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Suppression of soliton excitation by external field in quasi-one-dimensional Ising antiferromagnet RbFeCl3 · 2H2O
244
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 244-246 North- Holland
Suppression of soliton excitation by external field in quasi-one-dimensional Ising antiferromagnet RbFeCl 3 • 2H 20 M. Takeda, I. Magi, G. Kido, Y. Nakagawa, H. Okada
a
and N. Kojima
a
Institute for Materials Research, Tohoku University, Sendai 980, Japan Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
a
The absorption spectra of domain wall soliton in the one-dimensional canted Ising-like anti ferromagnet RbFeCI 3·2H 20 have been investigated in a continuous high magnetic field. The suppression of the soliton excitation was found in fields higher than 2 T applied parallel to the orthorhombic c-axis.
In low-dimensional magnetic materials nonlinear excitations, i.e, solitons, exist, as well as spin waves. In I-D Ising-like antiferromagnets a soliton is a thermally excited antiphase boundary. Such solitons in CsCoCl 3 and CsCoBr3 have been investigated by neutron scattering, ESR, Raman spectroscopy, magnetoabsorption and so on [1,2]. In RbFeCI 3 • 2H 20 , which is known to be a I-D Ising-like antiferromagnet with a canted spin structure [3], the presence of the soliton has been reported by Smit et al. [4]. They observed the anomalous line broadening in the Mossbauer spectra due to thermal excitation of solitons. However, the measurement of T1- 1 using a NMR technique has only shown preliminary and speculative evidence of the presence of the soliton [5]. RbFeCI 3·2H 20 belongs to the series of isomorphous transition-metal halides whose chemical formula is written as ABX 3 • 2H 20 (A = Rb, Cs; M = Mn, Fe, Co; X = CI, Br), A magnetic linear chain in RbFeCI 3·2H 20 consists of antiferromagnetically coupled spins running along the orthorhombic a-axis. The intrachain interactions (Ja/k n = -35 K) are two orders of magnitude stronger than interchain ones (Jb/k n = -0.76 K, Jclkn = -0.21 K) [6]. Below the Neel temperature (TN '= 11.96 K) the interchain interactions lead to a three dimensional ordered phase in which all Ising spins are situated in the ac-plane and the local easy axis of the spins makes an angle of 15 0 from the a-axis: each chain possessing weak ferromagnetic moment in the direction of the c-axis, In the isomorphous compound CsFeCI 3 • 2H 20 , magnetoabsorption spectra have been measured in a field parallel to the a-axis by Okada et al. [7]. They found the absorption line, which directly indicated the presence of soliton. The advantage of the optical investigation of the soliton in CsFeCI 3 • 2H 20 is that the intensity of the line is proportional to the soliton den-
sity. The same technique is employed in the present work to study the soliton in RbFeCI 3 • 2H 20 . In this paper we report an optical investigation of the soliton in RbFeCI 3 • 2H 2 0 . It has been found that the excitation of the soliton is significantly suppressed . by an external field parallel to the c-axis. Single crystals were obtained from a solution of FeCI 2 • 4H 20 and RbCI in a molar ratio of 3.2: 1 by slow evaporation. Typical dimensions of a sample used in these experiments were 3 X 3 X 1 mrrr', The sample was held between copper plates and fixed in the vacuum space of a metal cell immersed in liquid helium. The temperature was varied from 4.2 to 30 K by means of an electric heater and stabilized by a capacitance controller. An optical fiber was used for transmission of light. The intensity of monochromatic light through the sample was detected by a photomultiplier. A continuous high magnetic field up to 14 T was produced by a water-cooled resistive magnet at Tohoku University. Fig. 1 is the schematic representation of (a) the antiferromagnetic chain and (b) the soliton in RbFeCI J ·2H 20 . The arrow represents Ising spins along the magnetic chain and the vertical bar the soliton. The exciton line which indicates the presence of the soliton is different from the usual exciton line. The energy difference amounts to approximately a " since the dominant intrachain exchange interactions are canceled out at such sites as indicated by dots in fig. l(b).
'J
----- --
( 0 ) - - - - --- - - --- - - - - - - (b) ---~I~
Fig. 1. Schematic representation of the anti ferromagnetic chain of canted Ising spins in the anti ferromagnetic phase (a) and the soliton (b). The exciton at the dotted sites is different from that at usual sites.
