MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Gd 0.7Ca 0.3MnO 3
Electronic and Magnetic Measurements 73
Hwang et al.
Figure 3-17 Temperature - tolerance factor phase diagram
from reference [100], with the position of Gd 0.67 Ca 0.33 MnO 3
indicated.
This has been done for Gd 0.67 Ca 0.33 MnO 3 where the experimental results
are given in chapter 5. In Gd 0.67 Ca 0.33 MnO 3 the Mn atoms are on one
sublattice (B) and Gd atoms on the other (A). The dominant molecular field
constant is expected to be the ferromagnetic Mn-Mn coupling due to the
stronger 3d exchange interaction. From the trend found in the manganese
ferromagnetic ordering temperature as a function of the average of the R and
A atom size [100] in R 1-x A x MnO 3 , shown in Figure 3-17, the T C is expected to be
around 60K. The molecular field constant can then be approximated using
the relationship found for a ferromagnet in section 3.2.2.2.5, namely
2 2
λBB = 3kBTC /µ B
pB .
2
Here pi also includes the site occupancy for each type of atom on the i
sublattice. For Gd0.67Ca0.33MnO3 , the Mn (B) sublattice has two magnetic
contributions since the site is 67% occupied by Mn +3 (S = 2) and 33% occupied