Measuring magnetic profiles at manganite surfaces with monolayer resolution

Abstract

The performance of manganite-based magnetic tunnel junctions (MTJs) has suffered from reduced magnetization present at the junction interfaces that is ultimately responsible for the spin polarization of injected currents; this behavior has been attributed to a magnetic "dead layer" that typically extends a few unit cells into the manganite. X-ray magnetic scattering in resonant conditions (XRMS) is one of the most innovative and effective techniques to extract surface or interfacial magnetization profiles with subnanometer resolution, and has only recently been applied to oxide heterostructures. Here we present our approach to characterizing the surface and interfacial magnetization of such heterostructures using the XRMS technique, conducted at the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out in specular reflectivity geometry, switching the left/right elliptical polarization of light as well the magnetization direction in the scattering plane. Spectra were collected across the Mn L 2,3 edge for at least four different grazing angles in order to better analyse the interference phenomena. The resulting reflectivity spectra have been carefully fit to obtain the magnetization profiles, minimizing the number of free parameters as much as possible. Optical constants of the samples (real and imaginary part of the refractive index) in the interested frequency range are obtained through absorption measurements in two magnetization states and subsequent Kramers-Kronig transformation, allowing quantitative fits of the magnetization profile at different temperatures. We apply this method to the study of air-

The influence of surface roughness in X-ray resonant magnetic reflectivity experiments

“…XRMR signal as a function of photon energy is then collected at different grazing angles. As has been shown previously, oscillation in the sign of the XRMR spectra for different angle indicates inhomogeneity in the distribution of magnetic moments as a function of depth [5,6,10] that, for these materials, can be ascribed to the presence of dead layers at the interfaces. The period of these oscillations gives a measure of the dead layer thickness [5,6,11], and are usually discussed assuming an atomically flat surface and step-function like dead layer profile [5].…”

“…Because the variation of the magnetic moment due to the breaking of crystalline symmetry at surface or interface extends for a depth of few unit cells (a unit cell in perovskite oxides typically has a lattice parameter ∼4Å), for a spectroscopic investigation of the magnetism, a probe with a wavelength comparable to this dimension and sensitive to atomic magnetic moments is needed. The two most common techniques for measuring magnetic a e-mail: verna@tasc.infm.it depth profiles on the unit-cell scale are reflectivity measurements by polarized neutrons [3,4] and X-rays in resonance condition with the magnetic species [5][6][7][8][9][10]. The advantage of X-rays produced e.g.…”

Molecular orientation in soft matter thin films studied by resonant soft x-ray reflectivity

“…It is this unique depth profiling capability on the molecular length scale that distinguishes it from other established experimental methods such as nonlinear optical techniques. Analogous resonant X-ray reflectivity techniques that employ the contrast of magnetic circular dichroism are successfully used to study magnetization profiles in thin films 8,9 . Stoichiometry changes and distortions in the near surface region of strontium titanate were found by self-consistent modeling of X-ray reflectivity data in s-and p-incidence 10 .…”

A persistent metal–insulator transition at the surface of an oxygen-deficient, epitaxial manganite film

“…Complicating our understanding of the electronic properties of catalytically relevant complex oxide surfaces is the fact that at their surfaces and interfaces the broken translational symmetry fundamentally changes the properties 9,10 . While this has been shown to produce remarkable physics at interfaces [11][12][13] , at surfaces of three-dimensional (3D) perovskites the results are qualitatively different: many fully oxygenated 3D perovskite surfaces and interfaces exhibit a so-called nanoscale "dead layer" where the magnetic order and metal-insulator transitions that from the bulk are strongly reduced or even absent [14][15][16][17][18][19] . In first order, this can be explained by the decreased coordination at the surface that decreases the electronic band width, increases electronic correlations, and creates insulating phases.…”

“…Finally, the surface chemistry and surface defect concentration will affect the physical properties through their impact on the oxidation state of the transition metal cations. These changes thus not only influence the electronic and magnetic contact properties in heterostructure devices 16,17 , but most importantly will affect the surface chemical or catalytic characteristics as well. Moreover, the absence of good cleaving properties in 3D perovskites forces one to have in situ experimental surface analysis techniques in order to measure the properties of pristine surfaces and prevent further complications by contamination from exposure to the ambient atmosphere 22 .…”