Molecule-based Magnets Research Articles

New advances in organic spintronics

The basic components of spintronic devices are spin polarized ferromagnets and spin transporting non-magnetic spacers. Exploiting carbon-based materials for these components promises to extend functionality of information storage and processing as well as to improve device integration and fabrication. Here we present the magnetoresistance of organic semiconductor rubrene (C42H28) used as a spacer in La2/3Sr1/3MnO3 (LSMO)/organic semiconductor (OSC)/Fe heterojunctions. Efficient spin polarized tunneling through the thin layer of rubrene spacer (5 nm) was observed. As the thickness of rubrene layers is increased, device current is strongly limited by carrier injection resulting in strong temperature dependent device resistance. The carrier injection is described with thermionic field emission at the metal/OSC interface. As a next step toward organic spintronics we used an organic based magnet vanadium-tetracyanoethylene (V(TCNE)x, x∼2) in tandem with LSMO in a spin- valve with 5 nm rubrene spacer. V(TCNE)x is the earliest developed room temperature molecule-based magnet (Tc ∼ 400 K). Due to strong on-site Coulomb interaction and weak intermolecule overlapping their magnetic state can be described with a model of half-semiconductor in which valence and conduction bands are spin polarized. The magnetoresistance data for bulk V(TCNE)x is in agreement with the model of spin polarized valence and conduction bands. We demonstrated that an organic-based magnetic semiconductor V(TCNE)x functions very well as an electron spin polarizer in the standard spintronic device geometry.

A ferromagnetically coupled copper(II) trinuclear secondary building unit as precursor for the preparation of molecule-based magnets

The compounds 3D-{(Ph 4P) 2[ZnCu 3(Hmesox) 3Cl]·2.5H 2O} ( 1) and 3D-{(Ph 4P) 2[NiCu 3(Hmesox) 3Cl]·2.5H 2O} ( 2) have been prepared and their structure and magnetic properties investigated (H 4mesox = mesoxalic acid, 2-dihydroxymalonic acid). The compounds are obtained by means of the same procedure followed to prepare 3D-{(Ph 4P) 2[MnCu 3(Hmesox) 3Cl]·3.5H 2O}and 3D-{(Ph 4P) 2[CoCu 3(Hmesox) 3Cl]}, to which 1 and 2 are isostructural as deducted from the X-ray powder diffraction patterns and IR spectra. During the synthesis the {Cu 3(Hmesox) 3Cl} 4− unit is generated which acts as ferromagnetically coupled secondary building unit (SBU) to give 3D chiral networks with (10,3)-a topology, the [Zn{Cu 3(C 3HO 6) 3Cl}] 2 n − and [Ni{Cu 3(C 3HO 6) 3Cl}] 2 n −, in 1 and 2, respectively. The tetraphenylphosphonium cations are located in the voids of the 3D anion framework giving a supramolecular 3D cation net with the same (10,3)-a topology as the anion framework and both can be thought of forming interpenetrating supramolecular and covalent (10,3)-a nets. Because of the different nature of the interpenetrated 3D (10,3)-a frameworks a single crystal of 1 and 2 is chiral and enantiopure. The analysis of the magnetic properties in 1 by means of the isotropic spin Hamiltonian, H = − J( S 1 S 2 + S 2 S 3 + S 1 S 3), reveal a ferromagnetic coupling with J = +11.2 cm −1 in the copper(II) trinuclear unit of 1. Compound 2 exhibits long-range magnetic ordering with a T c of 7.0 K due to a ferrimagnetic coupling between the ferromagnetically coupled copper(II) ions of the trinuclear unit, antiferromagnetically coupled to the Ni(II) ions.

Molecule-based magnets

The employment of molecules, not atoms, as a basic building block to construct solids has lead to the development of new classes of materials exhibiting commercially useful properties.l,2 These include electrical conductivity, ferroelectricity as well as magnetic ordering. Molecules, in contrast to atoms, enables the modulation of the commercially useful properties by low-temperature organic-synthesis methodologies, that can lead to the improvement of the properties, and lead to the development of materials with a combination of properties that will expand their desirability. Herein, we focus solely upon work related to molecule-based magnets. Molecule-based magnets are defined as substances prepared from molecules (or molecular ions) that maintain aspects of the parent molecular framework, and magnetically order.

