Quantifying the critical thickness of electron hybridization in spintronics materials

ARTICLE Received 17 Nov 2016 | Accepted 15 May 2017 | Published 17 Jul 2017 DOI: 10.1038/ncomms16051 OPEN Quantifying the critical thickness of electron hybridization in spintronics materials T. Pincelli1,2,*, V. Lollobrigida1,3,*, F. Borgatti4, A. Regoutz5, B. Gobaut6, C. Schlueter7, T.-L. Lee7, D.J. Payne5, M. Oura8, K. Tamasaku8, A.Y. Petrov1, P. Graziosi4, F. Miletto Granozio9,10, M. Cavallini4, G. Vinai1, R. Ciprian1, C.H. Back11, G. Rossi1,2, M. Taguchi8,12, H. Daimon12, G. van der Laan7 & G. Panaccione1 In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depthdependent analysis of two paradigmatic systems, namely La1  xSrxMnO3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La1  xSrxMnO3, for fully restoring bulk properties. 1 Istituto Officina dei Materiali-CNR, Laboratorio TASC, Area Science Park, S.S. 14, Km 163.5, Trieste I-34149, Italy. 2 Dipartimento di Fisica, Università di Milano, Via Celoria 16, Milano I-20133, Italy. 3 Dipartimento di Scienze, Università degli Studi Roma Tre, Via della Vasca Navale 84, Roma I-00146, Italy. 4 Consiglio Nazionale delle Ricerche—Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), via P. Gobetti 101, Bologna I-40129, Italy. 5 Department of Materials, Imperial College London, South Kensington, London SW7 2AZ, UK. 6 Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, Area Science Park, Trieste 34149, Italy. 7 Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK. 8 RIKEN SPring-8 Center, Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan. 9 CNR-SPIN, Complesso Universitario Monte S. Angelo, Napoli 80126, Italy. 10 Dipartimento di Fisica, Università ‘Federico II’ di Napoli, Napoli, 80126, Italy. 11 Institut fur Experimentelle Physik, Universitat Regensburg, Regensburg D-93040, Germany. 12 Nara Institute of Science and Technology, 8-9165 Takayama, Ikoma, Nara 630-0192, Japan. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to G.P. (email: ). NATURE COMMUNICATIONS | 8:16051 | DOI: 10.1038/ncomms16051 | www.nature.com/naturecommunications 1 ARTICLE T NATURE COMMUNICATIONS | DOI: 10.1038/ncomms16051 he effectiveness of electron hybridization in solids and its competition with Coulomb interactions plays a fundamental role in novel physical phenomena, often termed as quantum properties1,2. In the context of spintronics, magnetic and electronic reconstructions at interfaces have been often reported, with their origin lying in the delicate interplay between charge, spin and orbital degrees of freedom1–5. Looking at the strength of electron hybridization and localization, near a surface or interface the reduced translational symmetry breaks or severely alters the electronic properties with important consequences for, for example, the magnetic order parameter, transition temperature and metallic/insulator character, thus potentially limiting the achievement of the desired performance in interfacebased devices6–8. Moreover, surface- and defect states play critical roles in mediating ferromagnetism, due to the modified chemistry of the first top layers. Prototypical spintronics systems displaying such effects are the rare-earth-doped manganites, in particular metallic La1  xSrxMnO3 (LSMO), and the most representative diluted magnetic semiconductor, (Ga,Mn)As. In both systems, the relationship between electronic reconstruction and magnetic properties and the competition between electron localization and hybridization are relevant ingredients in determining their Curie temperature (TC) and ferromagnetic state5–12. In LSMO, the mechanism and the reason for the modified electronic properties of the surface region are still open questions; in (Ga,Mn)As a carrier depletion zone up to 1 nm has been found in the vicinity of the surface, with modified ferromagnetic order11. Moreover, a remarkable example of altered electronic properties has been reported in the so-called magnetic ‘dead layer’ at the surface of otherwise ferromagnetic bulk systems13–17. To date, bulk sensitive techniques, exploiting the combination of aberration-corrected transmission electron microscopy and electron energy loss spectroscopy was recently able to quantify the role of the charge-transfer screening length at the interface LSMO/PZT (lead zirconate titanate)17, and revealed interfacial electronic reconstruction and a change in TC near the metal–insulator transition in both LSMO/STO and (Ga,Mn)As/GaAs (refs 11,18). Furthermore, surface-sensitive tools, such as angular resolved photoemission spectroscopy (PES) and scanning probes, gave clear indications of a negligible coherent spectral weight at the Fermi level in bilayer LSMO crystals, with a more fragile metallic and magnetic character at the surface than in the bulk16,19. Although general agreement has been reached on the observation that both metallicity and ferromagnetism of these systems are reduced at the surface, the determination of the crossover between surface and bulk properties, that is, what the ‘critical’ thickness of such an effect is, and whether the crossover is smooth or abrupt, needs a more complete, and preferably quantitative, description, with particular attention to the modification of the bulk electronic properties when approaching the surface. Here we report results obtained on thin films of metallic LSMO (with x ¼ 0.33 and x ¼ 0.35) and of (Ga,Mn)As (with Mn doping between 8 and 13%) using core-level X-ray PES. The large tuneability of the photon energy offered by synchrotron radiation is exploited to significantly vary the information depth from the surface region (o10 Å, corresponding to a few atomic layers) down to the bulk (4100 Å; refs 20,21). We provide direct and quantitative information of the evolution from metallic (bulk) to insulating (surface) character of these materials, together with a clear indication of the behaviour of hybridization/localization of the bulk electronic states upon both doping and depth. Core-level photoemission in the hard X-ray regime (hard X-ray photoelectron spectroscopy (HAXPES), with hv42 keV) supported by (...truncated)