Low and high frequency magnetic relaxations as markers of critical phenomena in chiral 2D metal–organic ferrimagnet

Chiral molecular magnets are important platforms for investigation of non-linear spin phenomena. Mapping of imaginary and real parts of magnetic susceptibility in coordinates field-temperature revealed two spin transitions at 15 K and 21 K in chiral 2D metal–organic ferrimagnet [Mn ^II(HL-pn)(H ₂O)][Mn ^III(CN) ₆]•2H ₂O titled Brown Needle crystals. Curie temperature corresponds to 21 K, while the transition between ferrimagnetic and spin canting incommensurate phase happens at 15 K. We found that the second and third harmonics of non-linear magnetic susceptibility correspond to time irreversible domain walls displacement. High frequency spin relaxation was monitored by electron spin resonance (ESR) technique. Resonant line at zero field was analyzed in the frame of formalism developed for non-Kramers ions coupled by dipole–dipole interaction. The zero field ESR spectra originates from the S = 2 ground state of the Mn ³⁺ ions. This line appears in a narrow temperature range 18–22 K indicating the intermediate magnetic phase arising in the vicinity of the Curie point. Microwave power saturation of the ESR line corresponds to a fast relaxation ∼10 ns. We found the significant difference in this relaxation time in racemic and chiral crystals. Resonant response of the Mn ³⁺ ions controlled by dipole–dipole magnetic interaction provides very sensitive indication of crystal fields their modification and it allows one to judge on origin critical phenomena at magnetic phase transition.