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Magnetic ordering in superconducting YBa2(Cu1−xFex)3Oy oxide
Journal of Nuclear Nor&h-Holland
~AGN~~C Masaaki Kusuo
Materials
211
170 (1990) 211-216
O~~~~ING
IN SUP~~CO~~U~iNG
MATSUI
‘, Kazuhito
NISHIYAMA
’ and
ISHIKAWA Kanetada
YBa,(Cu , _ x Fe, j30,, OXIDE
I, Hiroshi
NAGAMINE
MATSUOKA
‘, Masao
DOYAMA
‘,
2
’ Deportment of Materials Srience and Engineering, Nugoya University, Chikusa-ku, Nagoya, Japan ’ Mao~ Science Laboratory and Department 01 Ph_wics, university of Tokyo, Bunkyo-ku, Tokyo, Japan Received
22 February
1989; accepted
14 September
1989
Magnetic sus~ptib;l~ty m~surement and the zero-field muon-spin-relaxation experjment have been made for the YBa,(Cu,_XFeX)30, system. The susceptibility has been measured in the field strong enough for the magnetic flux to penetrate the superconductor and the resultant temperature dependence has demonstrated the field cooled effect and a cusp at low temperatures below T,. The cusp has been observed when the high field has been applied. The dynamical depolarization rate of the muon obtained by the muon-spin-relaxation experiment has shown the maximum at the same temperature as the cusp. The results suggest that the spin glass freezing of localized magnetic moment takes place at the temperature of the cusp, which is consistent with the previous Mossbauer effect experiments. The magnetic phase diagram for the system has been obtained.
I. Introduction Since the discovery of a new oxide superconductor by Bednorz and Mliller [l]. many types of pseudo-perovskite superconductors have been found. Theoretical approaches to clarify the mechanism of the high T, superconductivity have suggested that magnetic couphng between Cu(2) atoms in the Cu-0 plane plays an important role in the mechanism 12-51. Experimentally, Nishida et al. first reported the antiferromagnetic ordering in the quenched Y-Ba-CL-0 based on the muon-spin-relaxation measurement, while there was no evidence of magnetic ordering in the well-annealed superconducting Y-Ba-Cu-0 [6]. On the other hand, the Y-Ba-Cu-0 system doped by mag netic atoms such as Fe, Co or Ni has been well investigated [7--141. The crystal structure was transformed from orthorhombic to tetragonal by doping of Fe or Co and it was not changed by Ni doping. We also reported the crystal structure, the electrical resistivity and the magnetic susceptibility of YBa,(Cu,_,M,)30, (M = Fe, Co, Ni) system IS], where the magnetic ordering at low temperatures in doped samples was suggested. An existence of the magnetic ordering at low temperatures in the Fe-doped Y-Ba-Cu-0 sample was clarified by many authors utilizing the MSssbauer effect experiment [9-121. The hyperfine field of Fe was observed at low temperatures well below the superconducting transition 0022-3115/90/$03.50 0 Etsevier Science Publishers (North-Holland)
B.V.
temperature, but the spectrum of the well-annealed sample was very complicated [9]. Nasu et af. suggested based on the experiment for heat treated samples that Fe atoms mainly substitute the Cu(l) site in the Cu-0 chain and that the magnetic moment of the Fe atom at the Cu(1) site was frozen below 25 K to 16 K 191. Pankhurst et al. analyzed the spectrum assuming the dynamic spin relaxation effect of the local field at Fe site, though they did not specify a type of the magnetic ordering [Ill. Thus the experiment of the Mossbauer effect was very suggestive to the magnetic ordering, but the type of the ordering was hitherto unsolved, in the present paper, the magnetic properties, particularly the magnetic SusceptibiIity of Fe-doped YBa,(Cu, _,Fe,),O, oxides have been carefully investigated to clarify the type of the magnetic ordering involved. And we have observed a cusp and field cooling effect on the temperature dependence of susceptibility. The phenomena has been attributed to the spin glass freezing at low temperatures well below the superconducting transition temperature.
