Enhancement Of Magnetic Moment Research Articles

Identification and comparison of peculiarities in physical properties of multiferroic morphotrophic phase boundary sintered BiFeO3-xPbTiO3 nano-ceramics

In this paper a comprehensive study that includes structural, magnetic, ferroelectric, dielectric and electrical characterizations of as-prepared and quenched MPB 1-x(BiFeO3)-x(PbTiO3)(BF-xPT) solid solutions prepared by wet chemical method has been presented. Qualitative and quantitively structural analysis revealed coexistence of rhombohedral/monoclinic, tetragonal and orthorhombic phases. Sensitivity to the experimental variables such as composition, synthesis conditions, sintering temperature and particle size, confirms the existence of a complex energy phase diagram where minima associated with various polymorphs lies almost at same level for MPB compositions. Magnetic characterizations reveal the presence of three anomalies, i.e., originating from the Néel transition, iso-structural transition and ferroelectric phase transition. Second anomaly detected to be the most sensitive to the quenching. Enhancement of magnetic moment in quenched compositions is a manifestation of strong interaction between spins of Fe2+ ions with the pinned point defect dipoles. Interestingly, thermal quenching enhanced the ferroelectric properties by removing the domain wall pinning. Saturated ferroelectric loops were observed only for quenched samples which is a consequence of strong domain wall de-pinning. Low temperature dielectric relaxation has been observed while the high temperature dielectric data of as-prepared composition exhibited Maxwell-Wagner and iso-structural phase transition. In contrast, along with these two features, quenched samples showed additional Curie transition. Enhancement in large polaron hoping in the low frequency region for the quenched composition has been confirmed via frequency dependent profile of the ac conductivity. Low value of activation energy of charge carriers for quenched compositions indicates large polaron hoping characteristics in BF-xPT solid solution.

Absence of Magnetic Proximity Effect at the Interface of Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3} with EuS.

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)_{2}(Se,Te)_{3} family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10^{-3} μ_{B}/at.

Selection criteria for spintronic materials: Role of spin dependent band-width narrowing/broadening

The selection of a material of a particular application in spintronics requires that criteria of half-metallic should be fulfilled. The stability with adequate cohesive energy of the systems plays an important role. Apart from stability, magnetic moment must be adequate to store the information along with the appropriate presence of conducting channels at Fermi energy to conduct. Recently we have established a bridge between the results obtained by density functional theory (DFT) with those from mean field theory in the case of monatomic chains of 3-d series of periodic table. We have found that in the systems with less than half filled band, the up spin channel contributes for conduction and in case of more than half filled band the down spin channel contributes in conduction, while other channel is responsible for maintaining the magnetic state. It has been observed that both the conditions of stabilized magnetic state and conduction are required for a good spintronic material. Therefore in case of available systems, the materials with more than half filled would be better from application point of view.Therefore, we have extended our previous study to 4-d and 5-d series of periodic table for systems with more than half filled band only i.e. 4-d elements (Ru, Rh, Pd) and 5-d elements (Os, Ir, Pt) in 1 dimension (1D: monatomic chain), 2 dimension (2D: monolayer) and 3 dimension (3D: bulk) in addition to 3-d elements (Fe, Co, Ni). The study establishes a fact that as magnetization changes, for more than half filled band, the bandwidth or conduction also changes in such a way that bandwidth of up spin decreases and for down spin increases. The study also verifies that, in these systems, the onset of magnetism as well as enhancement of magnetic moment in low dimensions are driven by the combined role of band splitting and band-narrowing/broadening of the individual spin bands.

Flat-band splitting induced tunable magnetism in defective CrI3 monolayer

Chromium triiodide (CrI3) monolayer, as a two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductor, creates a large number of opportunities for the spintronic application of atomically thin magnets. Using density functional theory, we studied the electronic structures and carrier doping of CrI3 monolayer with vacancy defects. The energy band structures show that the flat-bands near the Fermi level in the spin-up conduction bands split into isolated flat-bands for the CrI3 monolayer with I vacancies, regardless of the vacancy concentration, whereas these flat-bands cross together in the perfect case. Further analysis suggests that the separation of the flat-bands results from the reduction of the Cr–I interaction induced by I vacancies. The introduction of I vacancies not only preserves the semiconducting feature, but also enhances the total magnetic moments of I-vacancy-contained CrI3, and the enhancement of magnetic moments is closely related with the number of vacancies. With the help of the flat-band splitting, the carrier doping could be used to further tune the magnetic moments, and simultaneously maintain the semiconducting characteristic in the I-vacancy-contained CrI3. Thus, our findings extend the realistic application of CrI3 monolayer in spintronics.

