Strain-modulated helimagnetism and emergent magnetic phase diagrams in highly crystalline MnP nanorod films

Abstract

We explore strain-modulated helimagnetism in highly crystalline MnP nanorod films grown on Si(100) substrates using molecular beam epitaxy. The strained MnP film exhibits a paramagnetic to ferromagnetic (FM) phase transition at ${T}_{C}\ensuremath{\sim}279\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the FM to helical phase transition at ${T}_{N}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The value of ${T}_{N}$ is greater than ${T}_{N}\ensuremath{\sim}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for the MnP single crystal, indicating strong strain-modulated helimagnetic states in the MnP nanorod film. The presence of significant thermal hysteresis in the helical phase indicates the coexistence of competing magnetic interactions, leading to the first-order metamagnetic transition. Similar to its single-crystal counterpart, an anisotropic magnetic effect is observed in the MnP film, which is independently confirmed by magnetic hysteresis loop and radio-frequency transverse susceptibility (TS) measurements. The evolution of the screw to the cone and fan phases is precisely tracked from magnetization vs magnetic field/temperature measurements. The temperature dependence of the anisotropy fields, extracted from the TS spectra, yields further insight into the competing nature of the magnetic phases. Unfolding of the different helical phases at $T<120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ $(\ensuremath{\sim}{T}_{\mathrm{N}})$ is analyzed by the temperature- and field-dependent magnetic entropy change. Based on these findings, the comprehensive magnetic phase diagrams of the MnP nanorod film are constructed for both the in-plane and out of plane magnetic field directions, revealing emergent strain/dimensionality-driven helical magnetic features that are absent in the magnetic phase diagram of the MnP single crystal.