2. Experimental procedure Using the raw oxides of 4N-YzOl, BaCO,, CuO and Fe,O,, we mixed and calcined them twice at 900 o C for
16 h and at 500 0 C for 2 h in air. Then we made pellets of IO ton/cm2 and calcined them in the same heat-treatment condition. It was cooled down very slowly to room temperature. The samples prepared were x = 0.0, 0.019, 0.041, 0.045, 0.058. 0.075, 0.087 and 0.116 for YBa,(Cu, _,Fe,),O,. The composition was analyzed by the EPMA method. Electrical resistivity was measured by the conventional four probe method. where the current direction was changed to compensate the thermal motive force at the voltage probe. The magnetic susceptibility was measured by means of a SQUID magnetometer for powder samples in a capsule. The X-ray diffraction was used to determine the crystal structure. The lattice parameters were calculated by the least mean square method using 36 diffraction peaks. The muon-spin-relaxation experiment was made for a pellet of 30 mm x 30 mm x 2 mm in dimension at the pulsed muon facility BOOM, Meson Science Laboratory, University of Tokyo of KEK. under a pressure
.qll.68 V C
Fig. 1. Lattice constants LI, h, c and c/u
3. Results and discussion 3. I. X-ruy diffruction
and electricul resistiuit~r
In order to characterize the samples, we measured the X-ray diffraction and electrical resistivity. The lattice parameters o, b and c are shown as a function of x in fig. 1. For .Y> 0.03. the crystal structure transforms from orthorhombic to tetragonal. The results are consistent to the previous papers [7,12.13]. The resistivity is shown as a function of temperature in fig. 2. With increasing x the superconducting transition temperature T, decreases monotonically. Since the transition is very sharp, it seems that the samples were well mixed by the sintering process. It is important to note that the resistivity in the normal state rapidly increases with x. so that the mean free path of electrons decreases. It is interesting that the superconductivity remains even in the tetragonal phase and no remarkable change takes place around the boundary of orthorhombic to tetragonal transition. Considering that the fundamental structure of Bi-Sr-Ca-Cu-0 superconductors such as Bi ,Sr,Ca ,Cu zOv is also tetragonal and does not include the Cu-0 chain [15j. the present result supports that the orthorhombic structure is not necessary for the superc~~nductivity. In other words, since the 0-O planes are common for high q oxide superconductors. Cu-0 planes surrounding an Y atom in the Y--Ba-CU-0 system might be responsible to the superconductivity.
as a function of Y at
R-l-.
_1..?.Mognerrc properties The magnetization in the low field is shown as a function of temperature in fig. 3. The diamagnetism at 4.2 K is abruptly decreased as a small amount of Fe, e.g. x = 0.019, is added. (Note that the trend is independent of the demagnetization factor because of the same shape of samples.) It seems that the abrupt decrease is due to the increase of penetration depth induced by the decrease of mean free path by impurity doping, which is
Y6a2Ku+xFex)30y
IO" i.
.-
0
100
200
300
T (K) Fig. 2. Electrtcal
resistivity as a function of temperature various compositions of Fe.
for
M. Matsui et a!. / Mug-netic ordering m superconducting YBu,(Cu,
x10- *
.-- ‘. _
-1.0
._
x Fe,)_@,. oxide
IYBa2(Cu&e&ly
213
’
/
YBq(Cu+,Fe,J30,.
;I_+zy%$ .J
0
50
0
T fKf
100
200
300
T (K)
100 Fig.
5. Inverse su~eptibiljty as a function of temperature above T, for various compositions of Fe.
Fig. 3. Temperature dependence of the low-field susceptibility for various compositions of Fe.
attributed to the decrease of the lower critical field &,. If Fe atoms mainly substitute the Cu(l) site as suggested by the Miissbauer experiment 191, indirect pair breaking in the CL(t)-0 plane might take place via the oxygen or Ba in the Ba-0 plane and thus decreases T,. Fig. 4 shows the magnetization curve at 4.2 K for the various composition of Fe. The apparent upper critical field NC2 significantly decreases with increasing Fe composition. Such a tendency was observed for Y substitution by magnetic rare earth elements such as Gd, Ho and Dy [16-181. In such cases, the susceptibility above T, satisfied a Curie--Weiss law (x = x0 + C/(T + 0)) and the paramagnetism existed independent of superconductivity. In the present case, the inverse susceptibility is also explained by the Curie-Weiss law as shown in fig. 5. We previously explained the field dependence of magnetization as shown in fig. 4 by means of the extrapolation of inverse susceptibility down to 4.2 K IS]. In the present work, the susceptibility was
H (kOe) Fig. 4. Magnetization curve at 4.2 K for various compositions of Fe.
