Hysteresis loops of exchange-biased permalloy/Fe50Mn50/permalloy trilayers
on glass were measured as a function of Fe50Mn50 and permalloy thicknesses
with the longitudinal Kerr effect employing a coherent light source. Kerr s
ignals originate from both permalloy layers and give a superposition of hys
teresis loops. In vibrating sample magnetometer or looptracer measurements
the contribution of a particular layer to a major hysteresis loop cannot be
identified. With the Kerr setup presented it is possible to identify the c
ontribution of each layer individually, since the finite optical path throu
gh the trilayer gives rise to optical attenuation and interference. For an
increasing total thickness of the trilayer, the signal of the buried permal
loy layer will become weaker due to attenuation. Kerr measurements of trila
yers up to 40 nm show a superposition of two equally oriented loops. Major
loops for trilayers of thickness greater than 40 nm show a superposition of
two oppositely oriented hysteresis loops. The transition is dependent only
on the total thickness of the trilayer, rather than the thickness of each
individual layer. This unusual effect can be explained by the phase differe
nce of the two Kerr signals. Additional measurements performed from the gla
ss side of the samples and measurements of a Fe50Mn50/permalloy bilayer con
firm that the sense of a hysteresis loop can change for a buried layer due
to optical effects. (C) 1999 American Institute of Physics. [S0021-8979(99)
06903-0].