CONTROL OF MAGNETORESISTANCE IN SPIN VALVES BASED ON NOVEL LANTHANIDE QUINOLINE MOLECULES

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A team of researchers from CIC Nanogune, Instituto de Ciencia Molecular (ICMol) and ALBA have developed a pathway to control the magnetoresistance of spin valve devices based on novel lanthanide quinolone molecules. The results, published in the journal of Advanced Functional Materials, demonstrate the interface-assisted control of the sign and magnitude of magnetoresistance and highlight the importance of the interfacial molecule–metal hibridization to engineer spin-dependent macroscopic parameters in spintronic devices.

Cerdanyola del Vallès, 16th January 2018 -  Molecules, due to their wide-ranging chemical functionalities that can be tailored on demand, are becoming increasingly attractive components for applications in materials science and solid-state physics. Remarkable progress has been made in the fields of molecular-based electronics and optoelectronics, with devices such as organic field-effect transistors and light emitting diodes. As for spintronics, a nascent field which aims to use the spin of the electron for information processing, molecules are proposed to be an efficient medium to host spin-polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin. Spin-polarized hybrid states induced by the interaction of the first molecular monolayers on ferromagnetic substrates are expected to govern the spin polarization at the molecule–metal interface, leading to changes in the sign and magnitude of the magnetoresistance in spin-valve devices. The formation of spin-polarized hybrid states has been determined by spin-polarized spectroscopy methods and principle-proven in nanosized molecular junctions, but not yet verified and implemented in large area functional device architectures.

The experimental results by Amilcar Bedoya-Pinto and co-workers at the CIC nanoGUNE and ICMol groups correspondingly lead by Professors L. Hueso and E. Coronado, report the fabrication of hybrid spin valve devices incorporating novel lanthanide quinoline molecules (Fig.1). The study of their magnetic and transport properties was combined with high-sensitivity multi-element specific X-ray spectroscopy (Fig.2), to evidence the key interfacial molecule-metal hybridization states possible at the BOREAS beamline of ALBA.

The results of the study show the ability of molecules to modify spin-dependent properties at the interface level via metal–molecule hybridization pathways. In brief detail, they demonstrate the control of the magnetoresistance sign in a fully functional molecular spin-valve system based on NaDyClq, a novel mononuclear quinoline molecule, combined with Co and NiFe as spin injector and detector electrodes. Contrary to other Co/AlOx/molecule/NiFe spin valve systems, the Co/AlOx/NaDyClq/NiFe devices exhibit negative magnetoresistance, evidence of a negative spin polarization at the NaDyClq/NiFe interface. Inverting the spin valve stack order –iFe/AlOx/NaDyClq/Co – leads to positive magnetoresistance, which points out to a positive spin polarization at the NaDyClq/Co interface. This change in spin polarization at the molecular/ferromagnetic interface is linked to the formation of specific hybrid electronic states, as confirmed by X-ray absorption spectroscopy. The formation of hybrid states determines the spin polarization at the relevant spin valve interfaces, allowing the control of macroscopic device parameters such as the sign and magnitude of the magnetoresistance. Thus, these results consolidate the application of the spinterface concept in a fully functional device platform as a pathway to deterministically engineer spin-dependent properties in spintronic devices.

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Figure 1: Schematic view of the Lanthanide quinolone molecule, NaDyClq (left) and the spin-valve device structure (right)

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Figure 2: Magnetoresistance (top) and X-ray spectroscopy (bottom) measurements, evidencing the control of the magnetoresistance sign and amplitude by engineering spin valves with NaDyClq/NiFe and NaDyClq/Co interfaces, and their corresponding interfacial molecule-metal hybridization states.


Reference: A. Bedoya-Pinto, S. G. Miralles, S. Vélez, A. Atxabal, P. Gargiani, M. Valvidares, F. Casanova,  E. Coronado and L. E. Hueso Adv. Funct. Mater. 2017, https://doi.org/10.1002/adfm.201702099


Acknowledgements: A.B.-P. and S.G.M. contributed equally to this work. The authors thank Roger Llopis for technical support at CIC nanoGUNE, as well as Gunnar Ohrwall from the beamline I1011 in Max Lab, Lund, for support in the acquisition of preliminary X-ray absorption data. Financial support from the EU (COST Action in Molecular Spintronics 15128, Projects HINTS FP7-263104, Spintros ERC Starting Grant ERC-2009-257654, and SpinMol ERC Advanced Grant ERC-2009-AdG-20090325), the Spanish Ministerio de Economía y Competitividad (Unit of Excellence Maria de Maeztu MDM-2015-0538 and MDM-2016-0618 as well as projects with FEDER cofinancing MAT2011-22785 MAT2012-37638, FIS2013-45469-C4-3-R, MAT2014-56143-R, and MAT2015-65159-R), Generalitat Valenciana (PROMETEO programme). S.G.M. thanks the Spanish MINECO for an FPI predoctoral grant. The authors acknowledge beamtime access at ALBA BL29 via proposal 2015-021263.

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