Enhanced magnetism in epitaxial SrRuO3 thin films

Abstract

We have observed enhanced magnetization in epitaxial SrRuO3 thin films compared to previously reported bulk and thin film values of 1.1-1.6 µB/Ru ion. The degree of enhancement is strongly dependent on the lattice distortions imposed on the SrRuO3 films by SrTiO3, (LaAlO3)0.3(SrTaO3)0.7 (LSAT), and LaAlO3 substrates. A larger enhancement of magnetization for coherently strained SrRuO3 films on SrTiO3 and LSAT compared to fully relaxed films on LaAlO3 confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. In particular, SrRuO3 films on (111) SrTiO3 exhibit exhanced moments as high as 3.8 µB/Ru ion, thus suggesting the stabilization of a high-spin Ru 4+ state.Transition metal complex oxides comprise a fundamentally intriguing and technonologically promising family of materials with a wide range of properties. The tunability of these properties via lattice distortions has been studied in detail in bulk and thin film form. While in the past it has often been difficult to separate out the effects of microstructure and defects from those of strain in epitaxial films, recent advances in atomically precise complex oxide thin film deposition techniques make it possible.Among the transition metal oxides, SrRuO 3 (SRO) is unique in that it is a 4d transition metal oxide that exhibits ferromagnetism and metallic conductivity. It has a distorted perovskite structure with a pseudocubic lattice parameter of 3.93Å.[1, 2] Bulk SrRuO 3 exhibits Curie temperature (T C ) ≈ 160 K and a moment of 1.1-1.6 µ B per Ru 4+ ion. [1,[3][4][5] These values are significantly reduced from the low-spin configuration value of 2 µ B /Ru 4+ ion. The reduction has been attributed to the delocalization of spin in an itinerant ferromagnet. SRO has also been identified as a technologically promising material as it has been incorporated both as an electrode in oxide devices and a structural template for other oxide films. [6][7][8][9] Recently, layer-by-layer growth of SrRuO 3 thin films has enabled the growth of near perfect epitaxial thin films. [10,11] However, the majority of this work examines films grown on (001) SrTiO 3 substrates. There has been some theoretical work that has predicted supressed magnetization for (i) SRO films under compressive epitaxial strain and (ii) SRO films compressed on the (110) plane in comparison with (001) SRO films.[12] Experimentally, remarkably little is known about the effects of the magnitude and symmetry of lattice distortions on the magnetization. [13,14] Epitaxial growth of SRO films provides model systems to study these distortions and their effects on magnetism.In this paper we demonstrate that magnetization can be enhanced in SRO films through lattice distortions generated by epitaxial strain. By modifying substrate choice and orientation as well as film thickness, we have systematically varied the magnitude and symmetry of the lattice distortions in SRO films imposed by the substrate. We find magnetization enhanced from bulk values that are consistent with the ...

Multiferroic properties of uniaxially compressed orthorhombic HoMnO3 thin films

“…For example, the transition temperature of ferromagnetic and ferroelectric (FE) ordering can drastically change, as can the magnitude of the associated order parameters. [6][7][8][9] Furthermore, when epitaxial strain is applied to a material which is close to a magnetic and/or electric phase boundary, the ordering motif itself can be changed. [10][11][12] The perovskite orthorhombic rare-earth manganites (o-REMnO3, RE = Tb -Lu, Y, Pbnm space group) are multiferroics with strong magnetoelectric coupling since they exhibit spin-induced FE.…”

Termination-dependent electronic and magnetic properties of ultrathinSrRuO3(111) films onSrTiO3

“…3,8,[28][29][30][31][32][33] As shown in Fig. 1 Intriguingly, from the M -H curve of SRO (111) films, Grutter et al 24 obtained the magnetization as large as 3.4 µ B /Ru, which far exceeds the low-spin (LS) moment value of 2 µ B /Ru. So they claimed the stabilization of the high-spin (HS) state (4µ B /Ru) in the SRO (111) films.…”

“…In fact, systematic growth of SRO (111) film on STO was accomplished only recently. [23][24][25][26] In the experimental aspect, delicate control of the layer growth is required, especially for the first ML, due to the conversion tendency of highly volatile Ru 4+ to SrO…”

Review on Electronic Correlations and the Metal-Insulator Transition in SrRuO3

“…Thus, in a metallic state where U and t (or band width W) are comparable, U promotes ferromagnetism. SRO is an itinerant ferromagnet with a magnetic moment 1.0~1.6 μ B per formula unit (Kanbayasi, 1976;Mazin & Singh, 1997;Rondinelli et al, 2008), which is smaller than net Bohr magnetron of an isolated unit cell, i.e., two-majority spin, 2 μ B (note that Grutter et al [2010] have reported an exceptional value). This discrepancy is attributed to the itinerancy of the four t 2g electrons.…”