Spin kinetics of liquid $^3$He in contact with a DyF$_3$ micropowder at ferromagnetic ordering of Dy$^{3+}$ ions

The spin kinetics of liquid $^3$He in contact with a mixture of LaF$_3$ ($99.67\%$) and DyF$_3$ ($0.33\%$) micropowders at temperatures of $1.5$–$3$ K has been studied by pulsed nuclear magnetic resonance (NMR). The DyF$_3$ is a dipolar dielectric ferromagnet with the phase transition temperature $T_c= 2.55$ K, whereas the diamagnetic fluoride LaF$_3$ is a diluting substance for the optimal observation conditions of $^3$He NMR in powder pores. The magnetic phase transition in DyF$_3$ is accompanied by a considerable change in the character of fluctuations of the magnetic moments of dysprosium ions, which affect the spin kinetics of $^3$He in contact with the substrate. Significant changes in the relaxations rates of the longitudinal and transverse magnetizations of $^3$He have been discovered in the region of magnetic ordering of the solid matrix. The technique of studying the static and fluctuating magnetic fields of a solid matrix at low temperatures using liquid $^3$He as a probe has been proposed.