Antiferromagnetic Alignment Research Articles

Magneto-optical effect of (Sm, Co) co-doping in ZnO semiconductor

In this paper, the effect of (Sm, Co) co-doping on the structure, optical and magnetic properties of ZnO nanoparticles was studied. Pure ZnO and (Sm, Co) co-doped nanoparticles, Zn1-2xSmxCoxO with x = 0.00, 0.005, 0.01, 0.02, 0.04 and 0.06, were synthesized by co-precipitation technique. The X-rays diffraction (XRD) scans were performed with 2θ varies from 20° to 80°.The XRD patterns of all samples confirmed the formation of wurtzite-type structure of ZnO with the formation of Sm2O3 and Co3O4 as minor secondary phases with x ≥ 0.005. The spherical morphology of pure ZnO is transformed to nanorods as the concentration of Sm3+ and Co2+ increases. The diameter and length of the nanorods were nearly 35.1–66.5 nm and 136.2–596 nm, respectively. UV–vis spectra were used for the calculation of the energy band gap (Eg) by different methods. Eg is found to increase with increasing (Sm, Co) doping up to x = 0.02. Energy band gap and Urbach energy are observed to have a reverse trend. Magnetic properties were investigated by the vibrating sample magnetometer. M − H hysteresis analysis reveals that for small doping, the samples have a ferromagnetic signal combined with diamagnetic contribution. The enhancement in ferromagnetism behaviour for 0.01 < x < 0.02 was justified by the formation of Bound Magnetic Polarons (BMPs) as it significantly enhances with the doping of Sm and Co by stabilizing Zn vacancy and other magnetic defects. Then the ferromagnetism is reduced for the doped samples up to x = 0.06, and an antiferromagnetic alignment is displayed.

Spin Alignment Studies on the Muon-Site Determination in La&lt;sub&gt;2&lt;/sub&gt;CuO&lt;sub&gt;4&lt;/sub&gt;

Here we report spin-alignment contributions to muon coordinate calculated utilizing density functional theory (DFT) calculation. We estimated four different antiferromagnetic (AF) spin alignments in La2CuO4. We observed small changes by adjusting spin configurations in DFT calculations. Cu-spin value of 0.61 µB is constant in all calculations. The insulating gap of 1.9 eV is unchanged in all configurations. Muon coordinate was defined as the most minimum energy in atomic potential distribution. By assuming that Cu-spin is a point dipole for each atom, internal fields for muon were calculated and compared to known experimental results.

Heat capacity signature of frustrated trimerons in magnetite

Recently it has been proposed that the long-range electronic order formed by trimerons in magnetite should be frustrated due to the great degeneracy of arrangements linking trimerons. This result has important consequences as charge ordering from the condensed minority band electrons leads to a complex 3D antiferro orbital order pattern. Further more, the corner sharing tetrahedra structure of spinel B-sites supports frustration for antiferromagnetic alignments. Therefore frustration due to competing interactions will itself induce disorder and very likely frustration in the spin orientations. Here we present very low temperature specific heat data that show two deviations to the magnons and phonons contributions, that we analyze in terms of Schottky-type anomalies. The first one is associated with the thermal activation across both ferroelastic twin and ferromagnetic anti-phase domains. The second Schottky-type anomaly displays an inverse (1/H) field dependence which is a direct indication of the disordered glassy network with macroscopically degenerated singular ground states.

Spin-dependent band-gap driven by nitrogen and oxygen functional groups in zigzag graphene nanoribbons

We investigated the role of nitrogen and oxygen functional groups on the electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) using spin-polarized first-principles density functional theory calculations. Twenty four functional groups of nitrogen, oxygen, and nitrogen-oxygen configurations were attached to the edge of ZGNRs. We analyze the band structure, spin-resolved bandgap, charge transfer, magnetic ordering, and binding energy. Similar to the pristine ZGNR, an indirect bandgap semiconducting behavior dominated in functionalized ribbons, except for carbonyl, pyrane, and pyridinium groups, that displays a metallic behavior. Spin-resolved band gap calculations revealed a strong dependence on the functional groups. Carbonyl groups showed a half-metallicity behavior with only spin up states at the Fermi level. In general, the functionalized ribbons displayed ferromagnetic alignment along the edges, but antiferromagnetic alignment between them, resulting in a total null magnetization. We showed some exceptions, the carbonyl, pyrane, lactam, pyridine N-oxide, and pyridinium functional groups induced significant changes in the magnetic arrangement, not only along the edge where they were attached but also to the opposite side. We have also incorporated a pentagon-heptagon pair or a single pentagon defect at the edge to explore the quinone, pyrone, and pyridone functional groups.

