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Linear and nonlinear magnetic responses of weakly sintered YBa2Cu4O8 ceramics of sub-micron scale grains
Physica C 412–414 (2004) 94–97 www.elsevier.com/locate/physc
Linear and nonlinear magnetic responses of weakly sintered YBa2Cu4O8 ceramics of sub-micron scale grains M. Hagiwara a
a,*
, T. Yamao b, T. Shima a, H. Deguchi c, M. Matsuura
d
Department Electronics and Information Sciences, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan b International Innovation Center, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan c Department of Electronics, Kyushu Institute of Technology, Kitakyushu, Fukuoka, 804-8550, Japan d Fukui University of Technology, Fukui, 910-8505, Japan Received 29 October 2003; accepted 15 December 2003 Available online 30 April 2004
Abstract Weakly sintered ceramic samples of YBa2 Cu4 O8 (124) are prepared as systems of Josephson-coupled sub-micron grains, with varying sintering time. The obtained ceramic systems are studied by observing fundamental and harmonic magnetic responses. Each specimen shows characteristic temperature dependence of the responses caused by intergrain ordering. The ordering temperature estimated by singular peak of the nonlinear response changes consistently with the sintering time, indicating that the coupling strength is systematically changed. Furthermore, dual anomalies of the nonlinear response are found. The results suggest that two different origins of nonlinear response inhere in the intergrain ordering. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.72.Bk Keywords: YBa2 Cu4 O8 ; Ceramic superconductor; Phase transition; Intergrain coupling; p junction; Successive ordering
1. Introduction YBa2 Cu4 O8 (124) is stoichiometric oxide superconductor, whose crystal is free from microscopic disorder by oxygen vacancy and twin formation [1]. When 124 ceramic is prepared by low temperature sintering using citrate pyrolysis precursor method [2], the material can be a system composed of super
*
Corresponding author. Tel.: +81-75-724-7416; fax: +81-75724-7400. E-mail address: [email protected] (M. Hagiwara).
fine superconductive grains for which adjacent ones are weakly coupled to each other [3]. Such a system forms a Josephson-coupled intergrain network, and the coupling energy essentially affects the physical properties. Earlier on, intergranule cooperative phenomenon has been discussed for granular superconductor as an analog of spin model [4,5]. Recently, frustration effect has been also regarded as essential in case of d-wave superconductor, because p-junction is included randomly in the network [6]. With this viewpoint, random freezing of intergrain loop currents, named ‘chiral glass’, has been predicted
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.027
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97
2. Experimental The samples were prepared by our improved method using citrate pyrolysis [11,12]. The precursor was calcined for 120 h at 776 °C to yield pure 124 phase. This yield was sieved and pressed and then sintered at 776 °C. Sintering time was set in three ranks: 34.5 h, 50 h, 80 h, for samples named A, B and C, respectively. From SEM observation, the grain size has been estimated to be 200–500 nm almost equally for all the samples [13]. Magnetic observations are carried out at absolute temperature (T ) range 100–4.2 K, with a dcSQUID sensor installed in a hand made cryostat. Digitally generated ac field signal H ðtÞ ¼ h sin xt is applied, and response magnetization wave at each temperature is digitized and stored in a personal computer to be analyzed by FFT algorithm. Such simultaneously derived fundamental and 0 harmonic components are represented by Mnx and 00 Mnx (or the values divided by h) defined in the formula: X 0 00 MðtÞ ¼ ðMnx sin nxt Mnx cos nxtÞ: ð1Þ n
Field amplitude h is set within a range 20–100 mOe, and frequency x=ð2pÞ is fixed at 0.2 Hz. Detail of our observation system will be introduced elsewhere.
