Strain-Engineered Ferromagnetism in LaMnO₃ Thin Films

Abstract

A systematic study of the growth process of LaMnO 3 (LMO) thin films, by pulsed laser deposition, on top of SrTiO 3 substrates under different oxygen partial pressures (PO 2) is reported. It is found that the accommodation of the orthorhombic LMO phase onto the cubic STO structure, i.e. the amount of structural strain, is controlled by background oxygen pressure. We demonstrate that magnetic behaviour can be continuously tuned from robust ferromagnetic (FM) ordering to an antiferromagnet. These results strongly point to a strain-induced selective orbital occupancy as the origin of the observed FM behaviour, in agreement with recent theoretical calculations.

Formation of Self-Organized Mn3O4 Nanoinclusions in LaMnO3 Films

“…Figure 3A shows a typical area scan for LMO sample grown on LAO. Similar results were obtained previously for films grown on STO [27]. We observe the presence of two peaks occurring at different 2θ angles that are the clear signature of twinned domains with two in-plane lattice parameters.…”

“…The temperature dependence of magnetization (in a magnetic field of 0.1 T) revealed two transitions, the first one is the expected antiferromagnetic transition of LMO at ∼140 K and the second one is the ferrimagnetic transition of manganese oxide secondary phase at around ∼40 K (Figure 10A). It is known that LMO thin films may exhibit a broad range of magnetic behaviors, from antiferromagnetic to strongly ferromagnetic, depending on the preparation conditions [27]. In our case, films are prepared in vacuum conditions leading to a residual moment below 0.3 µ B /f.u.…”

“…It is worth reminding here that this spontaneous regular formation of manganese oxide in LMO thin films has been obtained by laser ablation of single target and it is linked to the low oxidation conditions during the PLD process. Films grown at higher oxygen pressures exhibit flat surfaces with no sign of phase segregation [27]. More information about the arrangement of the secondary phase may be obtained from atomic force microscopy images as the shown in Figure 1B (on STO) and Figure 2B (on LAO).…”

“…In LMO bulk state, oxygen offstoichiometry (in fact, cation vacancy) is accommodated by modifying the Mn-O octahedral environment mainly resulting in a strong change in the orthorhombic lattice parameters that also influences the cooperative Jahn-Teller effect and, as a result, the magnetic ordering. Quite recently we have shown that a careful epitaxial analysis is required to fully understand the growth of LMO films under different oxygen conditions [27]. For this reason, we have performed reciprocal 2θ-φ area scans at grazing incidence around 220 and 004 orthorhombic in-plane reflections (corresponding to 200 reflections in pseudocubic notation).…”

“…The crystal structure of the bulk LMO is described by an orthorhombic Pbnm space group with cell parameters a = 5.533 Å, b = 5.727 Å, and c = 7.668 Å at room temperature [26]. Achieving the right LaMnO 3 stoichiometry in thin films leading to an antiferromagnetic (AF) ground state remains challenging regardless of the growth technique and it requires a fine control of growth conditions [27]. In particular, AF behavior is only obtained in films prepared at low oxygen pressures while the right stoichiometry is maintained.…”