Recently, lanthanum manganites $La_{1-x}A_xMnO_{3+\delta}$(A is a divalent alkaline-earth ion) have generated a intensive interest, which is stimulated by the observation of a colossal magnetoresistance(CMR). Most studies of these materials have focused on the investigation of electric and magnetic properties and the origin of the CMR effects. The undoped parent manganite $LaMnO_3$ is a insulating antiferromagnetic(AFM) state at low temperature, while $La_{1-x}A_xMnO_{3+\delta}$(x~0.3 or 2δ ~0.3) are metallic ferromagnetic(FM) states. The CMR effect have been mainly observed near $T_C$ in metallic FM materials, which roughly agrees with a metal-insulator transition temperature.
Some unsolved problems still remain in the research field related to lanthanum manganites. One is related to the microscopic state of the material for x ＜ 0.2. It is unclear whether its state is canted spin state, or phase separation (or magnetic polaron). It is also unclear whether phase separation exists near $T_C$, or not. In the case of non-stoichiometic materials $LaMnO_{3+\delta}$ and $La_{0.7}Ce_{0.3}MnO_{3+\delta}$, it has been unknown why macroscopic properties depends on the process of sample preparation. In addition, it is unclear whether the valence state of Ce ion in Ce-doped manganite is +3, or +4. Most of all, it is a very interesting problem whether phase separation plays an important role on the CMR effects, or not.
In this paper, the microscopic properties of lanthanum manganites, such as $LaMnO_{3+\delta}$, $La_{0.7}Ce_{0.3}MnO_{3+\delta}$, $La_{1-x}Ca_xMnO_3(x = 0.125-0.5)$, and $La_{1-x}Sr_xMnO_3(x=0.15-0.25)$ were studied by zero-field $^{55}Mn$ and $^{139}La$ nuclear magnetic resonance(NMR). Zero-field La NMR gives important informations for the local magnetic property around La ion, while Mn NMR gives ones for the local electric property. The obtained results from our study are as follows.
The main result of our study is that the mixed state - phase separation - are found in $La_{1-x}Ca_xMnO_3$ near any phase boundary induced by either temperature or hole doping. For instance, NMR results supported that the metallic FM phase coexists with insulating PM phase near $T_C$, or with insulating AFM phase in low hole doping range x ＜ 0.2 at low temperature. In x = 0.5, three phases maximally coexist near $T_N$; a stable metallic FM phase, an insulating AFM phase, and another metallic FM phase competing with the AFM phase. The undoped lanthanum manganites $LaMnO_{3+\delta}$ and $La_{0.7}Ce_{0.3}MnO_{3+\delta}$ with excess oxygen are also phase separated states coexisting with metallic FM phase and insulating AFM phase. In the Ce-doped manganite, the valence state of Ce ion isn't +4 as in $CeO_2$, but +3.
The volume ratio of the metallic region to the insulating region depends on the temperature and the hole doping ratio. Therefore, the macroscopic change of resistivity from an insulating state to a metallic state is attributed to the percolation between metallic FM clusters. This phenomenon is called a percolative phase separation. Because the external field helps the metallic FM ordering when it competes with the insulating AFM(or PM) ordering, we found that magnetoresistance is maximum at the temperature or at the hole doping where the system starts to form percolative paths through metallic FM clusters. This means that the percolative phase separation is one of the origins of the CMR effect in these systems.