Calculation of microscopic exchange interactions and modelling of macroscopic magnetic properties in molecule-based magnets

The state-of-the-art theoretical evaluation and rationalization of the magnetic interactions (J(AB)) in molecule-based magnets is discussed in this critical review, focusing first on isolated radical···radical pair interactions and afterwards on how these interactions cooperate in the solid phase. Concerning isolated radical pairwise magnetic interactions, an initial analysis is done on qualitative grounds, concentrating also on the validity of the most commonly used models to predict their size and angularity (namely, McConnell-I and McConnell-II models, overlap of magnetic orbitals,…). The failure of these models, caused by their oversimplified description of the magnetic interactions, prompted the introduction of quantitative approaches, whose basic principles and relative quality are also evaluated. Concerning the computation of magnetic interactions in solids, we resort to a sum of pairwise magnetic interactions within the Heisenberg Hamiltonian framework, and follow the First-principles Bottom-Up procedure, which allows the accurate study of the magnetic properties of any molecule-based magnet in an unbiased way. The basic principles of this approach are outlined, applied in detail to a model system, and finally demonstrated to properly describe the magnetic properties of molecule-based systems that show a variety of magnetic topologies, which range from 1D to 3D (152 references).

Framework-structured weak ferromagnets

Framework-structured weak ferromagnets are new rising stars in molecule-based magnetic materials. The framework structures are powerful carriers for long-range ordering of spins. And weak ferromagnetism due to spin canting is an effective approach for magnets because of its frequent occurrence and desired spontaneous magnetization as long as the canting angle γ is large enough. In this critical review, we provide an overview of the various framework-structured weak ferromagnets based on different grades of ligands (from mono-atom to three-atom-like ligands). Particular emphasis is given to the relationships between structural features and the properties, rational employment of the ligands, and weak ferromagnetic strategies for molecule-based magnets with exciting properties and applications (273 references).

Magnetically ordered molecule-based materials

Magnets composed of molecular components that provide both electron spins and spin-coupling pathways can stabilize bulk magnetic ordering. This was first reported for the ionic, zero-dimensional (0-D) electron transfer salt [Fe(C(5)Me(5))(2)](+)[TCNE]˙(-) (TCNE = tetracyanoethylene), which orders as a ferromagnet at T(c) = 4.8 K. Later V[TCNE](x) (x ∼ 2) was characterized to order above room temperature at 400 K (127 °C). Subsequently, numerous examples of organic- and molecule-based magnets have been characterized. In this critical review, after a discussion of the important aspects of magnetism pertaining to molecule-based magnets, including the determination of the magnetic ordering temperature (T(c)) these magnetically ordered materials are reviewed from a perspective of the structural dimensionality (208 references).

Thin films of coordination polymer magnets

Many applications of molecule based magnets, whether they are in information storage, displays, or as components in electronic or spintronic devices, will require putting the active materials on a surface or interfacing them with other components. Although there are many examples of molecule-based magnets, the families of tetracyanoethylene (TCNE) based magnets and Prussian-blue analogs possess materials properties that are close to those required for practical applications, and are the most advanced with respect to studies as thin films. This critical review focuses on fabrication and characterization of thin films of TCNE and Prussian-blue analog coordination polymer magnets. Emphasis is on current developments in thin film heterostructures and potential spintronics applications (135 references).

Magnetic anisotropies in paramagnetic polynuclear metal complexes

The study of the magnetic properties of highly anisotropic paramagnetic molecules is an area of intense current research interest. Of these, single-molecule magnets (SMMs) and single-chain magnets (SCMs) showing non-equilibrium magnetization have remained a key topic over the past two decades. The slow magnetization reversals found in SMMs and SCMs are contingent on two requirements: a large ground-state spin forbidding direct quantum transitions of spin reversal, and a series of excited spin levels, due to the anisotropy of the system, which can act as steppingstones for the thermal relaxation of the spin orientations (the Orbach process). In this critical review, the latter requirement, i.e. the existence of magnetic anisotropies in paramagnetic species, is reviewed with the aim of providing clues towards the rational design of molecule-based magnets (100 references).

