Magnetoresistance effect in bulk samples of La0.67Ca0.33MnO3 under a low magnetic field

Journal of Magnetism and Magnetic Materials 188 (1998) 180—184

Magnetoresistance effect in bulk samples of La Ca MnO 0.67 0.33 3 under a low magnetic field G.C. Bhar!,*, U. Chatterjee!, K. Tsukamoto", T. Yanagisawa", A. Obara", H. Shen# ! Physics Department, Laser Laboratory, Burdwan University, Burdwan-713104, India " Electrotechnical Laboratory, Optoelectronics Division, Umezono 1-1-4, Ibaraki-305, Tsukuba, Japan # State Key Laboratory of Functional Materials Informations, Shanghai Institute of Metallurgy, 865 Changning Road, Shanghai 200050, China Received 13 November 1997; received in revised form 6 March 1998

Abstract The magnetoresistance effect of bulk samples of La Ca MnO , prepared by the sol—gel method has been studied 0.67 0.33 3 under a low applied magnetic field. Quite a large (!37%) negative magnetoresistance ratio (MR) has been obtained at a relatively ‘higher’ temperature (190 K) with an applied external magnetic field intensity as low as 0.6 T. Observation of the MR peak at a temperature greater than the insulator—metal transition temperature has been made for the first time. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Magnetoresistance; Perovskites; Sol—gel method; Resistivity

Observations of very large negative magnetoresistance ratio (MR) near room temperature in doped Mn-oxides [A B MnO , where A is 1~x x 3 a rare earth element like La, Nd, Pr etc. and B is a divalent alkaline earth element like Ca, Sr, Ba etc.] has created renewed interest [1—9] both for theoretical understanding of the fundamental physics as well as for their device applications like in magnetic field sensors, hard disk drive heads, etc. Particularly interesting is the fact that MR values of some of these manganese perovskites are much larger than other materials like metallic multilayered films and also the properties of these

* Corresponding author. Tel.: #91 342 64374; fax: #91 342 64452/64374; e-mail: [email protected].

manganese perovskites can be suitably tailored by variation of a number of parameters like A/B ratio, processing conditions like oxygen content, local distribution of Mn3` and Mn4` ion pairs, etc. [10]. Large MR effects have been demonstrated in the bulk samples as well as in the thick and thin films of these materials. A magnetoresistive position sensor has been demonstrated [11] using a thick film of La Sr MnO . The possibilities of 2@3 1@3 3 utilising such MR materials as both the thermal storage and as the working material in an active magnetic regenerative refrigerator have been explored [12,13]. In almost all of the earlier reports to date, the temperature at which the MR peak appears has been reported to be slightly below the temperature at which the resistivity peak for the MR samples

0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 2 0 0 - 5

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occur. However, for their La Ca MnO sam0.67 0.33 x ples, prepared by the standard solid-state method, Zhang et al. [14] have reported the appearance of both the MR and resistivity peaks exactly at the same temperature. Here, we report the observation of a comparatively large negative MR effect (!37%) in the bulk La Ca MnO sample at 0.67 0.33 3 a considerably ‘higher’ temperature (190 K) under quite a low applied magnetic field (0.6 T) and which to our knowledge is the very first observation of the MR peak at temperature (¹) greater than the temperature (¹ ) at which the resistivity peak occurs in R the bulk sample with the same composition. The samples have been prepared by the sol—gel technique [8,9], preparing the required aqueous solution with La(NO ) ) 6H O, Ca(NO ) ) 4H O 33 2 32 2 and Mn(NO ) ) 6H O taken in stoichiometric pro32 2 portion and using urea as gelificant agent. The urea concentration was kept at 1 M. The aqueous solution was directly evaporated on a hot plate at a temperature of 190°C. An ash coloured gel formed after cooling and it was again preheated to 250°C in a closed furnace for 11 h. After cooling 2 inside the furnace, it was ground for 1 h in order 2 to powder the sample. The powdered sample appeared to be quite hygroscopic as was evident from the fact that the blackness of the sample became darker when kept outside. To remove the moisture, the powder was preheated again to 250°C inside the same furnace for 1 h and allowed to cool. The powdered sample was then pressed into several round disks of &20 mm diameter and 4.4 mm thick and then these disks were sintered at different temperatures for different durations inside another high-temperature furnace (M/S Yamada Denki, Model no. MSFT-1020). For all cases, in this second furnace, the temperature of the sample was raised at the rate of 10°C/min to the sintering temperature (¹ ), kept at ¹ for some time (t ) and S S S then the sample was allowed to cool at a rate of 1°C/min to room temperature. The cooling rate was kept lower to prevent cracking of the samples. We must mention here that all the samples have been sintered in this second furnace almost in an identical environment, except changing their ¹ S and t . No fresh oxygen or air was allowed to pass. S The furnace had not been evacuated before operating it, but all of its outlets remained closed. As such,

