Intrinsic Coercive Field Research Articles

A stable rhombohedral phase in ferroelectric Hf(Zr)1+xO2 capacitor with ultralow coercive field.

Hafnium oxide-based ferroelectric materials are promising candidates for next-generation nanoscale devices because of their ability to integrate into silicon electronics. However, the intrinsic high coercive field of the fluorite-structure oxide ferroelectric devices leads to incompatible operating voltage and limited endurance performance. We discovered a complementary metal-oxide semiconductor (CMOS)-compatible rhombohedral ferroelectric Hf(Zr)1+xO2 material rich in hafnium-zirconium [Hf(Zr)]. X-ray diffraction combined with scanning transmission electron microscopy reveals that the excess Hf(Zr) atoms intercalate within the hollow sites. We found that the intercalated atoms expand the lattice and increase the in-plane and out-of-plane stresses, which stabilize both the rhombohedral phase (r-phase) and its ferroelectric properties. Our ferroelectric devices, which are based on the r-phase Hf(Zr)1+xO2, exhibit an ultralow coercive field (~0.65 megavolts per centimeter). Moreover, we achieved a high endurance of more than 1012 cycles at saturation polarization. This material discovery may help to realize low-cost and long-life memory chips.

Magneto-mechanical coupling effect of ferromagnetic materials: Test characteristics and theoretical model

The magneto-mechanical coupling effect of ferromagnetic materials is the result of macroscopic influence of mechanical properties on magnetic properties under the combined action of stress field and applied magnetic field, which essentially can be attributed to the change of material properties caused by the transformation of microscopic magnetic domain structures. The effect of applied magnetic field is mainly manifested as the change of magnetic moment cluster (direction and size) inside microscopic magnetic domain; The stress field mainly causes the movement and rotation of the domain walls inside the ferromagnetic material, and further causes the change of magnetic moment cluster. Taking Q235 low carbon steel as an example, the effects of tensile stress σt on some important magnetic parameters of ferromagnetic materials under different excitation intensities (i.e., H0), such as the maximum value of applied magnetic field Hmax, intrinsic coercive field Hcj, hysteresis loss power per unit volume ph and anhysteretic magnetization Man, were obtained by setting macroscopic magneto-mechanical coupling test and fully considering the influence of demagnetizing field Hd (The standard ring specimen was used for demagnetization correction of the non-standard strip specimen). Meanwhile, the basic magnetization characteristics of ferromagnetic materials were discussed by introducing the basic magnetization curve F(H0, Ban) and considering the effect of σt as the equivalent stress field Hσ based on the theory of microscopic magnetic domain and the principle of thermodynamic. In the range of 635.4 A/m ≤ H0 ≤ 928.6 A/m and 0 MPa ≤ σt ≤ 350 MPa, the theoretical calculation models Man(H0, σt) and Hσ(H0, σt) in the magneto-mechanical coupling effect were obtained, which are in good agreement with the experimental results.

Property Regulation Principle in Mn‐Doped BF–BT Ceramics: Competitive Control of Domain Switching By Defect Dipoles and Domain Configuration

AbstractMn‐doping is reported to be an effective strategy to obtain better electrical property in bismuth ferrite‐barium titanate (BF‐BT)‐based ceramics, while the property regulation principle and its potential physical mechanism is still poorly understood. For disclose the veil of Mn‐doping enhancing properties, the typical material system BF‐BT–xMnO2mol% is designed and the effects of Mn‐doping on multilevel structures and external fields stimulated electrical properties are deeply investigated. Mn‐doping induced structure disorder and defect dipoles lead to the formation of nanodomain and the variation of local structure, contributing to intrinsic enhancement of polarization. Especially, electrical properties under different electric and temperature fields reveal that there is a competitive control of domain switching by defects and domain configuration, which is also verified by domain writing technology and switching spectroscopy piezore‐sponse force microscopy. That is, the defect dipoles have pinning effect on domain to hinder domain switching, leading to smaller polarization and indistinctive electrostrain at low fields. While the nanodomain reduces intrinsic coercive field and promotes domain switching, generating much larger polarization and electrostrain at high fields. lt is believed the decoding of domain switching behavior controlled by defect and domain can provide a paradigm to understand the property evolution in chemically modified BF‐BT.

