Anisotropic superconductivity and magnetism in single-crystal RbEuFe4As4

Abstract

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe4As4 using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of Tm = 15 K and a superconducting transition temperature of Tc = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Γ = 1.7. Ginzburg-Landau parameters of κc ∼ 67 and κ ab ∼ 108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and < 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.

Strongly fluctuating moments in the high-temperature magnetic superconductor RbEuFe4As4

“…In high fields, 0.4T < µ 0 H < 9T, the cusp of the magnetic transition evolves into a broad magnetic hump with its center moving to higher temperatures. At the highest field (9T) these magnetic fluctuations extend up to about 100Kfar above the superconducting transition-and provide a natural explanation for the reported negative, normalstate magneto-resistance 23 . We attribute this hump to a field-induced polarization of the Eu 2+ moments along the field direction and their associated fluctuations.…”

“…A more detailed justification shall be given below. The down-bending of the calorimetric data below ∼ 10K is attributed to the quantum nature of the high spin Eu 2+ moments [21][22][23] . In applied fields, the superconducting transition temperature is gradually suppressed; the effect is stronger, if the field is applied along the c axis.…”

Self-induced magnetic flux structure in the magnetic superconductor RbEuFe4As4

“…At small Ha, the diamagnetic signal at Tc~37K is very weak and is followed by a rapid increase in magnetization at Tm ~15K and saturation at T<Tm. For fields larger H||ab [13], which depict the positive growth of M at T<Tm due to the twist of Eu 2+ spins towards H. The M(H) loops for H||c-axis [13] are strongly tilted showing a large magnetic anisotropy, which locks the Eu 2+ moments in the ab-plane. Figure 2 shows a set of images for one of the samples cooled in a field of 220 Oe.…”

“…[15]). Previous work has shown [13] that this magnetic state is rather fragile and is easily polarized in the ab-plane by modest fields of less than 1 kG. Temperature and field dependences of the specific heat point to Berzinskii-Kosterlitz-Thouless nature of the magnetic transition at Tm, as described by twodimensional anisotropic Heisenberg model, with fine features caused by the three-dimensional effects [16].…”

“…In this Letter, we report observation of an intriguing magnetic flux behavior in the latest member of the stoichiometric iron-pnictide superconductor family, RbEuFe4As4, with a superconducting transition at Tc ~ 37 K and the onset of long-range magnetic order of the Eu moments near Tm ~15 K [3][4][5]13]. In this material, the Eu-moments order ferromagnetically within the ab-plane and have the in-plane orientation.…”