Spin transfer switching

When large current densities (10+7 A/cm2) cross CPP spin valves, spin transfer effects start to be observed. When a spin polarized DC current flows across the CPP device, both well defined spin wave modes, and chaotic spin wave generation has been observed as a function of the injected DC current [65], These spin-wave modes lead to excess magnetic noise. These effects were first predicted by Berger [66] and Slonczewski [67], and have been now experimentally observed by several groups. At low frequencies, spin transfer effects also affect the CPP device transfer curve [68], and can be used to induce free layer switching in magnetoresistive devices. 

Figure 23. Current induced magnetization switching (spin transfer driven) (a) resistance vs.

Polarization-transfer experiments which are based on a resonance condition, i.e. where a variable quantity in the experiment is matched to a parameter of the investigated spin system, can be carried out as a transient experiment or as an adiabatic experiment Figure 11.5 illustrates the differences between these two types of experiments. In a transient or sudden" experiment, the density operator is prepared in a state orthogonal to the effective polarization-transfer Hamiltonian (Fig. 11.5a). When the polarization-transfer Hamiltonian is switched on, the density operator starts precessing around the effective Hamiltonian, and usually maximum polarization transfer is reached after a 180° rotation. Since often the size of the effective Hamiltonian at the matching condition depends on... 

Keywords Charge transfer solid Electronic dimensionality Functional organic solid Ionicity diagram Organic metal Organic superconductor Phase transition Quantum spin liquid state Switching... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Switching on the 13C RF transmitter is represented by opening the valve between the reservoirs H and 13C. The relative powers of the proton and 13C RF transmitters are adjusted to maximize interactions between the two types of precessing nuclei. Polarization can then be transferred between neighboring nuclei through spin flip-flop processes. Optimization is achieved when the Hartmann-Hahn condition is met, i.e., the H and 13C RF field strengths are in a ratio set close to 1 4 (Pines et al. 1973). Magnetization is then transferred with a time constant TCH-... 

P450 saturated with substrates are reduced much faster than in the substrate-free state because of the positive shifts of the redox potential hy 100mV. Such control over the rate of the first electron transfer hy the substrate binding represents an important feature of the overall steady-state kinetics of many cytochromes P450 although some isozymes exist in a predominantly high-spin state, even without substrates, and they presumably lack this switch. 

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