Between single ion magnets and macromolecules: a polymer/transition metal-based semi-solid solution

Article abstract of DOI:10.1039/C8SC02277A

The creation of functional magnetic materials for application in high-density memory storage or in the new field of molecular spintronics is a matter of widespread interest among the material research community. Herein, we describe a new approach that combines the qualities of single ion magnets, displaying slow magnetic relaxations, and the merits of polymers, being easy to process and widely used to produce thin films. Basing the idea on cobalt(ii) ions and pyridine-based single ion magnets, a new macromolecular magnetic material was obtained-a polymeric matrix of poly(4-vinylpyridine) (P4VP) cross-linked by a cobalt(ii) salt bound within it, effectively forming a network of single ion magnets, with field-induced magnetic relaxations preserved in both bulk and thin film forms. The binding of cobalt is confirmed by a series of methods, like secondary ion mass spectroscopy or high-resolution X-ray photoelectron spectroscopy. The magnetic relaxation times, up to 5 × 10^?6 s, are controllable simply by dilution, making this new material a semi-solid solution. By this approach, a new path is formed to connect molecular magnetism and polymer science, showing that the easy polymer processing can be used in forming self-organizing functional magnetic thin films.

Full text of DOI:10.1039/C8SC02277A

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Between Single Ion Magnets and macromolecules:
polymer/transition metal–based semi–solid solution
Anna M. Majcher*a, Paweł Dąbczyńskia, Mateusz M. Marzecb, Magdalena Ceglarskaa, Jakub Rysza,
Andrzej Bernasikb,c, Shin–ichi Ohkoshid, Olaf Stefańczyk*d
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Creation of functional magnetic materials for application in high–density memory storage or in the new field of molecular
spintronics is a matter of widespread interest among the material research community. Herein, we describe a new
approach that combines the qualities of Single Ion Magnets that display slow magnetic relaxations, and the merits of
polymers, easy to process and widely used to produce thin films. Basing the idea on cobalt(II) ions and pyridine–based
Single Ion Magnets, a new macromolecular magnetic material was obtained – a polymeric matrix of poly(4–vinylpyridine)
(P4VP) cross–linked by cobalt(II) salt bound within it, effectively forming a network of Single Ion Magnets, with field–
induced magnetic relaxations preserved in both bulk and thin film form. Binding of cobalt is confirmed by a series of
methods, like Secondary Ion Mass Spectroscopy or high–resolution X–Ray Photoelectron Spectroscopy. The magnetic
relaxation times, up to 5·10-6 s, are controllable simply by dilution, making this new material a semi–solid solution. By this
approach, a new path is formed to connect molecular magnetism and polymer science, showing that the ease of polymers
www.rsc.org/
processing
can
be
used
in
forming
self–organizing
functional
magnetic
thin
films.
barriers.[7, 8]
Introduction
Efforts have been made to incorporate such systems into
layers (mostly the first SMM Mn12Ac and its variations). To do
that, two main strategies can be employed: chemisorption and
physisorption. Historically first was the latter – Mn12 clusters
have been incorporated into Langmuir–Blodgett films,
exhibiting a hysteresis of molecular origin.[9, 10] Electrospray ion
beam deposition of Mn12 was done on insulating surfaces.[11, 12]
Deposition of Mn12 complexes was also done from solution by
Every technological revolution has its roots in basic research.
This thought has long been the dream fuelling the scientists
who devote their efforts to synthesize and study the
properties of low dimensional molecular magnetic materials
that display slow relaxations of magnetization:[1] Single Ion
Magnets (SIM),[2,3,4] Single Molecule Magnets (SMM)[5] and
Single Chain Magnets (SCM).[6] Such low-dimensional systems
can be based on not only the historically first transition metals,
like cobalt, but recent focus is on lanthanide-based SMMs that
tend to exhibit higher blocking temperatures and energy
a
series of methods (breath–figure or lithographically–
controlled wetting as well as chemisorption by chemical
modification of the complex and/or surface).[13] Topographic
pattern of a DVD was recreated using a solution of polymer
and Mn12 molecules.[14] Self–organized growth of Co
ferromagnetic clusters of roughly 15 atoms on Pt surface was
done by deposition from vapour phase.[15] Other than the one
mentioned[13] reported examples of using chemisorption to
form layers of low-dimensional molecular magnets are:
chemical grafting of Fe4 SMMs[16] or Tb–based SMMs[17] to
surfaces. TbPc2 SMMs have successfully been coupled to
carbon nanotubes by supramolecular interactions.[18] Surface
anchoring of Co(II) and Ni(II) complexes has also recently been
done.[19] The recent focus is not only on memory storage but
also, even more importantly, on molecular spintronics.
