Order parameter of holmium in a magnetic field

The order parameter of the second order magnetic phase transition in Ho at 131 K has been measured in a magnetic field by neutron diffraction. It was expected that the applied field would change the number of components ( n) of the order parameter from four to two. Such a change would be visible in the order parameter exponent β. However, in a field of 40 kOe, β is found to be 0.38 ± 0.02, in agreement with the zero field measurement and with an n = 4 model and in contradiction to an n = 2 model.