measured more precisely than in our previous report since a SQUID magnetometer was used. Accordingly, the effective magnetic moment of Fe was estimated again. The effective moments per Fe atom obtained were 5.2, 5.0, 4.7, 4.3 and 4.3~~ for x = 0.019, 0.045, 0.058, 0.087 and 0.116, respectively, where we disregarded the Cu moment. These values are consistent with recent data reported by Westerholt et al. (191 and rather larger than the data reported by Noguchi et al. [14]. If the moment of the Cu atom P,,, = 0.17ps, estimated in the present measurement, is taken into consideration, above values should be revised by about -0.1~~. The effective moment for small x corresponds to the coexistence of 2+ and 3+ in the valence of Fe. Now let us proceed to the identification of the magnetic ordering. So far the Mossbauer effect could not clarify the type of ordering [9-121 and no change in susceptibility was observed around the magnetic ordering temperature as shown in fig. 3, because the magnetic field was too small to penetrate the sample. Accordingly, we measured the susceptibility in the field strong enough to make the magnetization of the sample almost zero (see fig. 4). Figs. 6 and 7 show the temperature dependence of susceptibility for zero-field-cooled (ZFC) and field-cooled (FC) samples in the field of 1, 3 or 5 kOe for x = 0.041 and 0.075, respectively. The procedure of the measurement was that the sample first cooked down to 4.2 K in zero field then the prescribed field was applied for ZFC. For FC, the susceptibility was measured with decreasing the temperature. Fieldcooled effects are seen in figs. 6 and 7, while there are cusps at 13.2 K and 21.9 K on the ZFC curves of 5 kQe for x = 0.041 and of 1 kOe for x = 0.075, respectively, as denoted by T,. Miiller et al. also observed the fieldcooled effect of the susceptibility in the weak field and reported the glass state of trapped Muxoids [20]. Though
1
YWW&.9Ge
0.041 MY +
ASYM
k/--++
-2.0’ 0
I 40
20
l’
Fig. 6. Temperature
80
60
(Kf
(FC)
in 3 and 5
and the zero-field-cooled
(ZFC)
sample of x = 0.041.
and dynamic depolarization
by the zero-field
muon-spin-relaxation
0.041.
the field-cooled effect observed for the ZFC curve of 3 kOe for x = 0.041 is consistent with that reported by them, the maximum of susceptibility like the cusp was not observed by them. It is noted that the field-cooled effect appeared below Tg for .x = 5.075 and the sample was no longer superconducting when 1 kOe was applied. The cusp is one of the typical phenomena of spin glass freezing of localized magnetic moments. It seems that the field-cooled effect observed on the 5 kOe curves for x = 0.041 contains both effects of trapped fluxoids and spin glass freezing of magnetic moments introduced by Fe. The reason that the cusp was not observed on the 3 kOe curve for x = 0.041, could be due to the insufficient field strength to penetrate the sample. The
J H=lkOe
I
0
50
100
T (Kf Fig. 7. Temperature for the field-cooled
dependence
31
Fig. 8. Asymmetry An arrow
indicates
rR_ Asymmetry
0'
20 “I- (n)
dependence of the susceptibility
kOe for the field-cooled
i
to
I50
-
of the susceptibility
(FC) and the zero-field-cooled ple of Y = 0.075.
in 1 kOe
(ZFC)
sam-
rate X obtained
experiment
the spin glass freezing is plotted in arbitrary
for
x =
temperature
units.
freezing temperatures denoted by Tg in figs. 6 and 7 are in good agreement with the ordering temperature reported by Mijssbauer effect experiments [12,21]. Accordingly. it is concluded that the magnetic ordering observed far below T, in the YBa,(Cu, .__lFe.X130, system arises from the spin glass state. Furthermore we made the zero-field muon-spin-relaxation experiment 1221 for two samples, i.e. x = 0.041 and 0.075. Fig. 8 shows the temperature dependence of the asymmetry of muon-spin-relaxation and the dynamic depolarization rate h for .v = 0.041. The relaxation function was fit by the root-exponential type function. The rate h showed a maxinlum at Tg.where the asymmetry decreased with lowering the temperature. This result is also a typical observation of magnetic ordering such as spin giass freezing 1231. When the frequency of the dynamical fluctuation of the local field in the sample becomes of the same order of the iife time of a muon, h should show maximum and then X decreases with decreasing the frequency. One of the typical characteristics of spin glass freezing in the muon-spin-relaxation is that h decreases gradually with decreasing the temperature below Ta, which suggests that the dynamical component of the magnetic field in the sample remains even below the freezing temperature. Such a gradual decrease of h belaw T& was observed as seen in fig. 8. The asymmetry also decreases due to the appearance of the static local magnetic field. Thus the present muon experiment supports the view derived from the above magnetic measurement. The spin glass freezing was also observed at 19.8 K for .Y= 0.075, The precise results of the muon-spin-refaxation experiment will be reported in ref. [22].
