{Dy(α-fur)3}n: from double relaxation single-ion magnet behavior to 3D ordering

Magnetic properties and magnetocaloric effect of Ln = Dy, Tb carborane-based metal-organic frameworks

We present the synthesis and magneto-thermal properties of carborane-based lanthanide metal-organic frameworks (MOFs) with the formula {[(Ln)3(mCB-L)4(NO3)(DMF)n]·Solv}, where Ln = Dy or Tb, characterized by dc and ac susceptibility, X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and heat capacity measurements. The MOF structure is formed by polymeric 1D chains of Ln ions with three different coordination environments (Ln1, Ln2, Ln3) running along the b-axis, linked by carborane-based linkers thus to provide a 3D structure. Static magnetic measurements reveal that these MOFs behave at low temperature as a system of S* = 1/2 Ising spins, weakly interacting ferromagnetically along the 1D polymeric chain (J*/kB = +0.45 K (+0.5 K) interaction constant estimated for Dy-MOF (Tb-MOF)) and coupled to Ln ions in adjacent chains through dipolar antiferromagnetic interactions. The Dy MOF exhibits slow relaxation of magnetization through a thermally activated process, transitioning to quantum tunneling of the magnetization at low temperatures, while both compounds exhibit field-induced relaxation through a very slow, direct process. The maximum magnetic entropy changes (-ΔSmaxm) for an applied magnetic field change of 2-0 T are 5.71 J kg-1 K-1 and 4.78 J kg-1 K-1, for Dy and Tb MOFs, respectively, while the magnetocaloric effect (MCE) peak for both occurs at T ∼ 1.6 K, approximately double that for the Gd counterpart.

Field-induced slow relaxation of magnetisation in two one-dimensional homometallic dysprosium(iii) complexes based on alpha- and beta-amino acids

Two one-dimensional dysprosium(iii) complexes based on α-glycine (gly) and β-alanine (β-ala) amino acids, with the formula {[Dy₂(gly)₆(H₂O)₄](ClO₄)₆·5H₂O}_n (1) and {[Dy₂(β-ala)₆(H₂O)₄](ClO₄)₆·H₂O}_n (2), have been synthesised and characterised structurally and magnetically. Both compounds crystallise in the triclinic system with the space group P1[combining macron]. In 1, two Dy^III ions are eight-coordinate and bound to six oxygen atoms from six gly ligands and two oxygen atoms from two water molecules, showing different geometries (bicapped trigonal prism and square antiprism). In 2, two Dy^III ions are nine-coordinate and bound to seven oxygen atoms from six β-ala ligands and two oxygen atoms from two water molecules in the same geometry (capped square antiprism). In the crystal packing of both compounds, cationic {[Dy₂(L)₆(H₂O)₄]⁶⁺}_n [L = α-glycine (1) and β-alanine (2)] chains, ClO₄^- anions, and water molecules generate a network connected through H-bonding interactions. The study of the magnetic properties of 1 and 2 through dc magnetic susceptibility measurements reveals different magnetic behaviour 1 and 2. In addition, ac magnetic susceptibility measurements show a field-induced slow relaxation of magnetisation for both compounds, pointing out that the single-molecule magnet (SMM) phenomenon occurs in both 1 and 2.

