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.