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Crystals
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Molecular Magnets
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Molecular Magnets

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Organic Crystalline Materials".

Deadline for manuscript submissions: closed (30 November 2018) | Viewed by 41345

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Maria Bałanda

E-Mail Website
Guest Editor
Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Kraków, Poland
Interests: magnetic and superconducting materials; phase transitions; magnetic relaxation; low-dimensional and functional molecular magnets
Dr. Magdalena Fitta

E-Mail Website
Guest Editor
Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Kraków, Poland
Interests: functional materials, molecular magnetism, molecular thin films, magnetocaloric effect
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

    Since the end of the 20th century, remarkable progress has been made in the study of the intriguing and diverse properties of magnetic molecular materials. As defined by Oliver Kahn, one of the founders of this area, “molecular magnetism deals with magnetic properties of isolated molecules and/or assemblies of molecules”. Basic research problems, as well as the potential functionality of the molecule-based systems in new technologies, present an important branch of modern science. At the first stage, research in the field of molecular magnetism was focused on intramolecular magnetic coupling in coordination complexes, the search for molecule-based long-range ordered magnets and the quest for magnetism in purely organic compounds. The discovery of magnetic bistability, quantum tunneling and slow relaxation in anisotropic high-spin molecules entered upon the era of molecular nanomagnets, which are the potential candidates for high-density magnetic storage, spintronics and quantum computing.

    Research into molecular magnetism develops rapidly. This interdisciplinary field encompasses the rational design, chemical synthesis, experimental characterization and theoretical modeling of a molecular material showing acquired properties. The properties of the potentially-functional materials (magnetic ordering temperature, magnetic moment, coercive field, color, etc.) can be controlled by external stimuli, such as temperature, light, applied pressure or sorption of guest molecules. Molecular magnets showing spin crossover transitions, photomagnetism, optical activity or magnetic sponge behavior are good candidates for sensitive and selective sensors and switches. In the last few years, there has also been significant interest in the magnetocaloric effect in molecular materials, motivated by the possibility of applying this effect to magnetic refrigeration. The important direction in the field of molecular magnetism is formation of thin films of magnetic molecules or using molecular entities in hybrid nanoscale devices.

    It is our pleasure to invite you to present the results of your work in this Special Issue devoted to molecular magnets. We hope to collect original papers as well as critical reviews and to gain a comprehensive report on the current work on this exciting and important topic.

Prof. Maria Bałanda
Dr. Magdalena Fitta
Guest Editors

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Keywords

  • molecule-based magnets
  • single-molecule magnets (SMM)
  • single-chain magnets (SCM)
  • spin crossover (SCO) compounds
  • slow relaxation in molecular systems
  • functionality in molecular magnetic materials
  • molecular magnetic sponges and porous magnets
  • magnetocaloric effect
  • photomagnetism and optical activity
  • thin layers of molecular magnets
  • molecular spintronics

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Published Papers (9 papers)

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Editorial

Jump to: Research, Review

3 pages, 168 KiB  
Editorial
Molecular Magnets
by Maria Bałanda and Magdalena Fitta
Crystals 2019, 9(3), 132; https://doi.org/10.3390/cryst9030132 - 6 Mar 2019
Cited by 3 | Viewed by 2305
Abstract
Molecular magnetism is an interdisciplinary research area, which deals with design, synthesis and physical characterization as well as the theoretical modeling of molecular materials showing acquired properties [...] Full article
(This article belongs to the Special Issue Molecular Magnets)

