XDC AND MAGNETIC MÖSSBAUER SPECTRA OF YBa2 (Cu1-xFex)3 O7-δ

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XDC AND MAGNETIC MÖSSBAUER SPECTRA OF

YBa2 (Cu1-xFex)3 O7-δ

S. Bhargava, G. Bhandage, J. Dormann, S. Sayouri, P. Renaudin, J. Jove, O.

Gorochov, M. Rateau, H. Pankowska, R. Suryanarayanan

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, dkembre 1988

XDC

AND MAGNETIC MOSSBAUER SPECTRA

OF

YBa2

_07-6

S. C. Bhargava (I), G. T. Bhandage (I' 4), J. L. Dormann (I), S. Sayouri (I' 5 ) , P. Renaudin (I), J. Jove

(2), 0. Gorochov (3), M. Rateau (3), H. Pankowska (3) and R. Suryanarayanan (3)

(I) Lab. de Magn6tism&, CNRS, 92195 Meudon Cedex, France

( 2 ) Lab. de Physique des Solides, CNRS, 98195 Meztdon Cedex, France

(3) Institzlt Curie, 75231 Paris Cedex 05, France

(4) Regional College of Education (NCERT), Mysore, lndia

( 5 ) Depart. de Physique, Undversite' de Fes, BP 1796, Fes, Maroc

Abstract.

-

We find the paramagnetic Mossbauer spectra of YBaz ( C U , F ~ ~ _ , ) ~ 07-6 can be fitted using symmetric

doublets, iron is in high spin ~ e state in the oxide, and the present study provides further support to the presence of ~ +

spin relaxation effects at low temperatures.

Mossbauer spectroscopy can be a usefui microscopic probe of the superconductivity of the oxide superconductors. However, for this three aspects must be clear: the site preferences of the probe Fe, the ionic state of Fe ions, and the cause of magnetic splitting of the Mossbauer spectra. The present study is concerned with these aspects.

We prepared the samples with

x

= 0.005, 0.01, 0.02,

and 0.04 in two different ways. In the first case (series A), the oxides were heated in air at 900 OC, after grinding reheated in oxygen at 950, cooled to 500 OC at 50 OC/hr and after sometime furnace cooled. In the second case (series B), after the air annealing at 900 OC, and heating in oxygen at 950 OC, the samples were cooled t o 500 at 100 OC/hr before furnace cooling after sometime. A part of the sample with composi-

tion

x

= 0.02 of the series B was reheated to 900 OC

in air and cooled, in air, to the room temperature. This sample will be further designated as "quenched" sample. All the slow cooled (SC) samples were found superconductors using resistivities and FC X measure- ments in field of 600 Oe. The quenched samples was found nonsuperconducting upto 4.2 K.

DC susceptibility in the nonsuperconducting state

was measured in the temperature (T) range from 4.2

to 300 K, using magnetic field of 5 kOe. The T de-

pendences were fitted to X = X o

+

C / (T

-

0) using

the least squares method. The results are summarised in table I. It has been assumed that the Cu ions carry no moment. The moment of the Cu ions in undoped sample is found to vary sensitively with the heat treat- ment. We ascribe this t o impurity phases, which are 'not indicated at all in the doped samples. Besides, the moment on Fe obtained using this assumption in samples obtained using widely different heat treatments is

always found to be same (% 4.7 p ~ )

.

The values are

appropriate for F'e3+ ion in the high spin state or I?e4+

than to low spin ~ estate. No anomaly to indicate a ~ +

magnetic ordering was observed in the T dependences

qf A in any of the samples, over the T range covered.

Table I. - Results of the analyses of DC susceptibility

measurenzents. Series x C x

lo4

emu/g A 0.005 0.74 0.01 1.21 B (SC) 0.01 1.26 0.02 2.52 0.04 5.07 B (Q) 0.02 2.19

The ambiguity in the cation distribution results from the many ways in which the paramagnetic spectra can be unfolded. We find that it is possible to fit all the spectral shapes with four symmetric doublets of equal

linewidths

(r

=

0.3 mm/s, Fig. 1). The isomer shifis

(IS) and quadrupole splittings (AE,) of the four dou-

blets do not vary, the relative intensities (RI) change

for different samples (Tab. 11). AE, for CuII site ob-

tained theoretically using the point charge model is low (0.12 mmjs), in agreement with the value found in the present study for one of the doublets. The analyses further show that the oxide is a better superconductor if RI of the doublet with largest splitting is low.