Circumferential confinement consequence on the magnetic properties of a punctured nanotube in the presence of an axial electric field

In this paper, we present magnetic properties of a finite graphene sheet with a triangle punctured vacancy, and its counterpart single-wall carbon nanotube as a rolled-up graphene sheet in the framework of the Hubbard model in the presence of an axial electric field, in order to form a comparison study between these two graphene samples. We have noticed that the tight-binding part of the Hamiltonian consists of two types of zero-energy states in the case of the graphene sheet, the strict zero-energy states, and the quasi zero-energy states. The first type takes part in a ferromagnetic coupling between the triangle edges and one edge of the rectangle graphene sheet, while the latter one has an antiferromagnetic alignment with the opposite edge of the rectangle graphene sheet. Involving the Coulomb interaction through Hubbard term, we have observed that the slope of the cluster edge states in nanotube is higher than the graphene sheet. Additionally, spin-depolarization happens in single-wall nanotube sooner than the graphene sheet by slightly increasing an axial electric field. Also, the graphene sheet is more robust than the single wall nanotube at low electric fields.

Role of 4f states in infinite-layer NdNiO2

Atomic $4f$ states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level $E_F$ as in the proposed topological Kondo insulator SmB$_6$, or further away as in the magnetic superconductor system ${\cal R}$Ni$_2$B$_2$C (${\cal R}$=rare earth ion) and in Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_7$, where the $4f$ shell of Pr has a devastating effect on superconductivity. In hole-doped ${\cal R}$NiO$_2$, the ${\cal R}$=Nd member is found to be superconducting while ${\cal R}$=La is not, in spite of the calculated electronic structures being nearly identical. We report first principles results that indicate that the Nd $4f$ moment affects states at $E_F$ in infinite-layer NdNiO$_2$, an effect that will not occur for LaNiO$_2$. Treating 20% hole-doping in the virtual crystal approach indicates that 0.15 holes empty the $\Gamma$-centered Nd-derived electron pocket while leaving the other electron pocket unchanged; hence Ni only absorbs 0.05 holes; the La counterpart would behave similarly. However, coupling of $4f$ states to the electron pockets at $E_F$ arises through the Nd intra-atomic $4f-5d$ exchange coupling $K\approx 0.5$ eV and is ferromagnetic (FM), i.e. anti-Kondo, in sign. This interaction causes spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ The Ni moments differ by 0.2$\mu_B$ for FM and antiferromagnetic alignment (the latter are larger), reflecting some itineracy and indicating that Heisenberg coupling of the moments may not provide a quantitative modeling of Ni-Ni exchange coupling.

Effect of confinement and octahedral rotations on the electronic, magnetic, and thermoelectric properties of correlated SrXO3/SrTiO3(001) superlattices ( X=V , Cr, or Mn)

By using density functional theory calculations with an on-site Coulomb repulsion term combined with Boltzmann transport theory, we explore the effect of ${t}_{2g}$ orbital occupation on the electronic, magnetic, and thermoelectric properties of ${(\mathrm{Sr}X{\mathrm{O}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{n}(001)$ superlattices with $n=1,3$ and $X=\mathrm{V}$, Cr, and Mn. In order to disentangle the effect of quantum confinement and octahedral rotations and to account for a wider temperature range, $P4/mmm$ (untilted) and $P{2}_{1}/c$ (tilted) phases are considered. We find that the ground-state superlattice geometries always display finite octahedral rotations, which drive an orbital reconstruction and a concomitant metal-to-insulator transition in confined ${\mathrm{SrVO}}_{3}$ and ${\mathrm{SrCrO}}_{3}$ single layers with ferro- and antiferromagnetic spin alignments, respectively. On the other hand, the confined ${\mathrm{SrMnO}}_{3}$ single layer exhibits electronic properties similar to bulk. We show that confinement enhances the thermoelectric properties, particularly for ${\mathrm{SrVO}}_{3}$ and ${\mathrm{SrCrO}}_{3}$ due to the emergent Mott phase. Large room-temperature Seebeck coefficients are obtained for the tilted superlattices, ranging from 500 to $600\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ near the band edges. The estimated attainable power factors of $27.9\phantom{\rule{0.28em}{0ex}}(26.6)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in plane for the ${({\mathrm{SrCrO}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{1}(001)$ superlattice with $P4/mmm\phantom{\rule{0.28em}{0ex}}(P{2}_{1}/c)$ symmetry and $28.1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ cross plane for the ${({\mathrm{SrMnO}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{1}(001)$ superlattice with $P{2}_{1}/c$ symmetry compare favorably with some of the best-performing oxide thermoelectrics. This demonstrates that the idea to use quantum confinement to enhance the thermoelectric response in correlated transition-metal oxide superlattices [Phys. Rev. Mater. 2, 055403 (2018)] can be applied to a broader class of materials combinations.

