MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

68 Chapter 3
ferromagnet will also show such hysteresis, as demonstrated in Figure 3-12
for SrRuO 3 . The field cooled curve may look like a ferromagnetic M vs. T
curve, however it is influenced greatly by the demagnetization field and
coercivity. For example in applied fields lower than the maximum
demagnetization field (4πNM S ) the net magnetization can only increase until
the demagnetization field cancels the applied field (M = H/4πN). The a plot
of M vs. T in this case will show a constant, low value for M, until the true
magnetization decreases near T C (see for example the low field data in [29]).
Reversible Field (H rev ) (Gauss)
100000
10000
1000
100
SrRuO 3 Irreversibility Line
Hrev (const T)
Hrev (Const H)
H rev = (1-T/T C ) 1.5
10
0.01 0.1
1-T/Tc
1
Figure 3-14 Magnetic irreversibility line for polycrystalline
SrRuO 3 . Above the line the magnetization is reversible,
below it is irreversible. The irreversibility exponent is about
1.5.
Another characteristic of a spin glass is an “S” shaped magnetization
curve, which gives an inflection point in M vs. H. Such a curve is also
observed for a ferromagnet, such as that seen for SrRuO 3 in Figure 3-13. The
curves for increasing and decreasing fields will meet at the irreversibility
point. The irreversibility points measured by the two methods (constant T,