Landau–Lifshitz–Gilbert equation

In physics, the Landau–Lifshitz–Gilbert equation, named for Lev Landau and Evgeny Lifshitz and T. L. Gilbert, is a name used for a differential equation describing the precessional motion of magnetization $M$ in a solid. It is a modification by Gilbert of the original equation of Landau and Lifshitz.

The various forms of the equation are commonly used in micromagnetics to model the effects of a magnetic field on ferromagnetic materials. In particular it can be used to model the time domain behavior of magnetic elements due to a magnetic field.^[1] An additional term was added to the equation to describe the effect of spin polarized current on magnets.^[2]

Landau-Lifshitz equation

The terms of the Landau-Lifshitz-Gilbert equation: precession (red) and damping (blue). The trajectory of the magnetization (dotted spiral) is drawn under the simplifying assumption that the effective field H_eff is constant.

In a ferromagnet, the magnetization $M$ can vary internally but at each point its magnitude is equal to the saturation magnetization $M s$ . The Landau–Lifshitz–Gilbert equation predicts the rotation of the magnetization in response to torques. An earlier, but equivalent, equation (the Landau-Lifshitz equation) was introduced by Landau & Lifshitz (1935):^[3]^[4]^[5]

$\frac{d\mathbf{M}}{d t}= -\gamma \mathbf{M} \times \mathbf{H_\mathrm{eff}} - \lambda \mathbf{M} \times \left(\mathbf{M} \times \mathbf{H_{\mathrm{eff}}}\right)$

(1)

where $γ$ is the electron gyromagnetic ratio. and $λ$ is a phenomenological damping parameter, often replaced by

\lambda = \alpha\frac{\gamma}{M_\mathrm{s}},

where $α$ is a dimensionless constant called the damping factor. The effective field $H eff$ is a combination of the external magnetic field, the demagnetizing field (magnetic field due to the magnetization), and some quantum mechanical effects. To solve this equation, additional equations for the demagnetizing field must be included.

Using the methods of irreversible statistical mechanics, numerous authors have independently obtained the Landau-Lifshitz equation.^[6]

Landau-Lifshitz-Gilbert equation

In 1955 Gilbert replaced the damping term in the Landau-Lifshitz (LL) equation by one that depends on the time derivative of the magnetic field:

$\frac{d \mathbf{M}}{d t}=-\gamma \left(\mathbf{M} \times \mathbf{H}_{\mathrm{eff}} - \eta \mathbf{M}\times\frac{d \mathbf{M}}{d t}\right)$

(2b)

This is the Landau-Lifshitz-Gilbert (LLG) equation, where $η$ is the damping parameter, which is characteristic of the material. It can be transformed into the Landau-Lifshitz equation:^[3]

$\frac{d \mathbf{M}}{d t} = -\gamma' \mathbf{M} \times \mathbf{H}_{\mathrm{eff}} - \lambda \mathbf{M} \times (\mathbf{M} \times \mathbf{H}_{\mathrm{eff}})$

(2a)

where

\gamma' = \frac{\gamma}{1 + \gamma^2\eta^2M_s^2} \qquad \text{and} \qquad\lambda = \frac{\gamma^2\eta}{1 + \gamma^2\eta^2M_s^2}.

In this form of the LL equation, the precessional term $γ'$ depends on the damping term. This better represents the behavior of real ferromagnets when the damping is large.^[7]

Landau-Lifshitz-Gilbert-Slonczewski Equation

In 1996 Slonczewski^[8] expanded the model to account for the spin-transfer torque, i.e. the torque induced upon the magnetization by spin-polarized current flowing through the ferromagnet. This is commonly written in terms of the unit moment defined by $\hat m = \vec M / M_S$ :

\dot{\hat m}=-\gamma \hat m\times \vec H_{\mathit{eff}}+\alpha \hat m\times \dot{\hat m}+\tau _{\parallel}\frac{\hat m\times (\hat x'\times \hat m)}{\left|\hat x'\times \hat m\right|}+\tau _{\perp}\frac{\hat x'\times \hat m}{\left|\hat x'\times \hat m\right|}

where $\alpha$ is the unitless damping parameter, $\tau_\perp$ and $\tau_\parallel$ are driving torques, and $\hat x'$ is the unit vector along the polarization of the current.

References and footnotes

↑ Yang, Bo. "Numerical Studies of Dynamical Micromagnetics". Retrieved 8 August 2011.
↑ http://wpage.unina.it/mdaquino/PhD_thesis/main/node47.html
1 2 Aharoni 1996
↑ Brown 1978
↑ Chikazumi 1997
↑ T. Iwata, J. Magn. Magn. Mater. 31–34, 1013 (1983); T. Iwata, J. Magn. Magn. Mater. 59, 215 (1986); V.G. Baryakhtar, Zh. Eksp. Teor. Fiz. 87, 1501 (1984); S. Barta (unpublished, 1999); W. M. Saslow, J. Appl. Phys. 105, 07D315 (2009).
↑ For details of Kelly's non-resonant experiment, and of Gilbert's analysis (which led to Gilbert's modifying the damping term), see T. L. Gilbert and J. M. Kelly, "Anomalous rotational damping in ferromagnetic sheets", Conf. Magnetism and Magnetic Materials, Pittsburgh, PA, June 14–16, 1955 (New York: American Institute of Electrical Engineers, Oct. 1955, pp. 253-263). http://people.physics.tamu.edu/saslow/MMMConf55_253GilbertKelly.pdf Text references to Figures 5 and 6 should have been to Tables 1 and 2. Gilbert could not fit Kelly's experiments with fixed usual gyromagnetic ratio $γ$ and a frequency-dependent $λ = αγ$ , but could fit that data for a fixed Gilbert gyromagnetic ratio $γ G = γ /(1+ α 2)$ and a frequency-dependent $α$ . Values of $α$ as large as 9 were required, indicating very broad absorption, and thus a relatively low-quality sample. Modern samples, when analyzed from resonance absorption, give $α$ 's on the order of 0.05 or less.
↑ Slonczewski, John C. (1996). "Current-driven excitation of magnetic multilayers". Journal of Magnetism and Magnetic Materials 159 (1): –1–L7. Bibcode:1996JMMM..159L...1S. doi:10.1016/0304-8853(96)00062-5. Retrieved 2014-09-25.

External links

Magnetization dynamics applet

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