Molecular-based Magnets Research Articles

Mixed Spin Ising Ferrimagnetic System in the Oguchi Approximation

It has been investigated an Ising ferrimagnet with mixed spin-2 and spin-5/2 and various crystal field parameters on a body-centered cubic lattice. In this research paper, ferrimagnetic compensation temperatures and phase transitions to clarify the characteristic features, in a series of molecular-based magnets AFeIIFeIII(C2O4)3[A=N(n-CnH2n+1)4, n=3-5] are examined in the Oguchi approximation (OA). The spin crystal domain dependence of compensation behavior acting on ions contain the Oguchi lattice, is mainly studied. It is discovered, specifically, that magnetic anisotropies in the disordered for the two sublattices of the model depend on the negative values of the crystal fields to produce a characteristic ferrimagnetic behavior. For DA/|J|=-1.5 and DB/|J|=-5.5, there are two compensation points are induced at an ordered phase(2,-1.5).

Chemical Tuning Meets 2D Molecular Magnets (Adv. Mater. 5/2023)

Molecular Magnets In article number 2208919, Yulong Huang, Shenqiang Ren, and co-workers report in situ chemical tuning to realize the controllable transformation from low-temperature magnetic order to room-temperature hard magnetism in a layered molecular-based magnet. A coercivity of 8500 Oe and energy product of 0.6 MG Oe are demonstrated, providing a universal approach to control organic magnetism.

Polarized Neutron Diffraction: An Excellent Tool to Evidence the Magnetic Anisotropy—Structural Relationships in Molecules

This publication reviews recent advances in polarized neutron diffraction (PND) studies of magnetic anisotropy in coordination compounds comprising d or f elements and having different nuclearities. All these studies illustrate the extent to which PND can provide precise and direct information on the relationship between molecular structure and the shape and axes of magnetic anisotropy of the individual metal sites. It makes this experimental technique (PND) an excellent tool to help in the design of molecular-based magnets and especially single-molecule magnets for which strong uniaxial magnetic anisotropy is required.

Spin compensation, critical and discontinuous transitions behaviors in a ferrimagnet of spins 5/2 and 7/2

We apply Monte Carlo simulation techniques to study the magnetic behavior of a mixed ferrimagnetic Ising model on sites of a square lattice of LxL size, with L = 120, where 5/2 spins are alternated with 7/2 spins. We consider an antiferromagnetic interaction next-neighbor, ferromagnetic interactions next-nearest-neighbor, and an external magnetic field in the Hamiltonian that defines the model. We characterize the effects of an external magnetic field on the behavior of the system through the analysis of spin compensation points, discontinuous transitions, and hysteresis loops at various temperatures in the total magnetization and magnetic susceptibility. At the end, we study the phase diagrams of the critical, compensation and discontinuous transition temperatures. The physical phenomena experienced by the system under study are relevant to the application in magneto-optical recordings, in the synthesis and construction of molecular-based magnets and have technological applications in the manipulation and storage of magnetic information. We found some phenomena not reported in previous works on the same system.

One-Dimensional Bimetallic Lantern Complex

As the scale of microelectronic circuit devices approaches the atomic limit, the study of molecular-based wires and magnets has become more prevalent. Lantern (or paddle-wheel) complexes with conventional {O(S),O} donor building block came into being, dozens of new lantern complexes have been prepared at present. The use of organic monomers to connect such complex building block to form metal coordination polymers with rigid open framework and tunable properties has become a research hotspot. Such polymers can be assembled in a one-dimensional array to produce unique electronic and magnetic properties for use in vapor color sensors, luminescent materials, magnetic resonance imaging, etc., to create diversified and multidisciplinary research fields.

Extremely well isolated two-dimensional spin- 12 antiferromagnetic Heisenberg layers with a small exchange coupling in the molecular-based magnet CuPOF

We report on a comprehensive characterization of the newly synthesized Cu$^{2+}$-based molecular magnet [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ (CuPOF), where pz = C$_4$H$_4$N$_2$ and 2-HOpy = C$_5$H$_4$NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, $\mu^+$SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the 2D square-lattice $S=1/2$ antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of $J/k_\mathrm{B} = 6.80(5)$ K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to $XY$ type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature $XY$ anisotropy under the influence of the interlayer coupling, occurs at $T_\mathrm{N} = 1.38(2)$ K, as revealed by $\mu^+$SR. In applied magnetic fields, our $^1$H-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at $T_\mathrm{N} =2.8$ K and 7 T.

