  Heterostructures from complex oxides allow one to combine various electronic
and magnetic orders as to induce new quantum states. A prominent example is the
coupling between superconducting and magnetic orders in multilayers from
high-Tc cuprates and manganites. A key role is played here by the interfacial
CuO2 layer whose distinct properties remain to be fully understood. Here, we
study with resonant inelastic X-ray scattering (RIXS) the magnon excitations of
this interfacial CuO2 layer. In particular, we show that the underlying
antiferromagnetic exchange interaction at the interface is strongly suppressed
to J ~ 70 meV, as compared to J ~ 130 meV for the CuO2 layers away from the
interface. Moreover, we observe an anomalous momentum dependence of the
intensity of the interfacial magnon mode and show that it suggests that the
antiferromagnetic order is accompanied by a particular kind of orbital order
that yields a so-called altermagnetic state. Such a two-dimensional altermagnet
has recently been predicted to enable new spintronic applications and
superconducting proximity effects.
