Instability driven formation of domains in the intermediate state of type-I superconductors

The formation of normal-state domains in type-I superconducting indium films is investigated using the high resolution magneto-optical imaging technique. The observed patterns consist of coexisting circular and lamellar normal-phase domains surrounded by the superconducting phase. The distribution of domain surface areas is found to exhibit a threshold, above which only the lamellar shape is observed. We show that this threshold coincides with the predicted critical surface area for the elongation instability of the circular shape. The partition of the normal phase into circular and lamellar domains is determined by the combined effects of the elongation instability and the penetration of magnetic flux by bursts at the early stage of pattern formation. It is not governed by mutual interactions between domains, as usually assumed for self-organized systems

Field-Driven Domain-Wall Dynamics in GaMnAs Films with Perpendicular Anisotropy

We combine magneto-optical imaging and a magnetic field pulse technique to study domain wall dynamics in a ferromagnetic (Ga,Mn)As layer with perpendicular easy axis. Contrary to ultrathin metallic layers, the depinning field is found to be smaller than the Walker field, thereby allowing for the observation of the steady and precessional flow regimes. The domain wall width and damping parameters are determined self-consistently. The damping, 30 times larger than the one deduced from ferromagnetic resonance, is shown to essentially originate from the non-conservation of the magnetization modulus. An unpredicted damping resonance and a dissipation regime associated with the existence of horizontal Bloch lines are also revealed

Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis. Connection with similar effects in systems with long-range magnetic interactions is emphasized

Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory

The magnetic domain structure and magnetic properties of a ferromagnetic (Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We show that, despite strong hysteresis, domain theory at thermodynamical equilibrium can be used to determine the micromagnetic parameters. Combining magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance measurements, we obtain the characteristic parameter for magnetic domains $\lambda_c$, the domain wall width and specific energy, and the spin stiffness constant as a function of temperature. The nucleation barrier for magnetization reversal and the Walker breakdown velocity for field-driven domain wall propagation are also estimated

Spin Drift Velocity, Polarization, and Current-Driven Domain-Wall Motion in (Ga,Mn)(As,P)

Current-driven domain wall motion is studied in (Ga,Mn)(As,P) ferromagnetic semiconducting tracks with perpendicular anisotropy. A linear steady state flow regime is evidenced over a large temperature range of the ferromagnetic phase (0.1Tc < T < Tc). Close to 0 K, the domain wall velocity is found to coincide with the spin drift velocity. This result is obtained below the intrinsic threshold for domain wall motion which implies a non-adiabatic contribution to the spin transfer torque. The current spin polarization is deduced close to 0 K and to Tc. It suggests that the temperature dependence of the spin polarization can be inferred from the domain wall dynamics.Fil: Curiale, Carlos Javier. Centre National de la Recherche Scientifique; Francia. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Lemaître, A.. Centre National de la Recherche Scientifique; FranciaFil: Ulysse, C.. Centre National de la Recherche Scientifique; FranciaFil: Faini, G.. Centre National de la Recherche Scientifique; FranciaFil: Jeudy, V.. No especifíca

Tuning Ginzburg-Landau theory to quantitatively study thin ferromagnetic materials

Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg-Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistical analysis of the domain wall width parameter, showing that our model can provide detailed nano-scale information while retaining the complex behavior of a statistical disordered model.Comment: 18 pages, 8 figure

Domain-wall roughness in GdFeCo thin films: crossover length scales and roughness exponents

Domain-wall dynamics and spatial fluctuations are closely related to each other and to universal features of disordered systems. Experimentally measured roughness exponents characterizing spatial fluctuations have been reported for magnetic thin films, with values generally different from those predicted by the equilibrium, depinning and thermal reference states. Here, we study the roughness of domain walls in GdFeCo thin films over a large range of magnetic field and temperature. Our analysis is performed in the framework of a model considering length-scale crossovers between the reference states, which is shown to bridge the differences between experimental results and theoretical predictions. We also quantify for the first time the size of the depinning avalanches below the depinning field at finite temperatures.Comment: 10 pages, 6 figures. The first two authors contributed equally to this wor

A complement to the modern crystallographer's toolbox: Caged gadolinium complexes with versatile binding modes

A set of seven caged gadolinium complexes were used as vectors for introducing the chelated Gd3+ ion into protein crystals in order to provide strong anomalous scattering for de novo phasing. The complexes contained multidentate ligand molecules with different functional groups to provide a panel of possible interactions with the protein. An exhaustive crystallographic analysis showed them to be nondisruptive to the diffraction quality of the prepared derivative crystals, and as many as 50% of the derivatives allowed the determination of accurate phases, leading to high-quality experimental electron-density maps. At least two successful derivatives were identified for all tested proteins. Structure refinement showed that the complexes bind to the protein surface or solvent-accessible cavities, involving hydrogen bonds, electrostatic and CH-π interactions, explaining their versatile binding modes. Their high phasing power, complementary binding modes and ease of use make them highly suitable as a heavy-atom screen for high-throughput de novo structure determination, in combination with the SAD method. They can also provide a reliable tool for the development of new methods such as serial femtosecond crystallography. © 2014 International Union of Crystallography.Peer Reviewe

Impeded Growth of Magnetic Flux Bubbles in the Intermediate State Pattern of Type I Superconductors

Normal state bubble patterns in Type I superconducting Indium and Lead slabs are studied by the high resolution magneto-optical imaging technique. The size of bubbles is found to be almost independent of the long-range interaction between the normal state domains. Under bubble diameter and slab thickness proper scaling, the results gather onto a single master curve. On this basis, in the framework of the "current-loop" model [R.E. Goldstein, D.P. Jackson and A.T. Dorsey, Phys. Rev. Lett. 76, 3818 (1996)], we calculate the equilibrium diameter of an isolated bubble resulting from the competition between the Biot-and-Savart interaction of the Meissner current encircling the bubble and the superconductor-normal interface energy. A good quantitative agreement with the master curve is found over two decades of the magnetic Bond number. The isolation of each bubble in the superconducting matrix and the existence of a positive interface energy are shown to preclude any continuous size variation of the bubbles after their formation, contrary to the prediction of mean-field models.Comment: \'{e}quipe Nanostructures Quantique