Giant Magnon-Polaron Anomalies in Spin Seebeck Effect in Double Umbrella-Structured ${\mathrm{Tb}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ Films

We report a giant hysteretic spin Seebeck effect (SSE) anomaly with a sign reversal at magnetic fields much stronger than the coercive field in a (001)-oriented ${\mathrm{Tb}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film. The high-field SSE enhancement reaches 4200% at approximately 105 K over its weak-field value and presents a nonmonotonic dependence on temperature. The unexpected high-field hysteresis of SSE is found to be associated with a magnetic transition of double-umbrella spin texture in TbIG. Nearly parallel dispersion curves of magnons and acoustic phonons around this neoteric transition are supported by theoretical calculations, leading to a high density of field-tuned magnon polarons and consequently an extraordinarily large SSE. Our study provides insight into the evolution of magnon dispersions of double-umbrella TbIG and could potentially boost the efficiency of magnon-polarons SSE devices.

Figure 1

(a) Device schematic for LSSE measurements in the $\mathrm{GSGG}(001)/\mathrm{TbIG}(140\mathrm{nm})/\mathrm{Pt}(5\mathrm{nm})$ sample. (b) Temperature dependence of the SSC at ${\mu }_{0}H=0.5\mathrm{T}$. (c) SSC as a function of magnetic field at typical temperatures.

Figure 2

The SSC as a function of the magnetic field along (a) [110] and (b) [111] directions, respectively.

Figure 3

(a) The simulated total magnetization $M$ along the field direction [100] and (b) the orientation angle ${\theta }_{\mathrm{Fe}}$ of the net magnetization from antiparallelly aligned $a$- and $d$-site Fe ions, as a function of the sweeping magnetic field in the [100] direction. (c) Schematic diagram of the zero-field $({\mu }_0{H}_1)$ double umbrella magnetic structure. The magnetic moments of three Tb sublattices, $c_1$, $c_2$, and $c_3$ ($c_1^{\prime }$, $c_2^{\prime }$, and $c_3^{\prime }$), form a cone magnetic structure in green (orange) around the [111] axis with 3-fold rotational symmetry. $c_1$ and $c_1^{\prime }$ are located in the $(0\overline{1}1)$ plane. The net magnetic moment of Fe lies in the [111] direction. (d) and (e) are schematic diagrams of calculated magnetic configurations at ${\mu }_0{H}_2=8.004$ and ${\mu }_0{H}_3=8.006\mathrm{T}$ for upsweeping, i.e., slightly before and after the magnetization jump at transition field, respectively.

Figure 4

The magnon spectra of bulk TbIG in the low-frequency regime ($<3.1\mathrm{THz}$) for the magnetic configurations at $H_1-H_3$ in Fig. 3. The color scale of the magnon dispersion curves indicates the projection of the polarization along the magnetic field. The orange curves represent the dispersion curves of the longitudinal (LA) and transverse (TA) acoustic phonons with sound speeds $c_l=6.08$ and $c_t=3.3\mathrm{km}/\mathrm{s}$ [31], respectively.

Figure 5

(a) SSC as a function of magnetic field at different temperatures around the anomaly region. $H_{C0}$ (indicated by blue arrow) is the position of the single precursory peak anomaly. $H_{C1}$ and $H_{C2}$ (indicated separately by yellow and green arrows) are the transition fields for down-sweeping and up-sweeping, respectively. The temperature dependence of (b) transition fields and (c) enhancement factor of the anomalies.