Ground-State Phase Diagram of a Mixed Spin-3/2 and Spin-7/2 Ising Model: A Study of Complex [Cr(CN)4(μ-CN)2Gd(H2O)4-(bpy)]n.4nH2O.1.5nbpy

In this work, the phase transitions, for a molecular-based magnet, are examined using mean field method (MFM). The longitudinal magnetizations can be achieved by minimizing the free energy is an interesting function for locating and induction of spin compensation temperatures as well. By applying Blume-Capel (BC) configuration to a system consisting of two sublattices for a mixed spin-3/2 and spin-7/2on a square lattice in the anisotropies plane, the ground-state phase diagram of this system has been built. On principles of Bogoliubov inequality (BI) of the free energy, it has been investigated the spin compensation phenomena. In particular, characteristic ferrimagnetic features have been shown depending on an unequal crystal field parameters (DA, DB<-1.0) applied on the A−sites and B−sites, respectively.

Molecular magnetism: from chemical design to spin control in molecules, materials and devices

The field of molecular magnetism is rapidly evolving towards the use of magnetic molecules and molecule-based magnetic materials in physics-driven and nanotechnology-driven fields, in particular molecular spintronics, quantum technologies, metal–organic frameworks (MOFs) and 2D materials. In molecular spintronics, the goal is the development of a new generation of spintronic devices based on molecular materials or, in the longer term, on one or a few molecules. In the area of quantum technologies, the milestones reached in the design of molecular spin qubits with long quantum coherence times and in the implementation of quantum operations have raised expectations for the use of molecular spin qubits in quantum computation. MOFs and 2D materials are two classes of materials for which magnetism has been, until very recently, an elusive property; molecular materials with attractive properties and functionalities are now starting to be developed in both areas. In MOFs, single-molecule magnets and spin crossover complexes can be integrated into the nodes of the framework, within the pores or both, sometimes giving rise to smart magnetic materials or to hybrid materials exhibiting synergistic combinations of properties. 2D molecular-based magnets can provide a platform to study magnetism in the 2D limit and exhibit superior properties compared with their inorganic analogues in terms of chemical stability and tunability. This Review discusses the expansion of the field of molecular magnetism from the chemical design and physical study of single-molecule magnets and multifunctional magnetic materials towards physics- and nanotechnology-driven areas, in particular molecular spintronics, quantum technologies, metal–organic frameworks and 2D materials.

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A\u2009=\u2009In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn2Mo3O8 have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li2AMo3O8 (A = In or Sc) is such a compound where spin-1/2 Mo3O13 clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the A site. Li2InMo3O8 undergoes conventional 120° long-range magnetic order below TN = 12 K whereas isomorphic Li2ScMo3O8 exhibits no long-range magnetic order down to 0.5 K. Here, we report exotic magnetisms in Li2InMo3O8 and Li2ScMo3O8 investigated by muon spin rotation (μSR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li2InMo3O8 and Li2ScMo3O8 show completely different behaviors observed in both μSR and INS measurements, representing their different ground states. Li2InMo3O8 exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the 120° spin structure. In contrast, Li2ScMo3O8 undergoes short-range magnetic order below 4 K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

Heterobimetallic Lantern Complexes and Their Novel Structural and Magnetic Properties.

