Quantitative Disentanglement of the Spin Seebeck, Proximity-Induced, and Ferromagnetic-Induced Anomalous Nernst Effect in Normal-Metal–Ferromagnet Bilayers

Abstract

We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe2O4-x (4 ≥ x ≥ 0) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe2O4 to metallic Ni33Fe67. A proximity-induced ANE could only be identified in the metallic Pt/Ni33Fe67 bilayer in contrast to Pt/NiFe2Ox (x > 0) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.In the emerging fields of spintronics Pt is employed frequently for generating and detecting pure spin currents, if adjacent to an FMI, although the possibility of magnetic proximity effects (MPEs) has to be taken into account. Due to its close vicinity to the Stoner criterion [11] the FM can potentially generate a Pt spin polarization at the interface. Consequently, this might induce additional parasitic effects preventing the correct interpretation of the measured ISHE voltage. Therefore, a comprehensive investigation regarding the magnetic properties of the NM/FM interface is required to distinguish the contributions of such parasitic voltages from the ISHE voltage generated by a pure spin current.In the case of SSE, the driving force for the spin current in the FM or FMI is a temperature gradient. When a spin current is generated parallel to a temperature gradient, it is generally attributed to the longitudinal spin Seebeck effect (LSSE) [4,5]. However, when using the ISHE in an adjacent NM for the spin current detection, not only a proximity-induced ANE [12] can contaminate the LSSE signal, but also an additional intrinsic ANE contribution could be present in case of studying ferromagnetic metals (FMMs) or semiconducting ferro(i)magnets [13,14]. Mainly NM/FMI bilayers have been investigated, while LSSE studies on NM/FMM are quite rare.However, Ramos et al. [14][15][16][17] and Wu et al. [18] individually investigated the LSSE in magnetite, which is conducting at room temperature (RT) and, thus, has an intrinsic ANE contribution. They identified the LSSE in Pt/Fe 3 O 4 [14] and CoFeB/Fe 3 O 4 bilayers [18] by using temperatures below the conductor-insulator transition of magnetite (Verwey transition at 120 K) in order to exclude any intrinsic ANE contribution. Ramos et al. further investigated the ANE in bulk magnetite without any Pt [15] and concluded that the ANE contributions for Pt/Fe 3 O 4 bilayers and multilayers should be quite small [16,17]. In addition, Lee et al. [19] and Uchida et al. [20,21] discussed that in Pt/FMM multilayers both LSSE and ANE contribute, but did not disentangle the effects quantitatively. Hence, a clear quantitative disentanglement of the LSSE in the FMM [22], the ...

Spincaloritronic Measurements: A Round Robin Comparison of the Longitudinal Spin Seebeck Effect

“…By using this procedure, the ordinary Seebeck effect which does not depend on the magnetic field can be treated as an additive constant and cancelled out. Other important challenges in the experimental research on the SSE are the study of some possible contributions of other magneto-thermoelectric effects, like for example the anomalous Nernst effect [19], [20], [40]- [46] and the geometry of the SSE (longitudinal vs. transverse configuration) [16], [47].…”

“…The electric observable depends on the ohmic contact that is fabricated on the Pt film, at the two opposite edges along the y direction. For the measurement of the temperature difference ∆T , some techniques are reported in the literature, including measurements of heat currents [46], [48]- [50] and measurements of the resistance of two Pt layers deposited on the two surfaces of the magnetic layer [51], [52]. For all the techniques, it is necessary to take into account that the presence of gas around the thermometer in contact with the thermal bath can limit the precision of the temperature measurement; in order to prevent this condition, it is possible to limit the source of noise that is coming from the gas by keeping the system under vacuum.…”

Electrical transport and optical band gap of NiFe2Ox thin films

“…For the investigation of such transport phenomena in conducting or semiconducting materials possible contributions from additional effects should be taken into account. For example, in case of studying the longitudinal spin Seebeck effect (LSSE) in metallic or semiconducting FMs, an additional anomalous Nernst effect (ANE) contribution can contaminate the LSSE signal [6][7][8][9]. Moreover, other spin caloritronic and spintronic effects, such as the recently observed spin Hall magnetoresistance [10,11] i.e., the magnetization orientation dependent absorption and reflection of a spin current density flowing along the direction normal to a FM insulator/normal metal (NM) interface, can benefit from the lack of mobile charge carriers in insulating materials, such as yttrium iron garnet (YIG).…”

“…A major advantage of NFO compared to other spinel ferrites is attributed to the manipulation of the parasitic effects like the ANE by changing the base temperature of the measurement, due to its semiconducting character. So far, several investigations have been performed on FM/NM bilayers such as NFO/Pt in order to study the transport phenomena of those systems [7,8,[21][22][23][24][25] providing information about the spin Seebeck effect coefficient, the proximity induced magnetization in the Pt layer and the magnon spin transport in NFO. More specifically, the investigation of the temperature dependent electrical resistivity of the films leads to a deeper insight of the localization of electronic states and the disorder of such systems which influences the transport phenomena in those materials.…”

“…Except from the electrical properties of the films we additionally analyzed the optical properties by determining the optical band gap of the materials verifying their semiconducting origin which enables a profound understanding of the transport phenomena in those films [7]. Our results reveal states within the band gap which display charge transfer, creating a framework for realizing the electronic structure of those complex oxides that promotes their use in spintronics applications.…”