Anomalous Ferromagnetism of Quasiparticle Doped Holes in Cuprate Heterostructures Revealed Using Resonant Soft X-ray Magnetic Scattering

We report strong ferromagnetism of quasiparticle doped holes both within the ab- plane and along the c- axis of Cu-O planes in low-dimensional Au/ d- La 1.8 Ba 0.2 CuO 4 /LaAlO 3 (001) heterostructures ( d = 4, 8 and 12 unit-cells) using resonant soft X-ray and magnetic scattering together with X-ray magnetic circular dichroism. Interestingly, ferromagnetism is stronger at a hole doped peak and at an upper Hubbard band of O with spin-polarization degree as high as 40%, revealing strong ferromagnetism of Mottness. For in- ab -plane spin-polarizations, the spin of doped holes in O2 p –Cu3 d –O2 p is a triplet state yielding strong ferromagnetism. For out-of- ab -plane spin-polarization, while the spins of doped holes in both O2 p –O2 p and Cu3 d –Cu3 d are triplet states, the spin of doped holes in Cu3 d –O2 p is a singlet state yielding ferrimagnetism. A ferromagnetic-(002) Bragg-peak of the doped holes is observed and enhanced as a function of d revealing strong ferromagnetism coupling between Cu-O layers along the c -axis.