ISSN 10628738, Bulletin of the Russian Academy of Sciences: Physics, 2010, Vol. 74, No. 12, pp. 1652–1654. © Allerton Press, Inc., 2010. Original Russian Text © S.A. Vyzulin, A.V. Gorobinskii, E.V. Lebedeva, N.E. Syr’ev, M.S. Shlapakov, 2010, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2010, Vol. 74, No. 12, pp. 1721–1723.
LowField Magnetic Resonance in Granular Nanostructures S. A. Vyzulina, A. V. Gorobinskiia, E. V. Lebedevab, N. E. Syr’evb, and M. S. Shlapakova a
Kuban State University, ul. Stavropol’skaya 49, Krasnodar, 350040 Russia b Faculty of Physics, Moscow State University, Moscow, 119991 Russia email:
[email protected]
Abstract—A series of (CoFeB) + (SiO2) magnetic film samples was investigated by ferromagnetic resonance. The magnetic resonance in low fields was experimentally observed. The influence on the position and width of the lowfield peak of the magnetic phase concentration in nanofilms and of the registration parameters of a spectrum were established. DOI: 10.3103/S1062873810120051
INTRODUCTION Granular magnetic nanostructures with inclusions of magnetic nanoparticles in a nonmagnetic matrix occupy a special place among nanoscale magnetic materials. The primary role in forming the magnetic properties of these materials is played by interaction effects between the particles and between the particles and the matrix. The value of interaction depends strongly on the distances between particles and on their number, i. e., on the size, shape and volume con centration of magnetic inclusions in a nonmagnetic matrix. When the size of the magnetic particles is reduced to a certain value, magnetic granules can transition from the multidomain state to the single domain state [1]. The position, size, and shape of the magnetic particles are virtually independent of the external magnetic field. It is known that in bulk ferro magnetic samples the phenomenon of ferromagnetic resonance (FMR) is observed at a certain correlation between the frequency of an external alternating mag netic field and the intensity of an external static bias field [2]. Resonance absorption by a ferromagnetic of electromagnetic field energy is observed not only in high fields (greater than the saturation field) but also in weak nonsaturating fields. The aim of this work was the experimental observa tion and investigation of properties of magnetic reso nance in granular nanostructures in weak bias fields. EXPERIMENTAL A number of samples of the magnetic film nanosys tem (CoFeB) + (SiO2) deposited on a quartz plate were studied by FMR. A magnetic layer contains gran ules of amorphous CoFeB alloy embedded into a non magnetic SiO2 matrix. The granules are around 3–
5 nm in size. The nanosystem was synthesized at Vor onezh State Technical University by ionbeam sput tering onto a quartz substrate in a vacuum using appropriately composed targets [3]. The width of the magnetic film was varied from 138 to 272 nm. The composition and concentration of magnetic granules in the nonmagnetic matrix were determined by Xray spectrum analysis using a JSM 7500F scan ning electron microscope with an INCAEnergy energy dispersive attachment. RESULTS AND DISCUSSION It was found that (a) the magnetic particles con sisted of cobalt, iron and boron alloy with atomic per centages of 40, 40, and 20%, respectively; (b) the vol ume concentration f of magnetic granules in the non magnetic matrix varied from 6 to 33%. FMR spectra were observed in the Xband at 9.13 GHz at room temperature using a standard JEOL FA300 ESR spectrometer. The first derivative of the absorption signal was recorded. FMR spectra for each of the samples were measured with different orienta tions of magnetic field H relative to the film surface, from α = 0 ( H || n ) to α = 90° ( H ⊥ n ), where n is the normal vector to the film surface. The spectrometer was capable of changing the bias field H from –100 to 20000 Oe. In the case of a tangent orientation of the bias field, two resonance peaks were found in the spectra of sam ples with f > 13% (Fig. 1). One of these (marked 1) was an absorption signal typical of the principal FMR mode. It is found in region of relatively high magnetic fields (from 980 to 2900 Oe). The other peak was in the region of very low magnetic fields, below 50–70 Oe (marked 2). The shape of the lowfield resonance is inverse to the principal FMR mode. The same peak
