Magnetic Ordering Effects Research Articles

Charge-controlled magnetism in colloidal doped semiconductor nanocrystals

Electrical control over the magnetic states of doped semiconductor nanostructures could enable new spin-based information processing technologies. To this end, extensive research has recently been devoted to examination of carrier-mediated magnetic ordering effects in substrate-supported quantum dots at cryogenic temperatures, with carriers introduced transiently by photon absorption. The relatively weak interactions found between dopants and charge carriers have suggested that gated magnetism in quantum dots will be limited to cryogenic temperatures. Here, we report the observation of a large, reversible, room-temperature magnetic response to charge state in free-standing colloidal ZnO nanocrystals doped with Mn(2+) ions. Injected electrons activate new ferromagnetic Mn(2+)-Mn(2+) interactions that are strong enough to overcome antiferromagnetic coupling between nearest-neighbour dopants, making the full magnetic moments of all dopants observable. Analysis shows that this large effect occurs in spite of small pairwise electron-Mn(2+) exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn(2+) ions in the same nanocrystal.

Ab initio study of formation energy and magnetism of sigma phase in Cr–Fe and Cr–Co systems

First-principles electronic structure calculations of total energy differences between the sigma phase and Reference States (RS) of pure constituents in Cr–Fe and Cr–Co systems performed by various methods are presented and compared with enthalpies of formation measured by calorimetry. Both measurements and calculations provide positive values of enthalpy of formation with respect to the RS. Negative values can be obtained when the pure constituents in the sigma phase structure are taken as the RS. Total energy differences of all sigma phase configurations involved are calculated at equilibrium volumes, reproducing well the experimental energy of formation of the sigma phase. The magnetic configurations in Cr–Fe and Cr–Co are also investigated and the stabilizing effect of magnetic ordering in sigma phase at 0 K is demonstrated. It turns out that the magnetic moment depends on the type of the occupied sublattice and total composition of alloy.

Magnetism in Nanoparticles: Beyond the Effect of Particle Size

A set of investigations on selected samples of nanosized cobalt ferrite are reviewed, aimed at studying the various factors affecting the magnetic properties of nanoparticles. Specifically, the effects of inter-particle interactions, of structural and magnetic order, both in the core and on the surface of the particle, have been examined. All factors render the control of the magnetic properties of nanosystems quite difficult, but, at the same time, they also offer the opportunity of tuning them properly, so that materials for specific applications may be created.

Electronic structure, linear, and nonlinear optical responses in magnetoelectric multiferroic material BiFeO3

BiFeO(3) has attracted great interest for its multiferroic property. The spontaneous electric polarization, multiferroism, and static magnetoelectric coupling have been widely studied both experimentally and theoretically. Here, in this paper, we report on the effects of magnetic ordering, spin fluctuation, and external magnetic field on the linear dielectric function and second-harmonic generation (SHG) in multiferroic BiFeO(3). First, our generalized gradient approximation plus Hubbard U calculations reproduce very well experimental data of linear dielectric function. In the mean time, it is revealed that SHG susceptibilities differ dramatically between antiferromagnetic configuration and ferromagnetic one, which is due to the enhanced contrast in the double-photon resonance absorption. Further Monte Carlo simulation of the coupling between electric order and spin-pair correlation function is presented for the spin fluctuation dependence of SHG. The significant nonlinear optical magnetoelectric effect around the Neel temperature provides novel route to enhancing magnetoelectric coupling in multiferroic materials.

Magnetic ordering effects in a Mn-modulation-doped high mobility two-dimensional hole system

We have studied the magnetotransport properties of a manganese (Mn)-modulation-doped high mobility two-dimensional hole system in a strained InAs quantum well (QW) structure. At precisely $T=600\text{ }\text{mK}$ a phase transition from paramagnetism to ferromagnetism can be observed by a change of the low-field magnetotransport behavior and hysteresis. In the magnetically ordered phase a superposition of positive magnetoresistance and weak antilocalization was detected in the longitudinal resistance ${R}_{xx}$ and in the Hall resistance ${R}_{xy}$ a superposition of normal, anomalous, and planar Hall effects demonstrating spontaneous magnetization in the QW plane was detected. From extensive analysis of the temperature and magnetic field dependence of the Shubnikov--de Haas oscillations we deduce the effective mass, transport, and quantum-scattering times. The latter indicates presence of small-range scattering potential. From corrections to the Drude conductivity we determine the impurity interaction time, which is significantly reduced in the ferromagnetic phase indicating interaction of the two-dimensional free holes' spin with the localized magnetic moments of 5/2 from Mn ions.

