Second and third harmonics of the spin density wave in chromium metal

Neutron-diffraction measurements with a single crystal of chromium in an approximately single-Q state have revealed the existence of a satellite reflection at three times the wave vector →Q of the spin density wave (SDW). This $3Q$ harmonic is itself an SDW with a polarization identical to that of the primary SDW and an amplitude →M3Q which is (1.65 ±{} 0.05) ×{} 10^-2 times the amplitude →M1Q of the primary SDW at 200 K. The amplitude |→M3Q| is found to be proportional to |→M1Q|³ for temperatures close to the N\'eel point; for temperatures below about 220 K there is a tendency for |→M3Q| to approach a saturated value. This behavior is contrasted with that of the recently discovered $2Q$ harmonic whose amplitude →A2Q is found to be proportional to |→M1Q|² even at low temperatures.