Researchers from Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and the ALBA Synchrotron have analyzed the microscopic origin of the so-called "magnetic proximity effect" occurring at the interface between a magnetic material (CoFe2O4, in the present case) and a nonmagnetic metal (Pt), which may induce a magnetic moment in the latter. The results, which are of relevance for the understanding of spin currents generated in metallic layers having a large spin orbit coupling are published in ACS Applied Materials & Interfaces. It is shown that, in Pt/CoFe2O4 bilayers, the growth conditions of Pt determine the presence or absence of a magnetic moment in the noble metal and therefore its magnetoresistance is controlled by either pure spin accumulation effects (Spin Hall) or a conventional anisotropic magnetoresistance resulting from interfacial chemical reconstruction. This work highlights the relevance of interfacial effects in spintronic devices based on ferromagnetic insulating oxides.
Physical proximity between neighboring magnetic and non-magnetic layers may induce magnetism in the latter for a number of reasons, ranging from the quantum nature of wave functions to processing effects. These magnetic proximity effects (MPEs) have received a renewed attention in recent years due to novel potential avenues for exploitation including the dissipation-less spintronics. Spin currents can be generated by a flow of charge along the nonmagnetic metal with large spin-orbit coupling. This produces a spin accumulation at the surfaces, controllable by the magnetization of an adjacent ferromagnetic layer (see Fig. 1). Paramagnetic metals typically used are close to ferromagnetic instability and thus MPEs can contribute to the observed angular-dependent magnetoresistance (ADMR). As interface phenomena govern spin conductance across the metal/ferromagnetic-insulator heterostructures, therefore, unraveling these distinct contributions is pivotal for a full understanding of the spin current conductance. Indeed, disentangling the origin of magnetic proximity effect and its control is a challenge that may hamper deep understanding and exploitation of interface-related spin devices.
Figure 1. Sketch of magnetization distribution at neighborhood of interfaces in paramagnetic-heavy metal/FMI heterostructures. (a) and (c) illustrate magnetic proximity effect and intermixing respectively. (b) In presence of charge flow, spin Hall effect promotes spin accumulation at interfaces.
To gain a clearest insight into the interface-related phenomena, in this work, researchers simultaneously grew two epitaxial CoFe2O4 (CFO) films on MgAl2O4(001) substrates under identical PLD growth conditions and subsequently covered them with sputtered Pt (4 nm) layers under different conditions, i.e. one at room temperature (sample RT) and other layer at a high temperature of 400 °C (sample HT). Magnetic measurements indicated that distinct growths produced some modification in the macroscopic magnetization; however, more dramatic changes were observed in their ADMR response. X-ray absorption spectroscopy (XAS) and X-ray absorption and magnetic circular dichroism (XMCD), as the depth-sensitive measurements, demonstrated that the growth conditions of Pt govern not only the appearance of a magnetic moment in the Pt layer, but also a rich electronic and magnetic reconstruction at the CFO interface that governs the magnetic and magnetotransport properties of the bilayer.
High-resolution scanning transmission electron microscopy (STEM) data provide further conclusive evidence of thermally-induced atomic reconstructions and Pt(Co, Fe) alloying at the Pt/CFO interface (Fig. 2). These results demonstrate that the magnetism in Pt does not result from a genuine proximity effect, but, at least partially, by the structural, electronic, and magnetic interface reconstructions. The new magnetic phases formed during HT capping, likely to be a metallic alloy, make an additional contribution to the ADMR that overrules and masks the spin Hall magnetoresistance. The XMCD experiments at (Fe, Co)-L, and Pt-M edges, together with the energy electron loss spectroscopy data, have been instrumental in unraveling the unexpected magnetic reconstructions that may occur at interfaces between Pt and insulating ferromagnetic oxides.
Figure 2. Left panel: (a,c) and (b,d) are Z-contrast STEM images of the Pt/CFO//MAO bilayers where the Pt was deposited at RT and HT, respectively. Right panel: Pt-M3 XAS and XMCD of (a) RT and (b) HT samples measured at 300 K.
Reference: Hari Babu Vasili, Matheus Gamino, Jaume Gàzquez, Florencio Sánchez, Manuel Valvidares, Pierluigi Gargiani, Eric Pellegrin, and Josep Fontcuberta, Magnetoresistance in Hybrid Pt/CoFe2O4 Bilayers Controlled by Competing Spin Accumulation and Interfacial Chemical Reconstruction, ACS Appl. Mater. Interfaces, 2018, 10 (14), pp 12031-12041, DOI: 10.1021/acsami.8b00384.
This work was supported by the Spanish MINECO grants MAT2014-56063-C2-1-R, MAT2017-85232-R, and the Severo Ochoa SEV-2015-0496 Projects and the Generalitat de Catalunya (2014 SGR 734) Project. Authors acknowledge fruitful discussions with F. Casanova. M.G. acknowledges his fellowship from CNPq - Brazil. J.G. acknowledges RyC contract (2012-11709).
Text by Hari Babu Vasili, at the ALBA Synchrotron website.