Microscopic origins for stabilizing room-temperature ferromagnetism in ultrathin manganite layers

Abstract

La 0.7 Sr 0.3 MnO 3 is a conducting ferromagnet at room temperature. Combined with thin SrTiO 3 layers, the resulting heterostructures could be used as highly spin-polarized magnetic-tunnel-junction memories. However, when shrunk to dimensions below an apparent critical thickness, the structures become insulating and ferromagnetic ordering is suppressed. Interface spin and charge modulations are thought to create an interfacial dead layer, thus fundamentally limiting the use of this material in atomic-scale devices. The thickness of this dead layer, and whether it is intrinsic, is still controversial. Here we use atomic-resolution electron spectroscopy to demonstrate that the degradation of the magnetic and transport properties of La 0.7 Sr 0.3 MnO 3 ∕SrTiO 3 multilayers correlates with atomic intermixing at the interfaces, and the presence of extended two-dimensional cation defects in the La 0.7 Sr 0.3 MnO 3 layers (in contrast to three-dimensional precipitates in thick films). When these extrinsic defects are eliminated, metallic ferromagnetism at room temperature can be stabilized in five-unit-cell-thick manganite layers in superlattices, placing the upper limit for any intrinsic dead layer at two unit cells per interface.electron energy loss spectroscopy | manganites | scanning transmission electron microscopy C olossal magnetoresistance, metal-insulator transitions, charge/orbital ordering, and half-metal ferromagnetism are only some of the intriguing phenomena that occur in manganites and have driven interest in the family of perovskite manganese oxides (ABO 3 perovskite structure, B site occupied by Mn) (1). With a Curie temperature (T c ) of ∼370 K, the half-metal La 0.7 Sr 0.3 MnO 3 (LSMO) has been considered a promising candidate for spintronics applications. However, although complete spin polarization in LSMO was inferred from photoemission measurements (2) and a record tunneling magnetoresistance (TMR) ratio of 1,800% was obtained at low temperature in tunnel junctions with manganite electrodes separated by a thin insulating layer of SrTiO 3 (STO) (3), the TMR decreased rapidly as the temperature increased and diminished while still far below T c . One possible origin for the reduced TMR is the reduction of ferromagnetic ordering at the interface between the manganite and the STO. These interfacial effects have been suggested to be dominant as the LSMO layer thickness decreases, causing the magnetization and T c to degrade and the resistivity to increase, ultimately resulting in films that are insulating at all temperatures when the layer thickness decreases below a reported critical value of 7-13 unit cells (4-11). This critical thickness for sustaining conductivity below T c is attributed to an inherent "dead layer" at the interface between . One origin for the reduced T c and saturation magnetization of thin LSMO films compared to the bulk is believed to be a result of spin canting at the LSMO/STO interface (6, 15). Alternatively, magnetic phase separation into ferromagnetic metallic and less-ord...

Engineered superlattices with crossover from decoupled to synthetic ferromagnetic behavior

“…The metal/ insulator transition temperature, T MI , can be defined as the . t * (LSMO) and t * (LSCO) refer to literature data of the critical thickness below which LSMO and LSCO single layer films display degraded magnetization and conductivity [16,[30][31][32][33].…”

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

“…In our case, the increase of the out-of-plane Mn-O-Mn bond angle with decreasing N is apparent in ( The current finding may provide a new insight to the 'dead' layer issue that are commonly found at the interface between manganite oxides and many perovskite substrates 36 , and has attracted renewed interests. In recent studies, orbital ordering, 37 polar discontinuity 38,39 , magnetic coupling to the titanate layer, 40 stoichiometry 41,42 and so on, have all been evidenced to couple to the interface magnetic property of the manganite layer, while a unified scenario is still lacking. Some recent studies on La 0.67 Sr 0.33 MnO 3 /SrTiO 3 interfaces have merged on the finding of increased amount of Mn 3 þ near the interface [43][44][45] and 3z 2 À r 2 orbital ordering irrespective of the strained state 37,45,46 .…”

Direct real space observation of magneto-electronic inhomogeneity in ultra-thin film La0.5Sr0.5CoO3−δ on SrTiO3(001)

“…This phenomenon manifests itself both in heterostructures, 1,2 where the difficulty of maintaining desired properties at interfaces has implications for oxide devices, 3,4 and in single films in the ultra-thin limit, [5][6][7][8] where it defines the lowest thickness to which functional phenomena such as ferromagnetism can be maintained. Given the numerous possible mechanisms at work (e.g., electronic and orbital reconstructions, 2,5,9 charge transfer, 2,9 and effects related to strain/defects/chemistry [6][7][8], it is important that the origin of this behavior be carefully elucidated in specific systems. Here, we focus on ultra-thin films of the perovskite cobaltite La 1Àx Sr x CoO 3 (LSCO) on SrTiO 3 (001) with low temperature Scanning Tunneling Microscopy (STM).…”