Our research, presented herein, takes a different approach: we
have combined the qualities of macromolecules that are
widely used to produce macromolecular thin films, and the
characteristics of Single Ion Magnets (SIMs) that exhibit slow
magnetic relaxations. This work proves that by incorporating a
transition metal–based SIM into a polymeric matrix we can
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Co(4vpy)2Br2 (2): the synthesis was analogical to the previous
case. After adding mmol of
4–vinylpyridine (215 μl) to a solution of 1 mmol CoBr2 in 5 ml
of ethanol, fine blue crystals formed almost instantly. Average
yield: around 73% based on amount of Co (≈ 315 mg).
preserve, or even enhance the magnetic relaxations crucial for
using the SIM as an information carrier, while placing it in a
material that can protect and stabilize the metal within and is
easy in processing. This was done both in bulk form, as well as
in the form of a macromolecular magnetic thin film. Although
in this particular case the temperature at which the relaxations
happen are low, and the relaxation times are yet far from
making the SIMs being able to store bytes of memory, the fact
2
Crystallites of
to assembly
2
1
were found to be much more stable in contrast
and even finely powdered do not change
that Single Ion Magnets and macromolecules together form a composition for several days in ambient atmosphere.
new class of materials is truly ground-breaking.
There have been numerous efforts to incorporate metals into
Crystal structures determination
diffraction data for and were collected at room
temperature on Rigaku R–AXIS RAPID diffractometer
. Single crystal X–Ray
polymers at the stage of polymerization, which prompted the
creation of a new class of materials, called metallopolymers.[20,
21]
1
2
a
There has also been a report of using block copolymers to
equipped with imaging plate area detector using graphite
monochromated Mo–Kα radiation (λ = 0.71075 Å). Single
crystals suspended in paratone–N oil were mounted with a
100 μm Dual Thickness Micro MountTM loop. The intensity data
were integrated by Rigaku RAPID AUTO. Structures were
solved by direct methods using SHELXS–97[26] incorporated in
the CrystalStructure 4.0 crystallographic software package[27]
and refined using a full–matrix least squares technique of
SHELXL–2014/7[28] included in the OLEX–2 1.2 software
package.[29]
form compounds with Prussian Blue Analogs,[22] they have,
however, never been studied from the point of view of their
magnetism, not to mention magnetic relaxations. Another clue
that prompted our research was the fact that P4VP–coated
carbon nanotubes have been used for magnetic wastewater
purification.[23]
Our approach was as follows: after synthesizing mononuclear
cobalt(II) compounds formed with the use of simple ligands, in
this case pyridine and 4–vinylpyridine, and confirming that
they are Single Ion Magnets, we employed an analogical
macromolecule with its structural units identical to the ligands
that form the studied SIM – the already polymerized poly(4–
vinylpyridine). This is a fundamental difference between the
material that we obtained and metallopolymers, as in the
latter the metal is incorporated at the stage of polymerization.
Table 1. Crystal data, data collection, and refinement
1
parameters for
and
2.
1
2
Experimental
Molecular formula C10H10Br2CoN2
C14H14Br2CoN2
Materials. All reagents were purchased from Sigma Aldrich or
Alfa Aesar and used without further purification. Anhydrous
cobalt(II) bromide was handled in inert atmosphere due to
extreme sensitivity to humidity.
Mr [g·mol-1]
T (K)
376.95
296(2)
429.02
296(2)
Caution! Cobalt(II) bromide as like as 4–vinylpyridine are toxic
and should be handled with care.