Ortho
100
/ Tetra
YBa&-.Fex)& o Tc (Hetssnereffect)
‘u\/
maximum at the same temperature of the cusp. The coexistence of sueprconductivity and spin glass state was discussed. The magnetic phase diagram for the system was obtained.
l P'SR
%
A Naw et al 0 Tamakl et al
c-50
Acknowledgements
0 0
0.05
0.10
0.15
We wish to thank to Grand-in-Aid for Special Project Research of The Ministry of Education, Science and Culture of Japan.
X
Finally, the magnetic and conductive phase diagram of the Y13az(Cu,_,FeX),0, system is summarized in fig. 9. The temperature T, in the figure was obtained by the low-field sus~tibiIity measurement. The spin glass freezing temperature is in good agreement with the temperature at which the hyperfine field was observed by the Mijssbauer effect experiment (12,211. The spin glass state coexists with superconductivity for 0 < x < 0.10 and the susceptibility above T, is Curie-Weiss like. It seems that the spin glass disappears for dense concentrations. Recently, Nakamichi et al. reported that the magnetic moments of Cu( 1) and Cu(2) were induced by the magnetic moment of Fe atoms [24]. Accordingly, it seems that the long range spin glass freezing of the present system takes place among the magnetic moment of Fe atoms and the induced moment of Cu atoms. In summary, the lattice constant, electrical resistivity and magnetic susceptibility were measured for the YBa,(Cu, _,Fe,),O, system. We carefully measured the susceptibility in the field strong enough for the magnetic flux to penetrate the superconductor. The cusp and the field-cooied effect were observed at low temperatures below T, in the temperature dependence of the susceptibility. It was concluded that the magnetic ordering observed in the previous Miissbauer experiments was due to the spin glass freezing of the localized magnetic moments. It was consistent with the results of the muon-spin-relaxation experiment where the dynamical depolarization rate of the muon demonstrated a
References
[l] J.G. Bednorz and K.A. Miiller, Z. Phys. B64 (1986) 188. [2] P.W. Anderson, Science 235 (1987) 1196. [3] P.W. Anderson, G. Baskaran, 2. Zho and T. Hue, Phys. Rev. Lett. 58 (1987) 2790. [4] G. Baskaran, 2. Zho and P.W. Anderson, Solid State Commun. 63 (1987) 973. [5] V.J. Emery, Phys. Rev. Lett. 29 (1987) 2794. [6] N. Nishida, H. Miyatake, D. Shimada, S. Ohkuma, M. Ishikawa, T. Takabatake, Y. Nakazawa, Y. Kuno, R. Keitel, J.H. Brewer, T.M. Riseman, D.L.I. Williams, Y. Watanabe, T. Yamazaki, K. Nishiyama, K. Nagamine, E.J. Ansaido and E. Torikai, Jpn. J. Appl. Phys. 26 (1987) L1856. (71 Y. Oda, H. Hujita, H. Toyada, T. Kaneko, T. Kohara, 1. Nakada and K. Asayama, Jpn. J. Appl. Phys. 26 (1987) L1660. [8] K. Ishikawa, H. Matsuoka, M. Matsui and M. Doyama, Proc. MRS Int. Meeting on Advanced Materials, Vol. 6 (1989) 347. [9] S. Nasu, H. Kitagawa, T. Kohara, Y. Oda, T. Shinjo, K. Asayama and F.E. Fujita, Proc. Int. Symp. on Non-equilibrium Solid Phases (JIMIS-5). March 1988, Kyoto, p. 14. [IO] X.Z. Zhou, M. Raudsepp, Q.A. Pankhurst, A.H. Morrish, Y.L. Luo and I. Maartense, Phys. Rev. B36 (1987) 7230. [ll] Q.A. Pankhurst, A.H. Morrish, M. Raudsepp and X.Z. Zhou, J. Phys. C 21 (1988) L7. 1121 T. Tamaki, T. Komai, A. Ito, Y. Maeno and T. Fujita, Solid State Commun. 65 (1987) 43. [13] Y. Maeno, M. Kato, Y. Aoki and T. Fujita, Jpn. J. Appl. Phys. 26 (1987) L1982. [I41 S. Noguchi, J. Inoue, K. Okuda, Y. Maeno and T. Fujita, Jpn. J. Appl. Phys. 27 (1988) L390. I151 J.M. Tarascon, Y. Le Page, G.W. Barboux, B.G. Bagley, L.H. Greene, W.R. McKinnon, G.W. Hull, M. Groud and D.M. Hwang. Phys. Rev. B37 (1988) 9382. 1161 J.M. Tarascon, W.R. McKinnon, L.H. Greene, G.W. Hull and E.M. Vogel, Phys. Rev. B36 (1987) 226. 1171 J.R. Thompson, D.K. Christen, B.C. Sales, L.A. Boatner and Y.C. Kim, Phys. Rev. B36 (1987) 836.
