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Submitted on 1 Jan 1988
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EFFECTS OF OXYGEN AND MAGNETIC
IMPURITIES ON NUCLEAR RELAXATION
ANOMALIES IN La-M-Cu-O (M = Ba, Sr) SYSTEMS
K. Kumagai, I. Watanabe, Y. Nakamura, H. Nakajima
To cite this version:
K. Kumagai, I. Watanabe, Y. Nakamura, H. Nakajima.
EFFECTS OF OXYGEN AND
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, dbcembre 1988
EFFECTS OF OXYGEN AND MAGNETIC IMPURITIES ON NUCLEAR
RELAXATION ANOMALIES IN La-M-Cu-0 (M
=
Ba, Sr) SYSTEMS
K. Kumagai,
I.
Watanabe, Y. Nakamura andH.
NakajimaDepartment of Physics, Faculty of Science, Hokkaido University, Sapporo, 060 Japan
Abstract. - Nuclear quadrupole resonance (NQR) of 13'~a in La2-,M,CuOr (M = Ba,Sr) have revealed unexpected enhancement of nuclear relaxation rate, 1
/
TI at T: N 10 K, suggesting the occurrence of magnetic instability in the lowtemperature region. In the undoped La2Cu04, oxygen deficiencies affect strongly the nuclear relaxation rate. Magnetic impurities (Fe etc.) suppress the enhancement of 1
/
TI, but non-magnetic Zn does not. In the superconducting region, fluctuations of Cu moments at low temperatures seem to remain extremely fast as same as that at high temperatures.A rich phase diagram involving the antiferrornag- 1000 t
.
-
, * , a a - ~.
8 s 3 v 3 t a ~ 8 s 8 L b q y netic (AF) order in new oxide superconductors has0 0
been confirmed by various experiments [I], suggesting A
that the strong Coulomb interaction in C U O ~ plane is crucial to understand the mechanism of the high- T, superconductors. In order to obtain microscopic
information on the electronic and magnetic proper- v
ties of high-T, superconductors, we have performed pulsed nuclear quadrupole resonance (NQR) of I 3 ' ~ a
in Laz-,M,Cu04 [2]. In the previous papers [3, 41, we
'cr
0 ~ 0 3 4 - I -reported the 139~a-nuclear relaxation anomalies with 2
Ba-doping a t low temperatures. The strong peak of 1 :
the nuclear relaxation rate a t Tz N 10 K indicates
magnetic instability and existence of a magnetic phase
(different from 3D-AF) such as a new type of spin glass
0.1 state [5]. In this paper, we report the effects of oxy-
gen defficiencies and impurities a t Cu sites on the dy- I 10 100
namical aspects of local moments of Cu in La-M-Cu-0 T ( K )
system.
Samples were prepared by solid-state reaction of mixture of La203, BaC03,SrC03 and CuO. A part of samples was heat-treated under flowing 0 2 and Ar atmosphere at various temperatures. The details for sample preparations were reported elsewhere [6]. Con- ventional phase coherent pulsed NMR apparatus was used for measurements of relaxation and spectra of 1 3 ' ~ a - N ~ ~ .
Figure 1 shows the temperature dependence of nu- clear spin-lattice relaxation rate, 1
/
TI, of the un- doped La~Cu04-,. The sample of No. 3 4 6 is heat- treated at Ar atmosphere and seem to be most oxygen- defective. In this sample with the highest TN, 1/
TI decreases as T~ with decreasing temperature. On the contrary, 1/
TI is enhanced below 70K
for the 0 2 annealed samples (No. 34-3 and 34-5) which seem to have less oxygen-deficiencies and show lower TN. The enhancement of 1/
TI below 70 K is strongly corre- lated with the decrease of TN. TI in usual antiferro- magnets is govenred through the spin wave excitations and becomes much longer at lower temperatures than that corresponding to spin wave gap. Therefore, the enhanced 1 / T I below 70K
<<
TN predicts that theFig. 1. - Tem erature dependence of nuclear relaxation rates, 1
/
TI, of P39La for variously heat-treated LaCuOl-,. NO. 34-1 (TN = 270 K),
NO. 3 4 2 (TN = 310 K),
NO. 34-3(TN
>
300 K),
No. 3 4 4 (TN = 210 K),
NO. 34-5 (TN 160 K, estimated value), No. 3 4 6 (TN>
300 K).
low lying excitations develop with decreasing oxygen deficiencies, and hence, decreasing TN.
We have no anomalies of 1
/
TI at TN for each sam- ple as seen in figure 1. We have also found the inde- pendence of 1/
TI on the oxygen content above 70 K. These suggest that the relaxation mechanism a t high temperature is attributed to the quadrupole relaxation by phonon scattering but is not attributed to the mag- netic origin. The contribution from magnetic fluctu- ations to nuclear relaxation is considered to be small at high temperatures above 70K
in this system. This interpretation is contradict with the result by Sasaki et al.[q.
