Transcript Multiferroic Behavior in the Double Pervoskite NaTbMnWO6

The Double Pervoskite NaTbMnWO6 : A Likely
Multiferroic Material
†
‡
‡
Alison Pawlicki, A. S. Sefat, David Mandrus
†
‡
Florida State University, Oak Ridge National Laboratory
Third Heat (in Ar/H2 gas flow)
Second Heat (in Ar/H2 gas flow)
Inverse Magnetic Susceptbility
Magnetic Susceptbility
24
0.4
20
0.4
10
20
30
2°
40
50
60
70
 (mol Oe/emu)
0
 (emu/mol Oe)
First Heat (in air)
 (emu/mol Oe)
18
0.2
0.2
10
20
30
40
T (K)
Powder X-Ray Diffraction: Sample Initially Heated
in Ar/H2 Gas Flow
Inital Ar/H2 Heat Sample
Inital Air Heat Sample
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Inital Ar/H2 Heat Sample
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12
10
8
6
4
2
0
0.0
0
50
100
150
200
250
300
350
400
0
450
50
100
Second Heat (in Ar/H2 gas flow)
Sample
TN (K)
Initial Heat in Air
Initial Heat in Ar/H2
First Heat (in Ar/H2 gas flow)
C (emu K/ mol Oe)
40
90
0
10
20
30
40
50
60
80
70
2 °
70
30
20
10
60
0
0
5
10
15
20
25
30
35
250
300
350
400
θ (K)
μeff/μB
Heat capacity of the initial Ar/H2
heated sample is only measured
because it is found more pure.
Two sharp anomalies were also
found at approximately the same
temperatures as in the magnetic
susceptibility, suggesting two
long range ordering temperatures.
50
100
200
10.730(3) −21.90(5) 9.265(6)
17.274(4) −25.43(7) 11.755(4)
15
15
Heat Capacity
110
150
T (K)
T (K)
40
T (K)
50
40
30
MnO + WO3 → MnWO4
20
10
0
Then we reacted stoichiometric amounts of MnWO4, Tb4O7, and an
approximate 8% excess of NaCO3.
0
20
40
60
80
100
T (K)
Conclusions
MnWO4 + ¼ Tb4O7 + ½ Na2CO3 → NaTbMnWO6
One sample was heated at 950° C in a dynamic flow of Ar/H2 gas
twice and another sample was heated in air at 800° C then heated at
950° C in a dynamic flow of Ar/H2 gas. We also attempted single
crystal growth by the use of floating zone image furnace pictured
above. The phase purity of the samples was analyzed by using Philips
Powder Diffractometer at room temperature. Magnetic susceptibility
data was collected using a Superconducting Quantum Interference
Device (SQUID) in an applied field of 1000 Oe. For the first time,
heat capacity measurements were done via Physical Property
Measurement System (PPMS).
Inital Air Heat Sample
22
Cp (J/mol K)
We synthesized polycrystalline samples of NaTbMnWO6 by using
two slightly different solid state routes. First we reacted stoichiometric
amounts of MnO and WO3 at 950° C to yield MnWO4.
Powder X-Ray diffraction
was used in every step of the
reaction
process.
The
diffraction pattern in purple
is from the sample initially
heated in air and it shows
excess Bragg peaks about 8°,
9°, 20°, and 21°. These peaks
are due to impurities and
were reduced in slightly
throughout the heating. The
diffraction pattern in orange
is from the sample initially
heated in a flow of Ar/H2 gas
and shows excess Bragg
peaks about 16° and 28°.
These peaks are reduced in
magnitude in the second
heating and are thought to be
from impurities. Because
there are less significant
excess peaks in the second
synthesis
process,
we
conclude that this method
produces less impurities.
The magnetic susceptibility for samples produced by the two different
heating processes is displayed below. Both samples show anomalies
at 15 and ~11 K. Using a high temperature fit to the Curie-Weiss Law,
the Curie-Weiss temperature, Curie constant, and effective magnetic
moment were found and listed in the table below. As noted, μeff
differs for both samples and the theoretical value is closer to the value
obtained for the initial Ar/H2 heated sample.
Cp (J/mol K)
Experimental Procedure
Powder X-Ray Diffraction: Sample Initially Heated in Air
Intensity
Double perovskites, of the chemical formula AA’BB’O6, are known
to show multiferroic behavior, exhibiting coupled ferroelectricity and
magnetism. Materials displaying both of these properties are unique
because they tend to be mutually exclusive since magnetism arises in
transition ions with partially filled d- or f-orbitals and all conventional
ferroelectricity arises in transition ions with empty d-orbitals.
However, some perovskites have been found to display both
behaviors, where the origin of ferroelectricty is unconventional. The
study of such materials is motivated by the possibility of controlling
electric charges by applied magnetic fields and controlling magnetic
fields by applied voltages. In this study, we focus on one particular
double perovskite, NaTbMnWO6.
Results and Discussion
Intensity
Introduction
Polycrystalline samples of NaTbMnWO6 of true orthorhombic phase
were synthesized by solid state methods. Two long-range magnetic
ordering transition temperatures were found in magnetic susceptibility
and heat capacity results at 15 K and at ~11 K. Because of the
interesting magnetic behavior, we would like to pursue further study of
magnetoelectric coupling and phase transitions in single crystals.
Powder Neutron diffraction measurements are currently underway.
Pictured above is one unit cell of the crystal structure of NaTbMnWO6.
The Na and Tb ions order simultaneously in layers. Powder x-ray
diffraction confirmed the samples to be of the true orthorhombic phase.
Acknowledgments
We would like to thank Brain Sales and Michael McGuire for laboratory assistance. This research was
carried out at Oak Ridge National Laboratory and was supported by the Division of Materials Sciences
and Engineering, The Office of Basic Energy Science, and the U.S. Department of Energy.
References
King, Graham; Thimmaiah, Srinvasa; Dwivedi, Akansha; Woodward, P. M. Chem.
Mater. 2007, 19, 6451-6458