Effect of the calcination process on the magnetotransport properties in co-precipitation derived La 0.67 Ca 0.33 MnO 3 ceramics

: La 0.67 Ca 0.33 MnO 3 (LCMO) attracts considerable attention as a quintessential example for colossal magnetoresistance ( CMR ), metal-insulator transition and related temperature coefficient of resistance ( TCR ) studies. Here, co-precipitation method was utilized to prepare the LCMO ceramics, whose magnetotransport properties as a function of calcination temperature ( T cal ) and calcination time (t cal ) were investigated. The magnetotransport properties of these LCMO ceramics were significantly enhanced compared with LCMO derived by sol-gel methods. The TCR of LCMO increased firstly and then decreased as the T cal increased, whereas the metal-insulator transition temperature (T MIT ) shifted towards to the lower temperature. Magnetoresistance ( MR ) increased as T cal rose and reached 82.4 % at T cal = 800 ℃. The mechanism of such magnetotransport properties with different temperature ranges was discussed. The optimal TCR of 32.3%·K -1 in LCMO was prepared with T cal = 500 ℃ and t cal = 8 h, showing that co-precipitation method would facilitate the potential application of LCMO in infrared detecting and magnetoresistive switching.


Introduction
The past decades have witnessed the intensive study of manganite R1-xBxMnO3 (R 2 is La, Sr, Nd etc., and B is divalent alkaline earth element) due to their rich electronic properties, which arise from interplay among lattice, spin, charge, and orbital [1] .
Among them, La1-xCaxMnO3 is a typical mixed-valence compound obtained by substituting La for Ca from the parent compound, antiferromagnetic (AFM) insulator LaMnO3. LCMO has been paid extensive attention, owing to the colossal magnetoresistive (CMR) effect and the potential applications for magnetic sensors, memory devices, etc. [2][3][4][5]. Furthermore, there exists a paramagnetism-ferromagnetism transition generally accompanied by a metal-insulator transition (MIT) in LCMO [6], which is well accepted as a result of competition between double exchange of Mn 3+ /Mn 4+ and Jahn-Teller effect [7,8]. A sharp MIT with a low resistivity can generate a large TCR, which makes the CMR manganites of great application potential in infrared detecting and bolometer, etc. [9,10], which motivated this study.
TCR is usually related to the grain size, crystallinity and homogeneity of the ceramic powders. These electrical and magnetotransport properties rely drastically on the preparation methods as well as preparation conditions. Many techniques can be used to prepare LCMO powders, such as the solid phase reaction [11], spray-drying [12], sol-gel method [13], co-precipitation method [14]. As for conventional solid phase reaction, a relative high calcination temperature, long calcination time and multiple grinding and sintering processes are necessary to obtain high-quality LCMO ceramics.
A solution of particular concentration and a co-current flow atomization system are required of spray-drying, which increased the preparation complexity and cost. Sol-gel method also has several disadvantages, the raw materials for the sol-gel method are expensive, the operation is complicated, the process time is long and the prepared samples are prone to crack. Notice that, the chemical co-precipitation method is simple in operation and mild in condition, which is utilized in this study to prepare LCMO ceramics.
Previous studies reported the electrical properties and TCR of La0.67Ca0.33MnO3, with 3 maximum TCR of 9.7%·K -1 and corresponding metal-insulator transition temperature (TMIT) of 267.59 K [21,22]. Therefore, it is of importance to further explore different calcination process to enhance MR and TCR in co-precipitation derived LCMO ceramics.
In present work, calcination process effect on the electrical transport and magnetotransport properties of LCMO ceramics. Tcal of 400, 500, 650 and 800 ℃ were used since the synthesis of the LCMO phase started at about 500°C and completed at about 800 °C [21]. The maximum TCR reached to 32.3 %·K -1 at Tcal = 500 ℃ and maximum MR is 82.8 % at Tcal = 800 ℃, which are largely enhanced compared with the sample derived by sol-gel method [23]. In the region of T < TMIT, the ρ-T could be demonstrated by the electron-electron scattering. In the region of T >TMIT, conductive mechanism was explained through the small polaron hopping (SPH) model. In addition, the resistivity (ρ) can be fitted by phenomenological percolation model over the entire temperature scope. Our study provides a new synthesis method for the application of LCMO materials in infrared detectors, contactless magnetoresistive switches, and memories to some extent.

Surface morphologies of LCMO samples
The surface morphologies of the LCMO samples are presented in Fig. 2(a-d). A large number of holes were found at the grains and grain boundaries (GBs) at Tcal = 400 5 ºC. This may be because the decomposition temperature of Ca(OH)2 is in the range of 500-650 °C. The excessively low Tcal cannot completely decompose the Ca(OH)2 in the precursor powder, and CO2 will be generated during the calcination process, causing the pores and defects at the grains and GBs. For Tcal above 500 °C, fewer defects were observed, while the densities of all samples increased. To explore the effects of grain size on different Tcal, the mean grain sizes of all samples were calculated with Nona Measurer 1.2 Software. The average grain sizes in Fig. 2(a-e) were 13.16, 14.89, 15.05, and 25 μm, respectively. This indicates that increased Tcal would lead to the growth of polycrystalline ceramic grains. This is mainly because with the increase of the Tcal, the sample activity will increase, resulting in higher crystallinity. However, when Tcal was 800 °C, the grain size of the powder can reach to 25 μm and the main crystal phase is formed, reducing the sintering activity and affecting the process of secondary recrystallization. In summary, the samples have good sintering activity and few defects at a Tcal of 500 °C.