Radical used, λ (Å)
Mo Kα (0.71075)
Syntheses of mononuclear compounds. Both syntheses were
inspired by an already long–known synthesis.[24]
Crystal system, Monoclinic; P 21/c
space group
Monoclinic; C 2/c
Co(py)2Br2 (1): 1 mmol of anhydrous CoBr2 (218.7 mg) was
dissolved in 5 ml of absolute ethanol resulting in a clear blue
solution. 2 mmols of pyridine (161 μl) were then added. After
about ten minutes, dark blue crystals began to form. Product
was collected by filtration and washed with small quantity of
cold absolute ethanol. Average yield: around 80% based on
amount of Co (≈ 300 mg). Pristine product was used
immediately for further experiments to prevent from eventual
change of composition. Crystallites of 1 are stable in ambient
conditions for a long time, however, finely powdered assembly
quickly absorbs humidity which results in the formation of
pink–coloured powder of Co(py)2(H2O)2Br2.[25]
a [Å]
b [Å]
c [Å]
8.6842(10)
18.1757(18)
8.5209(8)
9.1275(7)
12.1835(8)
14.8264(10)
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α [°]
90
90
Largest diff. peak / 0.58 / -0.41
hole (e·Å-3)
0.29 / -0.25
β [°]
100.878(7)
90
95.764(7)
90
Non–hydrogen atoms were refined anisotropically while
hydrogen atoms were positioned with an idealized geometry
and refined using a riding model. Crystal data, data collection,
γ [°]
and refinement parameters for 1 and 2 are listed in Table 1.
The structural data presented as figures were prepared with
the use of the CCDC Mercury 3.9 visualization software.[30]
V [Å3]
1320.8(2)
4, 1.896
7.320
724
1640.4(2)
4, 1.737
5.906
Z, dcalcd (g·cm-3)
μ (mm-1)
F(000)
Powder X–ray diffraction patterns of the polycrystalline
samples
of
1
and
2
were
collected
on
a Rigaku Ultima–IV equipped with Cu–Kα radiation (λ = 1.5418
Å). For these measurements, single crystals of
ground and packed into the sample holder.
1 and 2 were
836
Syntheses of bulk P4VP with CoBr2: Adequate amounts of
P4VP and CoBr2 were dissolved in anhydrous methanol
separately (in concentrations no higher than 50 mg/ml for the
polymer and 5 – 20 mg/ml for the salt). Then, while vigorous
stirring, the cobalt salt solution was pipetted into the polymer
solution. Immediate precipitation of threadlike blue product
was visible. Left to settle for at least one hour at room
temperature, the product was then collected by means of a
pipette from the bottom of the vial and dried under lowered
pressure for several hours. The resulting blue powder, the
colour intensity of which depended on the cobalt
concentration, was stable in ambient conditions in the vast
majority of cases, regardless of the time left in air.
2θ range for data 5.276 – 54.912
collection (°)
5.524 – 54.932
Index ranges
-11 ≤ h ≤ 11
-11 ≤ h ≤ 11
Reflections
collected / unique
6969 / 2987
12807 / 1870
(Rint = 0.0742,
Rsigma = 0.1003)
(Rint = 0.0449,
Rsigma = 0.0273)
Microelemental analysis and spectroscopic methods.
Elemental analysis was performed by the standard
microanalysis on Elementar Analysensysteme GmbH: vario
MICRO cube and ICP+MS was done by means of Agilent 7700
inductively coupled plasma mass spectrometer (ICP+MS).
Infrared absorption spectra were recorded with a JASCO IRT–
3100 FT–IR microscope.
Refinement
method
Full–matrix least–squares
Data / restraints / 2987 / 0 / 136
1870 / 0 / 105
1.107
Polymer and P4VP–CoBr2 structure modelling and
visualization. Possible molecular arrangements for P4VP and
P4VP with CoBr2 were obtained by using the 3D optimization
algorithm implemented in the ACD/ChemSketch software.[31]
Geometry optimization of one P4VP chain coordinating to one
CoBr2 unit was performed using ArgusLab software.[32] This
was done by fixing the number n of structural units between
the units coordinating to Co and calculating the minimum
potential energy of all implemented atoms and bonds. The
Hamiltonian used was Molecular Mechanics UFF (Universal
Force Field) available within the software. Attempts to
perform simulations for larger numbers of P4VP chains and/or
CoBr2 units did not deliver any reliable results (the structures
did not converge), however, we believe that the simple model
used is sufficient to explain the findings presented within this
Article. Results of the calculations for n = 0 to 6 and structure
visualizations can be found in ESI in Supplementary Table 4.
