School Papers

1 be slightly lower as novel methods were

1      
Introduction

1.1.         
Fundamentals of magnetism

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Magnetism is a property of matter that is under
investigation by scientists for centuries and is integrated into modern
technology with various applications. Thus, magnetic materials have been
fabricated for various applications. The most common materials that exhibit
magnetic properties are transition metal oxides and lanthanide based magnets.
Before molecular magnetic materials were introduced in the literature, bulk
magnets were studied and fabricated. These bulk magnets exhibited magnetic
behavior because of the unpaired electrons.

Electrons possess a spin which is an intrinsic
property of the particle and the spin generates a magnetic moment around the
electron which responds to a magnetic field.1 The magnetic moment that is generated from the spin
and angular momentum of the electron can be considered as analogous to a moving
charge in classical physics akin to current flowing through a copper wire.

The magnetic interaction of a compound depends on the
electronic structure of the material2. When all the electrons in a material are paired
antiparallel the magnetic moments cancel each other out and the result is a
diamagnetic interaction with a magnetic field. Diamagnetic materials are
repelled by magnetic fields. In case there are unpaired electrons in a compound
however the unpaired electron spins tend to align with the magnetic field and
this results in either a paramagnetic or ferromagnetic interaction. Which of
the two interactions takes place is dependent on the structure of the material.
Ferro and paramagnetic materials are attracted to a magnetic field.

The difference between paramagnetic and ferromagnetic
material is the strength that the material is attracted by the magnetic field
with ferromagnetic being the stronger of the two. The orientation of the
magnetic domains of a material determine whether a material is ferromagnetic or
paramagnetic. In bulk materials the domains of the unpaired spins can be
randomly orientated and align with the magnetic field but not with the exact
same orientation in respect to each other and then become random again when
they are no longer exposed to the magnetic field or align with perfect order
and exhibit a far stronger than a paramagnet. A famous example of a
ferromagnetic material is its namesake iron (Latin. ferrum) which is
strongly attracted to magnets. The reason that some materials with unpaired
spins orient in a way that are ferromagnetic or paramagnetic has do to how with
how the magnetic domains organize. The magnetic domains are parts of the bulk
magnet that contain spins with a certain orientation, thus if the domains
perfectly align in the same orientation with each other the material will be
ferromagnetic.

To quantify the magnetic properties of material
various methods were made to measure them. One of the oldest methods is the
Gouy balance1 that measures the attractive or repulsive force by
introducing a sample in a weight balance with and without the presence of a
magnetic field and then compared the weights. If the compound was ferro or
paramagnetic the weigh would be higher as it will be pulled down by the
magnetic field and if it was diamagnetic it would be slightly lower as novel
methods were developed however measurement of magnetic susceptibility were used
to describe the magnetic properties which is a measurement in the magnetization
of a material which is a measurement of the strength of response to the
magnetic field over the magnetic field imposed upon it.

1.2.         
The
beginning of Molecular nanomagnets

In 1980 the first documented case of a molecular
nanomagnet was published3. The molecule was a dodecanuclear Manganese complex
with the formula of Mn12O12(AcO)16(H2O)4.
This compound is also categorized as a single molecule magnet, usually
abbreviated in the literature as SMM. The difference between a nanomagnet and
an SMM is the size of the molecule, SMM being reserved for small paramagnetic
molecules consisting of a few nuclear centers making them a sub category and
nanomagnet for larger paramagnetic molecules that are in scales of nanometers4.

 The complex
consisted of eight Mn ions in the 3+ oxidation state with S=2 and
the other four with an oxidation state of 4+ with S=3/2 and the
total spin ground state is S=10. This complex also possessed axial anisotropy
because of the Jahn-teller distortion. The most important information from this
structure however is that it shows a hysteresis loop at 2.2 K. EPR was used and
the compound is also shown to have a large zero splitting energy and a high
degree of magnetic anisotropy resulting from the Jahn-Teller distortion. Unlike
previous materials the magnetic properties were not attributed to bulk magnetic
domains but to the spins of molecular orbitals. The compound has also exhibited
quantum tunneling magnetization (QTM)5.