3. Results and discussion Observed magnetic responses for the samples A, B and C show characteristic behavior reflecting intergrain ordering in weak-coupled ceramic superconductors. The overall feature is that Meissner signal gently appears below 80 K (Tc of 124) and the response is strikingly intensified at enough below Tc . Fig. 1 shows temperature dependence of 0 00 M1x and M1x for sample A as the typical one. From 80 K toward lower temperature, obvious 0 00 changes in M1x and M1x begin from around 43 K. The curves depend on h obviously around 40–25 K, indicating strong nonlinear effect. A new 0 findings are that M1x decreases once at 35 K and 0 forms double maxima and that M1x correspondingly shows two-step like change in its dropping change by intergrain ordering. Fig. 2 demonstrates qualitative change in wave form of the magnetic responses at several
Linear susceptibilities M' 1 /h, 2 M'' 1 /h (a.u.)
theoretically [6–8]. The phase transition toward this new phase has been supported by the observed divergent feature of nonlinear magnetic response [9,10]. However, the ordered state have not been well clarified experimentally. For instance, we have noticed preliminarily that sign of the nonlinear response below the critical temperature varies with specimen. In order to discuss the results, and to seek possible novel characteristic at the ordered state, systematic experiments for a series of specimens with well controlled sintered structure are necessary. Thus we prepare several 124 ceramics, which show each different coupling strengths but keep other chemical and structural characters, utilizing our synthetic technique [11,12]. Then we observe linear and nonlinear magnetic responses at weak excitation field conditions so as to examine both general nature and possible systematic change by the coupling strength.
95
1 :100 m e : 50 m e : 20 m e
2 M''1 /h
0
T c of 124
(Sample A) -1 0.05
0.00
-0.05
-2
M' 1 / h
-0.10
-0.15 30
-3 0
10
20
30
40
40
50
50
60
60
70
70
80
80
90 100
90
100
Temperature T / K
Fig. 1. Temperature dependencies of fundamental components of ac magnetic responses at various field amplitude h, for ceramic 124 of sample A. M, }, : in-phase components, , , : out-of-phase components.
96
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97 20
(Sample A)
15
Sample A
(a)
10
48.1 K
: h =10 0 m Oe : h = 20 m Oe
Induced magnetization M(t) (arbi. units)
5 0
43.5 K
M'3 ω /h (arbi. units)
-5
42.5 K
40.5 K
20 15
(b)
Sample B
(c)
Sample C
10 5 0 -5
33.7 K
20
32.0 K
15 10
4.3 K
5
period : 5 s
0
0
90
180
2 70
3 60
Time (Phase angle /degree)
-5 0
10
20
30
40
50
60
70
80
90
100
Temperature T / K
Fig. 2. Wave forms of magnetic response waves for ceramic 124 (sample A) at various temperature points. Field amplitude h ¼ 100 mOe. The horizontal value of time-axis is shown by phase angle of sinusoidal wave.
temperatures. The sinusoidal phase begins to advance forward continuously but steeply below 43 K, soon the wave distorts conspicuously. As temperature drops, the distortion alters qualitatively with increasing amplitude, and the response restores linear and non-shift form at the lowest temperature range. Such like temperature dependence is qualitatively common to the three samples. Seen quantitatively, however, some systematic differences by the sintering condition are found. For the sample A, B and C, in-phase third harmonic component 0 M3x is shown against T in Fig. 3(a)–(c) respectively. As expected, strong anomaly caused by intergrain ordering is seen in each figure. Further we 0 notice that each characteristic M3x ðT Þ curve is regarded as a composition of two kinds of anomaly; one is sharp symmetric peak, and another is antisymmetric-like anomaly at a little below the first peak temperature. When h is decreased, the upper
Fig. 3. Temperature dependencies of in-phase third harmonic 0 component M3x for ceramic 124. (a)–(c) correspond to sample A, B, and C, respectively. The changed parameter is field amplitude h.
