Materials Science and Engineering B77 (2000) 261 – 267 www.elsevier.com/locate/mseb
Colossal magnetoresistance in screen printed La0.67Ca0.33MnO3 thick films A.K.M. Akther Hossain a,*, L.F. Cohen b, A. Berenov c, J.L. Macmanus Driscoll c a
Department of Physics, Bangladesh Uni6ersity of Engineering and Technology, Dhaka 1000, Bangladesh b Blackett Laboratory, Imperial College, Prince Consort Road, London SW 7 2BZ, UK c Materials Department, Imperial College, Prince Consort Road, London SW 7 2BP, UK Received 13 December 1999; received in revised form 6 June 2000; accepted 17 June 2000
Abstract Thick films of La0.67Ca0.33MnO3 were fabricated on (100) oriented single crystal LaAlO3 (LAO), yttria stabilized zirconia (YSZ) and on polycrystalline Al2O3 by a screen printing technique. A range of sintering conditions were explored with temperature of 1200, 1300 and 1400°C, and in air, oxygen and nitrogen atmospheres. Magnetic and DC resistance properties were measured on all films. The so-called ‘colossal magnetoresistance’ (CMR) behaviour was found to occur for films on all substrates but for specific preparation conditions. The salient feature of the CMR observed in these thick films is that MR is not limited to a small temperature window near the metal–insulator transition (M – I), Tp1. The MR peak is very broad and for some films a temperature independent CMR is observed at temperatures below Tp1. Several repeat films were made and the reproducibility of the results obtained in the first batch was confirmed. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Colossal magnetoresistance; Thick film; Transition temperature; Screen printing; Sintering Keywords: 75.70.P; 75.30.K; 81.20.E
1. Introduction The manganites have received much attention recently due to their colossal magnetoresistance (CMR) properties close to the Curie temperature Tc in the presence of high magnetic fields [1,2]. For applications of magnetic materials, the technology needs to be made simple and cheap. Thick film fabricated via a powder route such as screen printing offer a good opportunity for future sensor applications. The screen printing technique offers a wide range of circuit complexities, from simple conductor patterns to highly complex hybrid integrated circuits, using the same basic materials and equipment. The substrates play an important role in the properties of materials. In this study, we have fabricated thick films on both polycrystalline and single crystal substrates. The thick films were fabricated using * Corresponding author. Tel.: +88-02-9665613; fax: + 88-028613046. E-mail address:
[email protected] (A.K.M. Akther Hossain).
the La0.67Ca0.33MnO3 composition on three different substrates (e.g. polycrystalline Al2O3, and on single crystal (100) LaAlO3 (LAO) and (100) yttria stabilized zirconia (YSZ)).
2. Experimental methods The La0.67Ca0.33MnO3 powders were synthesized using conventional solid state reaction. Powders of La2O3 (Aldrich 99.99%), CaCO3 (BDH 99.9%) and MnCO3 (Aldrich 99.9+%) were used to make the required compositions. The La2O3 powder was pre-heated at 1000°C for 6 h before mixing with the other ingredients, as this is a hygroscopic chemical and it was necessary to eliminate water. The ingredients were weighed in appropriate proportions and then dry ball milled for 24 h. The calcination was done in four steps. The mixed powders were calcined at 950°C for 6 h, the temperature ramps being 300°C h − 1 for both heating and cooling. This step was repeated three more times with
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ramp of 30°C h − 1 during both heating and cooling. The optimum sintering conditions and substrates for growing manganite thick films are yet to be determined. To explore the optimum sintering conditions, the films were sintered in different temperatures and under different atmospheres as shown in Table 1. To check for reproducibility in sintering behaviour some screen printed films were sintered under same conditions but in separate sintering runs.
intermediate grindings to obtain a homogeneous and phase pure composition. The final product was ultrasonically dispersed in water media to create powders of fine particle size. The dried, fine powders were mixed with Blythe vehicle (Cookson Matthey) to obtain ink. From this ink, thick films were screen printed onto different substrates. The thick films were kept in a desiccator overnight to settle, and then the organic vehicle burnt out at 300°C for 3 h using temperature
Table 1 Sintering conditions of thick films of La0.67Ca0.33MnO3 on single crystal (100) LAO, single crystal (100) YSZ and polycrystalline Al2O3, substrates Batch
Sample no.
