Competing interactions at the interface between ferromagnetic oxides revealed by spin-polarized neutron reflectometry

Abstract

We have investigated the magnetization profiles in superlattices composed of the two ferromagnets La 0.7 Sr 0.3 MnO 3 and SrRuO 3 using spin-polarized neutron reflectometry. In combination with magnetometry, the neutron data indicate a noncollinear spin configuration where orientation of the Ru moments changes from in plane at the interface to out of plane deep inside the SrRuO 3 layers. The spin structure originates in a competition between antiferromagnetic exchange interactions of Mn and Ru moments across the interface, and the magnetocrystalline anisotropy of the Ru moments, and it is closely related to the "exchange spring" structures previously observed in multilayers composed of ferromagnetic elements and alloys.The magnetotransport properties of metallic ferromagnetic oxides such as La 0.7 (Sr,Ca) 0.3 MnO 3 and SrRuO 3 have been the subject of a large-scale research effort. [1][2][3][4][5] Recent advances in the synthesis of oxide-based heterostructures have now opened up perspectives for the controlled manipulation of the magnetic properties of these compounds, and for the transfer of ferromagnetic spin polarization into other materials. Ferromagnetic oxides have thus been incorporated in heterostructures with a large variety of materials including high-temperature superconductors, 6-8 multiferroics, 9,10 and carbon nanotubes.11 The electronic properties of such heterostructures are determined by the complex interplay between a multitude of parameters including the charge-carrier density, spin and orbital polarization, lattice distortions, and disorder, all of which are in general different from those in the constituent bulk materials. Since these parameters are difficult to probe in an interface-specific manner, current research efforts aimed at a quantitative description of the interfacial electronic properties require simple model systems in which the influence of individual parameters can be determined separately.Recent experiments have identified epitaxial superlattices (SLs) composed of the two ferromagnets La 0.7 Sr 0. 14,15 Both materials share the same pseudocubic lattice structure, and they are metallic and orbitally degenerate at all temperatures. One therefore does not expect modulations of the charge-carrier density or interfacial orbital or lattice polarization to play major roles. Magnetometric measurements on LSMO/SRO SLs with 2-nm-thick LSMO and 5-nm-thick SRO layers (henceforth SL2/5) revealed an isotropically reduced net magnetization for T < T SRO C , suggesting antiferromagnetic (AFM) interlayer exchange coupling (Fig. 1). The out-of-plane magnetization at low temperatures increases with increasing thickness of the SRO layers. Data on SL2/8 samples showed a suppressed magnetization for magnetic fields applied in the SL plane only, whereas the magnetization in a perpendicular field increases upon cooling below T SRO C . The thickness-dependent magnetic properties have been interpreted as evidence of competing magnetic interactions at the interface. [14][15][16] Motivated by these res...

Properties of manganite/ruthenate superlattices with ultrathin layers

“…However, the magnetization direction in superlattices with thinner SRO layers need not be homogeneous. Indeed, spin polarized neutron reflectometry measurements on 5/12 and 5/20 LSMO/SRO superlattices show that the SRO magnetic moments close to the interface and the LSMO magnetic moments lie in the superlattice plane, whereas the SRO magnetic moments in the cores of the SRO layers are rotated toward the out-of-plane direction [41]. A similar conclusion was reached by Solignac et al [42] on LSMO/SRO bilayers by magnetometry and polarized neutron reflectometry measurements.…”

Spin transport in epitaxial magnetic manganite/ruthenate heterostructures with an LaMnO3 layer

“…According to fitting, the magnetization in the LSMO layer at T = 200 K is 4πM LSMO = 5.00 ± 0.03 kG, which corre sponds to 2.7μ B per manganese atom. Note that the values of SLD of the oxide layers and the magnetic moment of manganese atoms agree with the data in [17,21]. When the temperature decreases below 150 K, the spin asymmetry oscillation amplitude increases strongly near Q = 0.2 nm -1 (Fig.…”

“…The main assumption in the theoreti 10 L, μm cal calculations was the fact that LMO was an antifer romagnetic insulator. However, ferromagnetism with a Curie temperature of about 150 K appears in real epi taxial LMO films [13,21]. As a result, all three layers in the LSMO/LMO/SRO structure turn out to be fer romagnetic.…”

Polarization screening-induced magnetic phase gradients at complex oxide interfaces

“…Intuitively, one would expect that the transition from FM metallic to paramagnetic insulating phases at the top LSMO interface would significantly reduce room temperature magnetization. To probe this, we use polarized neutron reflectometry (PNR), a scattering technique that allows us to measure the depth-resolved magnetic structure and is used to isolate individual magnetic layers in a thin-film heterostructure 16,[51][52][53][54] . Figure 6 shows the result of PNR measurements conducted at 298 K with a 1 T field applied along the substrate [100] direction.…”