IT/ Nano • IFF Scientific Report 2006 132 I 133
Anisotropic FMR Linewidth of Triple- Domain Fe Layers on GaN(0001)
M. Buchmeier
1, D. E. Bürgler
1, P. A. Grünberg
1, C. M. Schneider
1, R. Meijers
2, R. Calarco
2, C. Raeder
3, and M. Farle
3CNI-Center of Nanoelectronic Systems for Information Technology (1IFF-9,2IBN)
3Fachbereich Physik, Universität Duisburg-Essen, Duisburg, Germany
We present a ferromagnetic resonance (FMR) study of 5 to 70 nm-thick Fe films on GaN(000), which grow in three differently oriented crystal- lographic Fe(110) domains. The samples show a strong (8 mT) 6-fold in-plane magnetic anisotropy with bulk-like thickness dependence and the easy axes oriented parallel to the Fe[001] di- rections. Thus, interface effects and averaged first order cubic or uniaxial anisotropies aris- ing from the three domain orientations can be excluded. We qualitatively explain the 6-fold anisotropy by spin relaxation inside the grains.
The FMR linewidthνs. frequency curves are lin- ear and indicate a good homogeneity of the mag- netic properties. However, the effective damp- ing parameter α shows pronounced anisotropy and thickness dependence. The additional damp- ing is explained by two-magnon scattering at de- fects, which are due to the triple domain struc- ture. These results are of importance for envis- aged spin injection devices based on Fe/GaN in- terfaces.
Spin damping in thin ferromagnetic films has re- cently gained attraction because of its importance for the switching of fast spintronic devices. In classical theory spinwave relaxation can be described via a scalar material parameterα in the Landau-Lifshitz- Gilbert equation. While experimental data on high- quality epitaxial, thin films have been successfully ex- plained by the classical intrinsic Gilbert-type damp- ing [1], there has been increasing evidence for the importance of non-classical damping mechanisms over the last years. The present ferromagnetic res- onance (FMR) study [2] deals with Fe films grown in a triple-domain structure on top of hexagonal GaN.
The structure of the films with thicknesses ranging between 5 and 70 nm has previously been investi- gated in detail [3].
We deposited 5 to 70 nm-thick Fe films on top of Al2O3/GaN(0001) templates [4] by electron-beam evaporation in ultra-high vacuum at room tempera- ture. After deposition the samples were annealedin- situ for one hour at 500◦C in order to improve the quality. The crystal structure was determined byin- situ low-energy electron diffraction (LEED) and ex- situ x-ray diffraction (XRD). It was found that bcc Fe grows with the (011) plane parallel to the in-
terface. The in-plane crystal orientation was ana- lyzed by comparing LEED pictures and XRD pole figures with simulations of different growth modes.
A perfect match with the simulation was found for a triple-domain growth in the so-called Nishiyama- Wassermann orientation [5] with Fe[001]GaN[11- 20] [3]. The magnetization dynamics has been in- vestigated by angle-dependent FMR with an in-plane static field. Measurements at resonance frequencies between 4.5 and 15.5 GHz have been performed us- ing a home-build cylindrical resonator with variable length and a Hewlett Packard network analyzer, while a commercial Bruker EPR spectrometer was used for the 24 GHz frequency measurements.
0 30 60 90 120 150
0 0.01 0.02 0.03 0.04 0.05 0.13 0.14
15.82GHz
7.98GHz
4.37GHz
Resonance field (T)
In-plane sample angle (°) e.a. Fe[001] h.a. Fe[011]
0 10 20 30 40 50 60 70 0
2 4 6 8
Anisotropy field (mT)
Fe thickness (nm)
FIG. 1: Resonance field as a function of the azimuthal sample angle for the 20 nm-thick Fe film. Red symbols with error bars are experimental data, blue solid lines are least square fits. Inset: Thickness dependence of the 6-fold anisotropy field2K6/MSefffor 7.98 GHz driving frequency.
The resonance fields as a function of the sample an- gle for the 20 nm film are shown in Fig. 1. The curves clearly show a 6-fold in-plane anisotropy. The minima of the resonance field at 30◦, 90◦, and 150◦ correspond to the easy axes, while the maxima at 0◦, 60◦, and 120◦indicate the directions of the hard axes. The easy axis has been determined from x- ray Laue pictures to lie parallel to GaN[11-20] and thus along the Fe[001] directions. The curves have been fitted taking into account an energy density EK6 =K6/9 sin2[3(ΘM−Θe.a.)]for the 6-fold in- plane anisotropy in order to extract the anisotropy strengthK6. The inset in Fig. 1 shows the thickness dependence of the anisotropy field 2K6/MSeff ex-
IFF Scientific Report 2006 • IT/ Nano
132 I133 tracted from the 7.98 GHz measurements. This data
excludes an interfacial origin, which should result in an approximate 1/thickness behaviour. The fact that the strength of the 6-fold anisotropy is reduced at high frequency and corresponding high fields sug- gests spin relaxation inside the grains into the local easy axes of the magneto-crystalline anisotropy as the origin of the 6-fold anisotropy. In analogy to Slon- czewskis explanation of the extrinsic biquadratic in- terlayer coupling by the fluctuation model [6], such a relaxation will lower the free energy of the system.
