Critical fields and spontaneous vortex state in the weak-ferromagnetic superconductor RuSr2GdCu2O8

TL;DR

This paper investigates the coexistence of superconductivity and weak ferromagnetism in RuSr2GdCu2O8, revealing a spontaneous vortex state between 30 K and 56 K, with detailed analysis of critical fields and vortex behavior.

Contribution

It provides the first detailed characterization of the spontaneous vortex state and critical magnetic fields in the weak-ferromagnetic superconductor RuSr2GdCu2O8.

Findings

Spontaneous vortex state observed between 30 K and 56 K.

Lower critical field Bc1(0) ~ 12 G, anisotropic parameter g ~ 7.

Vortex melting line follows (1 - T/Tm)^{3.5} with Tm = 39 K.

Abstract

A spontaneous vortex state (SVS) between 30 K and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with ferromagnetic Curie temperature TC = 131 K and superconducting transition temperature Tc = 56 K. The low field (20 G) Superconducting hysteresis loop indicates a narrow Meissner state region within average lower critical field Bc1(T)= Bc1(0)[1 - (T/T0)2], with average Bave c1 (0) = 12 G and T0 = 30 K. Full Meissner shielding signal in very low applied field indicates an ab-plane Bab c1(0) ~ 4 G with an estimated anisotropic parameter g ~ 7 for this layered system. The existence of a spontaneous vortex state between 30 K and 56 K is the result of weak-ferromagnetic order with a net spontaneous magnetic moment of ~ 0.1 muB/Ru, which generates a weak magnetic dipole field around 10 G in the CuO2 bi-layers. The upper critical field Bc2 varies linearly as (1 - T/Tc) up to 7-T field. The vortex melting line Bm varies as (1 - T/Tm)3.5 with melting transition temperature Tm = 39 K and a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.