As the scale of microelectronic circuit devices approaches the atomic limit, the study of molecular-based wires and magnets has become more prevalent. Compounds with quasi-1D geometries have been investigated for their electronic conductivity and magnetic properties with potential use as nanoscale circuit components and information storage devices. To increase the number of compositionally tailored molecular systems available to study, we have taken a building-block, bottom-up approach to the development of improved electronic structure and magnetic properties of quasi-1D arrays. Over the past decade, a large family of asymmetric complexes that can assemble into extended arrays has resulted. Lantern (or paddle-wheel) complexes with conventional {O, O} donor carboxylates are legion, but by the use of monothiocarboxylate ligands and hard-soft Lewis acid-base principles, dozens of new lantern complexes of the form [PtM(SOCR)4(L)] (M = Mg, Ca, Cr, Mn, Fe, Co, Ni, Zn; R = Ph (tba = thiobenzoate), CH3 (SAc = thioacetate); L = neutral or anionic ligand) have been prepared. Depending on M and L, new intermolecular arrangements have resulted, and the magnetic properties have proven particularly interesting. In the solid state, the [PtM(SOCR)4(L)] building blocks are sometimes isolated, sometimes form dimers, and can be induced to form infinite chains. The versatility of the lantern motif was demonstrated with a range of axial ligands to form both terminal and bridged complexes with various 3d metals and two different substituted thiocarboxylate backbone ligands. Within the dozens of crystallographically characterized compounds that make up this family of lanterns, several different structural motifs of solid-state dimerization were observed and divided into four distinct categories on the basis of their Pt···Pt and Pt···S distances and relative monomer orientations. Among all of these compounds, three novel magnetic phenomena were observed. Initially, long-range antiferromagnetic coupling between two metals more than 8 Å apart was observed in solid-state dimers formed via metallophilic Pt···Pt interactions and could induced by choice of the terminal L group. An infinite chain was prepared in [PtCr(tba)4(NCS)]∞ that displays ferromagnetic coupling between Cr centers with J/ kB = 1.7(4) K. Homobimetallic quasi-1D chains of the form [Ni2(SOCR)4(L)]∞ (R = Ph, CH3; L = DABCO, pyz) were also prepared with S = 1 {Ni2} building blocks in which the Ni centers have two different spin states with weak antiferromagnetic coupling along the chain, such that -0.18 > J/ kB > -0.24 K. In the [Ni2(tba)4(quin)] derivative, a solid-state dimer forms with a bridging square conformation by interlantern Ni2S2 interactions and displays unusual S = 1 configurations on both Ni centers and weak antiferromagnetic coupling between them.

Color of postponed magnetic noise inK0.4[Cr(CN)6][Mn(R/S)−pn](R/S)−pnH0.6molecular ferrimagnet

Exotic conditions for the existence and evolution of nonlinear spin ensembles (domain walls, spin solitons, skyrmions) in molecular-based magnets are incarnated in the macroscopic response of magnetization corresponding to collective stochastic behavior. The molecular ferrimagnet ${\mathrm{K}}_{0.4}[\mathrm{Cr}{(\mathrm{CN})}_{6}][\mathrm{Mn}(\mathrm{R}/\mathrm{S})\ensuremath{-}\mathrm{pn}](\mathrm{R}/\mathrm{S})\ensuremath{-}\mathrm{pn}{\mathrm{H}}_{0.6}$ manifests three types of magnetic relaxation: (a) continuous decay of magnetic moment, (b) stepwise relaxation by stochastic magnetization jumps, and (c) a single jump of magnetization in threshold magnetic field. Continuous relaxation at 20--50 K is provided by domain wall movement described in the frames of a strong pinning model, while a low-temperature continuous component of relaxation does not follow this model. Stepwise stochastic relaxation was observed below 8 K in both a sweeping reverse magnetic field and a stationary reverse magnetic field. Statistical treatment of the postponed magnetization jumps revealed a multimodal amplitude distribution of stochastic magnetization jumps corresponding to magnetic moment transitions between few clear distinguishable levels. Spectral density of magnetization jumps in a stationary magnetic field corresponds to white noise, while spectral density in a sweeping magnetic field manifests pink noise $\ensuremath{\sim}1/f$ provided by self-organized criticality. Postponed emission of magnetic noise in the ${10}^{\ensuremath{-}6}\ensuremath{-}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$ frequency range was observed in stationary conditions in contrast to Barkhausen noise.

Magnetic surfactants as molecular based-magnets with spin glass-like properties

This paper reports the use of muon spin relaxation spectroscopy to study how the aggregation behavior of magnetic surfactants containing lanthanide counterions may be exploited to create spin glass-like materials. Surfactants provide a unique approach to building in randomness, frustration and competing interactions into magnetic materials without requiring a lattice of ordered magnetic species or intervening ligands and elements. We demonstrate that this magnetic behavior may also be manipulated via formation of micelles rather than simple dilution, as well as via design of surfactant molecular architecture. This somewhat unexpected result indicates the potential of using novel magnetic surfactants for the generation and tuning of molecular magnets.

