Room temperature and low-field resonant enhancement of spin Seebeck effect in partially compensated magnets

Abstract

Resonant enhancement of spin Seebeck effect (SSE) due to phonons was recently discovered in YFeO (YIG). This effect is explained by hybridization between the magnon and phonon dispersions. However, this effect was observed at low temperatures and high magnetic fields, limiting the scope for applications. Here we report observation of phonon-resonant enhancement of SSE at room temperature and low magnetic field. We observe in LuBiFeGaO an enhancement 700% greater than that in a YIG film and at very low magnetic fields around 10 T, almost one order of magnitude lower than that of YIG. The result can be explained by the change in the magnon dispersion induced by magnetic compensation due to the presence of non-magnetic ion substitutions. Our study provides a way to tune the magnon response in a crystal by chemical doping, with potential applications for spintronic devices.

Magnon polarons in the spin Peltier effect

“…Importantly, the peak appears for the field H TA at which the magnon dispersion curve of BiGa:LuIG touches the TA-phonon dispersion curve, as with the magnonpolaron anomaly observed in the SSE measurement [29]. For BiGa:LuIG, the acoustic magnon dispersion reads…”

“…[55] According to our previous SSE study [29], the peak intensity induced by magnon−LA-phonon hybridization increases with decreasing T , so that we carried out the experiment at T = 100 K to well resolve the magnonpolaron features in the SPE.…”

“…Magnetoelastic coupling (MEC), the interaction between spin waves (magnons) and lattice waves (phonons), was first investigated more than half a century ago [1][2][3] and has renewed attention in spintronics . By the MEC, magnons and phonons, in the vicinity of the crossings of their dispersion relations, are hybridized into quasiparticles called magnon polarons that share mixed magnonic and phononic characters [6,8,[10][11][12][20][21][22][23][24][25][26][27][28][29][30]. Magnon polarons can convey spin information with velocities close to those of phonons, much faster than the magnon velocities in the dipolar magnon regime [7,8,14,15].…”

“…When measuring the SPE electrically by a thermocouple, the detected voltages are ∼ 10 2−3 times smaller than the typical SSE electric voltages measured via the ISHE [31,34,38]. Here, to overcome this issue, we use a Bi-and Ga-substituted lutetium iron garnet Lu 2 Bi 1 Fe 4 Ga 1 O 12 (BiGa:LuIG) film that exhibits large magnon-polaron peaks in SSE voltages [eight times greater than conventional Y 3 Fe 5 O 12 (YIG) films at room temperature, mainly due to its reduced magnon lifetime compared to YIG] [29]. By careful measurements, magnon polarons turned out to manifest as peaks in magnetic field-dependent SPE signals, as with the SSE.…”

Bi-reflection of spin waves

“…Just like a light wave, a spin wave refracts and reflects in a magnetic material [8][9][10] . Although a pure spin wave does not carry a polarization degree of freedom, it can hybridize with an elastic wave via the magneto-elastic coupling, and inherit a mode degree of freedom from an elastic wave, that is, longitudinal and transverse modes, when the frequency and wavelength of spin waves are matched with that of elastic waves [11][12][13][14][15][16][17][18] . For the hybridized waves, the two propagation modes can convert to each other when translational symmetry is broken down, because the two modes are no longer eigenstates 19,20 .…”