A New Magnetic Topological Quantum Material Candidate by Design.

Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. Here we report the characterization of high-quality crystals of EuSn2P2, a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSn2P2 has a layered, Bi2Te3-type structure. Ferromagnetic interactions dominate the Curie–Weiss susceptibility, but a transition to antiferromagnetic ordering occurs near 30 K. Neutron diffraction reveals that this is due to two-dimensional ferromagnetic spin alignment within individual Eu layers and antiferromagnetic alignment between layers—this magnetic state surrounds the Sn–P layers at low temperatures. The bulk electrical resistivity is sensitive to the magnetism. Electronic structure calculations reveal that EuSn2P2 might be a strong topological insulator, which can be a new magnetic topological quantum material (MTQM) candidate. The calculations show that surface states should be present, and they are indeed observed by angle-resolved photoelectron spectroscopy (ARPES) measurements.

Observation of Interfacial Antiferromagnetic Coupling between Magnetic Topological Insulator and Antiferromagnetic Insulator.

Inducing magnetic orders in a topological insulator (TI) to break its time reversal symmetry has been predicted to reveal many exotic topological quantum phenomena. The manipulation of magnetic orders in a TI layer can play a key role in harnessing these quantum phenomena toward technological applications. Here we fabricated a thin magnetic TI film on an antiferromagnetic (AFM) insulator Cr2O3 layer and found that the magnetic moments of the magnetic TI layer and the surface spins of the Cr2O3 layers favor interfacial AFM coupling. Field cooling studies show a crossover from negative to positive exchange bias clarifying the competition between the interfacial AFM coupling energy and the Zeeman energy in the AFM insulator layer. The interfacial exchange coupling also enhances the Curie temperature of the magnetic TI layer. The unique interfacial AFM alignment in magnetic TI on AFM insulator heterostructures opens a new route toward manipulating the interplay between topological states and magnetic orders in spin-engineered heterostructures, facilitating the exploration of proof-of-concept TI-based spintronic and electronic devices with multifunctionality and low power consumption.

Critical region phase transitions in the quasi-2D magnet CrCl3

CrCl3 has been known for a long time to be a quasi-2D ferromagnet (FM) with a specific-heat peak at 16.8K. Its 3D ordering is antiferromagnetic (AFM) with alternating 2D FM sheets perpendicular to the c-axis. Kuhlow [Phys. Stat. Sol. A 72, 161 (1982)] carried out Faraday rotation measurements on CrCl3 and concluded, on fairly tenuous grounds, that there are actually two closely spaced magnetic transitions, one at “16.7K” at which 2D FM sheets of in-plane spins start ordering, but do so with random alignment relative to the c-axis (the latter being perpendicular to the 2D FM sheets), and one in which a 3D AFM alignment of the 2D sheets of spins takes place, that being at TN ∼15.5K. We have carried out an extensive series of precision SQUID magnetometer measurements as a function of temperature, from very low fields (≥2.5G) and up to ∼2 kG on a high-purity single-crystal platelet, with the magnetic field both parallel and perpendicular to the c axis. The measurements provide more definitive evidence that there are two transitions, possibly three (at ∼16.8K, 16.0 and ∼14.3K, in our measurements). These data provide some support to Kuhlow’s conjecture that the initial phase transition is to a phase consisting of 3D-disordered, 2D-ordered, planes of spins. The very low field measurements provide the strongest support for this picture. To clarify the situation further neutron scattering (NS) measurements were carried out on a batch of the same material. The NS temperature-dependence measurements appear to provide additional support by showing a change in curvature in the magnetization (M) curve near 14K, and then an apparent linear decrease in M above 14K. Additionally, the NS data show a possible hysteresis near 16K during cooling. Possibly (though unlikely) the critical-region phase-transition behaviors observed in CrCl3 are unique to this quasi-2D system.

