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Spin-Peierls and spin-glass phases in pure and doped CuGeO3: a μSR study
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1687-1688
ELSEVIER
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magnetism and magnetic maicrlals
Spin-Peierls and spin-glass phases in pure and doped CuGeO3: a study O. Tchernyshyov a,*, A.S. Blaer a, A. Keren a, K. Kojima a,b, G.M. Luke a, W.D. Wu a, y.j. Uemura a, M. Hase c K. Uchinokura c, y. Ajiro d, T. Asano d, M. Mekata d a Dept. of Physics, Columbia University, New York, NY 10027, USA b University of Tokyo Meson Science Laboratory, Tokyo 113, Japan c Dept. of AppliedPhysics, University of Tokyo, Tokyo 113, Japan d Dept. of Applied Physics, Fukui University, Fukui 910, Japan Abstract We have conducted IxSR measurements in pure and Zn-doped CuGeO 3. The pure compound does not have static magnetic moments between 3 and 100 K, in agreement with its earlier proposed spin-Peierls structure. In Cuo.96Zn0.04GeO3, we found a spin-glass-like freezing of a small portion of Cu spins, presumably near the chain-cutting Zn atoms, occurring gradually between 3.5 and 6 K.
CuGeO 3, a compound with C u - O 2 - C u chains with antiferromagnetic interaction between Cu spins, undergoes a spin-Peierls (SP) transition at Tsp = 14 K [1]. Recent experiments (see, for example, Ref. [2]) support the simple picture found in organic SP compounds: neighboring Cu spins shift towards one another and form spin-singlet pairs; triplet excitations become separated by a gap; as a result, magnetic susceptibility quickly falls off with decreasing temperature. Cutting these chains by doping Zn reduces Tsp in CUl_xZnxGeO 3. Doping also produces unpaired Cu 2+ spins that freeze into a spin glass below a transition temperature Ts~. At the doping level x = 0.03, TsG and Tsp become equal, which was interpreted as a collapse of the SP state [3]. It was argued [4] that chain dimerization may still persist even though there is no gap in the excitation spectrum. No evidence of a spin-glass-like state has been found in a 3%-doped sample by ~SR methods [5], contrary to our results with 4% doping. We performed a series of ~SR measurements at TRIUMF (Vancouver, Canada) using sintered CuGeO 3 (Tsp = 14 K) and Cuo.96Zno.o4GeO 3 (TsG is about 5 K) in the range of temperatures from 0.05 to 100 K. The pure sample does not exhibit static magnetism in the temperature range 0.05-100 K. In a transverse field of
* Corresponding author. Fax: + 1-212-854-5888; email shev @phys.columbia.edu.
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T i m e , /~s Fig. 1. Corrected muon spin asymmetry. (a) CuGeO3 in zero magnetic field. T = 2 and 10 K. (b)Cuo.96Zno.o4GeOa in zero magnetic field. T varies from 2.1 to 10 K. (c) Cu0.96Zno.o4GeO 3 in longitudinal magnetic field at 2.1 K. Solid lines are curves fit from Eq. (1). The doped system has a much higher staggered magnetic field at T < 6 K than the pure one. Muon relaxation is completely static as longitudinal field easily decouples it.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 1 0 7 - 9
1688
O. Tchernyshyovet al./Journal of Magnetism and MagneticMaterials 140-144 (1995) 1687-1688
2 kG, the relaxation rate in G(t)= exp(-trZt 2) cos tot was tr = 0.065 Ixs -] above 10 K and rose to 0.085 p~s-1 by T = 3 K. Zero-field measurements also revealed some changes below approximately 5 K (Fig. la): the Gaussian shape of the relaxation function gradually deforms into exponential, which might be caused by presence of frozen spins if there are impurities a n d / o r defects in the sample. As temperature was lowered to 50 mK, no considerable changes were found. At all temperatures, it was possible to decouple the muon relaxation by applying a longitudinal magnetic field of 100 G. The 4%-doped sample was weakly magnetic above 6 K. As we lowered the temperature to 4 K, the relaxation rate increased roughly by a factor of 100 hinting at the appearance of frozen magnetic moments (Fig. lb). At T = 2.1 K, a longitudinal field of 100 G noticeably decoupied the relaxation process, while that of 1 kG suppressed it completely (Fig. lc). In zero field, the ix + polarization relaxes to 1 / 3 of its initial value for t > 2 - 3 Ixs. This proves the static character of the relaxation and provides an estimate for the magnetic field from frozen spins of about 100 G. Below 4 K, the relaxation is likely due to freezing electron spins, above 6 K it is caused by nuclear dipolar field. Remarkably, in the intermediate temperature range, we observe both slow (nuclear) and fast (electronic) signals with varying amplitudes. Given the static character of both relaxation mechanisms and a drastic difference of the field strengthes, we conclude that the freezing of spins in the sample is an inhomogeneous process: below 6 K, islands of freezing spins start to appear in the paramagnetic fluctuating sea. No further changes were observed as the temperature was lowered from 2 K to 50 inK. The data can be described very well by the following function representing coexistence of paramagnetic and spin-glass phases in the volume:
