In recent years, there was an increased interest in using antiferromagnetic (AFM) materials for THz frequency sources. The operation of such AFM structures is based on the AFM resonance, which can have THz frequencies, exceeding the frequency ferromagnetic (FM) resonances by order of magnitude [1,2]. Recent experiments showed that it is possible to generate such signal in AFM FeRh/Pt bi-layers by optically exciting them with a sub-picosecond laser pulse [3]. The signal in the AFM state can be originated from a partial phase transition induced by the laser heat from the AF to the FM phase [4]. Such transformation occurs on a sub picosecond time scale [5-7]. Using atomistic simulations, it is possible to reproduce such transition by including both the bilinear and biquadratic exchanges in the Hamiltonian [7,8]. However, to model larger spatial structures, a coarse-grained model for the dynamic of the magnetization is desirable.<br/>Here, we present a micromagnetic formulation to model the magnetization dynamics and phase transition in FeRh at low and high temperatures, including the transition near the Curie temperature. The formulation is based on a mean field approximation (MFA), where each macrospin is modeled by two coupled sublattices, including the atomistic biquadratic interactions described as a micromagnetic inter-sublattice molecular field with a cubic dependence of the magnetization. To validate the model, we compare our simulations of the phase transitions in the static and dynamic case with the existing literature. The results show an agreement with the experiments and atomistic modeling, displaying the characteristic hysteretic behavior for the magnetization (Fig. 1), and a similar dynamic response to an external thermal pulse (Fig.2).

![](https://assets.underline.io/uploads/markdown_image/1/image/514f2aed032312bb7acd410db166547c.jpg)

Equilibrium magnetization as a function of temperature.

![](https://assets.underline.io/uploads/markdown_image/1/image/25e2660f89da1d9cb11ba63fe22f9af2.jpg)

Dynamic response of the magnetization to an optical pulse (shaded area) for different values of the atomistic damping coefficient α



Macromagnetic Simulation of AFM-FM phase transition in FeRh using a modified Landau-Lifshitz-Bloch Equation

**Session chair:** Jean-Yves Chauleau, CEA-SPEC


HOF - Dynamics in AF-coupled Materials

technical paper

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This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Marco Menarini

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Macromagnetic Simulation of AFM-FM phase transition in FeRh using a modified Landau-Lifshitz-Bloch Equation

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