MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

102 Chapter 5
5.1 Ferrimagnetism
The polycrystalline pellets, crystals and thin films all display a
ferromagnetic transition and a compensation temperature (Figure 5-1), which
is characteristic of ferrimagnets. Both bulk materials, polycrystalline pellet
and float zone crystal, have very similar magnetic properties while the film is
slightly different.
Gd 0.67 Ca 0.33 MnO 3 displays the properties of a ferrimagnet, much like the
garnet ferrite Gd 3 Fe 5 O 3 [138]. At about 60K, the transition metal spins order,
dominating the magnetization. As the temperature is lowered, the opposing
magnetization of the weaker coupled rare earth ions increases, since they
behave almost as though they were free paramagnetic spins reacting to an
internal field (section 5.1.3, Figure 3-19).
At the compensation temperature, T Comp , the rare earth and transition
metal sublattice magnetizations exactly cancel (in zero field). Below T Comp , the
magnetization of the rare earth ions, which have larger spins, dominates.
This is best seen in zero field remnant data which shows the magnetization
changing sign at T Comp .
In the presence of magnetic fields greater than the coercive field, the
stronger sublattice flips to align parallel with the field, giving the minimum
in M at T Comp as seen the 5 kOe data.