Goodness–of–fit
on F2
1.250
Final
(I>2σ(I))
R
indices R1
=
wR2 = 0.0813
0.1064, R1
= 0.0515,
wR2 = 0.0651
R indices (all data) R1
=
wR2 = 0.0925
0.1847, R1
= 0.1037,
wR2 = 0.0753
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Chemical structures were visualized using CCDC Mercury 3.9 microelemental analysis and ICP+MS to show perfect
software.[30]
agreement (see ESI for details).
Preparation of thin macromolecular magnetic films. Thin films Atomic Force Microscopy. AFM images of film surface
of P4VP were obtained by spin–casting (2000 rpm) a 20 mg/ml topographies were measured at room temperature with
polymeric solution in chloroform onto cleaned silica wafers. Nanosurf FlexAFM microscope working in contact mode.
Several resulting layers were then treated with acetonitrile Setpoint and gains were adjusted to each measurement to
(previously tested not to dissolve the polymer) for 60 seconds obtain a clear image without noise. Each film topography was
to ensure that the final effect is not caused by the orthogonal investigated by collecting images at several randomly chosen
solvent alone. Several P4VP layers were immersed in a 20 areas. The images were processed using the WSxM 4.0
mg/ml CoBr2 solution in acetonitrile for 60 seconds and then software.[36]
rinsed with pure acetonitrile. The film was then dried in inert
atmosphere. The set of all films: as cast, treated with
X–ray Photoelectron Spectroscopy. The XPS analyses were
carried out in a PHI VersaProbeII Scanning XPS system using
monochromatic Al Kα (1486.6 eV) X–rays focused to a 100 µm
spot and scanned over the sample area of 400 µm x 400 µm.
The photoelectron take–off angle was 45° and the pass energy
in the analyzer was set to 23.50 eV to obtain high energy
resolution spectra for the C 1s, N 1s and Co 2p regions. A dual
beam charge compensation with 7 eV Ar+ ions and 1 eV
electrons were used to maintain a constant sample surface
potential regardless of the sample conductivity. All XPS spectra
acetonitrile alone and CoBr2 solution–treated was examined
by all the available methods except for magnetic
measurements, for which the thin polymeric film was
produced using a home–built H–dipping[33] setup on thin
plastic foil to minimize the diamagnetic contribution to the
signal. The thin film area was approximately 5 times 10 cm2,
later treated with CoBr2 acetonitrile solution identically as in
the previous case.
Magnetic measurements. All magnetic measurements were were charge referenced to the unfunctionalized, saturated
performed using a Quantum Design MPMS 5XL system, with carbon (C–C) C1s peak at 284.8 eV. The operating pressure in
the available range of temperatures 1.8 – 400 K and fields up the analytical chamber was less than 4x10-9 mbar.
to 50 kOe. All dry bulk products – SIM compounds and Deconvolution of spectra was performed using PHI MultiPak
macromolecular magnetic materials were pressed inside software (v.9.7.0.1). Spectrum background was subtracted
gelatine capsules and fitted into measurement straw without using the Shirley method.
using additional adhesives. The measurement of thin
macromolecular magnetic film was performed by compressing
Time–of–Flight: Secondary Ion Mass Spectrometry. The TOF–
SIMS experiments were performed on an ION:TOF TOF:SIMS V
(Munster, Germany) instrument, equipped with liquid metal
ion source and caesium ion source. Static SIMS measurements
a layer of size 5 cm x 10 cm produced by H–dipping on plastic
foil into a size suitable to fit within the plastic straw used for
standard measurements. All DC measurements were corrected
for the diamagnetic contribution of the sample holders and
constituent atoms (Pascal’s tables).[34]
+
were carried out with the use of Bi3 30keV ion beam. Both
negative and positive spectra were collected. Depth profiles of
samples were obtained in dual beam mode. 1keV Cs+ ion beam
+
Frequency dependence of the AC susceptibility was analysed was used to sputter a 500x500 μm2 area and Bi3 30keV ion
using the following Cole–Cole model:[35]
beam was used to analyse a 100x100 μm2 area concentric to
the sputtered surface.
