Magnetic properties and orbital anisotropy driven by Mn2+ in nonstoichiometric LaxMnO3-δ thin films

Magnetic measurements by complementary techniques have shown that the substitution of Mn2+ in the A site strongly influences the electronic and magnetic properties of nonstoichiometric LaxMnO3-delta thin films (x = 0.66 -> 1.07). We have studied a series of samples deposited on SrTiO3 (100) substrates by molecular beam epitaxy. SQUID magnetometer has been used to measure the dependence of magnetization on temperature and applied magnetic field between 400 K and 10 K. Mn L-2,L-3 x-ray absorption spectra (XAS) have revealed the presence of Mn2+ in the samples with smaller x, which are ferromagnetic. The dependence of XAS on linear polarization (linear dichroism) has revealed the magnetic anisotropy and the orbital preferential occupation as a function of x. Magnetic hysteresis loops and magnetic circular dichroism have allowed us to determine the ferromagnetic easy axis and the presence of a second coercive field in some of the samples. O K edge XAS has provided an insight into the band hybridization as a function of the La/Mn ratio and Mn2+ content. The experimental results reveal the double role of divalent Mn in these films. For x << 1 the Mn2+ ions at the A site stabilize to a robust ferromagnetic phase (T-C similar or equal to 350 K) and influence the orbital occupation of Mn3+ at the B site in the residual antiferromagnetic phase. These results indicate that substitution of La3+ with a magnetic divalent ion (Mn2+) gives rise to structural distortions overcoming the uniaxial strain induced by the substrate and to an unexpectedly strong superexchange interaction between A and B sites.