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Magnetic torsion measurements on hexagonal FeGe
Solid State Communications, Vol. 5, pp. 977-979, 1967. Pergamon Press Ltd. Printed in Great Britain
MAGNETIC TORSION MEASUREMENTS ON HEXAGONAL FeGe K.A. Blom and 0. Beckman Institute of Physics and M. Richardson Institute of Chemistry, University of Uppsala, Uppsala, Sweden (Received 6 November 1967 by S. Lundqvist)
The magnetic anisotropy of single crystal FeGe, with the hexagonal B 35 structure, has been studied by magnetic torsion measurements. The Néel temperature is (412 ±2)°K. The results indicate that the spins are parallel to the c-axis, probably with ferromagnetic ordering within each c-plane.
SINGLE crystals of FeGe, with the hexagonal B 35 structure’, have been measured in a torsion balance for determination of the magnetic anisotropy (~ i) in the temperature range 4.2 to 500°K. The B 35 phase was first studied by Ohoyama etal. 2, who reported antiferromagnetic behaviour with a Néel temperature of 410°K. No antiferromagnetic ordering was revealed in 3 the first neutron diffraction experiments however, but Mössbauer studies by Nikolaev et al. and by Tomiyoshi et al. ~ unambiguously ~1iowed that FeGe is ant1!~omagnetic. Later neutron diffraction experiments by Watanabe and Kunltomi~’ showed one superstructure line (1 0k), indicating antiferromagnetic ordering with the magnetic cell twice as large as the chemical cell. The spins were found to be directed parallel to the c-axis. ~.
is determined by rotating the torsion head back to the zero field position of the crystal. The torque ¶ on a unlaxial antiferromagnet in a homogenous magnetic field is given by
-
(V/2) BH (~ vii) sin2 cp where cc is the angle between the magnetic field and the magnetic axis. The torque is zero at four positions of the magnetic field, two of which are unstable and two are stable. The latter correspond to a minimum in potential energy with the spins oriented perpendicular to the field. The torsion measurements thus reveal the spin arrangement in the crystal. =
~,
-
Two crystals, weighing 0. 0151 and 0. 0159 g, were investigated. No torque could be detected when the crystals were mounted with the c-axis parallel to the axis of rotation, mdicating no magnetic anisotropy in a plane perpendicular to the c-axis. A torque proportional to sin2 cp was measured when the c-axis was perpendicular to the axis of rotation. The maximum torque was proportional to the square of the magnetic field (Fig. 1), showing that the hexagonal FeGe has no weak ferromagnetic moment. The sign of the torque as a function of magnetic field orientation revealed that the spins are parallel
Single crystals, approx 1 x 1 x 2 mm in size, were grown by the halogen transport method. For experimental details see Table 2, No. 3 of Ref. 1. The magnetic anisotropy was measured in a torsion balance with uniform magnetic field. The apparatus is similar to that used by Stout and Grlffel. e The crystal is mounted at the end of a long glass rod, which in turn is attached to a torsion head via a quartzfibre. A mirror on the rod determines the orientation of the sample with respect to a zero-field posttion. Alter applying the magnetic field the torque 977
978
MAGNETIC TORSION MEASUREMENTS
torque orb, units
Vol. 5, No. 12
3
(X
(X~X)g1O5
SI—units 2-X)10
Fe Ge hex B35
FeGe 1emu
hex B 35 4
4
3 3
2 0.2
0,1.
0.6
2
(Vs/m2)
2 2
FIG. 1 to the c-axis.
1~
The anisotropy in tue magnetic susceptibility (x ~ ~)was studied from 4. 2°K to temperatures above the Néel temperature. The latter was found to be T~ = (412 u 2)°K. The magnetic anisotropy plotted as a function of the square of the temperature gives a straight line in the temperature range of 200 390°K in reasonable agreement with spin wave theory9 (Fig. 2). There is no irregularity in the susceptibility curve, which indicates that the single crystals used in this investigation contain no contaminating phases. -
___________________________________ 10 T2I0’(K)2
15
T~412~K
FIG. 2
-
Extrapolation to 0°K gives (~i—XH)o
=
(4.1
± 0.1)
ment is shown in Fig. 3, where the ferrornagnetic interaction within each c-plane is much stronger than the antiferrornagnetic interaction between adjacent c-planes. This structure is the same as found from neutron diffraction experiments by Watanabe and Kunitomi.7
x 10~~ FeGe hex B35
SI units ~4. 5 x 103g susc. emuj. A single measurement at room temperature according to the Faraday method gave a perpendicular susceptibility ~ = (4. 1 ± 0.2) x 10~ SI units, indicating that ~ probably is independent of temperature. The susceptibility in the paramagnetic region is described by x=C/(T+e)
--
where according to Ohoyarna2 the Curie ternperature B is negative (e = 220°K)indicating a strong ferrornagnetic interaction within each sublattice. Assuming a molecular field in sublattice A:
I
-
I
____I~v~
HA = ~B LMA we get e 3~.i from the known values of Curie and Néel temperatures. A probable spin arrange-
_
-
-
• Ge-atoms
Fe atoms FIG. 3 -
Vol. 5, No. 12
MAGNETIC TORSION MEASUREMENTS
Acknowledgement This work was supported in part by the United States Air Force Office of Scientific Research under Contract AF 61(052)937 and monitored by the European Office, Office -
979
of Aerospace Research. A grant from the Swedish Natural Science Research Council is gratefully acknowledged.
