Ferromagnetic resonance ͑FMR͒ and magnetization ͑MAG͒ measurements were used to study the exchange interaction between the antiferromagnetic and ferromagnetic layers in an IrMn/ Cu/ Co system as a function of the Cu spacer thickness. Although the experimental angular variations of the exchange-bias fields H eb FMR and H eb MAG coincide, the coupling strengths J and the Co layers' anisotropy fields H U , obtained via numerical simulations, are different. For all Cu thicknesses J FMR Ͼ J MAG and H U FMR Ͻ H U MAG. The exchange coupling decreases exponentially with the spacer thickness and is a short-range interaction. These characteristics were explained in the framework of a model considering polycrystalline magnetic layers with independent easy axis distributions, taking into account the rotatable anisotropy. The role of antiferromagnetic grains at the interface with different sizes and different magnetic stabilities is essential for understanding the behavior of this exchange-biased system.
In the present study, we prepared two different magnetic biocatalysts of pectinase and cellulase: carrier-free magnetic CLEAs (CLEA-MP*) and immobilization on glutaraldehyde-activated magnetite (Enz-Glu-MP*). The biocatalysts were compared to their magnetic properties, immobilization parameters, stability and grape juice clarification. Enz-Glu-MP* presented higher magnetic properties than CLEA-MP*, whereas this presented higher surface area and pore volume. The K of the enzyme immobilized on Enz-Glu-MP* was 25.65mM, lower in comparison to the CLEA-MP* (33.83mM). On the other hand, CLEA-MP* was the most active and stable biocatalyst, presenting higher recovered activity (33.4% of cellulase), higher thermal stability (2.39 stabilization factor) and improved reusability (8cycles). The integration of magnetic technology with enzymatic immobilization emerges as a possibility to increase the recover and reuse of biocatalysts for application in juice technology.
Highlights
Magnetic-chitosan particles were prepared in nano, micro and macro sizes.
Particle size was important for biocatalyst properties.
Nano-CMag presented highest activity recovery for pectinase immobilization.
Pectinase immobilized on Macro-CMag showed highest thermal and operational stabilities.
Pectinase-Macro-CMag was able to clarify orange juice for at least 25 repeated batches.
In the framework of models that assume planar domain wall formed at the antiferromagnetic part of the interface of exchange-biased bilayers, one cannot distinguish between the cases of high or low ratios between the coupling and the antiferromagnet's anisotropy fields by using hysteresis loop measurement, ferromagnetic resonance, anisotropic magnetoresistance, or ac susceptibility techniques applied on one and the same sample. The analysis of the experimental data obtained on a series of FeMn/Co films indicated that once the biasing is established the variation in the coercivity with the FeMn layer thickness could be essential for solving this problem. If the coercivity decreases with the thickness then the interlayer exchange coupling is the parameter that varies while the domain-wall energy of the antiferromagnet remains practically constant.
Co ∕ Cu ∕ Ir Mn films were irradiated with 40keV He+ ions varying the fluence and the current, with magnetic field applied at 120° with respect to the original exchange-bias direction. The angular variations of the exchange-bias field of the irradiated samples were compared with those of the as-made and the thermally annealed films. Gradual deviation of the exchange-bias direction with the fluence increase was observed. Complete reorientation of the easy axes of both ferromagnet and antiferromagnet toward that of the field applied during irradiation was achieved for fluences higher than 1×1015ions∕cm2, accompanied with a significant enhancement of the exchange-bias field.
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