Magnetic chains of Fe3 clusters in the {Fe3YO2} butterfly molecular compound

The "butterfly" molecule [Fe3Y(μ3-O)2(CCl3COO)8(H2O)(THF)3] (in brief {Fe3YO2}) includes three Fe3+ ions which build a robust Fe3 cluster with a strong intracluster antiferromagnetic exchange and a total spin S = 5/2. It represents the starting magnetic system to study further interactions with magnetic rare earths when Y is replaced with lanthanides. We present heat capacity and equilibrium susceptibility measurements below 2 K, which show that each cluster has a sizeable magnetic anisotropy pointing to the existence of intercluster interactions. However, no phase transition to a long-range magnetically ordered phase is observed down to 20 mK. The intercluster interaction is analysed in the framework of the one-dimensional Blume-Capel model with an antiferromagnetic chain interaction constant J/kB = -40(2) mK between Fe3 cluster spins, and a uniaxial anisotropy with parameter D/kB = -0.56(3) K. This is associated to single chains of Fe3 clusters oriented along the shortest intercluster distances displayed by the crystal structure of {Fe3YO2}. Ac susceptibility measurements reveal that the magnetic relaxation is dominated by a quantum tunnelling process below 0.2 K, and by thermally activated processes above this temperature. The experimental activation energy of this single chain magnet, Ea/kB = 3.4(6) K, can be accounted for by the combination of contributions arising from single-molecule magnetic anisotropy and spin-spin correlations along the chains.

High relaxation barrier in neodymium furoate-based field-induced SMMs

Two new neodymium molecular magnets of formula {[Nd(α-fur)₃(H₂O)₂]·DMF}_n (1) and {[Nd_0.065La_0.935(α-fur)₃(H₂O)₂]}_n (2), α-fur = C₄H₃OCOO, have been synthesized. In (1) the furoate ligands, in bidentate bridging mode, consolidate zig-zag chains running along the a-direction. Compound (2) is a magnetically diluted complex of a polymeric chain along the b-axis. Heat capacity, dc magnetization and ac susceptibility measurements have been performed from 1.8 K up to room temperature. Ab initio calculations yielded the gyromagnetic factors g_x* = 0.52, g_y* = 1.03, g_z* = 4.41 for (1) and g_x* = 1.35, g_y* = 1.98, g_z* = 3.88 for (2), and predicted energy gaps of Δ/k_B = 125.5 K (1) and Δ/k_B = 58.8 K (2). Heat capacity and magnetometry measurements agree with these predictions, and confirm the non-negligible transversal anisotropy of the Kramers doublet ground state. A weak intrachain antiferromagnetic interaction J'/k_B = -3.15 × 10^-3 K was found for (1). No slow relaxation is observed at H = 0, attributed to the sizable transverse anisotropy component, and/or dipolar or exchange interactions enhancing the quantum tunnelling probability. Under an external applied field as small as 80 Oe, two slow relaxation processes appear: above 3 K the first relaxation mechanism is associated to a combination of Orbach process, with a sizeable activation energy U/k_B = 121 K at 1.2 kOe for (1), Raman and direct processes; the second, slowest relaxation mechanism is associated to a direct process, affected by phonon-bottleneck effect. For complex (2) a smaller U/k_B = 61 K at 1.2 kOe is found, together with larger g*-transversal terms, and the low-frequency process is quenched. The reported complexes represent rare polymeric Nd single-ion magnets exhibiting high activation energies among the scarce Nd(iii) family.

