NMR investigation of Gd(hfac)3NITEt fully frustrated helimagnet

The family of the quasi-1D molecular magnetic chains Gd(hfac)3NITR (where hfac is hexafluoro-acetylacetonate and NITR is 2-R-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-l-oxyl 3-oxyde) have assumed a great importance in the study of molecular magnets in the last 10 years. These chains develop along b crystallographic axis and consist in a regular pattern of Gd(hfac)3 moieties (S_Gd=7/2) alternated to nitronyl-nitroxide organic radicals NITR (R = iPr, Et, Ph, Me) (srad=1/2). In these derivatives the shortest interchain distance is about 10.5Å and, as a consequence, the ratio between interchain and intrachain exchange interactions is Jinter/Jintra<10-5, being very weak the dipolar interactions between the chains. It is possible to obtain two different classes of XY frustrated helimagnets due to the competition of nn and nnn exchange interactions, depending on the radical inserted. In particular in the compound containing Ethyl (in short Gd-Et) the frustration is strong, hence the name of fully frustrated system. Gd-Et fulfils the Villain’s conjecture presenting, in addition to a high temperature paramagnetic phase, an intermediate chiral ordered magnetic phase setting up at T(c)^h~2.2K and a further phase transition to the 3D long-range ordered helimagnetic phase at T(c)^3D~1.9K. These results have been experimentally demonstrated by means of coupled Muon Spin Rotation (MUSR), magnetic susceptibility and Specific Heat investigation, following theoretical predictions. To implement the study of spin dynamics begun with the MUSR experiments and to confirm the occurrence of a 3D phase transition, we present here a low temperature NMR investigation of Gd-Et performed through 1H nuclear spin-lattice relaxation rate (NSLR) 1/T1 and 1H absorption spectra measurements at low applied magnetic field H=0.1 Tesla. The choice of a low magnetic field was done to be far from the critical field (Hc=2.06T), where a distortion of the spin arrangement takes place. From both relaxation rate and spectra temperature behaviours, the transition to the 3D long-range magnetic helical phase is confirmed to occur at about 1.9K. In fact NSLR shows a sharp peak at the transition temperature T~1.9K, giving evidence of an anomaly in the two-spin correlation function typical of a phase transition to 3D long-range magnetic order. Also, the NMR 1H absorption spectra present a full width at half maximum (FWHM) that remains small (~100 kHz) for T>1.9K while just below the transition temperature T(c)^3D=1.9K becomes suddenly much larger (>1 MHz), due to the insurgence of a local field Hloc at protons sites, generated by the ordered arrangement of the electronic spins.