Showing papers on "Coercivity published in 2022"

Ultrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding

Abstract: Description Magnetic effects of lanthanide bonding Lanthanide coordination compounds have attracted attention for their persistent magnetic properties near liquid nitrogen temperature, well above alternative molecular magnets. Gould et al. report that introducing metal-metal bonding can enhance coercivity. Reduction of iodide-bridged terbium or dysprosium dimers resulted in a single electron bond between the metals, which enforced alignment of the other valence electrons. The resultant coercive fields exceeded 14 tesla below 50 and 60 kelvin for the terbium and dysprosium compounds, respectively. —JSY A single electron bond between lanthanide centers induces alignment effects that impart extremely high magnetic coercivity. Metal-metal bonding interactions can engender outstanding magnetic properties in bulk materials and molecules, and examples abound for the transition metals. Extending this paradigm to the lanthanides, herein we report mixed-valence dilanthanide complexes (CpiPr5)2Ln2I3 (Ln is Gd, Tb, or Dy; CpiPr5, pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5dz2 parentage, as determined by structural, spectroscopic, and computational analyses. Valence delocalization, wherein the d electron is equally shared by the two lanthanide centers, imparts strong parallel alignment of the σ-bonding and f electrons on both lanthanides according to Hund’s rules. The combination of a well-isolated high-spin ground state and large magnetic anisotropy in (CpiPr5)2Dy2I3 gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin.

A mechanically strong and ductile soft magnet with extremely low coercivity

Abstract: Abstract Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss 1 . The electrification of transport, households and manufacturing leads to an increase in energy consumption owing to hysteresis losses 2 . Therefore, minimizing coercivity, which scales these losses, is crucial 3 . Yet meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility 4 . This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteresis losses 5 . Here we introduce an approach to overcome this dilemma. We have designed a Fe–Co–Ni–Ta–Al multicomponent alloy (MCA) with ferromagnetic matrix and paramagnetic coherent nanoparticles (about 91 nm in size and around 55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency stress and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1,336 MPa at 54% tensile elongation, extremely low coercivity of 78 A m −1 (less than 1 Oe), moderate saturation magnetization of 100 A m 2 kg −1 and high electrical resistivity of 103 μΩ cm.

Development of non-rare earth grain boundary modification techniques for Nd-Fe-B permanent magnets

Abstract: The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure. Intergranular addition and grain boundary diffusion (GBD) process are effective approaches for enhancing coercivity with low material cost. This review summarizes the development of grain boundary modification techniques with emphasis on our recent work using cost-effective non-rare earth (non-RE) sources for GBD. Up to now, heavy rare earth (HRE) based compounds, metals and light rare earth (LRE) based alloys have been successfully employed as the diffusion sources for coercivity enhancement. Inspired from the previous investigations on the intergranular addition of non-RE compounds and alloys for Nd-Fe-B magnets, in 2015, we firstly proposed a novel GBD process based on diffusion source of MgO. After that, various non-RE diffusion sources have been developed. The fundamentals of non-RE additives and non-RE diffusion sources for hard magnetic properties enhancement of Nd-Fe-B magnets are summarized here based on both the experimental and computational results. In particular, the properties-microstructure relationships of non-RE GBD modified magnets are discussed. The non-RE alloys or compounds modify the composition and structure of the grain boundary by diffusing into the intergranular regions, resulting in enhanced coercivity and corrosion resistance. Recently, we used Al-Cr coatings for both coercivity enhancement and surface protection, which shortens the production process and makes non-RE diffusion sources more competitive. The opportunity and future directions for non-RE GBD are also discussed in this review.