The QTM and the hysteresis loop are the most important
properties that nanomagnets possess to make them different from bulk magnets in
terms of magnetic effects. The hysteresis is the ability of a molecular
nanomagnet to “remember” the magnetic field that was previously applied to it1.
Unlike a bulk paramagnetic material in which the spins immediately relax when
the magnetic field is no longer exerted a molecular nanomagnet can retain its
magnetization orientation after the magnetic field is no longer applied to the
material. This results in the material having a double well energy barrier of
magnetization that in order to change the orientation of the magnetization a
magnetic field of equal energy and opposite polarity is required. The quantum
tunneling magnetization is the ability of spins to go through the energy
barrier of the magnetization instead of acquiring the required energy and go
over it6. This occurs when there is an overlap between energy
states with same energy on the opposite wells. This cannot be observed on
larger structures because macroscopically the quantum effects disappear.

This information provided insight on the properties of
the compound and in general the properties that need to be investigated in
order to be able to classify a compound as a molecular nanomagnet. From that
point onward, the literature has more compounds that can be classified as nanomagnets
and the research concerns in synthesizing compounds which can potentially have
similar properties to this Mn12 cluster. Two approaches for making
compounds exist, the top-down approach which designs nanomagnets based on the chemical
structure of bulk magnets in an effort to create a miniaturized version of them
and the bottom-up approach that synthesizes molecules from scratch7.

1.3.         
Interest
for research and applications

Molecular nanomagnets is an interesting research topic
since unlike bulk magnets the spin systems are configurable7. Using molecules as a base for a magnetic material
means that coordination chemistry can be exploited to fine tune the spin system
of your metals or even design a ligand based spin system8. This could theoretically mean that extremely
specialized structures could be made for a specific application resulting in a
level of precision that was not possible with bulk magnets.

While in theory useful, in practice this effort faces
some obstacles as the chemistry of some metals and/or ligands may make a
desired material difficult to make7. This in result has sparked
research that aims to synthesize single molecule magnets that have desired
properties for various research goals.

Apart from SMMs research in molecular nanomagnets
spans also the synthesis of molecular magnet oligomers and polymers.

The added flexibility in the designing the spin system
that single molecule magnets and nanomagnets have has already been researched
for various applications that were not before possible with bulk magnetic
materials. Some of the applications that can potentially exploit the properties
of single molecule magnets include magnetic cooling because of the magnetocaloric
effect some SMMs that produce cooling upon demagnetisation7, quantum
information processing as a SMM with a total spin ground state of S=1/2 and
high enough relaxation time can act as a qubit and perform fundamental quantum
algorithms and also to perform research in understanding quantum behavior of
magnetic spin systems as well as MRI contrast agents due to the magnetization
they exhibit9. Also, due to the magnetic hysteresis they exhibit
they can theoretically be used to store date by exploiting the magnetization
barrier.

1.4.         
Addressing
the problem of blocking temperature

While the magnetic properties of SMMs are promising
for the development of novel technologies there is main practical concern which
is the blocking temperature. The blocking temperature is the maximum
temperature that a nanomagnet will exhibit the hysteresis loop10. Beyond that point the compound is rapidly
demagnetized and ceases to exhibit any magnetic behavior. This is a serious
concern as it can be seen with the Mn12 cluster because this
temperature is very low at 2.2K meaning that in order for a device based on it
to operate as expected it must be kept at that extremely low temperature. While
this is theoretically possible with liquid helium cooling, the practical costs
make these materials at the current state unsuitable for practical application.
Thus, a main part of research in this area is to overcome this limitation while
retaining the desirable properties. Fortunately, recent research has shown
promise regarding this problem as a Dysprocenium compound was synthesized that
had a blocking temperature of 60K11. The compound had a molecular formula of Dy(Cpttt)2B(C6F5)4,
where Cpttt = {C5H2 t Bu3-1,2,4}. 60 Kelvin is a high
enough temperature that a liquid nitrogen cooling system can be implemented
which is practically feasible for some applications.

