In this article, recent advances on polarized and unpolarized neutron diffraction under high pressure condition for frustrated antiferromagnet CuFeO₂ were reviewed. I provide two examples including (i) spherical neutron polarimetry analysis experiment under high pressure, and (ii) unpolarized neutron diffraction experiment in the multi-extreme conditions, high pressure, high field and low temperature. It is hoped that these techniques will help the elucidation of novel pressure-induced physical phenomena related to complex magnetic orderings.

Transition-metal intercalated dichalcogenides have attracted attention due to the observation of chiral helimagnetism (CHM) and chiral soliton lattice in CrNb₃S₆. It forms a chiral monoaxial crystal structure with a space group P6₃22. To examine the pressure effect on the period of CHM for CrNb₃S₆, we performed small-angle neutron scattering experiments with a piston-cylinder pressure cell up to 1.2 GPa. We observed a decrease in the magnetic transition temperature with increasing pressure. Moreover, the period of CHM decreased with increasing pressure. Compared to the pressure effect on the lattice constant c along the helical axis, the decrease of the period of CHM is much larger. This indicates that applying pressure weakens the amplitude of Dzyaloshinskii-Moriya interaction.

In this article, characteristics of the clamp-type cubic anvil high pressure cell, developed exclusively for neutron diffraction measurements, are presented. This pressure cell is extremely useful for magnetic studies that require a large volume of single crystal due to weak signal. Using the pressure cell, high pressure neutron diffraction measurements were performed using a single crystal of MnP, which exhibits superconductivity around 8 GPa. We could successfully observe magnetic Bragg peaks from Mn moments with 0.25 Bohr magneton at 7.5 GPa and 3 K.

A diamond anvil cell (DAC) for high-pressure neutron diffraction experiments has been developed at the PLANET beamline, Materials and Life Science Experimental Facility, in J-PARC. The conically supported diamond anvils were used for high-pressure generation. We succeeded in obtaining the neutron data for D₂O ice up to 69.4 GPa. In addition, the gasket materials suitable for the neutron diffraction measurements were investigated. 11 kinds of alloys were tested and SUS304, Inconel718 and M2052 (73Mn-20Cu-5Ni-2Fe, at%) alloys showed excellent performance. Especially, M2052 null-matrix alloy has proven to be useful for neutron diffraction experiments where the beam inevitably hits the gasket. We then obtained refinable neutron diffraction profiles in Rietveld analysis from D₂O ice at least up to 43.3 GPa.