Structure, transport, and magnetic properties of thin films and small particle composites

Two classes of magnetic materials with limited dimensions have been studied; spin valve type magnetic multilayers and small magnetic particles embedded in polymer matrix.; For the magnetic multilayers, NiO bottom spin valves and FeMn top spin valves were investigated. Exchange biased GMR behavior is demonstrated after field annealing. Novel cooling procedures were applied to separate the contribution from various AFM grains. It is found that a “memory effect” exists in all the samples, which can be ascribed to the existence of grain size distribution inside the AFM layer. Larger AFM grains show better temperature stability, while smaller ones show a larger exchange bias. This exchange bias not only depends on the FM-AFM exchange interaction, but also the interaction among different AFM grains. The nature of this latter interaction can be either FM or AFM type. To eliminate such temperature instability, Co/Ru/Co synthetic antiferromagnet was inserted. At Ru thickness about 7 to 9Å, the effective exchange bias is larger than 1000Oe. The memory effect is essentially eliminated, and the temperature stability was dramatically improved.; Another subject is the small magnetic particles embedded in polymer matrix. Under an applied inductive field, it demonstrates a Curie temperature controlled heating behavior, which is used for polymer and composite bonding. Two representative materials systems were selected, metallic Ni particles and oxide (Co _1−xZn_x)₂Ba₂Fe₁₂O ₂₂ with different Zn substitution. Curie temperature controlled heating behavior is demonstrated, varying from 100°C to 350°C. It is found the heating of the particle/polymer composite strongly depends on the dispersion of the magnetic particles inside the polymer matrix. SEM study demonstrated that the high temperature extrusion method is an appropriate dispersion technique. For the heat generation, a 2nd order field dependence is evident for initial low field and high field near saturation, reflecting the reversible domain wall motion dynamics. It has also been found that the magnetic particle size has a strong influence on the heat conduction from particles to the polymer matrix. An inverse dependence clearly demonstrates the importance of the interface between magnetic particles and polymer matrices.