  ZrMnP and HfMnP single crystals are grown by a self-flux growth technique and
structural as well as temperature dependent magnetic and transport properties
are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and
HfMnP are ferromagnetic with Curie temperatures around $370$~K and $320$~K
respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined
to be $1.9$~$\mu_\textrm{B}$/f.u. and $2.1$~$\mu_\textrm{B}$/f.u. respectively
at $50$~K. The magnetocaloric effect of ZrMnP in term of entropy change
($\Delta S$) is estimated to be $-6.7$ kJm$^{-3}$K$^{-1}$ around $369$~K. The
easy axis of magnetization is [100] for both compounds, with a small anisotropy
relative to the [010] axis. At $50$~K, the anisotropy field along the [001]
axis is $\sim4.6$~T for ZrMnP and $\sim10$~T for HfMnP. Such large magnetic
anisotropy is remarkable considering the absence of rare-earth elements in
these compounds. The first principle calculation correctly predicts the
magnetization and hard axis orientation for both compounds, and predicts the
experimental HfMnP anisotropy field within 25 percent. More importantly, our
calculations suggest that the large magnetic anisotropy comes primarily from
the Mn atoms suggesting that similarly large anisotropies may be found in other
3d transition metal compounds.
