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Low temperature magnetization of antiferromagnetic YBa2Cu3O6
Journal of Magnetism and Magnetic Materials 83 (1990) 517-518 North-Holland
LOW TEMPERATURE X. OBRADORS
MAGNETIZATION
a, J. TEJADA
J. GONZALEZ-CALBET
517
OF ANTIFERROMAGNETIC
b, J. RODRIGUEZ
‘, F. PEREZ
d, and M. MEDARDE
YBa,Cu,O,
a, M. VALLET
d,
a
a Ins. Cihcia
Materials, CSIC, Marti i Franqub s/n, 08028 Barcelona, Spain h Dpt. Fisica Fonamental, Univ. Barcelona, Diagonal 647, 08028 Barcelona, Spain
’ Inst. L.uue-Langevin,
156X,
’ Dpt. Quimica Inorgcinica,
38042
Grenoble,
Univ. Complutense
France de Madrid,
28040
Madrid,
Spain
The magnetic susceptibility and low temperature isothermal magnetization measured in an antiferromagnetic the reduction of YBa,Cu,O,.
YBa,Cu,O,
sample
obtained
during
in magentic fields up to 5.5 T have been
an “in situ” neutron
thermodiffractometry
study of
We give evidence of the intrinsic character of the low-temperature paramagnetic-like susceptibility even though the isothermal magnetization curves do not fit a simple Brillouin-like scaling. Some possible origins of this non-simple
magnetic
behaviour
are d&cussed.
The study of the magnetic properties of the antiferromagnetic “parents compounds” of the high temperature superconductors rely a undeniable interest in connection with the fundamental question of the electron pairing mechanism in these systems (for a recent overview of theories, see ref. [l]). For instance, it appears essential to understand the behaviour of holes introduced either by doping or by changing the oxygen stoichiometry [2,3]. In this work we present new results concerning the low temperature magnetic behaviour of an antiferromagnetic YBa,Cu,O, sample. There has been up to now a considerable controversy about the origin of the low temperature paramagnetic-like [4] behaviour in this system or the equivalent antiferromagnet La,CuO,. Surprisingly, there have fewer studies of their magnetization behaviour. A stoichiometric YBa,Cu,O, sample was prepared by means of an “in situ” neutron thermodiffractometric investigation of the reduction of YBa,Cu,O, [5]. This study clarified how a nominally pure YBa,Cu,O, sample losses the oxygen atoms down to a composition YBazCu306+.r with x smaller than 0.03. This experiment showed as well that the final sample contained only a tiny amount of an unidentified phase (different from BaCuO,, Yz BaCuO, or YzCu20,) having Bragg reflextions at d = 2.895 and 2.497 A. The magnetic susceptibility (H = 1 T) and several isothermal magnetization curves were measured by means of a Quantum Design SQUID magnetometer. Both, the careful preparation of our samples and the observation by other authors [4] of a good reproducibility of the magnetic susceptibility point to an intrinsic origin for the paramagnetic-like susceptibility observed at low temperatures (fig. 1). Moreover, it must be stressed that the non-observance of any susceptibility peak precludes the existence of impurities such as 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
Y, BaCuO, or YzCu20s which have been found to be antiferromagnetic at low temperatures [6] or antiferromagnetic CuO which has a flat susceptibility at low temperatures [7]. On the other hand, no evidence of BaCuO, impurities could be found neither in the neutron diffraction pattern of a fresh sample nor in an X-ray diffraction study of a YBa,Cu,O, sample degraded by the air contact during several months. These powder X-ray diffraction patterns give clear evidence of the formation of BaCO,, YzO, and a very small amount of CuO, but definitively not BaCuO,, Y,Cu,O, or Y,BaCuOs. Although these results seem to differ from those presented by other authors [S] for the hydrolisis of YBa,Cu,O,, they clearly show that extreme care must be taken in the sample handling. The whole magnetic susceptibility curve (fig. 1) was fitted to a law x = x0 + C/T where x0 is considered to
10,
Fig. 1. Magnetic
I
susceptibility of a YBa,Cu,O, sured at H = 1 T.
sample
mea-
X. Obradors el al. / Low temperature magnerirafion
518 0.24 7
A-4 4.2K
fY CT) Fig. 2. Isothermal magnetization curves of YBa,Cu,O,. connection line is only for guide of the eye.
