Q-band Ferromagnetic Resonance Research Articles

Measurements of Gilbert damping parameter α for CoPt-based and CoFe-based films for magnetic recording applications

Time-resolved MOKE (TRMOKE) and Q-band ferromagnetic resonance (FMR) are used to evaluate the damping parameter α and the magnetic anisotropy field (where K is the uniaxial anisotropy constant and is the saturation magnetization) of CoPtX–Y and CoPtX (where X is non-magnetic element and Y denotes oxide(s)) granular alloys with perpendicular magnetic anisotropy and variable Co/Pt atomic ratio R. It is found that α monotonically increases in the range of 0.03–0.07 with decreasing R in the range of 2.5–5.5. By comparing the CoPtX films sputter-deposited in Ar and in Ar+O2 atmosphere, it is found that α is less sensitive than HK to the presence of non-magnetic additives (Cr, B) in the grain core. This suggests that α is much less sensitive than K to perturbations of d-electron band structure. FMR and TRMOKE are also used to evaluate α of magnetically soft CoFeX alloys. It is shown that adding 2 at.% Tb to CoFeX leads to an increase of α from 0.012 (undoped case) to 0.048, and a negative shift of the resonance field which is linearly dependent on the RF frequency. This effect, similar to that previously reported on Tb-doped NiFe films, is attributed to the highly anisotropic Tb 4f–5d exchange coupling. A synthetic-antiferromagnetic trilayer CoFeX (7 nm)/Ru (0.8 nm)/CoFeX (7 nm) is investigated by FMR. The interlayer RKKY coupling constant, evaluated from the frequency shift between the acoustic (symmetric) and optic (antisymmetric) resonance modes is JEX = 0.11 erg cm−2. The α value corresponding to the acoustic mode (0.016) matches that of a single-layer CoFeX film whereas the optic mode shows a higher α (0.0265). This is interpreted to be a consequence of spin-pumping through the Ru interlayer. The evaluated spin mixing conductance for the CoFeX/Ru interface is = 22.5 nm−2.

Enhanced spin-pumping efficiency in hcp Co polycrystalline films obtained by sputtering deposition with high substrate temperature

The variation of the resonance field and the Gilbert damping parameter (G) with hcp Co film thickness was evaluated by measuring the X- and Q-band ferromagnetic resonance (FMR) for buffer-layer/Co (2–16 nm)/Cu (1 nm)/Pt (15 nm) films. The two types of hcp Co films were deposited at a substrate temperature (Tsub) of either 400 °C or room temperature using a magnetron sputtering system. The G values for the Co films with Tsub = 400 °C exhibited a more rapid increase with decreasing thickness. In addition, the effective linewidth of FMR spectrum exhibited more rapid increase in the Q-band FMR for Tsub = 400 °C. The experimental results suggest that the spin-pumping is enhanced to a greater extent in the Co films fabricated at Tsub = 400 °C. Judging from the structural analysis of the Co films, it is possible that grain growth involving atomically flat surfaces of Co films might enhance the spin-pumping efficiency.

Q-Band Ferromagnetic Resonance Study of Fe Thin Films on GaAs(001) Deposited by RF Magnetron Sputtering

The magnetic properties of epitaxial Fe(001) films, prepared on GaAs(001) substrates by RF magnetron sputtering, were investigated by ferromagnetic resonance spectroscopy operating at Q-band (35 GHz). The in-plane angle dependence of the resonance fields showed four-fold magnetic anisotropy with an easy (hard) direction along [100] ([110]). The anisotropy constant was slightly smaller than the magnetocrystalline anisotropy constant of bulk Fe. Ferromagnetic resonance linewidths, caused by the magnetic damping of Fe films, were narrow (<;70 Oe) and independent of the in-plane magnetic field directions. It will be shown that the observed linewidth gives good estimate of the upper bound of the Gilbert-type damping in Fe thin films.

Q-band ferromagnetic resonance for CoPt-based stacked perpendicular recording media with interlayer exchange coupling

The ferromagnetic interlayer exchange coupling Jinter for stacked perpendicular recording media with a granular layer (GL)/interlayer (IL)/alloy capping layer (CL) structure was quantitatively evaluated by Q-band ferromagnetic resonance (FMR). Two resonances with acoustic and optical precession modes were observed in the FMR signals from the stacked media. Fitting using the Landau–Lifshitz–Gilbert (LLG) equation indicated that Jinter increased from 0.55 to 1.83 erg/cm2 when the Pt IL thickness was reduced from 2.0 to 1.0 nm for media based on Co82Cr10Pt8-CL (4 nm) and Co74Pt16Cr10-8 mol (SiO2)-GL (16 nm). The optimum Pt IL thickness at which the switching field distribution was minimized due to a large reduction in the saturation field of the stacked media was found to correspond to the boundary condition between antiparallel and parallel precession of the magnetic moments of the GL and CL in FMR.