Strain-induced magnetic moment enhancement in frustrated antiferromagnet Cs2CuBr4

The structure and magnetic properties are studied in co-doped Cs2−xKxCuBr4−xClx and pressurized Cs2CuBr4 samples. No structural phase transition is found with doping concentration x ⩽ 0.1 and pre-compression pressure up to 4.5 GPa. The maximum susceptibility temperature Tmax of the zero-field-cooling (ZFC) susceptibility curves decreases slightly with increasing doping concentration and pre-compression pressure, indicating only small changes in the exchange coupling constants. However, an unusual enhancement of the magnetic moment deduced from the ZFC susceptibility is observed in both series samples. A maximum increase of 40% is obtained in Cs1.9K0.1CuBr3.9Cl0.1 sample. The magnetic moment increases almost linearly with decreasing Δ, i.e., defined as the wavenumber difference between the short- and long-bond stretching modes of the CuBr42− tetrahedra in the Raman spectra. The effect is likely due to the recovery of the Cu-3d orbital magnetic moments by strain-induced suppression of Jahn–Teller distortion in CuBr42− tetrahedra.

Elastic strain control of electronic structure, and magnetic properties of [Pr1−xCaxMnO3/SrTiO3]15 superlattices

We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr0.7Ca0.3MnO3/SrTiO3]15 and [Pr0.5Ca0.5MnO3/SrTiO3]15 on (001) oriented SrTiO3 and LaAlO3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO3 as compared to those grown on SrTiO3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (HK) and cubic anisotropy constant (K1) for all the investigated superlattices. This analysis yields the highest HK∼9 kOe and K1∼8×105 erg/cc for the [Pr0.7Ca0.3MnO3/SrTiO3]15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in TC (<100 K) was observed in the case of LaAlO3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of eg levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.

Strain healing of spin–orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 μB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 μB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin–orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin–orbit coupling.

Investigation of room temperature multi-functional properties of Nd doped mullite Bi2Fe4O9

Mullite type Bi2Fe4O9 (BFO) and Nd doped Bi2Fe4O9 (BNFO) systems were developed using conventional solid state route under normal atmospheric pressure. The structural phase compositions were analyzed using x-ray diffraction (XRD). The analysis revealed BFO orthorhombic primary phase with traces of secondary phase and compressive lattice structural distortion, the same being evident from theoretically calculated (LSDA + U and PBE + U)correlated systems, where LSDA is Local spin density approximation, Hubbard U = 4.5eV and PBE is Perdew, Burke and Ernzerh of exchange correlation functional. Raman spectral analysis showed changes in the frequency and line width with respect to Nd doping, which correlates the lattice distortion observed in XRD analysis. Henceforth, the Bi2-xNd2xFe4O9with x = 0, 0.01, 0.03, will be referred as BFO, BNFO1 and BNFO3. From the dielectric and optical studies, a significant increase in the dielectric constant and narrower band gap was observed in the BNFO samples, compared to BFO. The room temperature vibrating sample magnetometer (VSM) analysis of the BNFO samples showed weak ferromagnetic hysteresis behavior. Also, a significant enhancement of magnetic moment was observed with increased concentration of Nd in BNFO samples. This could be due to suppression of oxygen vacancies by the Nd dopant, that reduces the super-exchange interaction of Fe3+-O-Fe3+ and introduce Dzyaloshinskii-Moriya (D-M) interactions. The BNFO3 with control over its dielectric constant, optical band gap and magnetization by doping, makes it a viable material for magneto-electric and other multifunctional applications.

Defect-induced ferromagnetism in W18O49 nanowires

In this paper, strong room temperature ferromagnetism was observed in non-stoichiometric tungsten oxide nanostructure, sea urchin-like single crystalline W18O49 nanowires prepared by a solvothermal approach. The prepared W18O49 nanowires exhibited a 39-fold enhancement in saturation magnetic moment as compared to the transformed WO3 by annealing in air. By combining x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron-spin resonance (EPR) spectroscopy, a defect complex, i.e., W5+ ions accompanied by oxygen vacancies, has been identified in the prepared W18O49. The observed ferromagnetism in W18O49 is qualitatively explained through a model involving bound magnetic polarons which were induced by the coupling between the electron trapped by oxygen vacancy and the unpaired 5d electron of W5+ ion. The results provide us better insight into the defect engineering of room-temperature ferromagnetism in tungsten oxide.