The
be temperature independent (in our limited temperature range). The obtained values were x -4.6(l) X lo-’ emugg’ and C = 4.13(l) x 10e5 emu&g - ‘, equivalent to having 2.6% of paramagnetic S = : Cu*+ ions. These values are very near to that observed by other authors [4] thus giving further support to our feeling of an intrinsic origin of this low temperature paramagnetic behaviour. Finally, we have further investigated the low temperature paramagnetism by means of isothermal magnetization measurements in fields of up to 5 T. As can be observed in fig. 2, a strong non-linearity is observed below T- 40 K. When a Brillouin-like scaling is attempted a strong departure of a unique curve isobserved, thus indicating that the “magnetic defects” leading to these low temperature anomalies are not simple paramagnetic ions. Unfortunately, there has been up to now several experimental difficulties which preclude a clear separation of the intrinsic magnetic behaviour of these holes introduced in the antiferromagnetic CuO, planes [9]. For instance in La,CuO, a metamagnetic transition of a hidden weak ferromagnetic component lead to complex magnetization curves [lo]. In the YBa,Cu,O,+X system the difficulties arise from the existence of two different copper sublattices. It has been suggested that for low x values the holes concentrate in the Cul sublattice (chain layers) thus masking the intrinsic character of CuO, planes. If the paramagnetic centers we observe in YBa,Cu,O, may be attributed to Cu2+ ions in the Cul
of YBa,Cu,O,
sublattice (low x limit), this would mean that about 8% ofthe Cul ions are paramagnetic and consequently the oxygen content would correspond to x = 0.04. This is indeed the value of our uncertainty (0.03) in the oxygen content deduced from our neutron diffraction refinement [5]. A possible explanation of the non-scaling behaviour of the magnetization curves could be that there is a temperature dependent exchange field over these magnetic moments. This interpretation would be consistent with the neutron diffraction results [2,4] which show that the Cul magnetic moments lead to a modification of the magnetic structure. In this way, in the diluted limit (low x values) there is a magnetic frustration effect leading to an idle spin behaviour (loose spin). The exact nature of these magnetic defects are still unclear and probably only a detailed study of the magnetic moment distribution, as determined by polarized neutron diffraction [12], may give some clues to this problem. In summary, our observation of paramagnetic-like Cu moments (2.6 at%) is compatible with the oxygen detection limit of our powder neutron diffraction study. It is then not necessary to assume that these Cul magnetic moments lead to a cloud of disrupted antiferromagnetism within the CuO, planes. We recognize, however, that this hypothesis is very tempting because it would allow one to understand the continous reduction with increasing x of the magnetic moments deduced from neutron diffraction in Ba2YCu30,+, [2,4].
References
(11M.Cyrot,
J. de Phys. 49 (1989) C8-2215. et al., J. de Phys. 49 (1989) C8-2219. Y. Endoh, Physica B 156&157 (1989) 839. J.M. Tranquada et al., Phys. Rev. B 38 (1988) 2477. W.E. Farneth et al., Phys. Rev. B 39 (1989) 6594. J. Rodriguez et al., Physica C 153-155 (1988) 1671. T. Tsoe et al., Phys. Lett. A 125 (1987) 222. T. Chattopadhyay et al., Europhys. Lett. 8 (1989) 685. M. O’Keefe et al., J. Phys. Chem. Solids 23 (1962) 261. J.B. Forsyth et al., J. Phys. C 21 (1988) 2917. M.F. Yan et al., Appl. Phys. Lett. 51 (1987) 532. M. Inui et al., Phys. Rev. B 38 (1988) 6631. A. Aharony et al., Phys. Rev. Lett. 60 (1988) 1330. S.W. Cheong et al., Phys. Rev. B 39 (1989) 4395. H. Kadowaki et al., Phys. Rev. B 37 (1988) 7932. J.W. Lynn et al., Phys. Rev. Lett. 60 (1988) 2781. B. Gillon et al., Physica B 156&157 (1989) 851.
