Multiferroic behavior in EuTiO3 films constrained by symmetry
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99 GENERAL AND MISCELLANEOUS↗
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Magnetic structure of Fe 16 N 2 determined by polarized neutron diffraction on thin-film samples
Strain-free Fe 16 N 2 thin films were obtained on MgO (001) single-crystal substrates with Cr seed layers. Additionally, x-ray diffraction and transmission electron microscopy confirmed that the film is textured in all three spatial directions with crystallographic orientation determined by epitaxial constraints from the substrate. Average grain size was estimated to be in the range of tens of nanometers. Ten pieces of 40-nm-thick Fe 16 N 2 thin-film samples were stacked together for half-polarized neutron diffraction study at room temperature. A total of six reflections were usable, from which the magnetic structure of Fe 16 N 2 was determined and compared to existing models. Contrary to most electronic structure calculations, the present results suggest that the magnetic moment of Fe atoms at the 8h site is large and larger than that of the 4d site. Our results support the giant magnetization of Fe 16 N 2 .
36 MATERIALS SCIENCE↗
Unconventional magnetic order emerging from competing energy scales in the new R Rh 3 Si 7 intermetallics ( R = Gd-Yb)
The competition between Ruderman-Kittel-Kasuya-Yosida (RKKY), crystal electric field (CEF), and Kondo energy scales has recently emerged at the heart of complex magnetism in several Ce- or Yb-based intermetallics. Hard axis magnetic order has been observed in a handful of these compounds, independent of the crystal symmetry, size of the ordered moment, or the relative scale of the Kondo and magnetic ordering temperatures. This raises the question of the role of each energy scale in driving the ground state properties. In focusing on a single class of compounds, the rhombohedral RRh 3 Si 7 , we compare the anisotropy and magnetic ground states in members of this series with only RKKY interactions (R = Gd), or RKKY and CEF effects (R = Tb-Tm), with the behavior of the R = Yb compound, where all three energy scales (RKKY, CEF, Kondo) are at play. Moreover, we extend the comparison to two other isostructural Kondo systems YbIr 3 Si 7 and YbIr 3 Ge 7 , where hard axis magnetic order is also observed. The non-Kondo compounds RRh 3 Si 7 (R = Tb-Tm) lack the complexity of magnetic order along the hard CEF axis, pointing to the dominant role of the Kondo effect in driving this magnetic order. Furthermore, the CEF-RKKY competition is still responsible for complex magnetic ground states, and it appears that the electronic and magnetic degrees of freedom are entangled in all magnetic members of this series of compounds.
36 MATERIALS SCIENCE↗
Synthesis and behavior of bulk iron nitride soft magnets via high-pressure spark plasma sintering
Abstract In this study, dense bulk iron nitrides (Fe x N) were synthesized for the first time ever using spark plasma sintering (SPS) of Fe x N powders. The Fe 4 N phase of iron nitride in particular has significant potential to serve as a new soft magnetic material in both transformer and inductor cores and electrical machines. The density of SPSed Fe x N increased with SPS temperature and pressure. The microstructure of the consolidated bulk Fe x N was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRD revealed a primary phase of Fe 4 N with secondary phases of Fe 3 N and metallic iron. Finite element analysis (FEA) was also applied to investigate and explain localized heating and temperature distribution during SPS. The effects of processing on interface bonding formation and phase evolution were investigated and discussed in detail to provide insight into fundamental phenomena and microstructural evolution in SPSed Fe x N. Graphic abstract
36 MATERIALS SCIENCE↗
An effective field theory of magneto-elasticity
We utilize the coset construction to derive the effective field theory of magnon-phonon interactions in (anti)-ferromagnetic and ferrimagnetic insulating materials. The action is used to calculate the equations of motion which generalize the Landau-Lifshitz and stress equations to allow for magneto-acoustic couplings to all orders in the fields at lowest order in the derivative expansion. We also include the symmetry breaking effects due to Zeeman, and Dzyaloshinsky-Moriya interactions. This effective theory is a toolbox for the study of magneto-elastic phenomena from first principles. As an example we use this theory to calculate the leading order contribution to the magnon decay width due to its the decay into phonons.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
A Multiphysics Thermoelastoviscoplastic Damage Internal State Variable Constitutive Model including Magnetism
We present a macroscale constitutive model that couples magnetism with thermal, elastic, plastic, and damage effects in an Internal State Variable (ISV) theory. Previous constitutive models did not include an interdependence between the internal magnetic (magnetostriction and magnetic flux) and mechanical fields. Although constitutive models explaining the mechanisms behind mechanical deformations caused by magnetization changes have been presented in the literature, they mainly focus on nanoscale structure–property relations. A fully coupled multiphysics macroscale ISV model presented herein admits lower length scale information from the nanoscale and microscale descriptions of the multiphysics behavior, thus capturing the effects of magnetic field forces with isotropic and anisotropic magnetization terms and moments under thermomechanical deformations. For the first time, this ISV modeling framework internally coheres to the kinematic, thermodynamic, and kinetic relationships of deformation using the evolving ISV histories. For the kinematics, a multiplicative decomposition of deformation gradient is employed including a magnetization term; hence, the Jacobian represents the conservation of mass and conservation of momentum including magnetism. The first and second laws of thermodynamics are used to constrain the appropriate constitutive relations through the Clausius–Duhem inequality. The kinetic framework employs a stress–strain relationship with a flow rule that couples the thermal, mechanical, and magnetic terms. Experimental data from the literature for three different materials (iron, nickel, and cobalt) are used to compare with the model’s results showing good correlations.
36 MATERIALS SCIENCE↗
A Multiphysics Thermoelastoviscoplastic Damage Internal State Variable Constitutive Model including Magnetism
We present a macroscale constitutive model that couples magnetism with thermal, elastic, plastic, and damage effects in an Internal State Variable (ISV) theory. Previous constitutive models did not include an interdependence between the internal magnetic (magnetostriction and magnetic flux) and mechanical fields. Although constitutive models explaining the mechanisms behind mechanical deformations caused by magnetization changes have been presented in the literature, they mainly focus on nanoscale structure–property relations. A fully coupled multiphysics macroscale ISV model presented herein admits lower length scale information from the nanoscale and microscale descriptions of the multiphysics behavior, thus capturing the effects of magnetic field forces with isotropic and anisotropic magnetization terms and moments under thermomechanical deformations. For the first time, this ISV modeling framework internally coheres to the kinematic, thermodynamic, and kinetic relationships of deformation using the evolving ISV histories. For the kinematics, a multiplicative decomposition of deformation gradient is employed including a magnetization term; hence, the Jacobian represents the conservation of mass and conservation of momentum including magnetism. The first and second laws of thermodynamics are used to constrain the appropriate constitutive relations through the Clausius–Duhem inequality. The kinetic framework employs a stress–strain relationship with a flow rule that couples the thermal, mechanical, and magnetic terms. Experimental data from the literature for three different materials (iron, nickel, and cobalt) are used to compare with the model’s results showing good correlations.
36 MATERIALS SCIENCE↗
Magnetization-steps in Y₂CoMnO₆ double perovskite: The role of antisite disorder
Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y²CoMnO₆ which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T c ≈ 75 K. At 2 K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H c ≈ 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8 K. In the temperature range 2 K ≤ T ≤ 5 K, the hysteresis loops are anomalous as the virgin curve lies outside the main loop. The field-cooling conditions as well as the rate of field-sweep are found to influence the steps. Quantitative analysis of the neutron diffraction data shows that at room temperature, Y₂CoMnO₆ consists of 62% of monoclinic P2₁/n with nearly 70% antisite disorder and 38% Pnma. The bond valence sums indicate the presence of other valence states for Co and Mn which arise from disorder. We explain the origin of steps by using a model for pinning of magnetization at the antiphase boundaries created by antisite disorder. The steps in magnetization closely resemble the martensitic transformations found in intermetallics and display first-order characteristics as revealed in the Arrott's plots.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Impact of Ultra-Conducting Winding on the Power Density and Performance of Non-Heavy Rare Earth Traction Motors
The emergence of carbon nanotube-copper composite conductors has opened a new degree of freedom in the electric machine design to improve the machine's performance: the conductivity of the winding. The goal of the present study is: 1) to determine how much improvement in power per unit volume and performance can be achieved with the increase in winding conductivity; and 2) to investigate any physical limitations on the effectiveness of this degree of freedom in improving the power per unit volume and performance of electric traction motors. Conventional electric motor topologies affected by ferromagnetic saturation as well as slotless topology that is not affected by saturation are analyzed. It was found that the ferromagnetic saturation of the lamination materials reduces the effectiveness of highly conductive winding in improving the power per unit volume. For machine topologies that are not affected by ferromagnetic saturation, the power density improvement reaches the theoretical expectation. In both cases, highly conductive winding improves the efficiency map by enlarging high efficiency operating area into light load as well as high speed operations.
Raminosoa, Tsarafidy↗
A Minimal tight-binding model for ferromagnetic canted bilayer manganites
Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate and aside from a small minority-spin pocket observed in La 2–2 x Sr 1+2 x Mn 2 O 7 ( x = 0.38), transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO 2 planes and the mixing of the | x 2 – y 2 > and |3 z 2 – r 2 > Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of k z dispersion are included.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Magnetic resonance investigation of Zn{sub 1−x}Fe{sub x}O properties influenced by annealing atmosphere
ZnO is an attractive system for a wide variety of practical applications, being a chemically stable oxide semiconductor. It has been shown that Fe doping produces ferromagnetic semiconductor at room temperature. This material, therefore, has the potential for use in spintronic devices such as spin transistors, spin light emitting diodes, very high density nonvolatile semiconductor memory and optical emitters. It is believed that oxygen vacancies and substitutional incorporation are important to produce ferromagnetism in semiconductor oxide doped with transition metal ions. The present paper reports detailed electron paramagnetic resonance investigations (EPR) of the samples in order to investigate how annealing atmosphere (Air and Argon) influenced the magnetic behavior of the samples. X-band electron paramagnetic resonance (EPR) studies of Fe{sup 3+} ions in Zn{sub 1−x}Fe{sub x}O powders with x = 1%, 3% is reported. These samples are interesting to investigate as Fe doping produce ferromagnetism in ZnO, making a promising ferromagnetic semiconductor at room temperature.
36 MATERIALS SCIENCE↗
Investigation of the Substituting Effect of Chromium on the Electronic Structures and the Half-Metallic Ferromagnetic Properties of BaO
The first-principle methods of density functional theory were used to investigate the doping effect of 3d chromium (Cr) impurities on the electronic properties and the ferromagnetic arrangement of (BaO) barium oxide such as Ba{sub 1−x}Cr{sub x}O compounds at concentrations x = 0.25, 0.5, and 0.75. The p–d exchange coupling between 2p states of oxygen (O) and 3d (Cr) partially filled levels induces magnetism in the Ba{sub 1−x}Cr{sub x}O compounds. The majority- and minority-spin states exhibit respectively metallic and semiconducting features, leading to a half-metallic (HM) character with 100% spin polarization for all concentrations. The Ba{sub 0.75}Cr{sub 0.25}O, Ba{sub 0.5}Cr{sub 0.5}O, and Ba{sub 0.25}Cr{sub 0.75}O compounds have integral total magnetic moments of 4 μ{sub B} per Cr atom with HM gaps of 0.055, 0.218, and 0.308 eV, respectively. These behaviors form accurate half-metallic ferromagnets and make Ba{sub 1−x}Cr{sub x}O as potential materials for possible spintronics applications.
36 MATERIALS SCIENCE↗
Cr silicate as a prototype for engineering magnetic phases in air-stable two-dimensional transition-metal silicates
Abstract Identifying environmentally inert, ferromagnetic two-dimensional (2D) materials with high Curie temperatures ( T c ) down to the single layer limit has been an obstacle to fundamental studies of 2D magnetism and application of 2D heterostructures to spin-polarized devices. To address this challenge, the growth, structure and magnetic properties of a 2D Cr-silicate single layer on Pt(111) was investigated experimentally and theoretically. The layer was grown by sequentially depositing SiO and Cr followed by annealing in O 2 . Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and low energy electron microscopy all indicated a well-ordered layer that uniformly covered the surface, with STM and LEED indicating that the silicate relaxed to its favored lattice constant. Further experimental characterizations demonstrated that the Cr was nominally 3+ but with a lower electron density than typical trivalent Cr compounds. Comparison with theory identified a Cr 2 Si 2 O 9 structure that resembles a single layer of a dehydrogenated dioctahedral silicate. Magnetic circular dichroism in x-ray absorption spectroscopy revealed a ferromagnetically ordered state up to at least 80 K. Theoretical analysis revealed that the Cr in a dehydrogenated Cr-silicate/Pt(111) is more oxidized than Cr in freestanding Cr 2 Si 2 O 9 H 4 layers. This greater oxidation was found to enhance ferromagnetic coupling and suggests that the magnetism may be tuned by doping. The 2D Cr-silicate is the first member of a broad series of possible layered first-row transition metal silicates with magnetic order; thus, this paper introduces a new platform for investigating 2D ferromagnetism and the development of magnetoelectronic and spintronic devices by stacking 2D atomic layers.
2D materials↗
Synergy effect in sonodegradation of organic dye using graphene based magnetite composite materials
Owing to the unique physical properties, nanographene platelets could act as a good recombination inhibitor for functional materials. In this study, various concentrations of nanographene platelets are introduced to ferromagnetic Fe{sub 3}O{sub 4} nanoparticles to remove methylene blue in aqueous solution under ultrasonic irradiation. The coupling of nanographene platelets with Fe{sub 3}O{sub 4} nanoparticles is formed using hydrothermal method. Compared to pure Fe{sub 3}O{sub 4} nanoparticles, Fe{sub 3}O{sub 4}–nanographene platelets (Fe{sub 3}O{sub 4}–NGP) composites exhibited higher catalytic efficiency for methylene blue removal under ultrasonic irradiation. The catalytic efficiency increase with increasing nanographene platelets loading from 1 wt.% until 5 wt% and further decreased. From the scavenger technique, it is revealed that hole plays an important key role in sonocatalytic processes. The degradation efficiency decreased only 12% after 4 cycled.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Prediction of room-temperature multiferroicity in strained MoCr 2 S 6 monolayer
The contrasting d-orbital occupation required for ferroelectricity vs ferromagnetism makes it difficult for their coexistence in two-dimensional materials, especially at high temperature. To resolve this intrinsic contradiction, we propose a layered MoCr 2 S 6 multiferroics by alloying magnetic Cr element into the ferroelectric 1T phase of the MoS 2 matrix. First-principles calculations disclose that a spontaneous symmetry breaking, depending on the Mo atom displacement, leads to a robust ferroelectricity, which coexists with a ferromagnetic order originated from two neighboring Cr atoms. The effect can be further enhanced by tensile strain to bring about a room-temperature multiferroicity. Our results shed new light on the fundamental understanding of multiferroics and display promising applications in spintronics and multistate data storage.
2D materials↗
Final Technical Report Grant No. DE-FG02-97ER45653 Lance E. De Long, Principal Investigator, University of Kentucky Period of Performance: 09/01/97 to 05/14/15
Prior to 1997, the PI had studied the unusual upper critical magnetic field phase boundaries of several novel or exotic types of superconductors, including charge density wave materials such as NbSe 2 , organic superconductors such as κ-(ET) 2 Cu[N(CN) 2 ]Br, high-temperature oxide superconductors such as (Ba,K)BiO 3 and the cuprates, heavy fermion superconductors such as U 6 Fe, UBe 13 , URu 2 Si 2 and UPt 3 , and re-entrant Kondo alloys such as (La,Ce)Al 2 and ferromagnetic superconductors such as ErRh 4 B 4 . Most of these materials exhibited marked positive or negative curvature of H C2 (T) which could not be explained by traditional pair-breaking models. It became clear that many of these materials had very short coherence lengths that made quantized vortices highly mobile (depinned) near the phase boundary, and the fundamental, equilibrium H C2 (T) difficult to measure using finite field or current drives. These problems made the underlying physics obscure, and led to erroneous interpretations of experimental data in terms of models of exotic superconducting pairing mechanisms. Around 1995, these issues led the PI to take advantage of modern electron beam lithography techniques for patterning superconducting and ferromagnetic thin films on the nanoscale. Primarily due to strong magnetic shape anisotropy effects, EBL patterning has led to enhanced control of the spatial distribution and dynamics of topological defects such as domain walls and magnetic vortices, which can create serious energy dissipation and other limitations for modern devices. Moreover, finite size and interface effects also strongly alter phase transition temperatures and phase boundaries of superconducting and magnetic films, as well as introduce barriers to equilibration, enhanced fluctuations and alter magnetic relaxation. Geometrical frustration and spin ice behavior can also be systematically controlled in patterned film media. Film patterning thus provides an excellent tool for conducting highly-controlled, fundamental studies of cooperative phases and interactions in artificially structured condensed matter.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Efficient Optical Control of Magnon Dynamics in van der Waals Ferromagnets
Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe 3.6 Co 1.4 GeTe 2 . The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.
36 MATERIALS SCIENCE↗
From Complex Magnetism Ordering to Simple Ferromagnetism in Two-Dimensional LaCrSb 3 by Hole Doping
Competing orders widely exist in many material systems, such as superconductivity, magnetism, and ferroelectricity; LaCrSb 3 is a highly anisotropic magnetic material in which the spins are aligned ferromagnetically in one direction and canted antiferromagnetically in another in the Cr-Sb chains. Hole doping with Sr 2+ and Ca 2+ in the La site suppresses the antiferromagnetic correlations and transforms the anisotropic magnetic order into a ferromagnetic lattice in all directions. Here, first-principles density functional theory calculations show that the canted magnetic order becomes energetically less favorable compared to the FM order upon hole doping. Doping in the La site is an effective approach to modulate the competing orders in LaCrSb 3 .
36 MATERIALS SCIENCE↗