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arXiv 1003.5558cond-mat.mtrl-scicond-mat.other
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含缺陷氧化物及相关材料的铁磁性

Ferromagnetism in defect-ridden oxides and related materials

J. M. D. Coey, P. Stamenov, R. D. Gunning, M. Venkatesan, K. Paul

摘要:

高温铁磁性在含少量磁性掺杂物的氧化物薄膜和纳米颗粒中的存在仍然存在争议。有些人认为这些材料是稀磁半导体,而另一些人认为它们是铁磁性的,只是因为磁性掺杂剂形成了二次铁磁性杂质相,如钴金属或磁铁矿。也有d0系统和其他不含磁性离子的缺陷氧化物的报道。在这里,我们调查二氧化钛(金红石)含有1 - 5%的铁阳离子,并发现,室温铁磁性的脉冲激光沉积制备的薄膜是不是由于磁有序的铁。薄膜既不是稀磁半导体,也不是铁基铁磁杂质相的宿主。提出了一种新的缺陷相关的铁磁性模型,该模型涉及自旋分裂缺陷带,该缺陷带由来自邻近电荷库的电荷转移填充,在本例中,氧化物晶格中的Fe 2+和Fe 3+离子的混合物。该模型的相图显示了非均匀的Stoner铁磁性如何取决于电子总数Ntot、Stoner交换积分I和缺陷带宽W;带占有率由d-d库仑相互作用U决定。存在铁磁金属、半金属和绝缘体以及铁磁金属和绝缘体的区域。高温斯通纳磁性的一个特征是无滞后磁化曲线,其在室温以下实际上与温度无关。这与由局部偶极场确定的游荡铁磁轴有关。磁化强度受缺陷浓度的限制,而不受3d掺杂的限制。只有1 - 2%的薄膜是磁性有序的。

英文摘要:

The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1 - 5% of iron cations and find that the room-temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir in the present case a mixture Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d-d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an an anhysteretic magnetization curve which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1-2 % of the volume of the films is magnetically ordered.

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