Slow relaxation in a {Tb2Ba(α-fur)8}n polymer with Ln = Tb(iii) non-Kramers ions

We report the synthesis, crystal structure and magnetic properties of a new heteronuclear polymeric complex based on non-Kramers Tb ions and carboxylic α-fur = C4H3OCOO ligands: {[Tb2Ba(α-fur)8(H2O)4]·2H2O}n. The α-furoate ligands consolidate 1D zig-zag chains running along the c-axis, formed by Tb2 dimers separated by Ba ions. Ab initio calculations, in combination with the fit of experimental data, predict that the single-ion magnetic ground state is highly anisotropic () and consists of a quasi-doublet with a ΔTb/kB = 3.22 K gap, well separated from the next excited state, while the gap for the Tb2 dimer is Δ2Tb/kB = 2.58 K. Static magnetization and heat capacity measurements show that, magnetically, the system can be modeled as dimers of non-Kramers Tb ions, coupled by an antiferromagnetic intradimer interaction J'*/kB = -1.6 K. Dipolar interactions couple the Tb ions in the dimer with their first neighbour ions along the chain, with J''*/kB = -0.15 K, and with the surrounding ions out of the chain, with maximum J'''*/kB = -0.03 K. Ac susceptibility measurements in H = 0 performed down to 50 mK temperatures have enabled us to observe slow relaxation of magnetization, with an Orbach-like activation energy of U/kB = 1.1 K. It is assigned to the sluggish response of the 3D spin system due to a short-range ordering, possibly enhanced by the presence of disorder caused by defects in the polymeric chains. Under the application of a magnetic field, the system slowly relaxes by two distinct direct processes, strongly affected by a phonon bottleneck effect. We discuss the different relaxational phenomenology of the new complex in comparison with that of the isostructural {[Dy2Ba(α-fur)8(H2O)4]·2H2O}n, differing only in the Kramers nature of the ions, and the mononuclear {Ln(α-fur)3(H2O)3}n (Ln = Tb, Dy) complexes, previously reported.

Magnetization relaxation in the single-ion magnet DySc2N@C80: quantum tunneling, magnetic dilution, and unconventional temperature dependence

Quantum tunneling and relaxation of magnetization in single molecule magnet DySc₂N@C₈₀ is thoroughly studied as a function of magnetic dilution, temperature, and magnetic field.

Relaxation of magnetization in endohedral metallofullerenes DySc₂N@C₈₀ is studied at different temperatures, in different magnetic fields, and in different molecular arrangements. Magnetization behavior and relaxation are analyzed for powder sample, and for DySc₂N@C₈₀ diluted in non-magnetic fullerene Lu₃N@C₈₀, adsorbed in voids of a metal–organic framework, and dispersed in a polymer. The magnetic field dependence and zero-field relaxation are also studied for single-crystals of DySc₂N@C₈₀ co-crystallized with Ni(ii) octaethylporphyrin, as well as for the single crystal diluted with Lu₃N@C₈₀. Landau–Zener theory is applied to analyze quantum tunneling of magnetization in the crystals. The field dependence of relaxation rates revealed a dramatic dependence of the zero-field tunneling resonance width on the dilution and is explained with the help of an analysis of dipolar field distributions. AC magnetometry is used then to get access to the relaxation of magnetization in a broader temperature range, from 2 to 87 K. Finally, a theoretical framework describing the spin dynamics with dissipation is proposed to study magnetization relaxation phenomena in single molecule magnets.

Chemical tunnel-splitting-engineering in a dysprosium-based molecular nanomagnet

Total control over the electronic spin relaxation in molecular nanomagnets is the ultimate goal in the design of new molecules with evermore realizable applications in spin-based devices. For single-ion lanthanide systems, with strong spin-orbit coupling, the potential applications are linked to the energetic structure of the crystal field levels and quantum tunneling within the ground state. Structural engineering of the timescale of these tunneling events via appropriate design of crystal fields represents a fundamental challenge for the synthetic chemist, since tunnel splittings are expected to be suppressed by crystal field environments with sufficiently high-order symmetry. Here, we report the long missing study of the effect of a non-linear (C₄) to pseudo-linear (D_4d) change in crystal field symmetry in an otherwise chemically unaltered dysprosium complex. From a purely experimental study of crystal field levels and electronic spin dynamics at milliKelvin temperatures, we demonstrate the ensuing threefold reduction of the tunnel splitting.

