Magnetic relaxation for Mn-based compounds exhibiting the Larmor precession at THz wave range frequencies

Mn-based hard magnets are potentially suitable for advanced ultra-high frequency spintronics applications because they exhibit the Larmor precession of magnetization at THz wave-range frequencies with low magnetic relaxation. However, the low magnetic relaxation properties are not well understood; thus, a more detailed study is necessary. In this study, magnetization precessions in L1₀ Mn_1.54Ga, D0₂₂ Mn_2.12Ga, and C38 MnAlGe epitaxial films grown on MgO substrates were investigated using an all-optical pump-probe method under a magnetic field of ∼20 kOe. The coherent magnetization precessions at frequencies of more than 0.1, 0.2, and 0.3 THz for the C38 MnAlGe, L1₀ Mn_1.54Ga, and D0₂₂ Mn_2.12Ga films, respectively, were clearly measured. The effective damping constant for the C38 MnAlGe film was smaller than the previously reported value measured at 10 kOe by a factor of 2, whereas those showed a large angular dependence. The effective damping constants for the L1₀ Mn_1.54Ga and D0₂₂ Mn_2.12Ga films were independent of the field angle and approximately identical to the values measured at 10 kOe. Thus, it was concluded that the values for the Gilbert damping constant, α, were 0.008 and 0.012 for the L1₀ Mn_1.54Ga and D0₂₂ Mn_2.12Ga films, respectively. The possible origin of the discrepancy between the experimental and theoretical α values for these films is discussed.