Magnetic field sensitive gels, called ferrogels, are chemically cross-linked polymer networks
swollen by a ferrofluid. The monodomain magnetic particles, with a typical size of about 10 nm, couple
the shape of the polymer gel to the nonuniform external magnetic field. Shape distortion occurs
instantaneously and disappears abruptly when an external magnetic field is applied or removed,
respectively. This work provides a thermodynamic analysis of shape transition based on the free energy
of the swollen network that includes the elasticity of network chains as well as magnetic interactions of
finely dispersed solid particles with the external field. It is shown that noncontinuous shape transition
is due to a shift of equilibrium state from one local minimum to another one, similar to a first-order
phase transition. The discussions presented here may be useful for the design of magnetically active
soft polymeric actuators.
The magnetoresistance of the quasi-one-dimensional organic conductor (TMTSF) 2 PF 6 is studied under a hydrostatic pressure of about 0.8 GPa in magnetic fields up to 12 T perpendicular to the conducting a-b planes in the temperature range of 5 to 35 K. Both in the best conducting a and second best conducting b directions, the magnetoresistance follows a power law ⌬R/Rϭ(B/B 0 ) 3/2 . The a-b plane anisotropy is field independent. The scaling field B 0 follows an exponential temperature dependence B 0 ϰexp(T/T 0 ) with T 0 ϭ10 K. These findings are discussed in terms of recent theories of the magnetoresistance in (TMTSF) 2 PF 6 .
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