Unusual Properties of Hydrogen-Bonded Ferroelectrics: The Case of Cobalt Formate.

Hybrid organic-inorganic perovskites is a class of materials with diverse chemically tunable properties and outstanding potential for multifunctionality. We use first-principles simulations to predict room temperature ferroelectricity in a representative of the formate family, [NH_{2}NH_{3}][Co(HCOO)_{3}]. The ferroelectricity arises as a "by-product" of structural transition driven by the stabilization of the hydrogen bond. As a consequence the coupling with the electric field is relatively weak giving origin to large intrinsic coercive fields and making material immune to the depolarizing fields known for its detrimental role in nanoscale ferroelectrics. Insensitivity to the electric field and the intrinsic dynamics of the order-disorder transition in such material leads to the supercoercivity defined as significant increase in the coercive field with frequency. Room temperature polarization measurements provide further support for the predictions.

A new insight on evolution of texture and cellular structure of P/M processed Sm2Co17 permanent magnets and their effect on magnetic properties

Sm2Co17 type permanent magnets were prepared by a novel processing technique involving pulsed magnetization followed by cold isostatic pressing (CIP). The magnetic properties and microstructure of the CIPed magnetswere compared with those of the magnets processed through conventional uniaxial compaction in a transverse field press (TP)and heat treated under identical conditions. Enhanced values of remanence (Br ~ 10.7 kG) and energy product [(BH)max ~ 29 MGOe] were obtained in the CIPed magnetsas compared to those of the TP magnets[Br ~ 10 kG and (BH)max ~ 25 MGOe]. However, the values of intrinsic coercivity (iHc) and knee field (Hk) of CIPed magnet were found to be lower than those of the TP magnet.The results were explained on the basis of the difference in texture, microtexture and microstructure obtained in these magnets due to different processing methods adopted.

Identification of optimal solid solution temperature for Sm2Co17-type permanent magnets with different Fe contents

It is confirmed that the solid solution temperature range to obtain optimal magnetic properties is different for the magnets with different Fe contents, and the correlation between magnetic properties and microstructures influenced by solid solution temperature (Ts) has been systematically studied. The optimal solid solution temperature range is 1413–1463 K for the Sm(CobalFe0.213 Cu0.073Zr0.024)7.6 magnet, which is higher than that of the Sm(CobalFe0.262Cu0.073Zr0.024)7.6 magnet (1403–1453 K), and the optimal Ts range is about 50 K for both of the magnets. The solid solution temperature range shifting toward relatively high temperature is due to the increase in a phase transition temperature. The magnet solution-treated at proper temperature exhibits 1:7H single phase, and intact cell structure and high Cu concentration (23.12 at%) in the cell boundary are found after aging process, which makes the magnet shows high intrinsic coercivity (Hcj) and magnetic field at knee-point (Hknee). At a lower solid solution temperature, the 2:17H, 1:5H and Zr-rich precipitation phases appear, which affects the cell structure, density of lamellar phase and Cu concentration in the cell boundary, leading to the reduced magnetic properties. However, at a higher solid solution temperature, there exist obviously light gray and dark regions with different Sm, Cu and Fe contents in scanning electron microscopy observation, and the magnet shows low pinning field in the two regions and incomplete cell structure, resulting in an inferior Hcj and Hknee.

Dependence of macromagnetic properties on the microstructure in high-performance Sm2Co17-type permanent magnets