The slowing down of spin relaxation (SR) frequencies occurs in a superconductor at lower T. The spectrum of the quenched sample is similar to the spectrum

of the SC sample at 1.4 K, with lesser line broadenings.

This shows SR is slower in the quenched sample. In both cases, the symmetric paramagnetic doublet be-

comes asymmetric on lowering T before splitting mag-

netically, which is characteristic of lowering of SR frequencies. The small contribution of the central lines of the magnetic pattern t o the paramagnetic component in the spectra of the quenched sample will not alter this conclusion. This has been found unambiguously in other studies also. Furthermore, the magnetic spec-

trum of the quenched sample at low T clearly consists

of more than one component even though the param-

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C8 - 2206 JOURNAL DE PHYSIQUE

B - X = 0 . 0 1 ( S C ,

B : X = O 0 2 ( 0 1 -

3 0 . 0 100 0 1 5 0 . 0 280

CHANNEL N W B E R

Fig. 1. - Mossbauer spectra at 300 K.

Table 11. - Results of analyses of spectra at 300 K. The

uncertainty is IS, AEq and RI are 0.03,0.06 and 0.02,

respectively. Series IS AEq mm/s mm/s A 0.09 0.08- 0.08 1.17 -0.17 1.36 0.08 1.93 RI 0.01 0.02 0.04 91 K 88 K 84 K 0.05 0.03 0.03 0.44 0.45 0.45 0.26 0.20 0.17 0.25 0.32 0.35 0.02 0.02 ( Q ) 63 K 0.07 0.08 0.09 0.06 0.11 0.05 0.73 0.81

agnetic spectrum shows mainly one type of ion, of RI

80 %

.

The spectral shape resembles a paramagnetic

hyperfine spectrum which is obtained when SR is slow and more than one ionic state is thermally populated. The evolution of the spectral shape of the SC sample

on lowering of T shows that all ions behave similarly.

There is no magnetic component even at 4.2 K. At

1.4, magnetic splitting appears. In the case of the quenched sample, a magnetic component coexisting with the paramagnetic doublet can be seen even a t 30 K. The evolution of the spectral shape appears characteristic of an aggregate of superparamagnetic parti- cles of different sizes at least for one component. The

quenched sample corresponds t o 6 = 1 and has been

Fig. 2. - Mossbauer spectra versus temperature for 2 =

0.02 (Q), series B.

2 00 0

ee.5

** 0 - *a *

found to consist of magnetically ordered lattice of Cu ions. Cluster formation can occur because the quench- ing from 900 OC has been done in air. Nevertheless, more than one ionic state can be thermally populated at low T only if the Weiss field is smaller or comparable to the crystal field. This would imply Fe ions are not strongly coupled t o the Cu lattice. CuI sites indeed appear to be isolated in the structure. This would be close to the situation in the superconducting state too in which the Weiss field is zero, and the spectral shapes of the two would differ mainly due to changes in the

relaxation times. When the f 1/2 state is degenerate,

as is expected in the superconducting state, the spin spin relaxation frequencies of these levels is large. On

the other hand, if

f

1/2 levels are nondegenerate, due

to Zeeman splitting in non SC state, their relaxation times would become large. This qualitatively explains the shape experimentally observed.

. . . . . 2. ...,,:. .... ..,::_{" " ' .}.. _...... _.... . . : . . ... _...-....; .... _,.:.: . . . . i .. . . . . - . . . I . -. . . . 4 0 . 0 K - . . . . . -

[I] Xiao, G., Cieplak, M. Z., Gavrin, A., Streitz,

F. H., Bakhshai, A. and Chien, C. L., Phys. Rev. Lett. 60 (1988) 1446.

[2] Takano, M. and Takeda, Y., Jpn J. Appl. Phys.

26 (1987) L1862.

30 100. 1-0 2 0 0