Stability, Electronic and Magnetic Properties of Mn-Doped Copper Clusters: A Meta-GGA Functional Investigation

DFT calculations were performed to investigate the stability, reactivity, electronic and magnetic properties of the CunMn (n = 2–12) clusters. The obtained results show that the Cu6Mn and Cu9Mn clusters are found more stable than the neighboring clusters. The calculated HOMO–LUMO energy gaps vary from 0.471 to 2.444 eV, suggesting a semiconductor-like feature of these binary clusters. The condensed Fukui function (f k + ) for nucleophile attack has been calculated for each atom in CunMn clusters, and the results exhibit that the reactivity was mainly localized at the Mn atom. Accordingly, the CunMn clusters are more favourable to react with a nucleophilic reagent. On the basis of the ELF analysis, a low local localization function has been obtained between the atoms in clusters, indicating an ionic character of the chemical bonds in the CunMn clusters. The analysis of the magnetic properties indicates that the Cu atoms in CunMn clusters show an antiferromagnetic alignment with respect to the Mn atom’s magnetic moment, and the magnetic moment of these clusters mostly originates from Mn atom. Moreover, NPA analysis show that the 3d electrons of Mn atom play a substantial role in the magnetic properties for CunMn clusters, and the contribution of the 4s electrons is little.

Ab-initio studies of electronic, structural and thermophysical properties of the Sr2TiMoO6 double perovskite

Double perovskite-like material Sr2TiMoO6 (STMO) has been obtained by solid state synthesis. Analysis of the morphology by SEM reveals the occurrence of polyhedral grains with octahedral forms in its faces and hexahedron forms in the corners. The electronic structure and thermophysical properties are reported for the first time using the pseudopotential and plane-wave method. Theoretical calculations for different antiferromagnetic configurations showed that the antiferromagnetic spins alignment in planes interspersed along the direction [111] presents the most stable structure. Values of the coefficient of thermal expansion, the Debye temperature, and the Grüneisen parameter show that at low temperatures the specific heat at constant pressure and constant volume has a similar behavior, with a value for the Dulong-Petit limit expected for a triclinic perovskite-type structure. Finally, measures of susceptibility as a function of temperature suggest an antiferromagnetic behavior, with Néel temperature TN = 20.1 K. From the Curie-Weiss adjust, the temperature independent susceptibility was determined to be χo = 0.0038 emu/mol and the effective magnetic moment μ = Peff = μB = 1.30 μB.

Zero Hall conductivity and its electronic origin in a Cr-doped topological insulator

We investigate the electronic origin of zero Hall conductivity or the axion insulator phase of a Cr-doped $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ film by analyzing the evolution of its band structure upon a magnetic field sweep. By applying a small electric field or using asymmetric doping, we differentiate the coercivities of the two surfaces, so they can form either ferromagnetic (FM) or antiferromagnetic (AFM) alignments. Through the surface-resolved Berry curvature calculations, we demonstrate that the system manifests robust zero Hall plateaus when the two surfaces remain topologically nontrivial and enter a magnetic state which has intrasurface FM and intersurface AFM orderings. Our results provide insights for the understanding of the uncharted issues of axion insulators, complementary to the exciting experimental progress in this realm.

A New Original Conception in Rock Magnetism, Paleomagnetism and Geomagnetism: An Origin of the Reversed Magnetization of Rocks on Earth

