Tuning of competing magnetic and superconducting phase volumes in

The interplay between magnetism and superconductivity in LaFeAsO0.945F0.055 was studied as a function of hydrostatic pressure up to p�2.4 GPa by means of muon-spin rotation (�SR) and magnetization measurements. The application of pressure leads to a substantial decrease of the magnetic ordering temperature, reduction of the magnetic phase volume and, at the same time, to a strong increase of the superconducting transition temperature and the diamagnetic susceptibility. From the volume-sensitive �SR measurements it can be concluded that the superconducting and the magnetic areas, coexisting in the same sample, are inclined toward spatial separation and compete for phase volume as a function of pressure. DOI: https://doi.org/10.1103/PhysRevB.84.100501 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-50796 Submitted Version Originally published at: Khasanov, R; Sanna, S; Prando, G; Shermadini, Z; Bendele, M; Amato, A; Carretta, P; De Renzi, R; Karpinski, J; Katrych, S; Luetkens, H; Zhigadlo, N (2011). Tuning of competing magnetic and superconducting phase volumes in LaFeAsO0.945F0.055byhydrostaticpressure.PhysicalReviewB, 84(10) : 100501. DOI: https://doi.org/10.1103/PhysRevB.84.100501 ar X iv :1 10 5. 12 80 v1 [ co nd -m at .s up rco n] 6 M ay 2 01 1 Tuning of competing magnetic and superconducting phase volumes in LaFeAsO0.945F0.055 by hydrostatic pressure R. Khasanov, ∗ S. Sanna, G. Prando, 3 Z. Shermadini, M. Bendele, 4 A. Amato, P. Carretta, R. De Renzi, J. Karpinski, S. Katrych, H. Luetkens, and N.D. Zhigadlo Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Dipartimento di Fisica “A. Volta” and Unità CNISM di Pavia, I-27100 Pavia, Italy Dipartimento di Fisica “E. Amaldi”, Università di Roma3-CNISM, I-00146 Roma, Italy Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Dipartimento di Fisica and Unità CNISM di Parma, I-43124 Parma, Italy Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland The interplay between magnetism and superconductivity in LaFeAsO0.945F0.055 was studied as a function of hydrostatic pressure up to p ≃ 2.4 GPa by means of muon-spin rotation (μSR) and magnetization measurements. The application of pressure leads to a substantial decrease of the magnetic ordering temperature TN and a reduction of the magnetic phase volume and, at the same time, to a strong increase of the superconducting transition temperature Tc and the diamagnetic susceptibility. From the volume sensitive μSR measurements it can be concluded that the superconducting and the magnetic areas which coexist in the same sample are inclined towards spatial separation and compete for phase volume as a function of pressure. PACS numbers: 76.75.+i, 74.25.Ha, 74.62.Fj, 74.70.Xa The interplay between superconductivity and magnetism in high-temperature superconductors (HTS) remains an important open issue. In cuprate and Fe-based HTS the superconductivity can be induced in a magnetic parent compound by charge doping and/or by pressure (chemical or external). In most cuprate HTS the transformation from the magnetic into the superconducting state follows an almost common scenario. On increasing the doping level the antiferromagnetically ordered phase develops into a purely superconducting state through a region where a spin-glass type of magnetism coexists with superconductivity [1–3]. The situation with Fe-based HTS is, somehow, different. For some families of Febased HTS like e.g. SmFeAsO1−xFx, Ba(Fe1−xCox)2As, and FeSe1−xTex, the magnetism is continuously suppressed and superconductivity enhanced by changing the F, Co, or Se content. In the intermediate region, bulk magnetism and bulk superconductivity are coexisting in space [4–7]. In Ba1−xKxFe2As2 the magnetic and the superconducting areas are found to be separated microscopically as revealed, e.g., by atomic force microscopy experiments [8]. One of the most interesting cases is realized in the LaFeAsO1−xFx family of Fe-based HTS demonstrating an abrupt (first order like) transition between the magnetic and the superconducting phases. Muon-spin rotation (μSR) and Mössbauer experiments show that above a certain x the samples become purely superconducting without visible traces of magnetism [9]. Such a behavior seems to be rather different from the one observed for the other structurally related families of Fe-based HTS in which the La atom is replaced by Sm, Pr, Ce, etc. All of them demonstrate a coexistence between superconductivity and magnetism for a certain doping level [4, 5, 10]. Consequently the question if a similar coexistence is present in LaFeAsO1−xFx but within a much narrower, up to now not detected, doping region or if an abrupt change between the superconductivity and magnetism is a unique property of this particular family of Fe-based HTS needs to be resolved. Hydrostatic pressure experiments on LaFeAsO0.945F0.055, which is at the border to the superconducting state but still magnetic, were performed to distinguish between two above mentioned possibilities. This approach allows to follow the transformation of the material from the magnetic to the superconducting state in detail on one sample, i.e. without the necessity to synthesize a large number of samples with exactly defined stoichiometry near the phase boundary. Our measurements show that both the magnetic and superconducting states are most probably spatially separated in the crossover region of the phase diagram and compete for phase volume. The sample with the nominal composition LaFeAsO0.945F0.055 was prepared in cubic anvil high-pressure cell from the stoichiometric mixture of LaAs, FeAs, Fe2O3, Fe and LaF3 [11]. A pressure of ≃ 3 GPa was applied at room temperature. By keeping the pressure constant, the temperature was first ramped up to the maximum value of 1320C, kept constant for 5.5 h and then quenched to room temperature within a few minutes. The superconducting properties of LaFeAsO0.945F0.055 were studied by magnetization experiments. The zerofield-cooled and field-cooled (FC and ZFC) DC magnetization measurements up to p ≃ 1.1 GPa were performed by using the commercial SQUID magnetometer (MPMS-XL7) and a piston-cylinder CuBe pressure cell (”EasyLab Mcell 10“, [12]). The AC experiments up to p ≃ 2 GPa were performed by using a home-made AC magnetometer (AC frequency ν = 72 Hz, AC field amplitude μ0HAC ≃ 0.1 mT). The two pick-up and the