铁磁畴壁中自旋极化电流诱导的左旋极化自旋波

Left-handed polarized spin waves induced by spin polarized electric currents in ferromagnetic domain walls

极化指的是波偏振的方式,是波的一个基本性质.利用波的极化可以进行信息的编码,这一编码方式在光学和声学中得到广泛应用.利用自旋波进行信息的传递和处理是磁子学的主要研究课题.然而在铁磁材料中,由于只存在右旋极化的自旋波,利用波的极化进行信息的编码在铁磁自旋波器件中始终没有实现.前期研究发现,通过自旋极化电流可以在铁磁体中产生左旋极化自旋波,从而有望实现利用极化编码的自旋波器件.然而在一个均匀磁化铁磁体中产生左旋极化自旋波所需的电流密度过大,实验上难以实现.磁畴壁可作为自旋波波导,且畴壁中自旋波的截止频率趋近于零.本文从朗道-栗弗席兹方程出发,研究了在自旋极化电流存在的条件下磁畴壁中自旋波的色散关系和传播特性,证明只需要很小的自旋极化电流就可在畴壁中产生稳定的左旋极化自旋波.微磁学模拟证明了理论分析结果.该项研究为研制基于极化编码信息的自旋波器件提供了一个实际可行的方案.

Polarization refers to the orientation of the wave oscillation which is a fundamental property of wave.It has been used widely to encode information in photonics and phononics.In magnonics,spin wave also has been used for transmitting and processing information.However,exploiting the spin wave polarization to design devices has not been achieved yet in ferromagnets as only the right-handed polarized spin waves can be accommodated in ferromagnets.Our eariler study suggests that the left-handed polarized spin waves can be introduced into ferromagnets by appling a spin-polarized electric current,thus making it possible to design spin wave devices with polarization encoding.But the critical current needed to induce left-handed polarized spin wave in a uniformly magnetized ferromagnet is too high to be realized experimentally.Magnetic domain wall can serve as spin wave guide,and the cutoff frequency of spin wave in a domain wall approaches zero.In this work,the dispersion relationship and propagation characteristics of spin wave in a Bloch domain wall are studied based on the Landau-Lifshitz equation in the presence of a spin-polarized electrical current.It is found that the stable left-handed spin wave can be generated in the domain wall with only a small current density.Micromagnetic simulations confirm the theoretical analysis results.In addition,due to the different excitation efficiencies and spin transfer torque induced propagating nonreciprocity of left-and right-handed polarized spin wave,it is possible to excite selectively the left-and right-handed polarized spin wave,as well as nearly linearly polarized spin waves.This study provides a practical and feasible solution for designing spin wave devices based on the polarization coding technique.