Spin decoherence independent of antiferromagnetic order in IrMn

Abstract

We investigate the impact of pinned antiferromagnetic order on the decoherence of spin current in polycrystalline IrMn. In NiFe/Cu/IrMn/CoFe multilayers, we coherently pump an electronic spin current from NiFe into IrMn, whose antiferromagnetic order is globally pinned by static exchangebias coupling with CoFe. We observe no anisotropic spin decoherence with respect to the orientation of the pinned antiferromagnetic order. We also observe no difference in spin decoherence for samples with and without pinned antiferromagnetic order. Moreover, although there is a pronounced resonance linewidth increase in NiFe that coincides with the switching of IrMn/CoFe, we show that this is not indicative of anisotropic spin decoherence in IrMn. Our results demonstrate that the decoherence of electron-mediated spin current is remarkably insensitive to the magnetization state of the antiferromagnetic IrMn spin sink.A spin current is said to be coherent when the spin polarization of its carriers, e.g., electrons, is locked in a uniform precessional phase. How a spin current loses its coherence, particularly as it interacts with magnetic order, is a crucial fundamental question in spintronics [1]. In a ferromagnetic metal (FM), an electronic spin current polarized transverse to the magnetization dephases quickly in the uniform ferromagnetic exchange field [2,3]. Experiments of ferromagnetic resonance (FMR) spin pumping [4][5][6], where a coherently excited spin current propagates from a FM spin source to a FM spin sink [7], show the transverse spin coherence length in FMs to be as short as ≈1 nm [8]. The dephasing of transverse spin polarization s also gives rise to a spin-transfer torque, ∝ m × s × m, acting on the magnetization m of the FM spin sink [2,3,9,10].For antiferromagnetic metals (AFMs) with staggered exchange fields, a fundamental understanding of spin transport has yet to be developed by experiment. Although the transverse spin coherence length can in principle be 1 nm [11][12][13], an electronic spin current polarized transverse to the antiferromagnetic order parameter (Néel vector l) is expected to dephase in the diffusive limit of transport [12,14]. Such spin dephasing in AFMs generates a spin-transfer torque, ∝ l × s × l [13-15], which may be crucial for emerging antiferromagnetic spintronic technologies [16][17][18][19][20].Furthermore, spin dephasing in an AFM with a uniform Néel vector may yield anisotropic decoherence, where spin absorption by the AFM is enhanced when l ⊥ s [21].By contrast, polycrystalline thin films of AFMs by themselves do not exhibit anisotropic spin decoherence on a macroscopic scale, since the grains contain a distribution of Néel vector orientations that averages out the anisotropy [22]. While polycrystalline AFMs have found commercial applications (i.e., pinning ferromagnetic layers in spin valves) [23] and been used as spin sinks [8,22,[24][25][26][27][28], their nonuniform, unpinned antiferromagnetic order poses a challenge for gaining fundamental insight into spin decoherence.To ali...

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

“…Their robustness against external magnetic fields, potential for high-density data storage, absence of stray fields, ultrafast dynamics, and high energy efficiency make them an excellent candidate for ultrafast magnetic random access memory [7,8], particularly at deeply scaled technology nodes that are important to emerging applications in artificial intelligence and unconventional (non-von Neumann) computing [9]. At the same time, their large spin coherence length [1,5,10,11] and excellent magnon propagation [12][13][14][15] make antiferromagnets promising candidates for information transmission in emerging computing concepts based on spintronics. Beyond computing applications, the ultrafast exchange-dominated dynamics of antiferromagnets also make them potentially of interest for the realization of roomtemperature, electrically tunable, and narrowband terahertz sources and detectors [3,[16][17][18].…”

Dephasing of transverse spin current in ferrimagnetic alloys

“…show short coherence lengths of typically ≈1 nm [7,[23][24][25][26]. Nevertheless, a recent experimental study utilizing a spin galvanic detection method [27,28] has reported a long coherence length of >10 nm at room temperature in FIM CoTb [15].…”

“…In real materials, finite scattering and complex magnetic order may reduce λc significantly [18,20,21]. Most experiments on AFMs (e.g., polycrystalline IrMn) indeed show short λc of typically ≈1 nm [8,[22][23][24][25]. Nevertheless, a recent experimental study utilizing a spin galvanic detection method [26,27] has reported λc >10 nm at room temperature in FIMs of CoTb [18], consisting of antiferromagnetically coupled transition-metal (TM) and rare-earthmetal (RE) magnetic sublattices.…”

Independence of the spin current from the Néel vector orientation in antiferromagnet CoO

“…In this paper, we report our investigation of spin current pumped into AFM CoO from Fe (driven into FMR) across (permalloy). FMR has been a powerful mechanism for the study of spin pumping because the FM layer at FMR is a high-quality source of spin current that simultaneously serves as a spin-pumping indicator due to FMR linewidth broadening or damping enhancement [12,16,20,[32][33][34][35][36]. In order to avoid experimental artifacts mentioned previously, we designed our sample by considering the following factors.…”

“…It is obvious that whether or not there exists a spin current anisotropy is crucial to the understanding of spin current in AFM. Experimental verification of this anisotropic behavior of the spin current in AFM, however, seems to show inconclusive results with some reports supporting anisotropic behavior [12][13][14][15][16][17] and others that are contradictory [18][19][20]. The complexity in identifying the spin-current anisotropy can be traced to two critical experimental issues.…”