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although no specific measurements have been made, it is apparent that the oxygen content of the samples did not vary appreciably from sample to sample by this sintering process so far as the sintering in the second furnace was concerned. The scanning electron micrograph of the samples reveal that the particle size has increased from 600 nm to 2.5 lm when ¹ was changed from 950 to 1280°C, S t remaining the same (3 h). It should be noted that S while Sanchez et al. [8,9] had ground their samples sintered at high temperature again for an hour, we did not. Thus the particle size of our samples are larger than that obtained by them. The sintered disks are then cut to obtain rectangular shaped plates, the average dimensions (length]height] width) being approximately 1.97 cm]0.44 cm] 0.83 cm. From the residual pieces of each circular disk, at this stage, the structure of the powdered sample was examined using X-ray diffraction done with the Rigaku Rint X-ray diffractometer and all the obtained patterns show characteristic peaks of perovskite confirming single-phase perovskite material as has been reported in earlier works [8,9,15—17]. Fig. 1 shows such X-ray diffraction pattern for a sample sintered at ¹ "1280°C for S t "3 h. S In each of the obtained rectangular shaped plates, four silver wires were pasted with silver paste and each sample was then kept in the second furnace at 400°C for 30 min, both the rates of heating and cooling this time being maintained at 10°C/min. The measurement of the bulk electrical resistance and hence MR of the samples as a function of temperature and magnetic field were done using the standard four-probe method. Measurements were made without a magnetic field and with

Fig. 1. X-ray diffraction pattern of a powdered La Ca MnO sample sintered at 1280°C for 3 h. 0.67 0.33 3

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Fig. 2. Variation of normalised resistivity (curve a for H"0 and curve b for H"0.6 T) and MR (curve c) with temperature for the bulk La Ca MnO sample sintered at 750°C for 10 h. 0.67 0.33 3

a varying magnetic field intensity up to 6000 G, the temperature being varied in the range from 77 to 300 K, the magnetic field being applied parallel to the current direction. The measurement was carried out during the increase of temperature rather than during cooling, the average rate of increase being approximately 2 K/min. At first the resistance without a magnetic field was recorded and then keeping the magnetic field value fixed at H, the sample was allowed to cool down to 77 K. Measurement was then started again while heating the sample. From the experimentally obtained curves, the MR values at different temperatures have been calculated using the expression MR%"[100]Mo(0)!o(H)N]/ o(H) where o(0) and o(H) are respective values of resistivities of the sample for zero magnetic field strength and with magnetic field (H). Figs. 2—4 show variations of normalised resistivities both for the applied external field H"0 (curve a) and H"0.6 T (curve b), as well as the corresponding MR with temperature (¹) (curve c) for three different samples — sample I (¹ "750°C, S t "10 h), sample II (¹ "950°C, t "3 h) and S S S sample III (¹ "1280°C, t "10 h). The resistivity S S of the sample has increased by an order of magnitude when ¹ is decreased from 1280 to 750°C as S has been observed earlier [18]. It can also be seen that the MR peak value is not affected very much [18] by the particle size, changing from !25% to only !37% when ¹ changes from 750 to 1280°C, S t remaining the same (10 h). However, at this stage S

Fig. 3. Variation of normalised resistivity (curve a for H"0 and curve b for H"0.6 T) and MR (curve c) with temperature for the bulk La Ca MnO sample sintered at 950°C for 3 h. 0.67 0.33 3

Fig. 4. Variation of normalised resistivity (curve a for H"0 and curve b for H"0.6 T) and MR (curve c) with temperature for the bulk La Ca MnO sample sintered at 1280°C for 10 h. 0.67 0.33 3

we can confirm this only for the temperature range investigated (80—300 K) here, since both the samples have not shown any saturation effect in MR values in the low-temperature region and seem to attain higher values as ¹ becomes less than 80 K. But then one should also note that the MR values of the samples at ¹&80 K are almost the same, while for sample I this value is !25%, for sample III it is !28%. From the figures it is evident that sample I shows the usual observation of a single MR peak occurring at ¹(¹ . But for sample II, R we obtained double peaks both in the o(¹)/o ver0 sus ¹ curves as well as in the MR versus ¹ curves, the MR value becoming zero in between the peaks. o(¹) and o are, respectively, the resistivities of the 0

