Magnetic order in the pseudogap phase of HgBa ₂CuO _4+δ studied by spin-polarized neutron diffraction

Spin-polarized neutron diffraction experiments have revealed an unusual q=0 magnetic order in the model high-temperature superconductor HgBa ₂CuO _4+δ (Hg1201) below the pseudogap temperature T ^*. Together with results for the structurally more complex compound YBa ₂Cu ₃O _6+δ (YBCO), this establishes the universal existence of a genuine novel magnetic phase in underdoped cuprates with high maximal T _c (above 90 K at optimal doping). Here we report a systematic study of an underdoped Hg1201 sample (T _c=75 K), the result of which is consistent with the previously established doping dependence of the magnetic signal. We present an assumption-free analysis of all the data available for Hg1201. Depending on how the hole concentration is estimated, comparison with the results for YBCO leads to different scenarios for the competition between the q=0 magnetic order and the spin-density-wave order found in heavily underdoped YBCO.