0304-8853/90/$03.50 V 1990 - Elsevier Science Publishers B.Y. (North-Holland) and Yamada Science Foundation
245
M. Takeda et al. / Suppression of soli/on in RbFeClJ • 2JI10
Fig. 2 shows the absorption spectra at 14 K with the electric vector of the incident light, i, 110 and the external magnetic field, H, lie in the Voigt configuration. Even at 14 K ( > TN), there exist antiferromagnetic clusters in the linear chain. The absorption line, denoted by AO, is due to an usual exciton in the antiferromagnetic cluster and the H3 line is assigned as the exciton at the sites on the soliton. The soliton in a 1-0 Ising system is thermally excited so that the H3 line is observed as a hot band which appears above 10 K. The Al line and the HI line are the magnon sidebands of the AO line and of the H3 line, respectively. The details of the assignment are given for the isomorphous compound CsFeCI 3 • 2HzO in ref, [7]. As a field is increased, the AO line shows only an energy shift to higher energy. On the contrary, a splitting of the H3 line. can be observed in fields higher than 4 T. If there were no soliton in the antiferromagnetic chain, only the AO line would appear. Hence, the observation of the H3 line clearly indicates the presence of the soliton, which was not observed by Mossbauer experiments in a field of 6 T. The lower energy branch is unobservable in fields higher than 10 T, while the higher energy branch loses its intensity but can still be detected at 14 T. At temperatures lower than 14 K the H3 line begins to be suppressed in fields less than 4 T and completely disappears at 14 T. In the following discussion we concentrate on the H3 line and the AO line and ignore the other absorption lines which appear in fig. 2. In the antiferromagnetic cluster, the sublattice splitting of the AO line should be observed. However, the splitting is not observed, for the following reasons. It has been found that at 14 K the magnetization monotonically increases with increasing field, due to the flipping of the weak ferromagnetic moment, and almost saturates in fields higher than 4 T. Therefore, the magnetic field causes the sublattice splitting of the AO line, due to the weak ferromagnetic moment, but one of two branches disappears in the field higher than 4 T. The magnitude of the splitting in fields less than 4 Tis, however, very small compared with the line width of the AO line. Thus, the AO line only shows the energy shift without splitting. On the other hand, the splitting of the H3 line was observed, but the intensity of both branches became weak with increasing field. This splitting isattributed to the fact that the weak ferromagnetic moments of the two sites on the soliton are opposite to each other (see fig. l(b». The decrease in intensity of the H3 line can be explained as described below. The creation of one soliton in a canted antiferromagnetic chain gives rise to two antiferromagnetic regions whose weak ferromag-
Hllc Ella
14K
,uoH(T) 14
12 (\J
u
c .a....
10
(1
0 III
.a
8
6 4
2
o 24200 24250 Wave Number (cm") Fig. 2. The Cine structure of the magnetoabsorption spectra of RbFeCI)·2H 20 with ilia and JIlle at 14 K.
netic moments are opposite to each other. In fields higher than 4' T, almost all of the weak ferromagnetic moments are ' parallel to the field direction. In such a quasi-forced ferromagnetic state, some of the weak ferromagnetic moments must be flipped opposite to the field when the soliton is excited in the chain. Thus, the excitation of the soliton is suddenly suppressed by a field parallel to the c-axis between 2 and 4 T. Further work is needed to elucidate the difference in the intensity of two branches.
The authors are much indebted to all staff members in High Field Laboratory for the Superconducting Materials at Tohoku University for the operation of the water-cooled resistive magnet. References [1) M. Steiner and A.R. Bishop, in: Solitons. eds. V.L. Pokrovsky, S.E. Trullinger and V.E. Zakharov (Elsevier, Amsterdam. 1986) p. 783. [2) I. Mogi, M. Takeda, G. Kido, Y. Nakagawa. H. Kikuchi and Y. Ajiro, r, Phys. Soc. Jpn. 58 (1989) 2188. (3) l .AJ. Basten, Q.A.G. van Vlimmeren and W.J.M. de Jonge, Phys. Rev. B 18 (1978) 2179.
246
M. Takeda et al. / Suppression of soliton in RbFeCI J • 2/1P
(4] H.H.A. Srnit, H.J.M. de Groot. R.C. Thiel. LJ. de Jongh, C.E. Johnson and M.E Thomas, Solid State Commun. 53 (1985) 573. [5] A.M.C. Tinus, C.J.M. Denissen, H. Nishihara and W.J.M. de Jonge, J. Phys. C 15 (1982) L791.
(6] Q.A.G. van Vlimmeren, C.H.W. Swiiste. \V.J.M. de Jonge, M.J.H. van der Steeg. J.H.M. Stoelinga and P. Wyder, Phys. Rev. B 21 (1980) 3005. [7] H. Okada, N. Kojima, T. Ban and I. Tsujikawa, Phys. Rev. B (submitted).