216
M. Mama
et al. / Magnercc orderrng
m superconducrrng
[18] A. Poddar, P. Mandel, P. Choudhury, N. Das and B. Ghosh, J. Phys. C 21 (1988) 3323. [19] K. Westerholt, H.J. Wtiller. H. Bach and P. Stauche. Phys. Rev. B39 (1989) 11680. [20] K.A. Mtiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [21] S. Nasu, private communication. [22] M. Matsui, K. Ishikawa. H. Matsuoka. M. Doyama, K.
YBo,(Cu,
, Fe,
) ,O, ourde
Ntshtyama and K. Nagamine. to be published tn Jpn. J Appl. Phys. 1231 Y.J. Uemura. T. Yamazaki. D.R. Harshman, M. Senha and E.J. Ansaldo, Phys. Rev. B31 (1985) 546. [24] Y. Nakamichi, K. Kumagai. H. Nakajima and T. PuJtta. Proc. Int. MMS-HTSC Conf.. July 1989. Stanford, to be publtshed.
~AGN~~C Masaaki Kusuo
Materials
211
170 (1990) 211-216
O~~~~ING
IN SUP~~CO~~U~iNG
MATSUI
‘, Kazuhito
NISHIYAMA
’ and
ISHIKAWA Kanetada
YBa,(Cu , _ x Fe, j30,, OXIDE
I, Hiroshi
NAGAMINE
MATSUOKA
‘, Masao
DOYAMA
‘,
2
’ Deportment of Materials Srience and Engineering, Nugoya University, Chikusa-ku, Nagoya, Japan ’ Mao~ Science Laboratory and Department 01 Ph_wics, university of Tokyo, Bunkyo-ku, Tokyo, Japan Received
22 February
1989; accepted
14 September
1989
Magnetic sus~ptib;l~ty m~surement and the zero-field muon-spin-relaxation experjment have been made for the YBa,(Cu,_XFeX)30, system. The susceptibility has been measured in the field strong enough for the magnetic flux to penetrate the superconductor and the resultant temperature dependence has demonstrated the field cooled effect and a cusp at low temperatures below T,. The cusp has been observed when the high field has been applied. The dynamical depolarization rate of the muon obtained by the muon-spin-relaxation experiment has shown the maximum at the same temperature as the cusp. The results suggest that the spin glass freezing of localized magnetic moment takes place at the temperature of the cusp, which is consistent with the previous Mossbauer effect experiments. The magnetic phase diagram for the system has been obtained.
I. Introduction Since the discovery of a new oxide superconductor by Bednorz and Mliller [l]. many types of pseudo-perovskite superconductors have been found. Theoretical approaches to clarify the mechanism of the high T, superconductivity have suggested that magnetic couphng between Cu(2) atoms in the Cu-0 plane plays an important role in the mechanism 12-51. Experimentally, Nishida et al. first reported the antiferromagnetic ordering in the quenched Y-Ba-CL-0 based on the muon-spin-relaxation measurement, while there was no evidence of magnetic ordering in the well-annealed superconducting Y-Ba-Cu-0 [6]. On the other hand, the Y-Ba-Cu-0 system doped by mag netic atoms such as Fe, Co or Ni has been well investigated [7--141. The crystal structure was transformed from orthorhombic to tetragonal by doping of Fe or Co and it was not changed by Ni doping. We also reported the crystal structure, the electrical resistivity and the magnetic susceptibility of YBa,(Cu,_,M,)30, (M = Fe, Co, Ni) system IS], where the magnetic ordering at low temperatures in doped samples was suggested. An existence of the magnetic ordering at low temperatures in the Fe-doped Y-Ba-Cu-0 sample was clarified by many authors utilizing the MSssbauer effect experiment [9-121. The hyperfine field of Fe was observed at low temperatures well below the superconducting transition 0022-3115/90/$03.50 0 Etsevier Science Publishers (North-Holland)
B.V.
