Comparative study of magnetic behaviour in three classic molecular magnets (original) (raw)
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Role of the demagnetizing field on the EPR of organic radical magnets
Physical Review B, 1997
It has been long thought that microscopic mechanisms related to magnetic short-range order were responsible for the temperature dependence of the electron paramagnetic resonance g tensor in low-dimensional magnetic systems. We show here that the demagnetizing field can explain qualitatively and quantitatively the observed features, i.e., ͑i͒ the g shift, variation of the g value, ͑ii͒ the presence of magic angles where there is no g shift, and ͑iii͒ the reorientation of the g tensor with temperature. These features are discussed theoretically and supported experimentally in purely organic insulating compounds. Previous results obtained on two different nitroxide derivatives are revisited in this framework. The role of the demagnetizing field may probably be generalized to most low-dimensional molecular materials. ͓S0163-1829͑97͒07213-5͔
Magnetic Transition in Organic Radicals: The Crystal Engineering Aspects
Crystal Growth and Design, 2021
Spin−Spin interactions between unpaired electrons in organic radicals are of utter importance from the viewpoint of molecular magnetism and the development of smart materials. The diamagnetic to paramagnetic phase transition observed in some radicals often leads to "magnetic bistability," sometimes associated with a thermally accessible structural phase transition. The noncovalent interactions determining the solid-state packing arrangement are highly susceptible to external stimuli (temperature, pressure, light, electric field, etc.) and allow the radicals to respond reversibly. Thus, a qualitative understanding of the communication pathway of the spin centers and factors determining the solid-state packing arrangement for the radicals is most important. In this perspective, we mainly discuss the effect of noncovalent interactions rearranging the radicals' position with temperature determining the mechanistic pathway of such phase transitions. We focus on the importance of electronic parameters stabilizing different polymorphic phases of the radicals, secondary dynamic effects arising from the π-stacking in solid-state, and their role in a magnetic phase transition, along with the consequences of different external stimuli in fine-tuning the magnetic bistable states.
Recent Advances in Organic Radicals and Their Magnetism
Magnetochemistry, 2016
The review presents an overview of the organic radicals that have been designed and synthesized recently, and their magnetic properties are discussed. The π-conjugated organic radicals such as phenalenyl systems, functionalized nitronylnitroxides, benzotriazinyl, bisthiazolyl, aminyl-based radicals and polyradicals, and Tetrathiafulvalene (TTF)-based H-bonded radicals have been considered. The examples show that weak supramolecular interactions play a major role in modulating the ferromagnetic and antiferromagnetic properties. The new emerging direction of zethrenes, organic polyradicals, and macrocyclic polyradicals with their attractive and discrete architectures has been deliberated. The magnetic studies delineate the singlet-triplet transitions and their corresponding energies in these organic radicals. We have also made an attempt to collate the major organic neutral radicals, radical ions and radical zwitterions that have emerged over the last century.
Journal of the American Chemical Society, 2007
We present the synthesis, crystal structure, and temperature and field dependence of the magnetic properties of a new molecule-based magnet, [Co(hfac)2]‚BNO* (1), where hfac) 1,1,1,5,5,5-hexafluoroacetylacetonato and BNO* is the chiral triplet bis(nitroxide), 1,3-bis(N-tert-butyl-N-oxylamino)-5-{1′-methyl-1′-[2′′-(S)-methylbutoxy]ethyl}benzene. The presence of enantiomer-pure BNO induces the formation of chiral one-dimensional chains that are packed parallel to each other in the noncentrosymmetric P1 space group. 1 exhibits four magnetic ground states: paramagnetic; antiferromagnetic; forced ferrimagnetic; fieldinduced metastable ferrimagnetic. In the paramagnetic state (T > 20 K), it presents short-range antiferromagnetic interaction between Co ion and nitroxide radical and has a minimum of mT value at 220 K. The Weiss temperature estimated in the temperature range 220-300 K is found to be-89.9 K. At 20 K (TN), an antiferromagnetic long-range ordering is established. In the temperature range 4 K < T < 20 K, the isothermal magnetization curve show a spin-flip transition to the forced ferrimagnetic state at around 850 Oe. Below 4 K, this compound enters into a field-induced ferrimagnetic state, which is metastable and stabilized by the Ising character of the Co ion. In the low-temperature phase, the material becomes a very hard magnet with wide hysteresis loop whose coercive field reaches 25 kOe at 2 K. The magnetic phase diagram based on these magnetic data is presented.