Structural diversity and photo-physical and magnetic properties of dimeric to 1D polymeric coordination polymers of lighter lanthanide(iii) dinitrobenzoates

Single crystal diffraction studies reveal the formation of the following 10 new complexes of lighter Ln(iii) ions with general formulas {[Ln(μ2-L1)3·(H2O)2]·H2O}n (Ln = Nd (1) and Eu (2)), [Nd(μ2-L2)2·(CH3COO)·(H2O)2]n (3), [Ln2(μ2-L2)5·(L2)·(H2O)4]n (Ln = Sm (4), Ce (5), and Pr (6)), [La2(μ2-L2)6·(H2O)3·(DMF)]n (7) (DMF = dimethylformamide), [Ln(μ2-L2)2·(L2)·(H2O)3]2 (Ln = Eu (8) and Gd (9)) and [Gd(L2)·(CH3COO)2·(H2O)2]2 (10), where L1 and L2 are anions of 3,5- and 2,4-dinitrobenzoic acid, respectively. Complexes 1-7 are 1D coordination polymers, while 8-10 are dinuclear complexes. The luminescence properties of Nd(iii) and Eu(iii) analogues displayed metal-centred emission with L1 exhibiting weak but more efficient sensitization than L2. A study of the magnetic properties of the compounds clearly demonstrated the field-induced single ion magnet behaviour of the Nd(iii) compounds 1 and 3. Their behaviour has been compared to previously reported analogous Nd(iii) complexes and the role of the lattice solvent and polymorphism on the magnetic behaviour has been evaluated.

Field-Induced Slow Relaxation in a Dinuclear Dysprosium(III) Complex Based on 3-Methoxycinnamic Acid

We report the synthesis, structure, and magnetic properties of a new dinuclear dysprosium(III) complex based on a 3-methoxycinnamate ligand. The centrosymmetric complex exhibits a field-induced SMM behavior. In contrast to the previously reported lanthanide-based systems with cinnamate derivatives that relax through a combination of Raman and direct processes, an Orbach process is also involved in highlighting the role of the structural organization over the spin-lattice relaxations.

A belt-like one-dimensional Dy chain exhibiting slow magnetic relaxation behavior

A belt-like Dy chain compound was synthesized, which exhibited slow magnetic relaxation behavior and clear hysteresis loops below 0.6 K.

Single molecule magnetic behaviour in lanthanide naphthalenesulfonate complexes

A family of mononuclear lanthanide complexes exhibiting slow magnetic relaxation is reported, providing the first examples of pure 4f sulfonate-based single molecule magnets.

Dy(iii)-Carboxylate chain containing quasi-D5h sites exhibits enhanced energy barrier for magnetization reversal

A slight modification of the solvent induced a change in the local symmetry of one-third of the metal sites for a 1D Dy(iii) complex from quasi-D_5h to C_2v and resulted in a significant change in U_eff from 403.6 to about 7.5 K.

Cinnamic acid derivative rare-earth dinuclear complexes and one-dimensional architectures: synthesis, characterization and magnetic properties

Discrete and extended architectures based on two derivatives of cinnamic acid show slow relaxation of the magnetisation.

Slow magnetic relaxation in a dimeric Mn2Ca2 complex enabled by the large Mn(iii) rhombicity

A large single-ion transverse anisotropy at Mn(iii) sites induces slow magnetic relaxation at zero magnetic field of the ferromagnetic Mn dimers in a singular Mn₂Ca₂ complex.

Relaxation Dynamics and Magnetic Anisotropy in a Low-Symmetry Dy(III) Complex

The magnetic behaviour of a Dy(LH)3 complex (LH(-) is the anion of 2-hydroxy-N'-[(E)-(2-hydroxy-3-methoxyphenyl)methylidene]benzhydrazide) was analysed in depth from both theoretical and experimental points of view. Cantilever torque magnetometry indicated that the complex has Ising-type anisotropy, and provided two possible directions for the easy axis of anisotropy due to the presence of two magnetically non-equivalent molecules in the crystal. Ab initio calculations confirmed the strong Ising-type anisotropy and disentangled the two possible orientations. The computed results obtained by using ab initio calculations were then used to rationalise the composite dynamic behaviour observed for both pure Dy(III) phase and Y(III) diluted phase, which showed two different relaxation channels in zero and non-zero static magnetic fields. In particular, we showed that the relaxation behaviour at the higher temperature range can be correctly reproduced by using a master matrix approach, which suggests that Orbach relaxation is occurring through a second excited doublet.

