Generalized theoretical approach to quasi-one-dimensional molecular magnets (original) (raw)
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Modern studies in the area of molecular magnets: State, problems, and prospects
Russian Journal of Coordination Chemistry, 2015
The review considers the theoretical foundations and some recent results of investigations of the specific class of magnetically active materials: molecular magnets (single molecule magnets) and molecular magnets with the linear chain structure (single chain magnets). Possibilities and promising approaches to controlling their magnetic properties are analyzed.
Some new trends in the design of single molecule magnets
Pure and Applied Chemistry, 2017
In this review we briefly discuss some new trends in the design of single molecule magnets based on transition (3d, 4d, 5d) and rare-earth (4f) metal ions. Within this broad theme the emphasis of the present review is placed on the molecules which exhibit strong magnetic anisotropy originating from the unquenched orbital angular momenta in the ground orbitally degenerate (or quasi-degenerate) states. Along with the general concepts we consider selected examples of the systems comprising orbitally-degenerate metal ions and demonstrate how one can benefit from strong single-ion anisotropy arising from the first-order orbital angular momentum. The role of crystal fields, spin-orbit coupling and structural factors is discussed. Some observation stemming from the analysis of the isotropic exchange interactions, magnetic anisotropy and strongly anisotropic orbitally-dependent superexchange are summarized as guiding rules for the controlled design of single molecule magnets exhibiting high...
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
Comparative study of magnetic behaviour in three classic molecular magnets
Solid State Communications, 2012
We have studied the magnetic properties of three phenalenyl based organic neutral radicals. The first one is a Heisenberg chain antiferromagnet with one unpaired spin per molecule; second one is a diamagnetic, exhibiting a diamagnetic to paramagnetic phase transition at high temperature; the third one comprises of free neutral radicals and shows paramagnetic behaviour. Temperature dependent magnetic susceptibility measurements and isothermal magnetization measurements (as a function of magnetic field) were performed on all the three systems. In the case of the antiferromagnetic system, temperature dependent susceptibility and magnetization isotherms were fitted to the Bonner Fisher model. In the case of second system the diamagnetic to paramagnetic phase transition is investigated by performing isothermal magnetization measurements in the two different phases. The diamagnetic to paramagnetic phase transition seems to be of first order in nature.
Current Opinion in Solid State and Materials Science, 2001
This paper presents recent developments in the fields of molecular-based magnets and of single-molecule magnets. It concerns compounds made with open shell molecules which may be organic, inorganic or both.
Theoretical Calculations of Structure and Exchange Coupling of a Room-Temperature Molecular Magnet
2010
We report here quantum chemical calculations advancing structural hypotheses and corresponding methodological concerns devoted to the elucidation of magneto-structural features of the V[TCNE]2 molecular magnet (TCNE = tetracyanoethylene). V[TCNE]xy(CH2Cl2) is the first room-temperature molecular magnet discovered and the only one with an active organic component. Despite previous detailed magnetic, spectroscopic and conductivity studies, the mechanism for the strong exchange coupling in the family M[TCNE]xy(solvent) has remained an open question, the difficulty being related to the absence of structural data for these amorphous compounds. Starting from a structural model that we propose, we report the results of Density Functional Theory (DFT) calculations providing the optimized geometry of a periodic lattice. The DFT methods retrieve the long range magnetic ordering of the system, but overestimate the absolute values of exchange constants. We analyze the DFT results assessing the intrinsic limitations in the systematic account of the Broken Symmetry regime and long range effects. In this respect, Gaussian-type and Plane Wave calculations were corroborated, finding comparable results. We suggest and preliminarily checked improved treatments realized by multi-configuration ab initio methods, providing better estimates of the exchange constants.
