José J. Baldoví

Rational Design of Single-Ion Magnets and Spin Qubits Based on Mononuclear Lanthanoid Complexes

Here we develop a general approach to calculating the energy spectrum and the wave functions of the low-lying magnetic levels of a lanthanoid ion submitted to the crystal field created by the surrounding ligands. This model allows us to propose general criteria for the rational design of new mononuclear lanthanoid complexes behaving as single-molecule magnets (SMMs) or acting as robust spin qubits. Three typical environments exhibited by these metal complexes are considered, namely, (a) square antiprism, (b) triangular dodecahedron, and (c) trigonal prism. The developed model is used to explain the properties of some representative examples showing these geometries. Key questions in this area, such as the chemical tailoring of the superparamagnetic energy barrier, tunneling gap, or spin relaxation time, are discussed. Finally, in order to take into account delocalization and/or covalent effects of the ligands, this point-charge model is complemented with ab initio calculations, which provide accurate information on the charge distribution around the metal, allowing for an explanation of the SMM behavior displayed by some sandwich-type organometallic compounds.

A SIM‐MOF: Three‐Dimensional Organisation of Single‐Ion Magnets with Anion‐Exchange Capabilities

The formation of a metal-organic framework (MOF) with nodes that have single-molecule magnet (SMM) behaviour has been achieved by using mononuclear lanthanoid analogues, also known as single-ion magnets (SIMs), which enormously simplifies the challenging issue of making SMM-MOFs. Here we present a rational design of a family of MOFs, [Ln(bipyNO)4](TfO)3⋅x solvent (Ln=Tb (1); Dy (2); Ho (3); Er (4); TfO=triflate), in which the lanthanoid centres have an square-antiprismatic coordination environment suitable for SIM behaviour. Magnetic measurements confirm the existence of slow magnetic relaxation typical of SMMs, which has been rationalised by means of a radial effective charge model. In addition, we have explored the incorporation of bulky polyoxometalates (POMs) into the cavities of the SIM-MOF by anion exchange, finding that they do not interfere with the slow magnetic relaxation. This demonstrates the robustness of the frameworks and opens the possibility of incorporating non-innocent anions.

Modeling the properties of uranium-based single ion magnets

We analyze the magnetic behavior of the five uranium-based SIMs reported in the literature. By combining a corrected crystal field model with the magnetic experimental data, we obtain the lowest-lying magnetic levels and the associated wave functions of the nanomagnets, which are found to be compatible with the observed SMM behavior. Additionally, this approach has allowed us to propose some geometrical considerations and practical advice for experimentalists aiming for the rational design of SIMs and spin qubits based on uranium.

SIMPRE: a software package to calculate crystal field parameters, energy levels, and magnetic properties on mononuclear lanthanoid complexes based on charge distributions.

This work presents a fortran77 code based on an effective electrostatic model of point charges around a rare earth ion. The program calculates the full set of crystal field parameters, energy levels spectrum, and wave functions, as well as the magnetic properties such as the magnetization, the temperature dependence of the magnetic susceptibility, and the Schottky contribution to the specific heat. It is designed for real systems that need not bear ideal symmetry and it is able to determine the easy axis of magnetization. Its systematic application to different coordination environments allows magneto‐structural studies. The package has already been successfully applied to several mononuclear systems with single‐molecule magnetic behavior. The determination of effective point charge parameters in these studies facilitates its application to new systems. In this article, we illustrate its usage with two example studies: (a) an ideal cubic structure coordinating a lanthanoid ion and (b) a system with slow relaxation of the magnetization, LiHoxY(1‐x)F4.

Construction of a general library for the rational design of nanomagnets and spin qubits based on mononuclear f-block complexes. The polyoxometalate case.