Photocatalytic Degradation Properties of Li‐Cr Ions Substituted CoFe2O4 Nanoparticles for Wastewater Treatment Application

Abstract: Herein, the dual substitution of Li/Cr in CoFe2O4 ferrite nanoparticles synthesized through the sol‐gel auto combustion method is reported. X‐ray diffraction confirms mixed cubic spinel structure, while as dense morphology with well‐defined grain boundaries with the minimum grain size of 20 nm is obtained using FE‐TEM analysis. The thermal stability of the sample improves with an increase in dopant concentration. XPS study shows that Fe and Co exhibit +2 and +3 oxidation states with lower concentrations of oxygen vacancies. Raman spectroscopy confirms the substitution of Li/Cr ions at octahedral and tetrahedral sites, and the direct bandgap increases monotonously with doping. Saturation magnetization increases for x = 0.01, but it decreases for higher dopant concentration due to the spin canting effect, and the coercivity decreases for all the doping concentrations. The photodegradation studies show the enhanced activity against crystal violet (CV) dye when exposed to sunlight for 60 min but easily recoverable using a magnet. The increased activity by the doped nanoparticles is because of the decreased coercivity, particle size, and magnetic anisotropy. A 90.4% photocatalytic degradation is obtained for the composition, x = 0.03, which suggests that it can be an effective candidate for wastewater treatment.

Simultaneously achieving giant piezoelectricity and record coercive field enhancement in relaxor-based ferroelectric crystals

Abstract: A large coercive field (EC) and ultrahigh piezoelectricity are essential for ferroelectrics used in high-drive electromechanical applications. The discovery of relaxor-PbTiO3 crystals is a recent breakthrough; they currently afford the highest piezoelectricity, but usually with a low EC. Such performance deterioration occurs because high piezoelectricity is interlinked with an easy polarization rotation, subsequently favoring a dipole switch under small fields. Therefore, the search for ferroelectrics with both a large EC and ultrahigh piezoelectricity has become an imminent challenge. Herein, ternary Pb(Sc1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals are reported, wherein the dispersed local heterogeneity comprises abundant tetragonal phases, affording a EC of 8.2 kV/cm (greater than that of Pb(Mg1/3Nb2/3)O3-PbTiO3 by a factor of three) and ultrahigh piezoelectricity (d33 = 2630 pC/N; d15 = 490 pC/N). The observed EC enhancement is the largest reported for ultrahigh-piezoelectric materials, providing a simple, practical, and universal route for improving functionalities in ferroelectrics with an atomic-level understanding.

Lanthanum substitution effect on the structural, optical, and dielectrical properties of nanocrystalline BaFe2O4 ferrites

Abstract: The effects of varying the amount of lanthanum infusion on the barium ferrite characteristics were investigated in this study. An orthorhombic structure formed as a majority phase and traces of barium lanthanum iron oxide as segregated phases in the ferrites composites. The optical bandgap energy values of the BaLaxFe2−xO4 nanoferrites were 3.50–3.72 eV. SEM confirmed that the prepared nanoferrites had spherical morphology with size of 66.50–81.50 nm. The dielectric parameters of BaLaxFe2−xO4 nanoferrites were studied in the specified spectrum, which revealed the La doped barium nanoferrites had a low dielectric constant and conductivity properties compared to the barium nanoferrites. The saturation magnetization (60.03–25.98 emu g−1) and coercivity (454.85–80.65 Oe) of the prepared nanoferrites were improved due to an effect of La3+ ions. Accordingly, the observed optical bandgap energy, dielectric constant, and magnetism could be employed in high-frequency magneto-optoelectronic systems.

Impact of Sm3+ and Er3+ Cations on the Structural, Optical, and Magnetic Traits of Spinel Cobalt Ferrite Nanoparticles: Comparison Investigation

Abstract: In this study, we investigated a comparison of the structure, morphology, optic, and magnetic (room temperature (RT)) features of Er3+ and Sm3+ codoped CoFe2O4 (CoErSm) nanospinel ferrite (NSFs) (x ≤ 0.05) synthesized via hydrothermal (H-CoErSm NSFs) and sonochemical (S-CoErSm NSFs) approaches. The formation of all products via both synthesis methods has been validated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), along with energy-dispersive X-ray (EDX) and transmission electron microscopy (TEM) techniques. The single phase of the spinel structure (except for the Hyd sample with x = 0.03) was evidenced by XRD analysis. The DXRD (crystallite size) values of H-CoErSm and S-CoErSm NSFs are in the 10–14.7 and 10–16 nm ranges, respectively. TEM analysis presented the cubic morphology of all products. A UV–visible percent diffuse reflectance (DR %) study was performed on all products, and Eg (direct optical energy band gap) values varying in the 1.32–1.48 eV range were projected from the Tauc plots. The data of RT magnetization demonstrated that all prepared samples are ferromagnetic in nature. M–H data revealed that rising the contents of cosubstituent elements (Sm3+ and Er3+ ions) caused an increase in Ms (saturation magnetization) and Hc (coercive field) in comparison to pristine samples. Although concentration dependence is significant (x > 0.02), no strict regularity (roughly fluctuating) has been ruled out in Ms values for doped samples prepared via the hydrothermal method. However, sonochemically prepared samples demonstrated that Ms values increase with increasing x up to x = 0.04 and then decrease with the further rise in cosubstituent Sm3+ and Er3+ ions. The calculated values of Ms and Hc were found to be greater in H-CoErSm NSFs compared to those in S-CoErSm NSFs. The present investigation established that the distribution of cations and the variation in crystallite/particle sizes are efficient to control the intrinsic properties of all samples.