Substrates
1
LCL5
LAO
100
1400
1
Air
100
–
LCY5
YSZ Al2O3 LAO YSZ LAO
100
1400
5
Air
100
–
100
1400
1
N2
100 to 600°C hold 12 h
100
Reacted with Al2O3
100
1300
1
N2
100 to 600°C hold 12 h
100
Reacted with Al2O3
2 3
LCL6 LCY6 LCL8
LCY8
Heating rate °C h−1
Tmax (°C)
Time at Tmax Atmosphere (h)
Cooling rate (1) °C h−1
Cooling rate (2) °C h−1
Comments
Reacted with Al2O3
4
LCL9
YSZ Al2O3 LAO
5
LCY9 AHCa4 LCL10
YSZ Al2O3 LAO
100
1200
1
N2
100 to 600°C hold 12 h
100
6
LCY10 AHCa5 LCL11
YSZ Al2O3 LAO
100
1200
1
O2
100 to 600°C hold 12 h
100
7
LCY11 AHCa6 LCL12
YSZ Al2O3 LAO
100
1300
1
O2
100 to 600°C hold 12 h
100
Reacted with Al2O3
8
LCY12 AHCa7 LCL13
YSZ Al2O3 LAO
100
1400
1
O2
100 to 600°C hold 12 h
100
Reacted with Al2O3
9
LCY13 AHCa8 LCL14
YSZ Al2O3 LAO
100
1300
1
Air
100to 600°C hold 12 h
100
Reacted with Al2O3
10
LCY14 AHCa9 LCL15
YSZ Al2O3 LAO
100
1200
1
Air
100 to 600°C hold 12 h
100
11
LCY15 AHCa10 AHCa2
YSZ Al2O3 Al2O3
100
1200
24
Air
100 to 600°C hold 12 h
–
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magnetometer. The negligible diamagnetic signal for the LAO and YSZ substrates compared to magnetisation of the whole sample was not subtracted from the net magnetisation of the sample.
3. Results
3.1. Materials characterization Fig. 1. (a) SEM picture of LCL8, film on LAO, sintered at 1400°C in N2 flow 1 h, (b) SEM picture of LCL11, film on LAO, sintered at 1200°C in O2 flow 1 h.
Fig. 2. (a) SEM picture of LCY8, film on YSZ, sintered at 1400°C in N2 flow 1 h, (b) SEM picture of LCY11, film on YSZ, sintered at 1200°C in O2 flow 1 h.
