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ESR study of the residual magnetism in the spin–Peierls phase
Physica B 284}288 (2000) 1649}1650
ESR study of the residual magnetism in the spin}Peierls phase A.I. Smirnov *, V.N. Glazkov , L.I. Leonyuk, A.G. Vetkin P.L. Kapitza Institute for Physical Problems RAS, Kosygin str. 2, 117334 Moscow, Russia M.V. Lomonosov Moscow State University, 119899 Moscow, Russia
Abstract Weak ESR signals of pure CuGeO crystals found at low temperature indicate three types of defects in the spin}Peierls state: single spins, pairs of spins, spin clusters. These defects in high-quality crystals are ascribed to boundaries between domains arising due to breaks of the coherence of lattice dimerization. 2000 Elsevier Science B.V. All rights reserved. Keywords: CuGeO ; Low-dimensional magnetism; Spin clusters; Spin}Peierls crystal
The quasi-one-dimensional magnet CuGeO has been extensively studied as the "rst inorganic spin-Peierls (SP) material [1]. The SP phase transition occurs because the S" Heisenberg antiferromagnetic chains are unstable when coupled to a three-dimensional lattice. Below the SP-transition temperature ¹ "14.5 K mag1. netic chains are dimerized providing an energy gap. Therefore magnetic susceptibility should decrease and tends to zero below ¹ . Nevertheless, pure CuGeO 1. crystals do not show a perfect freezing-out of the susceptibility. There is a small residual susceptibility, which should be provided by defects. We found that the residual magnetic susceptibility is larger than the susceptibility of impurities and thus should be ascribed to some kind of intrinsic defects. The ESR study of residual magnetism is performed in the frequency range 9}75 GHz and a model of intrinsic defects is proposed. The ESR spectrum of a pure CuGeO single crystal in the residual susceptibility temperature range is shown in Fig. 1. The spectrum contains a triplet of lines 1,2,3 in the vicinity of the Cu>-ESR position and a separate line 4 at much higher "elds. Only a single line was observable at ¹"¹ . The integral ESR intensity equals the ESR 1. intensity of a paramagnet containing 10\ of the total number of Cu-ions. The concentrations of impurities of
* Corresponding author. E-mail address: [email protected] (A.I. Smirnov)
Fe, Ni, Co, Mn in our sample do not exceed 10\ per Cu-ion. The intensity of the line 4 is correlated with the amount of structural defects arising in di!erent samples at di!erent crystallization rates. Thus, all four ESR lines are due to Cu-ions and not to impurities. Samples of di!erent quality show the ESR lines 1}4 at the same "elds and frequencies while the total intensity and the spectral weight of components di!er for di!erent samples. Line 2 is placed at the "eld corresponding to Cu> ions and may be naturally associated with the undimerized Cuions at the breaks of spin chains. Within the frequency interval mentioned the lines 1 and 3 demonstrate frequency independent shifts of the resonance "eld with respect to line 2, but both lines correspond to the same g-factor value as line 2. These shifts and their angular dependencies are typical for a spin S"1 ESR in a crystal. Thus, we suppose that S"1 objects are exchange coupled pairs of Cu> ions. Close values of intensities of a single ion and pair resonances mean that concentrations of single spins and of pairs are nearly the same. For random defects distribution the number of pairs should be much smaller than of single spins. Line 4 has a remarkable power dependence of the ESR susceptibility. In the frequency range 20}23 GHz the susceptibility increases with power in a threshold manner, both at the peak of the resonance absorption and on the wings. It is a contradiction with the known saturation of the ESR susceptibility of noninteracting spins. A model of intrinsic defects of the SP state is based on the consideration of SP domains shown in Fig. 2. The
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 7 9 3 - 3
1650
A.I. Smirnov et al. / Physica B 284}288 (2000) 1649}1650
Fig. 1. 26.7 GHz ESR line (transmission versus "eld), ¹" 1.3 K, H "" c.
Fig. 2. Spin-Peierls domains and domain walls. Open and "lled circles are dimerized and undimerized Cu-ions, respectively.
breaking of dimers correlation between neighboring chains produces a type I domain wall, and breaking of dimers order within chains results in a domain wall of type II. These domain walls contain undimerized spins
and thus are magnetic defects. The domain walls are nonequilibrium objects, they survive due to pinning at defects of high-temperature structure. Therefore, the number of undimerized spins is much greater than the number of that defects. We ascribe the S"1 component of the ESR signal to the Cu-ions pairs arising at breaks of chains in domain walls of type I or at steps therein (see Fig. 2). Pairs of spins should occur here due to breaks of antiferromagnetic coupling within chains and to the known weak ferromagnetic exchange in a-direction. Paths of ferromagnetic exchange are shown on Fig. 2 by dashed lines. The ESR line 4 is probably due to type II walls because they contain undimerized spins within dimerized chains which result in spin-correlated clusters [2]. The anomalous g-factor of line 4 was explained on the basis of a 5-spins cluster model (two pairs of spins on both sides from the undimerized spin) including symmetric and antisymmetric exchange interactions [3]. The clusters in the domain wall should be correlated due to interchain exchanges. Therefore, the nonlinear ESR susceptibility may be explained by parametric excitation of the spin wave like excitations in 2D-correlated domain walls [3]. Thus, the described domain walls should be a special type of 2D magnets.
References [1] M. Hase et al., Phys. Rev. Lett. 70 (1993) 3651. [2] D. Khomskii et al., Chech. J. Phys. 46 (Suppl. S6) (1996) 3239. [3] A.I. Smirnov et al., Zh. Eksp. Teor. Fiz. 114 (1998) 1876 [JETP 87 (1998) 1019].
