Materials Science and Engineering B83 (2001) 79 – 83 www.elsevier.com/locate/mseb
Highly sensitive low-temperature low-field colossal magnetoresistance in screen printed La0.63Y0.07 Ca0.30MnO3 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 18 September 2000; accepted 4 December 2000
Abstract Thick films of La0.63Y0.07Cao.30MnO3 were fabricated on (100) oriented single crystal LaAlO3 (LAO) and on polycrystalline Al2O3 substrates by a screen printing technique. The films were sintered at 1200°C in air and oxygen atmosphere. All these thick films show a metal-insulator (M-I) transition at Tp1 220 K. For both substrates, the films sintered in oxygen flow have slightly higher Tp1. The film on polycrystalline Al2O3 sintered in oxygen flow shows 71% magnetoresistance (MR) at 220 K in the presence of an 8 T applied field. The salient feature of the colossal magnetoresistance observed in these thick films is that low-temperature low-field MR is highly sensitive than only Ca doped manganites. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Colossal magnetoresistance; Thick film; Transition temperature; Screen printing; Sintering
1. Introduction In the 1950s, the Perovskite compounds R1xAx MnO3 (where R is a rare earth ion and A is a divalent alkaline earth metal) were studied due to their remarkable magnetic behaviour [1,2]. Recently, the manganites have received much attention due to their so called colossal magnetoresistance (CMR) [3–5]. In most studies these materials were investigated in the form of bulk pellets or bar and thin films. For applications of these materials, technology needs to be simple and cheap. The screen printed thick film offers a good opportunity for future sensor and read head applications. This technique offers a wide range of circuit complexities, that is, 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 these studies, we have fabricated thick films of * Corresponding author. Tel.: + 88-02-9665613; fax: +88-028613046. E-mail address:
[email protected] (A.K.M. Akther Hossain).
La0.63Y0.07Ca0.30MnO3 on both polycrystalline Al2O3 and on LaAlO3 (LAO) single crystal substrates and investigated their magnetoresistive properties. We also made a comparative study of low temperature low field magnetoresistance of La0.63Y0.07Ca0.30MnO3 and La0.67Ca0.33MnO3 thick films on Al2O3 substrate.
2. Experimental methods The La0.63Y0.07Ca0.30MnO3 powders were synthesised using a conventional solid state reaction technique using the appropriate proportions of ingredients. Powders of La2O3 (Aldrich 99.99%), Y2O3 (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 the other ingredients, as this is hygroscopic chemical and it was necessary to eliminate water. The ingredients were weighed with appropriate proportions and then dry ball milled for 24 h. The calcination was performed in four steps. The mixed powders were calcined at 950°C for 6 h, the temperature ramps being
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Table 1 Sintering conditions of La0.63Y0.07Ca0.30MnO3 thick films on single crystal (100) LAO and polycrystalline Al2O3 substrates Sample no.
Substrates
Heating rate (°C/h)
Tmax (°C)
Time at Tmax (h)
Atmosphere
Cooling rate (1) (°C/h)
Cooling rate (2) (°C/h)
AHYL1
LAO
100
1200
1
Air
100 at 600°C hold 12 h
100
AHYA1
Al2O3
AHYL2
LAO
100
1200
1
O2
100 at 600°C hold 12 h
100
AHYA2
Al2O3
300°C per h for both heating and cooling. This step was repeated three more times with 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 screen printed thick films were kept in a desiccator overnight to settle and then the organic vehicle burnt out at 300°C for 3 h, using a temperature ramp of 30°C per hour during both heating and cooling. The various sintering conditions of growing manganite thick films on various substrates are given in Ref. [6]. The sintering conditions of La0.63Y0.07Ca0.30MnO3 thick films on different substrates are given in Table 1. We sintered these thick films only at 1200°C as the maximum sintering temperature for the films on polycrystalline Al2O3 substrates is 1200°C [6]. To check for reproducibility in sintering behaviour some screen printed films were sintered under same conditions but in separate sintering runs. The microstructures of the samples were investigated using a scanning electron microscope (SEM) (JEOL JSM-5300) with magnification in the range 750–5000X. 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 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 (z) was calculated from the simple geometrical relation R=zl/A. For magnetoresistance (MR) measurements, the applied field was perpendicular to the current flow in the sample. The MR was calculated using:
MR(%) = −
z(H= 0) − z(H) × 100 z(H= 0)
3. Results
3.1. Microstructure SEM pictures of some representative thick films of La0.63Y0.07Ca0.30MnO3 on single crystal LAO and polycrystalline Al2O3 substrates are shown in Fig. 1(a,b), respectively. The thick film on LAO sintered in oxygen flow is densely packed with a similar average grain size compared to the film on Al2O3 substrate. However, the grains for the thick film on LAO are nearly uniform size compared to the film on Al2O3 substrate.
