Unconventional Spin Pumping and Magnetic Damping in an Insulating Compensated Ferrimagnet

Abstract

Recently, the interest in spin pumping (SP) has escalated from ferromagnets into antiferromagnetic systems, potentially enabling fundamental physics and magnonic applications. Compensated ferrimagnets are considered alternative platforms for bridging ferro‐ and antiferromagnets, but their SP and the associated magnetic damping have been largely overlooked so far despite their seminal importance for magnonics. Herein, an unconventional SP together with magnetic damping in an insulating compensated ferrimagnet Gd3Fe5O12 (GdIG) is reported. Remarkably, the divergence of the nonlocal effective magnetic damping induced by SP close to the compensation temperature in GdIG/Cu/Pt heterostructures is identified unambiguously. Furthermore, the coherent and incoherent spin currents, generated by SP and the spin Seebeck effect, respectively, undergo a distinct direction change with the variation of temperature. The physical mechanisms underlying these observations are self‐consistently clarified by the ferrimagnetic counterpart of SP and the handedness‐related spin‐wave spectra. The findings broaden the conventional paradigm of the ferromagnetic SP model and open new opportunities for exploring the ferrimagnetic magnonic devices.

Temperature Evolution of Magnon Propagation Length in Tm3Fe5O12 Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping

“…In compensated ferrimagnetic insulators, e.g., GdIG, α increases drastically close to the magnetic compensation temperature. 32 However, most of the earlier reports indicate that TmIG films do not show magnetic compensation in the temperature range between 1.5 and 300 K. 47,48 Since our TmIG films also do not show magnetic compensation in the measured temperature range, the increased value of α TmIG at low temperatures in our TmIG films has a different origin. Sizeable increases in α and ΔH at low temperatures were also observed in YIG and different REIGs 92,97−99 including TmIG, 46 which was primarily attributed to Fe 2+ and/or RE 3+ impurity relaxation mechanisms.…”

“…Therefore, according to the expression , ξ = a 0 2 α J e x 2 K e f f ∝ 1 α false( H normalK normale normalf normalf false) 1 / 2 the observed decrease in ⟨ξ⟩ and hence the V LSSE signal at low temperatures, especially below 200 K in our TmIG films, has contributions from the temperature evolutions of both H K eff and α TmIG . The roles of magnetic anisotropy and damping in LSSE in different REIG-based MI/HM bilayers have been explored by different groups. ,,, All these studies indicated that the LSSE signal strength varies inversely with both magnetic anisotropy and damping. In this manuscript, we have not only highlighted the roles of the temperature evolutions of both magnetic anisotropy and damping in controlling the temperature dependent LSSE effect in TmIG/Pt bilayers but also attempted to establish possible correlations between ⟨ξ⟩, H K eff , and α.…”

“…29−31 Hariharan's group has explored the roles of bulk and surface magnetic anisotropies in LSSE in different REIG-based MI/ HM bilayers, 12,13 whereas a recent study highlighted the influence of damping on SPE and LSSE in a compensated ferrimagnetic insulator. 32 It has also been demonstrated that the LSSE in YIG/Pt bilayers varies inversely with intrinsic Gilbert damping of the YIG films; however, the LSSE coefficient does not show any significant correlation with the enhanced damping due to SPE in YIG/Pt bilayers. 33 All these studies highlight the important roles of both magnetic anisotropy and Gilbert damping in thermally generated magnon propagation in MI/HM bilayers.…”

“…Although YIG is considered as a benchmark system for LSSE, , there are only a limited number of studies that explore temperature-dependent LSSE in other iron garnets. ,,, For example, Gd 3 Fe 5 O 12 (GdIG), which is a ferrimagnetic insulator with magnetic compensation temperature ( T Comp ) close to room temperature, shows a sign inversion in the LSSE voltage as well as in the spin-Hall anomalous Hall effect around its magnetic compensation. However, the Gilbert damping in GdIG diverges over a broad temperature range around its T Comp , which makes it difficult to probe the temperature evolution of α and its contribution toward ⟨ξ⟩ over a wide temperature range around T Comp . Apart from YIG and GdIG, there has been a renaissance of research interest in another member of the REIG family, Tm 3 Fe 5 O 12 (TmIG), due to its wide-ranging extraordinary magnetic properties, e.g., strain-tunable perpendicular magnetic anisotropy (PMA), chiral and topological spin textures, and interfacial Dzyaloshinskii–Moriya interaction , combined with low coercivity, which make this system a promising candidate for numerous efficient spintronic applications, such as spin–orbit torque induced magnetization switching, , current-induced domain-wall motion, and spin Hall–topological Hall effects. , Recently, the LSSE has been investigated in TmIG/Pt bilayers with PMA at room temperature and shown to exhibit high interfacial spin transparency and spin-to-charge conversion efficiency at the TmIG/Pt interface .…”

“…Y 3 Fe 5 O 12 (YIG) has been a widely explored MI for generating and transmitting pure spin currents due to its ultralow damping (α ≈ 10 –4 –10 –5 ) and large ⟨ξ⟩ (∼100–200 nm). ,, This has led to a drastic increase in research over the last few decades, aimed at enhancing the spin current injection efficiency across the MI/HM interface by reducing the conductivity mismatch between the MI and HM layers by introducing atomically thin semiconducting interlayers − and enhancing the interfacial spin-mixing conductance. − Hariharan’s group has explored the roles of bulk and surface magnetic anisotropies in LSSE in different REIG-based MI/HM bilayers, , whereas a recent study highlighted the influence of damping on SPE and LSSE in a compensated ferrimagnetic insulator . It has also been demonstrated that the LSSE in YIG/Pt bilayers varies inversely with intrinsic Gilbert damping of the YIG films; however, the LSSE coefficient does not show any significant correlation with the enhanced damping due to SPE in YIG/Pt bilayers .…”