A giant enhancement of magnetic moment in a ternary three-shell icosahedral cluster: Fe@Mn12@Au20

ABSTRACTThe average magnetic moment per atom of Mn13 cluster is expected to be enhanced by doping or coating with a shell. Several ternary core–shell icosahedral clusters TM@Mn12@Au20 were constructed by combining substituting the central Mn with VIII elements (Fe, Co, Ni, Ru, Rh, Pd and Pt) and coating with a icosahedral Au20 shell, and systematically studied by using the first-principles density functional method. Compared to Mn13, Fe@Mn12@Au20 cluster shows a giant enhancement on total magnetic moment (52 µB) which can be greatly attributed to the ferromagnetic coupling between spin moments of atoms. Coating with Au20 shell enlarged the average distances of TM-Mn and Mn-Mn and is a useful way to change the magnetic coupling style. By analysis of density of states and electron localisation functional, we can conclude that the weak hybridisation between Fe and Mn in Fe@Mn12@Au20 is propitious to maintain their original direction of spin moments of atoms and then form ferromagnetic coupling.

Structure, magnetocaloric properties and thermodynamic modeling of enthalpies of formation of (Mn,X)-Co-Ge (X = Zr, Pd) alloys

Enthalpies of formation of (Mn,X)-Co-Ge (X = Zr, Pd) system in different structural states were analyzed utilizing the semi-empirical Miedema's model combined with the geometric one. Substitution of Mn by Zr significantly improves glass forming ability of Mn-Co-Ge, with parent equiatomic alloy among them. Chosen half-Heusler alloys, which are thermodynamically stable for the mentioned elemental compositions, were obtained by arc-melting. The x-ray diffraction studies confirmed coexistence of orthorhombic NiTiSi-type and hexagonal Ni2In-type structures for parent MnCoGe alloy and (Zr, Pd)-substituted samples. Thermomagnetic curves and magnetic isotherms allowed to reveal the Curie temperature TC and a magnetic entropy change ΔSM and allowed to analyze the influence of both substitutive elements. The Curie temperatures are equal to 293 K for MnCoGe, 278 K for Mn0.9Zr0.1CoGe alloy and 318 K for Mn0.9Pd0.1CoGe. The maximum magnetic entropy change ΔSM values, calculated for the change of external magnetic field of about 5 T, amount to 6.17, 2.94, and 11.11 J (kg K)−1 for MnCoGe, Mn0.9Zr0.1CoGe and Mn0.9Pd0.1CoGe, respectively. Differences in magnetic characteristics of the analyzed alloys are mainly caused by the changes of extent of undergoing first order magnetostructural transition with Pd and Zr atoms substitution. The presence of Pd d moments is also beneficial for the enhancement of total magnetic moment and maximum value of magnetic entropy changes in Mn0.9Pd0.1CoGe alloy.

Enhanced room temperature ferromagnetism and green photoluminescence in Cu doped ZnO thin film synthesised by neutral beam sputtering

The Cu (3 to 15 at%) is incorporated into ZnO thin film by atomic beam co-sputtering has been investigated for enhancement in room temperature ferromagnetism and green photo-luminance. These Cu-ZnO thin films examined with Raman spectroscopy, X-Ray Diffraction (XRD), UV-Visible spectroscopy, Hall measurement, magnetic force microscopy (MFM) and magnetic hysteresis. Raman spectroscopy, XRD confirms wurtzite structure and improvement in the crystallinity of ZnO upto 7% Cu. Further increase in Cu concentration results in growth in Cu nanoparticles. On increasing Cu concentration, there is decrement in transparency and increase in band gap with increase in n-type carrier concentration as confirmed from UV-Visible and Hall measurement studies. Magnetic measurement exhibited unique feature of room temperature ferromagnetic ordering in undoped and doped sample upto 3% Cu. The enhancement in magnetic moment as well as green emission in photoluminescence response with increase in Cu doping indicates that generation of large defects in ZnO by Cu doping, which can be attributed to combined effect of the presence of oxygen vacancies and/or structural inhomogeneity as well as formation of bound magnetic polarons. Importantly, synthesised Cu doped ZnO thin films can be used as spin LEDs and switchable spin-laser diodes.