Synthesis and Properties of Novel Photomodulation Magnetic Molecular-Based Magnet Based on Photochromic Biindenylidenedione Derivative

Molecular-based magnetic material can be used for information storage, magnetic imaging, photo-magnetic switching and so on. So the design and synthesis of the molecular-based magnet is a focus of contemporary science research. Sparked by the strategy of molecular magnets, the novel photomodulation magnetic molecular-based magnet photochromic biindenylidenedione derivative with 2-nitropropane radical and 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO) radical (4) was synthesized. The synthetic method of compound 4 is' that starting from photochromic biindenylidenedione derivative 7,7'-dimethyl-[2,2'-bi-1H-indene]-3,3'-diethyl-3,3'-di-hydroxy-1,1'-dione (1), via bromination reaction, 7,7'-bis(bromomethyl)-[2,2'-bi-1H-indene]-3,3'-diethyl-3,3'-dihydroxy-1,1'-dione (2) was obtained. Sodium ethoxide, 2-nitropropane and compound 2 were dissolved in absolute ethanol reaction to obtain biindenylidenedione derivative with 2-nitropropane radical and formyl 3. The structure of compound 3 was confirmed by H-1 NMR, MS and element analysis. The results indicated that 2-nitropropane group bonds with one methylene on the benzene ring of the compound 3. In further analysis to H-1 NMR and electron spin-resonace (ESR) spectra of compound 3 demonstrated that single electron spin exists in 2-nitropropane group. On the basis compound 3, stable nitroxide radical TEMPO was successfully introduced into the benzene rings of photochromic biindenylidenedione derivative to obtain compound 4. The properties of compounds 2, 3 and 4 were investigated. The testing results display that the solid compounds 2 similar to 4 possess photochromic property. ESR spectrum was found when irradiated solid compound 2. ESR spectrum was found and changed before and after photoirradiation in solid state of compound 3 and 4. The magnetic susceptibility measurement shows that the antiferromagnetic interaction for before and after photoirradiation in solid state of compound 4, and the antiferromagnetic interaction becomes stronger after photoirradiation. These testing results demonstrated that after photoirradiation in solid state of compound 3 and 4, there are two or three kinds of spin centres after photoirradiation: one is 2-nitropropane or TEMPO radical, and another is photo-generated radical of two indanione moieties. The color and magnetic of the solid compound 4 can be photomodulated.

Ferrimagnetism in the Mean-Field Approximation of the Mixed Spin-3/2 and Spin-5/2 Ising System

Using the Mean-field theory based on Bogoliubov inequality for the free energy, a ferrimagnetic mixed spin-3/2 and spin-5/2 Ising model with different anisotropies is investigated. The free energy of a mixed spin Ising ferrimagnetic system from MF approximation of the Hamiltonian is calculated. By minimizing the free energy, we obtain the equilibrium magnetizations and compensation points. In particular, we investigate the effect of a single-ion anisotropy on the magnetic properties including the compensation phenomenon, in order to clarify the characteristic behaviours in a series of molecular-based magnets . The phase diagram of the system is also discussed in the anisotropy dependence of transition temperature. Our results of this model predict the existence of many (two or three) compensation points in the ordered system on a simple cubic lattice.

Ferrimagnetism in the Mean-Field Approximation of the Mixed Spin-3/2 and Spin-5/2 Ising System

Using the Mean-field theory based on Bogoliubov inequality for the free energy, a ferrimagnetic mixed spin-3/2 and spin-5/2 Ising model with different anisotropies is investigated. The free energy of a mixed spin Ising ferrimagnetic system from MF approximation of the Hamiltonian is calculated. By minimizing the free energy, we obtain the equilibrium magnetizations and compensation points. In particular, we investigate the effect of a single-ion anisotropy on the magnetic properties including the compensation phenomenon, in order to clarify the characteristic behaviours in a series of molecular-based magnets . The phase diagram of the system is also discussed in the anisotropy dependence of transition temperature. Our results of this model predict the existence of many (two or three) compensation points in the ordered system on a simple cubic lattice.

Three-dimensional molecular-based magnet of octacyanometalate-based tungstate(V)-cobalt(II) bimetallic assembly with TC = 26 K

A three-dimensional (3D) octacyanotungstate(V)-based molecular based magnet containing Co(II) ions, {[CoII4(H2O)6(C2O4)][WV(CN)8]2·6H2O}n (1), has been synthesized in acidic solution and structurally characterized. X-ray determination indicates that in 1 each [W(CN)8]3− structural unit is linked by seven neighboring Co(II) ions forming 2D Co–W–CN inorganic connectivity layers. Further short conjugated oxalate bridges join these neighboring 2D layers ultimately forming the 3D framework. Magnetic investigation of complex 1 reveals that long-range ferromagnetic ordering occurs below the Curie temperature of 26K. The beautiful butterfly-shaped magnetic hysteresis loop at 10K between −70KOe and 70KOe exhibits a rather high coercive field (HC=3010Oe). The remnant magnetization Mr is equal to 3055cm3mol−1. To our best knowledge, complex 1 is the first example of 3D cyano-based magnets bridged by C2O42− anions.

