Magnetic phase diagram of Cr1/3NbS2: SANS study

The magnetic structure of the Cr1/3NbS2 compound was studied with help of small-angle neutron scattering under an applied magnetic field in a wide temperature range below TN = 130 K. At zero magnetic field the magnetic moments of Cr3+ ions build the helix structure with the wave vector k oriented along the c-axis of the chiral structure of P6322 space group. The detected diffraction ring is interpreted as scattering on the magnetic helices of randomly oriented crystallites within powder sample. The intensity of the diffraction peak decreases with temperature and dissolves at T = 115 K. The magnetic field differently affects the crystallites with helix wave vector k along the field and those with k perpendicular to the field. The magnetic field, when applied, leads to appearance of the chiral soliton lattice in the grains with k oriented perpendicularly to the field. The period of the chiral soliton lattice increases rapidly as H approaches HC1 = 0.25 T. The transition from chiral soliton lattice to the field-induced ferromagnet, however, is not homogeneous but is accompanied by strong ferromagnetic fluctuations in the wide field range around HC1. These fluctuations are well visible in the experiment as scattering centered at Q = 0. In the grains with k parallel to the field, the chiral soliton lattice undergoes a series of transitions from helimagnet to the conical state, and then to the field-induced ferromagnet at HC2 = 1.25 T. The period of the conical helix increases slightly (for 5%) with the field, while the transition to the field-induced ferromagnetic state is again accompanied by strong ferromagnetic fluctuations. The magnetic field-temperature phase diagrams for H perpendicular to the c-axis and for H parallel to the c-axis are plotted on the basis of the data obtained.