Transport, magnetic and thermal properties of $M$FePO $M$ = La, Pr, and Nd single crystals*

The recent discovery of $T_{c}$ values near 26 K in the compound LaFeAsO$_{1-x}$F$_{x}$ induced a torrent of publications on what are now recognized as a new class of Fe-based high temperature superconductors. To date, the phosphorus based versions of these compounds have recieved little attention due to their comparitively low $T_{c}$ values. In this work we report the low temperature electrical resistivity, magnetic susceptibility, and specific heat data of single crystalline PrFePO and NdFePO. We also report the effect of annealing on the properties of LaFePO, PrFePO, and NdFePO. A systematic comparison of the occurence of superconductivity in the series $M$FePO and $M$FeAsO (where $M$ is a lanthenide) points to a possible difference in the origin of the superconductivity in these two series of compounds.