Hysteresis Effects in Magnetovoltage Measurements in Superconducting ${\rm MgB}_{2}$

Hysteresis effects in superconducting ${\rm MgB}_{2}$ have been studied systematically by Magnetovoltage Measurements ($V-H$ curves) as a function of transport current $(I)$, temperature $(T)$ and sweep rate of external magnetic field $(H)$, $({\rm d}H/{\rm d}t)$. It was observed that time dependent effects appear as the field sweep rate $({\rm d}H/{\rm d}t)$ varies. This reflects itself as a decrease in the width of $V-H$ loops with decreasing of ${\rm d}H/{\rm d}t$. The dissipation measured in $V-H$ curves does not change as the sweep rate of external magnetic field is varied. It was observed that the measured dissipation in $V-H$ curves is independent of field orientation of $H$ with respect to $I$. Furthermore, it was observed that there is a significant asymmetry in $V-H$ loops between forward and reverse regions. The physical observations were interpreted mainly in terms of flux de-trapping/trapping and thus the difference in the local fields trapping in the grains between increasing and decreasing branches.