Editorial

3 pages, 168 KiB

Open AccessEditorial

by Maria Bałanda and Magdalena Fitta

Crystals 2019, 9(3), 132; https://doi.org/10.3390/cryst9030132 - 06 Mar 2019

Cited by 2 | Viewed by 1957

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

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13 pages, 15422 KiB

Open AccessArticle

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 7 | Viewed by 3835

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

Open AccessArticle

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 5 | Viewed by 3382

Abstract

The 1D {[Cu^II(cyclam)]₃[W^V(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 {[Cu^II(cyclam)]₃[W^V(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 {[Cu^II(cyclam)]₃[W^V(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 {[Cu^II(cyclam)(H₂O)]₂[Cu^II(cyclam)][W^V(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

Open AccessArticle

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 - 07 Jan 2019

Cited by 4 | Viewed by 5065

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

Open AccessArticle

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 7 | Viewed by 3284

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 Mn^III 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

Open AccessFeature PaperReview

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 7 | Viewed by 5214

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

\bar{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

\bar{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 (T_C) 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⁺-Cr^III-PBs and Cr²⁺-Cr^III-PBs with dominant antiferromagnetic super-exchange interaction increases with pressure in comparison with decrease of T_C in Ni²⁺-Cr^III-PBs and Co²⁺-Cr^III-PBs ferromagnets. T_C 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–Cr^III subsystem and Cr^III–C≡N–Ni²⁺ subsystem, and as a consequence T_comp 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⁺-Cr^III-PBs, Ni²⁺-Cr^III-PBs, and Co²⁺-Cr^III-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⁺-Cr^III-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

Open AccessReview

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 3096

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 |J_dd|/k_B and therefore the origin of the hump structure is related to the direct magnetic interaction, J_dd, 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

Open AccessReview

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 24 | Viewed by 5220

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 = W^V, Mo^V, Nb^IV), 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 = W^V, Mo^V, Nb^IV), 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 ΔS_m and adiabatic temperature change ΔT_ad, 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 −ΔS_m equal to 18.38 J·K⁻¹ mol⁻¹ at 4.3 K, while the corresponding maximum ΔT_ad = 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, T_c, were lower, i.e., −ΔS_m = 6.83 J·K⁻¹ mol⁻¹ (ΔT_ad = 1.42 K) and −ΔS_m = 4.9 J·K⁻¹ mol⁻¹ (ΔT_ad = 2 K) for {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n and{[Fe^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n, respectively. MCE results have been obtained also for other -[Nb(CN)₈]-based manganese polymers, showing significant T_c dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different T_c, due to dissimilar ligands or other phase of the material, the ΔS_m ~ T_c^−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ΔS_m ~ 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 {Mn^II(R-mpm)₂]₂[Nb^IV(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

Open AccessReview

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 5088

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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