Tailoring the coercive field in ferroelectric metal-free perovskites by hydrogen bonding

Abstract: The miniaturization of ferroelectric devices in non-volatile memories requires the device to maintain stable switching behavior as the thickness scales down to nanometer scale, which requires the coercive field to be sufficiently large. Recently discovered metal-free perovskites exhibit advantages such as structural tunability and solution-processability, but they are disadvantaged by a lower coercive field compared to inorganic perovskites. Herein, we demonstrate that the coercive field (110 kV/cm) in metal-free ferroelectric perovskite MDABCO-NH4-(PF6)3 (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) is one order larger than MDABCO-NH4-I3 (12 kV/cm) owing to the stronger intermolecular hydrogen bonding in the former. Using isotope experiments, the ferroelectric-to-paraelectric phase transition temperature and coercive field are verified to be strongly influenced by hydrogen bonds. Our work highlights that the coercive field of organic ferroelectrics can be tailored by tuning the strength of hydrogen bonding.

Low Operating Voltage, Improved Breakdown Tolerance, and High Endurance in Hf0.5Zr0.5O2 Ferroelectric Capacitors Achieved by Thickness Scaling Down to 4 nm for Embedded Ferroelectric Memory.

Abstract: The comparatively high coercive field in Hf0.5Zr0.5O2 (HZO) and other HfO2-based ferroelectric thin films leads to two critical challenges for their application in embedded ferroelectric memory: high operating voltage due to a large thickness-field product and poor endurance due to the high operating field close to the breakdown field. In this study, we demonstrate that the thickness scaling of ferroelectric HZO down to 4 nm is a promising approach to overcome these challenges. As the coercive voltage scales down almost linearly with the film thickness, the operating voltage of 4 nm HZO is reduced to 0.6 V for one-shot operation and 1.2 V for stable memory operation, which is in the voltage range compatible with scaled silicon technologies. Furthermore, it is found that the breakdown field is substantially improved in thinner HZO since the breakdown mechanism is dominated by the stress voltage, not the stress field, resulting in improved cycle-to-breakdown by more than 4 orders of magnitude when thinning from 9.5 to 4 nm. We identify two concerns accompanying thickness scaling: the increase in crystallization temperature and the pinched hysteresis behavior, which can be addressed by carefully preparing temperature-thickness mapping and applying strong-field wake-up cycling, respectively. Our optimal 4 nm-thick HZO ferroelectric capacitor exhibits an operating voltage of 1.2 V with over 10 year data retention and 1012 endurance cycles at 100 kHz, which can be further improved to more than 1014 with a smaller capacitor size and higher operating frequency.

A neutron diffraction investigation of high valent doped barium ferrite with wideband tunable microwave absorption

