摘要
Cr掺杂AlN是一种理论预测的高温稀磁材料,在实验上对其结构与磁性进行系统研究有助于探讨稀磁材料的磁性起源机理,也可以掌握控制其磁性的制备工艺。本文采用直流反应磁控溅射技术,制备了Cr掺杂AlN薄膜,系统研究了掺杂含量、铁电基片、基片负偏压、掺碳以及[TiN/Cr:AlN]多层膜结构等工艺条件对薄膜结构与磁性的影响。
研究表明:
(1)本文制备了掺杂含量小于11.5atom%的Cr掺杂AlN薄膜,通过XRD、X射线吸收谱等表征手段证明,单层膜结构的Cr掺杂AlN形成了成分均匀的纤锌矿固溶体,而小周期多层膜结构的Cr掺杂AlN则形成NaCl结构固溶体。其中Cr以+3价置换Al溶解于AlN基体中,没有发生第二相析出。
(2)所有Cr掺杂AlN样品均具有室温铁磁性,该磁性来自Al-Cr-N固溶体而非磁性第二相颗粒。在Si基片上生长的薄膜,随着掺杂含量从2.3atom%增加到7.7atom%,Cr掺杂AlN的平均原子磁矩从0.23μB/Cr下降到0.03μB/Cr。在LiTaO3、LiNbO3铁电基片上生长的薄膜,当铁电基片c轴与薄膜c轴的夹角减小时,Cr掺杂AlN薄膜的平均原子磁矩增加,其变化范围从0.01μB/Cr到0.20μB/Cr,该现象与AlN基体内部极化电场对磁偶极子的散射作用有关。
(3)施加基片负偏压使Si基片上的Cr掺杂AlN薄膜磁性从0.07μB/Cr提高到0.18μB/Cr,而掺碳使薄膜的磁性从0.10μB/Cr提高到0.42μB/Cr。通过分析表明,前者磁性的增强来自薄膜中晶界断键缺陷的增多,而后者磁性的增强来自C杂质缺陷的引入。
(4)减小Cr掺杂AlN在多层膜中的单层厚度至2nm,可以使它发生纤锌矿结构到NaCl结构的相变,新相也具有室温铁磁性,与原纤锌矿六方相相比,新相具有更大的剩磁,且更易被磁化。
Cr-doped AlN is a high curie temperature diluted magnetic material as predicted by theoretical studies. Systematical experimental studies on the structure and magnetism of Cr-doped AlN are helpful to find the origination of ferromagnetism of diluted magnetic materials, as well as to master the way to control their magnetic properties. In this paper, Cr-doped AlN films are prepared by DC reactive magnetron sputtering. The effects of preparation parameters, such as dopant concentration, ferroelectric substrates, substrate negative bias and C co-doping, are systematically studied. Additionally, [TiN/Cr:AlN] multilayers are prepared too. As the period of [TiN/Cr:AlN] multilayers changes, the variation of the structure and magnetism is studied.
As the results show,
Ⅰ. Cr-doped AlN films with content of Cr lower than 11.5atom% are prepared. The elaborate characterizations such XRD and XANES and so on show that the single layer Cr-doped AlN forms uniform solid solution with wurtzite structure, while the Cr-doped AlN in small period [TiN/Cr:AlN] multilayers forms solid solution with rocksalt structure. Dopant Cr dissolves into AlN matrix and substitutes Al with trivalance. No second phase precipitates.
Ⅱ. Room temperature (RT) ferromagnetism is observed in all the samples. It is ascribed to the Al-Cr-N solid solution but not the magnetic second phase. As to the films on Si substrate, the average atomic magnetic moment decreases from 0.23μB/ Cr to 0.03μB/ Cr when the dopant concentration increases from 2.3atom% to 7.7atom%. As to the films on LiTaO3 and LiNbO3 substrates, it is found that as the angle between the c-axis of film and c-axis of substrate decreases, the average atomic magnetic moment of Cr-doped AlN increases from 0.01μB/ Cr to 0.20μB/ Cr. This phenomenon is associated with the scattering of BMP by the internal polarization field of AlN matrix.
Ⅲ.Substrate negative bias leads in the increasing of the magnetism of Cr-doped AlN on Si substrate from 0.07μB/ Cr to 0.18μB/ Cr, while C co-doping leads in the increasing of the magnetism from 0.10μB/ Cr to 0.42μB/ Cr. According to the analysis, the increasing of magnetism is ascribed to the increment of boundary dangling-bond defects in the former case, and ascribed to the introduction of C impurity defects in the latter case.
Ⅳ.When the single layer of Cr-doped AlN in the multilayers is thinner than 2nm, a phase transition from wurtzite structure to rocksalt structure happens. The new phase is RT ferromagnetic too. It’s more susceptive to external magnetic field and has larger residual magnetization.
引文
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