Magnetic phase transition and low-temperature EPR spectra of a one-dimensional ferrimagnet formed by manganese(II) and a nitronyl nitroxide

The magnetic properties of single crystals of the chain compound formed by manganese(I1) hexafluoroacetylacetonate and the nitronyl nitroxide radical NIT-i-Pr (NIT-i-Pr = 2-isopropyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-l-o3x-yolx ide), Mn(hfa~)~NIT-i-Phra,v e been investigated at low temperature to characterize the magnetic phase transition. Measurements at low magnetic field (0-1 Oe) reveal that the system undergoes a transition to a 3-D ferromagnetic ordered state at 7.6 K. The observed magnetic anisotropy is of the XY type in agreement with a calculation of the dipolar energies. The dipolar interaction seems to be the origin of the phase transition as confirmed also by the value of the critical exponent y = 1.13. The single-crystal EPR spectra recorded in the range of temperature 4.2-300 K show the effect of short-range order with the resonance field depending on the crystal orientation. The g shifts are related to the magnetic susceptibility extending to the case of ferrimagnetic chains the model previously suggested by Nagata and Tazuke for one-dimensional ferro- and antiferromagnets. Mn(hfa~)~NIT-i-Pis r found to behave as the typical one-dimensional ferrimagnet