Design of 1D and 2D molecule-based magnets with the ligand 4,5-dimethyl-1,2-phenylenebis(oxamato)

Three new bimetallic oxamato-based magnets with the proligand 4,5-dimethyl-1,2-phenylenebis(oxamato) (dmopba) were synthesized using water or dimethylsulfoxide (DMSO) as solvents. Single crystal X-ray diffraction provided structures for two of them: [MnCu(dmopba)(H(2)O)(3)](n)·4nH(2)O (1) and [MnCu(dmopba)(DMSO)(3)](n)·nDMSO (2). The crystalline structures for both 1 and 2 consist of linearly ordered oxamato-bridged Mn(II)Cu(II) bimetallic chains. The magnetic characterization revealed a typical behaviour of ferrimagnetic chains for 1 and 2. Least-squares fits of the experimental magnetic data performed in the 300-20 K temperature range led to J(MnCu) = -27.9 cm(-1), g(Cu) = 2.09 and g(Mn) = 1.98 for 1 and J(MnCu) = -30.5 cm(-1), g(Cu) = 2.09 and g(Mn) = 2.02 for 2 (H = -J(MnCu)∑S(Mn),(i)(S(Cu,i) + S(Cu),(i-1))). The two-dimensional ferrimagnetic system [Me(4)N](2n){Co(2)[Cu(dmopba)](3)}(n)·4nDMSO·nH(2)O (3) was prepared by reaction of Co(II) ions and an excess of [Cu(dmopba)](2-) in DMSO. The study of the temperature dependence of the magnetic susceptibility as well as the temperature and field dependences of the magnetization revealed a cluster glass-like behaviour for 3.

Multifunctional Magnetic Molecular {[MnII(urea)2(H2O)]2[NbIV(CN)8]}n System: Magnetization-Induced SHG in the Chiral Polymorph

Incorporation of urea molecules into the magnetic system MnII-[NbIV(CN)8] results in formation of two magnetic polymorphs {[MnII(H2O)(urea)2]2[NbIV(CN)8]}n chiral α and centric β crystallizing in space groups P41 and P1̅, respectively. Both polymorphs exhibit extremely similar structures and are soft ferrimagnets with critical temperatures of 43 and 42 K, respectively. The presence of coordinated urea in the noncentrosymmetric α produces NLO functionality, second harmonic generation (SHG), whereas centric β is just another molecular magnet. As a result of interaction of both functionalities below magnetic ordering temperature, the magnetization-induced SHG (MSHG) is observed for polymorph α. The observed MSHG/SHG signal ratio was the largest among molecule-based noncentrosymmetric magnets. The presence of urea molecules in both polymorphs gives also rise to their interesting topotactic reactivity. Both polymorphs can be easily transformed into a new entity {[MnII(H2O)]2[NbIV(CN)8]·2.5MeOH}n 2b with significant shift of critical temperature Tc to 70 K. Subsequent hydration of 2b leads to {[MnII(H2O)2]2[NbIV(CN)8]·4H2O}n 3 with Tc of 47 K.

Origin of the Magnetic Bistability in Molecule-Based Magnets: A First-Principles Bottom-Up Study of the TTTA Crystal

The magnetic bistability present in some molecule-based magnets is investigated theoretically at the microscopic level using the purely organic system TTTA (1,3,5-trithia-2,4,6-triazapentalenyl). The TTTA crystal is selected for being one of the best-studied molecule-based systems presenting magnetic bistability. The magnetic properties of the high- and low-temperature structures (HT and LT phases, respectively) are accurately characterized by performing a First-Principles Bottom-Up study of each phase. The changes that the magnetic exchange coupling constants (J(AB)) undergo when the temperature is raised (LT → HT) or lowered (HT → LT) are also fully explored in order to unravel the reasons behind the presence of these two different pathways. The triclinic LT phase is diamagnetic due to the fact that the nearly eclipsed π dimer is effectively magnetically silent and not to formation of a covalent bond between two TTTA molecules. It is also shown that bistability in TTTA results from the coexistence of the monoclinic HT and triclinic LT phases in the temperature range studied.

