Transport and magnetic behaviors

Fig. 3.1－3.3 shows the field-cooled and zero-field-cooled temperature-dependent magnetizations M(T) measured at applied field of 0.1 T and 0.01 T together with the zero-field temperature dependence of resistivity ρ(T) for Fig. 3.1 the as-deposited (AD) and Fig. 3.2 the oxygen-annealed (PA) LSnMO films as well as for Fig. 3.3 the argon-annealed (ArPA) LSnMO film measured at 0.1 T, respectively. We note that in the paramagnetic (PM) state the resistivity of the AD-film is larger than that of the PA-films by nearly two orders of magnitude. While this may originate from the charge localization effects associated with lattice distortion [22, 23], the absence of a temperature-induced TIM in the AD-film can be more subtle and complicated. De Teresa et al. [24] argued that it might be related to the absence of long range FM order signified with a manifestation of spin-glass-like behavior at lower temperatures. This would imply that the large epitaxial strain originally existent in the AD-film not only induces enormous charge localization effect but also hinders the formation of long range FM ordering. If this argument is true, one expects to see the opposites for the PA-films. At the first glance, it seems to explain the over 200% enhancement in magnetization and dramatic increase in TIM rather consistently. However, as shown in Fig. 3.1－3.3, significant spin-glass-like behavior, characterized by the pronounced irreversibility between the field-cooled (FC) and zero-field-cooled (ZFC) M(T) curves, is evident for both cases. This implies that the insulator-metal transition and spin-glass state are not necessary mutually exclusive.

Furthermore, we note that the results shown in Fig. 3.1－3.3 also reveal some features deviating from that reported for low-doping LSnMO [18]. In that progressive suppression of spin-glass-like behavior with increasing Sn-doping was observed. It has been interpreted as a result of increasing Mn⁴⁺ ions driven by Sn-doping-induced La-vacancies, and the low-doping LSnMO’s were essentially regarded as hole-doped manganites.

However, according to the XAS results to be presented below, the features of Mn²⁺/Mn³⁺

mixed-valences indicate that the present AD-films might have effectively doped some electrons to the eg-band of Mn-3d orbitals. Furthermore, we note that, in our case (x ＝ 0.3) the spin-glass-like transition not only emerges at much higher temperature (T_g ≈ 150 K) than that reported in Ref. 14 (75 K → 20 K for x ＝ 0.04→ 0.18) but also is very sensitive to the applied field. Thus, we suspect that the Sn-doping-induced La-vacancies scenario may not apply to our case. The spin-glass-like behavior and CMR effect observed in the annealed LSnMO here is probably not arising from the divalent doping-enhanced ferromagnetic interaction and magnetic homogeneity but maybe related to the strain relaxation in a subtler manner. The other feature to be noted is the dramatic suppression of the low temperature magnetization for the AD-film when measured at 0.01 T. Since the basic ingredients for spin-glass to occur are disorder as well as the magnetic interaction randomness, anisotropy and frustrations¹⁹, we believe that these factors also account for the dramatic suppression of magnetization in the lower measuring fields for the AD-films. Finally, we note that very recent observations by Valencia et al. [25] have indicated that, in La2/3Ca1/3MnO3 films, the formation of Mn²⁺ ions due to the instability of Mn³⁺ subjected to prolonged air exposure might also lead to charge localization and, hence, the increasing resistivity and reducing magnetization. However, the relevance of

this non-ferromagnetic order originated from divalent Mn component to the observed spin-glass-like behaviors discussed above remains to be clarified.

On the other hand, since post annealing by argon is a common practice for preparing the tetravalent-doped CMR materials, it is important to clarify the effect of argon annealing on LSnMO films. The idea behind this practice was that the excessive oxygen may induce hole doping, and thus, may counteract the expected effect of electron-doped CMR materials. Therefore, annealing in the argon environment may avoid the introduction of holes induced by excess oxygen and could turn out to be a practical method of fabricating electron-doped CMR materials. As illustrated in Fig. 3.3, the M-T of the argon-annealed LSnMO (ArPA LSnMO) film does display a comparable magnetization to that of the PA-LSnMO film. Nonetheless, the zero-field temperature dependent resistivity, ρ(T), is about three times larger than that of the PA-LSnMO film.

