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Investigation of spin-glass behavior in the LnRu-1222 (Ln=Gd, Dy, Ho, Y) system
Physica C 408–410 (2004) 183–184 www.elsevier.com/locate/physc
Investigation of spin-glass behavior in the LnRu-1222 (Ln ¼ Gd, Dy, Ho, Y) system C.A. Cardoso a,*, F.M. Araujo-Moreira a, V.P.S. Awana b, E. Takayama-Muromachi b, O.F. de Lima c, H. Yamauchi d, M. Karppinen
d
a
b
Grupo de Materiais e Dispositivos, Rodovia Washington Luiz, CMDMC, DF, UFSCar, Caixa Postal 676, 13565-905 S~ao Carlos-SP, Brazil Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c Instituto de Fısica ‘‘Gleb Wataghin’’, UNICAMP, 13083-970 Campinas-SP, Brazil d Materials Science Laboratory, Tokyo Institute of Technology, Nagatsuta, Yokohama 226-8503, Japan
Abstract The dynamics of the magnetic properties of polycrystalline RuSr2 Ln1:5 Ce0:5 Cu2 O10 (LnRu-1222, Ln ¼ Gd, Y, Ho, Nd) have been studied by ac susceptibility. A frequency-dependent cusp was observed in vac vs. T measurements, which is interpreted as a spin-glass transition. These results are contrasted with the regular antiferromagnetic transition observed for YRu-1212. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Rutheno-cuprates; Magneto-superconductors; Spin glass
1. Introduction
2. Results and discussion
The coexistence of superconductivity and magnetism was first reported in RuSr2 Gd1:5 Ce0:5 Cu2 O10 (GdRu1222) [1] and later in RuSr2 GdCu2 O8 (GdRu-1212) [2]. This later compound has been rigorously studied and there is now a consensus that it presents at low temperatures an antiferromagnetic order with a weak ferromagnetic component due to spin canting. Although the magnetic behavior of GdRu-1222 seems, at a first glance, to be analogous to the one for GdRu-1212 samples, recent results point towards various differences [3,4]. In particular, it was found strong evidence of spinglass behavior in GdRu-1222 [4]. In this work we give further evidence supporting those findings by comparing the ac susceptibility, vac ¼ v0 þ iv00 for LnRu-1222, Ln ¼ Y, Gd, Ho, Dy, and YRu-1212.
The ac susceptibility technique is a very powerful method to provide evidence of a spin-glass behavior. In this case, both components v0 and v00 present a sharp, frequency-dependent cusp. The position of the cusp in v0 defines the freezing temperature Tf which is coincident with the temperature of the inflection point in v00 . In Fig. 1 we present the real component of the ac susceptibility, v0 , for the GdRu-1222 sample measured at Hdc ¼ 50 Oe. The main frame of Fig. 1 presents the ZFC/FC temperature dependence of v0 for the frequency m ¼ 10 000 Hz. v0 presents an abrupt drop at the superconducting transition temperature Tc and a sharp, frequencydependent peak at Tf 72 K. The peak shifts to lower temperatures and its intensity increases as the frequency of the excitation field is decreased (see inset, Fig. 1). For the v00 peak (not shown) we observe the shift to lower temperatures as well as a decrease of its intensity with decreasing the frequency of the excitation field. The frequency dependence of both components is a typical feature of the dynamics of spin-glass systems. Other results like magnetic relaxation and the observation of
*
Corresponding author. Tel.: +55-16-2608226/2608210/ 2608205; fax: +55-16-2614835. E-mail address: [email protected] (C.A. Cardoso).
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.02.159
184
C.A. Cardoso et al. / Physica C 408–410 (2004) 183–184
Fig. 1. Real component v0 of the complex susceptibility as a function of temperature for m ¼ 10 kHz (main panel) for GdRu1222. Inset shows the frequency dependence of the peak in v0 at the freezing temperature Tf .