The microstructures of the samples were investigated using a scanning electron microscope (SEM) (JEOL JSM-5300) with magnification ranging 750 –5000 ×. Images were acquired and analysed using image Tool 2.1 software. The thickness of the films was measured using a Zygo optical interferometer near the film edge. The X-ray diffraction was carried out in our samples using an X-ray diffractometer (PW 1710) operating at 40 kV and 40 mA with a Ni filter, using Cu–Ka radiation with wavelength of 1.54060 A, . A (111) plane cleaved silicon single crystal was used for calibration. The diffraction pattern was obtained by a step scanning technique from 10 – 90° in 2u with a step size of 0.02° and a count time of 3 s. Unit cell parameters were calculated using the non-linear least square refinement program, Unitcell. The DC electrical resistance of the thick films were measured by the standard four-point probe technique with a temperature range 295 – 20 K. The electrical contacts were made on contact pads on the films using conductive silver paint. Offset voltages were subtracted by reversing the current. The resistivity was calculated from the simple geometrical relation R = rl/A. For magnetoresistance measurements the applied field was perpendicular to the current flow in the sample. The magnetoresistance was calculated using r(H =0) −r(H) MR(%)= − × 100. r(H =0) Magnetisation measurements were made on the whole thick film sample using an OI-3001 vibrating sample
3.1.1. Microstructure SEM pictures of some representative thick films of La0.67Ca0.33MnO3 on single crystal LAO and YSZ substrates are shown in Fig. 1 and Fig. 2, respectively. Owing to more enhanced kinetics under more reducing conditions films on LAO sintered at 1400°C in nitrogen flow are more dense with a larger grain size compared to the film sintered in an oxygen atmosphere at the same temperature. This result agrees well with the literature [3]. Not surprisingly the lower sintering temperature (e.g. 1200°C) results in a similar grain size sample and a relatively more porous microstructure. The YSZ results are similar although the grain growth is larger than on LAO in the nitrogen atmosphere at 1400°C. Thick films on polycrystalline Al2O3 substrates (not shown) also have similar microstructures to the films on single crystalline (100) LAO and YSZ substrates sintered at similar temperatures. 3.1.2. X-ray analysis X-ray diffraction analysis was carried out on some of the representative thick films and also the powder from which the thick films were fabricated. It was found that all samples are single phase within the sensitivity of the X-ray measurements (: 5%). The unit cell parameters calculated for a pseudo-cubic unit cell as well as the unit cell volume of each sample are tabulated in Table 2. The La0.67Ca0.33MnO3 powder from which the ink of thick films were prepared has a pseudo-cubic structure with lattice parameter 7.7319 0.003 A, . From the Table 2, it was observed that the pseudo-cubic lattice parameter is the smallest for the films sintered in an oxygen flow. This is because sintering under more oxidising conditions creates more Mn4 + , which has a smaller ionic size compared to Mn3 + [4]. This result agrees well with those reported earlier [5,6]. The thick film on YSZ substrates which was sintered in a nitrogen flow has the largest lattice parameters, which is possibly due to incorporation of Y in the film from the YSZ substrate. 3.2. Physical properties 3.2.1. Resisti6ity The temperature dependence of the normalised resistivity, r(T)/r(RT), at zero field for the films deposited on single crystal (100) LAO substrates, sintered at
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Table 2 Lattice parameters and unit cell volume of the La0.67Ca0.33MnO3 thick films on various substrates sintered at various temperatures and atmospheresa Substrates
Sintering temperatures (°C)
LAO
1400
YSZ
1400
LAO
1300
YSZ Al2O3
1300 1200
a
Atmospheres
Pseudo-cubic lattice parameters (A, )
Unit cell volume (A, 3)
Air O2 N2 Air O2 N2 Air O2 N2 Air Air O2
7.7227 9 0.0009 7.7092 90.0018 7.7185 90.0014 7.7237 90.0008 7.7221 90.0014 7.7734 90.0007 7.7135 90.0037 7.7140 90.0014
460.5781 90.1674 458.1760 9 0.3171 459.8271 9 0.2479 460.7664 90.1472 460.4707 9 0.2512 469.7081 9 0.1359 458.9368 9 0.6647 459.0297 9 0.2545
7.7142 90.0029 7.7202 90.0013 7.7080 90.0027
459.0669 9 0.5201 460.1392 9 0.2271 457.9561 9 0.4847
The samples were sintered at the temperature indicated for 1 h.
