Nonlocal detection of out-of-plane magnetization in a magnetic insulator by thermal spin drag

Can Onur Avci

Perpendicularly magnetized ferrimagnetic insulators (FMI) have been drawing increasing attention in spintronics research. Recent achievements of efficient current-induced switching and domain wall motion [1-5] in perpendicular FMIs, combined with theirhighly tunable propertiesopen novel avenues for practical applications. Despite theFMIs’electrically insulating nature, the spin Hall magnetoresistance (together with its anomalous Hall component) [6] provides us with the relevant tools to detect theirmagnetization vector ina local geometry using a Hall bar device. Later on, it was discovered that thein-plane magnetization vectorofan FMIcan be detectedalsoin a nonlocal geometry by long distance magnon transport[7,8]. However, the detection of the perpendicular magnetization vector of an FMI in a nonlocal geometry remained a challenge for a long time. In this work[9],wedemonstratethat,by usingan engineered temperature gradient,one can detect theout-of-plane magnetization of anFMIby simply measuring the transverse voltage drop across the Ptstrip placed on top. This is due to a conceptually new mechanism that combines the spin currents driven by an out-of-plane(∇#$)and in-plane(∇%$)temperature gradientsin a Pt/FMI bilayer generated by a single nonlocal heat source. When the magnetization has a component oriented perpendicular tothe plane, ∇#$drives a spin current into Pt with out-of-plane polarization due to the spin Seebeck effect. ∇%$then drags the resulting spin-polarized electrons in Pt parallel to the plane against the gradient direction. This finally produces an inverse spin Hall effect voltage in Pt, transverse to ∇%$and proportional to the out-of-plane component of theFMI’smagnetization(see Fig.1). This simple method enables the detection of the perpendicular magnetization component in anFMIin a nonlocal geometryand opens up new routes towardsengineeringtemperature gradients togenerateandmanipulate thermal magnons and pure spin currents.

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