Research

Jump to: Editorial, Review

13 pages, 15422 KiB  
Article
Thermodynamics and Magnetic Excitations in Quantum Spin Trimers: Applications for the Understanding of Molecular Magnets
by Amelia Brumfield and Jason T. Haraldsen
Crystals 2019, 9(2), 93; https://doi.org/10.3390/cryst9020093 - 12 Feb 2019
Cited by 8 | Viewed by 4435
Abstract
Molecular magnets provide a playground of interesting phenomena and interactions that have direct applications for quantum computation and magnetic systems. A general understanding of the underlying geometries for molecular magnets therefore generates a consistent foundation for which further analysis and understanding can be [...] Read more.
Molecular magnets provide a playground of interesting phenomena and interactions that have direct applications for quantum computation and magnetic systems. A general understanding of the underlying geometries for molecular magnets therefore generates a consistent foundation for which further analysis and understanding can be established. Using a Heisenberg spin-spin exchange Hamiltonian, we investigate the evolution of magnetic excitations and thermodynamics of quantum spin isosceles trimers (two sides J and one side α J ) with increasing spin. For the thermodynamics, we produce exact general solutions for the energy eigenstates and spin decomposition, which can be used to determine the heat capacity and magnetic susceptibility quickly. We show how the thermodynamic properties change with α coupling parameters and how the underlying ground state governs the Schottky anomaly. Furthermore, we investigate the microscopic excitations by examining the inelastic neutron scattering excitations and structure factors. Here, we illustrate how the individual dimer subgeometry governs the ability for probing underlying excitations. Overall, we feel these calculations can help with the general analysis and characterization of molecular magnet systems. Full article
(This article belongs to the Special Issue Molecular Magnets)
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12 pages, 2723 KiB  
Article
Modification of Structure and Magnetic Properties in Coordination Assemblies Based on [Cu(cyclam)]2+ and [W(CN)8]3−
by Aleksandra Pacanowska, Mateusz Reczyński and Beata Nowicka
Crystals 2019, 9(1), 45; https://doi.org/10.3390/cryst9010045 - 16 Jan 2019
Cited by 6 | Viewed by 3795
Abstract
The 1D {[CuII(cyclam)]3[WV(CN)8]2.5H2O}n (1·5H2O) (cyclam = 1,4,8,11-tetraazacyclotetradecane) coordination polymer of ladder topology can be obtained in water-alcohol solution from [Cu(cyclam)]2+ and [W(CN)8] [...] Read more.
The 1D {[CuII(cyclam)]3[WV(CN)8]2.5H2O}n (1·5H2O) (cyclam = 1,4,8,11-tetraazacyclotetradecane) coordination polymer of ladder topology can be obtained in water-alcohol solution from [Cu(cyclam)]2+ and [W(CN)8]3− building blocks. Upon dehydration, 1·5H2O undergoes a single-crystal-to-single-crystal structural transformation to the anhydrous {[CuII(cyclam)]3[WV(CN)8]2}n (1) form, which retains the same topology, but is characterized by shorter Cu-W distances and significantly more bent CN-bridges. The deformation of the coordination skeleton is reflected in magnetic properties: the predominant intra-chain interactions change from ferromagnetic in 1·5H2O to antiferromagnetic in 1. The reaction between the same building blocks in water solution under slow diffusion conditions leads to the formation of a 0D {[CuII(cyclam)(H2O)]2[CuII(cyclam)][WV(CN)8]2}.3H2O pentanuclear assembly (2·3H2O). Full article
(This article belongs to the Special Issue Molecular Magnets)
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18 pages, 3126 KiB  
Article
Bis(triphenylphosphine)iminium Salts of Dioxothiadiazole Radical Anions: Preparation, Crystal Structures, and Magnetic Properties