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 244-246 North- Holland
Suppression of soliton excitation by external field in quasi-one-dimensional Ising antiferromagnet RbFeCl 3 • 2H 20 M. Takeda, I. Magi, G. Kido, Y. Nakagawa, H. Okada
a
and N. Kojima
a
Institute for Materials Research, Tohoku University, Sendai 980, Japan Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
a
The absorption spectra of domain wall soliton in the one-dimensional canted Ising-like anti ferromagnet RbFeCI 3·2H 20 have been investigated in a continuous high magnetic field. The suppression of the soliton excitation was found in fields higher than 2 T applied parallel to the orthorhombic c-axis.
In low-dimensional magnetic materials nonlinear excitations, i.e, solitons, exist, as well as spin waves. In I-D Ising-like antiferromagnets a soliton is a thermally excited antiphase boundary. Such solitons in CsCoCl 3 and CsCoBr3 have been investigated by neutron scattering, ESR, Raman spectroscopy, magnetoabsorption and so on [1,2]. In RbFeCI 3 • 2H 20 , which is known to be a I-D Ising-like antiferromagnet with a canted spin structure [3], the presence of the soliton has been reported by Smit et al. [4]. They observed the anomalous line broadening in the Mossbauer spectra due to thermal excitation of solitons. However, the measurement of T1- 1 using a NMR technique has only shown preliminary and speculative evidence of the presence of the soliton [5]. RbFeCI 3·2H 20 belongs to the series of isomorphous transition-metal halides whose chemical formula is written as ABX 3 • 2H 20 (A = Rb, Cs; M = Mn, Fe, Co; X = CI, Br), A magnetic linear chain in RbFeCI 3·2H 20 consists of antiferromagnetically coupled spins running along the orthorhombic a-axis. The intrachain interactions (Ja/k n = -35 K) are two orders of magnitude stronger than interchain ones (Jb/k n = -0.76 K, Jclkn = -0.21 K) [6]. Below the Neel temperature (TN '= 11.96 K) the interchain interactions lead to a three dimensional ordered phase in which all Ising spins are situated in the ac-plane and the local easy axis of the spins makes an angle of 15 0 from the a-axis: each chain possessing weak ferromagnetic moment in the direction of the c-axis, In the isomorphous compound CsFeCI 3 • 2H 20 , magnetoabsorption spectra have been measured in a field parallel to the a-axis by Okada et al. [7]. They found the absorption line, which directly indicated the presence of soliton. The advantage of the optical investigation of the soliton in CsFeCI 3 • 2H 20 is that the intensity of the line is proportional to the soliton den-
sity. The same technique is employed in the present work to study the soliton in RbFeCI 3 • 2H 20 . In this paper we report an optical investigation of the soliton in RbFeCI 3 • 2H 2 0 . It has been found that the excitation of the soliton is significantly suppressed . by an external field parallel to the c-axis. Single crystals were obtained from a solution of FeCI 2 • 4H 20 and RbCI in a molar ratio of 3.2: 1 by slow evaporation. Typical dimensions of a sample used in these experiments were 3 X 3 X 1 mrrr', The sample was held between copper plates and fixed in the vacuum space of a metal cell immersed in liquid helium. The temperature was varied from 4.2 to 30 K by means of an electric heater and stabilized by a capacitance controller. An optical fiber was used for transmission of light. The intensity of monochromatic light through the sample was detected by a photomultiplier. A continuous high magnetic field up to 14 T was produced by a water-cooled resistive magnet at Tohoku University. Fig. 1 is the schematic representation of (a) the antiferromagnetic chain and (b) the soliton in RbFeCI J ·2H 20 . The arrow represents Ising spins along the magnetic chain and the vertical bar the soliton. The exciton line which indicates the presence of the soliton is different from the usual exciton line. The energy difference amounts to approximately a " since the dominant intrachain exchange interactions are canceled out at such sites as indicated by dots in fig. l(b).
'J
----- --
( 0 ) - - - - --- - - --- - - - - - - (b) ---~I~
Fig. 1. Schematic representation of the anti ferromagnetic chain of canted Ising spins in the anti ferromagnetic phase (a) and the soliton (b). The exciton at the dotted sites is different from that at usual sites.