peak position tends to converge on each characteristic point, but the lower anomaly does not so. Seeing each upper peak for h ¼ 20 mOe, we can estimate the characteristic points to be 42 K, 46 K and 49 K, for samples A, B and C, respectively. These values, which agree with each inflection 00 ðT Þ are regarded as critical temperapoint of M1x ture Tc2 of each intergrain ordering. The estimated Tc2 changes systematically with the sintering time for our three samples. This result means that intergrain coupling energy is increased by progress of the sintering. The observed two step behavior in linear and nonlinear responses is thought to be intrinsic nature, considering our reproducibility confirmations for different sample pieces. Since the upper and lower anomaly positions are found to shift correlatively, some origin independent of intergrain structure is hard to be thought. Our results suggest
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97
that two kinds of mechanism of nonlinear magnetic response are involved in the intergrain ordering. The problem is whether the lower anomaly is a critical point at thermal equilibrium condition, or it is a non-equilibrium effect incidental on the uni-ordering. As an examination, v2 , the term of H 3 in ex0 panded series of MðH Þ is estimated from Mnx [14] for the sample B at h ¼ 100 mOe (Fig. 4). In the v2 T curve, peak position of the upper anomaly, 46 K, agrees with the critical point earlier esti0 mated by M3x at the smallest h, indicating certain singularity at the temperature point. As for the anomaly at lower temperature, the shape is unstably changed by accounted term number of 0 Mnx , meaning that singularity is not found within the present condition. For further inquiry, however, other experiments which are sensitive to local internal field are necessary. Seeing Fig. 3(a)–(c) again, we notice that the lower anomaly diminish as the sintering proceed. The reason of this behavior is not clear at present, though it will inform us about the two-step mechanism. But now we consider the following; 0 when the anomalies of M3x ðT Þ were broadened, variety of merged shape might be observed. Our
/10 4 (arbi. units)
0. 4 _ ( h 3/4) χ 2 ~ M' 3ω +5 M' 5ω+14M' 7ω +30M' 9ω
0. 2 0. 0
Sample B
-0.4
Approximated
χ2
-0.2
-0.6
f = 0.2 Hz h = 100 m O e
-0.8 -1.0 -1.2 0
10
20
30
40
50
60
70
80
90
100
Temperature T / K
Fig. 4. Temperature dependence of nonlinear susceptibility v2 estimated from first four terms series of in-phase odd-harmonic 0 responses Mnx for ceramic 124 of sample B. h ¼ 100 mOe.
97
preliminary results of complex feature might have suffered from such effect. In conclusion, we have clarified successfully the behavior of linear and nonlinear magnetic responses for a series of 124 ceramics which show each different intergrain coupling strength. The 0 , a divergent type found dual anomalies in M3x peak and an antisymmetric-like anomaly, change correlatively with the coupling strength, meaning that two different origins of nonlinear response inhere in the intergrain ordering. It has been also suggested that the upper anomaly tend to dominate the lower one. Preparing a specimen by longer sintering, we will be able to study the upper critical behavior separately. Acknowledgements This research was supported by the Ministry of Education, Culture, Sports, Science and Technology (Japan), Grant-in-Aid for Scientific Research. References [1] P. Marsh, R.M. Fleming, M.L. Mandish, A.M. DeSantolo, J. Kwo, M. Hong, L.J. Martinez-Miranda, Nature 334 (1988) 141. [2] K. Koyama, A. Junod, T. Graf, G. Triscone, J. Muller, Physica C 185–189 (1991) 461, and the references therein. [3] M. Kawachi, M. Hagiwara, K. Koyama, M. Matsuura, J. Phys. Soc. Jpn. 63 (1994) 3405. [4] J. Rosenblatt, Rev. de Phys. Appl. 9 (1974) 217. [5] M.Y. Choi, D. Stroud, Phys. Rev. B 35 (1987) 7109, and the references therein. [6] H. Kawamura, J. Phys. Soc. Jpn. 64 (1995) 711. [7] H. Kawamura, Mai Suan Li, J. Phys. Rev. B 54 (1996) 619. [8] H. Kawamura, Mai Suan Li, J. Phys. Soc. Jpn. 66 (1997) 2110. [9] M. Matsuura, M. Kawachi, K. Miyoshi, M. Hagiwara, K. Koyama, J. Phys. Soc. Jpn. 64 (1995) 4540. [10] T. Yamao, M. Hagiwara, K. Koyama, M. Matsuura, J. Phys. Soc. Jpn. 68 (1999) 871. [11] M. Hagiwara, T. Yamao, M. Matsuura, Physica C 392– 396 (2003) 66. [12] M. Hagiwara, T. Yamao, M. Matsuura, Ceram. Engin. Sci. Proc. (Am. Ceram. Soc.) 24 (3) (2003) 63. [13] T. Yamao, M. Hagiwara, T. Shima, M. Matsuura, Physica C, 2004, these Proceedings. [14] M. Hagiwara, T. Shimada, T. Shima, K. Miyoshi, M. Matsuura, J. Magn. Magn. Mater. 177–181 (1998) 175.
