XMCD measurements under pressure confirm ferromagnetism in YbCu2Si2 but find none in YbRh2Si2

Abstract

YbCu2Si2 and YbRh2Si2 have a similar phase diagram above 8GPa. While YbCu2Si2 is a well-known valence fluctuating compound that orders probably ferromagnetically above Pc=8GPa, in YbRh2Si2 the nature of the magnetic order at high pressure remains unclear. However, the fact that ferromagnetic (FM) fluctuations are found at low pressure leads to a robust speculation that the high pressure order could be FM. In this work, we have investigated the magnetic order induced by pressure in both compounds, by means of XMCD spectroscopy. Our measurements have confirmed the ferromagnetism in YbCu2Si2 through a clear saturation in the magnetization for pressures above Pc. On the other hand, the XMCD signal for YbRh2Si2 remains weak even at 25 GPa, pointing out a clear difference between the high pressure ordered phase of these compounds, and suggesting that FM order is unlikely if not completely excluded.