0304-8853/90/$03.50 V 1990 - Elsevier Science Publishers B.Y. (North-Holland) and Yamada Science Foundation
245
M. Takeda et al. / Suppression of soli/on in RbFeClJ • 2JI10
Fig. 2 shows the absorption spectra at 14 K with the electric vector of the incident light, i, 110 and the external magnetic field, H, lie in the Voigt configuration. Even at 14 K ( > TN), there exist antiferromagnetic clusters in the linear chain. The absorption line, denoted by AO, is due to an usual exciton in the antiferromagnetic cluster and the H3 line is assigned as the exciton at the sites on the soliton. The soliton in a 1-0 Ising system is thermally excited so that the H3 line is observed as a hot band which appears above 10 K. The Al line and the HI line are the magnon sidebands of the AO line and of the H3 line, respectively. The details of the assignment are given for the isomorphous compound CsFeCI 3 • 2HzO in ref, [7]. As a field is increased, the AO line shows only an energy shift to higher energy. On the contrary, a splitting of the H3 line. can be observed in fields higher than 4 T. If there were no soliton in the antiferromagnetic chain, only the AO line would appear. Hence, the observation of the H3 line clearly indicates the presence of the soliton, which was not observed by Mossbauer experiments in a field of 6 T. The lower energy branch is unobservable in fields higher than 10 T, while the higher energy branch loses its intensity but can still be detected at 14 T. At temperatures lower than 14 K the H3 line begins to be suppressed in fields less than 4 T and completely disappears at 14 T. In the following discussion we concentrate on the H3 line and the AO line and ignore the other absorption lines which appear in fig. 2. In the antiferromagnetic cluster, the sublattice splitting of the AO line should be observed. However, the splitting is not observed, for the following reasons. It has been found that at 14 K the magnetization monotonically increases with increasing field, due to the flipping of the weak ferromagnetic moment, and almost saturates in fields higher than 4 T. Therefore, the magnetic field causes the sublattice splitting of the AO line, due to the weak ferromagnetic moment, but one of two branches disappears in the field higher than 4 T. The magnitude of the splitting in fields less than 4 Tis, however, very small compared with the line width of the AO line. Thus, the AO line only shows the energy shift without splitting. On the other hand, the splitting of the H3 line was observed, but the intensity of both branches became weak with increasing field. This splitting isattributed to the fact that the weak ferromagnetic moments of the two sites on the soliton are opposite to each other (see fig. l(b». The decrease in intensity of the H3 line can be explained as described below. The creation of one soliton in a canted antiferromagnetic chain gives rise to two antiferromagnetic regions whose weak ferromag-
Hllc Ella
14K
,uoH(T) 14
12 (\J
u
c .a....
10
(1
0 III
.a
8
6 4
2
o 24200 24250 Wave Number (cm") Fig. 2. The Cine structure of the magnetoabsorption spectra of RbFeCI)·2H 20 with ilia and JIlle at 14 K.
netic moments are opposite to each other. In fields higher than 4' T, almost all of the weak ferromagnetic moments are ' parallel to the field direction. In such a quasi-forced ferromagnetic state, some of the weak ferromagnetic moments must be flipped opposite to the field when the soliton is excited in the chain. Thus, the excitation of the soliton is suddenly suppressed by a field parallel to the c-axis between 2 and 4 T. Further work is needed to elucidate the difference in the intensity of two branches.
The authors are much indebted to all staff members in High Field Laboratory for the Superconducting Materials at Tohoku University for the operation of the water-cooled resistive magnet. References [1) M. Steiner and A.R. Bishop, in: Solitons. eds. V.L. Pokrovsky, S.E. Trullinger and V.E. Zakharov (Elsevier, Amsterdam. 1986) p. 783. [2) I. Mogi, M. Takeda, G. Kido, Y. Nakagawa. H. Kikuchi and Y. Ajiro, r, Phys. Soc. Jpn. 58 (1989) 2188. (3) l .AJ. Basten, Q.A.G. van Vlimmeren and W.J.M. de Jonge, Phys. Rev. B 18 (1978) 2179.
246
M. Takeda et al. / Suppression of soliton in RbFeCI J • 2/1P
(4] H.H.A. Srnit, H.J.M. de Groot. R.C. Thiel. LJ. de Jongh, C.E. Johnson and M.E Thomas, Solid State Commun. 53 (1985) 573. [5] A.M.C. Tinus, C.J.M. Denissen, H. Nishihara and W.J.M. de Jonge, J. Phys. C 15 (1982) L791.
(6] Q.A.G. van Vlimmeren, C.H.W. Swiiste. \V.J.M. de Jonge, M.J.H. van der Steeg. J.H.M. Stoelinga and P. Wyder, Phys. Rev. B 21 (1980) 3005. [7] H. Okada, N. Kojima, T. Ban and I. Tsujikawa, Phys. Rev. B (submitted).