Quantifying The Longitudinal Spin Seebeck Effect Of Fe

Spintronics is a multidisciplinary field whose central theme is the manipulation of spin degrees of freedom in solid-state system,and it has attracted a lot of attention in recent years.The advantages of these new devices would be nonvolatility,increased data processing speed,decreased electric power consumption,and increased integration densities compared with conventional semiconductor devices.Major challenges in this field of spintronics that are addressed by experiment and theory include the optimization of electron spin lifetimes,the detection of spin coherence in nanoscale structures,transport of spin-polarized carriers across relevant length scales and heterointerfaces,and the manipulation of both electron and spins on sufficiently fast time scales.In recent years,motivated by newly discovered physical effects and strategies to improve existing thermoelectric devices,a new field termed as spin caloritronics which focuses on the interaction of spins with heat currents emerges.The spin Seebeck effect is an important method to generate spin current in this field.It describes the generation of pure spin current from temperature gradient.SSE can be measured with two geometries,i.e.,the transverse SSE(TSSE)and the longitudinal SSE(LSSE).Due to the different thermal conductivities of substrate and thin film,it is very challenging to established a well-defined in-plane temperature gradient.Therefore,there is still debate about the existence of TSSE.On the other hand,LSSE don't have this obstacle.However,in this configuration,anomalous Nerst effect(ANE)in ferromagnetic metal could potentially contaminate LSSE because it has exactly the same symmetry as LSSE.This makes them indistinguishable with each other.Therefore,most studies of LSSE are limited to ferromagnetic insulators where the ANE of the ferromagnet is absent.Despite the fact that the ferromagnetic metals are commonly used in current spintronic devices,the detection and study of LSSE in them,however,are still rare.Until very recently,only a few works begin to discuss the LSSE in ferromagnetic metals.For instance,Kannan et.al.,studied the LSSE in permalloy with zero ANE by tuning the composition,however,the quantitative information was not given.Holanda et.al.,studied the thermal signals in Py/NiO/Pt and Py/NiO/Ta trilayer structures,where NiO acts as the spin current channel and prevents the flow of electron current.The authors defined the SSE coefficient as the ratio between charge current in Pt layer with respect to the temperature gradient.One disadvantage of using this definition is that it varies with the property of FM-NM interface and NM material.In this paper,we report the quantitative study of the LSSE of Fe thin film utilizing Pt as a detecting layer.With varying the Fe thickness,we find that the Anomalous Nernst effect(ANE)of Fe changes sign and vanishes at?4.8 nm.This provides a unique opportunity to study the LSSE where the influence of ANE is absent.The observed opposite sign of the thermal voltages by inverse spin Hall effect in Pt/Fe and W/Fe bilayer structures also confirms that the signal is dominated by LSSE since Pt and W have opposite sign in spin Hall angle.With the effective spin-mixing conductance characterized by ferromagnetic resonance on Fe and Fe/Pt films,we further quantify the spin Seebeck coefficient of Fe.