Changes in Apparent Symmetry with Magnetic Field in Chromium

Abstract

The magnetic states of chromium are described in terms of fundamental wave vectors Q1, Q2, and Q3 and the polarizations associated with these Q vectors. If the magnetic reflections in neutron-diffraction experiments show Q1, Q2, and Q3 all present and contributing equally to the single-crystal diffraction pattern, we speak of the ``apparently cubic'' state. If only one Q vector is present, we speak of a ``single Q'' state. Cooling through the first-order paramagnetic-antiferromagnetic transformation at 38.5°C produces an ``apparently cubic'' state if no field is applied during cooling while the application of a field of 40 kG can produce a ``single Q'' state in some crystals. In other crystals 40 kG is not sufficient to produce an entirely ``single Q'' state. The preferred Q is parallel to the applied field; the polarization is then perpendicular to the field. If the field applied during cooling is less than necessary to completely suppress Q1 and Q3 while enhancing the contributions from Q2, the polarizations of Q1 and Q3 prefer the field direction. In the low-temperature phase of chromium, where the polarizations are parallel to the Q's, the application of 160 kG is sufficient to change an ``apparently cubic'' crystal into a state with one Q missing. The ``missing Q'' is parallel to the direction of the field. The critical scattering at 40°C shows that all three wave vectors Q1, Q2, and Q3 are equally represented in the wave-vector-dependent susceptibility and that the longitudinally polarized fluctuations are comparable to the transversely polarized fluctuations. The tetragonality of the Fermi surface of the ``single-Q'' state does not cause a large enough lattice distortion to be detected with an x-ray technique sensitive to 1 part in 2×104.