Electrical and Magnetotransport Properties of ${\rm La}_{{0.7}}{\rm Ca}_{{0.3}}{\rm Mn}_{{1-x}}{\rm Co}_{{x}}{\rm O}_{3}$

This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of La
_0.7
Ca
_0.3
Mn
_1-x
Co
_x
O
₃
(x = 0.09-0.17) prepared by solid-state reaction. Magnetic measurements versus temperature revealed gradual decrease of the magnetization (M) and Curie temperature (T
_C
) with increasing Co concentration (x). The T
_C
values vary from 194 to 159 K as changing x from 0.09 to 0.17, respectively. H/M versus M
²
performances around T
_C
prove the x = 0.09 sample undergoing a first-order magnetic phase transition (FOMT) while the samples with x ≥ 0.11 undergo a second-order magnetic phase transition (SOMT). The other with x = 0.10 is considered as a threshold concentration of the FOMT-SOMT transformation. Considering temperature dependences of resistivity, ρ(T), in the presence and absence of the magnetic field, the samples (excepting x = 0.17) exhibit a metal-insulator transition at T
_P
= 60-160 K, which shifts toward lower temperatures with increasing x. the metallic-ferromagnetic region, the ρ(T) data are well fitted to a power function ρ(T) = ρ
₀
+ ρ
₂
T
²
+ ρ
_4.5
T
^4.5
. This indicates electron-electron and electron-magnon scattering processes are dominant at temperatures T <; T
_P
. In addition, the conduction data at temperatures T > θ
_D
/2 (θ
_D
is the Debye temperature) and T
_P
<; T <; θ
_D
/2 obey the small-polaron and variable-range hopping models, respectively. The values of activation energy E
_p
, and density of states at the Fermi level N(E
_F
) were accordingly determined. Here, N(E
_F
) increases while E
_p
decreases when an external magnetic field is applied. We also have found that N(E
_F
) increases when materials transfer from the FOMT to the SOMT, and N(E
_F
) value becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.10).