriers for the rearrangement of the flux quanta become large and the system exhibits an activated behavior.

When external oscillating magnetic field is applied to the network, it causes jumps of fluxons between the neighboring cells. The system is intrinsically "noisy" due to a quasi-random spatial variation of the pinning barriers resulting from the local matching effects. This gives rise to a variety of interesting new phenomena such as formation of "domains" with a flux density "locked" to a certain filling factor, ratchet potential for flux motion, "telegraph" noise and stochastic amplification. We use magnetic noise spectroscopy and scanning Hall magnetic imaging to study ground state fluxon distributions and local magnetic dynamics in these systems.
 

Vortices and currents in superconductor/ferromagnet heterostructures

Superconductor/ferromagnet heterostructures exhibit a variety of novel vortex and magnetic phases as recently predicted theoretically. We image vortex structures in these systems using magnetic decoration technique (left) and study current flow patterns in superconductor/ferromagnet bilayers using scanning Hall microscopy (right).

 
Updated by M. Marchevsky on December 23, 2005