ageing effects for this same GdRu-1222 sample corroborate the spin-glass scenario [4]. Besides the spin-glass transition, it was observed also an anomaly in v0 at Tm 164 K, which is associated with the appearance of irreversibility in the magnetization measurement [4]. This is not fully understood yet, but it is believed that in this region the Ru spins present an incipient AFM ordering. Samples of the Ru-1222 with heavier lanthanides or yttrium present a similar magnetic behavior with two important differences. First, they do not present a superconducting transition. Second, the anomaly at high temperatures is much stronger. In this paper, however, we will limit the discussion to the strong peak in v0 . Fig. 2 presents a detailed study of the frequency dependence of the peak in v0 of LnRu-1222, Ln ¼ Y, Ho, Dy, and also for YRu-1212. The peak in v0 presented a clear frequency dependence for all LnRu-1222 samples. A quantitative measure of the frequency shift is obtained from DTf =ðTf logðxÞÞ. This quantity varies in the range of 0.004–0.018 for spin-glass systems [5], while for superparamagnets [5] it is of the order of 0.3. From the set of measurements presented in Figs. 1 and 2, we could estimate DTf =ðTf logðxÞÞ 0:005, 0.003, 0.0021 and 0.0017 for our Gd-, Dy-, Ho- and YRu-1222 sample. Therefore, our data indicate that all studied LnRu-1222 samples present a spin-glass phase. On the other hand, the peak in v0 for the YRu-1212 sample do not present any meaningful change for the frequencies studied, as expected for a regular antiferromagnetic transition. The AFM ordering for the Ru-1212 family is vastly supported by the literature. Our results indicate that these two families present a striking different magnetic behavior, although they may be easily confounded if only magnetization measurements are considered.
Fig. 2. Frequency dependence of the peak in v0 for LnRu-1222 (Ln ¼ Y, Ho, Dy) and YRu-1212.
3. Conclusions The frequency-dependent peak observed in the temperature dependence of the ac susceptibility vac provides strong evidence of the existence of spin-glass properties for samples of the LnRu-1222 family. This is to be contrasted with the usual interpretation of the existence of long-range antiferromagnetic order with spin canting. For the YRu-1212 sample, it was found that it presents the expected AFM transition. The microscopic reason why Ru-1212 presents a long-range order, while Ru1222 does not, is unclear at this time. It is also important to carefully analyze if sample quality is not affecting the results, since it is well known that the spin-glass behavior is very sensitive to sample quality.
Acknowledgements This work was supported by Brazilian agencies FAPESP and CNPq.
References [1] [2] [3] [4] [5]
I. Felner et al., Phys. Rev. B 55 (1997) R3374. C. Bernhard et al., Phys. Rev. B 59 (1999) 14099. I. Zivkovi c et al., Phys. Rev. B 65 (2002) 144420. C.A. Cardoso et al., Phys. Rev. B 67 (2003) 020407. J.A. Mydosh, Spin Glasses: An Experimental Introduction, Taylor & Francis, London, 1993.
Investigation of spin-glass behavior in the LnRu-1222 (Ln ¼ Gd, Dy, Ho, Y) system C.A. Cardoso a,*, F.M. Araujo-Moreira a, V.P.S. Awana b, E. Takayama-Muromachi b, O.F. de Lima c, H. Yamauchi d, M. Karppinen
d
a
b
Grupo de Materiais e Dispositivos, Rodovia Washington Luiz, CMDMC, DF, UFSCar, Caixa Postal 676, 13565-905 S~ao Carlos-SP, Brazil Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c Instituto de Fısica ‘‘Gleb Wataghin’’, UNICAMP, 13083-970 Campinas-SP, Brazil d Materials Science Laboratory, Tokyo Institute of Technology, Nagatsuta, Yokohama 226-8503, Japan
Abstract The dynamics of the magnetic properties of polycrystalline RuSr2 Ln1:5 Ce0:5 Cu2 O10 (LnRu-1222, Ln ¼ Gd, Y, Ho, Nd) have been studied by ac susceptibility. A frequency-dependent cusp was observed in vac vs. T measurements, which is interpreted as a spin-glass transition. These results are contrasted with the regular antiferromagnetic transition observed for YRu-1212. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Rutheno-cuprates; Magneto-superconductors; Spin glass
1. Introduction
2. Results and discussion
The coexistence of superconductivity and magnetism was first reported in RuSr2 Gd1:5 Ce0:5 Cu2 O10 (GdRu1222) [1] and later in RuSr2 GdCu2 O8 (GdRu-1212) [2]. This later compound has been rigorously studied and there is now a consensus that it presents at low temperatures an antiferromagnetic order with a weak ferromagnetic component due to spin canting. Although the magnetic behavior of GdRu-1222 seems, at a first glance, to be analogous to the one for GdRu-1212 samples, recent results point towards various differences [3,4]. In particular, it was found strong evidence of spinglass behavior in GdRu-1222 [4]. In this work we give further evidence supporting those findings by comparing the ac susceptibility, vac ¼ v0 þ iv00 for LnRu-1222, Ln ¼ Y, Gd, Ho, Dy, and YRu-1212.