PI J. Rossat-Mignot [31 [41 [51 [61 [71 PI [91 NOI u11 [121
LOW TEMPERATURE X. OBRADORS
MAGNETIZATION
a, J. TEJADA
J. GONZALEZ-CALBET
517
OF ANTIFERROMAGNETIC
b, J. RODRIGUEZ
‘, F. PEREZ
d, and M. MEDARDE
YBa,Cu,O,
a, M. VALLET
d,
a
a Ins. Cihcia
Materials, CSIC, Marti i Franqub s/n, 08028 Barcelona, Spain h Dpt. Fisica Fonamental, Univ. Barcelona, Diagonal 647, 08028 Barcelona, Spain
’ Inst. L.uue-Langevin,
156X,
’ Dpt. Quimica Inorgcinica,
38042
Grenoble,
Univ. Complutense
France de Madrid,
28040
Madrid,
Spain
The magnetic susceptibility and low temperature isothermal magnetization measured in an antiferromagnetic the reduction of YBa,Cu,O,.
YBa,Cu,O,
sample
obtained
during
in magentic fields up to 5.5 T have been
an “in situ” neutron
thermodiffractometry
study of
We give evidence of the intrinsic character of the low-temperature paramagnetic-like susceptibility even though the isothermal magnetization curves do not fit a simple Brillouin-like scaling. Some possible origins of this non-simple
magnetic
behaviour
are d&cussed.
The study of the magnetic properties of the antiferromagnetic “parents compounds” of the high temperature superconductors rely a undeniable interest in connection with the fundamental question of the electron pairing mechanism in these systems (for a recent overview of theories, see ref. [l]). For instance, it appears essential to understand the behaviour of holes introduced either by doping or by changing the oxygen stoichiometry [2,3]. In this work we present new results concerning the low temperature magnetic behaviour of an antiferromagnetic YBa,Cu,O, sample. There has been up to now a considerable controversy about the origin of the low temperature paramagnetic-like [4] behaviour in this system or the equivalent antiferromagnet La,CuO,. Surprisingly, there have fewer studies of their magnetization behaviour. A stoichiometric YBa,Cu,O, sample was prepared by means of an “in situ” neutron thermodiffractometric investigation of the reduction of YBa,Cu,O, [5]. This study clarified how a nominally pure YBa,Cu,O, sample losses the oxygen atoms down to a composition YBazCu306+.r with x smaller than 0.03. This experiment showed as well that the final sample contained only a tiny amount of an unidentified phase (different from BaCuO,, Yz BaCuO, or YzCu20,) having Bragg reflextions at d = 2.895 and 2.497 A. The magnetic susceptibility (H = 1 T) and several isothermal magnetization curves were measured by means of a Quantum Design SQUID magnetometer. Both, the careful preparation of our samples and the observation by other authors [4] of a good reproducibility of the magnetic susceptibility point to an intrinsic origin for the paramagnetic-like susceptibility observed at low temperatures (fig. 1). Moreover, it must be stressed that the non-observance of any susceptibility peak precludes the existence of impurities such as 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
Y, BaCuO, or YzCu20s which have been found to be antiferromagnetic at low temperatures [6] or antiferromagnetic CuO which has a flat susceptibility at low temperatures [7]. On the other hand, no evidence of BaCuO, impurities could be found neither in the neutron diffraction pattern of a fresh sample nor in an X-ray diffraction study of a YBa,Cu,O, sample degraded by the air contact during several months. These powder X-ray diffraction patterns give clear evidence of the formation of BaCO,, YzO, and a very small amount of CuO, but definitively not BaCuO,, Y,Cu,O, or Y,BaCuOs. Although these results seem to differ from those presented by other authors [S] for the hydrolisis of YBa,Cu,O,, they clearly show that extreme care must be taken in the sample handling. The whole magnetic susceptibility curve (fig. 1) was fitted to a law x = x0 + C/T where x0 is considered to
10,
Fig. 1. Magnetic
I
susceptibility of a YBa,Cu,O, sured at H = 1 T.
sample
mea-
X. Obradors el al. / Low temperature magnerirafion
518 0.24 7
A-4 4.2K
fY CT) Fig. 2. Isothermal magnetization curves of YBa,Cu,O,. connection line is only for guide of the eye.
