Spontaneous vortex phases in superconductor-ferromagnet Pb-Co nanocomposite films

Abstract

We report the appearance of spontaneous vortices in lead superconducting films with embedded magnetic nanoparticles and a temperature induced phase transition between different vortex phases. Unlike common vortices in superconductors, the vortex phase appears in the absence of applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. Transport measurements reveal dynamical phase transitions that depend on temperature (T) and applied field (H) and support the obtained (H-T) phase diagram. PACS numbers: 74.25.Dw, 74.25.Fy, 74.81.Bd The interplay between of superconductivity (SC) and ferromagnetism (FM) has been attracting the attention of the scientific community since the discovery of superconductivity [1]. Recently, there has been a resurgence of this interest due to new phenomena: for example, the increase of the critical current J c in hybrid systems containing sub-micron ferromagnetic particles on top of type II superconductors [2, 3] and the coexistence of superconductivity with long-range magnetic order in magnetic superconductors [4]. In a SC-FM hybrid, the magnets strongly affect the properties of the superconductor, leading to a change in the critical temperature T c and critical current J c . Also, they give an opportunity to observe new phenomena such as domain-wall superconductivity [5] and hysteresis pinning effect [6].In order to study the characteristics of the novel spontaneous vortex phases that arise from the interaction between vortices and embedded magnetic nanoparticles, we fabricated a hybrid system that consists of a 100 nm lead film (Pb) containing homogeneously distributed single domain cobalt (Co) particles (mean diameter about 4.5 nm) with randomly oriented magnetization.The samples are produced by the so-called inertgas(Ar) aggregation method with an Ar pressure of about 10 −1 mbar. It is a co-deposition of Pb and well-defined Co clusters directly onto a sapphire substrate without buffer and capping layer. The Ar is absorbed by a cryopump at the other end of the cluster chamber and only well-defined Co clusters can enter the main chamber. The substrate is mounted on a coldfinger of a rotatable 4 He cryostat and is cooled to ∼ 40 K during the deposition. The samples are deposited at low temperature in order to get high quality Pb films. The angle between the matrix and the cluster beams was 45 o . Due to the different beam directions, samples with different Co volume fraction can be made within one preparation. The de-position rates are controlled by three quartz balances in order to monitor the deposition rate at different positions of the substrate. Ag contacts for transport measurements are pre-deposited and connected to a multi-channel automatic measurement system. The typical dimensions of the sample were 10mm×3mm×100nm. After deposition, transport properties in both zero and non-zero magnetic field were investigated in-situ...

Coexistence of superconductivity and superparamagnetism in Pb–Co electrodeposited nanowires

“…The first term of the sum in equation (2) To fit the data above TC in Figure 3a), we used the value for µ that was obtained from the fit to the 7 data in Figure 2. We obtained VM n = (14±1)10 14 , which was in fair agreement with the same 8 parameter obtained from the data in Figure 2. These parameters, together with a superconducting 9…”

“…By fitting the curve in Figure 2 with equation (1), we obtained the following values for the Langevin 13 function parameters: µ= (17.8±0.1)µB and VM n = (9.28±0.04)10 14 , which correspond to a total Co 14 volume of (1.06±0.01)×10 -7 cm 3 . Since the magnetization of Co is 1.71 µB per atom [33], this value 15 for µ amounts to 10 atoms per cluster, if we neglect surface effects.…”

Ultrasensitive interplay between ferromagnetism and superconductivity in NbGd composite thin films

“…The kink types of features in R-T characteristics [ Fig. 4(a) ] can be of support to the picture of vortex formation 39 and hence the flux compression. However, for better understanding of the origin of the PME one needs to study the field dependence magnetization behaviour which is out of the scope of this study.…”

“…At relatively higher B-field, the sharpness of the transition gets distorted and it becomes wider. For example at B = 500 mT, the transition curve accompanies a kink type of structure (shown by an arrow) indicating an intermediate state possibly due to the vortex formation under the magnetic field for a type-II superconductor 39 . Thus the applied B-field above a threshold value causes similar types of features/kinks in the R-T characteristics for the Nb film as they appeared for the NbGd samples under no external field.…”

Visualization of the magnetic flux structure in phosphorus-doped EuFe2As2 single crystals

“…10 An important issue of the coexistence of superconductivity and magnetism from both theoretical [11][12][13][14] and experimental perspectives [15][16][17][18] relates to the microstructure of the magnetic flux, as well as to its dynamics upon variation of the temperature and external magnetic field. Until recently, low temperatures of superconducting and magnetic phase transitions of known single crystals, as well as the requirement of a high spatial resolution, have limited experimental capabilities for visualization of the magnetic flux structure employing e.g.…”