Local Spin Anisotropy Effects upon the Magnetization of Dimer Single Molecule Magnets
AIP Conference Proceedings, 2006
We present an exactly solvable model of equal spin s1 dimer single molecule magnets. The spins within each dimer interact via the Heisenberg and the most general quadratic global and local (single-ion) anisotropic spin interactions, and with the magnetic induction B. For antiferromagnetic couplings and s1 > 1/2, the low temperature T magnetization M (B) exhibits 2s1 steps of universal height and midpoint slope, the sth step of which occurs at the non-universal level-crossing magnetic induction B lc s,s 1 (θ, φ), where θ, φ define the direction of B. The specific heat CV exhibits zeroes as T → 0 at these B lc s,s 1 (θ, φ) values, which are equally surrounded by universal peak pairs as T → 0. The non-universal B lc s,s 1 (θ, φ) values lead to a rich variety of magnetization plateau behavior, the structure and anisotropy of which depend upon the various global and local anisotropic spin interaction energies. We solve the model exactly for s1 = 1/2, 1, and 5/2, and present M (B) and CV (B) curves at low T for these cases. For weakly anisotropic dimers, rather simple analytic formulas for M (B) and CV (B) at arbitrary s1 accurately fit the exact solutions at sufficiently low T or large B. An expression for B lc s,s 1 (θ, φ) accurate to second order in the four independent anisotropy energies is derived. Our results are discussed with regard to existing experiments on s1 = 5/2 Fe2 dimers, suggesting further experiments on single crystals of these and some s1 = 9/2 [Mn4]2 dimers are warranted.
A Diferrous Single-Molecule Magnet
European Journal of Inorganic Chemistry, 2007
The diferrous complex [Fe 2 (acpypentO)(NCO) 3 ] (1), (acpypentO -= the anion of 1,5-bis[2-pyridyl(1-ethylimino)]pentane-3-ol) was studied by spectroscopic (dual-mode Xband EPR and 57 Fe-Mössbauer) and magnetic (AC magnetic susceptibility) techniques. Complex 1 (A. K. Boudalis et al., Inorg. Chem. 2004, 43, 1574 was previously shown to exhibit an intramolecular ferromagnetic coupling with important single-ion anisotropies with marked differences in the singleion electronic parameters of the two iron sites. In this contribution, additional studies are carried out to probe its dynamic magnetic properties. Mössbauer spectra recorded at liquid helium temperatures indicate slow paramagnetic relaxation and a ground state of the system characterized by a non-Kramers doublet with an Ising-type anisotropy, a condition which favours single-molecule magnet (SMM) behaviour. A www.eurjic.org
Beilstein journal of nanotechnology, 2014
Single-crystal angular-resolved magnetometry and wavefunction-based calculations have been used to reconsider the magnetic properties of a recently reported Dy(III)-based single-molecule magnet, namely [Dy(hfac)3(L(1))] with hfac(-) = 1,1,1,5,5,5-hexafluoroacetylacetonate and L(1) = 2-(4,5-bis(propylthio)-1,3-dithiol-2-ylidene)-6-(pyridin-2-yl)-5H-[1,3]dithiolo[4',5':4,5]benzo[1,2-d]imidazole. The magnetic susceptibility and magnetization at low temperature are found to be strongly influenced by supramolecular interactions. Moreover, taking into account the hydrogen-bond networks in the calculations allows to explain the orientation of the magnetic axes. This strongly suggests that hydrogen bonds play an important role in the modulation of the electrostatic environment around the Dy(III) center that governs the nature of its magnetic ground-state and the orientation of its anisotropy axes. We thus show here that SMM properties that rely on supramolecular organization may not...
2009
The conventional magnetic materials used in current technology, such as, Fe, Fe 2 O 3 , Cr 2 O 3 , SmCo 5 , Nd 2 Fe 14 B etc are all atom-based, and their preparation/processing require high temperature routes. Employing self-assembly methods, it is possible to engineer a bulk molecular material with long-range magnetic order, mainly because one can play with the weak intermolecular interactions. Since the first successful synthesis of molecular magnets in 1986, a large variety of them have been synthesized, which can be categorized on the basis of the chemical nature of the magnetic units involved: organic-, metal-based systems, heterobimetallic assemblies, or mixed organic-inorganic systems. The design of molecule-based magnets has also been extended to the design of poly-functional molecular magnets, such as those exhibiting second-order optical nonlinearity, liquid crystallinity, or chirality simultaneously with long-range magnetic order. Solubility, low density and biocompatibility are attractive features of molecular magnets. Being weakly coloured, unlike their opaque classical magnet 'cousins' listed above, possibilities of photomagnetic switching exist. Persistent efforts also continue to design the ever-elusive polymer magnets towards applications in industry. While providing a brief overview of the field of molecular magnetism, this article highlights some recent developments in it, with emphasis on a few studies from the author's own lab.