Linear and nonlinear magnetic responses of weakly sintered YBa2Cu4O8 ceramics of sub-micron scale grains M. Hagiwara a
a,*
, T. Yamao b, T. Shima a, H. Deguchi c, M. Matsuura
d
Department Electronics and Information Sciences, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan b International Innovation Center, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan c Department of Electronics, Kyushu Institute of Technology, Kitakyushu, Fukuoka, 804-8550, Japan d Fukui University of Technology, Fukui, 910-8505, Japan Received 29 October 2003; accepted 15 December 2003 Available online 30 April 2004
Abstract Weakly sintered ceramic samples of YBa2 Cu4 O8 (124) are prepared as systems of Josephson-coupled sub-micron grains, with varying sintering time. The obtained ceramic systems are studied by observing fundamental and harmonic magnetic responses. Each specimen shows characteristic temperature dependence of the responses caused by intergrain ordering. The ordering temperature estimated by singular peak of the nonlinear response changes consistently with the sintering time, indicating that the coupling strength is systematically changed. Furthermore, dual anomalies of the nonlinear response are found. The results suggest that two different origins of nonlinear response inhere in the intergrain ordering. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.72.Bk Keywords: YBa2 Cu4 O8 ; Ceramic superconductor; Phase transition; Intergrain coupling; p junction; Successive ordering
1. Introduction YBa2 Cu4 O8 (124) is stoichiometric oxide superconductor, whose crystal is free from microscopic disorder by oxygen vacancy and twin formation [1]. When 124 ceramic is prepared by low temperature sintering using citrate pyrolysis precursor method [2], the material can be a system composed of super
*
Corresponding author. Tel.: +81-75-724-7416; fax: +81-75724-7400. E-mail address: [email protected] (M. Hagiwara).
fine superconductive grains for which adjacent ones are weakly coupled to each other [3]. Such a system forms a Josephson-coupled intergrain network, and the coupling energy essentially affects the physical properties. Earlier on, intergranule cooperative phenomenon has been discussed for granular superconductor as an analog of spin model [4,5]. Recently, frustration effect has been also regarded as essential in case of d-wave superconductor, because p-junction is included randomly in the network [6]. With this viewpoint, random freezing of intergrain loop currents, named ‘chiral glass’, has been predicted
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.027
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97
2. Experimental The samples were prepared by our improved method using citrate pyrolysis [11,12]. The precursor was calcined for 120 h at 776 °C to yield pure 124 phase. This yield was sieved and pressed and then sintered at 776 °C. Sintering time was set in three ranks: 34.5 h, 50 h, 80 h, for samples named A, B and C, respectively. From SEM observation, the grain size has been estimated to be 200–500 nm almost equally for all the samples [13]. Magnetic observations are carried out at absolute temperature (T ) range 100–4.2 K, with a dcSQUID sensor installed in a hand made cryostat. Digitally generated ac field signal H ðtÞ ¼ h sin xt is applied, and response magnetization wave at each temperature is digitized and stored in a personal computer to be analyzed by FFT algorithm. Such simultaneously derived fundamental and 0 harmonic components are represented by Mnx and 00 Mnx (or the values divided by h) defined in the formula: X 0 00 MðtÞ ¼ ðMnx sin nxt Mnx cos nxtÞ: ð1Þ n
Field amplitude h is set within a range 20–100 mOe, and frequency x=ð2pÞ is fixed at 0.2 Hz. Detail of our observation system will be introduced elsewhere.
3. Results and discussion Observed magnetic responses for the samples A, B and C show characteristic behavior reflecting intergrain ordering in weak-coupled ceramic superconductors. The overall feature is that Meissner signal gently appears below 80 K (Tc of 124) and the response is strikingly intensified at enough below Tc . Fig. 1 shows temperature dependence of 0 00 M1x and M1x for sample A as the typical one. From 80 K toward lower temperature, obvious 0 00 changes in M1x and M1x begin from around 43 K. The curves depend on h obviously around 40–25 K, indicating strong nonlinear effect. A new 0 findings are that M1x decreases once at 35 K and 0 forms double maxima and that M1x correspondingly shows two-step like change in its dropping change by intergrain ordering. Fig. 2 demonstrates qualitative change in wave form of the magnetic responses at several
Linear susceptibilities M' 1 /h, 2 M'' 1 /h (a.u.)