References 1.
RICHARDSON M., Acta Chem. Scand. (in press).
2.
OHOYAMA T.,
3.
SATYA MURTHY N. S., BEGUM R. J., SOMANATHAN C. S. and MURTHY M. R. L. N., Solid State Comm. 3, 113 (1965). ADELSON E. and AUSTIN A.E., J. Phys. Chem. Solids 26, 1795 (1965).
4.
KANEMATSU K. and YASUKOCHI K., J. Phys. Soc. Japan 18, 589 (1963).
5.
NIKOLAEV V.1., YAKIMOV S. S., DUBOVTSEV I. A. and GAVRILOVA Z. G., JETP Letters 2, 235 (1965).
6.
TOMIYOSHI S.,
7.
WATANABE H. and KUNITOMI N.,
8.
STOUT J.W. and GRIFFEL M., J. Chem. Phys.
9.
NAGAMIYA T., YOSIDA K. and KUBO R., Advances in Physics 4, 1 (1955).
YAMAMOTO H. and WATANABE H.,
J. Phys. Soc. Japan 21, 709 (1966).
J. Phys. Soc. Japan 21, 1932 (1966). 18, 1449 (1950).
L’anisotropie magnétique des monocristaux FeGe d’une structure hexagonale B 35 a étê étudie a l’aide des mesures de la torsion magnétique. La temperature de Nêel est (412 ±2)°K. Les résultats indiquent que les spins sont parallèles a l’axe c, probablement avec l’établissement d’un ordre ferromagnétique dana chaque plan c.
MAGNETIC TORSION MEASUREMENTS ON HEXAGONAL FeGe K.A. Blom and 0. Beckman Institute of Physics and M. Richardson Institute of Chemistry, University of Uppsala, Uppsala, Sweden (Received 6 November 1967 by S. Lundqvist)
The magnetic anisotropy of single crystal FeGe, with the hexagonal B 35 structure, has been studied by magnetic torsion measurements. The Néel temperature is (412 ±2)°K. The results indicate that the spins are parallel to the c-axis, probably with ferromagnetic ordering within each c-plane.
SINGLE crystals of FeGe, with the hexagonal B 35 structure’, have been measured in a torsion balance for determination of the magnetic anisotropy (~ i) in the temperature range 4.2 to 500°K. The B 35 phase was first studied by Ohoyama etal. 2, who reported antiferromagnetic behaviour with a Néel temperature of 410°K. No antiferromagnetic ordering was revealed in 3 the first neutron diffraction experiments however, but Mössbauer studies by Nikolaev et al. and by Tomiyoshi et al. ~ unambiguously ~1iowed that FeGe is ant1!~omagnetic. Later neutron diffraction experiments by Watanabe and Kunltomi~’ showed one superstructure line (1 0k), indicating antiferromagnetic ordering with the magnetic cell twice as large as the chemical cell. The spins were found to be directed parallel to the c-axis. ~.