Electrical transport properties
The temperature dependence of the resistivity (ρ-T) of LCMO was measured in the 150-300 K under 0 and 1 T (Fig. 3(a-d)). The ρ-T curves indicated that all samples underwent a metal to insulator transition [24,25]. In the low Tcal region of 400-650 °C, the TMIT and resistivity of LCMO ceramics showed a slight change. When Tcal was lifted from 650 to 800 °C, the ceramics simultaneously showed a sudden drop in TMIT and a drastic increase in ρ. The reduced grain size will enlarge the surface energy. For a Tcal of 400-650 °C, the grain size of the LCMO powder is small (13.16, 14.89, and 15.05 μm, respectively), and the surface energy is larger than the sample of Tcal of 800 °C. As a consequence, the ceramics with-size grains dense and high density was obtained after sintering. The scattering effect of the GBs on the electrons was weakened inducing a low resistivity. Combined with SEM image analysis, the high Tcal of 800 °C is believed to be detrimental to the TCR. 6 The resistivity for all samples substantially decreased under 1T field, Helman and Abeles 's model [26] can be used to explain. This model simulated that the resistivity should flat out gradually with the increase of the magnetic field and eventually vanish at certain critical field. The ρ-T curves showed that the resistivity decreased and the TMIT shifted towards the higher temperature. it will cause delocalization of charge carriers at 1T field, which may lead to the ordering of magnetic spins and decrease the resistivity.
The TCR of the samples prepared at different Tcal are shown in Fig. 3(e). The main electrical performance comparison of LCMO samples under different Tcal are presented in Table 2. Fig. 3(e) and Table 2 show that the peak TCR increased first and then decreased with the Tcal increased. When Tcal = 500 °C, the TCR reached 32.3%·K -1 , and TMIT shifted toward the low-temperature region, which show the advantages over the provious studies [27,28]. The reason for this phenomenon is that the lower Tcal (400 -500 °C) causes smaller-size precursor particles, produces denser ceramics, which involves fewer GBs and fewer pores, especially the latter; and boosts the transport properties. On the other hand, looser ceramics derived from higher Tcal (650-800 °C) can result into poorer conductivity, which is responsible for the low TCR observed in Fig. 3(e). However, for Tcal of 400 °C, the organics were not completely evaporated, which suppressed the TCR. In summary, optimal TCR can be found in the LCMO with Tcal of 500 °C. at Tcal = 800 °C, which is higher than the MR of LCMO ceramics prepared by the solgel method [29,30].
3.4. Mechanism of the transport properties 7 There are three temperature regimes considered in this study to understand the conduction mechanism of the ρ-T of LCMO ceramics prepared by co-precipitation at different temperatures [31,32].

Low-temperature range
Eq. (1) is a famous empirical equation that has provided a fitting method for ρ-T in the low-temperature region (T < TMIT): where ρ0 is the residual resistivity stemming from the GBs, as well as from the scattering mechanisms, which independent of temperature. The The ρ-T fitted by Eq. (1) are shown in Fig. 4(a) and Table 3. The squared linear correlation coefficients (R 2 ) are listed. The obtained values of R 2 were as high as 99.0% for all samples, which represents the high fitting quality. With application of 1T magnetic field, it can be seen from Table 3 that both the fitting values of ρ0 and ρ2 for all ceramics decreased. Additionally, the GB scattering decreases, which would be resulted in decreases of resistivity. The value of the 2 2 is greater than that of the 4.5 4.5 for all ceramics. Consequently, the scattering of electron-electron plays a major role in the conductivity of samples in the T < TMIT regime.

High-temperature range
The ρ-T of paramagnetic (PM) high-temperature region is fitted by SPH model. The SPH model could be approximated by an adiabatic Eq. (2).
where is the resistivity coefficient, is the polaron activation energy, and is the Boltzmann constant. 8 The ρ-T fitting results are presented in Fig. 4(b) and Table 4. The results revealed that the SPH model fits the ρ-T well. The fitting parameters are shown in Table 4. After 1T magnetic field was applied, the values of all ceramics are decreased, which would due to the addition of an external magnetic field changing the bond angle of Mn-O-Mn. In other words, the effective carrier mass is changed. As a result of this influence, the effective band gap is changed, and the carrier needs higher values of activation energy to cross this gap.

Entire temperature range
The phenomenological percolation model analyzed the ρ over the whole temperature scope, and the combined formula is as follows in Eq. (3).
where is the volume fraction of the FM phase and (1 − ) is the volume fraction of the PM phase. LCMO ceramics has been fitted over the whole temperature scope with Eq.
(3) and the fitting parameters are demonstrated in Fig. 4(c) and Table 5. Tc-mod stands for temperature at maximum resistivity. U ≈ 0 (1 − − ) ⁄ is the energy difference between the FM and PM states. There are two adjustable fitting parameters, namely, Tc-mod and 0 . The fitting parameters Tc-mod and U0/KB are shown in Table   1. Obviously, the value of the Tc-mod is very closed to TMIT. (that is, the temperature is lowered just after the sample is heated to 500 ºC), the resistivity is as high as 1.0412 Ω·cm, which is much higher than other study [33]. This may be that Ca(OH)2 etc. are not completely decomposed, resulting in a large number of pores in the target, uneven composition, and low density, which largely improves resistivity. The resistivity of the samples reached a maximum value of 271.3 K at 8 h.

Effect of tcal
From Fig. 5(b), the TCR first increased and then decreased. It reached a maximum value 9 of 32.3%·K -1 at tcal = 8 h. In general, the Tcal of 500 ºC and the tcal of 8 h are found to be the optimal sintering parameters for the co-precipitation derived LCMO ceramic.

Conclusion
A series of LCMO ceramics were prepared by co-precipitation method. The effect of calcination process on structure and magnetoelectric transport performance of LCMO ceramics was studied, and the conduction mechanism of the ρ-T was analyzed.