theoretically [6–8]. The phase transition toward this new phase has been supported by the observed divergent feature of nonlinear magnetic response [9,10]. However, the ordered state have not been well clarified experimentally. For instance, we have noticed preliminarily that sign of the nonlinear response below the critical temperature varies with specimen. In order to discuss the results, and to seek possible novel characteristic at the ordered state, systematic experiments for a series of specimens with well controlled sintered structure are necessary. Thus we prepare several 124 ceramics, which show each different coupling strengths but keep other chemical and structural characters, utilizing our synthetic technique [11,12]. Then we observe linear and nonlinear magnetic responses at weak excitation field conditions so as to examine both general nature and possible systematic change by the coupling strength.
95
1 :100 m e : 50 m e : 20 m e
2 M''1 /h
0
T c of 124
(Sample A) -1 0.05
0.00
-0.05
-2
M' 1 / h
-0.10
-0.15 30
-3 0
10
20
30
40
40
50
50
60
60
70
70
80
80
90 100
90
100
Temperature T / K
Fig. 1. Temperature dependencies of fundamental components of ac magnetic responses at various field amplitude h, for ceramic 124 of sample A. M, }, : in-phase components, , , : out-of-phase components.
96
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97 20
(Sample A)
15
Sample A
(a)
10
48.1 K
: h =10 0 m Oe : h = 20 m Oe
Induced magnetization M(t) (arbi. units)
5 0
43.5 K
M'3 ω /h (arbi. units)
-5
42.5 K
40.5 K
20 15
(b)
Sample B
(c)
Sample C
10 5 0 -5
33.7 K
20
32.0 K
15 10
4.3 K
5
period : 5 s
0
0
90
180
2 70
3 60
Time (Phase angle /degree)
-5 0
10
20
30
40
50
60
70
80
90
100
Temperature T / K
Fig. 2. Wave forms of magnetic response waves for ceramic 124 (sample A) at various temperature points. Field amplitude h ¼ 100 mOe. The horizontal value of time-axis is shown by phase angle of sinusoidal wave.
temperatures. The sinusoidal phase begins to advance forward continuously but steeply below 43 K, soon the wave distorts conspicuously. As temperature drops, the distortion alters qualitatively with increasing amplitude, and the response restores linear and non-shift form at the lowest temperature range. Such like temperature dependence is qualitatively common to the three samples. Seen quantitatively, however, some systematic differences by the sintering condition are found. For the sample A, B and C, in-phase third harmonic component 0 M3x is shown against T in Fig. 3(a)–(c) respectively. As expected, strong anomaly caused by intergrain ordering is seen in each figure. Further we 0 notice that each characteristic M3x ðT Þ curve is regarded as a composition of two kinds of anomaly; one is sharp symmetric peak, and another is antisymmetric-like anomaly at a little below the first peak temperature. When h is decreased, the upper
Fig. 3. Temperature dependencies of in-phase third harmonic 0 component M3x for ceramic 124. (a)–(c) correspond to sample A, B, and C, respectively. The changed parameter is field amplitude h.
