| Popis: |
The pressure derivative of the spin-density-wave Q vector in antiferromagnetic Cr, indicating an irreversible shift of the Q vector relative to the lattice on initial application of change in the vector Q in the unpressurised or 'virgin' sample is large and irreversible while the value for the derivative d ln Q/dp in a previously pressurised or 'hardened' sample, is essentially zero and reversible, apart from a small hysteresis at higher pressures, thus indicating an irreversible shift of the vector Q relative to the lattice on initial application of hydrostatic pressure, with subsequent locking. The values are independent of sample quality and origin and are reproducible within experimental error. This, as reported previously, is in essence the explanation for the historical discrepancy between the two classes of experimentally determined values for the derivative d ln Q/dp. The virgin state of the sample can be recovered by simply warming the sample to about 80K. The width of the hysteresis loops, expressed in units of external hydrostatic pressure, shows an initial exponential increase with the pressure ph up to which the sample has been hardened. It decreases linearly with magnetic field and is found to relax to its final value after initial application of ph with a time constant of approximately one hour. No evidence was found to support the prediction of Fenton (1980) that the loop width would be a function of specimen dimensions. An explanation of the hysteresis is proposed in terms of a progressive breakdown of the hardened condition. |
