Tuning of competing magnetic and superconducting phase volumes in LaFeAsO${}_{0.945}$F${}_{0.055}$ by hydrostatic pressure

The interplay between magnetism and superconductivity in LaFeAsO${}_{0.945}$F${}_{0.055}$ was studied as a function of hydrostatic pressure up to $p\simeq 2.4$ GPa by means of muon-spin rotation ($\mu$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 $\mu$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.

Figure 1

ac (solid curves) and ZFC ${\mu }_{0}H=5$ mT dc (open symbols) susceptibility curves as a function of temperature for different pressures of LaFeAsO${}_{0.945}$F${}_{0.055}$. The transition temperature $T_c$ is determined by the intersect of the linearly extrapolated magnetization curve with the zero line. The superconducting transitions of pure indium used as a pressure indicator in the ac magnetization experiment is highlighted by the oval for the highest pressure.

Figure 2

Temperature dependence of the magnetic volume fraction of LaFeAsO${}_{0.945}$F${}_{0.055}$ at various pressures. The magnetic ordering temperature $T_{\mathrm{N}}$ is defined as the temperature where the magnetic fraction reaches 50$\%$ of its maximum low-temperature value. The insets show the ZF muon-time spectra at $p=0.0$ and 2.36 GPa. The solid lines are fits by means of Eq. (1). The dotted lines represent the response of the pressure cell.

Figure 3

(a) Dependence of the magnetic ordering temperature $T_{\mathrm{N}}$ and the superconducting transition temperature $T_{\mathrm{c}}$ on pressure. The closed and the open circles correspond to $T_{\mathrm{c}}$ as obtained in ac and dc magnetization experiments, respectively. (b) The magnetic fraction at $T=5$ K and ZFC diamagnetic susceptibility $-4\pi \chi$ at $T=3.5$ K, ${\mu }_{0}H=5$ mT as a function of pressure. The closed and the open circles refer to the data obtained in ac and dc magnetization experiments, respectively. The lines are a guide for the eye.

Figure 4

(a) The ZF-$\mu$SR time spectra obtained after cooling the sample in zero magnetic field from $T\simeq 100$ K down to 2.6 K (red/gray symbols) and after sweeping the magnetic field to 0.1 T and then setting it back to 0.0 without changing the temperature (black symbols). (b) and (c) show the extended parts of the muon-time spectra at the lower and the higher time, respectively. The solid lines are fits by means of Eq. (1).