MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

104 Chapter 5
calculation predicts a constant moment for T < 5 K, while the data show an
almost linear change in M for 2 K < T < 5 K. Also, the peak in the
magnetization, observed at about 50 K in the 5 kOe data is less than the 1.9µ B
predicted in the mean field calculation.
5. 1. 2 High temperature susceptibility
Susceptibility data (Figure 5-2) above 200 K fit to the Curie-Weiss law:
2 2 2
χ = µ eff /(8(T-Θ)) with Θ = 34 K and µeff = 71 µB . The difference in the
susceptibility of the crystal compared to that of the pellet at high temperatures
2
may not be significant. For quantum mechanical spins µ eff per mole is
expected to be 63 µ B 2 : 2 2 µB 2 (0.67 × 4/2 × 6/2) = 16 µB 2 from Mn 3+ ; 2 2 µB 2 (0.33 ×
3/2 × 5/2) = 5 µ B 2 from Mn 4+ ; 2 2 µB 2 (0.67 × 7/2 × 9/2) = 42 µB 2 from Gd 3+ . This
extra moment, which is also observed in La 0.67 Ca 0.33 MnO 3 [103] section 4.1,
may be due to the orbital contribution in the manganese ions. Since the two
sublattices have different coupling energies, the susceptibility above T C will
not obey a simple Curie-Weiss law even in the mean field approximation
(Figure 5-5). The simple mean field model of the above ferrimagnetism
requires T C /Θ > 3, as described in section 3.2.2.4.1. This is clearly not the case
for the measured parameters. Alternatively, the addition of a Pauli-like
2
susceptibility χ0 to the fit, χ = χ0 + µ eff /(8(T-Θ)), will reduce the contribution
from µ eff . For example, an equally good fit to the data can be found with
2 2
µ eff = 63 µB , χ0 = 7.7 × 10 -6 emu G -1 g -1 , and Θ = 50 K.