矽化錳MnxSi晶體之超快動力學與磁性相圖研究

Abstract

我們對兩種Mn含量分別為0.945與0.998的MnxSi單晶進行低溫比熱量測、磁化強度量測、交流磁化率量測與時間解析可見光激發探測光譜量測。 根據比熱量測C/T – T 曲線上的相變特徵，可以推得Mn0.945Si與Mn0.998Si樣品中Conical phase、Skyrmion phase、IM phase、PM phase與FM phase的相變溫度。集合不同外加磁場下的C/T – T的相變溫度，可繪出Mn0.945Si與Mn0.998Si樣品對各外加磁場(H)與溫度(T)的磁性相圖。其中Mn0.998Si單晶Skyrmion phase可以存在的溫度及磁場區間較大(T=24~26 K, H=0~0.45 T)，且Skyrmion phase可以存在於零磁場下。在磁化率χac-T、M-T量測中，χac-T在由C/T – T推得的相變溫度上，都有發生相對應的磁相變化。 我們對可見光激發探測實驗中所得到激發後不同時間的瞬時反射率變化ΔR/R進行分析，並根據C/T – T得到的相變溫度觀察不同磁相下載子受激發後的弛緩行為。發現Mn0.945Si的自旋與晶格熱平衡時間(Spin-lattice thermalization time)、自旋重新排列時間(Spin reordering time)會隨相變發生產生變化。另外Mn0.998Si 在H = 0.26 T時，在激發瞬間的瞬時反射率變化，即ΔR/Rt=0，也會隨磁相變溫度產生變化。 本研究觀察了Mn0.945Si、Mn0.998Si兩種成分單晶在不同溫度及外加磁場下的磁相，並由可見光激發探測實驗觀察載子在不同磁相下受激發後的弛緩特性，而由載子弛緩特性隨磁相變的變化，可以得知各磁相中的準粒子超快動力學行為是有所差異的。

We have measured the specific heat, magnetization, AC susceptibility and time-resolved pump probe spectra of Mn0.945Si and Mn0.998Si. The specific heat C/T – T measurements showed the magnetic phase transition temperature of Mn0.945Si and Mn0.998Si. The phase transitions between Conical phase, Skyrmion phase, IM phase, PM phase and FM phase were observed. The phase diagrams of Mn0.945Si and Mn0.998Si were sketched by integrated several sets of C/T – T data in different applied magnetic field. We observed that in Mn0.998Si, the region of Skyrmion phase was relative large (T=24~26 K, H=0~0.45 T) compared with other Mn-Si ratio. More importantly, in Mn0.998Si, the Skyrmion phase stably exists at H=0 T. The magnetization measurements χac-T, M-T was also performed in this study. The χac-T data showed the magnetic phase transitions in each phase transition temperatures are corresponding to the C/T – T measurements. Moreover, the time-resolved pump probe spectroscopy was also carried out at various temperatures and magnetic fields. We used 400-nm circularly polarized light as the pump beam and 800-nm linearly polarized light as the probe beam. We analyzed the transient reflectivity changes (ΔR/R) of the 800-nm probe light to investigate the excited carrier relaxation processes in different magnetic phases. In Mn0.945Si, we observed that the spin-lattice thermalization time and spin reordering time change in different magnetic phases. For the case of Mn0.998Si at H = 0.26 T, the ΔR/R at zero time delay, i.e. ΔR/Rt=0, also changed in different magnetic phases. In summary, we investigated the magnetic phases in Mn0.945Si, Mn0.998Si single crystals at various temperatures and magnetic fields. Additionally, the excited carrier relaxation processes in each magnetic phase were also been studied, which indicated that quasiparticle properties are distinct in different magnetic phases.