This energy gain due to spin relaxation depends on the orientation of the external field and leads to an easy axis in the direction of largest energy gain. The effect has been addressed in detail by Heinrichet al. [7], who conclude that any system that exhibits a lateral variation of the anisotropic free energy char- acterized by a particular angular power will also ex- hibit an effective anisotropic free energy correspond- ing to a higher angular power. In our case, the 4-fold anisotropy of Fe results in a 6-fold in-plane anisotropy as observed. Assuming the spin relaxation mech- anism as the origin of the anisotropy, the spins get more and more aligned with increasing external field (i.e. increasing frequency) resulting in the observed decreasing anisotropy, which will vanish when the field is eventually high enough to completely saturate the sample.
0 3 0 60 9 0 120 150
0 5 1 0 1 5 20 25
FWHM of resonance line ∆B (mT)
In-plane sample angle (°) 15.82GHz
7.98GHz
4.37GHz
easy axis hard axis ∆B (mT)
0 5 10 15 20
Frequency (GHz) 25 0
5 10 15 20 25 30 35 40 nm
20 nm 10 nm 5 nm
FIG. 2: Angular dependence of the linewidth (FWHM)∆B for the 20 nm-thick Fe film. Inset:∆Bas a function of fre- quency for the 5, 10, 20, and 40 nm-thick Fe films. Solid and dashed lines correspond to easy and hard axes, re- spectively.
In Fig. 2 we plot the width of the resonance peak as a function of the sample angle for the 20 nm-thick Fe film and driving frequencies of 4.37, 7.98, and 15.82 GHz. All other samples show a qualitatively similar behaviour. At 7.98 and 15.82 GHz the width is max- imal for the field parallel to hard axes of the 6-fold anisotropy. For the field parallel to the easy axes the linewidths have minima and are reduced to about half the value of the hard axes. As expected an increase of the linewidth with frequency can be observed. The pronounced 6-fold anisotropy is in disagreement with the intrinsic isotropic Gilbert damping and must be due to additional extrinsic damping mechanisms. In the inset of Fig. 2 the FWHM of the resonance peak for the field parallel to an easy (solid lines) and a hard (dashed lines) axis of the 6-fold anisotropy is plot- ted as a function of the driving frequency for all sam- ples. The linear increase for Fe thicknesses of 5, 10, and 20 nm may be due to Gilbert damping. The ef-
fective damping parameterαeff = γ∆B/2νcan be extracted from the slope of the curves and ranges between 0.005 for the easy axis of the 5 nm-thick film and 0.035 for the hard axis of the thickest film at low frequency. A typical value for Fe(001) grown on GaAs(001) isα=0.004. The extrapolation of the curves to zero frequency yields very low offsets of about 1 mT, which means that there is no local reso- nance broadening and thus the samples are magnet- ically very homogeneous on a large scale. The two- magnon damping should saturate at high frequen- cies leading to a flattening of the curves in Fig. 2, which we indeed observe for the 40 nm-thick film. For the thinner films the highest employed frequency of 24 GHz is probably not high enough to saturate the broadening. The easy axis linewidth for the 40 nm film even decreases between 15 and 24 GHz. This decrease is compatible with realistic calculations of two-magnon scattering [8]. The observed linewidth variations must be attributed mainly to two-magnon scattering processes because (i) there is no signif- icant local resonance broadening, (ii) the observed effective damping is much larger than the typical lit- erature value for the Gilbert damping in Fe, (iii) the experimentalαeff increases strongly with film thick- ness, (iv) the damping is strongly anisotropic, and (v) a saturation of the linewidth is observed for the 40 nm-thick Fe film.
In summary, the peculiar triple-domain film struc- ture of Fe/GaN(001) gives rise to interesting mag- netic properties: (i) A 6-fold in-plane anisotropy with a strength of about 8 mT and bulk-like thickness de- pendence and (ii) a large FMR linewidth with 6-fold in-plane anisotropy. These specific magnetic proper- ties must be taken into account when Fe/GaN-like in- terfaces are to be employed in spin injection devices.
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