摘要
超薄膜磁学一直是近几十年来磁学领域研究的热点,一方面是因为超薄膜磁性材料在磁存储等信息产业上具有很大的应用价值,另一方面是由于超薄膜磁性材料由于其维度的降低本身就具有很多独特的磁学性质。对于这些性质的研究可以加深我们对于磁性机理的理解。其中低维下的磁性相变更是引起了磁学研究领域中理论和实验上的广泛兴趣。在本论文中,我们研究了磁性金属Ni的不同结构对其居里相变的影响,镍钯合金超薄膜的自旋重定向转变与居里相变的关联。此外,还研究了Fcc Fe超薄膜的磁性以及Bi/Cu(111)表面态的Rashba分裂等问题,得到了如下结果:
1.从第一性原理出发,利用线性缀加平面波(LAPW)结合固定自旋矩的算法,得到了Fcc Ni和Bcc Ni的交换关联积分。最后采用蒙特卡罗模拟得到了Fcc Ni和Bcc Ni的居里温度。通过理论计算我们可以很好的理解Bcc Ni的居里温度低于Fcc Ni居里温度的实验结果。
2.Fcc结构的Fe具有十分丰富的磁性相图,所以实验上Fcc Fe一直是超薄膜磁学研究的热点。我们在GaAs衬底上分别利用Au,Cu作为过渡层,外延得到了Fcc结构的Fe薄膜。利用磁光科尔效应的实验技术研究了Fcc Fe的磁性,并利用Rotmoke的方法得到了Fcc Fe的磁晶各向异性。
3.利用成分梯度的实验技术在Cu(001)衬底上外延得到了合金比例连续变化并且厚度均匀的Ni_x,Pd_(1-x)合金,并利用交流磁化率和磁光科尔效应的实验技术研究了Ni_xPd_(1-x)合金的自旋重定向转变与居里温度的关系。实验结果表明,在大于7层的厚度下,Ni_xPd_(1-x)合金会随着Ni含量的变化发生自旋重定向转变。合金的居里温度随着Ni含量的增加不断增加,并没有在自旋重定向转变发生的区域出现居里温度下降的现象。但是当自旋重定向转变发生后,Ni_xPd_(1-x)合金的居里温度随Ni含量增加的斜率要增大很多。这就表明对于薄膜体系,垂直各向异性相比于面内的四度各向异性更有助于提升铁磁薄膜的居里温度。
4.利用双光子光电子能谱,我们第一次在实验上观测到了Bi/Cu(111)表面合金在高于费米能级1.3eV的能量处具有未占据的自旋劈裂能态。这些能态可以用Rashba模型进行很好的拟合,从拟合结果可以得到Bi/Cu(111)表面合金未占据态的Rashba分裂的大小和有效质量等。
5.进行了一些探索性的实验,尝试利用电解液在磁性样品表面形成的电场,对薄膜样品的磁性进行电场调制。我们看到了一些有趣的现象,但没有得到理想的实验结果。
Ultrathin film magnetism is a hot topic during recent years in the research field of magnetism. One reason is that the ultrathin ferromagnetic films have a lot of applications in the industry. The other reason is that they have a lot of unique properties due to their low dimensionality. The research of such properties will improve our understanding to the mechanism of the ferromagnetism. Especially the critical behaviors of the low dimensional system are intensively investigated both in theory and experiment. In this thesis, I mainly focus on how the structure of Ni influences its Curie temperature, the correlation between the spin reorientation transition and Curie temperature of NiPd alloy. In addition, the magneto crystalline anisotropy of Fcc Fe and the Rashba splitting of the surface states for Bi/Cu (111) are investigated. The main results are as followed:
1. The exchange integrals of Fcc and Bcc Ni were obtained by combining the first principle LAPW calculation and fixed spin momentum method. And the Curie temperatures of them were calculated by the Monte-carlo simulation. The calculation result can well explain the experimental fact that the Curie temperature of Bcc Ni is much lower than that of the Fcc Ni.
2. Fcc Fe has a rich magnetic phase diagram, so its magnetic property is an interesting topic in the research of magnetism. We can obtain the Fcc Fe on GaAs substrate by growing Au or Cu seedlayer. Its magnetism was investigated by magneto-optic-kerr-effect. And the anisotropy of Fcc Fe was obtained by Rotmoke technique.
3. The correlation between spin reorientation transition (SRT) and Curie temperature of Ni_xPd_(1-x) alloy on Cu(001) was investigated by ac susceptibility and magneto-optic-kerr-effect (MOKE) measurements. The result shows that Ni_xPd_(1-x) alloy film exhibits a composition-driven spin reorientation transition above 7ML. And its Curie temperature increases monotonically but with different slopes upon the increase of Ni composition. The slope of the increase of Curie temperature versus Ni composition is getting much larger after SRT. This result suggests that for the film system, the perpendicular anisotropy can enhance the Curie temperature much higher than that of the in plane fourth-fold anisotropy.
4. The Rashba splitting of Bi/Cu(111) surface alloy has been investigated by the two-photon photoemission. Combined with the one-photon photoemission result, we can make sure that there are the unoccupied surface states above the Fermi level 1.3eV. And this is the first time to observe these surface states at this energy. These unoccupied surface states can be well fitted by the Rashba model. From this fitting, the magnitude of the Rashba splitting and the effective mass can be obtained.
5. We have performed some exploratory investigations on modulating the magnetism of the ultrathin ferromagnetic film by the electric field. We have observed some interesting phenomena, but didn' t get the ideal experimental result.
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