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sample at temperature ¹ and at 300 K. However, a similar measurement for a sample with ¹ " S 950°C but for longer t (e.g. 7 h), showed the obserS vation of the usual single peaks for both the o and MR curves, the peak for the latter occurring at ¹(¹ . The X-ray diffraction patterns for both the R samples confirm a single-phase perovskite structure. It can be seen from Fig. 4 that for sample III although the o(¹)/o versus ¹ curves have single 0 peaks, the corresponding MR versus ¹ curve has two peaks, one below ¹ while the other which R seems to be more prominent than the former is at ¹'¹ . An interesting observation for similar R measurement with this type of sample, which are sintered at ¹ '1000°C, is that if the sintering S temperature decreases or increases, the peak of the corresponding MR versus ¹ curve shifts towards lower or higher temperature for the same applied external magnetic field. For example, when H"0.6 T, for the sample with ¹ "1100°C the S MR peak occurs at ¹&180 K, for the sample with ¹ "1280°C it occurs at ¹&190 K, while for the S sample with ¹ "1350°C it occurs at ¹&200 K S although the nature of the MR versus ¹ curve for each sample remains the same in all cases as shown in Fig. 3 for sample III. Although an increase in t for these samples increases the resistivity as well S as the MR values; but for the same ¹ , the MR S peak occurs at the same temperature in all the cases. We observed that for the sample with ¹ "1280°C and t "10 h, the MR peak value is S S !37% while for the sample with ¹ "1280°C and S t "3 h, the MR peak value is !29%, although in S both cases the MR peak occurs almost at ¹"190 K. Fig. 5 shows the variation of resistance (normalised by the zero field value i.e. o(H)/o(0)) with the applied magnetic field H for sample III, for different fixed temperatures in the range 80—240 K. Earlier Hwang et al. [19] had studied and compared the MR and field dependent magnetisation in single crystal and polycrystalline La Sr MnO 2@3 1@3 3 samples. As expected, the nature of the curves in Fig. 5 can be compared more favourably with those in Fig. 2e of polycrystalline sample sintered at 1300°C than with those in Fig. 2a of single crystal. Apart from the fact that instead of a systematic decrease of o(H)/o(0) as the temperature increases

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Fig. 5. Magnetic field dependence of o(H)/o(0) (as defined in the text) for sample III at different temperatures ¹"80 K (1), 120 K (2), 160 K (3), 180 K (4), 190 K (5), 200 K (6) and 240 K (7).

as seen in their Fig. 2e, our sample shows an increase in between the temperature range 160—200 K, there are also some other marked differences as well — near room temperature our sample shows no detectable MR, the decrease of o(H)/o(0) is much slower in the low-field region and a saturation effect appears when H is increased beyond 0.5 T. On the other hand it definitely seems to confirm their observation that o(H)/o(0) has exactly similar field dependence for different temperatures when H is greater than 0.5 T, although while for their sample the value continues to increase with H, for our sample it appears to get saturated. Of course, it should also be pointed out that lack of higher magnetic field source prevented us from further investigating the saturation effect for H greater than 0.6 T on our sample. We have also observed that the sample with the same composition (La Ca MnO ) when prepared by 0.67 0.33 3 the standard ceramic process and sintered at 1280°C for 10 h, under the same condition, shows a single peak in the MR versus ¹ curve, the MR peak appearing at ¹(¹ , although the peak ocR curs at a slightly higher temperature than that obtained for the samples prepared similarly but sintered at 750°C for 10 h. In conclusion, a significant MR value (!37%) has been obtained in the bulk sample of La Ca MnO applying quite a low magnetic 0.67 0.33 3 field (0.6 T) and also at quite a ‘high’ temperature (190 K), all of which are desirable for practical

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device applications. Further work is proposed to be done to confirm whether the thin and thick films of the prepared samples show similar characteristics. The authors like to thank Dr. S. Okayama and Dr. S. Kimura of the Electrotechnical Laboratory, Japan for the useful discussions. The work has been supported by ITIT (Japan). The Indian authors also thankfully acknowledge the National Laser Programme, Government of India.

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