Switched external magnetic field

A significant step in the measurement of MFEs is the development of rapid field switching apparatusModem MOSFET transistors have made the possibility of field switching on the timescale of RP dynamics a reality. This technique was first used in a method called SEMF-CIDNP (switched external magnetic field chemically induced dynamic nuclear polarization). 

The magnetic moment of the 4s-valence electron of a chromium and a copper atom is cancelled out by the magnetic moment of a neighbouring atom s valence electron. All atoms in the table, except for copper, have unpaired electrons in d-orbits due to which you would expect a magnetic moment. They exhibit this behaviour when they are placed in an external magnetic field. As soon as this external magnetic field is switched off, the magnetism in the metals disappears. 

Fig. 37 Linear chain formation of DNA-coated paramagnetic polystyrene colloids with the different self-protection schemes displayed in Fig. 33. By using an external magnetic field, DNA-functionalized particles were brought together into linear chains, after which the temperature was lowered below the association temperature for beads, and the field turned off. (a) Representative microscopy picture of the resulting chain structures immediately after switching off the magnetic field, (b-d) Chains after 1 h at the specified temperature for particles functionalized with sticky end sequences able to form both loops and hairpins (b, c) or only loops (d). The degree of aggregation of chains in (d) is intermediate between the unprotected, branched chains in (b) and the perfectly linear, protected chains in (c). Adapted with permission from [157]...

The number of rotating planes corresponds to mi. For m = 0 the wave function (16) is real. In a spherically symmetrical environment there is no special direction in space and the wave function (18) acquires geometrical meaning only when an external magnetic field is switched on. Any of the... 

Fig. 30. Representation of information storage using a magnetic bit. a The bit can exist in either of two stable states - in this case, with parallel alignments of the individual dipoles, pointing either up or down, b The mean dipole direction can be switched repeatedly and reversibly at high and low external magnetic field, and interrogated at zero field, where the bit is not perturbed...

Switching the magnetic field during the life of the radical pairs is an experimentally much more demanding extension of CIDNP. A specially built external magnet and controller are needed experimental details for single and double switching can be found in the literature. 

Originally, the basic idea was realized without an FC switch by a continuous or one-tum rotation of the sample perpendicular to the external magnetic field direction, in order to induce a flow but such a mechanical rotation is, similar to a mechanical FC switch, much too slow to establish reproducible states of motions in common, low-viscosity anisotropic liquids such measurements were therefore restricted to high-viscosity liquid crystalline polymers. Fast electronic FC eliminates these restrictions, but to date the new tedmique has only been used with proton spin signals owing to instrumental limitations. ... 

Bayer and Zinn (W3), and Bayer (1975) have studied the switching phenomena in polycrystalline films of the type GdS/EuS/GdS and Gd/EuS/Gd. They have shown the characteristics of the S- or N-shaped current-controlled and voltage-controlled at different temperatures and with an external magnetic field. They have discussed the non-linearities of the current transport behavior. Ahn and Shafer (1971) have observed that the switching properties in their films are similar to those of the stoichiometric samples. 

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