ChemInform Abstract: Coordination Versatility of Pyrazole‐Based Ligands Towards High‐Nuclearity Transition‐Metal and Rare‐Earth Clusters

AbstractReview: 205 refs.

Octacyanotungstate (V)-based square [W2Mn2] complexes: Effect of intermolecular interaction on molecule-based magnetism

In this research, the synthesis, structure and magnetic properties of an ion-pair complex {[MnII(hmp)2]2(Cl)2}{[MnII(hmp)2WV(CN)8]2} (1) (hmpH = 2-hydroxymethylpyridine) are reported. Complex 1 was prepared by [W (CN)8]3− reacting with Mn(II) salt and hmpH in MeCN solution. Single-crystal X-ray diffraction shows that 1 is crystallized in triclinic system with a space group of P-1 and its structure consists of four parts of two chlorine-bridged [Mn2] cations and two tetrameric [Mn2W2] anionic clusters. The cationic [Mn2] and anionic [Mn2W2] moieties each are connected by π–π stacking to form their own layer. The packing of cations and anions layer by layer via molecular electrostatic potential, Van der Waals force and hydrogen bonding constructs a three-dimensional supramolecular framework. The magnetic analyses show the antiferromagnetic coupling interaction between MnII and WV or MnII and MnII in both parts. No evidence of single-molecule magnet or long-range ordering is observed in the data. By comparing to an analog with spin glass properties, we draw a conclusion that the differences of crystal packing mode, especially the π–π stacking within each layer, could be a key factor for the distinct magnetic properties.

New tricyanoiron(III) building blocks for the construction of molecule-based magnets

A series of tricyanoiron(III) complexes with the general formula mer-[FeIII(5-Xsap)(CN)3]2− (X = H, Me, MeO, Cl or Br, sapH2 = N-salicylidene-o-aminophenol) have been synthesized. These complexes were characterized by IR, ESI-MS, UV/Vis, elemental analysis and magnetic measurements. The structures of (PPh4)2[FeIII(sap)(CN)3] and (PPh4)2[FeIII(5-Mesap)(CN)3] have been determined by X-ray crystallography. These low-spin d5 tricyanoiron(III) complexes are potential building blocks for the construction of molecule-based magnets.

Critical behavior of two molecular magnets probed by complementary experiments

The fast developing field of molecule-based magnets involving organic and coordination chemistry provides the physicist with a multitude of novel compounds of unprecedented structure. The magnetic structure of ${\text{Cu}}_{4}(\text{tetren}){[\text{W}{(\text{CN})}_{8}]}_{4}$ $(\mathbf{1})$ was shown to consist of weakly coupled double layers. By contrast, in the structurally similar compound ${\text{Cu}}_{2+x}{\text{Cu}}_{4}{[\text{W}{(\text{CN})}_{8}]}_{4}$ $(\mathbf{2})$ the free spaces between the double layers are filled with paramagnetic copper(II) ions leading to a unique magnetic network. Both compounds exhibit the transition to a magnetically ordered phase at ${T}_{c}\ensuremath{\approx}33\text{ }\text{K}$ and ${T}_{c}\ensuremath{\approx}40\text{ }\text{K}$, respectively. The critical behavior of $\mathbf{1}$ and $\mathbf{2}$ is investigated using complementary methods: ac magnetometry, relaxation calorimetry, and muon spin-rotation spectroscopy. Apart from $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$, critical exponents $\ensuremath{\kappa}$ and ${\ensuremath{\kappa}}^{\ensuremath{'}}$ describing the combined scaling of excess entropy and order parameter are determined for both compounds. This type of scaling is verified for $\mathbf{1}$, the system revealing the signatures of the Berezinskii-Kosterlitz-Thouless transition. For $\mathbf{2}$ their values imply that the system is close to the universality class of the three-dimensional Heisenberg model. The relatively small value of exponent $\ensuremath{\gamma}=1.05$ for $\mathbf{2}$ indicates the presence of noncollinearity in the spin arrangement. Exponents $\ensuremath{\kappa}$ and ${\ensuremath{\kappa}}^{\ensuremath{'}}$ for $\mathbf{2}$ are also found consistent with noncollinear models. The shift of the heat-capacity anomaly toward higher temperatures with increasing applied field indicates the presence of ferromagnetic interactions in $\mathbf{2}$.

