Dynamic melting of confined vortex matter

We study {\em dynamic} melting of confined vortex matter moving in disordered, mesoscopic channels by mode-locking experiments. The dynamic melting transition, characterized by a collapse of the mode-locking effect, strongly depends on the frequency, i.e. on the average velocity of the vortices. The associated dynamic ordering velocity diverges upon approaching the equilibrium melting line $T_{m,e}(B)$ as $v_c \sim (T_{m,e}-T)^{-1}$. The data provide the first direct evidence for velocity dependent melting and show that the phenomenon also takes place in a system under disordered confinement. \pacs{74.25.Qt,83.50.Ha,64.70.Dv,64.60.Ht}Comment: Some small changes have been made. 4 pages, 4 figures included. Accepted for publication in Phys. Rev. Let

Depinning and dynamics of vortices confined in mesoscopic flow channels

We study the behavior of vortex matter in artificial flow channels confined by pinned vortices in the channel edges (CE's). The critical current $J_s$ is governed by the interaction with static vortices in the CE's. We study structural changes associated with (in)commensurability between the channel width $w$ and the natural row spacing $b_0$, and their effect on $J_s$. The behavior depends crucially on the presence of disorder in the CE arrays. For ordered CE's, maxima in $J_s$ occur at matching $w=nb_0$ ($n$ integer), while for $w\neq nb_0$ defects along the CE's cause a vanishing $J_s$. For weak CE disorder, the sharp peaks in $J_s$ at $w=nb_0$ become smeared via nucleation and pinning of defects. The corresponding quasi-1D $n$ row configurations can be described by a (disordered)sine-Gordon model. For larger disorder and $w\simeq nb_0$, $J_s$ levels at $\sim 30$ of the ideal lattice strength $J_s^0$. Around 'half filling' ($w/b_0 \simeq n\pm 1/2$), disorder causes new features, namely {\it misaligned} defects and coexistence of $n$ and $n \pm 1$ rows in the channel. This causes a {\it maximum} in $J_s$ around mismatch, while $J_s$ smoothly decreases towards matching due to annealing of the misaligned regions. We study the evolution of static and dynamic structures on changing $w/b_0$, the relation between modulations of $J_s$ and transverse fluctuations and dynamic ordering of the arrays. The numerical results at strong disorder show good qualitative agreement with recent mode-locking experiments.Comment: 29 pages, 32 figure

Evidence for a dynamic phase transition in [Co/Pt]_3 magnetic multilayers

A dynamic phase transition (DPT) with respect to the period P of an applied alternating magnetic field has been observed previously in numerical simulations of magnetic systems. However, experimental evidence for this DPT has thus far been limited to qualitative observations of hysteresis loop collapse in studies of hysteresis loop area scaling. Here, we present significantly stronger evidence for the experimental observation of this DPT, in a [Co(4 A)/Pt(7 A)]_3-multilayer system with strong perpendicular anisotropy. We applied an out-of-plane, time-varying (sawtooth) field to the [Co/Pt]_3 multilayer, in the presence of a small additional constant field, H_b. We then measured the resulting out-of-plane magnetization time series to produce nonequilibrium phase diagrams (NEPDs) of the cycle-averaged magnetization, Q, and its variance, Var(Q), as functions of P and H_b. The experimental NEPDs are found to strongly resemble those calculated from simulations of a kinetic Ising model under analagous conditions. The similarity of the experimental and simulated NEPDs, in particular the presence of a localized peak in the variance Var(Q) in the experimental results, constitutes strong evidence for the presence of this DPT in our magnetic multilayer samples. Technical challenges related to the hysteretic nature and response time of the electromagnet used to generate the time-varying applied field precluded us from extracting meaningful critical scaling exponents from the current data. However, based on our results, we propose refinements to the experimental procedure which could potentially enable the determination of critical exponents in the future.Comment: substantial revision; 26 pages, 9 figures; to appear in Phys. Rev.

Vortex dynamics in superconducting channels with periodic constrictions

Vortices confined to superconducting easy flow channels with periodic constrictions exhibit reversible oscillations in the critical current at which vortices begin moving as the external magnetic field is varied. This commensurability scales with the channel shape and arrangement, although screening effects play an important role. For large magnetic fields, some of the vortices become pinned outside of the channels, leading to magnetic hysteresis in the critical current. Some channel configurations also exhibit a dynamical hysteresis in the flux-flow regime near the matching fields

Depinning of a vortex chain in a disordered flow channel

We study depinning of vortex chains in channels formed by static, disordered vortex arrays. Depinning is governed either by the barrier for defect nucleation or for defect motion, depending on whether the chain periodicity is commensurate or incommensurate with the surrounding arrays. We analyze the reduction of the gap between these barriers as function of disorder. At large disorder, commensurability becomes irrelevant and the pinning force is reduced to a small fraction of the ideal shear strength of ordered channels. Implications for experiments on channel devices are discussed.Comment: 5 pages, 4 figures. Accepted for publication in Europhysics Letter

Mode locking of vortex matter driven through mesoscopic channels

We investigated the driven dynamics of vortices confined to mesoscopic flow channels by means of a dc-rf interference technique. The observed mode-locking steps in the $IV$-curves provide detailed information on how the number of rows and lattice structure in the channel change with magnetic field. Minima in flow stress occur when an integer number of rows is moving coherently, while maxima appear when incoherent motion of mixed $n$ and $n\pm 1$ row configurations is predominant. Simulations show that the enhanced pinning at mismatch originates from quasi-static fault zones with misoriented edge dislocations induced by disorder in the channel edges.Comment: some minor changes were made, 4 pages, 4 figures, accepted for publication in Phys. Rev. Let

Dynamic ordering of driven vortex matter in the peak effect regime of amorphous MoGe films and 2H-NbSe2 crystals

Dynamic ordering of driven vortex matter has been investigated in the peak effect regime of both amorphous MoGe films and 2H-NbSe2 crystals by mode locking (ML) and dc transport measurements. ML features allow us to trace how the shear rigidity of driven vortices evolves with the average velocity. Determining the onset of ML resonance in different magnetic fields and/or temperatures, we find that the dynamic ordering frequency (velocity) exhibits a striking divergence in the higher part of the peak effect regime. Interestingly, this phenomenon is accompanied by a pronounced peak of dynamic critical current. Mapping out field-temperature phase diagrams, we find that divergent points follow well the thermodynamic melting curve of the ideal vortex lattice over wide field and/or temperature ranges. These findings provide a link between the dynamic and static melting phenomena which can be distinguished from the disorder induced peak effect.Comment: 9 pages, 6 figure