Abstract

The CaMn₂X₂ (X = P, As, Sb, Bi) family of magnetic semiconductors provides a model platform for studying the effects of competing exchange interactions on a corrugated
honeycomb lattice in the presence of single-ion anisotropy. This dissertation focuses on CaMn₂As₂ and the solid solution CaMn2(P1-xAsx)2.

Single crystals of CaMn2(P1-xAsx)2 (0 ≤ x ≤ 1) were synthesized using the Sn-flux method. For CaMn₂As₂, temperature-dependent powder neutron diffraction measurements
confirmed the CaAl₂Si₂-type crystal structure and revealed long-range magnetic order below TN=62.3(5) K, characterized by the propagation vector k = (δ, δ, 0) with δ=0.0570(5) at 1.5 K. The temperature evolution of the magnetic intensities demonstrated a second-order transition, and critical-exponent analysis of prominent reflections yielded β ≈ 0.11(2), consistent with the value of 1/8 expected for the two-dimensionalIsing model.

DC resistivity measurements within the ab-plane confirmed insulating behaviour in both CaMn₂As₂ and CaMn₂P₂, with a clear resistive transition at 69(1) K observed in CaMn₂P₂. Complementary heat-capacity measurements using AC calorimetry revealed contrasting behaviour: CaMn₂P₂ displays a sharp first-order-like anomaly at 69(1) K, whereas CaMn₂As₂ exhibits a weaker, broader feature. In the doped series CaMn2(P1-xAsx)2, crystals with x < ~0.35 show distinct sharp peaks, while those with x > ~0.35 display broader anomalies with suppressed transition temperatures. The rapid change in behaviour between x = 0.314(10) and 0.360(10) indicates a first-order transformation of the underlying magnetic structure.

As part of this project, a simple AC calorimetry setup was developed, along with a custom LabVIEW routine for automated data collection, enabling systematic heat capacity measurements across the CaMn2(P1-xAsx)2 series.