Large Remanent Magnetization Research Articles

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation

ABSTRACT Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect (AEE). In addition to ANE/AEE, we discovered unique and superior material properties in these alloys: the amorphous structure, low thermal conductivity, and large in-plane coercivity and remanent magnetization. These properties make it advantageous over conventional materials to realize heat flux sensing applications based on ANE, as our Sm-Co-based films can generate thermoelectric output without an external magnetic field. Importantly, the amorphous nature enables the fabrication of these films on various substrates including flexible sheets, making the large-scale and low-cost manufacturing easier. Our demonstration will provide a pathway to develop flexible transverse thermoelectric devices for smart thermal management.

Microstructure and extrinsic magnetic properties of anisotropic Sm(Fe,Ti,V)12-based sintered magnets

Anisotropic SmFe12-based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity μ0Hc and remanent magnetization μ0Mr. In this study, we investigated the microstructure transformation and extrinsic performance change in the anisotropic Sm(Fe,Ti,V)12-based sintered magnets via reducing the Ti content and partially substituting Fe with Co. Reduction of Ti content in anisotropic sintered Sm8Fe73.5+xTi8-xV8Ga0.5Al2 magnet eliminated the secondary Fe2(Ti,V) phase, resulting in a remanence of μ0Mr = 0.8 T and energy density of (BH)max = 113 kJ/m3 (14.2 MGOe) in x = 3. A relatively high coercivity μ0Hc of 0.6 T is maintained owing to the formation of thin intergranular phase (IGP). Substitution of Co for Fe in the Sm8(Fe0.95Co0.05)73.5Ti8V8Ga0.5Al2 sintered magnet resulted in a low remanence and low coercivity μ0Hc. The electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations indicate the formation of twins inside 1:12 grains which deteriorate the texture and remanent magnetization. Magneto-optical Kerr effect (MOKE) microscopy and micromagnetic simulations revealed that although the magnetization reversal starts at the twinned grains, Sm-rich intergranular phase hinders propagation of reversed domains to the neighboring grains, preventing the deterioration of coercivity. However, the formation of ferromagnetic SmFe2-based phase of the MgCu2 type in Co-containing magnet is responsible for a further decrease of coercivity to μ0Hc = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe12-based sintered magnets, i.e., ones without twins and ferromagnetic SmFe2-based phase of the MgCu2 type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe12 grains. For SmFe12-type phase, reducing the nonmagnetic elements and increasing its saturation magnetization remain crucial to realize the potential to compete with the Nd2Fe14B phase.

Suppression of the Cycloidal Spin Arrangement in BiFeO3 Caused by the Mechanically Induced Structural Distortion and Its Effect on Magnetism

Bismuth ferrite (BiFeO3) particles are prepared by a combined mechanochemical−thermal processing of a Bi2O3 + α-Fe2O3 mixture. Structural, magnetic, hyperfine, morphological and chemical properties of the as-prepared BiFeO3 are studied using X-ray diffraction (Rietveld refinement), 57Fe Mössbauer spectroscopy, SQUID magnetometry, electron microscopy and energy dispersive X-ray spectroscopy. It is revealed that the structure of the ferrite exhibits the long-range distortion (significantly tilted FeO6 octahedra) and the short-range disorder (deformed FeO6 octahedra). Consequently, these structural features result in the suppression of a space modulated cycloidal spin arrangement in the material. The latter manifests itself by the appearance of only single spectral component in the 57Fe Mössbauer spectrum of BiFeO3. The macroscopic magnetic behavior of the material is interpreted as a superposition of ferromagnetic and antiferromagnetic contributions with a large coercive field and remanent magnetization. Taking into account the average particle size of the as-prepared BiFeO3 particles (∼98 nm), exceeding the typical period length of cycloid (∼62 nm), both the suppression of the spiral spin structure in the material and its partly ferromagnetic behavior are attributed to the crystal lattice distortion caused by mechanical stress during the preparation procedure.