The
be temperature independent (in our limited temperature range). The obtained values were x -4.6(l) X lo-’ emugg’ and C = 4.13(l) x 10e5 emu&g - ‘, equivalent to having 2.6% of paramagnetic S = : Cu*+ ions. These values are very near to that observed by other authors [4] thus giving further support to our feeling of an intrinsic origin of this low temperature paramagnetic behaviour. Finally, we have further investigated the low temperature paramagnetism by means of isothermal magnetization measurements in fields of up to 5 T. As can be observed in fig. 2, a strong non-linearity is observed below T- 40 K. When a Brillouin-like scaling is attempted a strong departure of a unique curve isobserved, thus indicating that the “magnetic defects” leading to these low temperature anomalies are not simple paramagnetic ions. Unfortunately, there has been up to now several experimental difficulties which preclude a clear separation of the intrinsic magnetic behaviour of these holes introduced in the antiferromagnetic CuO, planes [9]. For instance in La,CuO, a metamagnetic transition of a hidden weak ferromagnetic component lead to complex magnetization curves [lo]. In the YBa,Cu,O,+X system the difficulties arise from the existence of two different copper sublattices. It has been suggested that for low x values the holes concentrate in the Cul sublattice (chain layers) thus masking the intrinsic character of CuO, planes. If the paramagnetic centers we observe in YBa,Cu,O, may be attributed to Cu2+ ions in the Cul
of YBa,Cu,O,
sublattice (low x limit), this would mean that about 8% ofthe Cul ions are paramagnetic and consequently the oxygen content would correspond to x = 0.04. This is indeed the value of our uncertainty (0.03) in the oxygen content deduced from our neutron diffraction refinement [5]. A possible explanation of the non-scaling behaviour of the magnetization curves could be that there is a temperature dependent exchange field over these magnetic moments. This interpretation would be consistent with the neutron diffraction results [2,4] which show that the Cul magnetic moments lead to a modification of the magnetic structure. In this way, in the diluted limit (low x values) there is a magnetic frustration effect leading to an idle spin behaviour (loose spin). The exact nature of these magnetic defects are still unclear and probably only a detailed study of the magnetic moment distribution, as determined by polarized neutron diffraction [12], may give some clues to this problem. In summary, our observation of paramagnetic-like Cu moments (2.6 at%) is compatible with the oxygen detection limit of our powder neutron diffraction study. It is then not necessary to assume that these Cul magnetic moments lead to a cloud of disrupted antiferromagnetism within the CuO, planes. We recognize, however, that this hypothesis is very tempting because it would allow one to understand the continous reduction with increasing x of the magnetic moments deduced from neutron diffraction in Ba2YCu30,+, [2,4].
References
(11M.Cyrot,
J. de Phys. 49 (1989) C8-2215. et al., J. de Phys. 49 (1989) C8-2219. Y. Endoh, Physica B 156&157 (1989) 839. J.M. Tranquada et al., Phys. Rev. B 38 (1988) 2477. W.E. Farneth et al., Phys. Rev. B 39 (1989) 6594. J. Rodriguez et al., Physica C 153-155 (1988) 1671. T. Tsoe et al., Phys. Lett. A 125 (1987) 222. T. Chattopadhyay et al., Europhys. Lett. 8 (1989) 685. M. O’Keefe et al., J. Phys. Chem. Solids 23 (1962) 261. J.B. Forsyth et al., J. Phys. C 21 (1988) 2917. M.F. Yan et al., Appl. Phys. Lett. 51 (1987) 532. M. Inui et al., Phys. Rev. B 38 (1988) 6631. A. Aharony et al., Phys. Rev. Lett. 60 (1988) 1330. S.W. Cheong et al., Phys. Rev. B 39 (1989) 4395. H. Kadowaki et al., Phys. Rev. B 37 (1988) 7932. J.W. Lynn et al., Phys. Rev. Lett. 60 (1988) 2781. B. Gillon et al., Physica B 156&157 (1989) 851.
PI J. Rossat-Mignot [31 [41 [51 [61 [71 PI [91 NOI u11 [121