3.2. Physical properties 3.2.1. DC resisti6ity The normalised resistivity as a function of temperature for films on different substrates and sintered in different atmospheres is shown in Fig. 2. The results of a bulk sample prepared from the powder, which was sintered at 1400°C for 5 h in air is also shown in Fig. 2. The transport results are summarised in Table 2. All thick films show a peak in the z(T) curves at Tp1 (Table 2). There is a slight variation of Tp1 (2–5 K) among the thick films on different substrates sintered at 1200°C at different atmospheres. However, the bulk sample prepared from the thick film powder has a significantly lower Tp1. The quantity z(Tp1)/z(20 K), which measures the sharpness of the peak, is highest for the bulk
Fig. 1. (a) SEM picture of AHYL2, thick film of La0.63Y0.07Ca0.30MnO3 on single crystal LAO, sintered at 1200°C in oxygen flow; (b) SEM picture of AHYA2, thick film of La0.63Y0.07Ca0.30MnO3 on polycrystalline Al2O3, sintered at 1200°C in oxygen flow.
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samples. The activation energies (EA) are given in Table 2.
3.2.2. Magnetoresistance Fig. 3(a) shows the MR as a function of temperature for the films on polycrystalline Al2O3 sintered in oxygen flow. This film show 71% MR near Tp1 (220 K) in 8 T applied field and 46% MR at 20 K. The salient feature of the MR observed in these thick films is that the peak MR window is wider (35 K) and the low-temperature MR is temperature independent. The MR isotherms as a function of applied fields are shown in Fig. 3(b). Like the polycrystalline bulk samples [7,8] and thick films of La0.67Ca0.33MnO3 [6] this film also shows two MR slopes at a temperature much below Tp1. The comparison of low-temperature MR(H) of La0.63Y0.07Ca0.30MnO3 and La0.67Ca0.33MnO3 thick films at 40 K is shown in Fig. 4. It was observed that the low-temperature low-field MR sensitivity of La0.63Y0.07Ca0.30MnO3 is much sharper than the bulk or thick films of nominal composition La0.67Ca0.33MnO3 sintered at the same temperature and atmosphere. The low-temperature MR(H) show two slopes. The strong
Fig. 2. The normalised zero field resistivity as a function of temperature for the thick films of nominal composition La0.63Y0.07Ca0.30MnO3 on various substrates sintered at various atmospheres.
samples. This is because the bulk sample was sintered at a higher temperature and for a longer duration of time than the thick films. The z(T) above Tpl follows an activated behaviour z(T) 8 exp(EA/kBT) for all
Table 2 The resistivity peak temperature, Tp1, the R(Tp1)/R(20 K), thickness and the activation energy, EA, for the thick films of La0.63Y0.07Ca0.30MnO3 on different substrates sintered at 1200°C at different atmospheres and the bulk sample prepared from thick film powder Substrates
Sintering temperature (°C)
Atmosphere
Heading Thickness (m)
Tp1 (K)
R(Tp1)/R(20 K)
EA (meV)
LAO
1200
Air O2
50.25 48.98
221.9 222.9
10.2 10.2
125.9 124.2
Al2O3
1200
Air O2
48.5 48.43
219.2 224.5
8.9 8.2
125.1 129.3
Bulk
1400
Air
200.5
35.1
125.7
–
Fig. 3. (a) The MR(8 T) as a function of temperature; and (b) MR(T, H) for the thick film of La0.63Y0.07Ca0.30MnO3 on polycrystalline Al2O3 substrates, sintered at 1200°C in oxygen flow.
