Ferromagnetic spin-correlations in strained LaCoO3 thin films

Abstract

We present an element-resolved study of the valence and magnetic properties of LaCoO3 thin films grown via pulsed laser deposition. The Co L edge x-ray absorption shows that ferromagnetic (FM) order arises from a slight hole doping of the system presumably due to nonstoichiometry, which in the bulk system disrupts the low-spin state. However, even though the films are hole doped, the magnetic moments under tensile strain are much larger than the bulk system indicating that the strain can greatly increase the FM fraction observed in the spin-glass regime at low doping.

Strain-driven spin-state transition and superexchange interaction in LaCoO3:Abinitiostudy

“…This feature of our theory is due to a reduced mixing between the Co 3d and O 2p states in the HS CoO 6 units, leading to softer HS Co-O bonds, compared to that of the LS CoO 6 units [14]. The HS/LS state has an energy gap of 0.5 eV consistent with tensile-strained FM LCO being insulating [7]. On the other hand, we show that no magnetic state is stable under compressive strain [4,6,7,12].…”

“…In our previous work, using local spin density approximation combined with the Hubbard U correction (LSDA+U), we have shown that a ferromagnetic state based on a homogenous intermediate spin (IS) state (S=1) can be stabilized above 3.8% tensile strain [10]. The ferromagnetic IS state is, however, inconsistent with two experimentally determined properties of strained LCO: the IS state is half-metallic while experiment shows that strained LCO is insulating [7], and a rather high critical strain of 3.8% is required, which is somewhat higher than that in experiment (~2%) [5][6][7][8][9][10]. Most recently, using a LDA+U approach, Hsu et al have shown that a HS/LS mixed state has a lower energy than that of the IS state in tensile-strained LCO on STO [28].…”

“…A recent report by Metha et al also suggests that compressive strain by itself cannot produce a FM state in LCO [12], indicating the existence of an asymmetric orbital-lattice interaction [4]. Magnetization measurements of compressively strained LCO on LaAlO 3 (LAO) substrates show only weak to no ferromagnetism [6,7,12]. Most recently, Sterbinsky et al have shown that inter-site hybridization involving Co and O states in LCO on STO is weaker than that in LCO on LAO by comparing the pre-edge structure of the Co K-edge X-ray absorption spectra [14].…”

“…Since the first demonstration by Fuchs et al using films grown by pulsed laser deposition (PLD) [5] and by our group using molecular beam epitaxy (MBE) [10], several intriguing properties of strained LCO have been found experimentally. In addition to transport measurements showing insulating behavior for tensile-strained FM LCO [7], Fuchs and co-workers have also shown that both the population of higher spin states and the magnetization in LCO increase as tensile strain increases [6]. Using X-ray techniques, Merz and coworkers have suggested that the magnetic structure of tensile-strained LCO grown on SrTiO 3 (STO) is a mixture of Co 3+ high spin (HS) and Co 3+ LS states [13].…”

“…Furthermore, strain engineering in these oxide heterostructures opens up routes for creating novel electronic phases [2][3][4]. An exciting example is the recent demonstration of biaxial tensile strain stabilizing an insulating ferromagnetic (FM) ground state in LaCoO 3 (LCO) [5][6][7][8][9][10][11][12][13][14]. Though LCO is a classic example of a correlated 3d transition metal perovskite oxide [15,16], FM correlation has never been observed for the bulk ground state where the Co 3+ ions exist in the so-called low spin (LS) state (total spin per Co S=0) [15].…”