Abstract: Abstract The barium ferrite BaTi x Fe 12− x O 19 ( x = 0.2, 0.4, 0.6, 0.8) (BFTO- x ) ceramics doped by Ti 4+ were synthesized by a modified sol—gel method. The crystal structure and magnetic structure of the samples were determined by neutron diffraction, and confirm that the BFTO- x ceramics were high quality single phase with sheet microstructure. With x increasing from 0.2 to 0.8, the saturation magnetization ( M s ) decreases gradually but the change trend of coercivity ( H c ) is complex under the synergy of the changed grain size and the magnetic crystal anisotropy field. Relying on the high valence of Ti 4+ , double resonance peaks are obtained in the curves of the imaginary part of magnetic conductivity ( μ ″) and the resonance peaks could move toward the low frequency with the increase of x , which facilitate the samples perform an excellent wideband modulation microwave absorption property. In the x = 0.2 sample, the maximum reflection loss (RL) can reach −44.9 dB at the thickness of only 1.8 mm, and the bandwidth could reach 5.28 GHz at 2 mm when RL is less than −10 dB. All the BFTO- x ceramics show excellent frequency modulation ability varying from 18 ( x = 0.8) to 4 GHz ( x = 0.4), which covers 81% of the investigated frequency in microwave absorption field. This work not only implements the tunable of electromagnetic parameters but also broadens the application of high-performance microwave absorption devices.

Tailoring the coercive field in ferroelectric metal-free perovskites by hydrogen bonding

Abstract: The miniaturization of ferroelectric devices in non-volatile memories requires the device to maintain stable switching behavior as the thickness scales down to nanometer scale, which requires the coercive field to be sufficiently large. Recently discovered metal-free perovskites exhibit advantages such as structural tunability and solution-processability, but they are disadvantaged by a lower coercive field compared to inorganic perovskites. Herein, we demonstrate that the coercive field (110 kV/cm) in metal-free ferroelectric perovskite MDABCO-NH4-(PF6)3 (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) is one order larger than MDABCO-NH4-I3 (12 kV/cm) owing to the stronger intermolecular hydrogen bonding in the former. Using isotope experiments, the ferroelectric-to-paraelectric phase transition temperature and coercive field are verified to be strongly influenced by hydrogen bonds. Our work highlights that the coercive field of organic ferroelectrics can be tailored by tuning the strength of hydrogen bonding.

Impact of Nd3+ Substitutions on the Structure and Magnetic Properties of Nanostructured SrFe12O19 Hexaferrite

Abstract: In this study, SrFe12-xNdxO19, where x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, was prepared using high-energy ball milling. The prepared samples were characterized by X-ray diffraction (XRD). Using the XRD results, a comparative analysis of crystallite sizes of the prepared powders was carried out by different methods (models) such as the Scherrer, Williamson–Hall (W–H), Halder–Wagner (H–W), and size-strain plot (SSP) method. All the studied methods prove that the average nanocrystallite size of the prepared samples increases by increasing the Nd concentration. The H–W and SSP methods are more accurate than the Scherer or W–H methods, suggesting that these methods are more suitable for analyzing the XRD spectra obtained in this study. The specific saturation magnetization (σs), the effective anisotropy constant (Keff), the field of magnetocrystalline anisotropy (Ha), and the field of shape anisotropy (Hd) for SrFe12-xNdxO19 (0 ≤ x ≤ 0.5) powders were calculated. The coercivity (Hc) increases (about 9% at x = 0.4) with an increasing degree of substitution of Fe3+ by Nd3+, which is one of the main parameters for manufacturing permanent magnets.

Enabling ultra-low-voltage switching in BaTiO3

Abstract: Single crystals of BaTiO3 exhibit small switching fields and energies, but thin-film performance is considerably worse, thus precluding their use in next-generation devices. Here, we demonstrate high-quality BaTiO3 thin films with nearly bulk-like properties. Thickness scaling provides access to the coercive voltages (<100 mV) and fields (<10 kV cm-1) required for future applications and results in a switching energy of <2 J cm-3 (corresponding to <2 aJ per bit in a 10 × 10 × 10 nm3 device). While reduction in film thickness reduces coercive voltage, it does so at the expense of remanent polarization. Depolarization fields impact polar state stability in thicker films but fortunately suppress the coercive field, thus driving a deviation from Janovec-Kay-Dunn scaling and enabling a constant coercive field for films <150 nm in thickness. Switching studies reveal fast speeds (switching times of ~2 ns for 25-nm-thick films with 5-µm-diameter capacitors) and a pathway to subnanosecond switching. Finally, integration of BaTiO3 thin films onto silicon substrates is shown. We also discuss what remains to be demonstrated to enable the use of these materials for next-generation devices.