temperature, but the spectrum of the well-annealed sample was very complicated [9]. Nasu et af. suggested based on the experiment for heat treated samples that Fe atoms mainly substitute the Cu(l) site in the Cu-0 chain and that the magnetic moment of the Fe atom at the Cu(1) site was frozen below 25 K to 16 K 191. Pankhurst et al. analyzed the spectrum assuming the dynamic spin relaxation effect of the local field at Fe site, though they did not specify a type of the magnetic ordering [Ill. Thus the experiment of the Mossbauer effect was very suggestive to the magnetic ordering, but the type of the ordering was hitherto unsolved, in the present paper, the magnetic properties, particularly the magnetic SusceptibiIity of Fe-doped YBa,(Cu, _,Fe,),O, oxides have been carefully investigated to clarify the type of the magnetic ordering involved. And we have observed a cusp and field cooling effect on the temperature dependence of susceptibility. The phenomena has been attributed to the spin glass freezing at low temperatures well below the superconducting transition temperature.
2. Experimental procedure Using the raw oxides of 4N-YzOl, BaCO,, CuO and Fe,O,, we mixed and calcined them twice at 900 o C for
16 h and at 500 0 C for 2 h in air. Then we made pellets of IO ton/cm2 and calcined them in the same heat-treatment condition. It was cooled down very slowly to room temperature. The samples prepared were x = 0.0, 0.019, 0.041, 0.045, 0.058. 0.075, 0.087 and 0.116 for YBa,(Cu, _,Fe,),O,. The composition was analyzed by the EPMA method. Electrical resistivity was measured by the conventional four probe method. where the current direction was changed to compensate the thermal motive force at the voltage probe. The magnetic susceptibility was measured by means of a SQUID magnetometer for powder samples in a capsule. The X-ray diffraction was used to determine the crystal structure. The lattice parameters were calculated by the least mean square method using 36 diffraction peaks. The muon-spin-relaxation experiment was made for a pellet of 30 mm x 30 mm x 2 mm in dimension at the pulsed muon facility BOOM, Meson Science Laboratory, University of Tokyo of KEK. under a pressure
.qll.68 V C
Fig. 1. Lattice constants LI, h, c and c/u
3. Results and discussion 3. I. X-ruy diffruction
and electricul resistiuit~r
In order to characterize the samples, we measured the X-ray diffraction and electrical resistivity. The lattice parameters o, b and c are shown as a function of x in fig. 1. For .Y> 0.03. the crystal structure transforms from orthorhombic to tetragonal. The results are consistent to the previous papers [7,12.13]. The resistivity is shown as a function of temperature in fig. 2. With increasing x the superconducting transition temperature T, decreases monotonically. Since the transition is very sharp, it seems that the samples were well mixed by the sintering process. It is important to note that the resistivity in the normal state rapidly increases with x. so that the mean free path of electrons decreases. It is interesting that the superconductivity remains even in the tetragonal phase and no remarkable change takes place around the boundary of orthorhombic to tetragonal transition. Considering that the fundamental structure of Bi-Sr-Ca-Cu-0 superconductors such as Bi ,Sr,Ca ,Cu zOv is also tetragonal and does not include the Cu-0 chain [15j. the present result supports that the orthorhombic structure is not necessary for the superc~~nductivity. In other words, since the 0-O planes are common for high q oxide superconductors. Cu-0 planes surrounding an Y atom in the Y--Ba-CU-0 system might be responsible to the superconductivity.
as a function of Y at
R-l-.
_1..?.Mognerrc properties The magnetization in the low field is shown as a function of temperature in fig. 3. The diamagnetism at 4.2 K is abruptly decreased as a small amount of Fe, e.g. x = 0.019, is added. (Note that the trend is independent of the demagnetization factor because of the same shape of samples.) It seems that the abrupt decrease is due to the increase of penetration depth induced by the decrease of mean free path by impurity doping, which is
Y6a2Ku+xFex)30y
IO" i.