Evaluation valuation of Anisotropy Field and g-factor of c-plane Oriented CoPtCr Alloy Film Using Q-band Ferromagnetic Resonance

The uniaxial magnetic anisotropy field, Hk, and g-factor of a c-plane oriented CoPtCr alloy film was quantitatively evaluated by using Q-band (∼34 GHz) ferromagnetic resonance (FMR). An FMR signal with a high signal-to-noise ratio was successfully obtained by using a 2mmφ-Macor holder, which achieves phase matching, a decrease in the background of the signal, and accurate alignment of the external field parallel to the film normal or in-plane. An external field was applied perpendicularly or horizontally to the film, and a single magnetic domain was maintained during the measurements. The measured FMR signals demonstrated that 1) the Hk and g-factor were simultaneously derived for CoPtCr films with Co 75-90 at.%, 2) the magnitude of Hk was supported by that obtained with a Torque magnetometer, and 3) the g-factor increased from 2.11 to 2.19 when Co content was decreased from 85 to 65 at.% and the Cr content was increased from 5 to 20 at.%.

High-temperature ferromagnetism by means of oriented nanocolumns: Co clustering in (Zn,Co)O

In order to provide insight into the magnetic properties of semiconductors with ferromagnetic inclusions, we present a thorough quantitative analysis of (Zn,Co)O films with high Co concentration, which give rise to high saturation magnetization values (similar to 110 kA m(-1)) at room temperature. From the orientation-, temperature-, and field-dependent responses of the magnetization and of the Q-band ferromagnetic resonance modes, we give evidence of two types of nanosized ferromagnetic Co clusters and derive their respective contributions. The first type are spherical with diameters about 5 nm and blocking temperatures T(B) about 100 K. The second type are elongated along the surface normal and crystallographically oriented with respect to the ZnO host. Some are nanocolumns about 4 nm wide, whose height may reach up to 60 nm, leading to T(B) >= 300 K. These results are confirmed by high-resolution transmission electron microscopy analysis. In addition to provide transition from ferromagnetism to superparamagnetism above 300 K, the nanocolumns lead to strong anisotropic magnetic properties. We believe that they could find applications in spintronic devices.

Damping Constants for Permalloy Single-Crystal Thin Films

Angular dependence of Gilbert's damping constant alpha for Ni-Fe(001) single-crystal thin films were investigated by employing Q-band (35 GHz) and X-band (9 GHz) ferromagnetic resonance analysis in terms of the specimen's crystal structure. The alpha for 10-nm-thick specimen had constant values of 0.006, which were estimated by the Q-band ferromagnetic resonance (FMR) spectra, when the spectra were measured with changing external field directions in the film plane and out of plane. The alpha for the Ni-Fe specimens did not depend on the crystal directions. The alpha values estimated by X-band FMR spectra were in good agreement with those by Q-band FMR spectra, when the magnetization of the specimens was saturated at the X-band resonance field. The alpha values were overestimated, when the magnetization was not saturated and the specimens had multidomain structures at the resonance field.

Epitaxial growth of hcp/fcc Co bilayer films on Al2O3(0001) substrates

Co magnetic bilayer films with different stacking structures of fcc/hcp and hcp/fcc were successfully prepared on Al2O3(0001) substrates employing Au and Cu underlayers. The structure and the magnetic properties of these bilayer films were investigated. The crystal structure and the strain of Co layers are influenced by the underlayer. Q-band ferromagnetic resonance analysis has shown that the Co bilayer films consist magnetically of fcc and hcp phases. It has been shown possible to prepare hcp/fcc Co epitaxial magnetic bilayer films that can be used for patterned media applications.

Damping Constants of Ni-Fe and Ni-Co Alloy Thin Films

Gilbert's damping constants, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> , of Ni-Fe and Ni-Co thin films are determined by Q-band ferromagnetic resonance (FMR) analysis. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> is calculated from the resonance width of the FMR spectrum. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> of Ni <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_85$</tex> Fe <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_15$</tex> film is found to be 0.0034, which is one-fourth of those of Ni film <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(alpha =0.013)$</tex> and Co <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(alpha =0.013)$</tex> . <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> of the Ni <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_100 - x$</tex> Co <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_ x$</tex> specimens increases with decreasing the Co concentration from 100 to 68 at%, and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> value of Ni <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_32$</tex> Co <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_68$</tex> film has the maximum value of 0.0021. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$alpha$</tex> values of the Ni-Co specimens are nearly the same as those of the Co-Cr-Ta and the Co-Cr-Pt.