: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Single-molecule magnets (SMMs) containing only one metal center may represent the lower size limit for molecule-based magnetic information storage materials. Their current drawback is that all SMMs require liquid-helium cooling to show magnetic memory effects. We now report a chemical strategy to access the dysprosium metallocene cation [(CpiPr5)Dy(Cp*)]+ (CpiPr5 = penta-iso-propylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl), which displays magnetic hysteresis above liquid-nitrogen temperatures. An effective energy barrier to reversal of the magnetization of Ueff = 1,541 cm–1 is also measured. The magnetic blocking temperature of TB = 80 K for this cation overcomes an essential barrier towards the development of nanomagnet devices that function at practical temperatures.

Magnetic hysteresis up to 80 kelvin in a dysprosium metallocene single-molecule magnet

Toward promising candidates of quantum information processing, the rapid development of lanthanide-based single-molecule magnets (Ln-SMMs) highlights design strategies in consideration of the local symmetry of lanthanide ions. In this review, crystal-field theory is employed to demonstrate the electronic structures according to the semiquantitative electrostatic model. Then, specific symmetry elements are analysed for the elimination of transverse crystal fields and quantum tunnelling of magnetization (QTM). In this way, high-performance Ln-SMMs can be designed to enable extremely slow relaxation of magnetization, namely magnetic blocking; however, their practical magnetic characterization becomes increasingly challenging. Therefore, we will attempt to interpret the experimental behaviours and clarify some issues in detail. Finally, representative Ln-SMMs with specific local symmetries are summarized in combination with the discussion on the symmetry strategies, and some of the underlying questions are put forward.

Symmetry strategies for high performance lanthanide-based single-molecule magnets

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.

/pdf/molecular-magnetism-from-chemical-design-to-spin-control-in-2yza6q9lmp.pdf

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

A quantitative theory of the isotropic electron‐nuclear spin interactions of carbon 13 in pi‐electron radicals is presented and applied to the hyperfine splittings observed in the electron spin resonance spectra of these substances. The splittings arise from sigma‐pi interactions which polarize both the 1s and 2s electrons. The 1s‐orbital spin polarization is shown to contribute a term of negative sign with a magnitude comparable to that from the 2s electrons. For an sp2 hybridized carbon atom that is bonded to three atoms, Xi (i=1, 2, 3), the hyperfine constant aC has the form aC=(SC+ ∑ i=13QCXiC)ρπ+ ∑ i=13QXiCCρiπ, where ρπ and ρiπ(i=1,2,3) are the pi‐electron spin densities on atoms C and Xi, respectively. The contribution of the 1s electrons is determined by SC and that of the 2s electrons by the Q's, where QBCA is the sigma‐pi parameter for the nucleus of atom A resulting from the interaction between the bond BC and the pi‐electron spin density on atom B. Calculations for a planar CHC2 fragment model...

THEORETICAL INTERPRETATION OF CARBON-13 HYPERFINE INTERACTIONS IN ELECTRON SPIN RESONANCE SPECTRA. Technical Note Report No. 9

Magnetic hysteresis is observed in a dysprosocenium complex at temperatures of up to 60 kelvin, the origin of which is the localized metal–ligand vibrational modes unique to dysprosocenium. The discovery of molecules that exhibit magnetic bistability raised hopes for the use of such molecular systems as tiny building blocks for magnetic data storage. Despite a quarter of a century of research, however, the temperatures at which these molecules display their desirable magnetic properties remain frustratingly low. Conrad Goodwin et al. report the synthesis and characterization of a molecular dysprosocenium complex that shows magnetic bistability up to 60 kelvin—tantalizingly close to liquid nitrogen temperatures, the point at which applications would start to become a realistic possibility. Lanthanides have been investigated extensively for potential applications in quantum information processing and high-density data storage at the molecular and atomic scale. Experimental achievements include reading and manipulating single nuclear spins1,2, exploiting atomic clock transitions for robust qubits3 and, most recently, magnetic data storage in single atoms4,5. Single-molecule magnets exhibit magnetic hysteresis of molecular origin6—a magnetic memory effect and a prerequisite of data storage—and so far lanthanide examples have exhibited this phenomenon at the highest temperatures. However, in the nearly 25 years since the discovery of single-molecule magnets7, hysteresis temperatures have increased from 4 kelvin to only about 14 kelvin8,9,10 using a consistent magnetic field sweep rate of about 20 oersted per second, although higher temperatures have been achieved by using very fast sweep rates11,12 (for example, 30 kelvin with 200 oersted per second)12. Here we report a hexa-tert-butyldysprosocenium complex—[Dy(Cpttt)2][B(C6F5)4], with Cpttt = {C5H2tBu3-1,2,4} and tBu = C(CH3)3—which exhibits magnetic hysteresis at temperatures of up to 60 kelvin at a sweep rate of 22 oersted per second. We observe a clear change in the relaxation dynamics at this temperature, which persists in magnetically diluted samples, suggesting that the origin of the hysteresis is the localized metal–ligand vibrational modes that are unique to dysprosocenium. Ab initio calculations of spin dynamics demonstrate that magnetic relaxation at high temperatures is due to local molecular vibrations. These results indicate that, with judicious molecular design, magnetic data storage in single molecules at temperatures above liquid nitrogen should be possible.

/pdf/molecular-magnetic-hysteresis-at-60-kelvin-in-dysprosocenium-3wffprrfdi.pdf

Molecular magnetic hysteresis at 60 kelvin in dysprosocenium.