.-
0
100
200
300
T (K) Fig. 2. Electrtcal
resistivity as a function of temperature various compositions of Fe.
for
M. Matsui et a!. / Mug-netic ordering m superconducting YBu,(Cu,
x10- *
.-- ‘. _
-1.0
._
x Fe,)_@,. oxide
IYBa2(Cu&e&ly
213
’
/
YBq(Cu+,Fe,J30,.
;I_+zy%$ .J
0
50
0
T fKf
100
200
300
T (K)
100 Fig.
5. Inverse su~eptibiljty as a function of temperature above T, for various compositions of Fe.
Fig. 3. Temperature dependence of the low-field susceptibility for various compositions of Fe.
attributed to the decrease of the lower critical field &,. If Fe atoms mainly substitute the Cu(l) site as suggested by the Miissbauer experiment 191, indirect pair breaking in the CL(t)-0 plane might take place via the oxygen or Ba in the Ba-0 plane and thus decreases T,. Fig. 4 shows the magnetization curve at 4.2 K for the various composition of Fe. The apparent upper critical field NC2 significantly decreases with increasing Fe composition. Such a tendency was observed for Y substitution by magnetic rare earth elements such as Gd, Ho and Dy [16-181. In such cases, the susceptibility above T, satisfied a Curie--Weiss law (x = x0 + C/(T + 0)) and the paramagnetism existed independent of superconductivity. In the present case, the inverse susceptibility is also explained by the Curie-Weiss law as shown in fig. 5. We previously explained the field dependence of magnetization as shown in fig. 4 by means of the extrapolation of inverse susceptibility down to 4.2 K IS]. In the present work, the susceptibility was
H (kOe) Fig. 4. Magnetization curve at 4.2 K for various compositions of Fe.
measured more precisely than in our previous report since a SQUID magnetometer was used. Accordingly, the effective magnetic moment of Fe was estimated again. The effective moments per Fe atom obtained were 5.2, 5.0, 4.7, 4.3 and 4.3~~ for x = 0.019, 0.045, 0.058, 0.087 and 0.116, respectively, where we disregarded the Cu moment. These values are consistent with recent data reported by Westerholt et al. (191 and rather larger than the data reported by Noguchi et al. [14]. If the moment of the Cu atom P,,, = 0.17ps, estimated in the present measurement, is taken into consideration, above values should be revised by about -0.1~~. The effective moment for small x corresponds to the coexistence of 2+ and 3+ in the valence of Fe. Now let us proceed to the identification of the magnetic ordering. So far the Mossbauer effect could not clarify the type of ordering [9-121 and no change in susceptibility was observed around the magnetic ordering temperature as shown in fig. 3, because the magnetic field was too small to penetrate the sample. Accordingly, we measured the susceptibility in the field strong enough to make the magnetization of the sample almost zero (see fig. 4). Figs. 6 and 7 show the temperature dependence of susceptibility for zero-field-cooled (ZFC) and field-cooled (FC) samples in the field of 1, 3 or 5 kOe for x = 0.041 and 0.075, respectively. The procedure of the measurement was that the sample first cooked down to 4.2 K in zero field then the prescribed field was applied for ZFC. For FC, the susceptibility was measured with decreasing the temperature. Fieldcooled effects are seen in figs. 6 and 7, while there are cusps at 13.2 K and 21.9 K on the ZFC curves of 5 kQe for x = 0.041 and of 1 kOe for x = 0.075, respectively, as denoted by T,. Miiller et al. also observed the fieldcooled effect of the susceptibility in the weak field and reported the glass state of trapped Muxoids [20]. Though
1
YWW&.9Ge
0.041 MY +
ASYM
k/--++
-2.0’ 0
I 40
20
l’
Fig. 6. Temperature
80
60
(Kf
(FC)
in 3 and 5
and the zero-field-cooled
(ZFC)
sample of x = 0.041.
and dynamic depolarization
by the zero-field
muon-spin-relaxation
0.041.