1400°C in various atmospheres are shown in Fig. 3(a). All thick films on LAO substrates show a M –I transition at different peak temperatures, Tp1. The Tp1 values are given in Table 3. The thick films sintered at 1400°C in oxygen shows two peaks in the r – T curves like the peaks observed in the bulk samples prepared from the powder from which the ink of the thick films were made as shown in Fig. 4. The bulk sample, which was prepared from the thick film powder, has a very broad M–I transition and double peaks in the r– T curve. The thick films that were sintered in oxygen also show double peaks but r is an order of magnitude higher than that of the fully oxygenated bulk sample [7,8]. Most of the thick films on LAO substrates have very broad M–I transitions at Tp1 similar to the bulk samples prepared from the thick film powder. The Tp1 varies from 180 to 258 K for the thick films on LAO substrates. The film sintered at 1300°C in air (not shown) has the lowest Tp1 and the highest r(Tp1)/ r(RT) and r(Tp1)/r(20 K). The highest Tp1 was obtained for the film sintered at 1400°C in oxygen and this sample also has the lowest resistivity throughout the whole temperature range (295 – 20 K). The r above Tp1 follows an activated behaviour r(T) 8 exp(EA/ kBT) for all samples. The activation energies (EA) are given in Table 3. The origin of different behaviour of r – T in the present composition might be due to the fact that it was prepared using a slightly deviated method. We preheated the La2O3 powder before mixing with the other ingredients. Usually, La(OH)3 and La2O2CO3 coexist within the commercial La2O3. To eliminate La(OH)3 and La2O2CO3, we heat treated the commercial La2O3 at 1000°C for 6 h and then cooled and immediately stored in a desiccator. Recently, Sun et al. [9] reported that nearly stoichiometric composition can be obtained when commercial La2O3 is used and purified La2O3 (heat-treated) results in oxygen deficiency.
Fig. 3(b) shows the normalised r– T curves of the films fabricated on the single crystal (100) YSZ substrates for samples sintered at 1400°C only. All films except the film sintered at 1400°C in nitrogen flow show a M–I transition at Tp1. The Tp1 varies from 187–265 K for the thick films on YSZ substrates. The film sintered at 1400°C in oxygen has the lowest Tp1 and the highest r(Tp1)/r(RT) and r(Tp1)/r(20 K). The film sintered at 1300°C in oxygen has the highest Tp1. The r above Tp1 follows an activated behaviour with a slight variation of activation energies. The Tp1 values and the activation energies are given in Table 3. The normalised r–T curves for the films fabricated on the polycrystalline Al2O3 substrates are shown in Fig. 3(c). These films are sintered at 1200°C in air, oxygen and nitrogen flow. The films sintered at 1300 and 1400°C reacted with the substrates and were therefore not suitable for further investigations. The measured films showed a M–I transition at Tp1 The air and
Fig. 3. Normalised zero field r as a function of temperature for the La0.67Ca0.33MnO3 thick films (a) fabricated on single crystal (100) LAO substrates, (b) single crystal (100) YSZ substrates and (c) polycrystalline alumina substrates. The sintering temperature and atmospheres are shown in the figure.
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Table 3 The Tp1, Tc, activation energy, thickness and the r(Tp1)/r(20 K) of La0.67Ca0.33MnO3 thick films on different substrates, where Tp1 is the temperature correspond to the resistivity peak. Tc is the Curie–Weiss temperature and is taken from minimum in dM/dT. The M(T) were recorded in the presence of 1 T applied field (ZFC)a Substrates
Sintering temperature
LAO
1400
YSZ
1400
LAO
1300
YSZ
1300
LAO
1200
YSZ
1200
Al2O3
1200
Bulk sample
1400
a
Atmosphere
Thickness (mm)
Tp1 (K)
r(Tp1)/r(20 K)
Air O2 N2 Air O2 N2 Air O2 N2 Air O2 N2 Air O2 N2 Air O2 N2 Air O2 N2 Air
63.4 46.2 76.5 42.9 54.5 58.7 39.6 60.2 31.4 46.9 72.1 21.1 27.7 72.7 34.2 30.7 30.3 32.0 37.4 44.5 41.0
249.8 257.6 204.9 263.1 186.8 – 179.6 226.1 257.0 243.9 264.3 189.2 251.7 197.6 218.5 254.8 214.0 243.2 258.5 253.3 263.3 254.8
7.0 9.0 5.1 8.6 47.1 – 6.8 9.0 5.1 19.7 7.0 9.0 6.1 6.2 5.3 5.8 6.1 5.8 5.4 4.5 4.3 4.5
EA (meV) 127 108 167 116 133 110 124 152 120 126 113 131 142 117 130 130 114 115 119 109 110 119
Tc (K) 252 264 263 175 169
Results of a bulk sample prepared from this powder is also presented in this table.
