Magnetic and magnetoresistive properties of sodium-substituted lanthanum manganites

The magnetic, electric, and magnetoresistive properties of bulk samples of La1-xNaxMnO3 +/-delta (x =0.08-0.16) are studied. It is shown that at low temperatures all samples are ferromagnetic and the temperature of the transition from the paramagnetic into the ferromagnetic state increases with increasing sodium content. It is found that the saturation magnetization reaches its maximum value, close to that computed theoretically, in samples with x=0.12. A deviation of x from 0.12 decreases the saturation magnetization and broadens the magnetic transition. Analysis of the behavior of the magnetoresistance shows that there exists, together with the ferromagnetic conducting phase, a disordered weakly conducting phase, the volume fraction of the latter increasing as x deviates away from 0.12. The characteristic behavior of the magnetic and magnetoresistive properties of La1-xNaxMnO3 +/-delta samples are tied to the particulars of the evolution of the chemical composition and defectiveness of the samples as a function of the sodium concentration. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3331493]