The effect of magnetic order in amorphous LaSrMnO films on an anomaly in the variable range hopping

Anomalies of a variable range hopping (VRH) in amorphous LaSrMnO films containing ferromagnetic (FM) and antiferromagnetic (AFM) crystalline clusters are investigated. Anomalies are constituted, in the 190–130 K temperature interval, by an increase of the local activation energy ( ε a ) with decreasing temperature. It is established that the increase in ε a is caused by magnetic ordering of AFM clusters containing Mn 3+ and Mn 4+ centers participating in VRH.

Shape-controlled synthesis and cathodoluminescence properties of elongated α-Fe2O3 nanostructures

α -Fe 2 O 3 (hematite) nanostructures with various morphologies have been grown by thermal oxidation of compacted iron powder at temperatures between 700 and 900 °C. Different thermal treatments have been found to induce the growth of single-crystalline nanowires, nanobelts, nanoplates and featherlike structures, free and caped nanopillars, and pyramidal microcrystals or cactuslike microstructures. The experimental conditions leading to the different morphologies have been systematically investigated, as well as the possible growth mechanisms. The obtained nanostructures have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy, x-ray diffraction, and cathodoluminescence (CL) spectroscopy in the SEM. The formation of the nanostructures induces changes in the intensity and spectral distribution of the CL emission, as compared with the bulk material. Ligand to metal charge transfer transitions as well as Fe3+ ligand field transitions are thought to be involved in the observed luminescence. The evolution of the panchromatic CL intensity in the visible range as a function of temperature shows some anomalies that may be induced by magnetic ordering effects.

Itinerant Electron-Driven Chiral Magnetic Ordering and Spontaneous Quantum Hall Effect in Triangular Lattice Models

We study the Kondo Lattice and the Hubbard models on a triangular lattice. We find that at the mean-field level, these rotationally invariant models naturally support a noncoplanar chiral magnetic ordering. It appears as a weak-coupling instability at the band filling factor 3/4 due to the perfect nesting of the itinerant electron Fermi surface. This ordering is a triangular-lattice counterpart of the collinear Neel ordering that occurs on the half-filled square lattice. While the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature, causing a spontaneous quantum Hall effect in the absence of any externally applied magnetic field.

Low temperature transport and specific heat studies ofNd1−xPbxMnO3 single crystals

Electrical transport and specific heat properties ofNd1−xPbxMnO3 singlecrystals for 0.15≤x≤0.5 have been studied in the low temperature regime. The resistivity in the ferromagnetic insulating (FMI)phase for x≤0.3 has an activated character. The dependence of the activation gapΔ ondoping x has been determined and the critical concentration for thezero-temperature metal–insulator transition is determined asxc≈0.33. For a metallicsample with x = 0.42, a conventional electron–electron (e–e) scattering term is found in the low temperature electrical resistivity, although the Kadowaki–Woodsratio is found to be much larger for this manganite than for a normal metal.There is a resistivity minimum observed around 60 K for a metallic sample withx = 0.5. The effect is attributed to weak localization and can be described by a negativeT1/2 weak-localization contribution to resistivity for a disordered three-dimensional electronsystem. The specific heat data have been fitted to contributions from free electrons(γ), spinexcitations (β3/2), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The valueof γ is larger than for normal metals, which is ascribed to magnetic ordering effects involvingNd. Also, the Schottky-like anomaly appears broadened and weakened suggestinginhomogeneous molecular fields at the Nd-sites.

Comment on “Low-Lying States and Hidden Kinematic Collective Charge Instabilities in Parent Cobaltate Superconductors”

Qian et al [1] recently reported angular-resolved photoemission spectroscopy (ARPES) measurements for Na0.8CoO2 that show two concentric Fermi surfaces (FS) split by a delta k_F that varies by a factor of three around the Brillouin zone (BZ). The surfaces occupy 70+/-5% of the full 2D BZ and were interpreted as the bonding and antibonding splitting (BAS) of the a1g bands, with an unspecified effect of magnetic ordering. Below we show that this interpretation is not possible, and, in fact, no valid intepretation of the observed spectra in terms of the bulk electronic structure of Na0.8CoO2 can be found.

Magnetic ordering and quantum statistical effects in strongly repulsive Fermi-Fermi and Bose-Fermi mixtures

We investigate magnetic properties and statistical effects in one-dimensional (1D) strongly repulsive two-component fermions and in a 1D mixture of strongly repulsive polarized fermions and bosons. Universality in the characteristics of phase transitions, magnetization, and susceptibility in the presence of an external magnetic field $H$ are analyzed from the exact thermodynamic Bethe ansatz solution. We show explicitly that polarized fermions with a repulsive interaction have antiferromagnetic behavior at zero temperature. A universality class of linear field-dependent magnetization persists for weak and finite strong interaction. The system is fully polarized when the external field exceeds the critical value ${H}_{c}^{F}\ensuremath{\approx}\frac{8}{\ensuremath{\gamma}}{E}_{F}$, where ${E}_{F}$ is the Fermi energy and $\ensuremath{\gamma}$ is the dimensionless interaction strength. In contrast, the mixture of polarized fermions and bosons in an external field exhibits square-root field-dependent magnetization in the vicinities of $H=0$ and the critical value $H={H}_{c}^{M}\ensuremath{\approx}\frac{16}{\ensuremath{\gamma}}{E}_{F}$. We find that a pure boson phase occurs in the absence of the external field, fully polarized fermions and bosons coexist for $0<H<{H}_{c}^{M}$, and a fully polarized fermion phase occurs for $H\ensuremath{\geqslant}{H}_{c}^{M}$. This phase diagram for the Bose-Fermi mixture is reminiscent of weakly attractive fermions with population imbalance, where the interacting fermions with opposite spins form singlet pairs.