G(t) = A s G ( 1 + 2e-(rt)~) + (1 --Aso)GKT(t),
(1)
where GKT(t) is a fixed slowly relaxing function determined at T = 10 K when all spins are paramagnetic. The variable parameters are F , fl and ASG. The contribution from spin glass remarkably does not have a dip below the value of 1 / 3 characteristic of any static relaxation process. We intend to discuss this peculiarity in a more detailed paper. The results of the analysis are shown in Fig. 2. Above 6 K, the whole sample is paramagnetic and ASG ~ 0. Below 4 K, the spins are frozen and AsG ~ 1. Between 4 and 6 K, we observe inhomogeneous freezing of Cu e+ spins. It
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Temperature, K Fig. 2. Parameters in Eq. (1) for the doped system. (a) Fraction of the volume occupied by frozen spins vs. temperature. (b) Relaxation rate in the frozen part of the sample vs temperature. (c) Exponential /3. should be noted that the presence of frozen spins is still evident in our data at 5 K, whereas magnetic susceptibility measurements [3] yielded a lower freezing temperature of 4.7 K. At 2.1 K, the spin-glass staggered magnetic field was about 50 G. Given that magnetic field from one S = 1 spin at a distance of 1.9-2.7 A away from it reaches 500-1000 G, our findings support the idea that only chain ends freeze in the 4%-doped sample of Cu]_xZnxGeO 3.
References [1] M. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. [2] K. Hirota, D.E. Cox, J.E. Lorenzo, G. Shirane, J.M. Tranquada, M. Hase, K. Uchinokura, H. Kojima, Y. Shibuja and I. Tanaka, Phys. Rev. Lett. 73 (1994) 736. [3] M. Hase, I. Terasaki, Y. Sasago, IC Uchinokura and H. Obara, Phys. Rev. Lett. 71 (1993) 4059. [4] Z.Y. Lu, Z.B. Su and L. Yu, Phys. Rev. Lett. 72 (1993) 1276. [5] A. Lappas, K. Prassides, A. Amato, R. Feyerherm, F.N. Gygax and A. Schenck, Z. Phys. B, in press.
ELSEVIER
~
Jcnlna| @1
~i
magnetism and magnetic maicrlals
Spin-Peierls and spin-glass phases in pure and doped CuGeO3: a study O. Tchernyshyov a,*, A.S. Blaer a, A. Keren a, K. Kojima a,b, G.M. Luke a, W.D. Wu a, y.j. Uemura a, M. Hase c K. Uchinokura c, y. Ajiro d, T. Asano d, M. Mekata d a Dept. of Physics, Columbia University, New York, NY 10027, USA b University of Tokyo Meson Science Laboratory, Tokyo 113, Japan c Dept. of AppliedPhysics, University of Tokyo, Tokyo 113, Japan d Dept. of Applied Physics, Fukui University, Fukui 910, Japan Abstract We have conducted IxSR measurements in pure and Zn-doped CuGeO 3. The pure compound does not have static magnetic moments between 3 and 100 K, in agreement with its earlier proposed spin-Peierls structure. In Cuo.96Zn0.04GeO3, we found a spin-glass-like freezing of a small portion of Cu spins, presumably near the chain-cutting Zn atoms, occurring gradually between 3.5 and 6 K.
CuGeO 3, a compound with C u - O 2 - C u chains with antiferromagnetic interaction between Cu spins, undergoes a spin-Peierls (SP) transition at Tsp = 14 K [1]. Recent experiments (see, for example, Ref. [2]) support the simple picture found in organic SP compounds: neighboring Cu spins shift towards one another and form spin-singlet pairs; triplet excitations become separated by a gap; as a result, magnetic susceptibility quickly falls off with decreasing temperature. Cutting these chains by doping Zn reduces Tsp in CUl_xZnxGeO 3. Doping also produces unpaired Cu 2+ spins that freeze into a spin glass below a transition temperature Ts~. At the doping level x = 0.03, TsG and Tsp become equal, which was interpreted as a collapse of the SP state [3]. It was argued [4] that chain dimerization may still persist even though there is no gap in the excitation spectrum. No evidence of a spin-glass-like state has been found in a 3%-doped sample by ~SR methods [5], contrary to our results with 4% doping. We performed a series of ~SR measurements at TRIUMF (Vancouver, Canada) using sintered CuGeO 3 (Tsp = 14 K) and Cuo.96Zno.o4GeO 3 (TsG is about 5 K) in the range of temperatures from 0.05 to 100 K. The pure sample does not exhibit static magnetism in the temperature range 0.05-100 K. In a transverse field of
* Corresponding author. Fax: + 1-212-854-5888; email shev @phys.columbia.edu.