2      
Synthetic approaches and
candidate materials

In the literature there are various synthetic
strategies and compound classes that are researched for their molecular
magnetic properties and their possible applications. Some of them are based on
the initial Mn12 complex while others attempt chemically different
structures and experiment with alternative ways of synthesis

2.1.         
Homonuclear
d-block transition element complexes

Based on the Mn12O12(AcO)16(H2O)4
more complexes of homonuclear first row transition metal compounds were
synthesized. Manganese based compounds were made with examples being (PPh4)Mn12O12(O2CEt)16(H2O)412 that has a spin ground state of S = 19/2
and zero field spitting of D = –0.40 cm–1 with a hysteresis loop at 2.2K.
Dinuclear Mn complexes also exist with an example beingMn2(H2thme)2(bpy)4(ClO4)213 . This complex has a spin ground state
of S = 4 due to the ferromagnetic interaction of the Manganese unpaired
electrons and a zero-field splitting of D =
– 0.65 cm–1 and
a hysteresis loop was shown at 1.2K, lower than the initial Mn12
complex. In the family of Mn clusters the biggest SMM in literature exist the
Mn84O72(O2CMe)78(OMe)24(OH)6(MeOH)12(H2O)4214. It has a spin ground state of S = 6 and a hysteresis
loop at 1.5K.

Iron and Nickel compounds also exist in
the literature in the synthesis of homonuclear complexes. An example for iron is
Fe4(sae)4(MeOH)415 (sae = 2 salicylideneamino-1-ethanol).
The complex has a hysteresis loop blocking temperature at 1.1K with a spin
ground state of S = 8 and D = – 0.64 cm–1. For Nickel an example is Ni12(chp)12(O2CMe)
12(thf)6(H2O)616 with spin ground state of S = 12, D =
0.05 cm-1 a hysteresis at 0.55K and a magnetization energy barrier
at 9.6K. This is also the first Nickel based SMM reported.

Transition metal
SMMs seem to have either similar blocking temperature to the first Mn complex
which are low. Also, it is common to have antiferromagnetic coupling between
the metal centers17 which may hider their usefulness.

2.2.         
Microwave
heating

Synthesis of
molecular magnetic compounds using microwave heating is not very common, but it
is used in some extent.5 It can be used
in order to give a higher yield of products as well as to provide kinetic
products for some synthetic pathways. 
Using microwave heating the SMM compound Mn6O2(sao)6(O2CH)2-
(CH3OH)47 (saoH2 =
salicylaldoxime) had increased its yield and production time (80% in 24 hours versus
30% in 5 days). This experimental method shows promise and warrants further
research.

2.3.         
Polyoxometallate
templates

Another synthetic
method is the use of a polyoxometallate template that contains a cavity on the centre
to trap clusters of transition metal ions inside. An example of this kind of
structure is Cu20Cl(OH)24(H2O)12(P8W48O184)18. It has to be noted that the polyoxometallate itself
is not the SMM but the cluster that is in the cavity. Polyoxometallate templating is also successful in the
synthesis of lanthanide clusters such as the case of ErW10O369-
19 with the spin ground state of S = 15/2 and an energy
barrier of 55.2K.

This is promising
because it may give better synthetic control over the creation of a potential
SMMs and nanomagnets.

 

2.4.         
Giant
spin SMMs

Concerning the
structure of the compounds themselves one strategy employed is the synthesis of
molecules that have a very large spin ground state in order to have a strong
magnetic interaction. An example of this class of compounds is Mn3+12Mn2+7O8(N3)8(HL)
12(MeCN)6Cl2 (H3L =
2,6-bis(hydroxymethyl)-4-methylphenol) 
which has the spin of  S=83/2.
However due to the low energy barrier for the change of the magnetization
orientation it shows a hysteresis loop at 0.5K. The low blocking temperature
and energy barrier can be explained by the low anisotropy this compound has
because of the geometry of the molecule as the anisotropy for mixed oxidation
state Manganese compounds is from the Jahn-Teller distortion from Mn3+
and they need to be parallel with each other while they are not.

2.5.         
Heteronuclear
d block complexes

A promising class
of compounds that can be used are mixed d block transition metal complexes.
This can solve the problem that homonuclear metal complexes have when they
their spins couple antiferromagnetically and cancel each other out resulting in
lower or even zero spin ground states. Usually the different transition metals
are from different rows to be chemically distinguishable in crystallographic
techniques. An example of this compound is Ni{ReCl4(ox)}3420 with a spin ground state of S = 11/2 and an
anisotropy of D = 0.5 cm-1. The compound consists of a Ni(II) metal centre
and three Re(IV) metals outside bridged by oxalate ligands.