Since, in contrast to the AD films, both PA- and ArPA-LSnMO films exhibit signatures of typical paramagnetic-insulator (PI) to ferromagnetic-metal (FM) transition, annealing appears to have effects on driving the material from a ferromagnetic insulator to a ferromagnetic metal. However, there exist some differences in the detailed behaviors between the PA- and ArPA-films, as well.

Fig. 3.4－3.5 shows ρ(T) as a function of applied field for the PA film and ArPA film, respectively. The resistivity was measured with the field applied parallel to the film surface. For the AD-film, although there exists a typical PM-FM transition with T_c ≈ 190 K, the ρ(T) increases steeply with decreasing temperature (Fig. 3.1) and has no sign of metallic transition for applied field up to 8 Tesla. On the contrary, for the PA-film, in addition to having a nearly two orders of magnitude reduction in resistivity as compared

to the AD-film in the PM state, it also displays the typical CMR behavior with T_IM ~ 300 K at zero-field. The maximum magnetoresistance (MR) ratio, defined as Δρ/ρ＝ (ρ(0)- ρ(H))/ ρ(0) with ρ(0) and ρ(H) being the resistivity at zero field and at field H, appears around 250 K and reaches about 70% at a field of 8 Tesla. Together with the M(T) results shown in Fig. 3.1－3.3, the results demonstrate that annealing not only significantly enhances the low-temperature magnetization by more than 200% but also changes the magneto-transport properties of the LSnMO films dramatically. Guo et al.¹⁷, by varying the film thickness in their La0.9Sn0.1MnO3+δ films, have found similar enhancement in raising T_IM with increasing film thickness. However, there was no noticeable change in T_c and low temperature magnetization with film thickness variations, which led them to conclude that the enhancement of TIM was due to strain relaxation instead of formation of new phases introduced by oxygen- or La-deficiency [7-8, 20, 25-28]. Similarly, Thomas et al. [29] have attributed the improved magneto-transport properties observed in their

high temperature (900°C) annealed La0.7Ca0.3MnO3 films to the massive stress relaxation and improved film crystallinity accompanied with grain growth. However, they did not report how magnetization and Tc were affected by post-annealing. In comparison, for the ArPA-films, although it also displays the typical CMR behavior, the TIM ＝ 230 K at zero-field is somewhat lower than that of the PA films. The maximum MR ratio appears around 220 K and reaches nearly about 95% at a field of 8 Tesla. These results are, in fact, very similar to that of some La-deficient CMR materials [20, 24]. It appears that, from the magneto-transport properties alone, one cannot discern whether the typical CMR behaviors displayed by post-annealed films are indeed the genuine characteristics of electron-doped manganites or they are just manifestations of La-deficient manganites

induced by post annealing [11]. In addition, whether the lack of insulating-metallic transition in AD-LSnMO films is correlated to the lattice disorder or to other mechanisms (such as composition change) remains to be clarified. In order to give some further accounts on these issues, we performed further investigations on the electronic structures of the corresponding films by XAS measurements.

Figure 3.1: The field-cooled and zero-field-cooled temperature-dependent magnetizations (M(T)) measured at 0.1 T and 0.01 T for the as-deposited (AD) LSnMO film.

Figure 3.2: The field-cooled and zero-field-cooled temperature-dependent magnetizations (M(T)) measured at 0.1 T and 0.01 T for the oxygen-annealed (PA) LSnMO film.

Figure 3.3: The field-cooled and zero-field-cooled temperature-dependent magnetizations (M(T)) measured at 0.1 T for the argon-annealed (ArPA) LSnMO film.

Figure 3.4: ρ(T) as a function of applied field for the PA LSnMO film. The inset illustrates the field dependence of the MR ratio.

Figure 3.5: ρ(T) as a function of applied field for the ArPA LSnMO film. The inset illustrates the field dependence of the MR ratio.