The structure, electrical and magnetic properties of M-doped PbPdO2 (M = Cu, Co, Fe) thin films: A first-principles and experimental study

Abstract PbPdO2 and M-doped PbPdO2 (M = Cu, Co, Fe) films were prepared using pulsed laser deposition (PLD) method. The structures, electrical and magnetic properties were systematically investigated by XRD, Raman, SEM, XPS, AFM and VSM. Our results indicate that PbPdO2 and PbPd0.9M0.1O2 (M = Cu, Co, Fe) film with (0 0 2) preferred orientation can be well fabricated. The valence states of magnetic atoms in M-doped PbPdO2 (M = Cu, Co, Fe) thin films are identified to be Cu1+, Co2+,3+ and Fe3+, respectively. All Pb(Pd,M)O2 (M = Cu, Co, Fe) films exhibit ferromagnetism with high metal-insulator transition temperatures at around 330–370 K. Moreover, the resistivity of Fe doped PbPdO2 film is higher than that of the pristine PbPdO2 film, while the resistivities of Co and Cu doped PbPdO2 films are lower. At the same time, Fe, Co and Cu dopants do enhance the ferromagnetism of PbPdO2 film, while the enhancement of magnetic moment for Fe dopant is most evident. Lastly, based on Pb vacancy and O−1 observed experimentally, the band gaps and magnetic moments of Pb(Pd,M)O2 films were calculated by first-principles, and the results explain well the experimental facts.

First principles study of the ground state magnetic properties of hypothetical B2-Fe50Al50 multilayers

The ground state structural, electronic and magnetic properties of multilayers of B2 Fe-Al stoichiometric (Fe50Al50) phase were studied by first principles calculations in the framework of the density functional theory as a function of the number of layers. From the cohesive energy point of view all, the studied multilayers are more stable in the ferromagnetic state. The calculated equilibrium lattice parameter of the B2-Fe50Al50 phase in the multilayers is smaller than that in bulk. The calculations of the magnetic properties show an enhancement of the total magnetic moment of the B2-Fe50Al50 phase on going from bulk to multilayers structures. The total and partial density of states curves indicate changes in the electronic configuration of surface Fe atoms (relative to that observed in bulk Fe50Al50) in all the studied multilayers. For Fe atoms located at sites in the inner layers, there were observed significant modifications in their electronic distribution only for multilayers with low number of bcc unit cell layers (2 and 4). The increase of the magnetic moment of the multilayers is discussed in terms of the observed enhancement of the local magnetic moment of Fe atoms.

Magnetic and magnetotransport properties of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 spin valves

We report on the fabrication of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 (LSMO/SIO/LSMO) spin valve structures deposited on (0 0 1) (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates (LSAT) by high pressure sputtering. We present the structural, magnetic and magnetotransport properties of trilayers. Magnetization measurements indicate indirect exchange coupling between LSMO layers across the SrIrO3 layer. An enhancement of saturation magnetic moment was observed for trilayers with one and six unit cell thick SrIrO3 spacing layers. Low field magnetoresistance measurements show standard spin switching behavior and inverse spin switching in epitaxial LSMO/SIO/LSMO spin valve structures depending on the orientation of magnetic field to the plane and magnetic field to the current.

Pressure induced anomalous magnetic behaviour in nanocrystalline YCrO3 at room temperature

High pressure behaviour of nanocrystalline YCrO3 is investigated up to 10 GPa using electrical, magnetic, synchrotron x-ray diffraction and Raman spectroscopy measurements. High pressure dielectric constant measurements show a sharp peak at 4.5 GPa, though the sample is found to be in ferroelectric phase up to the highest pressure of our study from piezoelectric current measurements. X-ray diffraction measurements show absence of any structural phase transition, however anomalies are observed in the unit cell structural parameters at about 4.3 GPa and the Y-atom position shows a maximum shift at the same pressure. In the absence of any structural transition, anomalous behaviour of relevant Raman modes with minimum in the Raman band width at about same pressure indicate towards a spin-phonon interaction. AC magnetic measurements in the toroid anvil cell show an anomalous enhancement of magnetic moment above 4 GPa indicating a collective magnetic response of nanoparticles.

