Magnetic and Electrical Properties of (La1−xCax)MnO3

Samples of polycrystalline (La
1−x
Ca
x
)MnO
3
, where x takes values ranging from 0.005 to 0.3, have been studied experimentally. Specimens were carefully prepared in the stoichiometric forms by changing the oxygen partial pressure conditions for each value of x. For specimens having x smaller than 0.1, the Néel temperature T
N
is almost constant, 141°K, while the paramagnetic Curie temperature θ increases as x increases. On the other hand, for specimens in which x is greater than 0.1, both T
N
(or T
c
) and θ are almost equal and increase with the value of x. The Mn
⁵⁵
NMR spectra associated with Mn
³⁺
and Mn
⁴⁺
ions were observed in specimens having x smaller than 0.175, while motionally narrowed Mn
⁵⁵
spectra were observed where the values of x were 0.2 or 0.3. All the experimental results could be qualitatively explained by postulating that the d holes at the sites around Ca
²⁺
ions contribute to both the electrical conduction and the magnetic interaction. The ferromagnetic interaction induced by mobile d holes can not be ascribed to the double-exchange interaction, if this interaction has the usual form b cos (θ/2).