Anisotropic quantum magnetism in the Pr⁴⁺ oxide Sr₂PrO₄

Interest in anisotropic quantum magnetism resurged in 2006 with the discovery that the Kitaev model hosts a quantum spin liquid ground state. In recent years, high-valent lanthanides, like Pr⁴⁺, have been used as building blocks for anisotropic quantum magnets. The high oxidation state leads to enhanced covalency, which increases the energy scale of the crystal electric field and ultimately leads to the breakdown of LS-coupling.¹ This can give rise to unusual magnetic properties like the strong and anisotropic exchange interactions found in Na₂PrO₃.^2-4 Here, we combine X-ray and neutron spectroscopy to investigate this phenomenon in Sr₂PrO₄. Resonant inelastic X-ray scattering (RIXS) and X-ray magnetic circular dichroism (XMCD) at the Pr M_4,5-edge revealed the details of Pr⁴⁺ single-ion physics, which allowed us to reexamine the composition of the ground state wave function. On this basis, we have determined the magnetic order from neutron diffraction experiments and used inelastic neutron scattering (INS) to probe the spin wave excitations. Together, these experiments provide a holistic picture of the anisotropic magnetism of Sr₂PrO₄.

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