Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below T(V)=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (approximately 80 meV) are shifted upwards in energy above T_{V} due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Delta(5) symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above T(V).     

Pressure-induced magnetic order in golden SmS

High pressure 149Sm nuclear forward scattering experiments have been performed on the nonmagnetic semiconductor SmS. We present the first clear evidence that the closure of the insulating gap at p(Delta) approximately 2 GPa coincides with the appearance of magnetic order. The pressure-induced magnetic phase transition has some first order character and suggests that the Sm ions are nearly trivalent at p(Delta). A Gamma(8) quartet crystal field ground state with a value of approximately 0.5 micro(B) for the samarium magnetic moment is inferred from our results. Considerable magnetic short range order is observed above the ordering temperature inferred from macroscopic measurements.     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

典型磁性材料价电子结构研究面临的机遇与挑战

目前在磁性材料磁有序现象研究中广泛使用的交换作用、超交换作用和双交换作用模型形成于1950年代及其以前,这些模型都涉及材料中的价电子状态,但那时还没有充分的价电子状态实验依据.1970年代以来,有关价电子结构实验结果的报道越来越多,这些实验结果表明传统的磁有序模型需要改进.首先,大量电子谱实验表明,在氧化物中除存在负二价氧离子之外,还存在负一价氧离子,并且负一价氧离子的含量可达30%或更多.这说明以所有氧离子都是负二价离子为基本假设的超交换和双交换作用模型需要改进.其次,一些实验证明,铁、钴、镍自由原子的一部分4s电子在形成铁磁性金属的过程中变成了3d电子,这为探讨金属磁性与电输运性质的关系提供了依据.此外,即使在现代的密度泛函计算中,仍不能给出磁性交换作用能的函数表达式,只能采取各种不同模型进行模拟计算,从而使磁性材料的模拟计算遇到严重困难.寻求一个磁有序能的函数表达式可能是解决这个困难的途径.这些研究表明磁性材料价电子结构研究面临着重大的机遇与挑战.本文首先介绍一些典型的实验例证,然后介绍了基于这些实验结果的一套典型磁性材料的磁有序新模型,随后介绍了基于新模型的磁性材料价电子结构与旧模型的主要区别,最后指出了未来研究工作面临的挑战.     

Ferromagnetism in ZnO with（Mn,Li） codoping

First-principles calculations were performed to investigate the magnetic properties of Zn(Mn,Li)O based on the Perdew-Burke-Ernzerhof form of generalized gradient approximation. Antiferromagnetic (AFM) ordering is the ground state in Mn-doped ZnO system without the codopant of Li, while seven different geometrical configurations of Zn(Mn,Li)O prefer stable ferromagnetic (FM) ordering. We found that dopant Li can effectively change the magnetic coupling in the ZnMnO system. The Curie temperature (T_C) of FM ordering depends on the geometric configuration, and the highest T_C is about 1388 K. The FM stabilization is greatly affected by Mn-Mn distance rather than by the position of dopant Li. We propose that dopant Li mediates FM coupling through a double exchange interaction or an RKKY interaction when Li is located, respectively, near or far from Mn ions.     

Raman scattering studies of the magnetic ordering in hexagonal HoMnO3 thin films

We present the results of the temperature dependence of the Raman spectra of hexagonal HoMnO₃ thin films in the 13–300 K temperature range. The films were grown on Pt(111)//Al₂O₃ (0001) substrates using the laser ablation method. In the HoMnO₃ thin films, we initially observedseveral broad Raman peaks at ∼510, 760, 955, 1120, and 1410 cm⁻¹. These broad Raman peaks display an anomalous behavior near the magnetic transition temperature, and the intensity difference of the Raman spectra at different temperatures shows several pairs of negative and positive peaks as the temperature is lowered below the Néel temperature. Our analyses indicate that all the broad peaks are correlated with magnetic ordering, and we have assigned the origin of all the broad peaks. Purely on the basis of the Raman analysis, we have deduced the Néel temperature and the spin exchange integrals of HoMnO₃ thin films. We also investigated the effects of the growth condition on the strongest broad peak at ∼760 cm⁻¹, which is related with pure magnetic ordering. This result indicates that the oxygen defect in the HoMnO₃ sample has negligible effect on magnetic ordering. Copyright © 2009 John Wiley & Sons, Ltd.     

Zero temperature phase diagram of the ferromagnetic Kondo lattice

We study numerically the one-dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic islands induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion and presents an insulating gap associated to the charge structure. We study the metal–insulator transition induced by a magnetic field which removes simultaneously both charge and spin orderings. This new mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.     

Anomalous magnetic and superconducting properties in a Ru-based double perovskite

We have studied the double perovskite [1] structure Sr₂Y(Ru_1-x Cu_x)O₆ system. The parent compound is an antiferromagnetic insulator with Neel temperature ～ 26 K. Partially substituted the Ru ion by Cu the compounds increase their conductivity drastically and eventually become superconducting. More intriguingly is the observation of the coexistence of superconductivity and magnetic ordering. The superconducting transition temperature T _c and the magnetic ordering temperature T _m are of the same order. The observed magnetic structure and superconductivity of these compounds can be understood in terms of a plausible theoretical model based on the double exchange idea.     

