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e117601 - 01(2007 PHYSICAL REVIEW LETTERS week ending 16...

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Low Relaxation Rate in Epitaxial Vanadium-Doped Ultrathin Iron Films C. Scheck, 1 L. Cheng, 1 I. Barsukov, 2 Z. Frait, 2 and W. E. Bailey 1, * 1 Materials Science, Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA 2 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic (Received 29 December 2006; published 12 March 2007) The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate G . For many years, pure iron (Fe) has had the lowest known value of G 57 MHz for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of G which are significantly reduced to 35 5 MHz at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number Z . DOI: 10.1103/PhysRevLett.98.117601 PACS numbers: 76.30.Fc, 72.25.Rb, 75.40.Gb, 75.50.Bb Ultrafast magnetization dynamics comprise a major area of current research in magnetism. Novel dynamical phe- nomena have been observed recently in confined structures [ 1 ], with programmed field pulses [ 2 , 3 ], through interac- tions with intense light pulses [ 4 , 5 ], and under the influ- ence of spin polarized currents [ 6 , 7 ]. In all cases, the observed phenomena compete against ferromagnetic relaxation in the magnetic material. Relaxation aligns magnetization M with applied fields H , bringing dynamics to a stop. The lowest limit of the relaxation rate is intrinsic to a given material and given by G M s , where is the related dimensionless damping constant. In metals, the damping has seen renewed theo- retical interest [ 8 ] motivated particularly by its formal relationship with spin momentum transfer torques [ 9 , 10 ], or by its enhancement with impurities [ 11 ]. Low relaxation rates are of particular interest for low critical currents in spin momentum transfer [ 12 ], narrow band response in frequency domain devices [ 13 ], and reduced thermal noise in nanoscale magnetoresistive sensors [ 14 ]. Pure iron (Fe) has long been known to exhibit the lowest measured intrinsic relaxation rate of all elemental ferro- magnetic metals or binary alloys [ 15 ]. Lowest values of 57 MHz ( 0 : 002 ) have been found in both single- crystal whiskers and epitaxial films [ 16 ] at room tempera- ture. Elemental Ni, Co, and standard alloys such as Ni 81 Fe 19 show much higher values ( G 220 , 170, 114 10 MHz , respectively.) In this Letter, we show that the intrinsic relaxation rate G in a low- Z ferromagnetic alloy can be substantially lower than that known for pure Fe. Epitaxial MgO 100 = Fe 1 x V x 8 nm 100 ultrathin films, deposited by UHV sputtering, exhibit values of G to 35 MHz, reduced by 40% . While a comparable value has been identified recently in NiMnSb [ 7 ], the low damping has been attrib- uted to the special electronic characteristics of this ordered, half -metallic compound [ 17 ], including a low orbital com- ponent of the magnetic moment. We show that in Fe 1 x V x the observed effect can be understood as the reduced
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