Room Temperature Metalation of 2H-TPP Monolayer on Iron and Nickel Surfaces by Picking up Substrate Metal Atoms

Here, it is demonstrated, using high-resolution X-ray spectroscopy and density functional theory calculations, that 2H-tetraphenyl porphyrins metalate at room temperature by incorporating a surface metal atom when a (sub)monolayer is deposited on 3d magnetic substrates, such as Fe(110) and Ni(111). The calculations demonstrate that the redox metalation reaction would be exothermic when occurring on a Ni(111) substrate with an energy gain of 0.89 eV upon embedding a Ni adatom in the macrocycle. This is a novel way to form, via chemical modification and supramolecular engineering, 3d-metal-organic networks on magnetic substrates with an intimate bond between the macrocycle molecular metal ion and the substrate atoms. The achievement of a complete metalation by Fe and Ni can be regarded as a test case for successful preparation of spintronic devices by means of molecular-based magnets and inorganic magnetic substrates.

Magnetic and Mössbauer spectra observed for mixed-metal magnets NBu4 ${\bf Fe}^{\bf II}_{\bf n}$ MA II 1 − n [FeIII(OX)3] (MA=Mn, Fe)

Mixed-metal molecular-based magnets NBu4\({\rm Fe}^{\rm II}_{\rm n}\)MAII1 − n[FeIII(OX)3] (MA=Mn, Fe) were investigated by magnetic and Mossbauer measurements. The magnetic susceptibility of NBu4\({\rm Fe}^{\rm II}_{0.07}{\rm Mn}^{\rm II}_{0.93}\) [FeIII(OX)3] can be fitted to a Curie-Weiss law with a Weiss paramagnetic Curie temperature of θ = −114.76 K. The negative Weiss constant indicates an intramolecular antiferromagnetic coupling interaction between the adjacent Fe(II) and Fe(III) ions through the oxalate bridge. In the complex NBu4FeII[FeIII(OX)3], the Mossbauer results indicate that the FeII and FeIII sublattices experience spontaneous magnetizations. The compound contains two different spin carriers; i.e. FeII(S = 2), FeIII(S = 5/2). Two magnetic sublattices are defined. The appearance of nuclear Zeeman splittings suggests that long range magnetic ordering takes place below 50 K.

The fruitful introduction of chirality and control of absolute configurations in molecular magnets

In this critical review, it is shown how the introduction of chirality and the control of the absolute configurations of chiral elements in molecular magnets allow obtaining enantiopure chiral magnets (ECM), an archetype of multifunctional materials. This task has been recognised as a major challenge for both chemists and physicists of molecular magnetism. To reach this goal, the former have combined the rational approaches towards molecular-based magnets and of enantiopure metal-organic frameworks. They have used enantiopure stable radicals, ligands from the chiral pool, enantiopure coligands associated with achiral connectors or enantioselective self-assembly to successfully reach their synthetic targets. They were motivated by the will to obtain suitable systems for the experimental demonstration of the influence of enantiomeric purity on the physico-chemical properties. This influence can be found in the magnetic properties themselves but, most interestingly, in the coexistence and interaction between the properties arising from controlled non-centrosymmetry. Thus the combination of natural circular dichroism, second harmonic generation or ferroelectricity with long-range magnetic ordering can give birth to new properties like magneto-chiral dichroism, magnetisation induced second harmonic generation or multiferroicity. The two former synergetic effects have already been demonstrated in enantiopure chiral magnets. The third one remains a challenging target that can be reached by adapting strategies developed towards enantiopure molecular ferroelectrics (119 references).

Magnetic hysteresis loops in molecular-based magnetic materials AFe IIFe III(C 2O 4) 3

A molecular-based magnetic material AFe IIFe III(C 2O 4) 3 (A = organic cation) with a honeycomb structure is studied. The molecular-based magnet system consists of mixed spin-2 and spin- 5/2 honeycomb lattices with ferrimagnetic interlayer coupling. The magnetization, hysteresis loops and initial susceptibility have been calculated using a numerical method which includes both the longitudinal and transverse fields. We investigated the magnetic reversal of the system and found the existence of triple hysteresis loop patterns, affected by the anisotropy, longitudinal and transverse fields, and interlayer and intralayer exchange.