e117601 - PRL 98, 117601 (2007) PHYSICAL REVIEW LETTERS...

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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 signi±cantly reduced to 35 ² 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 con±ned structures [ 1 ], with programmed ±eld pulses [ 2 , 3 ], through interac- tions with intense light pulses [ 4 , 5 ], and under the in²u- 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 ±elds H , bringing dynamics to a stop. The lowest limit of the relaxation rate is intrinsic to a given material and given by G ± ±²M s , where ² 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 ( ² ± 0 : 002 ) have been found in both single- crystal whiskers and epitaxial ±lms [ 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 ² 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 ³ 100 ´ = Fe 1 ÿ x V x ³ 8nm ´³ 100 ´ ultrathin ±lms, deposited by UHV sputtering, exhibit values of G to 35 MHz, reduced by µ 40% . While a comparable value has been identi±ed 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 ÿ x V x
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e117601 - PRL 98, 117601 (2007) PHYSICAL REVIEW LETTERS...

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