Polyhedron, 2007
The mechanism of the magnetic interaction in the a phase of the organic radical p-cyano-tetrafluorophenyl-dithiadiazolyl has been studied using a first-principles bottom-up theoretical procedure. Six J AB radical-radical magnetic interactions are computed to be larger than j0.05j cm À1 (two, with values +10.91 and À10.25 cm À1 , dominate over the others, whose absolute values are always smaller than 1.5 cm À1 ). The connectivity of these non-negligible J AB interactions creates a complex 3D magnetic topology within the crystal. The computed magnetic susceptibility curve, v, was also calculated by diagonalizing the matrix representation of the Heisenberg Hamiltonian, whose J AB parameters are set to their computed values. This fully reproduces the shape of the experimental curve and is consistent with the bulk antiferromagnetism reported experimentally (reflected in a sharp maximum observed in v at 10 K). Attempts to model the magnetic susceptibility data using a simple model based only upon the two dominant interactions proved impossible.
Magnetocaloric effect in molecular magnet
Journal of Magnetism and Magnetic Materials, 2014
Magnetocaloric effect in {[Fe(pyrazole) 4 ] 2 [Nb(CN) 8 ]·4H 2 O} n molecular magnet is reported. It crystallizes in tetragonal I4 1 /a space group. The compound exhibits a phase transition to a long range magnetically ordered state at T c ≈8.3 K. The magnetic entropy change ∆S M as well as the adiabatic temperature change ∆T ad due to applied field change µ 0 ∆H=0.1, 0.2, 0.5, 1, 2, 5, 9 T as a function of temperature have been determined by the relaxation calorimetry measurements. The maximum value of ∆S M for µ 0 ∆H = 5 T is 4.9 J mol −1 K −1 (4.8 J kg −1 K −1 ) at 10.3 K. The corresponding maximum value of ∆T ad is 2.0 K at 8.9 K. The temperature dependence of the exponent n characterizing the field dependence of ∆S M has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class.
Studies of a linear single-molecule magnet
Dalton Transactions, 2007
Reaction of the dinuclear complex [Mn 2 O 2 (bpy) 4 ](ClO 4 ) 3 with H 3 cht (cis,cis-1,3,5-cyclohexanetriol) in MeCN produces the complex [Mn 3 (Hcht) 2 (bpy) 4 ](ClO 4 ) 3 ·Et 2 O·2MeCN (1·Et 2 O·2MeCN). Dc magnetic susceptibility measurements reveal the existence of weak ferromagnetic exchange between the three Mn ions, leading to a spin ground state of S = 7, with D = −0.23 cm −1 . W-Band (94 GHz) EPR measurements on restrained powdered crystalline samples confirm the S = 7 ground state and determine the ground state zero-field splitting (ZFS) parameters of D = −0.14 cm −1 and B 4 0 = +1.5 × 10 −5 cm −1 . The apparent 4th order behaviour is due to a breakdown of the strong exchange limit approximation (J ≈ d, the single-ion ZFS). Single crystal dc relaxation decay and hysteresis loop measurements reveal the molecule to have an appreciable energy barrier to magnetization relaxation, displaying low temperature sweep rate and temperature-dependent hysteresis loops. Density functional studies confirm the ferromagnetic exchange coupling between the Mn ions. † Electronic supplementary information (ESI) available: In-phase ac susceptibility plot and W-band EPR spectra for complex 1. See
Synthetic Metals, 1996
Our interest is the study of magnetic properties on organic compounds. We have previously reported a new class of molecular magnets based on aminonaphthalenesulfonic acid and aniline. In this work we explore theoretically a simple model based on superposed aniline molecules and the goal is to determine the mechanism that stabilizes the parallel alignment of the spin component. Calculations were performed within the local density approximation (LDA). The results obtained show a high sensitivity of the magnetic coupling relative to translations and rotations of one aniline molecule with respect to the other. Distances between both NH units have also a great importance in the spin stabilization. These results are in good accordance with some synthesis difficulties.