Biaxial magnetic structures in rare-earth compounds: GdMg and HoP

The flopside spin structure, where the magnetic moments form two sublattices which at low temperatures are mutually perpendicular was first found in HoP and then in other rare-earth pnictides. There are large orbital contributions to the magnetic moments of these compounds and it had been thought that they play an important role in stabilizing the flopside spin structure. However, recently this spin structure has been found in GdMg. As Gd³⁺ is an S-state ion, there are negligible orbital effects. We have developed a model Hamiltonian which is able to explain both the occurence of initially a ferromagnetic phase and then at low temperature the flopside spin structure in two very dissimilar compounds GdMg and HoP. For GdMg we find that the competition between the near neighbor ferromagnetic and antiferromagnetic bilinear exchange interactions is such that while they produce a transition to a ferromagnetic phase at 110 K, an unusually small amount of biquadratic (quadrupolar) coupling is able to stabilize a flopside phase at low temperature which is able to resist collapse in a field as large as 150 kOe. For HoP we find that although anisotropic bilinear pair interactions - as for example pseudo-dipole - exist, they cannot be the primary origin of the flopside phase; quadrupole pair interactions are essential to explain the appearance of first the ferromagnetic and then the flopside phases found in HoP. On the basis of our model calculations we are able to explain the data extant on these compounds and we make some predictions which are open to experimental verification.