Exchange magnon induced resistance asymmetry in permalloy spin-Hall oscillators

Abstract

We investigate magnetization dynamics in a spin-Hall oscillator using a direct current measurement as well as conventional microwave spectrum analysis. When the current applies an anti-damping spin-transfer torque, we observe a change in resistance which we ascribe to the excitation of incoherent exchange magnons. A simple model is developed based on the reduction of the effective saturation magnetization, quantitatively explaining the data. The observed phenomena highlight the importance of exchange magnons on the operation of spin-Hall oscillators.The combination of the spin-Hall effect and the spintransfer torque may be used to compensate the magnetic damping of a ferromagnet, facilitating precession of the macroscopic magnetization by the application of a direct current 1-5 . Such a device, a spin-Hall oscillator, is patterned in a simple current in-plane geometry and contains no tunnel barriers. The ability of the spin-Hall effect to apply a spin-current transverse to the chargecurrent enables the use of conducting magnetic materials as well as low-damping, insulating materials like yttriumiron-garnet (YIG) 5 . This is in contrast to conventional spin-torque nano-oscillators where a spin-polarized current is passed into a conducting ferromagnetic layer 6 . Despite being attractive due to their geometric simplicity and flexibility in choice of magnetic materials, for spin-Hall oscillators to become competitive with conventional spin-torque nano-oscillators, sources of damping such as spin-pumping 7 and multi-magnon scattering via exchange magnons 8,9 should be addressed in order to reduce the linewidth and increase the power generation efficiency. In this Letter we show that the current induced excitation of incoherent exchange magnons can be observed using a direct current measurement. We find a change in the sample resistance which correlates with the reduction of saturation magnetization taken from our spectroscopy measurements, similar to previously reported results 9 . Both phenomena are consistent within a simple model only utilizing the precession cone-angle of the auto-oscillatory exchange magnons.The sample is a Pt(3)/Py(4)/AlO x (1.6) trilayer deposited by d.c. magnetron sputtering onto a MgO substrate. Numbers in parentheses give thicknesses in nanometers. The multilayer is patterned into a 500 nm × 6 µm bar via electron-beam lithography and subsequent Ar ion milling. Adjacent evaporated Cr(5)/Au(50) pads a) current address: Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany b) ajf1006@cam.ac.uk serve as contact pads. When current is applied to this structure, the portion of the current flowing inside the heavy metal (platinum) generates an out-of-plane spincurrent via the spin-Hall effect (SHE). This spin-current exerts a spin-transfer torque (STT) on the magnetization of the ferromagnetic layer (permalloy) 10,11 , which can oppose the magnetic damping, leading to a change in the Gilbert damping parameter α 12,13 :Here, α 0 is the pristine damping parameter, j c,hm is the charge-curren...

Relation between unidirectional spin Hall magnetoresistance and spin current-driven magnon generation

“…Furthermore, the value of ∆RUSMR is inverted when the direction of the transverse field is reversed, whereas reversal of the longitudinal field does not influence R||. These symmetries are consistent with the prior studies of USMR, [18][19][20][21][22][23] and mirror those of SHE, indicating a close relationship between these two effects.…”

“…While the dependence of resistance on the magnetization direction for this effect differs from AMR, both are uniaxial -the resistance is symmetric with respect to the reversal of the magnetization or the current direction. In contrast, the recently discovered unidirectional spin Hall 3 magnetoresistance (USMR) [18][19][20][21][22][23] is determined by the product (j × �) , where j is the density of the electric current, M is the magnetization of the FM layer, and � is the unit vector normal to the plane of the bilayer. Consequently, USMR changes sign when either the magnetization or the electric current is reversed.…”

“…However, adequate understanding of USMR, necessary for the implementation of efficient memory devices utilizing this effect, has not yet been achieved. The proposed interpretations of USMR include the contribution to resistance of spin accumulation at the FM/NM interface 18 and spin-current driven excitation of magnons in the FM layer 21,23 .…”

Origins of the Unidirectional Spin Hall Magnetoresistance in Metallic Bilayers

“…These results reconcile the interpretation of the UMR in terms of spin-dependent conductivity [8,30] and magnon excitations [22,23,26].…”

“…Recent studies have shown that an additional currentdependent unidirectional magnetoresistance (UMR) emerges in FM/NM systems due to either the SHE or REE [8,9,[21][22][23][24][25][26][27]. Unlike the most common magnetoresistive effects, including the anisotropic magnetoresistance (AMR), SMR, and EMR, the UMR is a nonlinear effect that violates Onsager reciprocity, being odd under either magnetization or current reversal.…”