Optical control of magnetization in a room-temperature magnet: V-Cr Prussian blue analog

We report reversible photoinduced magnetic phenomena for a V-Cr Prussian blue analog $({T}_{c}\ensuremath{\sim}350\text{ }\text{K})$. This molecule-based magnet exhibits a decrease in magnetization upon illumination with UV light $(\ensuremath{\lambda}\ensuremath{\sim}350\text{ }\text{nm})$ and reaches a metastable state that has a long lifetime at low temperatures ($>{10}^{6}\text{ }\text{s}$ at 10 K). This photoexcited magnetic state totally recovers back to the ground state by warming above 250 K, and partially recovers with green light $(\ensuremath{\lambda}\ensuremath{\sim}514\text{ }\text{nm})$ illumination. The effect of green light is triggered only when the sample is previously UV irradiated, suggesting a hidden metastable magnetic state. The photoinduced magnetic effects are proposed to originate from structural distortion that alters the magnetic exchange coupling and/or anisotropy.

Crystal Structure and Magnetic Properties of Organic Radical Cation Salt, α-(Benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole)2ReBr6

Abstract A new molecule-based magnet, α-(benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole (BBDTA))2ReBr6, comprised a one-dimensional network composed of an alternative alignment of BBDTA+ (S = 1/2) and ReBr62− (S = 3/2) and an S = 1/2 regular chain network of the other BBDTA+. This material became a ferromagnet below 12.2 K. The magnetization curve showed complex field dependence, which might be related to the orientation of the one-dimensional alternative network in its crystal.

ChemInform Abstract: Spin Polarization in 1,3,5‐Trihydroxybenzene‐Bridged First‐Row Transition Metal Complexes

AbstractReview: 125 refs.

ChemInform Abstract: First Isolation and Characterization of an Electron Transfer Salt of a Tetracyanoquinodimethane with Porphyrinatomanganese(II) Having Novel cis-μ-Coordination Manner: A Molecule-Based Magnet with a 2.3 K Tc.

An electron transfer salt comprised of 2,5-dimethyl-7,7,8,8-tetracyano-p-quinodimethane (DMTCNQ) with porphyrinatomanganese(II) was structurally and magnetically characterized. The [DMTCNQ]−• exhibits a novel cis-μ-coordination motif.

Pressure-induced phase transition in a molecule-based magnet with interpenetrating sublattices

The molecule-based magnet ${[{\text{Ru}}_{2}{({\text{O}}_{2}\text{CMe})}_{4}]}_{3}[\text{Cr}{(\text{CN})}_{6}]$ contains two interpenetrating sublattices with sublattice moments confined to the cubic diagonals. At ambient pressure, a field of about 850 Oe rotates the antiferromagnetically coupled sublattice moments toward the field direction, producing a wasp-waisted magnetization curve. Up to 7 kbar, the sublattice moments increase with pressure due to the enhanced exchange coupling between the Cr(III) and $\text{Ru}{(\text{II}/\text{III})}_{2}$ spins on each sublattice. Above 7 kbar, the sublattice moment drops by about half and the parallel linear susceptibility of each sublattice rises dramatically. The phase transition at 7 kbar is most likely caused by a high-to-low-spin transition on each ${\text{Ru}}_{2}$ complex.