nitrogen sintered samples showed a peak with an additional shoulder. The oxygen sintered sample shows a broad peak only. The Tp1 varies from 253 – 264 K. The film sintered in oxygen has the lowest Tp1 and the sample sintered in nitrogen has the highest Tp1. The nitrogen and oxygen sintered samples show the lowest resistivity values. The resistivity above Tp1 for these films also follows an activated behaviour. The Tp1 and the activation energies are given in Table 3. In this study we have observed a wide variation of Tp1’s for thick films and different substrates sintered at different atmospheres. Choice of substrates and sintering conditions clearly have a great influence on the properties of manganite materials [10,11]. Recently, Balcells and collaborators [12 – 14] reported magnetoresistive properties of La2/3Srl/3MnO3 thick films, prepared by either spray printing or screen printing techniques on polycrystalline Al2O3 substrates, sintered at 1400°C in oxygen. However, our La0.67Ca0.33MnO3 thick films on Al2O3 substrates, sintered at 1400°C in air, oxygen and nitrogen reacted heavily with the substrates. This suggests that the chemical composition also has an influence on the properties of manganite thick films.
3.2.2. Magnetoresistance (MR) The MR(8T) curves as a function of temperature for some representative thick films on LAO are shown in Fig. 5(a). We observed that in addition to MR peaks
Fig. 4. The zero field r as a function of temperature for a bulk sample of La0.67Ca0.33MnO3 sintered in air and a thick film on LAO sintered in oxygen flow. The sintering temperature was 1400°C.
Fig. 5. (a) The MR(8T) curves as a function of temperature. (b) The MR(T, H) curves for thick films of La0.67Ca0.33MnO3 on single crystal (100) LAO substrates sintered at various temperatures and atmospheres.
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sintered at 1400°C in oxygen atmosphere. For the thick film on polycrystalline Al2O3 substrate, the maximum MR, 65%, is observed at Tml = 252 K. This sample also shows a low temperature MR, 43%, at 20 K.
Fig. 6. Magnetic moment as a function of temperature for a few thick films of La0.67Ca0.33MnO3 on different substrates sintered at 1400°C in different atmospheres. The data were taken in the presence of 1 T applied field (ZFC).
around Tp1 all samples show low-temperature MR. The maximum peak MR, 80%, was obtained for the sample which was sintered at 1300°C in air at Tml =180 K. The salient feature of the MR of our present thick films is that MR is not limited to a small temperature window, as observed in thin epitaxial films [15 – 17]. The MR peaks are either: (i) very broad; or (ii) have a slightly smaller magnitude of low-temperature MR which is essentially temperature independent. These properties arise from impurities (reaction with substrates) and grain boundaries in our film. The origin of this low temperature MR in the polycrystalline bulk sample was described extensively elsewhere as a function of grain size and oxygen [7,8,18]. The MR isotherms as a function of applied field for some representative thick films on LAO substrates are shown in Fig. 5(b). This graph indicates the MR sensitivities as a function of temperature and field for the films. MR has four different types of sensitivity as a function of temperature and field. 1. Highly sensitive low- temperature (Tp1) and lowfield (B1 T) MR. 2. Weakly sensitive low-temperature MR with high applied field (1 – 3 T). 3. MR around Tp1. 4. High temperature (T \Tp1) MR. The MR(8T) curves as a function of temperature and MR isotherms as a function of field for the thick films on YSZ and polycrystalline Al2O3 substrates have shown similar characteristics. The maximum MR, 94%, at Tml (184 K) was obtained for the film on YSZ,