by Paweł Pakulski, Mirosław Arczyński and Dawid Pinkowicz
Crystals 2019, 9(1), 30; https://doi.org/10.3390/cryst9010030 - 7 Jan 2019
Cited by 5 | Viewed by 5597
Abstract
Phenanthroline dioxothiadiazoles are redox active molecules that form stable radical anions suitable for the construction of supramolecular magnetic materials. Herein, the preparation, structures and magnetic properties of bis(triphenylphosphine)iminium (PPN) salts of [1,2,5]thiadiazole[3,4-f][1,10]phenanthroline 1,1-dioxide (L), [1,2,5]thiadiazole[3,4-f][4,7]phenanthroline 1,1-dioxide (4,7-L), 5-bromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide [...] Read more.
Phenanthroline dioxothiadiazoles are redox active molecules that form stable radical anions suitable for the construction of supramolecular magnetic materials. Herein, the preparation, structures and magnetic properties of bis(triphenylphosphine)iminium (PPN) salts of [1,2,5]thiadiazole[3,4-f][1,10]phenanthroline 1,1-dioxide (L), [1,2,5]thiadiazole[3,4-f][4,7]phenanthroline 1,1-dioxide (4,7-L), 5-bromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (BrL), and 5,10-dibromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (diBrL) are reported. The preparation of new bromo derivatives of the L: 5-bromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (BrL) and 5,10-dibromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (diBrL)—suitable starting materials for further derivatization—are described starting from a commercially available and cheap 1,10-phenanthroline. All PPN salts show antiferromagnetic interactions between the pairs of radical anions, which in the case of PPN(diBrL) are very strong (−116 cm−1; using Ĥ = −2JSS type of exchange coupling Hamiltonian) due to a different crystal packing of the anion radicals as compared to PPN(L), PPN(4,7-L), and PPN(BrL). Full article
(This article belongs to the Special Issue Molecular Magnets)
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10 pages, 3769 KiB  
Article
Ferromagnetic Oxime-Based Manganese(III) Single-Molecule Magnets with Dimethylformamide and Pyridine as Terminal Ligands
by Carlos Rojas-Dotti, Nicolás Moliner, Francesc Lloret and José Martínez-Lillo
Crystals 2019, 9(1), 23; https://doi.org/10.3390/cryst9010023 - 31 Dec 2018
Cited by 8 | Viewed by 3636
Abstract
Two new members of the [Mn6] family of single-molecule magnets (SMMs) of formulae [Mn6(μ3-O)2(H2N-sao)6(dmf)8](ClO4)2 (1) and [Mn6(μ3-O)2(H2 [...] Read more.
Two new members of the [Mn6] family of single-molecule magnets (SMMs) of formulae [Mn6(μ3-O)2(H2N-sao)6(dmf)8](ClO4)2 (1) and [Mn6(μ3-O)2(H2N-sao)6(py)6(EtOH)2][ReO4]2·4EtOH (2), (dmf = N,N′-dimethylformamide, py = pyridine, H2N-saoH2 = salicylamidoxime) have been synthesized and characterized structurally and magnetically. Both compounds were straightforwardly prepared from the deprotonation of the H2N-saoH2 ligand in the presence of the desired manganese salt and solvent (dmf (1) vs. py (2)). Compound 1 crystallizes in the triclinic system with space group Pī and 2 crystallizes in the monoclinic system with space group P21/n. In the crystal packing of 1 and 2, the (ClO4)− (1) and [ReO4]− (2) anions sit between the cationic [Mn6]2+ units, which are H-bonded to –NH2 groups from the salicylamidoxime ligands. The study of the magnetic properties of 1 and 2 revealed ferromagnetic coupling between the MnIII metal ions and the occurrence of slow relaxation of the magnetization, which is a typical feature of single-molecule magnet behavior. The cationic nature of these [Mn6]2+ species suggests that they could be used as suitable building blocks for preparing new magnetic materials exhibiting additional functionalities. Full article
(This article belongs to the Special Issue Molecular Magnets)
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Review

