Magnetic Archaeology of Early-type Stellar Dynamos

Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields (≳1 kG) and others very weak fields (≲10 G). Recently, we proposed that this reflects the processing or lack thereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as is. We now expand on this theory and explore the timescales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-generated magnetic fields evolve on the timescale of the mass loss, not that of the dynamo.

Unveiling unconventional magnetism at the surface of Sr2RuO4

Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr2RuO4, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr2RuO4 is still ongoing, a deeper understanding of the Sr2RuO4 normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr2RuO4 in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr2RuO4 surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry.

Electrical Properties of Superconductive Thick Film Wires Obtained from a Melt Process

YBaCuO superconductive thick film wires were fabricated by employing a melt process with a peak temperature of 1100 °C. Transition temperature and peak critical current density of these YBaCuO superconductive thick film wires were 90 K and 3.5 × 104 A/cm2, respectively. Their magnetic lev-itation force measured at a temperature of 77 K with a permanent magnet was 65.45 N during magnetic cooling. The repulsion force in the case of field cooling was 10.12 N. A permanent magnet with surface magnetism of 5.25 kG was used to cool down superconductive specimens, from which magnetic force of 15.62% of peak magnetic field was trapped. A single crystal YBaCuO superconductive thick film wire was obtained after coating powders of raw materials from a melt process employed for the fabrication of YBaCuO superconductive thick film wire.

Competing ferro- and antiferromagnetic exchange drives shape-selective $$\hbox{Co}_3\hbox{O}_4$$ nanomagnetism

We have synthesized three different shapes of $$\hbox{Co}_{3}\hbox{O}_{4}$$ Co 3 O 4 nanoparticles to investigate the relationships between the surface Co$$^{2+}$$ 2 + and Co$$^{3+}$$ 3 + bonding quantified by exploiting the known exposed surface planes, terminations, and coordiations of $$\hbox{Co}_{3}\hbox{O}_{4}$$ Co 3 O 4 nanoparticle spheres, cubes and plates. Subsequently this information is related to the unusual behaviour observed in the magnetism. The competition of exchange interactions at the surface provides the mechanism for different behaviours in the shapes. The cubes display weakened antiferromagnetic interactions in the form of a spin-flop that occurs at the surface, while the plates show distinct ferromagnetic behaviour due to the strong competition between the interactions. We elucidate the spin properties which are highly sensitive to bonding and crystal field environments. This work provides a new window into the mechanisms behind surface magnetism.

Method of boundary integral equations in magnetostatic damping of thin shells

Object and purpose of research. Calculation of the surface current density needed to compensate magnetic signature of thin ferromagnetic shell. Materials and methods. Numerical methods for boundary integral equations. Main results. Numerical solutions are considered for the densities of the inner and outer current layers which compensate external magnetostatic signature of closed ferromagnetic shells of arbitrary shape. The effect of mesh size and surface magnetism approximation upon the compensation error was investigated on test models. Conclusion. The results of the research can be used to optimize the location of degaussing coils aboard offshore objects (the geometry is taken into account).