(Invited) Development of Anisotropic Magnetic Nanocomposites for Miniaturized RF Device Applications

The miniaturization of radio frequency (RF) components has become increasingly in-demand due to the popularity of mobile devices, such as smartphones, tablets, and other ad-hoc networking products. Many state-of-the-art RF components, due to the typical use of ferrite materials and a bulky permanent magnet that magnetizes the ferrites, are too cumbersome to be used in fully integrated monolithic microwave integrated circuits (MMICs). Thus, passive RF components that do not require external magnetic fields are needed.Here we present a class of anisotropic magnetic nanocomposites (AMNs) that are capable of working as the magnetic layer within passive RF components. Our AMNs are based on high aspect ratio (>1000) magnetic nanowires and functional matrices, and show a square shaped easy-axis M(H) loop, a large remanent magnetization, and a tunable ferromagnetic resonant frequency. With the AMNs, miniaturized passive circulators were prototyped. In this presentation, we will discuss the synthesis of AMNs and the tailoring of the AMNs magnetic and RF properties to improve circulator performance.This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.Distribution Statement A - Approved for Public Release, Distribution Unlimited.

Microwave response of coplanar waveguide based on electrodeposited CoFe2O4 nanowires

Well-crystallized CoFe2O4 nanowires are grown by electrodeposition into anodic alumina oxide (AAO) membrane. The XRD and EDS analysis confirmed the synthesis of pure CoFe2O4 with average grain size ranging from 30 to 22 nm. The SQUID investigations revealed that the nanowires electrodeposited at −1.2 V exhibit a large remanent magnetization and high permanent internal magnetic field along the wire axis. The microwave studies emphasized a gyromagnetic resonance around 42 GHz. In addition, numerical simulations performed by HFSS showed good agreement with experimental results. As a consequence, such electrodeposited nanocomposite is a promising candidate for high-frequency microwave devices.

Molten salt synthesis, formation mechanism and greatly enhanced magnetic properties of randomly oriented BaM ferrite

Due to the influence of specific preparation process and product microstructure characteristics, it is a difficult task to prepare the randomly oriented BaFe12O19 (BaM) ferrite with large remanent magnetization (Mr) and high saturation magnetization (Ms), which hinder the practical application of the BaM ferrite. One of the means to solve this problem is to prepare well-developed hexagonal plate-like structure and single magnetic domain particles by lower synthesis temperature. In this study, the BaM ferrite were prepared using molten salt synthesis (MSS) method, and the preparation conditions of excellent magnetic properties and the formation mechanism of BaM ferrite synthesized by molten salt were discussed. Powder X-ray diffraction (PXRD), scanning electron microscope (SEM) and magnetic properties analyses indicated that the BaM ferrite with greatly enhanced magnetic properties could be obtained by adjusting the calcination temperature, Fe3+/Ba2+ molar ratio and the salts to reactants mass ratio R. The Ms and Mr of the prepared BaM ferrite were 71.9 emu/g and 39.5 emu/g, respectively. The Ms of the prepared BaM ferrite was significantly enhanced, and the Mr was much higher than that of BaM ferrite prepared by other methods. Using hydrothermal method synthesized spindle-like α-Fe2O3 as a raw material confirmed that the formation mechanism of molten salt synthesis of BaM ferrite was dissolution and precipitation mechanism. The formation of BaM ferrite underwent a series of processes including BaFe2O4 and BaFe4O7 formation, iron oxide dissolution, BaFe2O4 and BaFe4O7 desolvation of Ba2+, ion diffusion, nucleation growth and precipitation.

Large remanent magnetization in Bi doped CaMn7O12 compounds: Magnetic and X-ray absorption spectroscopic studies

The structural and magnetic properties of Bi doped CaMn7O12 (CMO) multiferroics are correlated with the results of electronic structures obtained from X-ray absorption spectroscopy. These compounds were prepared by using wet sol–gel technique and their X-ray diffraction patterns confirm the single phase formation. The solid solubility limit of Bi in CMO is found to be 10% at Ca site. A remarkable enhancement in magnetization is observed for 10% Bi doped Ca1-xBixMn7O12. This increase in remanence magnetization is ~10 times in comparison to CMO. Doping of divalent Ca ions by trivalent Bi ions cause an increase in trivalent Mn ions and hence increase in unpaired electrons leading to this enhancement of magnetization. The X-ray absorption studies at the O K-, and Mn K-, and Mn L3,2 -edges of Ca1-xBixMn7O12 (x = 0, 0.05 and 0.10) clearly indicate an increase in the ratio of Mn3+/Mn4+ along with increase in interstitial oxygen atoms. This significant increase in magnetic remanence for optimum Bi doping is very promising for magnetic and microwave applications.

