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Nonlinear spin-wave excitations at low magnetic bias fields.

Bauer HG, Majchrak P, Kachel T, Back CH, Woltersdorf G - Nat Commun (2015)

Bottom Line: Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance.Our data show that the common model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behaviour in the low magnetic field limit.In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Regensburg, Universitätsstrasse 31, 93040 Regensburg, Germany.

ABSTRACT
Nonlinear magnetization dynamics is essential for the operation of numerous spintronic devices ranging from magnetic memory to spin torque microwave generators. Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance. Here we use X-ray magnetic circular dichroism to determine the number density of excited magnons in magnetically soft Ni80Fe20 thin films. Our data show that the common model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behaviour in the low magnetic field limit. Here we derive a model of parametric spin-wave excitation, which correctly predicts nonlinear threshold amplitudes and decay rates at high and at low magnetic bias fields. In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes.

No MeSH data available.


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Time-resolved ferromagnetic resonance measurements at 2.5 GHz.(a) In-phase and (b) out-of-phase components of the normalized dynamic XMCD signal in units of degrees corresponding to the real and imaginary parts of the susceptibility. The maximum of the imaginary part of the susceptibility shifts to lower fields when a critical excitation level is reached15. Owing to the normalization procedure with static XMCD hysteresis loops, the absolute error for the excursion angles is below 5%. (c) From data as shown in a and b, the phase angle of magnetization with respect to the driving field is determined at the low amplitude FMR field, HFMR. The main mechanism limiting growth of the precession amplitude with an increasing driving field is a shift in the phase of the precessing magnetization. The instrumental limitations during these measurements lead to a phase error of up to 10°.
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f2: Time-resolved ferromagnetic resonance measurements at 2.5 GHz.(a) In-phase and (b) out-of-phase components of the normalized dynamic XMCD signal in units of degrees corresponding to the real and imaginary parts of the susceptibility. The maximum of the imaginary part of the susceptibility shifts to lower fields when a critical excitation level is reached15. Owing to the normalization procedure with static XMCD hysteresis loops, the absolute error for the excursion angles is below 5%. (c) From data as shown in a and b, the phase angle of magnetization with respect to the driving field is determined at the low amplitude FMR field, HFMR. The main mechanism limiting growth of the precession amplitude with an increasing driving field is a shift in the phase of the precessing magnetization. The instrumental limitations during these measurements lead to a phase error of up to 10°.

Mentions: Our experiments are performed using Permalloy (Ni80Fe20) films deposited on top of the signal line of coplanar waveguide structures. In all measurements, a magnetic bias field HB forces the static magnetization to be oriented in the x direction. A magnetic r.f. field oriented along the y direction leads to a forced precession of M, as illustrated in Fig. 1. The precession of the magnetization vector is strongly elliptical due to the demagnetizing field. As indicated in Fig. 1, the X-ray beam can be oriented at an angle θ=30° with respect to the film normal. In this geometry, the precession of the magnetization causes slight changes of the absorption of circularly polarized X-ray photons detected by a photo diode in transmission. In a first set of measurements, the X-ray beam is tilted in the y direction as illustrated in Fig. 1 (). A continuous wave r.f. excitation is synchronized to the X-ray flashes. Owing to the large ellipticity of the magnetization precession, the detected signal is mostly given by the in-plane magnetization My projected onto the X-ray beam direction. When the phase of the magnetic r.f. driving field is set to 90° or 0° with respect to the X-ray pulses, the measured signal represents either the real or the imaginary part (χ′ or χ′′) of the dynamic magnetic susceptibility 17 (cf. Fig. 2).


Nonlinear spin-wave excitations at low magnetic bias fields.

Bauer HG, Majchrak P, Kachel T, Back CH, Woltersdorf G - Nat Commun (2015)

Time-resolved ferromagnetic resonance measurements at 2.5 GHz.(a) In-phase and (b) out-of-phase components of the normalized dynamic XMCD signal in units of degrees corresponding to the real and imaginary parts of the susceptibility. The maximum of the imaginary part of the susceptibility shifts to lower fields when a critical excitation level is reached15. Owing to the normalization procedure with static XMCD hysteresis loops, the absolute error for the excursion angles is below 5%. (c) From data as shown in a and b, the phase angle of magnetization with respect to the driving field is determined at the low amplitude FMR field, HFMR. The main mechanism limiting growth of the precession amplitude with an increasing driving field is a shift in the phase of the precessing magnetization. The instrumental limitations during these measurements lead to a phase error of up to 10°.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595629&req=5

f2: Time-resolved ferromagnetic resonance measurements at 2.5 GHz.(a) In-phase and (b) out-of-phase components of the normalized dynamic XMCD signal in units of degrees corresponding to the real and imaginary parts of the susceptibility. The maximum of the imaginary part of the susceptibility shifts to lower fields when a critical excitation level is reached15. Owing to the normalization procedure with static XMCD hysteresis loops, the absolute error for the excursion angles is below 5%. (c) From data as shown in a and b, the phase angle of magnetization with respect to the driving field is determined at the low amplitude FMR field, HFMR. The main mechanism limiting growth of the precession amplitude with an increasing driving field is a shift in the phase of the precessing magnetization. The instrumental limitations during these measurements lead to a phase error of up to 10°.
Mentions: Our experiments are performed using Permalloy (Ni80Fe20) films deposited on top of the signal line of coplanar waveguide structures. In all measurements, a magnetic bias field HB forces the static magnetization to be oriented in the x direction. A magnetic r.f. field oriented along the y direction leads to a forced precession of M, as illustrated in Fig. 1. The precession of the magnetization vector is strongly elliptical due to the demagnetizing field. As indicated in Fig. 1, the X-ray beam can be oriented at an angle θ=30° with respect to the film normal. In this geometry, the precession of the magnetization causes slight changes of the absorption of circularly polarized X-ray photons detected by a photo diode in transmission. In a first set of measurements, the X-ray beam is tilted in the y direction as illustrated in Fig. 1 (). A continuous wave r.f. excitation is synchronized to the X-ray flashes. Owing to the large ellipticity of the magnetization precession, the detected signal is mostly given by the in-plane magnetization My projected onto the X-ray beam direction. When the phase of the magnetic r.f. driving field is set to 90° or 0° with respect to the X-ray pulses, the measured signal represents either the real or the imaginary part (χ′ or χ′′) of the dynamic magnetic susceptibility 17 (cf. Fig. 2).

Bottom Line: Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance.Our data show that the common model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behaviour in the low magnetic field limit.In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Regensburg, Universitätsstrasse 31, 93040 Regensburg, Germany.

ABSTRACT
Nonlinear magnetization dynamics is essential for the operation of numerous spintronic devices ranging from magnetic memory to spin torque microwave generators. Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance. Here we use X-ray magnetic circular dichroism to determine the number density of excited magnons in magnetically soft Ni80Fe20 thin films. Our data show that the common model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behaviour in the low magnetic field limit. Here we derive a model of parametric spin-wave excitation, which correctly predicts nonlinear threshold amplitudes and decay rates at high and at low magnetic bias fields. In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes.

No MeSH data available.


Related in: MedlinePlus