ꢀꢄꢅ ꢀꢃ
ꢀ = ꢀꢁ − ꢂꢀꢁꢁ = ꢀꢃ + ꢆꢇ ꢂꢉꢊ
,
ꢋꢆꢌꢍ
(1)
ꢈ
Ellipsometry. The thin film thicknesses were measured by a
spectroscopic ellipsometer (SENTECH SpectraRey/3) equipped
with a micro spot. The ellipsometric angles Ψ and Δ were
determined for wavelengths λ in the spectral range between
320 and 800 nm. The measurements were taken at angles of
incidence and detection of 70°. The polymer layers were
modelled as a Cauchy layer of thickness d with a refractive
index n(λ) of the form:
that was fitted to the in–phase and out–of–phase
magnetization simultaneously in each case. The resulting
values of τ were then plotted as their logarithms versus inverse
temperature, and a weighted linear fit of the Arrhenius law
was performed for each case, thereby yielding the values of τ0
and the heights of the energy barrier for specific
concentrations of Co atoms within the macromolecular matrix.
All the data obtained from magnetic measurements were
processed using OriginPro 9.1 software.
ꢎ ꢏ = ꢎꢐ + ꢎꢆ ꢏꢅꢑ + ꢎꢑ ꢏꢅꢒ
ꢈ ꢋ
.
(2)
Layer thickness and the parameters n0, n1 and n2 were varied
numerically in order to achieve the best agreement between
the model and the measured values ∆(λ) and Ψ(λ). The
thickness of silicon oxide layer on the top of substrate was
measured independently and accounted for in the model.
The concentration of Co for each sample was estimated by
assuming low–temperature magnetization (1.8 K) in 50 kOe
identical to the total magnetic moment value produced by
CoBr2(py)2 in the same conditions. The validity of this method
was cross–checked with the concentrations calculated from
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Figure 1 in ESI). It is worth to emphasize that in both structures
there are no crystallization solvent molecules. Furthermore,
none of the assemblies show any hydrogen bonds, π–π or
anion–π interactions that would form a long-range network,
which is uncommon for molecular materials with aromatic
organic ligands, and which provides good isolation of the single
molecules from each other within the crystal.
Results and discussion
Syntheses of the mononuclear compounds. Starting from
cobalt (II) salts, pyridine and 4–vinylpyridine, two mononuclear
compounds were synthesised (1 and 2). Their synthesis has
been known for many years,[37] however, they have never
been investigated for the occurrence of magnetic relaxations
despite their tetrahedral geometry.[4] The scheme of the
synthesis and the structure visualizations of the obtained
compounds can be found in Figure 1 on the left. For details of
the synthesis and structure visualization, see Experimental
Magnetic properties of the mononuclear compounds.
Measurements of alternating current (AC) magnetic
susceptibility proved the existence of field–induced (HDC = 2.5
kOe) slow magnetic relaxations, typical for Single Ion Magnets
(Supplementary Figure 3, ESI). Cole–Cole model was fitted to
the real and imaginary component of AC magnetic
Section. Single crystal structures of compounds
of mononuclear distorted tetrahedral CoL2Br2 complexes,
where L is pyridine for and 4–vinylpyridine for (a more
detailed structural diagram can be found in Supplementary
1 and 2 consist
1
2
susceptibility χ’ and χ’’ simultaneously, delivering the height of
Figure 1 Synthetic routes for obtaining the described materials: combining CoBr2 salt (middle) with pyridine leads to mononuclear compound 1 (top left arrow) and with 4–
vinylpyridine to mononuclear compound 2 (bottom left arrow). Reaction of poly(4–vinylpyridine) with CoBr2 leads to the formation of the new macromolecular magnetic material
(right arrow). Structures of 1 and 2 were obtained via single crystal XRD measurements while the P4VP and P4VP–CoBr2 were modelled using geometry optimization algorithm in
ACD/ChemSketch software. Hydrogen atoms are omitted for clarity.
the energy barriers equal to 28(2) and 35(3) K, and relaxation
time τ0 values of 8·10-10 s and 1·10-11 s for
1
and
2, respectively.
The
times ranged from 0.13 to 0.28 and from 0.027 to 0.36 for
and
Single Ion Magnets (for details see ESI).