Magneto-electric interactions in composites of self-biased Y- and W-type hexagonal ferrites and lead zirconate titanate: Experiment and theory

This report is on the observation and theory for strong mechanical strain mediated magneto-electric (ME) coupling in composites of lead zirconate titanate (PZT) and self-biased Y- or W-type hexagonal ferrites. Polycrystalline Y-type (Ni1−xZnx)2 Y, and W-type (Co1−xZnx)2 W, hexagonal ferrites for x = 0–0.4 prepared by ceramic processing techniques showed a large remanent magnetization due to uniaxial or in-plane magneto-crystalline anisotropy. The strength of ME coupling in symmetric trilayer composites of the ferrites and PZT was measured by the ME voltage coefficient (MEVC) at low-frequencies and at longitudinal electromechanical resonance. The bias magnetic field H-dependence of MEVC at low-frequencies in the composites with (Ni, Zn) Y showed hysteresis with its value under self-bias 90% or more of the value for the optimum bias field. In the case of composites with W-type ferrites, the MEVC under zero-external bias was 60%–80% of its value for the optimum bias field. Both types of composites when subjected to an ac magnetic field at the EMR frequency showed an order of magnitude enhancement in the MEVC compared to low-frequencies and the peak value at EMR for zero-bias was 90% of its value under the optimum bias. A model has been developed for the large ME response under the self-bias provided by the remanent magnetization and estimated values of MEVC are in good agreement with the data. The hexaferrite-ferroelectric composites showing ME response without the need for an external magnetic bias are of importance for use as sensors and sensor arrays of magnetic fields.

Heavily Fe-doped ferromagnetic semiconductor (In,Fe)Sb with high Curie temperature and large magnetic anisotropy

We present high-temperature ferromagnetism in heavily Fe-doped ferromagnetic semiconductor (FMS) (In1−x,Fex)Sb (x = 20%–35%) thin films grown by low temperature MBE. The Curie temperature (TC) of (In1−x,Fex)Sb reaches 385 K at x = 35%, which is significantly higher than room temperature and the highest value so far reported in III–V based FMSs. Moreover, large coercive force (HC = 160 Oe) and large remanent magnetization (Mr/MS = 71%) have been observed at low temperature (10 K) for an (In1−x,Fex)Sb thin film with x = 35%, indicating that the FMS (In1−x,Fex)Sb is very promising for spintronics devices.

BaFe12O19 films prepared on Al2O3 (0 0 0 1) by direct current magnetron sputtering

M-type hexaferrite BaFe12O19 films that exhibit a uniaxial magnetic-crystalline anisotropy hold great application promise in the field of magnetic data-storage and microwave devices. In this letter, BaFe12O19 thin films that show an out-of-plane magnetic anisotropy were prepared on c-plane Al2O3 substrates by direct current magnetron sputtering, which is high-efficiency but seldom adopted in the fabrication of ceramic materials including most hexaferrites. In addition, the textured BaFe12O19 film with hexagonal c-axis lying in the film surface was also discussed in this work. The epitaxial-like growth of the BaFe12O19 films was characterized by the combination of x-ray diffractions and pole figures. Self-biasing properties of the films induced by the epitaxial-like structure were reflected in the magnetic measurement of hysteresis loops. The film sample deposited at high temperature of 600 °C exhibited a relatively large remanent magnetization (65% of the saturation magnetization) normal to the film surface, and a small one of <10% along in-plane directions, showing their potential for application in planar magnetic devices with light weight.

Ferromagnetic insulator Cr2Ge2Te6 thin films with perpendicular remanence

Ferromagnetic van der Waals (vdW) insulators possess robust magnetic order even in a few layers of two-dimensional sheets. The heterostructures of such vdW materials prepared by molecular-beam epitaxy (MBE) are one of the ideal platforms for exploration of novel electronic/spintronic functionalities based on their ferromagnetism via an abrupt hetero-interface. Here we report successful MBE-growth of a vdW magnetic insulator Cr2Ge2Te6 thin film on a topological insulator (Bi,Sb)2Te3. Metal to insulator transition is observed in Cr–Ge–Te alloy films with increasing Ge content as tuned by the Ge and Cr flux ratio, corresponding to the structural phase change from Cr2Te3 to Cr2Ge2Te6. In the nearly stoichiometric Cr2Ge2Te6 films, a large remanent magnetization with perpendicular magnetic anisotropy appears in contrast to the bulk crystals with no discernible hysteresis. The perpendicular remanence with high Curie temperature of about 80 K remains in the thinnest 6-nm film prepared in this study. Designing of magnetic vdW heterostructures based on the Cr2Ge2Te6 thin films offers great opportunities for exploring unusual physical phenomena via proximity effect at the vdW hetero-interface.

Three-Dimensional Self-Assembly of Core/Shell-Like Nanostructures for High-Performance Nanocomposite Permanent Magnets.

Core/shell nanostructures are fascinating for many advanced applications including strong permanent magnets, magnetic recording, and biotechnology. They are generally achieved via chemical approaches, but these techniques limit them to nanoparticles. Here, we describe a three-dimensional (3D) self-assembly of core/shell-like nanocomposite magnets, with hard-magnetic Nd2Fe14B core of ∼45 nm and soft-magnetic α-Fe shell of ∼13 nm, through a physical route. The resulting Nd2Fe14B/α-Fe core/shell-like nanostructure allows both large remanent magnetization and high coercivity, leading to a record-high energy product of 25 MGOe which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. Our approach is based on a sequential growth of the core and shell nanocrystals in an alloy melt. These results make an important step toward fabricating core/shell-like nanostructure in 3D materials.

Effect of Ba and Mn doping on microstructure and multiferroic properties of BiFeO3 ceramics

Ba, Mn single and co-doped BiFeO3 ceramics were prepared by combination of solid-state reaction and rapid sintering method. Enhanced ferroelectric and ferromagnetic properties were observed in Ba, Mn co-doped ceramics. Large remnant polarization (Pr∼40.87 μC/cm2) for Bi0.9Ba0.1FeO3 and large remanent magnetization (Mr∼0.834 emμ/g) and coercive field (Hc∼938 Oe) for Bi0.95Ba0.05Fe0.9Mn0.1O3 were acquired, respectively. The analysis of structure and valence measured by X-ray diffraction (XRD), Raman spectrum and X-ray photoelectron spectroscopy (XPS) show that an appropriate doping ratio of Ba and Mn ions in BiFeO3 (BFO) plays an important role in facilitating microstructure distortion and transition, which are favorable for enhancing ferroelectric properties of BFO ceramics. Furthermore, the existence of Fe2+ and Mn4+ ions in BaMn co-doped BFO contributes to the ferromagnetic property. These results demonstrate that ions co-doping is an effective way to modulate the room-temperature ferroelectric and ferromagnetic properties for multiferroic applicability of BFO.

Substitution-driven structural, optical and magnetic transformation of Mn, Zn doped BiFeO3

Mn, Zn doped and pure BiFeO3 were prepared by sol–gel method. The 5% and 10% Zn-doped phase crystallites have a rhombohedral symmetry (R3c) structure. Doped with 5% Mn, BiFeO3 contains both rhombohedral (R3c) and tetragonal phase (P4mm) while 10% Mn gives a rise to tetragonal phase (P4/mbm). Raman and Fourier Transformed Infrared spectra also confirmed the structure transformation. Compared with pure BiFeO3, Mn-Zn doped samples present intense blue shift as shown in UV-Visible diffuse absorption together with the decreased direct optical band gap. The substitution of Mn ions for Fe enhances ferromagnetic properties while the Zn-doped ones show linear paramagnetism. Owing to the interactions between the ferromagnetic and antiferromagnetic microdomains, Mn and Zn co-doping results in a large remanent magnetization (Mr) of 0.045 emu/g.

Large electric tunable remanent magnetization and in-plane anisotropy field in (110) PMN-0.32PT/Cu/Fe65Co35 multiferroic heterostructures

Large electric field tunable remanent magnetization and in-plane anisotropy in a Fe65Co35 film was observed in sputtered ferromagnetic/ferroelectric heterostructures. Two stable magnetization directions were established through a 90° transition of the in-plane uniaxial anisotropy of FeCo film under a positive electric field, and large nonvolatile remanence ratio tunability between 0.50 and 0.12 was realized using electric field pulses. With the increase of negative electric field, the anisotropy field increases at a rate of around 31.1 Oe/(kV cm−1). The large tunable anisotropy field and remnant magnetization may have potential applications in electrically tunable microwave magnetic devices and information storage memories.

Self-biased cobalt ferrite nanocomposites for microwave applications

Oriented CoFe2O4 nanoparticles, dispersed in polymethyl methacrylate (PMMA) matrix, were fabricated by magnetophoretic deposition of functionalized nanocolloidal cobalt ferrite particles into porous alumina membrane. Their magnetic behavior exhibits an out-of-plane easy axis with a large remanent magnetization and coercitivity. This orientation allows high effective internal magnetic anisotropy that contributes to the permanent bias along the wire axis. The microwave studies reveal a ferromagnetic resonance at 46.5 and 49.5GHz, depending on the filling ratio of the membrane. Ansoft High Frequency Structure Simulator (Ansoft HFSS) simulations are in good agreement with experimental results. Such nanocomposite is presented as one of the promising candidates for microwave devices (circulators, isolators, noise suppressors etc.).

Low-temperature synthesis, structural and magnetic properties of self-dopant LaMnO3+δ nanoparticles from a metal-organic polymeric precursor

Self-dopant LaMnO3+δ nanoparticles have been successfully synthesized by metal citrate complex method based on Pechini-type reaction route, at low temperature (773K). Powder X-ray diffraction and transmission electron microscope revealed pure and nanostructured phase of LaMnO3+δ (δ=0.125) with an average grain size of ∼72nm (773K) and ∼80nm (1173K). DC-magnetization measurements under an applied magnetic field of H=±60kOe showed an increase in the magnetization with the increase of calcination temperature. Ferromagnetic nature shown by non-stoichiometric LaMnO3+δ was verified by well-defined hysteresis loop with large remanent magnetization (Mr) and coercive field (Hc). Surface areas of LaMnO3+δ nanoparticles were found to be 157.4 and 153 m2g−1 for the samples annealed at 773K and 1173K, respectively.

Spin-glass magnetism of surface rich Au cluster film

The magnetic response of two-dimensional uncapped Au islands tens of nanometers in size was measured using a superconducting quantum interference device magnetometer. The magnetization vs. field was found to exhibit a clear ferromagnetic-like hysteresis loop and a large positive remanent magnetization was acquired after momentary application of a strong magnetic field. The Au magnetic response is assumed to be due to the surface effects in the nanosized islands and the large fraction of canted surface magnetic moments which interaction and mutual orientation differ from the bulk diamagnetic order.

Large magnetic coercive field in Bi0.9La0.1Fe0.98Nb0.02O3 polycrystalline compound

Single phase Bi0.9La0.1FeO3 (BLFO) and Bi0.9La0.1Fe0.98Nb0.02O3 (BLFNO) polycrystalline ceramics were prepared by conventional solid state reaction followed immediately by the quenching process. An obvious structure transition from the rhombohedral-type to the monoclinic-type with the doping of Nb was observed. The average grain size decreased from 7 µm for BLFO to 500 nm for BLFNO. The magnetic ordering temperature of BLFNO decreased in comparison with the sample free of Nb. BLFNO shows a large coercive field, 1.35 T, and a large remanent magnetization, 0.23 A m2 kg−1, at 300 K. The possible reasons for the remarkable magnetic performance of BLFNO are discussed.

Effects of Chemically Induced Contraction of a Coordination Polyhedron on the Dynamical Magnetism of Bis(phthalocyaninato)disprosium, a Single-4f-Ionic Single-Molecule Magnet with a Kramers Ground State

Chemically induced longitudinal contraction of the square-antiprism coordination polyhedron of a peripherically substituted bis(phthalocyaninato)dysprosiumate(III), a dysprosium-based single-4f-ionic single-molecule magnet having a J z = +/- (13)/ 2 Kramers doublet ground state, resulted in drastic changes in dynamical magnetism including a doubling of the energy barrier, a 2-order-of-magnitude decrease of the spin reversal rate, a significant rise of the blocking temperature, and the first observation of the emergence of a large remanent magnetization.