Microstructure characteristics are important for the high-performance Sm2Co17-type permanent magnets to achieve high intrinsic coercivity (Hcj) and large magnetic field at the knee-point (Hknee). In this work, the correlation between microstructures and magnetic properties has been systematically studied. The different microstructures were obtained by two procedures: by adjusting the aging temperature and optimizing the Zr content. On one hand, with increase of aging temperature from 1063 K to 1103 K, the cell size and the lamellar phase density increase from 75.1 nm and 0.014 1/nm to 101.5 nm and 0.043 1/nm, respectively, and the average peak Cu concentration at the cell boundaries reaches 21.9 at% which is helpful for obtaining relatively high Hcj and Hknee. As the aging temperature increases further to 1143 K, a much larger cell size and with incomplete cell boundaries results in an irregular distribution of Cu concentration at the cell boundaries. In this case, the Hknee is low even though the magnet shows a high Hcj. On the other hand, the increase of Zr content also promotes an increase in both the lamellar phase density and the average peak Cu concentration at the cell boundaries, giving rise to high Hcj and Hknee. Taking into consideration the aging temperature and Zr content, one can conclude that a lower aging temperature and lower Zr content produce a lower lamellar phase density and average Cu concentration at the cell boundaries and this leads to a lower Hcj and Hknee of the magnets. To further confirm this correlation between the microstructures and magnetic properties, a magnet of Sm(CobalCu0.062Fe0.285Zr0.016)7.6 was aged at a higher temperature of 1143 K to increase the lamellar phase density. As expected, the lamellar phase density increases from 0.008 to 0.013 1/nm; however, the cell size increases simultaneously from 142.5 to 247 nm. The magnet exhibits a much higher but uneven Cu concentration at the cell boundaries. As a result, the Hcj increases from 1.64 to 10.18 kOe, while Hknee just increases from 1.09 to 3.91 kOe.

Origin of the Intrinsic Coercivity Field Variations and Magnetic Study of ε-Fe2O3

Origin of the intrinsic coercivity field variations observed by hysteretic measurements established by Nikolic et al. was investigated in detail by ZFC/FC measurements. In this study, magnetic behavior of the samples is further examined by calculation of the ZFC $${{M}_{n}}(T)$$ and $${{M}_{n}}(H)$$ first derivatives. Performed study showed that the main contribution to the measured coercivity comes from the e-Fe2O3 particles, characterized by $$Hci = 24~\;{\text{kOe}}$$ , although both samples posses $${\text{SPM}}$$ e-Fe2O3 fraction. As a result, Hci values of the samples are differed dependent on the quantity of the SPM e-Fe2O3 particles in the sample. Mechanism of the intrisic coercivity field variations is discussed.

Origin of the Intrinsic Coercivity Field Variations of ε-Fe2O3

e-Fe2O3 phase is recognized as an attractive material, in both technological and scientific point of view, since it can achieve very high room-temperature coercivity (10–20 kOe). In this paper, multi-phase samples Fe2O3/SiO2 with slightly different $${\text{Fe/Si}}~$$ molar ratio were produced by sol–gel synthesis route. The obtained samples were characterized by various experimental techniques including XRD, TA, FTIR, and $${\text{SQUID}}$$ (DC and AC magnetic measurements). It was found that both samples consisted of α-Fe2O3 and e‑Fe2O3 phases embedded in the silica matrix, and showed very similar structural and magnetic properties, except that displayed significantly different room-temperature intrinsic coercivity field values: HciS1 = 14.3 kOe and HciS2 = 7.5 kOe. We have discussed possible origin of thus high intrinsic coercivity field variation.

Correlation of Microstructure with Hard Magnetic Properties of Glass-Coated MnBi Microwires

ABSTRACTIn this work, we study the hard-magnetic properties and its influence on the microstructure of MnBi-based glass-coated microwires obtained by Taylor-Ulitovsky process, as well as their magnetic and thermal dependence in the range of 200 K-360 K. We obtained glass-coated microwires pieces trough of the Taylor-Ulitovsky process. Glass-coated microwires exhibited the formation of Low-Temperature Intermetallic Phase (LTIP)-MnBi, as confirmed by scanning electron microscopy (SEM). Energy Dispersive Spectroscopy (EDS) elemental chemical composition analysis showed LTIP regions interspersed within Bi- and Mn-rich areas. Magnetic properties were determined by vibrating sample magnetometry (VSM), for which a considerable intrinsic coercivity field (iHc) up to 6000 Oe at 360 K, together with a saturation magnetization (Ms) of 57.49 emu/cm3. Therefore, this combination of properties renders LTIP is a promising precursor with potential for applications at high temperatures.

Coercivity mechanism of rare-earth free MnBi hard magnetic alloys

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic ofreal materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi- and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronm¨uller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

Superlattice-like structure and enhanced ferroelectric properties of intergrowth Aurivillius oxides.

