Explanation of magnetic behavior in Ru-based superconducting ferromagnets

Abstract

We have investigated ${\text{RuSr}}_2{\text{Eu}}_{1.5}{\text{Ce}}_{0.5}{\text{Cu}}_2{\text{O}}_{10}$ (Ru-1222) and ${\text{RuSr}}_2{\text{EuCu}}_2{\text{O}}_8$ (Ru-1212) samples by using x-ray diffraction, scanning electron microscopy, dc magnetization, ac susceptibility, and resistivity measurements. Based on the results obtained, we propose an explanation of the magnetic behavior of the Ru-based systems. Our model is capable of describing controversial observations of multiple magnetic transitions on temperature dependent dc magnetization measurements as well as the reentrance of irreversibility in hysteresis loops at high temperatures, which enables the bell-shaped behavior of the coercive field within $90\text{K}\lesssim T\lesssim 200\text{K}$. The experimental results suggest that Ru-based samples always contain a small amount of at least one additional magnetic phase with its own magnetic behavior, which is similar yet distinct from the main Ru phase. The presence of these phases and the superposition of their magnetic contributions can produce different transport properties and lead to features that are inherent to various magnetic states, such as ferromagnetic, antiferromagnetic, and spin glass, and still exhibit a coexistence of magnetism and superconductivity at low temperatures. This variety of possible states has led to different controversial models proposed in the literature, reflecting one or another feature observed. The model proposed in this work does not contradict but rather unifies the existing scenarios for the Ru-based systems in a common picture.