Optical Excitation of Single- and Multimode Magnetization Precession in Fe - Ga Nanolayers

Abstract

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120nm layer, and a single mode with effective damping α ef f = 0.005 and frequency up to 100 GHz in a 4nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetisation precession approaching THz frequencies should be possible with Galfenol nanolayers.

Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer

“…The dominating role of the thermal modulation of the magnetocrystalline anisotropy is also confirmed by the dependence of the precession amplitude excited by a single laser pulse on the direction of the external magnetic field. This dependence demonstrates a four-fold in-plane symmetry with a slight uniaxial distortion, which corresponds to the magnetocrystalline anisotropy of the studied layer 12 .…”

“…The chosen composition of Fe and Ga is characterized by a high Curie temperature ( T c ≈ 900 K) and large saturation magnetization M 0 11 . Experiments were carried out at ambient conditions at room temperature with an external magnetic field, B , applied in the layer plane at an angle − π /8 from the [100] crystallographic direction, which corresponds to the maximal sensitivity of the magnetization, M , to the temperature-induced changes of the magnetic anisotropy 12 . In the studied layer, the lowest, fundamental mode of the quantized magnon spectrum is well separated from the higher-order modes due to the large exchange mode splitting 12 .…”

“…Experiments were carried out at ambient conditions at room temperature with an external magnetic field, B , applied in the layer plane at an angle − π /8 from the [100] crystallographic direction, which corresponds to the maximal sensitivity of the magnetization, M , to the temperature-induced changes of the magnetic anisotropy 12 . In the studied layer, the lowest, fundamental mode of the quantized magnon spectrum is well separated from the higher-order modes due to the large exchange mode splitting 12 . As a result, the magnon spectrum consists of a narrow single spectral line of Lorentzian shape with a width of ≈500 MHz.…”

“…3d. The precession amplitudes are obtained from the absolute values of the KR angle in the transient KR signals 12 . The field-independent small amplitude background, which is due to the response of the electron system and not related to the magnetization 21 , has been subtracted from the experimental values.…”

“…(4) the precession amplitude and phase are determined as A n ðBÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 n B ð Þ þ b 2 n ðBÞ p and φ n B ð Þ ¼ arctan a n ðBÞ=b n ðBÞ ð Þ , where a n B ð Þ ¼ Àω n ξQ b n þ 2τ À1 M ω 2 n Q a n À Á =G and b n B ð Þ ¼ ω n ξQ a n þ 2τ À1 M ω 2 n Q b n À Á =G, with G ¼ ξ 2 þ 4ω 2 n τ À2 M , and ξ ¼ ω 2 n À ω 2 . Here ω ¼ 2πf ðBÞ and τ À1 M ¼ α 0 γ 0 ðF θθ þ F ϕϕ Þ=2 are the field-dependent frequency and decay time of the fundamental magnon mode 12 . The Fourier series expansion coefficients of the effective driving force Q a n and Q b n are determined by the temporal modulation of the temperature given by Eq.…”