Magnetic Domain Imaging of Ni-Mn-Ga Heusler Alloys Using Lorentz TEM
Magnetic Domain Imaging of Ni-Mn-Ga Heusler Alloys Using Lorentz TEM
复制标题
使用洛伦兹 TEM 对 Ni-Mn-Ga Heusler 合金进行磁域成像
DOI：
10.1017/s1431927616009442
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发表时间：
2016
影响因子：
2.8
通讯作者：
M. De Graef
中科院分区：
文献类型：
--
作者：
Isha Kashyap;M. De Graef
Ni-Mn-Ga合金是一种铁磁形状记忆合金（FSMA），在外加磁场的作用下，Ni-Mn-Ga合金的马氏体孪晶变体发生重排，从而产生高的磁场诱导应变（MFIS）。在Ni-Mn-Ga合金中观察到9.5%的非常高的可恢复应变[1]，并且由于该特性，它们在致动器和传感器中找到应用。NiMn-Ga合金是多铁性材料，表现出一种以上的铁性，在这种情况下，铁磁性和铁弹性。此外，合金在低于800°C的温度下也经历原子有序化。所有这些属性引起了有趣的功能，在这种合金，即孪晶变体和孪晶界;反相界（APBs）;磁畴和畴壁。然而，在纳米尺度上缺乏这种合金的磁性微观结构研究。此外，在这种合金中的各种微观结构缺陷和磁畴之间的相互作用知之甚少。在本研究中，我们利用洛仑兹透射电子显微镜（LTEM）技术来研究Ni-Mn-Ga合金中的磁畴微结构和不同铁性畴之间的相互作用。菲涅耳或散焦洛伦兹模式是一种简单的成像磁畴壁的方法。当一个电子通过一个有磁场的磁性材料区域时，它会受到洛伦兹力的作用，因此会向一个特定的方向偏转。只有面内磁感应强度对偏转有贡献。这种偏转在磁畴壁的位置处产生电子不足的区域或电子过量的区域，这又在所得图像中表现为暗特征或亮特征。在菲涅尔模式下拍摄三种不同的图像，即：焦点内、焦点不足和焦点过焦图像。当透镜从欠焦模式转到过焦模式时，墙壁的对比度被反转。菲涅耳模LTEM仅提供关于畴壁的定性信息。然而，通过基于强度输运方程的相位重建，可以从菲涅耳图像序列中提取有关磁感应强度的定量信息。相位重建的彩色图给出了样品内部的积分磁感应强度的方向。洛伦兹技术和相位重构的详细描述可以在[2]中找到。
Ni-Mn-Ga alloys are Ferromagnetic Shape Memory Alloys (FSMAs) in which a high Magnetic Field Induced Strain (MFIS) can be obtained by inducing the rearrangement of martensite twin variants under the influence of an external magnetic field. A very high recoverable strain of 9.5% has been observed in Ni-Mn-Ga alloys [1], and because of this property, they find applications in actuators and sensors. NiMn-Ga alloys are multi-ferroic materials exhibiting more than one ferroic property, in this case, ferromagnetism and ferroelasticity. Moreover, the alloy also goes through an atomic ordering at a temperature below 800°C. All these properties give rise to interesting features in this alloy, namely twin variants and twin boundaries; anti-phase boundaries (APBs); magnetic domains and domain walls. However, there is a lack of magnetic microstructure studies of this alloy at a nanometer scale. In addition, the interaction between various microstructural defects and magnetic domains in this alloy is poorly understood. In this study, we utilize the Lorentz Transmission Electron Microscopy (LTEM) technique to investigate the magnetic domain microstructure and interaction between different ferroic domains in Ni-Mn-Ga alloys. The Fresnel or out-of-focus Lorentz mode is a simple way of imaging magnetic domain walls. When an electron passes through a region of magnetic material with a magnetic field, it experiences a Lorentz force and as a result gets deflected in a specific direction. Only the inplane magnetic induction will contribute to the deflection. This deflection gives rise to a region of deficiency of electrons or a region of excess of electrons at the locations of magnetic domain walls which in turn appears as a dark feature or a bright feature in the resulting image. Three different images are taken in the Fresnel mode namely: in-focus, under-focus and over-focus image. When the lens goes from under-focus to over-focus mode, the contrast of walls is reversed. The Fresnel mode LTEM only provides qualitative information about the domain walls. However, quantitative information about the magnetic induction can be extracted from the Fresnel series of images by doing phase reconstruction based on transport-of-intensity equation. The phase reconstructed color map gives the direction of integrated magnetic induction inside the sample. A detailed description of Lorentz technique and phase reconstruction can be found in [2].