The observation of a charge density wave in the underdoped cuprate high T c superconductors (Cu-CDW) raised a debate about its relationship with superconductivity. In bulk YBa 2 Cu 3 O 7−δ the Cu-CDW is incipient and mainly pinned by defects. Nevertheless, a large magnetic field can induce a true long-range Cu-CDW order as it suppresses superconductivity. An enhanced Cu-CDW order was also observed in YBa 2 Cu 3 O 7 /La 2/3 Ca 1/3 MnO 3 multilayers. Here, we show that the magnitude of the Cu-CDW in YBa 2 Cu 3 O 7−δ / Nd 0.65 (Ca 1y Sr y) 0.35 MnO 3 multilayers can be varied by adjusting the strength of the manganite charge and orbital order via the Sr content (tolerance factor). Furthermore, we resolve the reconstruction of the crystal field levels of the interfacial Cu ions that are also affected by the manganite charge and orbital order. This tuneable interfacial coupling and Cu-CDW in YBa 2 Cu 3 O 7−δ can be used for studying the relationship between the Cu-CDW and superconductivity and, possibly, for inducing new intertwined quantum states.

Anti-ferromagnetic spintronic devices could offer ultra fast dynamics and a higher data density than conventional ferromagnetic devices. One of the challenges designing such devices is the control and detection of the magnetisation of the anti-ferromagnet due to its lack of stray fields, and this is often achieved through the exchange bias effect. In exchange biased systems the pinned spins are known to comprise a small fraction of the total number of interface spins, yet their exact nature and physical origin has so far been elusive. Here we show that in the technologically important disordered γ IrMn 3 /CoFe structure the pinned spins arise from the small imbalance in the number of spins in each magnetic sublattice in the antiferromagnet due to the naturally occurring atomic disorder. These pinned spins are strongly coupled to the bulk antiferromagnet explaining their stability. Moreover, we find that the ferromagnet strongly distorts the interface spin structure of the antiferromagnet, causing a large reversible interface magnetisation that does not contribute to exchange bias but does increase the coercivity. We find that the uncompensated spins are not localised spins which occur due to point defects or domain walls but instead constitute a small motion of every AFM spin at the interface. This unexpected finding resolves one of the long standing puzzles of exchange bias and provides a route to developing optimized nanoscale antiferromagnetic spintronic devices.

With dc magnetization and polarized neutron reflectometry we studied the ferromagnetic response of YBa 2 Cu 3 O 7 /La 2/3 Ca 1/3 MnO 3 (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples, for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order, since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers.