Hongtao Shi

Date of Graduation

2002

Document Type

Dissertation/Thesis

Abstract

When materials with antiferromagnetic (AF)/ferromagnetic (F) interfaces are cooled through the Neel temperature (TN) of the AF in a magnetic field, a unidirectional magnetic exchange anisotropy (EA) is induced in the F layers. Such anisotropy is manifested by a shift of the center of the F hysteresis loop by an amount known as exchange bias HE. Exchange anisotropy has attracted much attention in the last decade due to its applications in magnetic random access memory, permanent magnets, and magnetic recording media. Despite this interest, the basic mechanism responsible for EA is not well understood. In this thesis the EA is studied in dilute Ising antiferromagnets as a function of dilution, interface structure and angle of the various applied magnetic fields. In the first set of experiments, antiferromagnetic FexZn 1-x[special characters omitted]F2/ferromagnetic, polycrystalline Co bilayers were grown on MgO (100) substrates via molecular beam epitaxy (MBE). X-ray diffraction showed that the dilute antiferromagnet is (110)-oriented with two perpendicular crystallographic twins in the film plane. After field cooling samples with and without 1.0 nm pure FeF2 at the interface between the FexZn1-xF 2 layer and the Co layer through the TN of the AF layer, hysteresis loops were measured using a superconducting quantum interference device (SQUID) magnetometer to determine HE in the bilayers. The blocking temperature TB, at which HE = 0, was found to have a linear dependence on the concentration of Fe, x, in the dilute layer for x > 0.25, which coincides with the TN of the bulk dilute crystals. The exchange bias was increased by 65% for x = 0.84 compared to pure FeF2/Co bilayers. This enhancement is presumably due to the formation of domain states inside the dilute antiferromagnetic layers, and the pure interface layer is crucial in increasing the coupling between the F and the AF layers. The angular dependence of HE, as well as the coercivity and the remanence, on the cooling field direction was studied in the twinned samples using a vibrating sample magnetometer (VSM). The cooling field was applied at an angle α with respect to the twins' perpendicular bisector in the plane of the samples. The most negative HE was found to occur along the AF easy c-axis for 0 ≤ α ≤ 300 ∼ 400. An exchange bias flop occurred if α was increased further. The 1.0 nm pure FeF2 interface layer resulted in a sharper exchange bias flop transition, indicating that the pure interface layer acts as a buffer for the interface interaction. As a comparison, single crystal FeF2/Co bilayers were also prepared. A large HE was observed with the sample field-cooled along the easy axis of FeF2, whereas two loops with the same HE magnitude but of opposite signs were observed when the cooling field was applied 900 to the AF easy c-axis. Changing the cooling field direction to 910 caused the sample to acquire a significant positive HE parallel to the AF easy axis. These experiments demonstrate that the interface coupling responsible for HE is extremely sensitive to the underlying magnetic anisotropy of the AF layer, and that the direction of the cooling field does not necessarily determine the direction of HE.

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