Explanation of magnetic behavior in Ru-based superconducting ferromagnets

Abstract

We have investigated RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10 ͑Ru-1222͒ and RuSr 2 EuCu 2 O 8 ͑Ru-1212͒ 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 K Շ T Շ 200 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.

Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru0.9Sr2YCu2.1O7.9

“…I, there were many different interpretations 2,14-16 reviewed in Ref. 4 . In present work, we employ these results in combination with structural magnetic characterisations to derive a consistent magnetic phase diagram with corresponding structural transformations, resolving existing contradictions.…”

“…We note that García et al 22 also observed change in the behaviour of the superconducting region at H = 1 kOe in their field dependent resistivity and ac susceptibility measurements. In the phase diagram, the decreasing T irr (H) and T N (H) curves in increasing field may indicate a metamagnetic transition separating AFM from FM state, likely stipulated by spin exchange interactions described in our phase mixture model 4 and built upon in the following works [10][11][12] . The types of temperature and field dependent magnetic orderings shown in the phase diagram have been determined from the NPD measurements, which will be discussed in detail in Sections A and B.…”

“…Magnetization measurements can provide rather unambiguous indication of at least two ferromagnetic phase mixture 4 , which creates grounds for a diversity of magnetic transitions and states due to various form of spin and/or charge exchanges as considered in Refs. [10][11][12] .…”

“…Coexistence of two mutually exclusive spin ordering phenomena, superconductivity (SC) and magnetism has sparked the interest in high temperature cuprate superconductors, RuSr 2 RCu 2 O 8−δ (Ru-1212) [1][2][3][4][5][6] (R indicates a rare earth element), where, the RuO 2 layer is responsible for magnetic order and the CuO 2 layer gives rise to superconductivity. Originally, bulk magnetization measurements 1,7,8 and nuclear magnetic resonance (NMR) 9 reported weak-ferromagnetic order in the system.…”

Element-specific probe of Ru magnetism and local structure inRuSr2Eu1.5Ce0.5CuSouza-Neto1, Haskel2, Lang3 et al. 2009Phys. Rev. B9070View full textAdd to dashboardCite

“…[4][5][6][7][8] Phase purity, in particular ruling out the presence of magnetic impurities with similar ordering temperature ͓such as SrRuO 3 ͑SRO͔͒, is important in order to support assertions of microscopic uniform coexistence of magnetism and superconductivity. Lattice distortions such as rotations of RuO 6 octahedra can affect the magnetic structure through spin-orbit coupling ͑e.g., Dzyaloshinsky-Moriya interactions 9,10 ͒, antisymmetric exchange interactions, or single-ion anisotropy.…”