Aurivillius oxides with an intergrowth structures have been receiving increasing interest because of their special structures and potential outstanding ferroelectric properties. In this work, Bi3LaTiNbFeO12–Bi5Ti3FeO15 and Bi3TiNbO9–Bi3LaTiNbFeO12 compounds were successfully synthetised using a simple solid-state reaction method. X-Ray diffraction patterns and scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) images confirm the 2–3 and the 3–4 intergrowth structures in Bi3TiNbO9–Bi3LaTiNbFeO12 and Bi3LaTiNbFeO12–Bi5Ti3FeO15 compounds, respectively. A superlattice-like distortion in these oxides was proposed resulting from the combination of sub-lattices with different a and b parameters, which was validated by XRD refinements and Raman spectra. Polarization-electric field tests and pulsed polarization positive-up negative-down measurements demonstrate that such superlattice-like structures can effectively enhance the intrinsic ferroelectric polarization and coercive field of these oxides, especially when compared with their component oxides Bi3TiNbO9, Bi3LaTiNbFeO12 and Bi5Ti3FeO15. Simultaneously, ferroelectric Curie temperatures of Bi3TiNbO9–Bi3LaTiNbFeO12 and Bi3LaTiNbFeO12–Bi5Ti3FeO15 oxides are lowered because of the internal stress in the superlattice-like structure. Nevertheless, the paramagnetism of the samples is hardly influenced by their structure, while mainly related to their iron content, in which iron has a similar effective magnetic moment around 3.4–3.9.

The effect of poly vinyl alcohol (PVA) surfactant on phase formation and magnetic properties of hydrothermally synthesized CoFe2O4 nanoparticles

Nanoparticles of CoFe2O4 were synthesized by hydrothermal process at 190°C with and without poly vinyl alcohol (PVA) addition using treatment durations of 1.5–6h. The synthesized powders were characterized with X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), Field emission scanning electron microscope (FESEM) and vibration sample magnetometer (VSM) techniques. XRD results show presence of CoFe2O4 as the main phase and Co3O4 as the lateral phase in some samples. The results show that in the samples synthesized without PVA addition considerable amount of lateral phase is present after 3h of hydrothermal treatment while with PVA addition this phase is undetectable in the XRD patterns of the sample synthesized at the same conditions. Microstructural studies represent increasing of particle size with increasing of hydrothermal duration and formation of coarser particles with PVA addition. The highest maximum magnetization (Mmax) values in both of the samples that were synthesized with and without PVA addition are about 59emu/g that were obtained after 4.5h of hydrothermal treatment. Intrinsic coercive field (iHc) value of the sample without PVA addition increases from 210 to 430Oe. While with PVA addition the iHc value changes from 83Oe to 493Oe. The mechanism of changes in Mmax and iHc values has been explained.

Intrinsic Ferroelectric Coercive Field Calculation for BZT Films Doped by Indium and Lanthanum

The intrinsic coercive field is predicted by using Landau Ginzburg theory of ferroelectricity. A routine program created on Delphi platform is used to make simulation of the calculation of hysteresis loops and intrinsic coercive field. There are shifting in the values of intrinsic ferroelectric field of BZT on adding the zirconium composition, and the maximum intrinsic coercive field of BZT is found at 0.25 mol zirconium. The calculation shows that the best intrinsic coercive field is found at 1 mol % dopant.

The peak in anomalous magnetic viscosity

Anomalous magnetic viscosity, where the magnetization as a function of time exhibits non-monotonic behaviour, being seen to increase, reach a peak, and then decrease, is observed on recoil lines in bulk amorphous ferromagnets, for certain magnetic prehistories. A simple geometrical approach based on the motion of the state line on the Preisach plane gives a theoretical framework for interpreting non-monotonic behaviour and explains the origin of the peak. This approach gives an expression for the time taken to reach the peak as a function of the applied (or holding) field. The theory is applied to experimental data for bulk amorphous ferromagnet alloys of composition Nd60−xFe30Al10Dyx, x = 0, 1, 2, 3 and 4, and it gives a reasonable description of the observed behaviour. The role played by other key magnetic parameters, such as the intrinsic coercivity and fluctuation field, is also discussed. When the non-monotonic behaviour of the magnetization of a number of alloys is viewed in the context of the model, features of universal behaviour emerge, that are independent of alloy composition.