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24 pages, 13795 KiB  
Review
The Effect of Pressure on Magnetic Properties of Prussian Blue Analogues
by Maria Zentkova and Marian Mihalik
Crystals 2019, 9(2), 112; https://doi.org/10.3390/cryst9020112 - 20 Feb 2019
Cited by 8 | Viewed by 5832
Abstract
We present the review of pressure effect on the crystal structure and magnetic properties of Cr(CN)6-based Prussian blue analogues (PBs). The lattice volume of the fcc crystal structure space group Fm 3 ¯ m in the Mn-Cr-CN-PBs linearly decreases for p [...] Read more.
We present the review of pressure effect on the crystal structure and magnetic properties of Cr(CN)6-based Prussian blue analogues (PBs). The lattice volume of the fcc crystal structure space group Fm 3 ¯ m in the Mn-Cr-CN-PBs linearly decreases for p ≤ 1.7 GPa, the change of lattice size levels off at 3.2 GPa, and above 4.2 GPa an amorphous-like structure appears. The crystal structure recovers after removal of pressure as high as 4.5 GPa. The effect of pressure on magnetic properties follows the non-monotonous pressure dependence of the crystal lattice. The amorphous like structure is accompanied with reduction of the Curie temperature (TC) to zero and a corresponding collapse of the ferrimagnetic moment at 10 GPa. The cell volume of Ni-Cr-CN-PBs decreases linearly and is isotropic in the range of 0–3.1 GPa. The Raman spectra can indicate a weak linkage isomerisation induced by pressure. The Curie temperature in Mn2+-CrIII-PBs and Cr2+-CrIII-PBs with dominant antiferromagnetic super-exchange interaction increases with pressure in comparison with decrease of TC in Ni2+-CrIII-PBs and Co2+-CrIII-PBs ferromagnets. TC increases with increasing pressure for ferrimagnetic systems due to the strengthening of magnetic interaction because pressure, which enlarges the monoelectronic overlap integral S and energy gap ∆ between the mixed molecular orbitals. The reduction of bonding angles between magnetic ions connected by the CN group leads to a small decrease of magnetic coupling. Such a reduction can be expected on both compounds with ferromagnetic and ferrimagnetic ordering. In the second case this effect is masked by the increase of coupling caused by the enlarged overlap between magnetic orbitals. In the case of mixed ferro–ferromagnetic systems, pressure affects μ(T) by a different method in Mn2+–N≡C–CrIII subsystem and CrIII–C≡N–Ni2+ subsystem, and as a consequence Tcomp decreases when the pressure is applied. The pressure changes magnetization processes in both systems, but we expect that spontaneous magnetization is not affected in Mn2+-CrIII-PBs, Ni2+-CrIII-PBs, and Co2+-CrIII-PBs. Pressure-induced magnetic hardening is attributed to a change in magneto-crystalline anisotropy induced by pressure. The applied pressure reduces saturated magnetization of Cr2+-CrIII-PBs. The applied pressure p = 0.84 GPa induces high spin–low spin transition of cca 4.5% of high spin Cr2+. The pressure effect on magnetic properties of PBs nano powders and core–shell heterostructures follows tendencies known from bulk parent PBs. Full article
(This article belongs to the Special Issue Molecular Magnets)
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16 pages, 4122 KiB  
Review
Magnetic and Electronic Properties of π-d Interacting Molecular Magnetic Superconductor κ-(BETS)2FeX4 (X = Cl, Br) Studied by Angle-Resolved Heat Capacity Measurements
by Shuhei Fukuoka, Sotarou Fukuchi, Hiroki Akutsu, Atsushi Kawamoto and Yasuhiro Nakazawa
Crystals 2019, 9(2), 66; https://doi.org/10.3390/cryst9020066 - 26 Jan 2019
Cited by 6 | Viewed by 3456
Abstract
Thermodynamic picture induced by π-d interaction in a molecular magnetic superconductor κ-(BETS)2FeX4 (X = Cl, Br), where BETS is bis(ethylenedithio)tetraselenafulvalene, studied by single crystal calorimetry is reviewed. Although the S = 5/2 spins of Fe [...] Read more.
Thermodynamic picture induced by π-d interaction in a molecular magnetic superconductor κ-(BETS)2FeX4 (X = Cl, Br), where BETS is bis(ethylenedithio)tetraselenafulvalene, studied by single crystal calorimetry is reviewed. Although the S = 5/2 spins of Fe3+ in the anion layers form a three-dimensional long-range ordering with nearly full entropy of Rln6, a broad hump structure appears in the temperature dependence of the magnetic heat capacity only when the magnetic field is applied parallel to the a axis, which is considered as the magnetic easy axis. The scaling of the temperature dependence of the magnetic heat capacity of the two salts is possible using the parameter of |Jdd|/kB and therefore the origin of the hump structure is related to the direct magnetic interaction, Jdd, that is dominant in the system. Quite unusual crossover from a three-dimensional ordering to a one-dimensional magnet occurs when magnetic fields are applied parallel to the a axis. A notable anisotropic field-direction dependence against the in-plane magnetic field was also observed in the transition temperature of the bulk superconductivity by the angle-resolved heat capacity measurements. We discuss the origin of this in-plane anisotropy in terms of the 3d electron spin configuration change induced by magnetic fields. Full article
(This article belongs to the Special Issue Molecular Magnets)