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Fig. 4. The MR as a function of field at 40 K for the thick films of La0.67Ca0.33MnO3 and La0.63Y0.07Ca0.30MnO3 on polycrystalline Al2O3 substrates sintered at 1200°C in oxygen flow.
Fig. 6. The normalised zero field resistance as a function of temperature for the first and repeat sintered thick films of La0.63Y0.07Ca0.30MnO3 on polycrystalline Al2O3 substrate.
field dependence of MR exists for an applied field below H* as shown in Fig. 3(b). It was observed previously that H is a function of temperature and H*(CT)$ Hs(T) determined magnetically, where Hs is the saturation magnetising field [7]. The lowtemperature, low-field (below H*) MR has been attributed to either spin-polarised tunneling [9], spin dependent scattering [10– 12] or micromagnetic behaviour associated with alignment of magnetic domain at the grain boundaries [13]. The origin of lowtemperature, high field (above H*) MR might be due to the disordered or canted spin in the grain boundary region. Fig. 5 represents one possible explanation of both the strong and weak field dependence of MR at low-temperatures. At TTc (where Tc is the paramagnetic to ferromagnetic transition temperature, which is close to the Tp1) the material is in the ferromagnetic regime. However, in the absence of field the magnetisation of the grain of the polycrystalline material will be similar to that in Fig. 5(a). Also, the individual spins at the grain boundary region are randomly oriented. In the absence of field, a carrier will be suffer scattering from the unaligned magnetic domain, as well as disordered spin at the grain boundary region. By applying a low
magnetic field, the magnetisation of each grain starts to align towards the direction of the external magnetic field direction, as shown in Fig. 5(b). However, a large magnetic field is required to align the spins of the grain boundaries, as shown in Fig. 5(c).
3.2.3. Reproducibility of thick films We have repeated the 1200°C oxygen sintered for thick films of La0.63Y0.07Ca0.30MnO3 on polycrystalline Al2O3 substrate, to check the reproducibility of the thick films. The electrical transport results for the first and second batch sintered films under the same condition on Al2O3 substrate, are shown in Fig. 6. Results of two thick films (represented by (a) and (b)) of the second batch sintered thick films, are presented in Fig. 6. It was observed that Tp1 decreases from 225 to 217 K and z/z(RT) at Tp1 increases from 3.4 to 4.1, for the second batch sintered films compared to first sintered film. It is possible that this difference is due to the fact that second sintering was performed in a different furnace and the temperature calibration has slightly deviated from the first furnace. However, the two film of the second batch has identical z(T), as shown in Fig. 6. From the random reproducibility check and assum-
Fig. 5. Explanation of the two slopes MR at low-temperature (T Tc) with the help of grain and grain boundary magnetisation: (a) zero applied field Hext =0; (b) Hext = H *; and (c) Hext \ H*. We assumed tetrakaidecahedral (truncated octahedral) grain shape.
A.K.M. Akther Hossain et al. / Materials Science and Engineering B83 (2001) 79–83
ing MR(H, T) is also similar, it is concluded that the thick films are reproducible.
4. Conclusions Thick films of La0.63Y0.07Ca0.30MnO3 show CMR near Tp1 and also low temperature MR. The low temperature MR exhibits two slopes. Highly sensitive low temperature, low-field MR persists up to certain applied field H*. The low temperature, low field MR sensitivity is much sharper compared to La0.67Ca0.33MnO3 thick film and bulk sample. At low temperature, two slopes MR is explained with the help of grain and grain boundary magnetisation. Highly sensitive low temperature and low field MR is due to the grain boundary. This sensitive grain boundary related MR found in the polycrystalline thick film may be very useful for practical applications.
Acknowledgements Dr A.K.M.A. Hossain would like to thank the Association of Commonwealth Universities for the scholarship tenable at Imperial College, London.
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