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Magnetic field effects on the optical spectrum of FeCl2
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
15 April 1973
MAGNETIC FIELD EFFECTS ON THE OPTICAL SPECTRU&f OF FeCI, D.J. ROBBINS and P. DAY University of Oxford, Inorganic Clremistry Laboratory, Oxford, UK Received 31 January
1973
The intensity of the &and-field zero-phonon line of FeC12 at 4272 A has been measured in mwctic fields up to 30 kOe at tcmperntures of 8 and 1 1°K. TXe intensity first rises and then falls with increasing field, as the crystaI passes through a phase transition from an antiferromagnetic (mctamaqctic) to a ferromenctic state.
The intensification of formally spin-forbidden ligand field transitions in antiferromagnetic crystals by co-excitation of magnons is well known [ 11. On the other hand, as we have recently demonstrated [2], the analogous intensification in ferromagnetic materials can only take place by annihilation of thermally created magnons. Metamagnetic compounds, in which ferromagnetic layers are weakly coupled to yield a threedimensionally antiferromagnetic crystal might be expected to occupy an intermediate position between these two extremes. Of such materials, the layer compound FeC12 is particularly interesting since in moderate magnetic fields (about 10 kOe at 4”K, with fll [OOOl]) it undergoes a transition from the antiferromagnetic to a three-dimensional ferromagnetjc phase [3]. At low temperatures the transition is of first order, but at T z 0.85 TN it becomes second- or higher order, resulting in a point on the phase diagram which has been called a tricritical point. Optical spectra should be very sensitive to a change in the magnetic ordering as cold magnon-exciton combinations cannot be created in the ferromagnetic, but only in the antiferromagnetic phase. For this reason we have undertaken a study of the optical spectrum of FeCI, as a function both of temperature and external magnetic field. Schnatterly and Fontana [4] recently reported the temperature dependence in zero magnetic field of a sharp zero-phonon line centred at 4270 A, and showed that the unusual temperature dependence of the dipole strength could be exp!ained in terms of coupling between the exciton and thermally excited magnons propagating in an effectively two-
dimensional ferromagnetic lattice. We have also measured the temperature dependence of this line, with results which agree with those of Schnatterly and Fontana 141. The behaviour of the line as a function of temperature is in agreement with recent inelastic neutron scattering experiments on FeCLZ [5], which demonstrated that the spin-waves simulate those of a two-dimensional ferromagnet with large anisotropy. The intensity of the optical transition thus increases with the disorder in the system. In this preliminary note we report the magnetic field dependence of the same zero-phonon line. Experiments were condl:cted on two double-beam spectrophotometers, a Gary 14 and an instrument designed and built in this laboratov [6], empIoying two different superconducting magnet systems with Oxford Instruments Harwell temperature controllers. The spectrum
between
4290
and 426C! a is complex,
showing
at leasi three lines with different
temperature dependences. In fig. 1 we show the variation of the dipole strength of the first sharp line, at 4272 j, (normalized to unity at H = 0) with magnetic field for two temperatures within the first order phase transition region. From the results in fig. 1 the foilowing points may be made. (1) The intensity of the line increases on approaching the transition field. This effect is to be expected since, in the presence of an external magnetic field; the magnon branches arising from sublattices with spins parailel and antiparallel to the fieId direction- are no longer degenerate. Consequently, with increasing magnetic field, the disorder in the “antiparallel” sub529
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
15 April 1973
,
x
‘. “X
-. -. *-__
- - - .,+___
-_
--XL_
T-1lK T-8K
*
10
0
30
20
40
H(kOe) Fig. 1. Variation 3f ti-,ieintegrated 11°K; 0.8”K.
band arca of the 4272 A Lice of F&12 (axial s$ec&n)
lattice increases until the phase transition is reached.
