Magnetic properties and orbital anisotropy driven by Mn2+in nonstoichiometric LaxMnO3−δthin films

Abstract

Magnetic measurements by complementary techniques have shown that the substitution of Mn${}^{2+}$ in the $A$ site strongly influences the electronic and magnetic properties of nonstoichiometric La${}_{x}$MnO${}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films ($x=0.66\ensuremath{\rightarrow}1.07$). We have studied a series of samples deposited on SrTiO${}_{3}$ (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 ${\mathrm{L}}_{2,3}$ x-ray absorption spectra (XAS) have revealed the presence of Mn${}^{2+}$ 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 Mn${}^{2+}$ content. The experimental results reveal the double role of divalent Mn in these films. For $x\ensuremath{\ll}1$ the Mn${}^{2+}$ ions at the $A$ site stabilize to a robust ferromagnetic phase (${T}_{C}\ensuremath{\simeq}350$ K) and influence the orbital occupation of Mn${}^{3+}$ at the $B$ site in the residual antiferromagnetic phase. These results indicate that substitution of La${}^{3+}$ with a magnetic divalent ion (Mn${}^{2+}$) 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.