Mechanisms and origins of half-metallic ferromagnetism in CrO2
Mechanisms and origins of half-metallic ferromagnetism in CrO2
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DOI：
10.1103/physrevb.92.144407
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发表时间：
2015-10-07
影响因子：
3.7
通讯作者：
Mazurenko, V. V.
中科院分区：
文献类型：
--
作者：
Solovyev, I. V.;Kashin, I. V.;Mazurenko, V. V.
利用第一性原理电子结构计算得到的真实低能模型，研究了正则半金属(HM)铁磁性材料CrO2中原子间交换作用的行为。为此，我们采用了基于有效Anderson杂质哈密顿量的精确对角化的动力学平均场理论(DMFT)，并进一步补充了交换相互作用的无限小自旋旋转理论。为了阐明静态和动态电子关联所起的相对作用，我们将所得到的结果与几种静态方法进行了比较，其中包括无限制的Hartree-Fock(HF)近似、静态DMFT(对应于自能的无限频率限制)以及在随机相近似下处理关联相互作用的优化有效势方法。我们的结果表明，CrO2中HM铁磁性的起源是非常重要的。对于相邻配位球中的相互作用，HF和DMFT方法产生了非常相似的结果，这是因为部分抵消了铁磁(FM)双交换和反铁磁(AFM)超交换贡献，这代表了交换相互作用的(Delta(Sigma)over Cap)(-1)展开(Delta(Sigma)over Cap)(-1)展开(Delta(Sigma)over Cap是原子内自旋分裂)。这两个贡献在HF近似下都较弱，这分别是由于t(2g)态的额外轨道极化和忽略了动力学关联。高阶项在(Delta(Sigma)Over Cap)(-1)展开中的作用是双重的。一方面，它们对邻近的交换作用产生额外的FM贡献，从而趋于稳定FM态。另一方面，它们产生了原子力显微镜的长程相互作用，这使得在由t(2g)带组成的最小模型的单点DMFT计算中FM态不稳定。因此，最小模型中的稳健铁磁性是偶然的，而且这一图景在更严格的DMFT方法的水平上得到了很大的修正。我们认为，最小模型中缺少的主要成分是氧2p带的直接交换相互作用和磁极化。我们在局域自旋密度近似下评价了这些贡献，并认为它们对CrO2中FM基态的稳定性起着非常重要的作用。
Using a realistic low-energy model, derived from the first-principles electronic structure calculations, we investigate the behavior of interatomic exchange interactions in CrO2, which is regarded to be one of the canonical half-metallic (HM) ferromagnetics. For these purposes we employ the dynamical mean-field theory (DMFT), based on the exact diagonalization of the effective Anderson impurity Hamiltonian, which was further supplemented with the theory of infinitesimal spin rotations for the exchange interactions. In order to elucidate the relative roles played by static and dynamic electron correlations, we compare the obtained results with several static techniques, including the unrestricted Hartree-Fock (HF) approximation, static DMFT (corresponding to the infinite frequency limit for the self-energy), and optimized effective potential method for treating the correlation interactions in the random-phase approximation. Our results demonstrate that the origin of the HM ferromagnetism in CrO2 is highly nontrivial. As far as the interactions in the neighboring coordination spheres are concerned, HF and DMFT methods produce very similar results, due to the partial cancellation of ferromagnetic (FM) double-exchange and antiferromagnetic (AFM) superexchange contributions, which represent two leading terms in the (Delta(Sigma) over cap)(-1) expansion for the exchange interactions (Delta(Sigma) over cap being the intra-atomic spin splitting). Both contributions are weaker in the HF approximation due to, respectively, additional orbital polarization of the t(2g) states and neglect of dynamic correlations. The role of higher-order terms in the (Delta(Sigma) over cap)(-1) expansion is twofold. On the one hand, they give rise to additional FM contributions to the neighboring exchange interactions, which tend to stabilize the FM state. On the other hand, they produce AFM long-range interactions, which make the FM state unstable in the single-site DMFT calculations for the minimal model, consisting of the t(2g) bands. Thus, the robust ferromagnetism in the minimal model, which can be easily obtained using static approximations, is fortuitous and this picture is largely revised at the level of more rigorous DMFT approach. We argue that the main ingredients, which are missing in the minimal model, are the direct exchange interactions and the magnetic polarization of the oxygen 2p band. We evaluate these contributions in the local-spin-density approximation and argue that they play a very important role in stability of the FM ground state in CrO2.