Enhancing hard magnetic properties in Fe69.5-xNd7B21Nb2.5Zrx (x = 0-3) bulk magnets by magnetic field heat treatment

•Zr addition can inhibit the growth of α-Fe grains and increase the volume fraction of hard Nd2Fe14B magnetic phase.•Hard magnetic properties can be improved by magnetic field heat treatment.•The magnetic field heat treatment can contribute to a uniform microstructure and benefits precipitation of α-Fe.•Moreover, magnetic field heat treatment can decrease the volume fraction of B-rich regions in the alloys.