CU1_xZn×GeO 3
1 L. . . . . . .
b) x=0.04, zero field )
1 [] Jo~uu~out21uoa~uodou~moo~ou~uE lkG
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c) x=0.04, LF, T=2.1K 0
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2
T i m e , /~s Fig. 1. Corrected muon spin asymmetry. (a) CuGeO3 in zero magnetic field. T = 2 and 10 K. (b)Cuo.96Zno.o4GeOa in zero magnetic field. T varies from 2.1 to 10 K. (c) Cu0.96Zno.o4GeO 3 in longitudinal magnetic field at 2.1 K. Solid lines are curves fit from Eq. (1). The doped system has a much higher staggered magnetic field at T < 6 K than the pure one. Muon relaxation is completely static as longitudinal field easily decouples it.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 1 0 7 - 9
1688
O. Tchernyshyovet al./Journal of Magnetism and MagneticMaterials 140-144 (1995) 1687-1688
2 kG, the relaxation rate in G(t)= exp(-trZt 2) cos tot was tr = 0.065 Ixs -] above 10 K and rose to 0.085 p~s-1 by T = 3 K. Zero-field measurements also revealed some changes below approximately 5 K (Fig. la): the Gaussian shape of the relaxation function gradually deforms into exponential, which might be caused by presence of frozen spins if there are impurities a n d / o r defects in the sample. As temperature was lowered to 50 mK, no considerable changes were found. At all temperatures, it was possible to decouple the muon relaxation by applying a longitudinal magnetic field of 100 G. The 4%-doped sample was weakly magnetic above 6 K. As we lowered the temperature to 4 K, the relaxation rate increased roughly by a factor of 100 hinting at the appearance of frozen magnetic moments (Fig. lb). At T = 2.1 K, a longitudinal field of 100 G noticeably decoupied the relaxation process, while that of 1 kG suppressed it completely (Fig. lc). In zero field, the ix + polarization relaxes to 1 / 3 of its initial value for t > 2 - 3 Ixs. This proves the static character of the relaxation and provides an estimate for the magnetic field from frozen spins of about 100 G. Below 4 K, the relaxation is likely due to freezing electron spins, above 6 K it is caused by nuclear dipolar field. Remarkably, in the intermediate temperature range, we observe both slow (nuclear) and fast (electronic) signals with varying amplitudes. Given the static character of both relaxation mechanisms and a drastic difference of the field strengthes, we conclude that the freezing of spins in the sample is an inhomogeneous process: below 6 K, islands of freezing spins start to appear in the paramagnetic fluctuating sea. No further changes were observed as the temperature was lowered from 2 K to 50 inK. The data can be described very well by the following function representing coexistence of paramagnetic and spin-glass phases in the volume:
G(t) = A s G ( 1 + 2e-(rt)~) + (1 --Aso)GKT(t),
(1)
where GKT(t) is a fixed slowly relaxing function determined at T = 10 K when all spins are paramagnetic. The variable parameters are F , fl and ASG. The contribution from spin glass remarkably does not have a dip below the value of 1 / 3 characteristic of any static relaxation process. We intend to discuss this peculiarity in a more detailed paper. The results of the analysis are shown in Fig. 2. Above 6 K, the whole sample is paramagnetic and ASG ~ 0. Below 4 K, the spins are frozen and AsG ~ 1. Between 4 and 6 K, we observe inhomogeneous freezing of Cu e+ spins. It
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I
I
,
1.8
1.8
1.4
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1 2
3
4
5
Temperature, K Fig. 2. Parameters in Eq. (1) for the doped system. (a) Fraction of the volume occupied by frozen spins vs. temperature. (b) Relaxation rate in the frozen part of the sample vs temperature. (c) Exponential /3. should be noted that the presence of frozen spins is still evident in our data at 5 K, whereas magnetic susceptibility measurements [3] yielded a lower freezing temperature of 4.7 K. At 2.1 K, the spin-glass staggered magnetic field was about 50 G. Given that magnetic field from one S = 1 spin at a distance of 1.9-2.7 A away from it reaches 500-1000 G, our findings support the idea that only chain ends freeze in the 4%-doped sample of Cu]_xZnxGeO 3.
References [1] M. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. [2] K. Hirota, D.E. Cox, J.E. Lorenzo, G. Shirane, J.M. Tranquada, M. Hase, K. Uchinokura, H. Kojima, Y. Shibuja and I. Tanaka, Phys. Rev. Lett. 73 (1994) 736. [3] M. Hase, I. Terasaki, Y. Sasago, IC Uchinokura and H. Obara, Phys. Rev. Lett. 71 (1993) 4059. [4] Z.Y. Lu, Z.B. Su and L. Yu, Phys. Rev. Lett. 72 (1993) 1276. [5] A. Lappas, K. Prassides, A. Amato, R. Feyerherm, F.N. Gygax and A. Schenck, Z. Phys. B, in press.