The ac susceptibility technique is a very powerful method to provide evidence of a spin-glass behavior. In this case, both components v0 and v00 present a sharp, frequency-dependent cusp. The position of the cusp in v0 defines the freezing temperature Tf which is coincident with the temperature of the inflection point in v00 . In Fig. 1 we present the real component of the ac susceptibility, v0 , for the GdRu-1222 sample measured at Hdc ¼ 50 Oe. The main frame of Fig. 1 presents the ZFC/FC temperature dependence of v0 for the frequency m ¼ 10 000 Hz. v0 presents an abrupt drop at the superconducting transition temperature Tc and a sharp, frequencydependent peak at Tf 72 K. The peak shifts to lower temperatures and its intensity increases as the frequency of the excitation field is decreased (see inset, Fig. 1). For the v00 peak (not shown) we observe the shift to lower temperatures as well as a decrease of its intensity with decreasing the frequency of the excitation field. The frequency dependence of both components is a typical feature of the dynamics of spin-glass systems. Other results like magnetic relaxation and the observation of
*
Corresponding author. Tel.: +55-16-2608226/2608210/ 2608205; fax: +55-16-2614835. E-mail address: [email protected] (C.A. Cardoso).
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.02.159
184
C.A. Cardoso et al. / Physica C 408–410 (2004) 183–184
Fig. 1. Real component v0 of the complex susceptibility as a function of temperature for m ¼ 10 kHz (main panel) for GdRu1222. Inset shows the frequency dependence of the peak in v0 at the freezing temperature Tf .
ageing effects for this same GdRu-1222 sample corroborate the spin-glass scenario [4]. Besides the spin-glass transition, it was observed also an anomaly in v0 at Tm 164 K, which is associated with the appearance of irreversibility in the magnetization measurement [4]. This is not fully understood yet, but it is believed that in this region the Ru spins present an incipient AFM ordering. Samples of the Ru-1222 with heavier lanthanides or yttrium present a similar magnetic behavior with two important differences. First, they do not present a superconducting transition. Second, the anomaly at high temperatures is much stronger. In this paper, however, we will limit the discussion to the strong peak in v0 . Fig. 2 presents a detailed study of the frequency dependence of the peak in v0 of LnRu-1222, Ln ¼ Y, Ho, Dy, and also for YRu-1212. The peak in v0 presented a clear frequency dependence for all LnRu-1222 samples. A quantitative measure of the frequency shift is obtained from DTf =ðTf logðxÞÞ. This quantity varies in the range of 0.004–0.018 for spin-glass systems [5], while for superparamagnets [5] it is of the order of 0.3. From the set of measurements presented in Figs. 1 and 2, we could estimate DTf =ðTf logðxÞÞ 0:005, 0.003, 0.0021 and 0.0017 for our Gd-, Dy-, Ho- and YRu-1222 sample. Therefore, our data indicate that all studied LnRu-1222 samples present a spin-glass phase. On the other hand, the peak in v0 for the YRu-1212 sample do not present any meaningful change for the frequencies studied, as expected for a regular antiferromagnetic transition. The AFM ordering for the Ru-1212 family is vastly supported by the literature. Our results indicate that these two families present a striking different magnetic behavior, although they may be easily confounded if only magnetization measurements are considered.
Fig. 2. Frequency dependence of the peak in v0 for LnRu-1222 (Ln ¼ Y, Ho, Dy) and YRu-1212.
3. Conclusions The frequency-dependent peak observed in the temperature dependence of the ac susceptibility vac provides strong evidence of the existence of spin-glass properties for samples of the LnRu-1222 family. This is to be contrasted with the usual interpretation of the existence of long-range antiferromagnetic order with spin canting. For the YRu-1212 sample, it was found that it presents the expected AFM transition. The microscopic reason why Ru-1212 presents a long-range order, while Ru1222 does not, is unclear at this time. It is also important to carefully analyze if sample quality is not affecting the results, since it is well known that the spin-glass behavior is very sensitive to sample quality.
Acknowledgements This work was supported by Brazilian agencies FAPESP and CNPq.
References [1] [2] [3] [4] [5]
I. Felner et al., Phys. Rev. B 55 (1997) R3374. C. Bernhard et al., Phys. Rev. B 59 (1999) 14099. I. Zivkovi c et al., Phys. Rev. B 65 (2002) 144420. C.A. Cardoso et al., Phys. Rev. B 67 (2003) 020407. J.A. Mydosh, Spin Glasses: An Experimental Introduction, Taylor & Francis, London, 1993.