Characterization of the magnetic properties of Fe[(Htrz) 2(trz)](BF4)
using quartz tuning forks
Nicholas J. Lavini1
Carrie Schindler2, Elisabeth S. Knowles, Daniel M. Pajerowski, and Mark W. Meisel
Department of Physics, University of Florida, Gainesville, FL 32611-8440
7/27/09
Abstract
The material, Fe[(Htrz)2(trz)]BF4, has been of special interest in the scientific
community over the last few decades due to its unique spin transition properties near room
temperature. Analysis of the magnetic properties of Fe[(Htrz)2(trz)]BF4 transpired from the use
of quartz tuning forks commonly found in wrist watches. The resonant frequency and quality
factor values were determined for each fork in the presence of zero magnetic field. The material
was tested in two different magnets with field strengths ranging from 0 T to 1 T. Profiles of the
magnetic fields were acquired by using a Toshiba Hall Sensor THS118. With a known magnetic
field strength, the researchers compared resonant frequencies and quality factor values of the
forks with and without Fe[(Htrz)2(trz)]BF4 nanoparticles attached. Data were also acquired from
the material using a superconducting quantum interference device (SQUID) magnetometer. The
magnetic susceptibility (χ) was then calculated along with the magnetic moment for the
micrograms of Fe[(Htrz)2(trz)]BF4 placed on the fork.
1
2
Department of Physics, Manhattan College, Riverdale, NY 10471
Department of Physics, University of Evansville, Evansville, IN 47722
Introduction
Spin is an intrinsic property of all electrons, usually equated to angular momentum. An
electron can be in one of two spin states: up or down. The magnetic properties of matter are
determined by these spin states. Electrons orbiting molecular nuclei must be aligned in the same
direction for a material to be magnetic. The alignment of electron spins produces an overall spin
for the molecule. Molecules must then align with other molecules in the substance to produce a
magnetic effect. Groups of molecules form domains. The total magnetic spin of the domains
must line up with each other in order for a material to be magnetic [1]. Solids have the best
chance at being magnetic because of their rigid structure of molecules. The molecules in most
liquids and gasses have too much freedom of motion and rarely remain aligned.
Spin transition compounds, discovered about fifty years ago, have provided a new area of
chemistry/physics exploration, especially in the past two decades. These compounds possess the
unique quality of occupying a low spin or a high spin state depending on variations of
temperature or pressure [2,3]. The resulting microscopic spin of the particles provides the
macroscopic magnetic properties of the material, which can be diamagnetic, paramagnetic, or
ferromagnetic. These properties can be incorporated into new devices such as switches and
sensors.
Spin crossover compounds containing iron(II) are of special interest to the science
community because these compounds exhibit spin transitions induced by the irradiation of light.
Unfortunately, this reaction only occurs at temperatures below 50 K, posing a serious problem to
the practical applications of this material [4, 5]. We researched Fe[(Htrz)2(trz)]BF4. The
magnetic effects are associated with the iron atoms concentrated in the center of the structure as
seen in Figure 1. After some adjusting using chemical methods, the spin transition of iron(II)
2
compounds can be shifted close to room temperature. A
thermochromic effect is seen when the transition from low to high
spin occurs. The material changes color from purple to white.
Regrettably, after many cycles and/or excessively heating the sample
a shift is observed at the transition temperature and hysteresis
curve [2].
Experiment
In our experiment, the set-up is designed to test the spin
Figure 1: (taken from [6]):
Molecular structure of
Fe[(Htrz)2(trz)]BF4.
transition material on quartz tuning forks [7] at their resonant
frequencies. A clean tuning fork will be passed through a magnet
while taking data on the amplitude of the fork’s vibration at millimeter intervals. These data will
provide a base to compare the forks with the material on them. A frequency generator (Agilent
33220A) drove the fork while a lock-in amplifier (Stanford Research Systems SR530) was
utilized to record the amplitude of the oscillations (Figure 2). At the resonant frequency, the
amplitude suddenly increases projecting a sharp peak onto the graphing program connected to
the amplifier. After fitting the graph to a Lorentzian curve, we determined the exact resonant
frequency (f0), along with
the full-width-halfmaximum (Δf). The fullwidth-half-maximum is a
measure of the width of
the resonant peak at the
value equal to one half of
Figure 2: Circuit diagram for measuring tuning fork resonance.
3
the amplitude of the resonant frequency. We then determined the quality factor (Q) of the tuning
fork (f0/ Δf). The quality factor is the ratio of the reactance to the resistance of an electric circuit.
A high Q value describes a circuit where energy is dissipated slowly, while a low Q valued
circuit loses energy rapidly. When purchased, the tuning forks are enclosed in a canister
containing a very low pressure. This pressure value produces a very high Q value for the forks.