This paper belongs to a series of contributions aiming at establishing a general library that helps in the description of the crystal field (CF) effect of any ligand on the splitting of the J ground states of mononuclear f-element complexes. Here, the effective parameters associated with the oxo ligands (effective charges and metal-ligand distances) are extracted from the study of the magnetic properties of the first two families of single-ion magnets based on lanthanoid polyoxometalates (POMs), formulated as [Ln(W5O18)2](9-) and [Ln(β2-SiW11O39)2](13-) (Ln = Tb, Dy, Ho, Er, Tm, Yb). This effective CF approach provides a good description of the lowest-lying magnetic levels and the associated wave functions of the studied systems, which is fully consistent with the observed magnetic behavior. In order to demonstrate the predictive character of this model, we have extended our model in a first step to calculate the properties of the POM complexes of the early 4f-block metals. In doing so, [Nd(W5O18)2](9-) has been identified as a suitable candidate to exhibit SMM behavior. Magnetic experiments have confirmed such a prediction, demonstrating the usefulness of this strategy for the directed synthesis of new nanomagnets. Thus, with an effective barrier of 51.4 cm(-1) under an applied dc field of 1000 Oe, this is the second example of a Nd(3+)-based single-ion magnet.

Molecular anisotropy analysis of single-ion magnets using an effective electrostatic model.

Simple electrostatic models have been shown to successfully rationalize the magnetic properties of mononuclear single molecule magnets based on f-elements and even to predict the direction of the magnetic anisotropy axis in these nanomagnets. In this Article, we go a step forward by showing that these models, conveniently modified to account for the covalency effects, are able to predict not only the easy axis direction but also the three components of the magnetic anisotropy. Thus, by using a lone pair effective charge (LPEC) model we can fully reproduce the angular dependence of the magnetic susceptibility in single crystals of pentamethylcyclopentadienyl-Er-cyclooctatetraene single-ion magnet. Furthermore, the parametrization of the ligands obtained in this study has been extrapolated to successfully reproduce spectroscopic data of a set of mononuclear lanthanoid complexes based on the same kind of ligands, thus emphasizing the predictive character of this model.

An updated version of the computational package SIMPRE that uses the standard conventions for Stevens crystal field parameters

The crystal field approach used by SIMPRE is analyzed, verifying the exactness of the results concerning energy levels and magnetic properties calculated by the package. To coincide with the prevailing conventions, we reformulate the presentation of the crystal field parameters, so that the results are now, also from a formal point of view, strictly correct. New calculations are presented to test the influence of neglecting the excited J states, a common but critical approximation employed by SIMPRE. For that, we examine the case of Er(trensal) complex (H3trensal = 2,2′,2″‐tris(salicylideneimino)triethylamine) where the influence of this approximation is found to be minimal. A patched version of the code, SIMPRE 1.1, and an updated version of the user manual are now available. Finally, we comment on “Software package SIMPRE – revisited,” which apparently revisits a software package without inspecting or using the code.

Coherent manipulation of spin qubits based on polyoxometalates: the case of the single ion magnet [GdW30P5O110]14−

Polyoxometalate single ion magnet [GdW30P5O110](14-) (1) has been studied by generalized Rabi oscillation experiments. It was possible to increase the number of coherent rotations tenfold through matching the Rabi frequency with the frequency of the proton. Achieving high coherence with polyoxometalate chemistry, we show its excellent potential not only for the storage of quantum information but even for the realization of quantum algorithms.

Magnetic Properties of the Layered Lanthanide Hydroxide Series YxDy8-x(OH)20Cl4·6H2O: From Single Ion Magnets to 2D and 3D Interaction Effects

The magnetic properties of layered dysprosium hydroxides, both diluted in the diamagnetic yttrium analogous matrix (LYH:0.04Dy), and intercalated with 2,6-naphthalene dicarboxylate anions (LDyH-2,6-NDC), were studied and compared with the recently reported undiluted compound (LDyH = Dy8(OH)20Cl4·6H2O). The Y diluted compound reveals a single-molecule magnet (SMM) behavior of single Dy ions, with two distinct slow relaxation processes of the magnetization at low temperatures associated with the two main types of Dy sites, 8- and 9-fold coordinated. Only one relaxation process is observed in both undiluted LDyH and intercalated compounds as a consequence of dominant ferromagnetic Dy-Dy interactions, both intra- and interlayer. Semiempirical calculations using a radial effect charge (REC) model for the crystal field splitting of the Dy levels are used to explain data in terms of contributions from the different Dy sites. The dominant ferromagnetic interactions are explained in terms of orientations of easy magnetization axes obtained by REC calculations together with the sign of the superexchange expected from the Dy-O-Dy angles.

Rational Design of Lanthanoid Single‐Ion Magnets: Predictive Power of the Theoretical Models

We report two new single-ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised.