Weak Ferromagnetic State Research Articles

Evidence of new magnetic ordering at high temperatures in Pb-based multiferroics perovskites

Presented here are the results of magnetic data that reveal that both Pb-based perovskites samples, Pb(Fe1/2Nb1/2)O3 (lead iron niobate – PFN) and Pb(Fe2/3W1/3)O3 (lead iron tungstate – PFW), exhibit a change in magnetic ordering from paramagnetic (PM) to weak ferromagnetic state at high temperature: a weak ferromagnetic order occurs in the samples around 530 K. The experimental results suggest a possible involvement of the Pb ions into the superexchange of antiferromagnetic (AFM) phase.

Epitaxial strain induced magnetic transitions and phonon instabilities in tetragonal SrRuO3

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both tensile and compressive strain in the xy-plane can induce an abrupt change in the magnetic moment of the Ru atom. In particular, under an in-plane compressive strain of ∼4%, a ferromagnetic to nonmagnetic transition is induced, whereas for a tensile strain larger than 3%, the magnetic moment of Ru drops gradually with increase of the strain, exhibiting a weak ferromagnetic state. We find that these magnetic transitions can be qualitatively explained by the Stoner model. In addition, frozen-phonon calculations at the Γ point and phonon dispersion calculations reveal that structural instabilities can occur under both compressive and tensile strain. These instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains metallic in the range we studied. This might have an influence on the physical properties of oxide supercells having SRO as a constituent.

Weak ferromagnetic polar phase in the BiFe1−xTixO3 multiferroics

Neutron powder diffraction and magnetization measurements of selected samples of the BiFe1−xTixO3 series were performed. Ti4+ substitution was shown to induce the appearance of weak ferromagnetism in the initial polar R3c phase stable at x ≤ 0.1. In the concentration range 0 ≤ x ≤ 0.1, room-temperature residual magnetization increases from 0 to 0.25 emu/g (the latter is characteristic of the field-induced weak ferromagnetic state in pure BiFeO3). The calculated ferroelectric polarization decreases from ~70 μC/cm2 (x = 0) to ~60 μC/cm2 (x = 0.1) at room temperature. Magnetic ordering coexists with the large spontaneous polarization in a broad temperature range to make the BiFe1−xTixO3 (x → 0.1) perovskites promising for multiferroic applications.

Interplay of 4f-3d magnetism and ferroelectricity in DyFeO3

DyFeO3 exhibits a weak ferromagnetism (TNFe ∼ 645 K) that disappears below a spin-reorientation (Morin) transition at TSRFe ∼ 50 K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy ions (TNDy ∼ 4.5 K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (TSRFe <T <TN,C) without applying a magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57Fe Mössbauer studies show that a hyperfine field (Bhf) deviates from the mean-field–like behaviour that is observed in the weak ferromagnetic state and decreases below the onset of the spin-reorientation transition (80 K), implying that the Bhf above TSR had an additional contribution from Dy ions due to the induced magnetization by the weak ferromagnetic moment of the Fe sublattice and below TSR this contribution decreases due to collinear ordering of the Fe sublattice. These results clearly demonstrate the presence of magnetic interactions between Dy(4f) and Fe(3d) and their correlation with ferroelectricity in the weak ferromagnetic state of DyFeO3.

Evolution of crystal structure and ferroic properties of La-doped BiFeO3 ceramics near the rhombohedral-orthorhombic phase boundary

Bi1−xLaxFeO3 ceramics (0.15⩽x⩽0.2) across the rhombohedral-orthorhombic phase boundary have been studied using X-ray and neutron diffraction, magnetization measurements and piezoresponse force microscopy (PFM). The regions of structural stability of the polar, anti-polar and non-polar phases have been identified depending on the dopant concentration and a temperature and the structural phase diagram has been further clarified. The factors influencing phase transitions (size effects, chemical bonds peculiarities, local chemical inhomogeneities, etc.) have been estimated. PFM measurements testified a maximal piezoelectric response for a compound with the dominant rhombohedral phase in a metastable state. Magnetic properties have been discussed assuming weak ferromagnetic state with a major contribution from the orthorhombic phase. An evolution of structural parameters across the phase boundary decisive for improved ferroelectric and magnetic properties has been analyzed.

Spin-orbital state induced by strong spin-orbit coupling

To clarify a crucial role of a spin-orbit coupling in the emergence of novel spin-orbital states in 5d-electron compounds such as Sr2IrO4, we investigate ground state properties of a t2g-orbital Hubbard model on a square lattice by Lanczos diagonalization. In the absence of the spin-orbit coupling, the ground state is spin singlet. When the spin-orbit coupling is strong enough, the ground state turns into a weak ferromagnetic state. The weak ferromagnetic state is a singlet state in terms of an effective total angular momentum. Regarding the orbital state, we find the so-called complex orbital state, in which real xy, yz, and zx orbital states are mixed with complex coefficients.

Neutron diffraction and magnetic study of the low-temperature transitions in SrMo1−xFexO3−δ

SrMo1−xFexO3−δ (x=0, 0.1, 0.2 and 0.3) perovskites have recently been described as performing anode materials in solid-oxide fuel cells. In this work, we describe the structural phase transitions they undergo below room-temperature (RT), studied “in-situ” from neutron powder diffraction data and DSC measurements. At RT all the studied compositions are cubic, space group Pm-3m, with unit-cell parameters that decrease with Fe doping. Upon cooling the samples, two structural phase transitions are identified: one to a tetragonal structure with I4/mcm space group (around T1=240K), and the second one to an orthorhombic Imma phase below T2=100K. The magnetic properties have also been evaluated; the Fe substitution drives an evolution from a Pauli-paramagnetic state (x=0) to a weak ferromagnetic state combined with antiferromagnetic interactions; the susceptibility and the saturation magnetization increases monotonically with increasing the Fe-doping content.

Growth Mechanism and Magnetic Properties of Highly Crystalline NiO Nanocubes and Nanorods Fabricated by Evaporation

A new approach to the preparation of regularly shaped NiO nanocubes and nanorods by an infrared heating evaporation method is presented. The growth model is proposed to be a vapor–solid mechanism. The morphology of the nanocrystals can be tuned by the carrier gas flow rate. The samples consist of nanocrystals that are highly crystallized with atomic-scale smooth surfaces. This novel method could be extended to nanocrystal growth of other oxides with low volatility or high melting points. The results of magnetic characterization indicate that the NiO nanorods and nanocubes grown from a mixed target both show weak ferromagnetic states at low and room temperature due to uncompensated spins at the surfaces of the nanocrystals.

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO3 (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO3 has a weak ferromagnetic ground state below 356 K—this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO3.

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.

Anomalous Weak Ferromagnetism in Electron-doped Nd1-xSrxMnO3 (0.50 ¡? x ¡? 0.62) Thin Films

In the present work, we demonstrate that an anomalous weak ferromagnetism occurs in the composition range 0.50 <= x <= 0.62 in nanostructured thin films of Nd1-xSrxMnO3 (NSMO). Nanocrystalline thin films of overdoped manganite Nd1-xSrxMnO3 (x similar to 0.50, 0.55, 0.60, and 0.62) are grown on single-crystal LaAlO3 (001) substrates by using a nebulized chemical spray pyrolysis technique. These single phase films possess an average crystallite size similar to 15 nm, and the width of the grain boundaries is similar to 1 - 2 nm. In the composition range 0.50 <= x <= 0.62, the ground state of NSMO is an A-type antiferromagnetic (AFM) metallic state. However, in the present films, a broad paramagnetic (PM)-to-ferromagnetic (FM) transition is observed in all the compositions, at T-C similar to 226 K for x = 0.50 and at 235 K for rest of the samples. All the films show a well-defined M-H hysteresis loop at 5 K. The coercivity (H-C) of these films is much larger than those having compositions in the range 0.35 < x < 0.45. For x = 0.50 (0.62), the films H-C = -1210 (-1484) Oe and +1162 (+1476) Oe. The asymmetry in the coercivity suggests the presence of a weak exchange bias effect in these films. The FM ground state in these films is observed to have a smaller magnetic moment per Mn atom than the expected full moment from the rigid model; we term this as a weak ferromagnetic (WFM) state arising due to the destabilization of the AFM ordering. We propose a possible scenario based on the combined effect of spin reorganization and enhanced orbital disordering in nanosized manganites to explain the observed anomalous weak ferromagnetism in the A-type AFM spin ordered region.

Mössbauer spectroscopic study of spin reorientation in Mn-substituted yttrium orthoferrite

The first-order spin-reorientation transition in the Mn-substituted yttrium orthoferrites, YFe1−xMnxO3 (x = 0.1, 0.15 and 0.2), has been investigated using 57Fe Mössbauer spectroscopy. Owing to its large anisotropy, substitution of Mn3+ ions in YFeO3 induces a spin-reorientation transition from the low-temperature antiferromagnetic state to a high-temperature weak ferromagnetic state. With increasing x, the spin-reorientation transition temperature (TSR) increases whereas the Néel temperature (TN) decreases. Analysis of the Mössbauer spectra unambiguously confirms the occurrence of spin reorientation relative to crystal axes. At a given temperature, the mean hyperfine field decreases with the increasing Mn concentration. The variation of canting angle with temperature for YFe0.85Mn0.15O3 has been estimated.

Phase Transitions, Magnetic and Piezoelectric Properties of Rare‐Earth‐Substituted BiFeO3 Ceramics

The concentration range of the stability of polar (R3c) and antipolar phases in the Bi1−xRExFeO3 (RE–La ‐ Dy) solid solutions has been determined by X‐ray study of the polycrystalline samples. Both polar and antipolar phases become less stable with a decrease of the rare earth ionic radii. It is stimulated by a reduction of the rare‐earth ions polarizability with a decrease in ionic radii. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar one exhibits dominantly antiferromagnetic behavior near the polar‐antipolar morphotropic boundary. The local piezoelectric response decreases with increase in antipolar phase content in the mixed polar‐antipolar structural state. It is suggested that the piezoelectric activity is associated with polar (R3c) phase.

Structural stability and magnetic properties of Bi 1− xLa(Pr) xFeO 3 solid solutions

In this work, X-ray diffraction data taken on Bi 1− x La x FeO 3 solid solutions are used to verify the following structural phase transitions: “polar rhombohedral–antipolar orthorhombic” at x ≈ 0.16 and “commensurate–incommensurate” within the orthorhombic phase at x ≈ 0.18 . In contrast, in the Bi 1− x Pr x FeO 3 series, the polar rhombohedral phase transforms into an antipolar orthorhombic one at x ≥ 0.13 . The polar rhombohedral phase near the morphotropic phase boundary exhibits an isothermal transformation into an antipolar orthorhombic phase, though the transformation occurs much faster in the case of La-doped compounds. The incommensurate structural phase was not detected in Bi 1− x Pr x FeO 3 solid solutions. The ternary structural phase diagram is constructed for (Bi,La,Pr)FeO 3 systems. In addition, the polar rhombohedral phase exhibits a magnetic field-induced transition from the modulated antiferromagnetic state into a homogeneous weak ferromagnetic state whereas the antipolar phase is a weak ferromagnetic state in the absence of an external field.

Conditions favoring the polar weak ferromagnetic state in BiFeO3-type multiferroics

The crystal structure and magnetic properties of Bi1 − x A x FeO3 − x/2 (A = Ca, Sr, Pb, Ba), Bi1 − x A x (Fe1 − x Ti x )O3, and Bi1 − x A x (Fe1 − x/2Nb x/2)O3 solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.

Enhanced piezoelectric and magnetic properties of Bi1−xCaxFe1−x/2Nbx/2O3 solid solutions

An investigation of the crystal structure, the magnetic and the piezoelectric properties of polycrystalline Bi1-xCaxFe1-x/2Nbx/2O3 (I) and Bi1-xPbxFe1-x/2Nbx/2O3 (II) systems was performed by x-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometry, and piezoresponse force microscopy. It is shown that an increasing niobium content induces a polar-to-nonpolar morphotropic boundary near x = 0.19 (I) and x = 0.3 (II). Within the polar region of the system (I), (0.15 ≤x ≤0.18), the solid solutions are homogeneous weak ferromagnets whereas compounds of the system (II) do not exhibit an appreciable spontaneous magnetization. It is assumed that chemical substitutions leading to a decrease of the initial volume of the unit cell favor the stabilization of the weak ferromagnetic state within the rhombohedral ferroelectric phase. The piezoresponse is significantly enhanced near the morphotropic boundary. The piezoelectric properties of the parent antiferromagnet BiFeO3, harboring a cycloidal spatially modulated spin structure, are compared with those of the polar weak ferromagnet Bi0.82Ca0.18Fe0.91Nb0.09O3.

Charge states and magnetic ordering in LaMnO3/SrTiO3superlattices

We investigated the magnetic and optical properties of [$({\mathrm{LaMnO}}_{3}){}_{n}$/$({\mathrm{SrTiO}}_{3}){}_{8}$]${}_{20}$ ($n$ $=$ 1, 2, and 8) superlattices grown by pulsed-laser deposition. We found that a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the ${\mathrm{LaMnO}}_{3}$ layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the ${\mathrm{LaMnO}}_{3}$ layer thickness up to eight unit cells, suggesting that the effect is not limited to the interface. On the other hand, the magnetic ordering became less stable as the ${\mathrm{LaMnO}}_{3}$ layer thickness decreased, probably due to a dimensional effect.

Bitriazole bridged manganese(II) framework with weak ferromagnet behavior

An asymmetric bridging ligand 3,4′-bi-1,2,4-triazole (Hbtr) was used for constructing transition metal coordination polymer, and yielding a manganese(II) compound [Mn(btr) 2] n ( 1) with a unique three-dimensional (3D) framework. Single-crystal X-ray diffraction reveals that 1 crystallizes in the orthorhombic space group Fdd2 and the structure is 3D (3, 6)-connected topological framework constructed from manganese ions and μ 3-btr ligands. The magnetic investigation for compound 1 shows that it is an antiferromagnetically coupled 3D array of high-spin manganese(II) ions with long-range ordering below T = 2.6 K, which is a weak ferromagnetic state due to spin-canting.

Isothermal structural transitions, magnetization and large piezoelectric response in Bi1−xLaxFeO3perovskites

We report on the discovery of an isothermal structural transition observed in Bi${}_{1\ensuremath{-}x}$La${}_{x}$FeO${}_{3}$ ($0.17\ensuremath{\leqslant}x\ensuremath{\leqslant}0.19$) ceramics. At room temperature, an initially pure polar rhombohedral phase gradually transforms into a pure antipolar orthorhombic one. The polar phase can be recovered by annealing at $T&gt;300 {}^{\ifmmode^\circ\else\textdegree\fi{}}$C. In accordance with neutron powder diffraction data, an inverse isothermal antipolar-polar transition takes place at $T&gt;300$ ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C, where the polar phase becomes more stable. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar phase has been obtained in a mixed antiferromagnet--weak ferromagnet state. The relatively low external pressure induces polar-antipolar transition, but there is no evidence of electric-field-driven antipolar-polar transition. The observed large local piezoelectric response is associated with structural instability of the polar phase, whereas local multistate piezoelectric loops can be related to the domain wall pinning effect.

Tunable Magnetism in Strained Graphene with Topological Line Defect

We examine the magnetic properties of two-dimensional graphene with topological line defect using first-principles calculations and predict a weak ferromagnetic ground state with spin-polarized electrons localized along the extended line defect. Our results show that tensile strain along the zigzag direction can greatly enhance local magnetic moments and ferromagnetic stability of the system. In sharp contrast, tensile strain applied along the armchair direction quickly diminishes these magnetic moments. A detailed analysis reveals that this intriguing magnetism modulation by strain stems from the redistribution of spin-polarized electrons induced by local lattice distortion. It suggests a sensitive and effective way to control magnetic properties of graphene which is critical for its applications in nanoscale devices.