Ordering of magnetic impurities and tunable electronic properties of topological insulators

We study collective behavior of magnetic adatoms randomly distributed on the surface of a topological insulator. Interactions of an ensemble of adatoms are frustrated, as the RKKY-type interactions of two adatom spins depend on the directions of spins relative to the vector connecting them. We show that at low temperatures the frustrated RKKY interactions give rise to two phases: an ordered ferromagnetic phase with spins pointing perpendicular to the surface, and a disordered spin-glass-like phase. The two phases are separated by a quantum phase transition driven by the magnetic exchange anisotropy. The ordered phase breaks time-reversal symmetry spontaneously, driving the surface states into a gapped state, which exhibits an anomalous quantum Hall effect and provides a realization of the parity anomaly. We find that the magnetic ordering is suppressed by potential scattering.     

Magnetic behavior and clustering effects in Mn-doped boron nitride sheets

Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS.     

Magnetic and transport properties of bilayered manganites La1.2Sr1.8Mn2-xGaxO7（x=0,0.08）

The magnetic and electrical properties of nonmagnetic Ga +3 ion substitution for Mn site are investigated in the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7.When the Mn is substituted by Ga,the ferromagnetic property obviously weakens,the magnetic transition temperature decreases and a spin-glass behaviour occurs at low temperature.Meanwhile,doping causes the resistivity to dramatically increase,the metal-insulator transition temperature to disappear,and a greater magneto-resistance effect to occur at low temperature.These effects result from the fact that Ga substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction,and further suppresses ferromagnetic ordering and metallic conduction.     

The magnetic structure of M-type hexaferrites

A symmetry analysis is presented of the possible types of magnetic ordering in hexagonal ferrimagnets having an (M type) magnetoplumbite structure. In the analysis the transition channel to a magnetically ordered state according to the k=0 star is considered. It is shown that the only general commuting representation for all regular point systems in such compounds is the unitary representation τ₁ corresponding to ferromagnetic ordering of the magnetic moments in sublattices. Antiferromagnetic exchange between ions of different sublattices leads to a collinear ferrimagnetic structure.     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

Magnetic pair making and breaking effect of Ru in La0.7Sr0.3Mn0.9Ru0.1O3 and La0.5Sr0.5Co0.9Ru0.1O3

The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La_0.7Sr_0.3MnO₃ and La_0.5Sr_0.5CoO₃ has been investigated. It is found that substitution of 10% Ru at the Mn site of La_0.7Sr_0.3MnO₃ decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔT_c80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La_0.5Sr_0.5CoO₃. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru⁺⁴/Ru⁺⁵. The e_g and t_2g parentage of Ru⁺⁵ is similar to Mn⁺⁴ and therefore, Ru⁺⁵ ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co⁺³: t_2g⁵e_g¹), forcing a double exchange FM state to anti-FM state.     

Thermal characteristics of phase transformations in Fe-Pt alloy

The effect of ordering on the thermal characteristics of the magnetic and martensite transitions in the Fe-Pt alloy with a composition close to stoichiometric Fe₃Pt composition is analyzed. It is found that a transition to a partly ordered state reduces the specific heat variation and thermal effect of the material due to the magnetic transition.     

The theory of atomic and magnetic ordering in ternary alloys with fcc lattice

We consider the theory of the mutual influence of atomic and spin ordering phenomena in ternary alloys of transition elements with fcc lattice. The case of quasibinary alloys B₃A-B₃D is considered in detail. Concentration dependences of the critical temperature of the order-disorder phase transition are considered for cases of atomic and magnetic ordering.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 48–57, December, 1970.     

Charge-orbital ordering and Verwey transition in magnetite

Local density approximation + Hubbard U (LDA + U) band structure calculations reveal that magnetite (Fe3O4) forms an insulating charge-orbital-ordered state below the Verwey transition temperature. The calculated charge ordering is in good agreement with that inferred from recent experiments. We found an associated t(2g) orbital ordering on the octahedral Fe2+ sublattice. Such an orbital ordering results primarily from the on-site Coulomb interaction. This finding unravels such fundamental issues about the Verwey transition as the mechanism for the charge ordering and for the formation of the insulating gap, as well as the nonobedience of the Anderson's criterion for the charge ordering.     

Spin nematics and magnetization plateau transition in anisotropic kagome magnets

We study S=1 kagome antiferromagnets with an isotropic Heisenberg exchange J and strong easy-axis single-ion anisotropy D. For D>J, the low-energy physics can be described by an effective S=1/2 XXZ model with antiferromagnetic Jz approximately J and ferromagnetic J perpendicular approximately J2/D. Exploiting this connection, we argue that nontrivial ordering into a "spin-nematic" occurs whenever D dominates over J, and discuss its experimental signatures. We also study a magnetic field induced transition to a magnetization plateau state at magnetization 1/3 which breaks lattice translation symmetry due to ordering of the Sz and occupies a lobe in the B/Jz-Jz/J perpendicular phase diagram.     

Configurations, Electronic Structure and Magnetic Ordering of Small Manganese Clusters

We investigate the configurations, electronic structures, and magnetic ordering of MnN （N = 2-13） clusters based on all-electron density functional theory. The Jahn-Teller effect plays an important role in determining the ground state of certain geometries. The magnetic ordering of the MnN dusters transits from ferromagnetic ordering for the smallest （ N = 2, 3） dusters to a near degeneracy state including ferromagnetic and ferrimagnetic ordering in the vicinity of N = 4-6 and to a clear ferrimagnetic ordering at N = 7 or beyond. N = 6 and 10 are the magic numbers for neutrai MnN （N = 2-13） dusters.