Evidence of ferromagnetic coupling for manganese pairs in a layered van der Waals GaS semiconductor

Abstract

Using high-frequency electron paramagnetic resonance (EPR), we have observed the ferromagnetic coupling of manganese Mn2+ pairs in a layered van der Waals GaS semiconductor. The EPR spectra of Mn2+ pairs (Mn24+) [replacing covalently bonded Ga24+ pairs oriented along the chain axis (c axis) of two gallium ions and being situated in the center of the layer] were recorded at 94 and 130 GHz. The fine structure parameters for the lower multiplets with spin S = 5 and S = 4 were determined equal to D = −0.040 cm−1 and D ≅ −0.035 cm−1, respectively. Based on the observation of additional EPR lines in the region of intersection of the levels in the magnetic field of the multiplet with S = 5 and S = 4, the energy of the isotropic exchange interaction J was estimated to be ∼−0.5 cm−1. For all transitions, a well-resolved hyperfine structure was observed, due to the interaction with two equivalent 55Mn nuclei, leading to the appearance of 11 lines with a hyperfine interaction constant A = 30(1) × 10−4 cm−1, which is approximately half the hyperfine interaction constant for single Mn2+ ions. In addition, a signal from single manganese ions Mn2+ with spin S = 5/2 and a hyperfine interaction constant A = 60(1) × 10−4 cm−1 was observed, which are characterized by extremely large fine structure splitting of D = −0.15 cm−1. Simultaneously with EPR studies, the crystals were monitored by photoluminescence and micro-Raman scattering measurements using a microscope with confocal optics.