Low-temperature anomaly of the magnetization in alloys (Pr, Dy, $M$)$_{2}$(Fe, Co)$_{14}$B ($M$ = Gd, Sm, Nd)

It has been found that temperature dependences of the saturation magnetization of sintered hard magnetic (Pr, Dy, $M$)$_{2}$(Fe, Co)$_{14}$B ($M$ = Gd, Sm, Nd) alloys demonstrate an increase at a temperature lower than a critical temperature (150 K for Sm and Nd and 70 K for Gd). An additive of copper does not influence the critical temperature. It has been assumed that there is a low-temperature phase in which cobalt is replaced with boron that diffuses from the (Pr, Dy, Gd)(Fe, Co)$_{4}$B phase to the near-surface region of grains of the main magnetic (Pr, Dy, Gd)$_{2}$(Fe, Co)$_{14}$B phase.