with magnetic ficlci (HiI (00011); X,
observed in zero field as T + TN, so that a high degree of near-neighbour spin correlation is maintained through the phase transition. (2) The intensity of the line reaches a maximum at
the line. This may explain the scatter of observed points in this region. In addition to an increase in the dipole strength of the 4272 a line through the critical field we also observe a rise in the absorbance of the crystal throughout the entire visible and near infrared regions, even at
about
frequencies
However, the increase
is only a small fraction
15- 16 kOe. The criticai
transition
of that
field at the
temperature of our two experiments has been determined [3] by single crystal susceptibility measurements as lo-12 kOe. However, the susceptibility experiments were carried out on a cylindrical sample, for which the demagnetizing factor is smaller than in our thin platelike sample. We also note that the change of absorbance with field in the critica! region appears sharper at the lower temperature. (3) At higher fields in the ferromagnetic phase the line intensity decreases with increasing magnetic field. This probably reflects the increased energy of the inplane magnon branch in an external magnetic field parallel to the unique crystal axis. (4) Between 25-30 kOe the lineshape becomes complex, perhaps reflecting a Zeeman splitting of 530
outside
those
of the electronic
absorption
bands. This phenomenon we interpret as critical scattering occurring near the phase transition. Fig. 2 demonstrates the effect by plotting the observed absorbance at 4260 A (Le., at a wavelength outside the zero-phonon line) as a function of magnetic field. The initial rise in scattering exhibits a profile similar to that [3] of the magnetisation isotherms. The scattering was observed near the transition field 21 a number of different temperatures. However, at 19 + l”K, a further unusual phenomenon was observed. As the magnetic field was increased through the region of the phase transition the light level registered by the photomultiplier began to oscillate as a function of time. The oscillations continued until the field had reached about 15 kOe, at which point the
Volume 19, number
CHEMICAL PHYSICS LETTERS
4
Effective
1973
, _
absorbance
10
0
1.5 April
15
20
H (kOe) Fig. 2. Effective absorbance
of Fe&
at 19°K as a function of magnetic field. .41sa plotted are experimental magnetisatian curves
131. spectral baseline had returned approximately to its level at zero field. In fig. 2, the region of oscillations is indicated by the dotted line, and their amplitude by the error bar. Although we cannot at the present time entirely rule out the possibility that the oscillations are caused by an instrumental effect, we emphasise that they occur only within a small range of magnetic field, and only with appreciable amplitude at temperatures approaching r, of FeCl,. Neither has any comparable effect been observed with any other sample in the same instrument in this region of field and temperature. These arguments suggest strongiy that the phenomenon is indeed associared with the second-order phase transition region of FeCI,, and may reflect a property of the crystal itself, or of its interaction with the external magnetic field. We are at present undertaking more detailed experiments on the whole range of optical phenomena associated with this unusual type of mag netic ordering.
We are grateful to Dr. A.J. Thomson for the use of the Gary 14 and superconducting magnet at the University of East Anglia, and for help with the experiments there. Acknowledgement is also made to SRC for an equipment grant and a Fellowship (to D..i.R.).
References [l] D.S. McClure, in: Excitons, magnons and phonons in molecular crystals, ed. A.B. Zahlan (Cambridge Uniu. Press, London, 1968). [2] P. Day, A.K. Gregson and D.H. Leech, Phys. Rev. Le~tcrs 30 (1973) 19. [3] IS. Jacobs and P.E. Lawrence, Phys. Rev. 164 (1967) 866. [4] S.E. Schnatterly and M. Fontana, J. Phys. (Paris) 33
(1972) 691. [S] R.J. Birgenean. W.E. Yelon, E. Cohen and I. hiakovsky, Phys. Rev. B 5 (1972) 2607. [6] J.C. Collingwood, R.G. Denning and P. Quested, unpublished work.
531
CHEMICAL PHYSICS LETTERS
15 April 1973
MAGNETIC FIELD EFFECTS ON THE OPTICAL SPECTRU&f OF FeCI, D.J. ROBBINS and P. DAY University of Oxford, Inorganic Clremistry Laboratory, Oxford, UK Received 31 January
1973
The intensity of the &and-field zero-phonon line of FeC12 at 4272 A has been measured in mwctic fields up to 30 kOe at tcmperntures of 8 and 1 1°K. TXe intensity first rises and then falls with increasing field, as the crystaI passes through a phase transition from an antiferromagnetic (mctamaqctic) to a ferromenctic state.
The intensification of formally spin-forbidden ligand field transitions in antiferromagnetic crystals by co-excitation of magnons is well known [ 11. On the other hand, as we have recently demonstrated [2], the analogous intensification in ferromagnetic materials can only take place by annihilation of thermally created magnons. Metamagnetic compounds, in which ferromagnetic layers are weakly coupled to yield a threedimensionally antiferromagnetic crystal might be expected to occupy an intermediate position between these two extremes. Of such materials, the layer compound FeC12 is particularly interesting since in moderate magnetic fields (about 10 kOe at 4”K, with fll [OOOl]) it undergoes a transition from the antiferromagnetic to a three-dimensional ferromagnetjc phase [3]. At low temperatures the transition is of first order, but at T z 0.85 TN it becomes second- or higher order, resulting in a point on the phase diagram which has been called a tricritical point. Optical spectra should be very sensitive to a change in the magnetic ordering as cold magnon-exciton combinations cannot be created in the ferromagnetic, but only in the antiferromagnetic phase. For this reason we have undertaken a study of the optical spectrum of FeCI, as a function both of temperature and external magnetic field. Schnatterly and Fontana [4] recently reported the temperature dependence in zero magnetic field of a sharp zero-phonon line centred at 4270 A, and showed that the unusual temperature dependence of the dipole strength could be exp!ained in terms of coupling between the exciton and thermally excited magnons propagating in an effectively two-
dimensional ferromagnetic lattice. We have also measured the temperature dependence of this line, with results which agree with those of Schnatterly and Fontana 141. The behaviour of the line as a function of temperature is in agreement with recent inelastic neutron scattering experiments on FeCLZ [5], which demonstrated that the spin-waves simulate those of a two-dimensional ferromagnet with large anisotropy. The intensity of the optical transition thus increases with the disorder in the system. In this preliminary note we report the magnetic field dependence of the same zero-phonon line. Experiments were condl:cted on two double-beam spectrophotometers, a Gary 14 and an instrument designed and built in this laboratov [6], empIoying two different superconducting magnet systems with Oxford Instruments Harwell temperature controllers. The spectrum
between
4290
and 426C! a is complex,
showing
at leasi three lines with different
temperature dependences. In fig. 1 we show the variation of the dipole strength of the first sharp line, at 4272 j, (normalized to unity at H = 0) with magnetic field for two temperatures within the first order phase transition region. From the results in fig. 1 the foilowing points may be made. (1) The intensity of the line increases on approaching the transition field. This effect is to be expected since, in the presence of an external magnetic field; the magnon branches arising from sublattices with spins parailel and antiparallel to the fieId direction- are no longer degenerate. Consequently, with increasing magnetic field, the disorder in the “antiparallel” sub529
Volume 19, number 4
CHEMICAL PHYSICS LETTERS
15 April 1973
,
x
‘. “X
-. -. *-__
- - - .,+___
-_
--XL_
T-1lK T-8K
*
10
0
30
20
40
H(kOe) Fig. 1. Variation 3f ti-,ieintegrated 11°K; 0.8”K.