1
/
TI of the samples with divalent elements (Ba, Sr) are enhanced and diverge a t T,* and nearx
= 0.02 at which the AF order disappears [2, 31. The temperature dependence of 1/
TI
is fitted by the universal relation of 0.5t/
(t-
1)2 and 15t (1-
t) (ms-l) above and be- low T,*, respectively, wheret
is the reduced tempera-C8 - 2146 JOURNAL DE PHYSIQUE ture oft = T/T£ [4]. Figure 2 shows the temperature
dependence of 1 / Ti of Lao.9sBao.02 (Cuo.99Ao.01) O4 where A is non-magnetic Zn and magnetic Fe and Co. Non-magnetic impurity of Zn does not affects the re-laxation rate, but magnetic ions of Fe and Co sup-press the enhancement of 1 / Ti. In the Fe-doped case, the well defined internal field at La site is observed at temperatures far above T* (up to near 70 K). As the internal field originats from the dipole field due t o the AF-ordered Cu moments, this fact shows that TN increases by the Fe-dpoing, which causes the sup-pression of 1 / Ti due to the long-range 3D-magnetic correlation.
Finally, 1 / Ti at 4.2 and 1.5 K are shown in figure 3 as a function of Sr concentration. Surprisingly, 1 / Ti
Fig. 2. — Temperature dependence of nuclear relaxation rates, 1 / T i , of 139La in Lai.98Bao.02 (Cuo.99To.01) O4
(T = Fe, Ni, Zn).
A possible origin for the divergence of 1 / Ti is due to a critical slowing-down of magnetic fluctuations which are originated from a kind of phase transition such as a spin glass type or new state where quantum effects on spin dynamics becomes important. The magnetic phase diagram determined from the divergence of the 1 / Ti is different from the 3D-AF one obtained by magnetic susceptibility measurements [1]. As the re-cent investigation of heat capacity and magnetic sus-ceptibility have not revealed any anomalies at T* [8], t h e new phase is not associated with a free energy dif-ference at T*. This might imply that the new phase transition is like a spin glass.
Aharony et al. [5] have pointed out that the doped hole on oxygen sites introduce a strong effective ferro-magnetic coupling between connecting Cu spins, which may cause a frustration leading to a 3D spin glass phase. Our NQR spectra show that the well defined in-ternal field exists in the new phase below T*. This fact rules out a short range 3D-spin glass state of classical spins, but suggests a rather long range spin correla-tion exist at low temperature region. The experimen-tal anomalies of the NQR relaxation support this the-oretical argument in some aspects such as re-entrant behavior of the 3D-AF order and the existence of a kind of some frustration state between the AF and SC state.
Fig. 3. - Concentration dependence of 1 / T\ of La at 4.2 and 1.5 K for La2-a,Sra;Cu04.
in the SC state decreases over 5 decades in order of magnitude with increasing of only few percent of Sr-content. This indicates that the fluctuation time of Cu moments changes drastically with hole-doping and do not show any slowing-down with decreasing tem-perature in the SC state. It is interesting to clarify whether such fast spin fluctuations are responsible for the appearance of high-Tc superconductivity or not.
Extended study on the temperature dependence of the nuclear relaxation in the SC state is now in progress. A k n o w l e d g e m e n t s
This work is supported in part by Grant-in-Aid for Scientific Research from the Minister of Education and Culture of Japan.
[1] Kato, M., Maeno, Y. and Fujita, T., Physicq C
152 (1988) 116.
[2] Kitaoka, Y. et al., Jpn J. Appl. Phys. 26 (1987) 397.
[3] Watanabe, I., Kumagai, K., Nakamura, Y., Kimura, T., Nakamichi, Y. and Nakajima, H., J.
Phys. Soc. Jpn 56 (1987) 3028;
Kumagai, K., Watanabe, I., Aoki, H., Nakamura, Y., Kimura, T., Nakamichi, Y. and Nakajima, H.,
Physica B 148 (1987) 480.
[4] Kumagai, K., Aoki, H., Watanabe, I., Nakamura, Y. and Nakajima, H., J. Phys. Soc. Jpn 57 (1988)
1155.
[5] Aharony, A., Birgeneau, R. J., Coniglio, A., Kast-ner, M. A. and Stanley, H. E., Phys. Rev. Lett. 6 0 (1988) 1330.
[6] Watanabe, I., Nakamura, Y. and Kumagai, K.,
Physica C 152 (1988) 262.
[7] Sasaki, S. et al, J. Phys. Soc. Jpn 57 (1988) 1151.