Abstract So far the field-reversal theory has been accepted to account for the reversed remanent magnetization (RM) of rocks on the Earth. Orlickỳ (2014) revealed a frequent occurrence of the antiferromagnetic (AFM) Fe-Ti oxides in the rocks. Now I have renamed these minerals as the Fe-Ti ferrimagnetic-antiferromagnetic chemical phases (Fe- Ti FriM-AFM ChPs). They may have either cubic spinel, or tetragonal spinel symmetry, respectively. They behave as the Fe-Ti polycrystalline materials. These Fe-Ti FriMAFM ChPs are the two sublattice A and B ChPs, with some specific magnetic behaviour. The titanomagnetite (Ti-Mt, Curie temperature, TC = 230 oC; FriM alignment) and the titanomaghemite (Ti-Mgh, Néel temperature, TN = 450 oC; AFM alignment) containing rocks have been identified as the representants of the the Fe-Ti FriM-AFM cubic spinel ChPs. The interactions with the magnetizing field, with the Weis molecular fields (Weiss-Heisenberg forces) have generated the reversally oriented internal field. The reversally oriented spontaneous magnetization has arosen in the rock. This internal field has been identified as the most important phenomenon leading to the production of the reversally oriented magnetization in the Fe-Ti FriM-AFM ChPs containing rocks. The equations expressing the magnetic behaviour of the magnetic susceptibility (κ) of rocks versus temperature have been derived. The Fe-Ti FriM-AFM cubic spinel can undergone the transition in favour of the Fe-Ti FriM-AFM tetragonal spinel in the rocks, due to a change of the thermodynamic conditions in nature. The reversed RM has supposed been inparted from the Fe-Ti FriM-AFM cubic spinel during this alteration-transition processess. Such tetragonal spinel is more stable and it is able to survive in the rocks in nature. The results of laboratory magnetization of the selected groupings of rocks have been presented below. The basic laboratory methods for the detection of the magnetic behaviour of the Fe-Ti FriM-AFM ChPs containing rocks are described as well. The presented results have shown that we do not need to apply the field reversal theory, because I have revealed the realistic mechanism which is able to generate the reversed RM of rocks under a presence of the normal geomagnetic field.

Potentiality of CuFeO2-delafossite as a solar energy converter

The CuFeO2 semiconductor, a delafossite compound with a lower band gap, has electronic and optical properties of interest for absorbing solar radiation in solar converters. In order to evaluate its potential the optical properties and their decomposition into atomic contributions are obtained using first-principles. Both ferromagnetic and antiferromagnetic spin alignments are considered. According to the results the structure, the Fe-Cu transitions makes almost no contribution to the absorption. Then, by using the absorption coefficient, the maximum absorption efficiencies, voltages, and currents of the solar conversion are found. The results indicate it as being a good candidate for solar cells but it does not generate enough output voltage for water-splitting.

O-O Bond Formation in Photosystem II Oxygen Evolving Complex

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. Light driven water oxidation is facilitated in Nature by the Oxygen Evolving Complex (OEC) - Mn4Ca cluster embedded in the photosystem II protein. During its function, the OEC cycles through five redox states termed S0-S4 after which, oxygen is evolved. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 μs) changes to the Mn Kβ XES spectrum. However, no oxidation of the Mn4Ca core above the all MnIV state was detected to precede O-O bond formation, and the observed changes were therefore assigned to O-O bond formation dynamics. Our mechanism proposes O-O bond formation occurs prior to the transfer of the final (4th) electron from the Mn4Ca cluster to the oxidized TyrZ. This model resolves the kinetic limitations associated with O-O bond formation, and suggests an evolutionary adaptation to avoid releasing of harmful peroxide species. Atomistic model for the S3 state reported by us [1, 2] is indistinguishable (within the resolution of XRD) from one of the recently reported by Shen and co-workers [3]. This S3-state model incorporates a MnIV=O fragment of radicaloid character and exhibits antiferromagnetic alignment with opposite spin orientations between two Mn centers and associated oxygens creating an opportunity for a low barrier to O-O bond formation. 1. Davis et al. 2015, arXiv:1506.08862 2. Jensen et al. J Phys. Chem. Letters, 2017, 8, 2584−2589. 3. Suga et al. Nature, 2017, 543(7643), 131-135.

