Pressure tuning of magnetic states in elemental thulium

The magnetic and structural properties of elemental thulium were studied by means of neutron powder diffraction at pressures up to 5.5 GPa in the temperature range 7 – 290 K. At ambient pressure below T _N = 52 K, the incommensurate magnetic state with a longitudinal sinusoidal modulation and propagation vector q = (0, 0, q _z) was formed. The q _z value increases on cooling from 0.279(2) and reaches the commensurate value of 2/7 at the temperature T ₁ = 24 K. The new magnetic arrangement is of an antiphase domain structure, composed of structural blocks involving three or four hexagonal layers with magnetic moment directions parallel within a given block and oppositely alternating from block to block along the c axis. Under high pressure, the T _N value increases slightly up to 57 K at 5.5 GPa with a pressure coefficient dT_N/dP = 0.9 K/GPa. The pressure dependence of the T ₁ temperature is more pronounced with a pressure coefficient dT₁/dP = 2.9 K/GPa. The arrangement of commensurate magnetic structures formed below T ₁ is strongly pressure dependent. A formation of the complex magnetic states composed of structural blocks involving three or four hexagonal layers with magnetic moment signs alternating from block to block along the c axis with q _z = 7/23 at 2 GPa and q _z = 7/22 at 5.5 GPa was observed. The lattice compression of Tm at T = 7 K demonstrates a weak anisotropy with the most compressible c axis.