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was observed in granular nanostructures CoFeZr + Al2O3 and in multilayered nanostructures in which a composite CoFeZr + Al2O3 was used as the magnetic layers and the interlayers were made of hydrogenated silicon. The position of the second type of peaks depended strongly on the history of magnetic field change. Sev eral spectra with different field sweeping ranges were recorded for this type of peaks. The lower value of the bias field, –100 Oe, was kept constant; whereas the upper value was varied from 50 to 8000 Oe. When the value of upper sweeping limit Hu is increased, the res onance field shifts monotonously to the high field region from 5–10 Oe at Hu= 50 Oe to 50–70 Oe at Hu = 8000 Oe. The width and intensity of the lowfield peak remain almost constant when Hu is varied. The value of the resonance field of the lowfield peak depends on the orientation of the sample relative to the direction of the bias field. We investigated vary ing the bias field in the fixed range from –100 to +250 Oe. The resonance field grows as the magneti zation changes from tangential (α = 90°) to normal (α = 0°) orientation. The width of the lowfield peak also depends on the direction of the external bias field. When the orienta tion of the sample is changed from tangential to nor mal, the width of the lowfield peak increases by more than an order of amplitude. The intensity of the low field peak attains its maximum value at α = 90°. In changing from α = 90° to α = 0°, the intensity of the lowfield peak tends to zero. We studied the concentration dependences of reso nance field value and peak widths at tangential magne tization (α = 90°) for both types of resonance. The bias field sweeping range was –100 to +5000 Oe. A monotonous decline in resonance field value (1) H r from 2900 to 980 Oe is observed for the principal FMR mode as f increases (Fig. 2, curve 1). For the sig nals in the low field, the concentration dependence is reversed (Fig. 2, curve 2). As f increases from 13 to (2) 33%, the resonance field value H r rises from ≈0 to 20 Oe. The experimental dependences of peak width on f for the principal (curve 1) and the lowfield (curve 2) signals are shown in Fig. 3. Both curves can be charac terized by the presence of two regions that differ from one another by the dependence of the peak width on f. At f of 6 to 20%, the width of the principal FMR mode, (1) Δ H r , is nearly constant and has a value of 1000 Oe. (1)
As f increases to 33%, Δ H r declines monotonously to 200 Oe. For the lowfield peaks, at f of 13 to 20% the (2) value of Δ H r declines swiftly from 1000 to 0.68 Oe; (2)
and at f > 20% the value of Δ H r remains almost con stant.
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Intensity, arb. units 1.0 2 0.5 1 0 −0.5 −1.0 −100
900
1900 H, Oe
Fig. 1. An FMR spectrum.
Hr(1), Oe 3000
Hr(2), Oe 20
2000
2
1
10 1000 0 0
10
30
20 f, %
Fig. 2. Dependence of Hr on f.
ΔH, Oe 1000
1 2
500
0
10
20
30 f, %
Fig. 3. Dependence of the peak widths on f.
The nature of the lowfield peak is not yet fully understood. However, we may confidently state that it does not appear due to resonance of the magnetostatic precession type or to spin waves. It is known that upon variation of the magnetic phase concentration, a structural change in a mate rial’s magnetic properties from superparamagnetic to ferromagnetic can take place. The range of concentra tions at which such a phenomenon can occur is called the percolation transition. This transition manifests itself in a change in the ferromagnetic resonance line width in particular [4]. An analysis of FRM spectra,
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however, usually does not allow us to define exactly the range of concentrations in which structural changes in the magnetic properties of a material take place. The observed abrupt break in the concentration depen dence of ΔH for lowfield resonance at the volume concentration f ≈ 20% can apparently be interpreted as the percolation transition. CONCLUSIONS Magnetic resonance in low fields was experimen tally observed. The influence of the magnetic phase concentration in the film nanosystem (CoFeB) + (SiO2) and spectral parameters (SHF signal power, sweeping time, modulatin amplitude, and bias field sweeping range) on the position and width of the low field peak was established. It was shown that the width
of the lowfield resonance peak is more sensitive to change in the magnetic phase concentration than the analogous parameter of the principal FMR mode. REFERENCES 1. Suzdalev, I.P. and Suzdalev, P.I., Usp. Khim., 2001, vol. 70, pp. 203–240. 2. Gurevich, A.G., Magnitnyi rezonans v ferritakh i anti ferromagnetikakh (Magnetic Resonance in Ferrites and Antiferromagnetics), Moscow: Nauka, 1973. 3. Zolotukhin, I.V. and Kalinin, Yu.E., Sovremennye problemy fiziki tverdogo tela i materialovedeniya (The Modern Problems of Solid Physics and Material Sci ence), Voronezh, 2005, pp. 44–54. 4. Vyzulin, S.A., et al., Izv. Vyssh. Uchebn. Zaved. Fiz., 2006, vol. 49, no. 3, pp. 47–53.
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