Modelling paramagnetism in EELS: a study of magnetic order

Magnetic ordering has been shown to have a significant effect on the shape of the oxygen K-edge energy-loss near edge structure (ELNES) in a series of chromite spinels. However, the ELNES of these materials has only been simulated using a rough approximation of antiferromagnetism - the true nature of the magnetic interactions responsible for the detailed oxygen K-edge shape is still unknown. Chromite spinels typically undergo transitions to long range ordered antiferromagnetic (AFM) structures at temperatures below ∼15K. Dynamic short range magnetic order (SRO) has been observed at temperatures up to 150K using neutron powder diffraction (NPD). It is not clear whether long range magnetic order, short range magnetic order or paramagnetic effects are responsible for the characteristic oxygen K-edge ELNES observed at room temperature. Here we discuss the possibility of carrying out paramagnetic simulations using the real space multiple scattering program FEFF8.2, and show preliminary results of paramagnetic simulations of the oxygen K-edge ELNES of magnesium chromite.

Enhancement of paramagnetic effects during spin alignment in 2D semiconductors

Effects of magnetic ordering associated with the Coulomb exchange interaction of free electrons in a 2D Fermi system are considered. It is shown that the paramagnetic response is substantially enhanced by Fermi-liquid effects. The phase transition to a state with spontaneous polarization of spins can be observed when Heisenberg parameter J is not smaller than μF/3.06 (approximately one-third of the Fermi energy) and not larger than half the Fermi energy (J ≤ μF/2).

Magnetic ordering effects in the Raman spectra of Sn1−xCoxO2

The room temperature ferromagnetism is observed in chemically synthesized powderlike samples of Sn1−xCoxO2 compound (x=0.07 and 0.1). The Raman spectra of the Sn1−xCoxO2 films were analyzed in terms of their structural and magnetic properties. In the Raman spectra of the Sn1−xCoxO2 films additional features were revealed which are associated with the excitation of phonon modes in the cobalt-oxide-defect complex. The intensities of the Raman bands showed a strong dependence on the doping concentration and on applied magnetic field. The softening of the ∼630cm−1 symmetric A1g mode and the field dependence of the intensity Raman scattering observed for Co-doped SnO2 films was ascribed to spin-phonon interaction. The absorption spectra show that there are changes of absorption edge toward low energy due to the influence of incorporated Co atoms on the electronic states of SnO2. These results can be described by the bound magnetic polarons theory.

Magnetic property and charge ordering effect in polycrystalline LuFe2O4

The charge ordering effect in LuFe2O4 powder is investigated with the vibrating sample magnetometer and Mössbauer spectroscopy. The crystal structure of LuFe2O4 shows the two-dimensional layered type rhombohedral(R3¯m) structure. The lattice constants are found to be a0=3.439Å and c0=25.258Å by the Rietveld refinement. The Néel temperature (TN) is determined to be 250K. The Mössbauer spectra consist of four sextets indicating the magnetic ordering below TN, where three sets are assumed to be Fe3+ phases and the other is Fe2+ at 4.2K, and two doublets splitting in a paramagnetic region. At room temperature, the electric quadrupole splittings (ΔEQ) of two doublets are 0.22±0.01 and 0.67±0.01mm∕s, respectively. The isomer shift value of a doublet with smaller ΔEQ is 0.18±0.01mm∕s relative to the Fe metal, which is consistent with the Fe3+ valence state, while the value of a doublet with larger ΔEQ is 0.83±0.01mm∕s indicating Fe2+ state. The Mössbauer spectra suggest that the observed asymmetry below 370K is due to the charge ordering effect between Fe2+ and Fe3.

Doniach diagram for ordered, disordered and underscreened Kondo lattices

The Doniach's diagram has been originally proposed to describe the competition between the local Kondo effect and the inter-site RKKY interactions in cerium compounds. Here we discuss the extension of this diagram to different variations of Kondo lattice model. We consider (a) ordered cerium compounds where the competition between magnetic order and Kondo effect plays an important role, as in CeRh 2Si 2, (b) disordered cerium systems with competing spin glass phase, magnetic ordered phases and a Kondo phase, as in the heavy fermion cerium alloy CeCu x Ni 1 - x and (c) uranium compounds where a coexistence between Kondo effect and ferromagnetic order has been observed, as in UTe. We show that all these cases can be described by a generalized Doniach phase diagram.