α parameter that describes the distribution of relaxation
1
2, respectively, which is well within the range typical for
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Syntheses of the macromolecular magnetic material. The structural units per one CoBr2) to almost 4.0 cm3Kmol-1 for
same approach was undertaken to prepare magnetic lower concentrations of cobalt (more than 10 P4VP structural
a
macromolecular matrix (synthesis visualized in Figure 1, right). units per one CoBr2). Both these observations can be explained
Structural characterization was not possible in this case as by the fact that polymer chains become elastic at high
there is no ordered crystal structure
– powder X–ray temperatures, which allows the anisotropic cobalt ions to
diffraction showed a single amorphous peak (not shown). The orient towards the direction of the applied magnetic field. This
visualizations of P4VP and P4VP with CoBr2 in Figure 1 were hypothesis would also explain the higher χT values for lower
obtained by geometry optimization (for details, see cobalt concentrations – the less Co atoms cross–link the
Experimental Section and ESI). Detailed calculations performed polymeric matrix, the less rigid the network is and the more
on a simple model of one CoBr2 unit coordinating to one P4VP freely the cobalt ions can orient towards the field direction
chain in different geometries (varying number of structural without breaking the chemical bonds in the structure. This is
units between the units coordinating to the Co atom) showed suppressed at low temperatures (below 100 K), where the
that concentrations higher than ca. 4 P4VP structural units in polymer chains freeze and their elasticity decreases. It should
one chain coordinating to one cobalt would be energetically be noted that the decrease in χT for all the compounds
unfavourable. This explains the results of the reactions in high observed at low temperatures is typical for all Co(II)
Co:P4VP structural units molar ratio, resulting in the maximal compounds and is due to single–ion anisotropy.[41]
achievable effective cobalt concentration no higher than one
cobalt per ca. 5 P4VP structural units (for details of the
performed syntheses, see Experimental Section and ESI). The
solution of cobalt salt added to a solution of the poly(4–
vinylpiridine) in the same solvent causes an immediately
visible precipitation of blue threadlike product indicating
cross–linking of the polymer. The product of the reaction –
bright blue powder – by colour itself suggests binding of the
cobalt ion in tetrahedral geometry,[38] the same as in the
mononuclear compounds. The material, dried under lowered
pressure, is stable in ambient conditions, which is another very
important feature of this substance, as tetrahedral cobalt
complexes are often unstable in air and susceptible to oxygen
and moisture. The reaction was performed with varying molar
ratios (from 1 mole of cobalt salt per two moles of polymer
structural units up to 1:40). As the amount of the cross–linked
polymer was difficult to control, the cobalt concentration in
the final product was in each case estimated by comparing the
low–temperature high–field magnetization with the value
produced by 1. The validity of this method of Co amount
estimation was checked by microelemental analysis and
ICP+MS (for details, see ESI).
High resolution X–ray Photoelectron Spectroscopy of the
bulk. Doping the polymer with CoBr2 produces a new, highly
energetic nitrogen N 1s peak at binding energy about 400 eV
(Figure S5, ESI) which has previously been detected for P4VP
doped with either Pd[39] or Pt[40] complexes, and was ascribed
to nitrogen atoms coordinated with these metals.
Magnetic properties of the macromolecular magnetic
material. Magnetic measurements for a series of P4VP with
CoBr2 both in constant and alternating magnetic fields
revealed several surprising features. Magnetization measured
in
HDC = 1 kOe, depicted as χT per one cobalt(II) ion vs
Figure 2 on the top show that, compared to the results for
and
.
T
in
1
2
(
χT at room temperature equal 2.4 and 2.5 cm3Kmol-1,
respectively, within the range typical for similar SIMs),
However, for the macromolecular magnetic material χT
behaves in a different way, gradually reaching higher values at
higher temperatures. What is even more interesting is that
there is an abrupt change from around 3.0 cm3Kmol-1 at room
temperature for high cobalt concentrations (below 10 P4VP
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AC magnetic measurements revealed other interesting
features of this material. Firstly, the field–induced (HDC = 2.5
kOe) magnetic relaxations of Co(II) are indeed preserved
within the polymeric matrix, with one slight, but very
important improvement – a visible in a drop in the
α
parameter, which in every case stayed within the range 0.15 –
0.20. This in average is comparable or lower with the values
for 1 and 2. This means that despite a completely disordered
structure of the polymeric matrix, the distribution of the
relaxation times for all Co(II) centres is very narrow. The
second intriguing property of this new material is that with
lowering concentration of the cobalt centres within the
polymer, a significant increase of the relaxation times can be
observed.