ESR study of the residual magnetism in the spin}Peierls phase A.I. Smirnov *, V.N. Glazkov , L.I. Leonyuk, A.G. Vetkin P.L. Kapitza Institute for Physical Problems RAS, Kosygin str. 2, 117334 Moscow, Russia M.V. Lomonosov Moscow State University, 119899 Moscow, Russia
Abstract Weak ESR signals of pure CuGeO crystals found at low temperature indicate three types of defects in the spin}Peierls state: single spins, pairs of spins, spin clusters. These defects in high-quality crystals are ascribed to boundaries between domains arising due to breaks of the coherence of lattice dimerization. 2000 Elsevier Science B.V. All rights reserved. Keywords: CuGeO ; Low-dimensional magnetism; Spin clusters; Spin}Peierls crystal
The quasi-one-dimensional magnet CuGeO has been extensively studied as the "rst inorganic spin-Peierls (SP) material [1]. The SP phase transition occurs because the S" Heisenberg antiferromagnetic chains are unstable when coupled to a three-dimensional lattice. Below the SP-transition temperature ¹ "14.5 K mag1. netic chains are dimerized providing an energy gap. Therefore magnetic susceptibility should decrease and tends to zero below ¹ . Nevertheless, pure CuGeO 1. crystals do not show a perfect freezing-out of the susceptibility. There is a small residual susceptibility, which should be provided by defects. We found that the residual magnetic susceptibility is larger than the susceptibility of impurities and thus should be ascribed to some kind of intrinsic defects. The ESR study of residual magnetism is performed in the frequency range 9}75 GHz and a model of intrinsic defects is proposed. The ESR spectrum of a pure CuGeO single crystal in the residual susceptibility temperature range is shown in Fig. 1. The spectrum contains a triplet of lines 1,2,3 in the vicinity of the Cu>-ESR position and a separate line 4 at much higher "elds. Only a single line was observable at ¹"¹ . The integral ESR intensity equals the ESR 1. intensity of a paramagnet containing 10\ of the total number of Cu-ions. The concentrations of impurities of
* Corresponding author. E-mail address: [email protected] (A.I. Smirnov)
Fe, Ni, Co, Mn in our sample do not exceed 10\ per Cu-ion. The intensity of the line 4 is correlated with the amount of structural defects arising in di!erent samples at di!erent crystallization rates. Thus, all four ESR lines are due to Cu-ions and not to impurities. Samples of di!erent quality show the ESR lines 1}4 at the same "elds and frequencies while the total intensity and the spectral weight of components di!er for di!erent samples. Line 2 is placed at the "eld corresponding to Cu> ions and may be naturally associated with the undimerized Cuions at the breaks of spin chains. Within the frequency interval mentioned the lines 1 and 3 demonstrate frequency independent shifts of the resonance "eld with respect to line 2, but both lines correspond to the same g-factor value as line 2. These shifts and their angular dependencies are typical for a spin S"1 ESR in a crystal. Thus, we suppose that S"1 objects are exchange coupled pairs of Cu> ions. Close values of intensities of a single ion and pair resonances mean that concentrations of single spins and of pairs are nearly the same. For random defects distribution the number of pairs should be much smaller than of single spins. Line 4 has a remarkable power dependence of the ESR susceptibility. In the frequency range 20}23 GHz the susceptibility increases with power in a threshold manner, both at the peak of the resonance absorption and on the wings. It is a contradiction with the known saturation of the ESR susceptibility of noninteracting spins. A model of intrinsic defects of the SP state is based on the consideration of SP domains shown in Fig. 2. The
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 7 9 3 - 3
1650
A.I. Smirnov et al. / Physica B 284}288 (2000) 1649}1650
Fig. 1. 26.7 GHz ESR line (transmission versus "eld), ¹" 1.3 K, H "" c.
Fig. 2. Spin-Peierls domains and domain walls. Open and "lled circles are dimerized and undimerized Cu-ions, respectively.
breaking of dimers correlation between neighboring chains produces a type I domain wall, and breaking of dimers order within chains results in a domain wall of type II. These domain walls contain undimerized spins
and thus are magnetic defects. The domain walls are nonequilibrium objects, they survive due to pinning at defects of high-temperature structure. Therefore, the number of undimerized spins is much greater than the number of that defects. We ascribe the S"1 component of the ESR signal to the Cu-ions pairs arising at breaks of chains in domain walls of type I or at steps therein (see Fig. 2). Pairs of spins should occur here due to breaks of antiferromagnetic coupling within chains and to the known weak ferromagnetic exchange in a-direction. Paths of ferromagnetic exchange are shown on Fig. 2 by dashed lines. The ESR line 4 is probably due to type II walls because they contain undimerized spins within dimerized chains which result in spin-correlated clusters [2]. The anomalous g-factor of line 4 was explained on the basis of a 5-spins cluster model (two pairs of spins on both sides from the undimerized spin) including symmetric and antisymmetric exchange interactions [3]. The clusters in the domain wall should be correlated due to interchain exchanges. Therefore, the nonlinear ESR susceptibility may be explained by parametric excitation of the spin wave like excitations in 2D-correlated domain walls [3]. Thus, the described domain walls should be a special type of 2D magnets.
References [1] M. Hase et al., Phys. Rev. Lett. 70 (1993) 3651. [2] D. Khomskii et al., Chech. J. Phys. 46 (Suppl. S6) (1996) 3239. [3] A.I. Smirnov et al., Zh. Eksp. Teor. Fiz. 114 (1998) 1876 [JETP 87 (1998) 1019].