Anomalous quadrupole feature in the mixed state of YNi₂B₂C

Abstract

We demonstrate the observation of paramagnetic-like anomaly in the mixed state of YNi₂B₂C crystal from the magnetization measurements for H ∼ 20 kOe. In the LuNi₂B₂C borocarbide superconductor, observation of such an anomaly was explained on the basis of symmetry reorientation transition of the flux line lattice; i.e., transition from square to triangular lattice due to non-local effects. However, a detailed magnetization study on YNi₂B₂C sample reveals that such a type of paramagnetic anomaly can arise in the magnetization measurements due to the admixture of a signal from quadrupolar moment with that from dipole moment of the mixed state, if the sample is not positioned precisely at the central position in a VSM coil. When we intentionally shifted the sample position from the centering position to preferentially record the quadrupole moment only, we noted that the merging of the forward and the reverse legs of the loop near the superconductor-normal boundary is much better defined in the Q–H measurements, rather than in the M–H measurements. The Q–H plots, thus, provide a more accurate recipe to determine the Hc₂ values. We have also examined the timescale disorder experienced by the flux line lattice (FLL) during the field ramping process in isothermal M–H measurements. Our study revealed the existence of non-monotonic dependence of spatial disorder within FLL due to the change in the ramp rate. The field ramp rate acts as an additional source of disorder in the formation of the FLL.