3MnO3 manganite

Sensors and Actuators A 132 (2006) 52–55

On–off magnetoresistive sensor based on screen-printed La2/3Sr1/3MnO3 manganite D. Rubi a,∗ , J. Fontcuberta a , M. Lacaba b , A.M. Gonz´alez b , J. Bazt´an b , A. Calleja c , Ll. Aragon´es c , X.G. Capdevila d , M. Segarra d a

Institut de Ci`encia de Materials de Barcelona, Campus UAB, E-08193 Bellaterra, Spain b PIHER NACESA, Navarra 31500, Spain c Quality Chemicals, Catalunya 08292, Spain d Facultat de Qu´ımica, Universitat de Barcelona, Barcelona 08028, Spain

Received 14 September 2005; received in revised form 23 May 2006; accepted 27 May 2006 Available online 10 July 2006

Abstract Half-metallic ferromagnetic oxides present a remarkable potential for the development of magnetoresistive sensors. We report on the design, construction and test of a contact-less on–off position sensor based on thick La2/3 Sr1/3 MnO3 films. Films were fabricated on polycrystalline Al2 O3 substrates by means of the standard screen-printing technique. The temperature dependence of the output signal of the sensor has been monitored from room temperature up to 120 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: Manganites; Thick films; Sensors

1. Introduction Manganese perovskites, usually called “manganites”, show a substantial reduction of their resistivity when a relatively low magnetic field, of the order of a few kOe, is applied [1]. This phenomenon is known as “low field magnetoresistance”. Manganites have been intensively investigated in the past years in order to obtain an accurate description of their underlying physics, as well as to develop possible applications. In particular, the manganite La2/3 Sr1/3 MnO3 (LSMO, presenting a Curie temperature (TC ) of about 360 K) has been successfully used to build analogic magnetoresistive sensors operative at room temperature [2,3]. This work describes the design and test of a new one: a digital (on–off switch) position sensor, based on LSMO thick films prepared by standard screen-printing technique. It is found that the sensor is operative from room temperature up to 100 ◦ C, which is slightly above the Curie temperature of LSMO. This work provides further evidence of the potential of this kind of materials in the construction of low-cost magnetic devices.

∗

Corresponding author. Tel.: +34 93 580 1853; fax: +34 93 580 5729. E-mail address: [email protected] (D. Rubi).

0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.05.034

2. Fabrication and characterization of LSMO thick films LSMO powders were prepared by a water-soluble polymer combustion method, starting from electronic grade raw materials [4]. The process avoids the use of extensive milling, as it produces an homogeneous, fine free-flowing powder. The ceramic grains were suspended in an organic vehicle (56.5/43.5% w/w) [5], and screen printed onto commercial Al2 O3 (0.65 mm thick). The mask used to define the sensor geometry is a screen made of 200 mesh stainless-steel fabric, having an open area (defined as the ratio between the screen aperture and the string thickness) of 48%. For the final socked application, the sensor will be composed by two magnetoresistive elements (A and B, with dimensions 6 mm × 1 mm, as seen in Fig. 1(a)), which can be operated at the same time and independently from each other. The duplicity of the output signal is intended to accomplish the redundance requirements of the designed sensor. After screen-printing the LSMO based ink, the films were dried in a conveyor furnace for 12 in air, at 200 ◦ C [5]. Films were sinterized at 1100 ◦ C, in air, for 8 h. After sintering, the films resulted well adhered, mechanically stable and with a sur-

D. Rubi et al. / Sensors and Actuators A 132 (2006) 52–55

Fig. 1. (a) Photograph of the screen printed sensor. The two LSMO tracks are labelled as A and B. (b) Hall map of the axial component of the hexapolar Sm–Co magnet. The contrast between magnetic poles with opposite polarities can be clearly appreciated. Full lines indicate the maximum field zones, while dotted lines show the minimum field areas.

face free of fissures or visible cracks. Their microstructure can be seen in the scanning electron microscopy – SEM – of Fig. 2(a). It is found the presence of a porous network of well connected submicrometric grains. The SEM cross-section image of Fig. 2(b) shows that films are ∼15 ␮m thick. In order to perform the transport characterization, contacts have been made by using commercial silver paste. The room temperature 4-point resistance was found to be of about 40 . This value leads to a resistivity of 0.01  cm, which is in good agreement with reported values in ceramic LSMO samples with similar grain size that our films [3]. Fig. 3 shows the magnetoresistance (defined as MR = R(H) − R(H = 0)/R(H = 0)) of one of the tracks as a function of the applied field. The resistance was measured by means of the standard 4-probe technique, while the magnetic field was provided by a conventional electro-magnet. The magnetoresistance at the highest applied field (H = 5.5 kOe) is of about 1.5%. This value is smaller than that previously reported in thick LSMO films sinterized at the same temperature (2.6% at 5.5 kOe [2]), relying this difference on the higher sintering time – which enhances the grain connectivity and lowers the magnetoresistance – used for films reported here.

