MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

66 Chapter 3
3.2.2.2.9 Irreversibility
Below T C , a ferromagnet must undergo a first order phase transition when
the field direction is changed. Like all first order processes, this involves
nucleation and growth (of magnetic domains) and can be hysteretic.
Technically there is only hysteresis when the internal field changes sign; if
the field increases or decreases while retaining the same sign (direction), the
change in the magnetization is continuous. However, in real systems, there
exist internal anisotropy and demagnetization fields. So in low fields
irreversible behavior can be found.
Magnetization (µ B )
0.12
0.10
0.08
0.06
0.04
0.02
SrRuO 3 Pellet irreversibility
Field Cooled
20 Oersted Magnetization
Zero Field Cooled
FC
ZFC
0.00
0 50 100
Temperature (K)
150 200
Figure 3-12 SrRuO 3 showing spin-glass like irreversibility of
zero-field-cooled and field-cooled measurements in a small
field. The field cooled curve may look saturated, but is
actually less than 1/10 saturated at low temperatures. A small
peak is observed in the zero-field-cooled measurement when
the reversibility point is reached.
The field required to reduce to zero the magnetization of a saturated
sample, is called the coercive field. When the coercive field is large, such as