Landau–Lifshitz–Gilbert equation

In physics, the Landau–Lifshitz–Gilbert equation (usually abbreviated as LLG equation), named for Lev Landau, Evgeny Lifshitz, and T. L. Gilbert, is a name used for a differential equation describing the dynamics (typically the precessional motion) of magnetization $M$ in a solid. It is a modified version by Gilbert of the original equation of Landau and Lifshitz. The LLG equation is similar to the Bloch equation, but they differ in the form of the damping term. The LLG equation describes a more general scenario of magnetization dynamics beyond the simple Larmor precession. In particular, the effective field driving the precessional motion of $M$ is not restricted to real magnetic fields; it incorporates a wide range of mechanisms including magnetic anisotropy, exchange interaction, and so on.

The various forms of the LLG equation are commonly used in micromagnetics to model the effects of a magnetic field and other magnetic interactions on ferromagnetic materials. It provides a practical way to model the time-domain behavior of magnetic elements. Recent developments generalizes the LLG equation to include the influence of spin-polarized currents in the form of spin-transfer torque.

Landau–Lifshitz equation


In a ferromagnet, the magnitude of the magnetization $M$ at each spacetime point is approximated by the saturation magnetization $M_{s}$ (although it can be smaller when averaged over a chunk of volume). The Landau-Lifshitz equation, a precursor of the LLG equation, phenomenologically describes the rotation of the magnetization in response to the effective field $H_{eff}$ which accounts for not only a real magnetic field but also internal magnetic interactions such as exchange and anisotropy. An earlier, but equivalent, equation (the Landau–Lifshitz equation) was introduced by :

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, and various internal magnetic interactions involving quantum mechanical effects, which is typically defined as the functional derivative of the magnetic free energy with respect to the local magnetization $M$. To solve this equation, additional conditions for the demagnetizing field must be included to accommodate the geometry of the material.

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 magnetization:

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:

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.

Landau–Lifshitz–Gilbert–Slonczewski equation
In 1996 John Slonczewski 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 $γ$:
 * $$\dot{\mathbf{m}}=-\gamma \mathbf{m}\times \mathbf{H}_{\mathrm{eff}}+\alpha \mathbf{m}\times \dot{\mathbf{m}}+\tau _{\parallel}\frac{\mathbf{m}\times (\mathbf{x}\times \mathbf{m})}{\left|\mathbf{x}\times \mathbf{m}\right|}+\tau _{\perp}\frac{\mathbf{x}\times \mathbf{m}}{\left|\mathbf{x}\times \mathbf{m}\right|}$$

where $$\alpha$$ is the dimensionless damping parameter, $$\tau_\perp$$ and $$\tau_\parallel$$ are driving torques, and $λ=αγ$ is the unit vector along the polarization of the current.