Neutron investigation of the magnetic scattering in an iron-based ferromagnetic superconductor

Neutron diffraction and small angle scattering experiments have been carried out on the double-isotopic polycrystalline sample $\left({}^7\mathrm{L}{\mathrm{i}}_{0.82}\mathrm{F}{\mathrm{e}}_{0.18}\mathrm{OD}\right)\mathrm{FeSe}$. Profile refinements of the diffraction data establish the composition and reveal an essentially single phase material with lattice parameters of $a=3.7827\AA$ and $c=9.1277\AA$ at 4 K, in the ferromagnetic-superconductor regime, with a bulk superconducting transition of $T_C=18\mathrm{K}$. Small angle neutron scattering measurements in zero applied field reveal the onset of ferromagnetic order below $T_F\approx 12.5K$, with a wave vector and temperature dependence consistent with an inhomogeneous ferromagnet of spontaneous vortices or domains in a mixed state. No oscillatory long range ordered magnetic state is observed. Field-dependent measurements establish a separate component of magnetic scattering from the vortex lattice, which occurs at the expected wave vector. The temperature dependence of the vortex scattering does not indicate any contribution from the ferromagnetism, consistent with diffraction data that indicate that the ordered ferromagnetic moment is quite small.

Figure 1

Rietveld refinement of the neutron diffraction pattern for the ${}^7{\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\mathrm{ODFeSe}[x\approx 0.183\left(6\right)]$ sample collected at 4 K using the Cu(311) monochromator $(\lambda =1.540\AA )$ at the BT-1 diffractometer. Structural refinement of the tetragonal superconducting phase gave lattice constants $a=3.7827\left(1\right)\AA$ and $c=9.1277\left(3\right)\AA$. Top and bottom ticks indicate the superconducting phase and about $10\%\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3$ impurity phase, respectively. The * indicates a (Fe-Se) impurity estimated at less than $1\mathrm{wt}\%$, whose peaks are considerably broader than instrumental resolution. The tetragonal unit cell is shown as an inset.

Figure 2

(a) Temperature dependence of magnetic moment of the ${}^7{\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\mathrm{ODFeSe}$ ($x\approx 0.18$) sample measured in a superconducting quantum interference device magnetometer in an applied field of 1 mT. The zero-field-cooled data (ZFC) were taken upon heating and the field-cooled (FC) data upon cooling. Inset shows the data below 30 K revealing the $T_c\approx 18\mathrm{K}$. Note that $1\mathrm{emu}={10}^{-3}\mathrm{A}{\mathrm{m}}^2$. (b) Magnetic field dependence of isothermal magnetization per formula unit of the same sample measured in hysteresis mode and ZFC. The inset expands the low field region to highlight the hysteresis, as expected for a ferromagnet.

Figure 3

(a) Magnetic scattering as a function of wave vector $Q$ for several temperatures. No magnetic field is applied. At 12.5 K very little magnetic scattering is observed, as is the case at higher temperatures (not shown). Below the ferromagnetic transition magnetic scattering intensity develops, which increases monotonically with both decreasing $Q$ and $T$. No peak in this $Q$ range is observed, ruling out the formation of a long range ordered oscillatory magnetic state. (b) Integrated intensity as a function of temperature, revealing a magnetic transition temperature of $\sim 12.5\mathrm{K}$. Uncertainties are statistical in origin and represent one standard deviation.

Figure 4

(a) Magnetic intensity at 5 K after cooling from 25 K in an applied field of 0.4 T. The ferromagnetic scattering has shifted to smaller $Q$, indicating that the length scale for this scattering has increased or the strength has decreased, as would be expected when a field is applied. The peak at $Q=0.0077{\AA }^{-1}$ is due to vortex scattering. (b) Integrated intensity of the vortex scattering as a function of temperature. The onset of scattering occurs at $T_C=18\mathrm{K}$. No evidence of the ferromagnetic ordering is observed, indicating that the ordered moment is small (see text).