is determined by rotating the torsion head back to the zero field position of the crystal. The torque ¶ on a unlaxial antiferromagnet in a homogenous magnetic field is given by
-
(V/2) BH (~ vii) sin2 cp where cc is the angle between the magnetic field and the magnetic axis. The torque is zero at four positions of the magnetic field, two of which are unstable and two are stable. The latter correspond to a minimum in potential energy with the spins oriented perpendicular to the field. The torsion measurements thus reveal the spin arrangement in the crystal. =
~,
-
Two crystals, weighing 0. 0151 and 0. 0159 g, were investigated. No torque could be detected when the crystals were mounted with the c-axis parallel to the axis of rotation, mdicating no magnetic anisotropy in a plane perpendicular to the c-axis. A torque proportional to sin2 cp was measured when the c-axis was perpendicular to the axis of rotation. The maximum torque was proportional to the square of the magnetic field (Fig. 1), showing that the hexagonal FeGe has no weak ferromagnetic moment. The sign of the torque as a function of magnetic field orientation revealed that the spins are parallel
Single crystals, approx 1 x 1 x 2 mm in size, were grown by the halogen transport method. For experimental details see Table 2, No. 3 of Ref. 1. The magnetic anisotropy was measured in a torsion balance with uniform magnetic field. The apparatus is similar to that used by Stout and Grlffel. e The crystal is mounted at the end of a long glass rod, which in turn is attached to a torsion head via a quartzfibre. A mirror on the rod determines the orientation of the sample with respect to a zero-field posttion. Alter applying the magnetic field the torque 977
978
MAGNETIC TORSION MEASUREMENTS
torque orb, units
Vol. 5, No. 12
3
(X
(X~X)g1O5
SI—units 2-X)10
Fe Ge hex B35
FeGe 1emu
hex B 35 4
4
3 3
2 0.2
0,1.
0.6
2
(Vs/m2)
2 2
FIG. 1 to the c-axis.
1~
The anisotropy in tue magnetic susceptibility (x ~ ~)was studied from 4. 2°K to temperatures above the Néel temperature. The latter was found to be T~ = (412 u 2)°K. The magnetic anisotropy plotted as a function of the square of the temperature gives a straight line in the temperature range of 200 390°K in reasonable agreement with spin wave theory9 (Fig. 2). There is no irregularity in the susceptibility curve, which indicates that the single crystals used in this investigation contain no contaminating phases. -
___________________________________ 10 T2I0’(K)2
15
T~412~K
FIG. 2
-
Extrapolation to 0°K gives (~i—XH)o
=
(4.1
± 0.1)
ment is shown in Fig. 3, where the ferrornagnetic interaction within each c-plane is much stronger than the antiferrornagnetic interaction between adjacent c-planes. This structure is the same as found from neutron diffraction experiments by Watanabe and Kunitomi.7
x 10~~ FeGe hex B35
SI units ~4. 5 x 103g susc. emuj. A single measurement at room temperature according to the Faraday method gave a perpendicular susceptibility ~ = (4. 1 ± 0.2) x 10~ SI units, indicating that ~ probably is independent of temperature. The susceptibility in the paramagnetic region is described by x=C/(T+e)
--
where according to Ohoyarna2 the Curie ternperature B is negative (e = 220°K)indicating a strong ferrornagnetic interaction within each sublattice. Assuming a molecular field in sublattice A:
I
-
I
____I~v~
HA = ~B LMA we get e 3~.i from the known values of Curie and Néel temperatures. A probable spin arrange-
_
-
-
• Ge-atoms
Fe atoms FIG. 3 -
Vol. 5, No. 12
MAGNETIC TORSION MEASUREMENTS
Acknowledgement This work was supported in part by the United States Air Force Office of Scientific Research under Contract AF 61(052)937 and monitored by the European Office, Office -
979
of Aerospace Research. A grant from the Swedish Natural Science Research Council is gratefully acknowledged.
References 1.
RICHARDSON M., Acta Chem. Scand. (in press).
2.
OHOYAMA T.,
3.
SATYA MURTHY N. S., BEGUM R. J., SOMANATHAN C. S. and MURTHY M. R. L. N., Solid State Comm. 3, 113 (1965). ADELSON E. and AUSTIN A.E., J. Phys. Chem. Solids 26, 1795 (1965).
4.
KANEMATSU K. and YASUKOCHI K., J. Phys. Soc. Japan 18, 589 (1963).
5.
NIKOLAEV V.1., YAKIMOV S. S., DUBOVTSEV I. A. and GAVRILOVA Z. G., JETP Letters 2, 235 (1965).
6.
TOMIYOSHI S.,
7.
WATANABE H. and KUNITOMI N.,
8.
STOUT J.W. and GRIFFEL M., J. Chem. Phys.
9.
NAGAMIYA T., YOSIDA K. and KUBO R., Advances in Physics 4, 1 (1955).
YAMAMOTO H. and WATANABE H.,
J. Phys. Soc. Japan 21, 709 (1966).
J. Phys. Soc. Japan 21, 1932 (1966). 18, 1449 (1950).
L’anisotropie magnétique des monocristaux FeGe d’une structure hexagonale B 35 a étê étudie a l’aide des mesures de la torsion magnétique. La temperature de Nêel est (412 ±2)°K. Les résultats indiquent que les spins sont parallèles a l’axe c, probablement avec l’établissement d’un ordre ferromagnétique dana chaque plan c.