band arca of the 4272 A Lice of F&12 (axial s$ec&n)
lattice increases until the phase transition is reached.
with magnetic ficlci (HiI (00011); X,
observed in zero field as T + TN, so that a high degree of near-neighbour spin correlation is maintained through the phase transition. (2) The intensity of the line reaches a maximum at
the line. This may explain the scatter of observed points in this region. In addition to an increase in the dipole strength of the 4272 a line through the critical field we also observe a rise in the absorbance of the crystal throughout the entire visible and near infrared regions, even at
about
frequencies
However, the increase
is only a small fraction
15- 16 kOe. The criticai
transition
of that
field at the
temperature of our two experiments has been determined [3] by single crystal susceptibility measurements as lo-12 kOe. However, the susceptibility experiments were carried out on a cylindrical sample, for which the demagnetizing factor is smaller than in our thin platelike sample. We also note that the change of absorbance with field in the critica! region appears sharper at the lower temperature. (3) At higher fields in the ferromagnetic phase the line intensity decreases with increasing magnetic field. This probably reflects the increased energy of the inplane magnon branch in an external magnetic field parallel to the unique crystal axis. (4) Between 25-30 kOe the lineshape becomes complex, perhaps reflecting a Zeeman splitting of 530
outside
those
of the electronic
absorption
bands. This phenomenon we interpret as critical scattering occurring near the phase transition. Fig. 2 demonstrates the effect by plotting the observed absorbance at 4260 A (Le., at a wavelength outside the zero-phonon line) as a function of magnetic field. The initial rise in scattering exhibits a profile similar to that [3] of the magnetisation isotherms. The scattering was observed near the transition field 21 a number of different temperatures. However, at 19 + l”K, a further unusual phenomenon was observed. As the magnetic field was increased through the region of the phase transition the light level registered by the photomultiplier began to oscillate as a function of time. The oscillations continued until the field had reached about 15 kOe, at which point the
Volume 19, number
CHEMICAL PHYSICS LETTERS
4
Effective
1973
, _
absorbance
10
0
1.5 April
15
20
H (kOe) Fig. 2. Effective absorbance
of Fe&
at 19°K as a function of magnetic field. .41sa plotted are experimental magnetisatian curves
131. spectral baseline had returned approximately to its level at zero field. In fig. 2, the region of oscillations is indicated by the dotted line, and their amplitude by the error bar. Although we cannot at the present time entirely rule out the possibility that the oscillations are caused by an instrumental effect, we emphasise that they occur only within a small range of magnetic field, and only with appreciable amplitude at temperatures approaching r, of FeCl,. Neither has any comparable effect been observed with any other sample in the same instrument in this region of field and temperature. These arguments suggest strongiy that the phenomenon is indeed associared with the second-order phase transition region of FeCI,, and may reflect a property of the crystal itself, or of its interaction with the external magnetic field. We are at present undertaking more detailed experiments on the whole range of optical phenomena associated with this unusual type of mag netic ordering.
We are grateful to Dr. A.J. Thomson for the use of the Gary 14 and superconducting magnet at the University of East Anglia, and for help with the experiments there. Acknowledgement is also made to SRC for an equipment grant and a Fellowship (to D..i.R.).
References [l] D.S. McClure, in: Excitons, magnons and phonons in molecular crystals, ed. A.B. Zahlan (Cambridge Uniu. Press, London, 1968). [2] P. Day, A.K. Gregson and D.H. Leech, Phys. Rev. Le~tcrs 30 (1973) 19. [3] IS. Jacobs and P.E. Lawrence, Phys. Rev. 164 (1967) 866. [4] S.E. Schnatterly and M. Fontana, J. Phys. (Paris) 33
(1972) 691. [S] R.J. Birgenean. W.E. Yelon, E. Cohen and I. hiakovsky, Phys. Rev. B 5 (1972) 2607. [6] J.C. Collingwood, R.G. Denning and P. Quested, unpublished work.
531