A 3D MOF constructed from dysprosium(iii) oxalate and capping ligands: ferromagnetic coupling and field-induced two-step magnetic relaxation

Field-induced two-step magnetic relaxation was observed in the first ferromagnetic 3D lanthanide(iii) oxalate framework with a capping ligand.

Building 1D lanthanide chains and non-symmetrical [Ln2] “triple-decker” clusters using salen-type ligands: magnetic cooling and relaxation phenomena

Three series of related lanthanide polymers/dimers have been structurally and magnetically investigated.

Magnetic relaxation versus 3D long-range ordering in {Dy₂Ba(α-fur)₈}n furoate polymers

A novel Dy-complex formulated as {[Dy2Ba(α-C4H3OCOO)8·(H2O)4]·2H2O}n, {Dy2Ba(α-fur)8}n, has been synthesized, structurally characterized, and magnetically and thermally investigated as a function of field and temperature, down to 85 mK. The α-furoate ligands consolidate 1D zig-zag chains formed by Dy2 dimers separated by Ba ions. Ab initio calculations were used to determine the easy anisotropy axis direction, the gyromagnetic tensor components and the energy levels of each Dy. The heat capacity and susceptibility measurements allowed us to conclude that intradimer and interdimer interactions are ferromagnetic and of the same order, J'/k(B) ≈ J''/k(B) = +0.55 K. In the absence of an applied magnetic field, the dynamic relaxation of the magnetization occurs through the fast (τ(T) ~ 10(-5) s) spin-reversal of each of the individual Dys through a quantum tunneling (QT) process. A long-range 3D ordered state is achieved at T(N) = 0.25 K, in which the ferromagnetically coupled zig-zag chains (J'/k(B) ≈ J''/k(B) = +0.528(1) K) running along the c-axis are antiferromagnetically coupled to the adjacent chains (J'''/k(B) = -0.021(1) K). Critical slowing down of the QT time constant is observed when the temperature approaches T(N). Under the application of a magnetic field, the QT relaxation is replaced by an Orbach process (with energy barrier U/k(B) = 68 K and τ0 ~ 10(-9) s at H = 2 kOe) and a very slow (τ(s) ∼ 0.2 s) relaxation process. We propose and demonstrate the proof of concept of a spintronic device, in which two different relaxation rates can be selected, and on/off switched by magnetic field biasing. The dynamical behavior of this compound is compared with another furoate to discuss the effect of competitive interactions.

Field and dilution effects on the magnetic relaxation behaviours of a 1D dysprosium(iii)-carboxylate chain built from chiral ligands

A novel enantiomer-pure dysprosium chain shows various slow magnetic relaxation behaviours depending on the field and dilution effects.

Structural and magnetic properties of some lanthanide (Ln = Eu(iii), Gd(iii) and Nd(iii)) cyanoacetate polymers: field-induced slow magnetic relaxation in the Gd and Nd substitutions

The cyanoacetate complexes, {[Ln₂(CNCH₂COO)₆(H₂O)₄]·2H₂O}_n, with Ln = Gd, Nd, show field-induced slow relaxation: resonant phonon trapping and Orbach process for Gd, and Orbach and direct processes for Nd.

Experimental and theoretical evidence that electrostatics governs easy-axis orientation in DyIII-based molecular chains

Magnetic slow relaxation is observed in a Dy^III-based molecular chain and the magnetic easy-axis is determined via single-crystal magnetometry and ab initio calculations.

Preparation and properties of a calcium(ii)-based molecular chain decorated with manganese(ii) butterfly-like complexes

The room temperature reaction of [Mn₂O₂(bipy)₄](ClO₄)₃ (bipy = 2,2′-bipyridine) with Ca(CHCl₂COO)₂ in methanol produced a yellow crystalline material.