The effect of magnetic fields on chemical reactions through the RP (radical pair) mechanism is well established, but there are few examples, in the literature, of biological reactions that proceed through RP intermediates and show magnetic field-sensitivity. The present and future relevance of magnetic field effects in biological reactions is discussed.
A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.
Here we describe an electronic circuit capable of producing rapidly switched dc magnetic fields of up to 20 mT with a rise time of 10 ns and a pulse length variable from 50 ns to more than 10 micros, suitable for use in the study of magnetic field effects on radical pair (RP) reactions. This corresponds to switching the field on a time scale short relative to the lifetime of typical RPs and maintaining it well beyond their lifetimes. Previous experiments have involved discharging a capacitor through a low inductance coil for a limited time using a switching circuit. These suffer from decaying field strength over the duration of the pulse given primarily by the ratio of the pulse width to the RC constant of the circuit. We describe here a simple yet elegant solution that completely eliminates this difficulty by employing a feedback loop. This allows a constant field to be maintained over the entire length of the pulse.
Gyroremanent magnetization (GRM) has previously been studied in connection with the well-known technique of ac demagnetization, which is used extensively in rock magnetism, palaeomagnetism and magnetic materials research. The "normal" GRM effect was explained by Stephenson in 1980, but recently a "reversed" GRM was obtained in a CrO 2 particle system by Madsen. We have studied samples of -Fe 2 O 3 particles (commercial magnetic recording tapes) and have identified samples with both "normal" and "reversed" GRM. The amplitude of the ac field was found to essentially influence the character of the GRM. In this paper, we also present micromagnetic simulations of the GRM in systems of single-domain particles and the results are discussed. Both "normal" and "reversed" GRM have been obtained in the simulations.Index Terms-Gyroremanent magnetization (GRM), magnetization processes, micromagnetic model, particulate media, rock magnetism.
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