3.2.3. Magnetic properties of La0.67,Ca0.33MnO3 thick films Fig. 6 shows M–T curves for a few representative thick films on different substrates sintered only at 1400°C in different atmospheres for an hour. This data was taken in presence of 1 T applied field. The samples were cooled down to 20 K in the absence of field and then warmed up in 1T applied field. All samples show a paramagnetic to ferromagnetic transition at Tc. The Tc’s are defined by the minima in dM/dT and are given in Table 3. It was observed that the Tc’s are very close to Tp1. However, the thick film on YSZ sintered at 1400°C in nitrogen atmosphere has a Tc 169 K, but no Tp1 was present in the r–T curve. 3.2.4. Post-annealing of La0.67,Ca0.33MnO3 thick films The thick films sintered at 1400°C in air were postannealed in a controlled oxygen atmosphere, as transport results of the film indicated oxygen deficiency in the thick films due to preparation route of the thick film powder. The film on LAO was annealed at 600°C and film on YSZ was annelid at 900°C. Post-annealing conditions are given in Table 4. There is not any significant change in the shape of the r–T curve for the film on LAO substrates; however, the film on YSZ exhibits a broad peak with an additional shoulder. There is a decrease (15%) of the r(T) as a result of post-annealing, as expected, since higher oxidation state gives more Mn4 + and greater ferromagnetic coupling. 3.2.5. Reproducibility of La0.67,Ca0.33MnO3 thick films We have repeated the 1400°C oxygen sintered sample on LAO substrate and the 1200°C oxygen sintered sample on polycrystalline Al2O3 substrate. The transport results of the first and second batch sintered thick films under same condition on LAO and Al2O3 are shown in Fig. 7. From Fig. 7, we found that the form of r(T) is very similar for both first and repeat sintered thick films. From this random reproducibility check, and assuming that R(H, T) is also similar, it is concluded that the thick films are reproducible.
Table 4 Post-annealing conditions of La0.67Ca0.33MnO3 thick films on various substrates Sample no.
Substrates
LCL5 LCY5
LAO YSZ
Heating rate °C h−1 100 100
Tmax (°C) 600 900
Time at Tmax (h) 6 6
Atmosphere 0.8 pO2 0.8 pO2
Cooling rate °C h−1 100 100
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have shown the highest MR and the highest r(Tp1)/ r(20 K).
Acknowledgements Dr A.K.M.A. Hossain would like to thank Association of Commonwealth Universities for the scholarship. This work was also funded by the EPSRC. Fig. 7. The normalised zero field resistivity as a function of temperature for the first and repeat sintered thick films of La0.67Ca0.33MnO3 on various substrates.
4. Conclusions We have fabricated manganite thick films on various substrates using a screen printing technique. Screen printing has proven to be successful technique for the preparation of high quality, reproducible thick films. We have observed a wide variation of Tp1 for the thick films on different substrates sintered at different temperatures and atmospheres. This variation is due to reactivity with substrate (which increases in the order YSZ, LAO and Al2O3) amounts of Mn4 + in films, grain size and connectivity of the samples. The physical properties of the thick films are either similar or show improve MR properties compared to bulk polycrystalline samples prepared from the same powders. The highest MR obtained for the thick films is 94% in 8 T applied field near Tp1 for La0.67Ca0.33MnO3 thick film fabricated at 1400°C in flowing oxygen on single crystal YSZ substrate. In addition, a temperature independent low-temperature MR of 51% has been obtained at 20 K for this film. The highest sintering temperature for the thick films on polycrystalline Al2O3 is 1200°C in all atmospheres (e.g. air, O2, and N2). The highest sintering temperature for the single crystal substrates was 1400°C. However, the thick films on YSZ, sintered at 1400°C in nitrogen atmosphere depressed the M – I transition below 70 K. The thick film on YSZ sintered at 1400°C in oxygen and film on LAO sintered at 1300°C in air
.
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