Spin transfer appears to be a promising tool for improving spintronics devices. Experiments that quantitatively access the magnitude of the spin transfer are required for a fundamental understanding of this phenomenon. By inductively measuring spin waves propagating along a permalloy strip subjected to a large electrical current, we observed a current-induced spin wave Doppler shift that we relate to the adiabatic spin transfer torque. Because spin waves provide a well-defined system for performing spin transfer, we anticipate that they could be used as an accurate probe of spin-polarized transport in various itinerant ferromagnets.
Principle of the spin wave measurements. (A) Sketch of a spin wave subjected to spin transfer torque (case of a spin wave propagating against the dc current – along the electron flow- with a spin polarization P > 0). The red arrows represent the flow of spin-polarized electrons (the spin current Q). (B) Optical micrograph of the device with a w = 2µm permalloy strip (t = 20 nm) and a pair of lamba = 0.8 µm antennae. ( C) Scanning electron micrograph of the central region. (D) Fourier transform of the microwave current density for the antenna shown in ( C). It was calculated by assuming a uniform current density across each branch of the meander. (E) Sketch of the operating principle of propagating spin wave spectroscopy.
Influence of a dc current on the spin wave propagation. (A) Mutual inductance measurement in the presence of a I =+6mA dc current for the w=2µm, 
= 0.8 µm sample under 
= 1.029 T. 
is shown as a red curve and corresponds to spin waves propagating from antenna 1 to antenna 2. 
is shown as a blue curve and corresponds to spin waves propagating from antenna 2 to antenna 1. The orientations of the spin wave wave vector and of the electron flow are shown in the inset. The measured frequency shift 
is indicated on the graph. For clarity, only the imaginary part and only the frequency range corresponding to the main peak (
) are shown. (B) Idem for I = –6 mA.
References:
V. VLAMINCK, M. BAILLEUL ,
Spin-wave transduction at the submicrometer scale : experiment and modelling,
Phys. Rev. B 81, 014425/1-13 (2010).
V. VLAMINCK, M. BAILLEUL,
Current-induced spin-wave Doppler shift ,
Science 322, 410-413 (2008).