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Register a new userUltrafast Measurements of the Interfacial Spin Seebeck Effect in Au and Rare-Earth Iron Garnet Bilayers
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We investigate picosecond spin-currents across Au/iron-garnet interfaces in response to ultrafast laser heating of the electrons in the Au film. In the picoseconds after optical heating, interfacial spin currents occur due to an interfacial temperature difference between electrons in the metal and magnons in the insulator. We report measurements of this interfacial longitudinal spin Seebeck effect between Au and rare-earth iron-garnet insulators, i.e. RE$_3$ Fe$_5$O$_{12}$, where RE is Y, Eu, Tb, Tm. We use time domain thermoreflectance (TDTR) measurements to characterize the thermal response of the bilayer to ultrafast optical heating. We use time-resolved magneto-optic Kerr effect (TR-MOKE) measurements of the Au layer to measure the time-evolution of spin accumulation in the Au film. We observe a spin Seebeck effect between Au/TmIG that is three times larger than for an Au/YIG bilayer. The interfacial thermal conductance between electrons in the Au and magnons in the TmIG layer is ~ 3 $frac{MW}{m^2 K}$.
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For longitudinal spin Seebeck effect (LSSE) devices, a multilayer structure comprising ferromagnetic and nonmagnetic layers is expected to improve their thermoelectric power. In this study, we developed the fabrication method for alternately stacked yttrium-iron-garnet (YIG)/Pt multilayer films on a gadolinium gallium garnet (GGG) (110) substrate, GGG/[YIG(49 nm)/Pt(4 nm)]$_n$ ($n =$ 1 - 5) based on room-temperature sputtering and $ex$-$situ$ post-annealing method and we evaluated their structural and LSSE properties. The fabricated [YIG/Pt]$_n$ samples show flat YIG/Pt interfaces and almost identical saturation magnetization $M_{rm s}$, although they contain polycrystalline YIG layers on Pt layers as well as single-crystalline YIG layers on GGG. In the samples, we observed clear LSSE signals and found that the LSSE thermoelectric power factor (PF) increases monotonically with increasing $n$; the PF of the [YIG/Pt]$_5$ sample is enhanced by a factor of $sim 28$ compared to that of [YIG/Pt]$_1$. This work may provide a guideline for developing future multilayerbased LSSE devices.
The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Until recently, most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a non-local spin Seebeck geometry, we provide additional evidence that the total magnon spin current in the ferrimagnetic insulator yttrium iron garnet (YIG) actually contains two distinct terms: one proportional to the gradient in the magnon chemical potential (pure magnon spin diffusion), and a second proportional to the gradient in magnon temperature ($ abla T_m$). We observe two characteristic decay lengths for magnon spin currents in YIG with distinct temperature dependences: a temperature independent decay length of ~ 10 ${mu}$m consistent with earlier measurements of pure ($ abla T_m = 0$) magnon spin diffusion, and a longer decay length ranging from about 20 ${mu}$m around 250 K and exceeding 80 ${mu}$m at 10 K. The coupled spin-heat transport processes are modeled using a finite element method revealing that the longer range magnon spin current is attributable to the intrinsic spin Seebeck effect ($ abla T_m eq 0$), whose length scale increases at lower temperatures in agreement with our experimental data.
Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI) which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7~nm-thick YIG film by measuring the nonreciprocal spin wave propagation in terms of frequency, amplitude and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e. the film normal direction, applied field and spin-wave wavevector. By measuring the asymmetric group velocities, we extract a DMI constant of 16~$mu$J/m$^{2}$ which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultra-low spin-wave damping can be achieved.
We report on the structure, magnetization, magnetic anisotropy, and domain morphology of ultrathin yttrium iron garnet (YIG)/Pt films with thickness ranging from 3 to 90 nm. We find that the saturation magnetization is close to the bulk value in the thickest films and decreases towards low thickness with a strong reduction below 10 nm. We characterize the magnetic anisotropy by measuring the transverse spin Hall magnetoresistance as a function of applied field. Our results reveal strong easy plane anisotropy fields of the order of 50-100 mT, which add to the demagnetizing field, as well as weaker in-plane uniaxial anisotropy ranging from 10 to 100 $mu$T. The in-plane easy axis direction changes with thickness, but presents also significant fluctuations among samples with the same thickness grown on the same substrate. X-ray photoelectron emission microscopy reveals the formation of zigzag magnetic domains in YIG films thicker than 10 nm, which have dimensions larger than several 100 $mu$m and are separated by achiral N{e}el-type domain walls. Smaller domains characterized by interspersed elongated features are found in YIG films thinner than 10 nm.
We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{textrm{4-x}}$ ($4geq x geq 0$) 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 NiFe$_2$O$_4$ to metallic Ni$_{33}$Fe$_{67}$. A proximity-induced ANE could only be identified in the metallic Pt/Ni$_{33}$Fe$_{67}$ bilayer in contrast to Pt/NiFe$_2$O$_{rm x}$ ($x>0$) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.
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