2.6.         
Lanthanide
complexes

Another
interesting class of compounds are lanthanide based complexes. These are
candidate material due to the high amount of f electrons that lanthanides
possess as well as the high degree of anisotropy their compounds can offer21. This class of compounds has seen a major
breakthrough with the synthesis of the Dysprocenium compound that has a
blocking temperature of 60K as well as high relaxation time at that temperature.
Apart from this recent breakthrough the literature has other complexes based on
lanthanide such as the polynuclear Dy complexes Dy8(OH)8(phendox)6(H2O)8Cl2(OH)2·18H2O·18MeOH
and Dy11(OH)11(phendox)6(phenda)3(OAc)3(OH)·40H2O22 which exhibit low relaxation time. Since the
lanthanide complexes exhibit desirable properties for SMMs research is
promising in this class of compounds.

2.7.         
Transition
metal lanthanide complexes

Heteronuclear d-f
complexes are also an area that is research for the synthesis of potential
SMMs. Like the heteronuclear d-d complexes they also attempt to solve the
problem of creating polynuclear complexes that result in a spin ground state of
S=0. Apart from that there is a degree of configurability in the spin ground
state and anisotropy due to the various combinations of metal lanthanide
complexes. In the literature there are numerous examples of d-f complexes. An
example of a d-f complex is the Copper Terbium complex CuLTb(hfac)2223 (H3L = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxy-benzylideneamino)
ethane and Hhfac = hexafluoroacetylacetone) with an energy barrier of 21K. It
has a low blocking temperature of 1.2K. Another example is the metallocrown
complex of {(GdCu5(Glyha)5(H2O)2)(GdCu5(Glyha)5(H2O)3)-(1,3-bdc)3·16H2O}n24 which is a polymeric structure that shows exchange
interactions between the monomers. This indicates the ability to synthesize d-f
polymers that are based on monomeric interactions.

3      
Aims of the project

This project
focuses on the d-f complex and f block lanthanide SMMs. The aim is to
synthesize d-f and f bock complexes with various combinations of transition
metals and lanthanides and then characterize them using x-ray diffraction and
superconductive quantum interference device (SQUID) magnetometry to obtain
structural and magnetic data and determine if they can be used as suitable SMMs
for various applications Since previous work in the Murrie group has demonstrated
that Cu(II)Gd(III) complexes using polyalcohol and polycarboxylic ligands can
be synthesized the aim is synthesize similar compounds based on this research.

4      
Conclusions

The field of
molecular nanomagnets is a promising one in terms of possible applications to
emerging technologies. From possible magnetic refrigerants, data storage
miniaturization and to radical improvements in the information processing with
the possibility of designing qbits and quantum logic gates the field has
sparked a high amount of interest. Apart from the technological application
molecular nanomagnets and SMMs can also be used to improve our understanding of
fundamental physics by investigating the quantum effects of magnetization. The
field also benefits from the fact that there are various ways of designing
nanomagnetic materials using different ways to synthesize a compound that can
have the desired properties adding a high degree of configurability and
potentially allowing the ability to be able to synthesize compounds that are
prepared for specific application. While theoretically very useful there are
certain limitations to be considered. One of them is the chemical nature of the
metals. Since different metals prefer different coordination environment some
compounds can be more challenging to make than others7. The other important
limitation in the field is the low blocking temperature. This makes any
potential application of the materials unlikely as the practical cost of
cooling is problematic. Recently however due to the breakthrough of the Dysprocenium
complex that has a 60K blocking temperature the possibility of practical
application starts to emerge as this demonstrates that significantly higher
blocking temperatures can be achieved. It is also noteworthy that a 60K can be
reached using liquid nitrogen that while not ideal for every application can
still be practically used. This has the potential for emerging technologies
based on these materials to be implemented in the near future and resulting in
an increased interest. However, most of the compounds still demonstrate low
blocking temperatures and thus more research is required in order to overcome
this obstacle. Additionally, even higher blocking temperatures should be
pursued in order to possibly develop compounds that are operable in room
temperature and higher. Overall this makes molecular nanomagnets a field that
is not sufficiently research and further research is required to be able to
synthesize desired compounds.

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