Description Magnetic effects of lanthanide bonding Lanthanide coordination compounds have attracted attention for their persistent magnetic properties near liquid nitrogen temperature, well above alternative molecular magnets. Gould et al. report that introducing metal-metal bonding can enhance coercivity. Reduction of iodide-bridged terbium or dysprosium dimers resulted in a single electron bond between the metals, which enforced alignment of the other valence electrons. The resultant coercive fields exceeded 14 tesla below 50 and 60 kelvin for the terbium and dysprosium compounds, respectively. —JSY A single electron bond between lanthanide centers induces alignment effects that impart extremely high magnetic coercivity. Metal-metal bonding interactions can engender outstanding magnetic properties in bulk materials and molecules, and examples abound for the transition metals. Extending this paradigm to the lanthanides, herein we report mixed-valence dilanthanide complexes (CpiPr5)2Ln2I3 (Ln is Gd, Tb, or Dy; CpiPr5, pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5dz2 parentage, as determined by structural, spectroscopic, and computational analyses. Valence delocalization, wherein the d electron is equally shared by the two lanthanide centers, imparts strong parallel alignment of the σ-bonding and f electrons on both lanthanides according to Hund’s rules. The combination of a well-isolated high-spin ground state and large magnetic anisotropy in (CpiPr5)2Dy2I3 gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin.

https://www.science.org/cms/asset/b56441db-e6ad-4504-bffb-155259009e28/science.abl5470.v1.pdf

Ultrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding

The use of alternating current (AC) magnetometry to measure magnetic relaxation times is one of the most fundamental measurements for characterising single-molecule magnets (SMMs). These measurements, performed as a function of frequency, temperature and magnetic field, give vital information on the underlying magnetic relaxation process(es) occurring in the material. The magnetic relaxation times are usually fitted to model functions derived from spin–phonon coupling theories that allow characterisation of the mechanisms of magnetic relaxation. The parameters of these relaxation models are then often compared between different molecules in order to find trends with molecular structure that may guide the field to the next breakthrough. However, such meta-analyses of the model parameters are doomed to over-interpretation unless uncertainties in the model parameters can be quantified. Here we determine a method for obtaining uncertainty estimates in magnetic relaxation times from AC experiments, and provide a program called CC-FIT2 for fitting experimental AC data as well as the resulting relaxation times, to obtain relaxation parameters with accurate uncertainties. Applying our approach to three archetypal families of high-performance dysprosium(III) SMMs shows that accounting for uncertainties has a significant impact on the uncertainties of relaxation parameters, and that larger uncertainties appear to correlate with crystallographic disorder in the compounds studied. We suggest that this type of analysis should become routine in the community.

Uncertainty estimates for magnetic relaxation times and magnetic relaxation parameters.

Understanding quantum tunnelling of the magnetisation (QTM) in single-molecule magnets (SMMs) is crucial for improving performance and achieving molecule-based information storage above liquid nitrogen temperatures. Here, through a field- and temperature-dependent study of the magnetisation dynamics of [Dy(tBuO)Cl(THF)5][BPh4]·2THF, we elucidate the different relaxation processes: field-independent Orbach and Raman mechanisms dominate at high temperatures, a single-phonon direct process dominates at low temperatures and fields >1 kOe, and a field- and temperature-dependent QTM process operates near zero field. Accounting for the exponential temperature dependence of the phonon collision rate in the QTM process, we model the magnetisation dynamics over 11 orders of magnitude and find a QTM tunnelling gap on the order of 10−4 to 10−5 cm−1. We show that removal of Dy nuclear spins does not suppress QTM, and argue that while internal dipolar fields and hyperfine coupling support QTM, it is the dynamic crystal field that drives efficient QTM. Understanding quantum tunnelling of the magnetisation in single-molecule magnets is crucial for their potential application in information storage. Here the authors conduct a field- and temperature-dependent study of the magnetisation dynamics of a dysprosium-based SMM, finding four distinct relaxation processes that dominate in different regimes.

/pdf/field-and-temperature-dependent-quantum-tunnelling-of-the-c35dv8xisb.pdf

Field- and temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule magnet

The origin of 60 K magnetic hysteresis in the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1-Dy) remains mysterious, thus we envisaged that analysis of a series of [Ln(Cpttt)2]+ (Ln = lanthanide) cations could shed light on these properties. Herein we report the synthesis and physical characterization of a family of isolated [Ln(Cpttt)2]+ cations (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu), synthesized by halide abstraction of [Ln(Cpttt)2(Cl)] (2-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu). Complexes within the two families 1-Ln and 2-Ln are isostructural and display pseudo-linear and pseudo-trigonal crystal fields, respectively. This results in archetypal electronic structures, determined with CASSCF-SO calculations and confirmed with SQUID magnetometry and EPR spectroscopy, showing easy-axis or easy-plane magnetic anisotropy depending on the choice of Ln ion. Study of their magnetic relaxation dynamics reveals that 1-Ho also exhibits an anomalously low Raman exponent similar to 1-Dy, both being distinct from the larger and more regular Raman exponents for 2-Dy, 2-Er, and 2-Yb. This suggests that low Raman exponents arise from the unique spin-phonon coupling of isolated [Ln(Cpttt)2]+ cations. Crucially, this highlights a direct connection between ligand coordination modes and spin-phonon coupling, and therefore we propose that the exclusive presence of multihapto ligands in 1-Dy is the origin of its remarkable magnetic properties. Controlling the spin-phonon coupling through ligand design thus appears vital for realizing the next generation of high-temperature single-molecule magnets.

Daniel Reta

Papers

Molecular magnetic hysteresis at 60 kelvin in dysprosocenium.

Ultrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding

Uncertainty estimates for magnetic relaxation times and magnetic relaxation parameters.

Field- and temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule magnet

Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations