Origin Of Magnetic Order Research Articles

Progress in designing novel single-phase room temperature multiferroics

Magnetoelectric multiferroics, because of their coupled electric and magnetic ordering in a single phase, are eminent for applications in logic and nonvolatile memory devices. Recent advances in computational modeling, synthesis, and characterization techniques are spurring significant advances in the study of these materials. During last several years, our group at University of Puerto Rico has been involved in synthesizing novel single-phase room temperature multiferroic materials that included not only solid solutions of lead zirconate titanate with lead iron tungstate, lead iron niobate, and lead iron tantalate, but also recently discovered palladium substituted room temperature multiferroics, namely PbPd0.3Ti0.7O3 and Pb(Zr0.2Ti0.8)Pd0.3O3. In this review, we have reported thin films of the above materials that were synthesized using pulsed laser deposition technique on various substrates at Speclab, University of Puerto Rico. We describe in detail thin film fabrication, structural diffraction analyses (XRD, TEM), dielectric, ferroelectric, piezo-response force microscopy, and magnetization results on recently grown Pd-substituted lead titanate and lead zirconate titanate thin films in an elaborate manner. The films were phase pure and stabilized in c-axis oriented tetragonal phase (P4mm) with surface roughness RQ of 2–4 nm. Temperature-dependent dielectric studies on metal-dielectric-metal heterostructure capacitors indicate that ferroelectric to paraelectric phase transition is above 500 K. A proof of ferroelectricity was confirmed from strong domain switching responses using piezo-response force microscopy. These films showed ferromagnetic ordering below 395 K. The origin of magnetic ordering in these materials was attributed to the existence of Pd2+ and Pd4+ mixed oxidation states of palladium dispersed in the polar matrices. We conclude with the above findings that these multiferroics may have potential applications.in nonvolatile memory and other multifunctional devices.

Absence of Ferromagnetism in VSe2 Caused by Its Charge Density Wave Phase

How magnetism emerges in low-dimensional materials such as transition metal dichalcogenides at the monolayer limit is still an open question. Herein, we present a comprehensive study of the magnetic properties of single crystal and monolayer VSe$_{2}$, both experimentally and \emph{ab initio}. Magnetometry, X-ray magnetic circular dichrosim (XMCD) and \emph{ab initio} calculations demonstrate that the charge density wave in bulk stoichiometric VSe$_{2.0}$ causes a structural distortion with a strong reduction in the density of sates at the Fermi level, prompting the system towards a non-magnetic state but on the verge of a ferromagnetic instability. In the monolayer limit, the structural rearrangement induces a Peierls distortion with the opening of an energy gap at the Fermi level and the absence of magnetic order. Control experiments on defect-induced VSe$_{2-\delta}$ single crystals show a breakdown of magnetism, discarding vacancies as a possible origin of magnetic order in VSe$_{2}$.

Revealing Insights into LixFePO4 Nanocrystals with Magnetic Order at Room Temperature Resulting in Trapping of Li Ions.

A systematic study on the structures, magnetic properties, and electrochemical performance of LixFePO4 was carried out. Interestingly, it was found that the magnetic phase transition temperature is around 295 K in LixFePO4 (x ≤ 0.12) nanocrystals, which is higher than that of LiFePO4 (TN = 51(1) K) and FePO4 (TN = 114(1) K) phases. The origin of magnetic order around room temperature (RT) in LixFePO4 (x ≤ 0.12) is proposed to be due to both Fe2+-Fe3+ superexchange and the broken local symmetries induced by Fe2+-Li+ antisite defects. Ab initio calculations were adopted to calculate the total energy change of the ferromagnetic state and antiferromagnetic state (ΔE) to qualitatively estimate TN and proved the existence of the magnetic order around room temperature. Because of the strong Lorentz force in RT magnetic domains, Li ions will be trapped and form sub-10 nm solid solution domains of Li0+δFePO4, which have a direct relationship with electrochemical performance during (de)lithiation procedures.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6

Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.

Temperature-Mediated Magnetism in Fe-Doped ZnO Semiconductors

We have employed first-principles calculations with PBE0 hybrid functional to study the magnetic origin of Fe-doped ZnO semiconductors. Density functional theory predicts antiferromagnetic ordering for Fe2+-substituted ZnO materials. Origins of magnetic ordering are attributed directly to the local ordering of Fe in the ZnO matrix. Fe3+ induced magnetism is studied for models exhibiting a zinc vacancy or an interstitial oxygen atom. In both cases Fe3+ couples antiferromagnetically. Taking into account the temperature-dependent relative Gibbs energy, the magnetic ordering of Fe2+ and Fe3+ with interstitial oxygen atoms is changed from anti- to ferromagnetic with increasing T. This indicates that the Fe-doped ZnO magnetism is highly dependent on temperature.

First-Principles Calculations of the Electronic Structure of Tetragonalα-FeTe andα-FeSe Crystals: Evidence for a Bicollinear Antiferromagnetic Order

By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal alpha-FeTe is in a bicollinear antiferromagnetic order, in which the Fe local moments (approximately 2.5 microB) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This novel bicollinear order results from the interplay among the nearest, the next-nearest, and the next-next-nearest neighbor superexchange interactions, mediated by Te 5p band. In contrast, the ground state of alpha-FeSe is in a collinear antiferromagnetic order, similar to those in LaFeAsO and BaFe2As2. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors.

Scaling the Kondo lattice

The origin of magnetic order in metals has two extremes: an instability in a liquid of local magnetic moments interacting through conduction electrons, and a spin-density wave instability in a Fermi liquid of itinerant electrons. This dichotomy between 'local-moment' magnetism and 'itinerant-electron' magnetism is reminiscent of the valence bond/molecular orbital dichotomy present in studies of chemical bonding. The class of heavy-electron intermetallic compounds of cerium, ytterbium and various 5f elements bridges the extremes, with itinerant-electron magnetic characteristics at low temperatures that grow out of a high-temperature local-moment state. Describing this transition quantitatively has proved difficult, and one of the main unsolved problems is finding what determines the temperature scale for the evolution of this behaviour. Here we present a simple, semi-quantitative solution to this problem that provides a basic framework for interpreting the physics of heavy-electron materials and offers the prospect of a quantitative determination of the physical origin of their magnetic ordering and superconductivity. It also reveals the difference between the temperature scales that distinguish the conduction electrons' response to a single magnetic impurity and their response to a lattice of local moments, and provides an updated version of the well-known Doniach diagram.

π-Electron Ferromagnetism in Metal-Free Carbon Probed by Soft X-Ray Dichroism

Elemental carbon represents a fundamental building block of matter and the possibility of ferromagnetic order in carbon has attracted widespread attention. However, the origin of magnetic order in such a light element is only poorly understood and has puzzled researchers. We present a spectromicroscopy study at room temperature of proton irradiated metal-free carbon using the elemental and chemical specificity of x-ray magnetic circular dichroism. We demonstrate that the magnetic order in the investigated system originates only from the carbon pi-electron system.

Anisotropy of Susceptibility and Optical investigation of the Antiferromagnetic phase Transition in U1-xCexRu2Si2

Temperature dependence of magnetic susceptibility of U 1 _x Ce.xRu2 Si2 single crystals (x = 0, 0.025, 0.05, 0.075) has been studied at temperatures from 4.2 K up to room temperature and in magnetic fields applied along and perpendicular to the c-axis. The attention was paid to anomalies connected with the antiferromagnetic phase transition, which are present in crystals with x < 0.05. Far-infrared measurements of U0,99 Ce0,01 Ru e Sit single crystal have been studied for the first time. The measurements support the assumption that the origin of magnetic ordering in U0.99 Ce0.01 Rue Sit is the same as in heavy-fermion superconductor URu2Si2 and can be interpreted as being due to a spin-density-wave gap.

Mössbauer study on Fe-sitting site and origin of magnetic order in YBa2(Cu0.95Fe0.05)3O7−δ

Fe-sitting sites and the origin of magnetic order in YBa2(Cu0.95Fe0.05)3O7−δ have been studied by comparing the Mossbauer spectra observed before and after heat-treatments at 300°C in vacuum. Both of the major quadrupole doublets D-1 and D-2 are assigned to originate from Fe’s replacing Cu1’s. It is also suggested that fluctuations of Cu2 spins play an important role for inducing the magnetic order.