Abstract
The temperature-dependent longitudinal spin Seebeck effect (LSSE) in heavy metal (YIG) hybrid structures is investigated as a function of YIG film thickness, magnetic field strength, and different HM detection materials. The LSSE signal shows a large enhancement with reductions in temperature, leading to a pronounced peak at low temperatures. We find that the LSSE peak temperature strongly depends on the film thickness as well as on the magnetic field. Our result can be well explained in the framework of magnon-driven LSSE by taking into account the temperature-dependent effective propagation length of thermally excited magnons in the bulk of the material. We further demonstrate that the LSSE peak is significantly shifted by changing the interface coupling to an adjacent detection layer, revealing a more complex behavior beyond the currently discussed bulk effect. By direct microscopic imaging of the interface, we correlate the observed temperature dependence with the interface structure between the YIG and the adjacent metal layer. Our results highlight the role of interface effects on the temperature-dependent LSSE in HM/YIG system, suggesting that the temperature-dependent spin current transparency strikingly relies on the interface conditions.
- Received 28 August 2015
DOI:https://doi.org/10.1103/PhysRevX.6.031012
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Popular Summary
The spin Seebeck effect—in which a voltage is generated by a thermal gradient across a magnetic material—exhibits a temperature dependence in magnetically insulating (YIG) films capped with heavy metals. Here, we explain the origin of the spin Seebeck effect by considering a combination of bulk and interfacial properties. Our key discovery is that the widely accepted phonon-magnon coupling mechanism in the bulk YIG is not dominant and cannot explain the low-temperature enhancement of the spin Seebeck effect signal. Instead, we show that previously neglected effects at the interface between YIG and the capping layer play a key role.
We experimentally investigate YIG films with thicknesses ranging from 150 nm to . By combining full electrical characterization with transmission electron microscopy, we show that phonon-magnon interactions do not significantly modulate the temperature dependence of the spin Seebeck effect. We show that the peak temperature depends on the film thickness and that higher peak temperatures are associated with thinner films. Our results complete the picture of the spin Seebeck effect by showing that it is a combination of the bulk spin current and interfacial effects that govern the size of the measured signal. This finding allows one to, for instance, optimize the signal for a device operating at a given temperature.
We expect that our findings will pave the way to engineering the spin Seebeck effect for applications in which a large signal is necessary, such as energy harvesting.