Experiments are described on surface gravity-capillary waves in a 70 cm × 2.8 cm rectangular tank undergoing small vertical oscillations. The large aspect ratio of the tank permits study of several neighbouring two-dimensional wave modes; their onset, hysteresis and instability. Two water depths, close to one and two centimetres, are investigated in detail. Thresholds are determined for wave onset from a flat surface and the hysteretic return to a flat surface from finite-amplitude waves. Also, conditions at which a pure standing wave becomes unstable to other modes are established. Our finding of hysteresis below the minimum forcing for linear wave-onset suggests that nonlinear forcing and nonlinear damping are both significant, as recently suggested by Miles (J. Fluid Mechanics 248 (1993) 67l.) A theoretical model, incorporating these effects, is analysed and the corresponding hysteresis boundaries established. These are broadly consistent with our experimental results.

The forced volume magnetostriction and the NMR of ferromagnetic FeN and ferrimagnetic Mn4N have been measured in the ordered state. At 4.2 K the magnetization ( sigma ) per unit mass and the effective field (HI) at the nucleus of the Fe(I) site of Fe4N are found to change with pressure such that delta ln sigma / delta P approximately= delta ln mod HI mod / delta P<0, while for Mn4N delta ln sigma / delta P approximately=6 delta ln mod HI mod / delta P>0. The result for Mn4N is interpreted as showing that the magnitude of the small (negative) moment on the Mn(II) site decreases more rapidly under pressure than does the large Mn(I) moment. The value of delta ln mod HI mod / delta P for Mn4N was found to be a strong function of temperature such that delta lnTN/ delta P approximately=5.2 Mbar-1. An analysis of the Mn4N NMR data suggests that the discrepancy between the calculated (spin) and measured moment at the Mn(I) site may be due to a small orbital moment at that site.

The pressure dependence of the magnetisation of Y2Fe17, Y6Fe23 and YFe2 at 4.2 K has been deduced from forced volume magnetostriction measurements in fields up to 12 T. The pressure dependence of the 89Y hyperfine field in the above compounds and YFe3 has been measured using NMR. The results are consistent with a model in which a moment of approximately=-0.4 mu B exists at the Y site of all four compounds as found in computer calculations for YFe2.

The anomalous properties of the Kondo lattice are compared with the well behaved Kondo system, CePd. The low-temperature thermal expansion of CePd is in agreement with previous heat capacity data, showing a ferromagnetic transition at and a second transition at 3.5 K probably due to reorientation of the magnetic moments. The Grüneisen factor, calculated from thermal-expansion and heat capacity is , the same as the Grüneisen factor calculated from the pressure dependence and comparable to a previous measurement of . Thermal expansion measurements (in fields of up to 8 T) and ac susceptibility measurements (under pressures of up to 7.33 kbar) have been made on in the temperature range 1.6-40 K. The zero-pressure ac susceptibility measurements confirm that there is an antiferromagnetic transition at , as previously reported. The magnetic contribution to the linear thermal-expansion coefficient of polycrystalline in zero magnetic field has a maximum value near 17 K and becomes small by 29 K. There is no peak in at the Néel temperature. The Néel temperature is found to decrease under pressure at the rate of , which indicates that is a magnetic Kondo lattice (with ) on the right-hand side of the Doniach diagram.

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