Figure 2 Magnetic properties of the bulk new material: results are presented for bulk
P4VP with CoBr2 in varying P4VP structural units:Co molar ratios. Top: ꢓT vs.
T
(measured in 1 kOe) additionally compared with the same data for 1 and 2. Bottom: 3D
plot of AC magnetization measured in DC field of 2.5 kOe at four different frequencies
as a function of temperature and of the molar ratio of P4VP structural units per one
CoBr2. HAC = 3 Oe in each case. Top inset: phase diagram of ꢔꢕvalue and height of the
energy barrier as a function of the P4VP structural units per one CoBr2 molar ratio.
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Figure 3 Properties of thin films of the macromolecular magnetic material compared with films of P4VP as cast and treated with the orthogonal solvent only: representative
results of Atomic Force Microspcopy measurements (left column), Secondary Ion Mass Spectroscopy (middle column) and high resolution X-Ray Photoelectron Spectroscpy N 1s
spectra (right column) for thin films of: P4VP as cast (top row), P4VP treated with acetonitrile alone (middle row) and P4VP film treated with CoBr2 acetonitrile solution (bottom
The results of the AC magnetic measurements in the DC field
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of 2.5 kOe are gathered in Figure
2 (bottom). Each wrinkling of a cross–linked rigid layer on a layer of a softer
measurement of mAC versus temperature was performed at polymer.[44,45]
four different frequencies of the alternating magnetic field.
The difference between the values of mAC’ for different
Secondary Ion Mass Spectrometry (SIMS) of the thin films.
frequencies and the value of mAC’’, small for high Co
concentrations, becomes notably larger with increasing Co
dilution. This means that at low temperatures the lower the
concentration of Co centres within the polymer, the slower the
magnetic relaxations become. Such features are often seen in
the so–called solid solutions.[42] However, as our material does
not have an ordered crystal structure, we will call it a semi–
solid solution – as we are on the border of soft matter and
ordered molecular compounds. Fits of the Arrhenius law to
plots of the natural logarithm of relaxation times (each value
being a result of a Cole–Cole model fit to isothermal AC
susceptibility frequency dependence – see Supplementary
The SIMS data for the obtained films deliver a confirmation
that CoBr2 is bound to the pyridine rings within the polymeric
layer (see Figure 4, middle column). There is a notable increase
-
of CoBr2N2 units in CoBr2 solution–treated films compared to
the films as cast and treated with acetonitrile. This suggests
binding of the cobalt centre to the nitrogen atoms found in the
pyridine structural units. This is supported by a sharp drop in
the number of unbound pyridines (seen as the decrease of
C5N- units in CoBr2 solution–treated films compared to
untreated samples). For more results of the SIMS
measurements, please see ESI. Depth profiling shows that only
the surface layer is occupied by cobalt ions (no more than 10%
of the total film thickness in this case – thickness was
determined by an ellipsometry measurement, for details see
Experimental Section and ESI).
Figure
interesting results concerning the height of the energy barrier
kB and the τ0 parameter – depicted as the inset in Figure 2
4 in ESI) versus inverse temperature produced
Δ
/
(top). The pattern that emerges is as thus: decreasing cobalt
concentration has a much less significant influence on the
height of the energy barrier, which keeps the same order of
magnitude and shows no monotonous change, than on the
relaxation time – at higher ratios of P4VP structural units per
one cobalt ion, the value of τ0 increases significantly. Similar
High–resolution X–Ray Photoelectron Spectroscopy of the
thin films. The XPS data provided an insight into the chemical
bonds within the obtained magnetic macromolecular films. An
example of the obtained high–resolution XPS spectra for
nitrogen 1s regions can be seen in Figure 4 in the right column.
A modification of the N 1s band in the sample treated with the
cobalt salt solution compared with the samples as cast and
treated with the orthogonal solvent only – which agrees with
the data obtained for the bulk product (Supplementary Figure
7, ESI) – is consistent with Co atoms coordinating nitrogen in
pyridine units. For a more detailed insight into the XPS results,
please see ESI.
effect
has
already
been
observed
for
K(Ph4P)[Co0.06Zn0.94(OPh)4], which required dilution with
diamagnetic species to induce slow magnetic relaxations.[43]
Here, the mechanism is different in the sense that the polymer
chains are flexible – therefore, they can effectively adjust to
coordinate the CoBr2 units in almost any way, and no
diamagnetic salt is needed in order to achieve the desired
dilution.
