Magnetic and superconducting anisotropy in Ni-doped RbEuFe4As4 single crystals

Abstract

We investigate the effect of Ni doping on the Fe-site in single crystals of the magnetic superconductor RbEuFe4As4 for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific heat, resistivity and magnetization measurements. Upon Ni-doping, the resistivity curves shift up in a parallel fashion indicating a strong increase of the residual resistivity due to scattering by charged dopand atoms while the shape of the curve and thus the electronic structure appears largely unchanged. The observed step ∆C/Tc at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multi-band superconductivity and strong inter-band pairing. The upper critical field slopes are reduced upon Ni doping for in-as well as out-of-plane fields leading to a small reduction in the superconducting anisotropy. The specific heat measurements of the magnetic transition reveal the same BKT behavior close to the transition temperature Tm for all doping levels. The transition temperature is essentially unchanged upon doping. The in to out-of-plane anisotropy of Eu-magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound. arXiv:1908.00773v1 [cond-mat.supr-con]

When superconductivity does not fear magnetism: Insight into electronic structure of RbEuFe$_{4}$As$_{4}$

“…Muon spin rotation (µSR) measurements of polycrystaline samples [22] indicated that under hydrostatic pressure the superconducting transition temperature T c decreased, while the magnetic transition temperature T m increased, suggesting no coupling between s ± superconductivity typical for IBSC and exotic magnetic order of Eu. A recent study of the magnetism and superconductivity in Ni-doped RbEuFe 4 As 4 showed that upon doping the SC transition critical temperature T c decreased, but the Eu magnetic transition temperature T m is hardly affected [23]. This suggests that Eu magnetism and superconductivity might be weakly dependent from each other.…”

Evolution from helical to collinear ferromagnetic order of the Eu$^{2+}$ spins in RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$

“…On the other hand, according to the prediction by Anderson and Suhl, the periodicity of the spin helix d is correlated with the supercondcuting coherence length ξ 0 in the form of d ∝ (ξ 0 ) 1/3 . 42 As the superconducting transition temperature T SC and the upper critical field H c2 decrease with increasing Ni doping, 23 ξ 0 increases according to the Ginzburg-Landau formalism H c2 = Φ 0 /2πξ 0 2 , which is consistent with the diminishing θ and increasing helix periodicity d . Although some recent spectroscopic measurements seem to suggest the decoupling of magnetism from Eu from superconducting FeAs layers, 43,44 we note that a recent scanning Hall microscopy experiment has revealed a pronounced suppression of the superfluid density near the Eu magnetic ordering temperature in Eu1144, indicating a pronounced exchange interaction between the superconducting and magnetic subsystems.…”

“…Γ 1 allows the c-axis aligned ferromagnetic Eu layers stacking with modulated moment size values at different layers, which is not consistent with the easy-plane magnetization as revealed from the single-crystal sample with a similar Ni doping level. 23 On Bragg reflections from the RbEu(Fe0.91Ni0.09)4As4 main phase, EuFe2As2 impurity, RbFe2As2 impurity, vanadium sample container, as well as the magnetic Bragg reflections from the RbEu(Fe0.91Ni0.09)4As4 main phase and the EuFe2As2 impurity, respectively. (e), (f) and (g) show the enlarged high-resolution diffraction patterns at 2 and 20 K around the (0 0 1), (0 0 2) and (0 0 3) nuclear peak positions, respectively, illustrating the commensurate magnetic contributions with k = 0 at 2 K. the other hand, Γ 5 allows the in-plane aligned ferromagnetic Eu layers to stack helically along the c axis, with a constant moment size value at different layers.…”

“…22 As the helical magnetic order of the Eu 2+ spins with a twodimensional (2D) character in undoped Eu1144 is proposed to be associated with the presence of superconductivity, 24,25 it is of great interest to clarify how the magnetic structure of RbEu(Fe 1−x Ni x ) 4 As 4 develops against the weakening of the superconductivity induced by Ni doping. Fitting to the magnetic suceptibility in the paramagnetic state yields comparable positive values of Currie-Weiss temperature for samples with different x , 22,23 reflecting dominant in-plane ferromagnetic interactions between the Eu 2+ moments. Detailed neutron diffraction measurements on RbEu(Fe 1−x Ni x ) 4 As 4 will deliver important information regarding how the stacking pattern of the ferromagnetic Eu layer along the c axis changes with x and how it is correlated with the suppression of superconductivity.…”

“…Systematic macroscopic characterizations including resistivity, magnetization, and specific heat measurements have been performed on polycrystalline and single-crystal samples of RbEu(Fe 1−x Ni x ) 4 As 4 to establish the superconducting and magnetic phase diagram. 22,23 It is figured out that T SC descreases rapidly with the Ni doping, while the magnetic ordering temperature of the Eu sublattice, T m , remains essentially unchanged. Consequently, RbEu(Fe 1−x Ni x ) 4 As 4 transforms from the ferromagnetic superconductor (FSC) with T SC > T m for x < 0.07, to the so-called "superconducting ferromagnet" (SFM) with T m > T SC for 0.07 x 0.08, and finally to the ferromagnetic non-superconductor for x > 0.09.…”