Spin Symmetry Breaking: Superparamagnetic and Spin Glass-Like Behavior Observed in Rod-Like Liquid Crystalline Organic Compounds Contacting Nitroxide Radical Spins

Abstract

Liquid crystalline (LC) organic radicals were expected to show a novel non-linear magnetic response to external magnetic and electric fields due to their coherent collective molecular motion. We have found that a series of chiral and achiral all-organic LC radicals having one or two five-membered cyclic nitroxide radical (PROXYL) units in the core position and, thereby, with a negative dielectric anisotropy exhibit spin glass (SG)-like superparamagnetic features, such as a magnetic hysteresis (referred to as ‘positive magneto-LC effect’), and thermal and impurity effects during a heating and cooling cycle in weak magnetic fields. Furthermore, for the first time, a nonlinear magneto-electric (ME) effect has been detected with respect to one of the LC radicals showing a ferroelectric (chiral Smectic C) phase. The mechanism of the positive magneto-LC effect is proposed and discussed by comparison of our experimental results with the well-known magnetic properties of SG materials and on the basis of the experimental results of a nonlinear ME effect. A recent theoretical study by means of molecular dynamic simulation and density functional theory calculations suggesting the high possibility of conservation of the memory of spin-spin interactions between magnetic moments owing to the ceaseless molecular contacts in the LC and isotropic states is briefly mentioned as well.

Enhanced Magnetization in a Frustrated Spin System by Ni+ Ion Beam Irradiation

“…In a typical FC curve for a superparamagnetic system, we may thus expect the magnetization of the particles would be increased with decreasing temperatures. 19,22,23 However, in our FC curves, the magnetization shows a decrease with decreasing temperature, and exhibits no appreciable change below ∼200 K, featuring the spin-glass-like behavior. The spin glass behavior arises from intraparticle interaction or magnetic order frustration at the particle surface, enhancing the effective anisotropy.…”

Magnetically Manipulable Ionic Liquid Crystals Incorporating Neutral Radical Moiety

“…Thus, the origin of such unique magnetic properties observed in all phases of ionic 6 a could be mesoscopically rationalized in terms of (i) the preferential formation of spin glass-like inhomogeneous magnetic domains consisting of magnetic spin domains surrounded by paramagnetic spins in the original ionic solid phase, (ii) the subsequent enlargement of the size and/or number of magnetic spin domains by thermal effect, and (iii) preservation of the overall increased magnetic spin domains in the ionic solid phase by cooling until cryogenic temperatures (Figure 7C) . [57] Intuitively, we predict that the correlation length of spin polarization or the magnetic domain size corresponds to the molecular orientation. However, since the positive magneto-LC effect depends on the ratio of the life time of the spin polarization to that of the intermolecular contacts, [36] the molecular orientation does not directly relate to the positive magneto-LC effect.…”

Spirocyclic Nitroxides as Versatile Tools in Modern Natural Sciences: From Synthesis to Applications. Part I. Old and New Synthetic Approaches to Spirocyclic Nitroxyl Radicals

“…For the synthesis of a paramagnetic discotic compound, trans -diradical 214 , dinitrone 211 is treated with an excess of 4-(diethoxymethyl)phenylmagnesium bromide in THF at ambient temperature for 24 h; in this case, the yield of corresponding SNDR 212 (X = CH(OEt) 2 ) increases up to 37%. Subsequent hydrolysis of trans -acetal 212 , selective oxidation of diformyl derivative 215 to paramagnetic dicarboxylic acid 216 , and the final amidation step involving amine 217 and appropriate condensation reagents (DMT-MM and N -methylmorpholine) result in α,α′-SNDR 214 ( Scheme 38 ), showing both discotic and smectic liquid crystalline phases and manifesting superparamagnetic-like behavior [ 81 , 148 ].…”