Once removed the Q value decreases and the resonant frequency is also shifted, as depicted in
Figure 3, meaning that a loss in signal resolution occurs when opening the forks. The loss of
resolution could be caused by the fact that the pressure in the can is much lower than the
atmospheric pressure outside the can. The resolution could also decrease in the process of
opening the fork. In order to open the canisters we used a pair of diagonal pliers. The pliers
were used to gently break the seal holding the can to the base of the fork. This procedure
required some force and very well could have damaged the forks. The application of particles in
nail polish also contributed to the decreased resolution. The change in resolution could also have
arisen from a combination of these reasons.
The magnetic
250
Large Fork out of Can
field was found by using
200
Large Fork in Can
a Toshiba Hall Sensor
THS118 to record field
strength at different
points along the axis.
The Hall sensor operates
maximally when a current
of 5 mA is passed
Amplitude (mV)
150
100
50
0
32.754
32.756
32.758
32.760
32.762
32.764
32.766
32.768
32.770
Frequency (kHz)
Figure 3: Resonance peaks for an opened and unopened fork.
4
through it. In order to achieve this scenario, the sensor was connected in series with a resistor of
know resistance (2 kΩ). A variable power supply drove the circuit, with digital multi-meters
measuring the voltage across the two elements. Applying Ohm’s Law permitted the calculation
of the current (Figure 4). A voltage of 12.7 V powered the circuit with a 5 mA current. With a
known Hall voltage the magnetic field strength could be calculated at each point, forming a
profile of the magnetic field. Knowing the field profile allows comparison of field strength to
any change/shift in the resonant amplitude. The material was tested in a 0.3 T magnet and a 1 T
superconducting magnet (Oxford Teslatron) at room temperature (Figure 5). The material was
also analyzed with a superconducting
quantum interference device
(SQUID) magnetometer. The
SQUID acquires data utilizing small
magnetic fields at a range of
temperatures (Figure 6).
Data
Figure 4: Circuit diagram for the Hall sensor.
The tuning forks were then
Outside
Magnet
Inside Magnet
Figure 5a:
Field strength as a function of
distance for the 0.3 T magnet.
5
Figure 5b:
Field strength as a function of
distance for the 1 T magnet.
loaded with Fe[(Htrz)2(trz)]BF4
0.4
1.0
0.3
0.8
Magnetic Field B (T)
Magnetic Field B (T)
0.2
0.1
0.0
-0.1
-0.2
0.6
0.4
0.2
-0.3
-15
-10
-5
0
5
10
15
0.0
20
-25
-20
-15
-10
-5
0
5
10
15
20
25
Z-axis distance from center (cm)
Z-axis distance into magnet (mm)
nanoparticles. The particles were
-6.0
Moment (emuG) x 10
-5
used in solution with clear nail
-6.2
polish (Sally Hanson: Hard as
-6.4
Nails). A needle was dipped into
-6.6
the well mixed solution and then
-6.8
used to apply a small amount of
Moment (emuG) (300 to 100)
Moment (emuG) (100 to 310)
substance to each prong of the
H=1T
-7.0
100
125
150
175
200
225
250
275
Temperature (K)
300
fork individually.
Figure 6: Magnetic moment as a function of temperature for
Fe[(Htrz)2(trz)]BF4 taken in a Quantum Design MPMSXL
SQUID magnetometer in a 1 T field.
The fork was left to dry
overnight. Once the fork was loaded with nanoparticles, the fork was placed in the same circuit
and run through both magnets. The frequency and amplitude were again measured at different
magnetic field strengths. The runs were graphed in a waterfall style to see if the resonant
frequency shifted at all as observed in Figure 7. A graph of a clean fork would have all resonant
frequencies equal, along with equal amplitudes. The particles did not really shift the resonant
6
4.0
-0.2459 T
-0.2443 T
-0.2366 T
-0.2222 T
-0.1653 T
0.0796 T
0.3054 T
0.3181 T
0.3210 T
0.3265 T
0.3442 T
0.3652 T
0.37078 T
3.6
3.2
Amplitude
2.8
2.4
2.0
1.6
1.2
32000
32100
32200
32300
32400
Frequency (Hz)
frequency as much as we had hoped. However, the Q values varied greatly. The magnetic force
on the particles must have affected the fork in order to explain this variation.