peak position tends to converge on each characteristic point, but the lower anomaly does not so. Seeing each upper peak for h ¼ 20 mOe, we can estimate the characteristic points to be 42 K, 46 K and 49 K, for samples A, B and C, respectively. These values, which agree with each inflection 00 ðT Þ are regarded as critical temperapoint of M1x ture Tc2 of each intergrain ordering. The estimated Tc2 changes systematically with the sintering time for our three samples. This result means that intergrain coupling energy is increased by progress of the sintering. The observed two step behavior in linear and nonlinear responses is thought to be intrinsic nature, considering our reproducibility confirmations for different sample pieces. Since the upper and lower anomaly positions are found to shift correlatively, some origin independent of intergrain structure is hard to be thought. Our results suggest
M. Hagiwara et al. / Physica C 412–414 (2004) 94–97
that two kinds of mechanism of nonlinear magnetic response are involved in the intergrain ordering. The problem is whether the lower anomaly is a critical point at thermal equilibrium condition, or it is a non-equilibrium effect incidental on the uni-ordering. As an examination, v2 , the term of H 3 in ex0 panded series of MðH Þ is estimated from Mnx [14] for the sample B at h ¼ 100 mOe (Fig. 4). In the v2 T curve, peak position of the upper anomaly, 46 K, agrees with the critical point earlier esti0 mated by M3x at the smallest h, indicating certain singularity at the temperature point. As for the anomaly at lower temperature, the shape is unstably changed by accounted term number of 0 Mnx , meaning that singularity is not found within the present condition. For further inquiry, however, other experiments which are sensitive to local internal field are necessary. Seeing Fig. 3(a)–(c) again, we notice that the lower anomaly diminish as the sintering proceed. The reason of this behavior is not clear at present, though it will inform us about the two-step mechanism. But now we consider the following; 0 when the anomalies of M3x ðT Þ were broadened, variety of merged shape might be observed. Our
/10 4 (arbi. units)
0. 4 _ ( h 3/4) χ 2 ~ M' 3ω +5 M' 5ω+14M' 7ω +30M' 9ω
0. 2 0. 0
Sample B
-0.4
Approximated
χ2
-0.2
-0.6
f = 0.2 Hz h = 100 m O e
-0.8 -1.0 -1.2 0
10
20
30
40
50
60
70
80
90
100
Temperature T / K
Fig. 4. Temperature dependence of nonlinear susceptibility v2 estimated from first four terms series of in-phase odd-harmonic 0 responses Mnx for ceramic 124 of sample B. h ¼ 100 mOe.
97
preliminary results of complex feature might have suffered from such effect. In conclusion, we have clarified successfully the behavior of linear and nonlinear magnetic responses for a series of 124 ceramics which show each different intergrain coupling strength. The 0 , a divergent type found dual anomalies in M3x peak and an antisymmetric-like anomaly, change correlatively with the coupling strength, meaning that two different origins of nonlinear response inhere in the intergrain ordering. It has been also suggested that the upper anomaly tend to dominate the lower one. Preparing a specimen by longer sintering, we will be able to study the upper critical behavior separately. Acknowledgements This research was supported by the Ministry of Education, Culture, Sports, Science and Technology (Japan), Grant-in-Aid for Scientific Research. References [1] P. Marsh, R.M. Fleming, M.L. Mandish, A.M. DeSantolo, J. Kwo, M. Hong, L.J. Martinez-Miranda, Nature 334 (1988) 141. [2] K. Koyama, A. Junod, T. Graf, G. Triscone, J. Muller, Physica C 185–189 (1991) 461, and the references therein. [3] M. Kawachi, M. Hagiwara, K. Koyama, M. Matsuura, J. Phys. Soc. Jpn. 63 (1994) 3405. [4] J. Rosenblatt, Rev. de Phys. Appl. 9 (1974) 217. [5] M.Y. Choi, D. Stroud, Phys. Rev. B 35 (1987) 7109, and the references therein. [6] H. Kawamura, J. Phys. Soc. Jpn. 64 (1995) 711. [7] H. Kawamura, Mai Suan Li, J. Phys. Rev. B 54 (1996) 619. [8] H. Kawamura, Mai Suan Li, J. Phys. Soc. Jpn. 66 (1997) 2110. [9] M. Matsuura, M. Kawachi, K. Miyoshi, M. Hagiwara, K. Koyama, J. Phys. Soc. Jpn. 64 (1995) 4540. [10] T. Yamao, M. Hagiwara, K. Koyama, M. Matsuura, J. Phys. Soc. Jpn. 68 (1999) 871. [11] M. Hagiwara, T. Yamao, M. Matsuura, Physica C 392– 396 (2003) 66. [12] M. Hagiwara, T. Yamao, M. Matsuura, Ceram. Engin. Sci. Proc. (Am. Ceram. Soc.) 24 (3) (2003) 63. [13] T. Yamao, M. Hagiwara, T. Shima, M. Matsuura, Physica C, 2004, these Proceedings. [14] M. Hagiwara, T. Shimada, T. Shima, K. Miyoshi, M. Matsuura, J. Magn. Magn. Mater. 177–181 (1998) 175.