Synthesis and Characterization of Er-Doped Nano ZnO Samples

Pure and erbium-doped ZnO nanoparticles (Zn1−xErxO), (0.00 ≤ x ≤ 0.10), were synthesized by wet chemical co-precipitation method. The structural, optical, and magnetic properties of the prepared samples were investigated using x-ray powder diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV), Fourier transform infrared spectroscopy (FTIR), M-H magnetic hysteresis, and electron paramagnetic resonance (EPR). XRD studies exhibit the presence of a single ZnO wurtzite hexagonal crystal structure for 0.00 ≤ x ≤ 0.06. A secondary phase of Er2O3 appears for x > 0.06. This means that the solubility limit for doping Zn2+ ions by Er3+ ions is about x = 0.06 under our preparations condition. The lattice parameter a is not affected by the Erbium doping. On the other hand, the lattice parameter c and the unit cell volume V increase with the increase of x up to x = 0.06. This is attributed to the larger ionic size of Er3+ ions (0.88 A) compared to Zn2+ ions (0.74 A). Both c and V decrease for x > 0.06. TEM micrographs indicate that the shape and the size of the ZnO nanoparticles are modified by changing the doping level of Er. The UV measurements point out that band gap energy Eg decreases with the increase of x up to x = 0.06. Then, it increases for both x = 0.08 and 0.10. FTIR spectra confirm the presence of O–H and Zn–O stretching modes at 3451.963 and 428.901 cm− 1, respectively, in pure and doped ZnO samples. The Zn–O stretching mode shifts toward a lower wavenumber for x = 0.06 and toward a higher wavenumber for x = 0.10. M-H hysteresis analysis, at room temperature, reveals that the pure ZnO has a ferromagnetic signal combined with diamagnetic and paramagnetic contributions. This ferromagnetism is reduced for the doped samples up to x = 0.02, and an antiferromagnetic alignment appears for 0.04 ≤ x ≤ 0.10. The saturation magnetization (Ms), the coercivity (Hc), the retentivity (Mr), the anisotropy constant (Ka), and the magnetic moment (μm) were estimated and discussed in terms of erbium doping for the different samples. EPR spectra for Zn1−xErxO were measured at room temperature in order to study the effect of Er substitution on the g value, resonance field (Hr), peak to peak line width (ΔHpp) and spin–spin relaxation time constant (T2).

Double-spiral magnetic structure of the Fe/Cr multilayer revealed by nuclear resonance reflectivity

We have studied the magnetization depth profiles in a [57Fe(dFe)/Cr(dCr)]x30 multilayer with ultrathin Fe layers and nominal thickness of the chromium spacers dCr 2.0 nm using nuclear resonance scattering of synchrotron radiation. The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has been detected at zero external field. The joint fit of the reflectivity curves and Mossbauer spectra of reflectivity measured near the critical angle and at the "magnetic" peak reveals that the magnetic structure of the multilayer is formed by two spirals, one in the odd and another one in the even iron layers, with the opposite signs of rotation. The double-spiral structure starts from the surface with the almost antiferromagnetic alignment of the adjacent Fe layers. The rotation of the two spirals leads to nearly ferromagnetic alignment of the two magnetic subsystems at some depth, where the sudden turn of the magnetic vectors by ~180 deg (spin-flop) appears, and both spirals start to rotate in opposite directions. The observation of this unusual double-spiral magnetic structure suggests that the unique properties of giant magneto-resistance devices can be further tailored using ultrathin magnetic layers.

Unconventional superconductivity in iron pnictides: Magnon mediated pairing

We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe2As2 and BaFe2As2 in pure form for umklapp scattering with scattering wave vector Q=(1,1), in the unit of π/a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe2As2 and BaFe2As2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.

Carrier-driven coupling in ferromagnetic oxide heterostructures

Transition metal oxides are well known for their complex magnetic and electrical properties. When brought together in heterostructure geometries, they show particular promise for spintronics and colossal magnetoresistance applications. In this letter, we propose a new mechanism for the coupling between layers of itinerant ferromagnetic materials in heterostructures. The coupling is mediated by charge carriers that strive to maximally delocalize through the heterostructure to gain kinetic energy. In doing so, they force a ferromagnetic or antiferromagnetic coupling between the constituent layers. To illustrate this, we focus on heterostructures composed of SrRuO$_3$ and La$_{1-x}$A$_{x}$MnO$_3$ (A=Ca/Sr). Our mechanism is consistent with antiferromagnetic alignment that is known to occur in multilayers of SrRuO$_3$-La$_{1-x}$A$_{x}$MnO$_3$. To support our assertion, we present a minimal Kondo-lattice model which reproduces the known magnetization properties of such multilayers. In addition, we discuss a quantum well model for heterostructures and argue that the spin-dependent density of states determines the nature of the coupling. As a smoking gun signature, we propose that bilayers with the same constituents will oscillate between ferromagnetic and antiferromagnetic coupling upon tuning the relative thicknesses of the layers.