Magnetoelastic-coupling-assisted first-order antiferromagnetic transitions in the corrugated honeycomb-lattice compounds ( Ca , Sr ) Mn 2 P 2

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Abstract
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 ) Mn 2 P 2 , electrical resistivity ρ measurements in the a b plane are reported to exhibit sharp discontinuities at the respective PM to AFM ordering temperatures T N = 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 CaMn 2 P 2 and SrMn 2 P 2 in the a b plane and along the c axis using capacitance dilatometry. The thermal expansion measurements show a sharp discontinuity in Δ L / L at T N , yielding a diverging behavior in the thermal expansion coefficient α ( T ) associated with strong magnetoelastic coupling, driving the first-order AFM transition in CaMn 2 P 2 . However, this effect is weaker in SrMn 2 P 2 , consistent with the magnetic entropy changes S ( T ) obtained from the heat-capacity measurements. Temperature-dependent a b -plane x-ray diffraction measurements for CaMn 2 P 2 near its T N yield a volume thermal expansion coefficient consistent with literature data for other materials. The uniaxial pressure derivatives of T N in the two compounds are found to be respectively opposite in sign, presumably due to the different magnetic structures.
Funding Information
  • U.S. Department of Energy (DE-AC02-07CH11358)
  • Basic Energy Sciences
  • Deutsche Forschungsgemeinschaft (CRC/TRR 288)
  • Deutsche Forschungsgemeinschaft (A02)

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