Introduction
The study of molecular magnetism has offered an unrivaled opportunity to bring together the disciplines of synthetic chemistry and applied physics. Within this research field, single molecule magnets (SMMs) have emerged as an attractive class of materials owing to their potential applications in nanotechnology. SMMs are superparamagnetic molecules characterized by a large magnetic moment (S ) and sizeable uniaxial magnetic anisotropy, that gives rise to a bistable magnetic ground state and slow relaxation of magnetization.[1] Because of these remarkable properties controllable at the molecular scale, SMMs have been proposed as potential candidates for high-density magnetic storage, as well as qubits for the practical implementation of quantum computing.[2] One figure of merit for SMMs is the “blocking temperature” (TB ), often defined as the temperature at which the characteristic magnetic relaxation time (\(\tau\)) is 100 seconds (\(\tau=100\ s\)), though there are other metrics with which to compare the performance of these materials.
Pinpointing the birth of molecular magnetism is a very challenging task[3] and beyond the scope of this perspective article. When SMMs started being studied in the 1980s, this new research theme brought together physicists and chemists with the common intent to explore a new space at the intersection between the two disciplines.[1] Since the beginning, scientists from diverse backgrounds with a shared interest in magnetism set the scene for fruitful collaboration based on achieving at a minimum, two clear goals: 1) to create tunable materials following specific requirements and design criteria; 2) to measure and model magnetic properties in order to develop useful physical models. These goals function as a feedback loop that has inspired cohorts of scientists: we make materials and study their properties, then point towards new directions through an iterative process, thus solving old problems and furnishing new models. This synergic collaborative process is demonstrably valuable to both communities. The design, implementation and realization of any technological advances from these works will require yet more collaborative work in device engineering, fabrication and plant-scale chemical engineering.
A firm landmark in the history of SMM is the report in 1993 of magnetic remanence in a polymetallic manganese cluster, [Mn12O12(OAc)16(H2O)4] (OAc = CH3COO; ‘Mn12’),[4] whose synthesis and first magnetic characterization were originally reported in the 1980s.[5] Its magnetic properties are dominated by super-exchange between the twelve Mn atoms leading to a combinedS = 10 ground state that is subject to strong uniaxial anisotropy. This triggered a concerted experimental and theoretical effort to understand and control the origins of this new physics, culminating in the description of the energy barrier that controls the slow relaxation of magnetization in terms of chemically modifiable parameters (\(U_{\text{eff}}=\left|D\right|S^{2}\) for integer\(S\), where \(D\) and \(S\) are the zero-field splitting (ZFS) parameter, and total spin respectively). This led to a synthetic arms race of sorts towards increasing S , and hence \(U_{\text{eff}}\), reasoning that larger energy barriers would lead to retention of magnetization at higher temperatures.[6] However, \(D\) and \(S\) were showed to not be independent,[7] after more than two decades of research even the best performing 3d-based SMMs only showed magnetic hysteresis up to 6.5 K,[8] limiting possible technological applications.
The start of a new direction in SMM research was spurred by the observation of slow relaxation of magnetization in a monometallic Tb(III) phthalocyanide complex in 2003.[9] In this new approach, magnetic anisotropy could be orders of magnitude larger because it is introduced as a first-order effect via the unquenched orbital angular momentum of 4f orbitals in lanthanides,[10] rather than as a second-order effect via interaction with excited states in 3d-based compounds. An additional advantage of a monometallic approach is the elimination of the requirement for ferromagnetic interactions in polymetallic compounds. The synthetic and theoretical communities were focused by the popularization of design criteria of Ln(III) (Ln = lanthanide) coordination environments to stabilize the magnetic states of interest and therefore tailor \(U_{\text{eff}}\),[11] based on pioneering work by Sievers in the 1980s.[12]
Broadly speaking, this was the state of the field when in 2016, when Dr David Mills, Dr Nicholas Chilton, and Profs. Stephen Liddle, Eric McInnes and Richard Winpenny (all at The University of Manchester) were awarded the EPSRC grant “Designing Highly Axial Lanthanide Single Molecule Magnets”, aimed at obtaining SMMs withTB that could be amenable for real-world applications i.e. above 77 K nitrogen boiling temperature. In this perspective article we will focus on our collaborative experimental and theoretical approach to this task during our time working in these research groups, which saw in the era of “high-temperature” Ln SMMs, that now show magnetic hysteresis at temperatures as high as 80 K.[13,14] We will first address the synthetic challenges posed by highly axial Dy(III) SMMs and the measurement of their magnetic properties; secondly, we will give an account of our efforts to understand and predict magnetic relaxation.