the field-cooled effect observed for the ZFC curve of 3 kOe for x = 0.041 is consistent with that reported by them, the maximum of susceptibility like the cusp was not observed by them. It is noted that the field-cooled effect appeared below Tg for .x = 5.075 and the sample was no longer superconducting when 1 kOe was applied. The cusp is one of the typical phenomena of spin glass freezing of localized magnetic moments. It seems that the field-cooled effect observed on the 5 kOe curves for x = 0.041 contains both effects of trapped fluxoids and spin glass freezing of magnetic moments introduced by Fe. The reason that the cusp was not observed on the 3 kOe curve for x = 0.041, could be due to the insufficient field strength to penetrate the sample. The
J H=lkOe
I
0
50
100
T (Kf Fig. 7. Temperature for the field-cooled
dependence
31
Fig. 8. Asymmetry An arrow
indicates
rR_ Asymmetry
0'
20 “I- (n)
dependence of the susceptibility
kOe for the field-cooled
i
to
I50
-
of the susceptibility
(FC) and the zero-field-cooled ple of Y = 0.075.
in 1 kOe
(ZFC)
sam-
rate X obtained
experiment
the spin glass freezing is plotted in arbitrary
for
x =
temperature
units.
freezing temperatures denoted by Tg in figs. 6 and 7 are in good agreement with the ordering temperature reported by Mijssbauer effect experiments [12,21]. Accordingly. it is concluded that the magnetic ordering observed far below T, in the YBa,(Cu, .__lFe.X130, system arises from the spin glass state. Furthermore we made the zero-field muon-spin-relaxation experiment 1221 for two samples, i.e. x = 0.041 and 0.075. Fig. 8 shows the temperature dependence of the asymmetry of muon-spin-relaxation and the dynamic depolarization rate h for .v = 0.041. The relaxation function was fit by the root-exponential type function. The rate h showed a maxinlum at Tg.where the asymmetry decreased with lowering the temperature. This result is also a typical observation of magnetic ordering such as spin giass freezing 1231. When the frequency of the dynamical fluctuation of the local field in the sample becomes of the same order of the iife time of a muon, h should show maximum and then X decreases with decreasing the frequency. One of the typical characteristics of spin glass freezing in the muon-spin-relaxation is that h decreases gradually with decreasing the temperature below Ta, which suggests that the dynamical component of the magnetic field in the sample remains even below the freezing temperature. Such a gradual decrease of h belaw T& was observed as seen in fig. 8. The asymmetry also decreases due to the appearance of the static local magnetic field. Thus the present muon experiment supports the view derived from the above magnetic measurement. The spin glass freezing was also observed at 19.8 K for .Y= 0.075, The precise results of the muon-spin-refaxation experiment will be reported in ref. [22].
Ortho
100
/ Tetra
YBa&-.Fex)& o Tc (Hetssnereffect)
‘u\/
maximum at the same temperature of the cusp. The coexistence of sueprconductivity and spin glass state was discussed. The magnetic phase diagram for the system was obtained.
l P'SR
%
A Naw et al 0 Tamakl et al
c-50
Acknowledgements
0 0
0.05
0.10
0.15
We wish to thank to Grand-in-Aid for Special Project Research of The Ministry of Education, Science and Culture of Japan.
X
Finally, the magnetic and conductive phase diagram of the Y13az(Cu,_,FeX),0, system is summarized in fig. 9. The temperature T, in the figure was obtained by the low-field sus~tibiIity measurement. The spin glass freezing temperature is in good agreement with the temperature at which the hyperfine field was observed by the Mijssbauer effect experiment (12,211. The spin glass state coexists with superconductivity for 0 < x < 0.10 and the susceptibility above T, is Curie-Weiss like. It seems that the spin glass disappears for dense concentrations. Recently, Nakamichi et al. reported that the magnetic moments of Cu( 1) and Cu(2) were induced by the magnetic moment of Fe atoms [24]. Accordingly, it seems that the long range spin glass freezing of the present system takes place among the magnetic moment of Fe atoms and the induced moment of Cu atoms. In summary, the lattice constant, electrical resistivity and magnetic susceptibility were measured for the YBa,(Cu, _,Fe,),O, system. We carefully measured the susceptibility in the field strong enough for the magnetic flux to penetrate the superconductor. The cusp and the field-cooied effect were observed at low temperatures below T, in the temperature dependence of the susceptibility. It was concluded that the magnetic ordering observed in the previous Miissbauer experiments was due to the spin glass freezing of the localized magnetic moments. It was consistent with the results of the muon-spin-relaxation experiment where the dynamical depolarization rate of the muon demonstrated a
References
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