  Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs
with a finite center-of-mass momentum. The resultant spatially modulated
superconducting order parameter is able to stabilize Josephson junctions (JJs)
with pi phase difference in superconductor-ferromagnet heterostructures and
realize 'quiet' phase qubits. The emergence of two-dimensional (2D) layered
superconducting and magnetic materials promises a new platform for realizing pi
JJs with atomically sharp interfaces by van der Waals stacking. Here we
demonstrate a thickness-driven 0-pi transition in JJs made of NbSe2 (an Ising
superconductor) with a Cr2Ge2Te6 (a ferromagnetic semiconductor) weak link. By
systematically varying the Cr2Ge2Te6 thickness, we observe a vanishing
supercurrent at a critical thickness around 8 nm, followed by a re-entrant
supercurrent upon further increase in thickness. Near the critical thickness,
we further observe unusual supercurrent interference patterns with vanishing
critical current around zero in-plane magnetic field. They signify the
formation of 0-pi JJs (with both 0 and pi regions) likely induced by the
nanoscale magnetic domains in Cr2Ge2Te6. Our work highlights the potential of
van der Waals superconductor-ferromagnet heterostructures for the explorations
of unconventional superconductivity and superconducting electronics.
