Magnetism and superconductivity

Project coordinator: Vagov A.V.

Researchers: Vagov A.V., Shanenko A.A., Saraiva T.T.

External cooperation: Stolyarov V.V., Agiar J.A.

Recent results and publications

Superconductivity between type-I and type-II in ferromagnetic superconductors

Recent advances in the study of ferromagnetic superconductors have opened up many new and interesting aspects of the physics of superconductivity, in particular concerning magnetic properties of superconductors. The coexistence of superconductivity and magnetism in such materials depends to a large extent on how the magnetic and superconducting subsystems are coupled. Namely, the most important thing is which of the two subsystems is the "strongest", i.e. which of the two critical temperatures is the largest - the Curie temperature of magnetic ordering Tm or the critical temperature of superconductivity Tc. In uranium-based compounds such as UGe2, URhGe, or UCoGe, or in Ho1.2Mo6S8, ErRh4B4, and ZrZn2 materials, the interaction between the magnetic moments and spins of free electrons occurs via the exchange mechanism. In this case, singlet pairing is suppressed and Tc < Tm, and superconductivity with triplet pairing gives only minor corrections to the predominantly ferromagnetic state.

Iron pnictides, such as EuFe{Rb,Rh,Cs}AsP, are an opposite example, where the connection between the superconducting and magnetic subsystems is carried out through a magnetic field. The weak ferromagnetism in these materials does not suppress the usual singlet pairing, and the relationship between critical temperatures is opposite to Tc > Tm. In particular, this takes place in EuFe₂(As1-xP_x)2 compounds for a large range of the P-doping parameter x. In these compounds, superconductivity is associated with Fe-3d electrons, and ferromagnetic ordering is created by Eu-4f spins. At x=0.21, this compound exhibits Tc=24.2K and Tm=19K. An even greater difference between the critical temperatures Tc=36K and Tm=15K is observed in the EuRbFe4As4 compound.

The coexistence of two order parameters (superconducting and ferromagnetic) in such materials at temperatures T < Tm < Tc, which describe superconductivity and magnetism, leads to many physical phenomena not observed in conventional superconductors and ferromagnets. Experimentally measured spatial magnetization profiles in such materials have revealed a wide variety of spatial exotic patterns that are sensitive to the slightest changes in temperature, applied magnetic field, and current. These patterns appear to be similar to a family of self-organized patterns emerging in many systems in nature, ranging from geological structures to embryonic cells. A distinctive feature of such exotic structures in ferromagnetic superconductors is the existence of non-trivial topological excitations, for example, skyrmions associated with vortices in ferromagnet-superconductor heterostructures [shown by research groups at the Moscow Institute of Physics and Technology (Stolyarov) and the University of Antwerp (Milosevic)]

On the contrary, the interval of intermediate temperatures Tm < T < Tc used to attract much less attention. In this case, ferromagnetic ordering does not exist by itself, acting as a perturbation of the superconducting state. However, here, too, the influence of the magnetic subsystem cannot be neglected, since the paramagnetic response of spins can strongly modify the superconducting characteristics. This follows already from the London theory of superconductivity when the coupling to the magnetic subsystem is taken into account [see works by Bulaevsky et al.]. Theoretical calculations predict that the vortex­–vortex interaction, initially repulsive, becomes attractive at low temperatures, indicating a transition from type–II to type–I superconductivity.

Recent studies in our Center demonstrated that a temperature-controlled crossover from type-I to type-II superconductivity in ferromagnetic superconductors takes place when the temperature drops from Tc to Tm. The temperature change allows the system to pass through the entire interval separating type-I and type-II superconductivity. It scans the entire phase diagram that cannot be achieved in conventional compounds. Thus, ferromagnetic superconductors with Tm < Tc represent a unique class of materials that can be tuned to any desired point between the two conventional superconductivity types. Such materials provide a universal testing ground for studying details of the intertype superconductivity and exotic vortex matter (vortex clusters, vortex chains, and liquid vortex drops). This may open prospects for technological applications where a controlled change in the magnetic response type can be utilized to develop novel class of the field, current, or temperature sensors.