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Fig. 2. (a) SEM micrograph showing the microstructure of a LSMO thick film. (b) Cross-section image of the same film.

of 18 mm. Fig. 1(b) shows a Hall map of the axial component of the used magnet, where dark and clear zones correspond to the two possible polarities of the magnetic field. The mean magnetic field acting on each track at a given angle is obtained by integrating the local magnetic field (established from the Hall map) on a surface similar to that of an LSMO film. The maximum mean magnetic field (zones centred around the full lines showed in Fig. 1(a)), at a distance of 1 mm from the magnet surface, is expected to be of about MAX ∼ 2.6 kOe, while the minimum mean field (zones around the dashed lines in Fig. 1(a)) is of about MIN ∼ 0.2MAX . Notice that due to the finite size of the sensing elements, the average minimum

3. Testing the sensor response The magnetic field in the final socked sensor is provided by an hexapolar Sm–Co permanent magnet, with a diameter

Fig. 3. Room temperature magnetoresistance (MR) as a function of the applied field for a LSMO thick film.

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D. Rubi et al. / Sensors and Actuators A 132 (2006) 52–55

magnetic field is non-zero. The rotating magnet is placed on top (at a distance of about 1 mm) of one of the LSMO tracks. In order to perform a preliminary test, the sensor was appropriately encapsulated, while one of the LSMO tracks was connected in series with a 220  ceramic resistance and a 1 V bias voltage was set between them. The output was measured on the magnetoresistive element by using a conventional multimeter. When switching-on the rotation of the magnet, a clear output is obtained, as can be seen in Fig. 4. The output shows a periodic voltage oscillation of the MR as a function of the magnet swept angle. Two resistance states can be established: a low (high) resistance state taking place when the highest (minimum) magnetic field acts on the LSMO track. The high resistance state is achieved when the track faces the transition zone between opposite polarities (minimum field), while the low resistance state takes place when the track is in front of the center of each pole (maximum field). As can be seen from Fig. 1(b), both situations occur six times per revolution, the output showing then the existence of six peaks every 360◦ . Naturally, each resistance state can be assigned to “on” and “off” states, being possible to convert the analogical output to a digital “on–off” square signal by means of appropriate electronics. Fig. 4 shows that the peak-to-peak output, normalized by the 1 V input, is of about 0.08%. This corresponds to a LSMO magnetoresistance of ∼0.4%, which is a smaller value than that expected from the MR(H) data presented in Fig. 3; according to this figure, the resistance variation between applied fields of MAX ∼ 2.6 kOe and MIN ∼ 0.2MAX is of about 0.7%. Fig. 4 also shows that the measured signal presents a somewhat irregular structure that may be attributed to the critical dependence of the output on the alignment between the track and the magnet. Afterwards, we proceed to test the sensor performance in its on–off digital configuration. The magnet used in this occasion was a bipolar ferrite (18 mm × 5 mm × 5 mm), which provides a magnetic field of about 0.5 kOe at the sensor position (about 1 mm below the magnet), oriented in the plane of the sensor. The lower magnetic field of the ferrite, when compared to the Sm–Co magnet, may affect the intensity of the output signal. However, this effect should be partially counter backed by the existence of the so-called anisotropic magnetoresistance (AMR): in this configuration, both the bias current and the applied magnetic field

Fig. 5. (a) Room temperature and (b) 100 ◦ C digital output for the socked sensor (see text for details).

are in the same plane; as the angle between them is not a constant but changes when the ferrite magnet is rotated, the appearance of an AMR effect is expected. This can induce a noticeable variation of the LSMO resistance, as previously reported in Ref. [6], that adds to the conventional magnetoresistance. Experiments were done at temperatures between 0 and 120 ◦ C, in an industrial climatic chamber. The magnet rotation was controlled by means of a motor-encoder system (3200 microsteps). The outputs of both tracks were converted, by means of appropriate electronics (comparator and amplifier circuits), into square signals. Fig. 5 shows two of the measurements, performed at 25 and 100 ◦ C. It can be seen the existence of two 90◦ dephased square signals, reflecting the contribution of both LSMO tracks. In spite of the reduction of the magnetoresistance when the temperature approaches to the LSMO paramagnetic state, it is worth noting that well defined on–off output signals can be obtained up to 100 ◦ C (see Fig. 5(b)). We should notice that a similar or better performance is expected when using the designed hexapolar Sm–Co magnet, as it will provide a stronger magnetic field acting on the magnetoresistive layers than in the bipolar ferrite case. 4. Conclusions

Fig. 4. Room temperature output of the sensor, as a function of the magnet rotation. The magnet was rotated at a constant velocity. A 1 V bias voltage was used.

