Strong Interfacial Exchange Field in a Heavy Metal/Ferromagnetic Insulator System Determined by Spin Hall Magnetoresistance

Abstract

Spin-dependent scattering transport at heavy metal (HM)/magnetic insulator (MI) interface can be described in terms of three parameters: the spin-sink conductance Gs and the real and imaginary part of the spin-mixing conductance, G↑↓=Gr+iGi [1,2]. Each parameter is relevant for different spin-dependent phenomena. For instance, Gs originates from spin-flip processes and therefore is the leading parameter in electrical biasing of magnons [3], whereas Gr accounts for the spin-transfer (Slonczewski) torque to the magnetization and plays a fundamental role in spin-pumping experiments [4]. On the other hand, Gi quantifies the interfacial exchange field, which induces a field-like torque in the conduction electrons of the HM and is important for example in spin-splitting field experiments in superconductivity [5]. These conductances are broadly studied in ferrimagnetic insulators, where usually the contribution of Gr is much larger than that of Gi [6], leading to only a few reports on the exchange field at HM/MI interfaces [7]. In this work [8], we study the three spin conductance terms by spin Hall magnetoresistance (SMR) in a new system: a ferromagnetic insulator (FMI) such as EuS. By SMR measurements as a function of the temperature, and taking advantage of the newly developed microscopic theory for SMR [2], we can extract relevant microscopic parameters such as the exchange interaction between the 1s electrons in Pt and the localized magnetic moments in EuS (Jint~18 meV) [8]. An interfacial exchange field of the order of 1 meV (~15 T) acting upon the conduction electrons of Pt can be estimated from Gi, which is at least three times larger than Gr below the Curie temperature [8]. Our work provides an easy method to quantify this interfacial spin-splitting field, which is of interest in different areas of Condensed Matter Physics, such as proximity effects in superconducting hybrid systems.