Exchange bias and magnetic structure in modulation-doped manganite superlattices

Abstract

Superlattices of antiferromagnetic (AF) La1∕3Ca2∕3MnO3 and ferromagnetic (F) La2∕3Ca1∕3MnO3 were grown using sputtering technique. The AF layer thickness ranged between 3.9nm⩽tAF⩽15.6nm while the F layer thickness was kept constant (7.8 nm). Hysteresis measurements after field cooling, show an exchange bias up to a temperature of ∼100K. Surprisingly, the saturation magnetization increases with antiferromagnetic layer thickness and can exceed the expected bulk value for the La2∕3Ca1∕3MnO3 composition, whereas the interface coupling energy between the AF and F layers derived from the exchange bias is constant. This suggests that magnetization depth profile extends beyond the chemical doping interface.

Magnetic depth profile of a modulation-dopedLa1−xCaxMnO3exchange-biased system

“…It should be noted that the presence of a constant ferromagnetic region in the first antiferromagnetic layer is consistent with the experimental observations in Ref. 16, where the normalized saturation magnetization increased with antiferromagnetic layer thickness. The samples used in Ref.…”

“…6 Some of the more thoroughly studied magnetic heterostructures are exchange-bias systems, 11,12 where the coupling between an antiferromagnet and a ferromagnet results in an asymmetric hysteresis loop shift. Mixed valence manganite heterostructures with modulated composition are a particularly interesting realization of exchange-bias systems, [13][14][15][16] since the magnetic order in these materials can be tuned via cation doping, which shifts the balance from antiferromagnetic superexchange to ferromagnetic double exchange coupling. 17 Thus in an exchange-bias system consisting of differently doped antiferromagnetic and ferromagnetic manganite layers all the spins occupy the same lattice sites, but only their interaction is varied by local doping.…”

“…In fact, recent experiments in modulation-doped La 1−x Ca x MnO 3 superlattices ͑x =1/ 3 and 2/3 for ferromagnetic and antiferromagnetic ordering, respectively͒ showed that the saturation magnetization of these superlattices depends on the thickness of the antiferromagnetic layer, and, if normalized by the nominal thickness of the ferromagnetic layer, the saturation magnetization can even exceed bulk values for La 2/3 Ca 1/3 MnO 3 . 16 This suggests that the ferromagnetic magnetization may extend beyond the chemical doping interface into the antiferromagnetic layer. In order to test this hypothesis we performed polarized neutron reflectometry on a La 2/3 Ca 1/3 MnO 3 / La 1/3 Ca 2/3 MnO 3 superlattice and found that the magnetization profile is in general commensurate with the doping profile, but surprisingly an additional layer with net ferromagnetic magnetization is formed at the interface between the first nominally antiferromagnetic La 1/3 Ca 2/3 MnO 3 layer with the SrTiO 3 substrate.…”

“…[24][25][26] First, we considered a homogeneous net moment in the antiferromagnet, since this would naturally explain the increase in the saturation magnetization with increasing antiferromagnetic layer thickness as was observed in Ref. 16. It is evident from Fig.…”

Detailed magnetic and structural properties of exchange‐biased La1‐xCaxMnO3

“…Since this manganite F/AF superlattice is an epitaxial system, the interfacial coupling should be identical for the different samples and it should be independent of individual layer thicknesses. However, we observed in our samples that saturation magnetization shows strong dependence on antiferromagnetic layer thickness [13]. Thus, although this EB system has a simple chemical structure in comparison to most other EB systems, there are additional interactions due to the complex electronic structure at the interface.…”