In summary, we have shown that thick screen-printed LSMO films can be successfully used in the development of contact-less magnetoresistive sensors. In particular, we have demonstrated

D. Rubi et al. / Sensors and Actuators A 132 (2006) 52–55

the design and construction of an on–off switch, which showed being operative between room temperature and 100 ◦ C. Further materials engineering is needed to improve the room temperature functional properties of these kinds of half-metallic magnetoresistive oxides. This achievement would eventually lead to a new generation of low-cost devices. Acknowledgments

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potential for spintronics applications, such as manganites, double perovskites, magnetite or diluted magnetic semiconductors. Josep Fontcuberta was born in Barcelona (Spain) in 1953. He received his PhD degree in Physics at the University of Barcelona in 1982. After a post-doc stage in U.K. he became assistant professor at Univ. of Barcelona. In 1991 he moved to the Institut de Ci`encia de Materials de Barcelona (CSIC). He is a Research Professor with interest on functional properties of materials suitable for magnetoelectronics. He has co-authored more than 300 scientific papers and directed several PhD Thesis works. He is the director of the Laboratory of Magnetism and Magnetic Materials of ICMAB.

We acknowledge financial support from AMORE (U.E., Contract G5RD-CT2000-00138) and DIMOS (MCyT Spain, Contract MAT2002-03431) projects.

Marta Lacaba is currently working for PIHER-NACESA, a company located in Navarra (Spain) focused on the production of custom sensors and control solutions.

References

Ana Mar´ıa Gonz´alez is currently working for PIHER-NACESA, a company located in Navarra (Spain) focused on the production of custom sensors and control solutions.

[1] J. Fontcuberta, Colossal magnetoresistance, Phys. World (1998) 33–38. [2] Ll. Balcells, J. Cifre, A. Calleja, J. Fontcuberta, M. Varela, F. Benitez, Room-temperature magnetoresistive sensor based on thick films manganese perovskite, Sens. Actuators A 81 (2000) 64–66. [3] Ll. Balcells, A.E. Carrillo, B. Mart´ınez, F. Sandiumenge, J. Fontcuberta, Room temperature magnetoresistive sensor based on thick films manganese perovskite, J. Mag. Mag. Mater. 221 (2000) 224–230. [4] Quality Chemicals, product reference 066619, Technical data can be found in http://www.qualitychemicals.com. [5] Industrial process developed by PIHER-NACESA (http://www.pihernacesa.com). [6] Ll. Balcells, E. Calvo, J. Fontcuberta, Room-temperature anisotropic magnetoresistive sensor based on manganese perovskite thick films, J. Mag. Mag. Mater. 242–245 (2002) 1166–1168.

Biographies Diego Rubi was born in Buenos Aires (Argentina) in 1973. He received his degree in Physical Sciences from the Universidad de Buenos Aires in 2002, and he is about to obtain his PhD in Materials Sciences from the Institut de Ci`encia de Materials de Barcelona and the Universidad Aut´onoma de Barcelona (Spain). His research topics during the last years included the synthesis and structural, magnetic, transport and spectroscopic characterization of materials with

´ Bazt´an is currently working for PIHER-NACESA, a company located Jesus in Navarra (Spain) focused on the production of custom sensors and control solutions. Alberto Calleja L´azaro was born in Barcelona in 1971. He received his degree in Chemistry and PhD in Chemical Engineering from the University of Barcelona in 1994 and 1999, respectively. Currently he is a R&D manager at QUALITY CHEMICALS SL. His fields of interests are new organic and inorganic chemicals development with special emphasis on advanced electronic materials. Lluis Aragon`es Canaldas was born in Tarragona in 1956. He earned his degree in Chemistry from the University of Barcelona in 1978. Now he is the general manager of QUALITY CHEMICALS SL, company committed to customeroriented chemicals with high purity requirements. Xavier G. Capdevila was born in Barcelona in 1973. He received his degree in Chemistry from the University of Barcelona in 1991. Nowadays he is finishing his PhD studies in combustion synthesis of technical ceramics. His research interests include conductor and superconductor ceramics as well as fuel cells. Merc´e Segarra Rubi was born in Badalona in 1965. She obtained her degree in Chemistry and PhD in Metallurgy from the University of Barcelona in 1989 and 1994, respectively. Presently she has a permanent position in the Materials Science Department of University of Barcelona. Her main scientific interests are metallurgy, advanced ceramics and environmental research.