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33 pages, 10816 KiB  
Review
Multifunctional Molecular Magnets: Magnetocaloric Effect in Octacyanometallates
by Magdalena Fitta, Robert Pełka, Piotr Konieczny and Maria Bałanda
Crystals 2019, 9(1), 9; https://doi.org/10.3390/cryst9010009 - 22 Dec 2018
Cited by 27 | Viewed by 5742
Abstract
Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)8]n− complexes, (M = WV, MoV, NbIV), are universal building blocks as [...] Read more.
Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)8]n− complexes, (M = WV, MoV, NbIV), are universal building blocks as they lead to various spatial structures, depending on the surrounding ligands and the choice of the metal ion. One of the functionalities of the octacyanometallate-based coordination polymers or clusters is the magnetocaloric effect (MCE), consisting in a change of the material temperature upon the application of a magnetic field. In this review, we focus on different approaches to MCE investigation. We present examples of magnetic entropy change ΔSm and adiabatic temperature change ΔTad, determined using calorimetric measurements supplemented with the algebraic extrapolation of the data down to 0 K. At the field change of 5T, the compound built of high spin clusters Ni9[W(CN)8]6 showed a maximum value of −ΔSm equal to 18.38 J·K−1 mol−1 at 4.3 K, while the corresponding maximum ΔTad = 4.6 K was attained at 2.2 K. These values revealed that this molecular material may be treated as a possible candidate for cryogenic magnetic cooling. Values obtained for ferrimagnetic polymers at temperatures close to their magnetic ordering temperatures, Tc, were lower, i.e., −ΔSm = 6.83 J·K−1 mol−1 (ΔTad = 1.42 K) and −ΔSm = 4.9 J·K−1 mol−1 (ΔTad = 2 K) for {[MnII(pyrazole)4]2[NbIV(CN)8]·4H2O}n and{[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n, respectively. MCE results have been obtained also for other -[Nb(CN)8]-based manganese polymers, showing significant Tc dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different Tc, due to dissimilar ligands or other phase of the material, the ΔSm ~ Tc−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ΔSm ~ ΔHn dependence, and the critical exponents, related to n, were determined, pointing to the 3D Heisenberg model as the most adequate for the description of these particular compounds. At last, results of the rotating magnetocaloric effect (RMCE), which is a new technique efficient in the case of layered magnetic systems, are presented. Data have been obtained and discussed for single crystals of two 2D molecular magnets: ferrimagnetic {MnII(R-mpm)2]2[NbIV(CN)8]}∙4H2O (mpm = α-methyl-2-pyridinemethanol) and a strongly anisotropic (tetren)Cu4[W(CN)8]4 bilayered magnet showing the topological Berezinskii-Kosterlitz-Thouless transition. Full article
(This article belongs to the Special Issue Molecular Magnets)
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27 pages, 6552 KiB  
Review
Interplay of Spin and Spatial Anisotropy in Low-Dimensional Quantum Magnets with Spin 1/2
by Alžbeta Orendáčová, Róbert Tarasenko, Vladimír Tkáč, Erik Čižmár, Martin Orendáč and Alexander Feher
Crystals 2019, 9(1), 6; https://doi.org/10.3390/cryst9010006 - 21 Dec 2018
Cited by 9 | Viewed by 5618
Abstract
Quantum Heisenberg chain and square lattices are important paradigms of a low-dimensional magnetism. Their ground states are determined by the strength of quantum fluctuations. Correspondingly, the ground state of a rectangular lattice interpolates between the spin liquid and the ordered collinear Néel state [...] Read more.
Quantum Heisenberg chain and square lattices are important paradigms of a low-dimensional magnetism. Their ground states are determined by the strength of quantum fluctuations. Correspondingly, the ground state of a rectangular lattice interpolates between the spin liquid and the ordered collinear Néel state with the partially reduced order parameter. The diversity of additional exchange interactions offers variety of quantum models derived from the aforementioned paradigms. Besides the spatial anisotropy of the exchange coupling, controlling the lattice dimensionality and ground-state properties, the spin anisotropy (intrinsic or induced by the magnetic field) represents another important effect disturbing a rotational symmetry of the spin system. The S = 1/2 easy-axis and easy-plane XXZ models on the square lattice even for extremely weak spin anisotropies undergo Heisenberg-Ising and Heisenberg-XY crossovers, respectively, acting as precursors to the onset of the finite-temperature phase transitions within the two-dimensional Ising universality class (for the easy axis anisotropy) and a topological Berezinskii–Kosterlitz–Thouless phase transition (for the easy-plane anisotropy). Experimental realizations of the S = 1/2 two-dimensional XXZ models in bulk quantum magnets appeared only recently. Partial solutions of the problems associated with their experimental identifications are discussed and some possibilities of future investigations in quantum magnets on the square and rectangular lattice are outlined. Full article
(This article belongs to the Special Issue Molecular Magnets)
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