Fishtail effect and vortex dynamics in LiFeAs single crystals

We investigate the fishtail effect, critical current density ($J_c$), and vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak (SP) in the magnetization loop only with the field $\left|\right|$ $c$ axis. We calculate a reasonably high $J_c$, however, values are lower than in Ba-122- and 1111-type FeAs compounds. Magnetic relaxation data imply a strong pinning which appears not to be due to conventional defects. Moreover, we find that the calculated magnetic relaxation rate is very low in LiFeAs. Our data suggest that the origin of the SP may be related to a vortex lattice phase transition. We have constructed the vortex phase diagram for LiFeAs on the field-temperature plane.

Figure 1

Temperature dependence of ${\chi }_{\mathrm{vol}}$ for LiFeAs deduced from dc $M$ measured following ZFC and FC protocols. The data have been collected in $H\left|\right|c$ $=$20 Oe. The inset shows $M$ vs $H$ plots measured at 10 K with the field parallel to both $c$ axis and $\mathit{ab}$ plane.

Figure 2

(a) The isothermal $M$ vs $H$ loops recorded at different temperatures with $H\left|\right|c$ for LiFeAs. (b) The critical current density $J_c$ as calculated from the $M(H)$ loops in (a) as function of field for different temperatures; for details see text.

Figure 3

(a) The energy barrier $U_0/k_B$ obtained by fitting of Eq. (1) as a function of temperature. (b) The magnetic relaxation rate $S$ as a function of temperature in $H\left|\right|c$ $=$10 kOe as calculated using Eq. (2) at $t$ $=$1000 s. The inset exemplary shows the best fit of the magnetic relaxation data at $T$ $=$10 K and $H\left|\right|c$ $=$10 kOe using Eq. (1).

Figure 4

(a) The field dependence of $U_0/k_B$ and $S$ (at 1000 s) extracted from the magnetic relaxation data at 10 K exploiting Eqs. (1) and (2), respectively. The vertical dashed lines mark $H_m$ and $H_p$; for details see text. (b) The same physical properties $U_0/k_B$ and $S$ shown for 5 K.

Figure 5

The vortex phase diagram on the $H$-$T$ plane with $H\left|\right|c$ for LiFeAs. Solid lines represent the fit to the experimental data using the functional form $H_x(T)=H_x(0)(1-T/T_c){}^n$ (see text).