Preparation of thin macromolecular magnetic films. As the
thin
Magnetic properties of the macromolecular magnetic thin
film. To investigate the magnetic properties, a large–area P4VP
film was produced on thin plastic foil and treated with CoBr2
solution to ensure the amount of cobalt needed to produce a
satisfactory signal. Measurements of AC magnetization
delivered a confirmation that the cobalt centres within the film
show field–induced (HDC = 2.5 kOe) magnetic relaxations
(Figure 4). The relaxation times derived from Cole–Cole model
main goal of this research was obtaining
a
macromolecular film that would combine the magnetic
properties of SIM compounds with the qualities of polymers
(such as ease of processing), the same reaction was
reproduced as described herein. Because the product of the
previous synthesis is a cross–linked polymer network which is
impossible to dissolve without its destruction, a different
approach was employed. First, we produced thin films of the
polymer alone. This step was followed by immersing the
produced layers in a solution of cobalt salt in a solvent
orthogonal for the polymer (acetonitrile). The layer was then
washed with the same solvent to ensure that no unbound
cobalt salt remained in the produced films. To exclude the
effect of the solvent, a layer of P4VP treated with acetonitrile
only was also produced for comparison.
fits plotted logarithmically as
a function of inverse
temperature (Figure 4 inset) with a linear fit performed on
them deliver a rough estimate of the τ0 value of 3·10-6 s and
the height of the energy barrier 8(2) K, the order of magnitude
of which stays within the pattern that emerged for the bulk
samples. The considerable errors are caused by the fact that
this measurement was done on the verge of the
magnetometer sensitivity (10-8 emu compared with the 10-7
emu signal), however, the qualitative confirmation that field-
induced relaxations are preserved also in the form of a thin
film is undeniable. In order to probe the electronic structure of
such thin films it would generally be useful to employ X-ray
absorption spectroscopy and its derived methods (X-ray
Magneto-Chiral Dichroism and X-ray Natural Linear Dichroism)
to deliver information on both orbital and spin components of
the magnetic moment – however, the short relaxation times
will most probably prevent getting reliable information with
these methods.[46] Using our data, it is also impossible to
obtain information on the orientation of magnetic centres
Atomic Force Microscopy (AFM) of the thin films. AFM was
used to characterize the films topography. As can be seen in
Figure 3 in the left column, the films processed with the use of
the cobalt salt reveal a tenfold increase in the surface
roughness (see Supplementary Table 5 in ESI for roughness
analysis results), compared to the P4VP as cast and treated
with acetonitrile only. The pasta–shaped features in the
topography of the CoBr2–doped P4VP film are consistent with
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within the layer, however, it is highly unlikely that the
produced films are epitaxial. To obtain films with a more
oriented magnetic structure is another challenge which we will
attempt to solve in near future – the research presented
herein is just a starting point.
Conflicts of interest
The authors declare no competing financial interests.
Acknowledgements
This research was financed by the Polish National Science
Centre
within
the
SONATA
Project
UMO–
2015/19/D/ST5/01936. The authors express their gratitude
towards Michał Rams (Institute of Physics, Jagiellonian
University, Krakow, Poland) for valuable discussions about the
magnetism of the described materials. The present research
was partially supported by the Grant–in–Aid for Specially
Promoted Research of JSPS (JSPS Kakenhi grant number
15H05697), Grants-in-Aid for Scientific Research on Innovative
Areas Soft Crystals (area No. 2903, 17H06367) and Advanced
Photon Science Alliance (APSA) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan (MEXT). The
Global Science course from MEXT, the Cryogenic Research
Center in The University of Tokyo, and the Center for Nano
Lithography & Analysis in The University of Tokyo supported by
MEXT are acknowledged.
Figure 4 AC magnetization measured in HDC = 2.5 kOe versus AC field frequency for a
thin film of P4VP treated with CoBr2 solution. HAC = 3 Oe. Solid lines represent
respective Cole–Cole model fits performed simultaneously for mAC' and mAC''. Inset:
relaxation times obtained from the Cole–Cole fits as a function of inverse temperature
with a linear Arrhenius law fit.
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