Results
The next step was to analyze whether or not the Q values changed and whether or not any
hysteretic behavior was observed from the fork. Fitting and graphing the frequencies and Q
values against field strength was where we hoped to see results (Figure 8a). Unfortunately, the
data did not show a hysteresis loop in the 0.3 T magnet. The gradient may have also been
responsible for providing the force on the particles that caused the frequency shifts. Similarly to
the magnetic field strength, the graphs of resonant frequency and Q against B*(dB/dZ) did not
7
clearly show a hysteresis loop (Figure 8b). While taking data we observed major interference
2500
Q (In) (1)
Q (In)
Q (Out)
2500
Q (Out) (2)
2000
1500
1500
Q
Q
2000
1000
1000
500
500
(f0(B)-f0(0))/f0(0) in
(f0(B)-f0(0))/f0(0) out
(f0(B)-f0(0))/f0(0) in
(f0(B)-f 0(0) )/f 0(0) x 10 -3
(f0(B)-f0(0))/f0(0) x10
-3
2
1
0
-1
-2
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
2
(f0(B)-f0(0))/f0(0) out
1
0
-1
-2
-0.3
Magnetic Field B (T)
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
B*(dB/dZ) (T2/m)
from the environment. The scatter in the data caused by the noise from the lab may well have
1000
affected the data to a
Q in
Q out
degree. We then moved
Q
900
800
to the 1 T magnet and
( f 0(B)-f 0(0) ) /f 0(0) x 10 -3
700
-0.50
-0.52
preformed the same
(f0(B)-f0(0))/f0(0) in
(f0(B)-f0(0))/f0(0) out
analysis (Figures 9a and
-0.54
9b). Since the 1 T
-0.56
-0.58
-0.60
0.0
magnet has an enclosed
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Magnetic Field (B)
coil, some of the
Figure 9a: Resonant frequency in field divided by resonant frequency with no
field as a function of magnetic field strength. Quality factor as a function of
field strength. Both taken in the 1 T magnet.
8
environmental noise
was filtered out. As
1000
Q in
expected, Q values
Q out
Q
900
decreased in the stronger
800
(f0(B)-f 0(0) )/f0(0) x 10 -3
700
magnetic field. There
-0.50
(f0(B)-f0(0))/f0(0) in
(f0(B)-f0(0))/f0(0) out
-0.52
seems to be a stronger
-0.54
correlation between the
-0.56
-0.58
data in the 1 T magnet as
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
B*dB/dZ
Figure 9b: Resonant frequency in field divided by resonant frequency with no
field as a function of B*(dB/dZ). Quality factor as a function of B*(dB/dZ).
Both taken in the 1 T magnet.
opposed to the smaller
0.3 T magnet. However,
the data appear to form a flat band when accounting for the uncertainty. In order for the data to
be acquired, we had to extend the wire connections to almost three feet in length. The
lengthening of the wires added more capacitance to the circuit, further affecting the Q values.
Our next conclusions are to be drawn from the magnetic susceptibility (χ) of the material.
This value, equal to the magnet moment divided by magnetic field strength, provided important
information regarding the reliability of our experiment. The magnetization of the nanoparticles
is about -1.5x10-3 emuG/g (derived from figure 6). The change in magnetization of the
nanoparticles on the fork is about -1x10-2 emuG/g (derived from figure 8a). These data yield a
signal to noise ratio of approximately 0.15. The magnitude of signal we receive from the lock-in
amplifier is 15% of the magnitude of the noise received from the environment. We will not be
able to conclusively see the affect of the magnetic fields on the Fe[(Htrz)2(trz)]BF4 nanoparticles
with our current set-up.
Further research which can be done in this field will need to place a loaded fork in the
same magnetic field, but at a different temperature. We want to see what effects if any occur if
9
the sample is in a different spin state. We need to devise a system for taking data from the fork
while it is in a dry ice bath. The ice will bring the temperature of the set up down to slightly
below 200 K, far below the transition temperature of nominally 275 K. A thermal couple will
also have to be included in this next set up in order to measure the exact temperature, making
sure the sample is in the low spin state. Another possible area to explore is the introduction of an
operational amplifier into the circuit. The operational amplifier will enlarge the signal coming
from the nanoparticles, which will increase the signal to noise ratio allowing more definite
conclusions to be drawn.
This work was supported by the National Science Foundation through the University of
Florida Physics REU program (DMR-9820518, DMR-0139579, and DMR-0552726), jointly
supported by the National Science Foundation and the Department of Defense (DMR-0851707),
and the single-investigator grant to Mark W. Meisel (DMR-0701400). We are grateful to the
Telham Group, UF Chemistry, for supplying samples and to Selman Hershfield and others for
making the UF REU program successful.
References
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(http://www.school-for-champions.com/science/magnetic_factors.htm).
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[4] S. Bohommeau, G. Molnar, A. Galet, A. Zwick, J. Real, J. McGarvey, and A. Bousseksou,
Agnew, Chem Int. Ed. 44, 4069 (2005).
[5] V. Neil, J. Martinez-Agudo, M. Munoz, A. Gaspar, and J. Real, Inorg. Chem. 40, 3838
10
(2001).
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