Magnetoelastic-coupling-assisted first-order antiferromagnetic transitions in the corrugated honeycomb-lattice compounds ( Ca , Sr ) Mn$_2$P$_2$

Understanding the origin of unconventional magnetic order is a central theme in the research field of functional quantum materials for spin-dependent applications. In general, a paramagnetic (PM) to antiferromagnetic (AFM) transition is of thermodynamically second order in nature in the absence of a magnetic field. In the magnetically frustrated corrugated-honeycomb lattice compounds (Ca, Sr)Mn2P2, electrical resistivity rho measurements in the ab plane are reported to exhibit sharp discontinuities at the respective PM to AFM ordering temperatures TN = 69.5 and 53 K, respectively, signifying the unusual first-order nature of the AFM transition. To reveal the origin of such anomalous transitions, high-resolution linear thermal-expansion measurements are reported for both CaMn2P2 and SrMn2P2 in the ab plane and along the c axis using capacitance dilatometry. The thermal expansion measurements show a sharp discontinuity in AL/L at TN, yielding a diverging behavior in the thermal expansion coefficient alpha(T) associated with strong magnetoelastic coupling, driving the first-order AFM transition in CaMn2P2. However, this effect is weaker in SrMn2P2, consistent with the magnetic entropy changes S(T) obtained from the heat-capacity measurements. ... mehrTemperature-dependent ab-plane x-ray diffraction measurements for CaMn2P2 near its TN yield a volume thermal expansion coefficient consistent with literature data for other materials. The uniaxial pressure derivatives of TN in the two compounds are found to be respectively opposite in sign, presumably due to the different magnetic structures.