Anomalous critical fields and the absence of Meissner state in Eu(Fe_0.88Ir_0.12)₂As₂crystals

Abstract

We report electrical and magnetic measurements of an optimally doped Eu(Fe 0.88 Ir 0.12 ) 2 As 2 crystal which shows superconductivity at T sc = 22 K and magnetic ordering of the Eu 2+ spins below 20 K. The results suggest that the Eu 2+ spins lie flat ferromagnetically in the ab plane at T ab m = 20 K, and then tilt toward the c-axis at T tilt m = 17.4 K. The isothermal magnetization loop at low temperatures shows both ferromagnetic behavior and superconducting characteristics, unambiguously demonstrating the coexistence of ferromagnetism and superconductivity. The upper critical field measured is remarkably reduced, as compared to other Fe-based superconductors with the same T sc , and it exhibits abnormal temperature dependence featured by the existence of an inflection point around T ab m , where the anisotropy ratio γ (≡ H ab c2 /H c c2 ) shows a minimum value smaller than 1.0. These observations can be explained by a ferromagnetic exchange field of ∼30 T which tilts its direction toward the c-axis below T tilt m . The strong internal field, much higher than the intrinsic lower critical field expected, leads to the absence of Meissner state, which is confirmed by the magnetic measurements under ultra-low fields. IntroductionSuperconductivity (SC) and ferromagnetism (FM) are two antagonistic cooperative phenomena in solid-state systems, thus it is very rare that SC coexists with FM (or even other magnetic states with ferromagnetic component) in a single material 2 [1,2]. Such material showing both SC and ferromagnetic components was earlier called 'ferromagnetic superconductor' (FMSC) [1] or 'magnetic superconductor' [3]. Lorenz and Chu [4] classified this special material into two categories according to its superconducting transition temperature T sc and ferromagnetic transition temperature T m . When T m > T sc , it is termed 'superconducting ferromagnet'; and for T m < T sc , it is called an FMSC.Recently, possible FMSCs were found in the EuFe 2 As 2 -related system, where the Eu spins and the FeAs layers are respectively responsible for spontaneous magnetization and SC, by applying pressure [5,6] or by chemical doping with phosphorus [7], cobalt [8,9] and ruthenium [10]. The parent compound EuFe 2 As 2 undergoes two antiferromagnetic transitions at ∼20 and ∼195 K, respectively, corresponding to the Eu and the Fe sublattices [11,12]. The magnetic structure was revealed by a neutron diffraction study [13] which shows an A-type antiferromagnetism (AFM) for the Eu 2+ spins, and a stripe-like AFM (also called spindensity wave (SDW)) for the Fe magnetic moment. Both Eu and Fe moments are parallel to the crystallographic a-axis in the undoped EuFe 2 As 2 . SC at 5-30 K emerges when the Fe-site SDW is sufficiently suppressed, either by applying high pressures or by an extrinsic doping. Simultaneously, the Eu 2+ spins turn to the c-axis at low temperatures [14,15], which leads to FM [7, 16, 17], helimagnetism [8] or spin canting [18,19], all with significant ferromagnetic component. The coexistence of SC and FM ...

Superconductivity and ferromagnetism in hole-dopedRbEuFe4As4

“…Simultaneously, the Eu spins become ferromagnetically ordered at T Curie ∼ 20 K, accompanying with a spin reorientation towards the [001] (or c-axis) direction [2]. Similar coexistence of Fe-based SC and Eu-spin ferromagnetism (FM) was also observed and verified in several Fe-site electron-doped EuFe 2 As 2 systems [12][13][14][15][16].…”

Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1−xCox)2AsJin1, Xiao2, Bukowski3 et al. 2016Phys. Rev. B292460View full textAdd to dashboardCite

“…Such coexistence of strong ferromagentism and superconducivity was also observed in other doped EuFe 2 As 2 compounds including EuFe 2 (As 1−x P x ) 2 , 10,34,51 Eu(Fe 1−x Ru x ) 2 As 2 , 12 and Eu(Fe 1−x Ir x ) 2 As 2 . 13,35,52 Interestingly, in the phase diagram shown in Fig. 9, the onset of superconductivity seems to accompany the formation of strong ferromagnetism.…”

“…6 This compound exhibits a spin-densitywave (SDW) ordering of the itinerant Fe moments concomitant with a tetragonal-to-orthorhombic structural phase transition below 190 K. In addition, the localized Eu 2+ spins order below 19 K in an A-type antiferromagnetic (AFM) structure (ferromagnetic layers stacking antiferromagnetically along the c direction). 7,8 Superconductivity can be achieved in this system by suppressing the SDW ordering of Fe in the form of chemical substitution [9][10][11][12][13] or applying external pressure. 14,15 For example, by hole doping with partial substitution of K for Eu, Eu 0.5 K 0.5 Fe 2 As 2 shows SC below the superconducting transition temperatureT sc = 32 K. 9 Isovalent substitution of P into the As sites can also give rise to the SC with T sc over 20 K. 10,16 Nevertheless, for the electron-doped Eu(Fe 1−x Co x ) 2 As 2 system, the reports about its physical properties remain quite controversial.…”

Reentrant phases in electron-dopedEuFe2As2: Spin glass and superconductivity

“…In case of Eu(Fe 0.91 Ir 0.09 ) 2 As 2 , the normal state resistivity depends linearly on T , which is typical for unconventional superconductors close to optimal doping level (x = 0.12) [20]. Such a behavior is often ascribed to the vicinity of a quantum critical point [21], but could be also explained by spin fluctuations [22].…”