Description
Damping enhancement in ferromagnetic resonance to study spin current injection from FM1 and spin absorption in FM2 or NM in magnetic heterostructures FM1/Cu/FM2 or FM1/Cu/NM.

Partners
Prof. W. E. Bailey, Columbia University, New York, USA
F. Wilhelm, A. Rogalev, European Synchrotron Radiation Facility (ESRF), Grenoble

Financing
Accueil Pro Rhone-Alpes (2009-2010
UJF Programme Chercheur invité (2012)
ANR-PNANO 2010 MILESTONE (2010-2013)
FP7-NMP-2011-Small CRONOS (2012-2015)

Objectives
Spin pumping : The precessing magnetization in FM1 injects (pumps) a spin current into adjacent layers, where it can be absorbed (e.g. via spin flip scattering). This leads to a loss of angular momentum for the precessing magnetization and thus enhanced damping. This additional damping is non-local, Gilbert type and depends as the inverse of the FM1 thickness.
Spin current absorption : In spin torque driven magnetization dynamics the spin current is expected to be absorbed at the interface on a lengthscale of 1 nm due to a classical dephasing. Varying the FM2 thickness in heterostructures FM1/Cu/FM2 this absorption length can be studied via the enhanced damping in FM1.

Latest results
Demonstration of non-local damping in FM1/Cu/NM, FM1/Cu/FM2 and FM1/NM heterostructures for FM1,2=Co, CoFeB and FeNi and NM=Pt, Pd, Ru with a 1/t_FM1 thickness dependence. Extraction of the interface spin mixing conductances g↑↓ of individual interfaces .
Linear increase with thickness t_FM2 and a cutoff length of 1nm for the spin current absorption in FM2 (Co, CoFeB, FeNi) and IrMn indicating dephasing. Exponential increase with thickness t_NM for NM (Pt, Pd, Ru) with a characteristic length l_c < Lsf
Induced moments in direct contact FM1/NM and indirect contact FM1/Cu/NM heterostructures from XMCD studies, help to understand the spin current absorption mechanism.

Staff
Abhijit Ghosh – PhD, Bill Bailey , Ursula Ebels, Stéphane Auffret (Materials Deposition), Eric Gautier (TEM studies), F. Wilhelm, A. Rogalev (ESRF, XMCD studies)

Related content
A. Ghosh, J. Sierra, S. Auffret, U. Ebels, W. E. Bailey, Appl. Phys. Lett. 98, 052508 (2011)
A. Ghosh, S. Auffret, U. Ebels, W. E. Bailey, Phys. Rev. Lett. 109, 127202 (2012)
W. E. Bailey, A. Ghosh, S. Auffret, E. Gautier, U. Ebels, , to appear in Phys. Rev. B

Figure 1

(a) Typical FMR absorption derivative,
(b) peak-peak field linewidth DHpp versus frequency and
(c) resonance frequency versus field. The experiments are done using a field swept, broadband, coplanar waveguide FMR spectrometer

Figure 2

(a) Linewidth vs frequency of a FeNi/Cu/Pt heterostructure for different FeNi thicknesses. The slope provides the damping alpha that decreases with thickness as plotted in (b). The difference of the damping without and with Pt overlayer, yields the additional non-local damping due to spin pumping that is plotted in (c) on a linear and in (d) on a log scale, revealing the exact 1/thickness dependence as predicted from spin pumping theory

Figure 3

Schematic illustration of spin pumping and the spin absorption in a ferromagnet FM (left) and a non-magnetic paramagnet PM (right) and the corresponding experimental results of the enhanced damping alpha (bottom), showing a linear increase in the case of ferromagnets FM (left) and an exponential increase in the case of non-magnetic paramegnets (right)