A comparison study of the entropy changes in materials with and without short-rangemagnetic order

The effects of short-range magnetic order (SRMO) on magnetic entropy changes have been studied for thecompounds Sm0.55Sr0.45MnO3, Eu0.55Sr0.45MnO3 and the rare-earth metal Gd. Ferromagnetic clusters composed of∼8 Mn ions are found to be formed in the paramagnetic phase of the manganites based on theanalysis of susceptibility. As a result, the entropy change for a field variation of 0–5 T is∼6 J kg−1 K−1 forSm0.55Sr0.45MnO3 and∼7 J kg−1 K−1 forEu0.55Sr0.45MnO3, wellbelow the values expected without considering the effect of SRMO. (The maximum entropy change willbe ∼56 J kg−1 K−1 for both compounds.) Assuming the presence of sizable magnetic clusters, the field-inducedentropy change, not only its peak value but also its temperature dependence, can bewell reproduced by the mean field theory. The average cluster size is found to bem≈8, in agreement with the result obtained from a Curie–Weiss analysis ofthe susceptibility. The presence of SRMO reduces the entropy change by∼80%. In contrast, the mean field theory gives a good description of the entropy change in Gd,for which no significant SRMO is observed. The present work reveals the important role ofSRMO in modifying the magnetocaloric properties of the magnetic materials.

Structural distortions and model Hamiltonian parameters: From LSDA to a tight-binding description ofLaMnO3

The physics of manganites is often described within an effective two-band tight-binding TB model for the Mn eg electrons, which apart from the kinetic energy includes also a local “Hund’s rule” coupling to the t2g core spin and a local coupling to the Jahn-Teller JT distortion of the oxygen octahedra. We test the validity of this model by comparing the energy dispersion calculated for the TB model with the full Kohn-Sham band structure calculated within the local spin-density approximation LSDA to density functional theory. We analyze the effect of magnetic order, JT distortions, and “GdFeO3-type” tilt rotations of the oxygen octahedra. We show that the hopping amplitudes are independent of magnetic order and JT distortions and that both effects can be described with a consistent set of model parameters if hopping between both nearest and next-nearest neighbors is taken into account. We determine a full set of model parameters from the density functional theory calculations, and we show that both JT distortions and Hund’s rule coupling are required to obtain an insulating ground state within LSDA. Furthermore, our calculations show that the GdFeO3-type rotations of the oxygen octahedra lead to a substantial reduction of the hopping amplitudes but to no significant deviation from the simple TB model.

The effect of magnetic order and thickness in the Raman spectra of oriented thin films ofLaMnO3

Low-temperature Raman studies on (100)-orientedLaMnO3 thin films of different thicknesses deposited onLaAlO3(100) substrate by using the pulsed laser deposition technique are presented. The phonon modefrequencies depend strongly on the thickness of the film and in comparison to theLaMnO3 single crystal shift to higher values with decreasing thickness. The broadening of Ramanline widths was also observed with decreasing film thickness. Anomalous softening of theB2g mode (Jahn–Teller distortion activated) was observed for all films below the magnetic orderingtemperature. A signature of lattice anomalies in the form of changes in line width of theB2g mode correlated with the magnetic ordering temperature was also seen. The results wereinterpreted by considering strong spin–phonon coupling and reduction in Jahn–Tellerdistortion driven polarons due to magnetic order.

Crossover from S–I–S to S–F–S junctions in composites Y 3/4Lu 1/4Ba 2Cu 3O 7 + Y 3(Al 1− xFe x) 5O 12

The transport properties of the two phases composites based on Y 3/4Lu 1/4Ba 2Cu 3O 7 (YBCO) and Y 3(Al 1− x Fe x ) 5O 12 ( x = 0.0 … 1.0) have been studied to investigate the effect of magnetic ordering in garnet on superconducting properties of the composites. The composites represent network of Josephson junctions where the non-superconducting ingredient (garnet) acts as barriers separating YBCO grains. It is found that magneto-active ions (Fe) in garnet are responsible for an additional suppression of Josephson coupling in the composites. At some critical concentration of Fe atoms full suppression of super current is observed in a temperature range from T C of YBCO grains down to the characteristic temperature T m. This behavior is attributed to the interaction of super current carriers with spins of Fe atoms located in the barriers. The crossover from S–I–S (S – superconductor, I – insulator) to S–F–S (F – ferrimagnet) junctions is observed in the composites Y 3/4Lu 1/4Ba 2Cu 3O 7 + Y 3(Al 1− x Fe x ) 5O 12 with x increase.