Effect of carbon on the magnetic properties of Dy-implanted GaN films

In this paper, samples were prepared by co-implanting Dy and carbon ions into c-plane (0001) GaN films and a subsequent rapid thermal annealing at 800 °C. The room temperature magnetic properties of Dy and C co-implanted GaN samples were investigated by experiments and theoretical calculations. Dy and C co-implanted GaN samples had the larger room temperature magnetic moment than that of the Dy-implanted GaN. The calculated results showed that the spin polarization of C-2p electrons leads to the local magnetic moment due to the hybridization between Dy-4f electron states and C-2p electron, which explains the enhancement of magnetic moment in co-implanted GaN samples. Our results suggest that co-implantation is a potential approach to enhance the room temperature ferromagnetism.

Magnetism of CrO overlayers on Fe(001)bcc surface: first principles calculations

Riva et al. [Surf. Sci. 621, 55 (2014)] as well as Calloni et al. [J. Phys.: Condens. Matter 26, 445001 (2014)] have studied the oxydation of Cr films deposited on Fe(001)bcc through low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy. In the present work we perform a density functional approach within Quantum Expresso code in order to study structural and magnetic properties of CrO overlayers on Fe(001)bcc. The calculations are performed using DFT+U. The investigated systems include O/Cr/Fe(001)bcc, Cr/O/Fe(001)bcc, Cr0.25O0.75/Fe(001)bcc, as well as the O coverage O x /Cr/Fe(001)bcc (x = 0.25; 0.50). We have found that the ordered CrO overlayer presents an antiferromagnetic coupling between Cr and Fe atoms. The O atoms are located closer to the Fe atoms of the surface than the Cr atoms. The ground state of the systems O/Cr/Fe(001)bcc and Cr/O/Fe(001)bcc corresponds to the O/Cr/Fe(001)bcc system with a magnetic coupling c(2 × 2). The effect of the O monolayer on Cr/Fe(001)bcc changes the ground state from p(1 × 1) ↓ to c(2 × 2) and produces an enhancement of the magnetic moments. The O x overlayer on Cr/Fe(001)bcc produces an enhancement of the Cr magnetic moments.

Interface-induced perpendicular magnetic anisotropy of Co nanoparticles on single-layer h-BN/Pt(111)

Ferromagnetism with perpendicular magnetic anisotropy (PMA) was observed at room temperature in cobalt nanoparticles (NPs) grown on hexagonal boron nitride (h-BN) on a Pt(111) surface. It was shown that the Co NPs have planar hexagonal shapes with a mean diameter of ∼20 nm and a mean height of ∼1.6 nm. The depth-resolved analysis of X-ray magnetic circular dichroism at the Co L2,3-edges revealed that in the ferromagnetic Co NPs, the ratio of the orbital magnetic moment to the spin magnetic moment in the out-of-plane direction becomes larger at the Co NP/h-BN interface than the ratio in bulk Co. The B and N K-edge near edge X-ray absorption fine structures showed the orbital hybridization between the π orbitals of h-BN and d orbitals of Co at the interface, as an origin of the orbital magnetic moment enhancement possibly giving rise to PMA in the Co NPs.

Structural modification, photoluminescence, and magnetic property enhancement with Er3+ doping, of sol–gel TiO2 thin films

Nanocrystalline undoped and Er3+ doped TiO2 thin films were prepared by sol–gel dip-coating technique onto glass substrates. Post-deposited films were annealed at 450 °C for 1 h. Differential scanning calorimetric analysis showed that undoped film crystallization occurred at 339 °C, while it appears at lower temperature for Er:TiO2. X-ray diffraction and Raman spectroscopy results indicate that the films crystallize only into anatase phase, and the crystallite size decreases from 24.3 to 21.16 nm with increasing Er3+ doping ratio. The photoluminescence (PL) spectra are composed of two broad peaks at 520–570 nm and 640–680 nm assigned to (2H11/2, 4S3/2)–4I15/2 and 4F9/2–4I15/2 transitions of Er3+ ions, respectively. PL emission intensities increase gradually raising the erbium doping ratio. Vibrating sample magnetometer results of undoped TiO2 thin films reveal a ferromagnetic behavior at room temperature and erbium doping leads to an enhancement of the saturated magnetic moment (Ms).