Molecular modeling and molecular dynamics simulation of the polarization switching phenomena in the ferroelectric polymers PVDF at the nanoscale

The molecular modeling and molecular dynamics of polarization switching for the ferroelectric films model of polyvinylidene fluoride (PVDF) are investigated at the nanoscale. We consider a molecular model of PVDF film, consisting of two and four a chains [–CH2–CF2–]n limited by n=6 elementary units. The first-principle approach is applied to the switching and kinetics of these models. Two types of behavior were established for PVDF chains: simultaneous and sequential rotation in high and low electric fields. Kinetics of sequential polarization switching shows a homogeneous critical behavior in the low electric field with a critical point at Landau–Ginzburg–Devonshire (LGD) coercive field E=EC. This type of kinetics demonstrates a kink-like behavior for polarization solitary wave propagation. The simultaneous type of kinetics demonstrates the total domain-like polarization switching, corresponding to exponential behavior of switching time in high electric field as for bulk samples. Corresponding LGD intrinsic coercive field for a two-chain and four-chains model is EC~2.0GV/m with revealing size effect. Obtained results show common quantum nature of PVDF chains switching phenomena—the quantum interaction of the PVDF molecular orbitals under applied electric field at the nanoscale level. The results obtained are compared with experimental data.

Crystallization and Magnetic Properties of Melt-Spun SmCo<sub>7-x</sub>(Cr<sub>3</sub>C<sub>2</sub>)<sub>x</sub> Alloys

The amorphous formation ability, crystallization behavior and magnetic properties of SmCo7 magnets melt-spun at 30 and 40 m/s have been studied. The ribbons melt-spun at 40 m/s exhibit amorphous structure in the range of 0.15x0.25 for SmCo7-x(Cr3C2)x alloys. In the amorphous state, these alloys are soft magnetic with intrinsic coercive field iHc of the order of 40-70 Oe. After annealing at 750 °C for 5 min, the alloy shows enhenced magnetic properties with iHc of 7.98 kOe, Mr of 55.05 emu/g, and Mr of 70.99 emu/g. The MFM image reveals a uniform distribution of individual grains with average size about 20 nm and a domain size about 100250 nm in width. This interaction domain results in the enhancement of the magnetic properties.

Magnetic and microstructural properties of (Nd,Pr)-(Tb,Dy,Gd)-(Fe,Co,Al,Cu)-B type magnets

(Nd,Pr)-(Tb,Dy,Gd)-(Fe,Co,Al,Cu)-B magnets were heat treated in two different ways and investigated in the temperature range 4.2 to 400 K. After optimal heat treatment intrinsic coercive field iHc = 1640 and 5040 kA/m for room temperature and 10 K was obtained and the temperature coefficient of remanence α = −0.01 for temperature range 80 to 375 K was found. On basis of magnetic measurements and 57Fe Mossbauer spectroscopy, the possible reasons for the increase of iHc during multi-step heat treatment are discussed.

Hard ferromagnetism in melt-spun Hf2Co11B alloys

Hard ferromagnetic behavior is reported for crystalline Hf2Co11B produced by melt-spinning. For the highest-performing material, remanent magnetization, intrinsic coercive field, and maximum energy product at room temperature are 6.2 kG, 4.5 kOe, and 6.7 MGOe, respectively. This is the highest reported energy product for this class of alloys, and is about half that of optimized Nd2Fe14B ribbons. The Curie temperature of both crystalline and amorphous Hf2Co11B is near 770 K. The results suggest further optimization of magnetic properties should be attainable, and indicate Hf2Co11B to be a promising material for rare-earth-free permanent magnets.

Effect of La Doping on Structural and Magnetic Properties of SrLa<sub>x</sub>Fe<sub>12−x</sub>O<sub>19</sub> Ceramics from Aqueous Solution Method

Influences of different amount of La-substituted on structural and magnetic properties of M-type strontium ferrites SrLaxFe(12−x)O19 compounds prepared by an aqueous solution method have been systematically investigated by XRD, VSM and TEM. In this work, it is found that only magnetoplumbite structure exists in the sample rather minorphases from the XRD patterns. When the doping content is x≤0.06mol, substituting of La3+ for Fe3+ occurs, forming SrLaxFe(12-x)O19. In contrast, when the doping content is x>0.06mol, Sr2+ is substituted by La3+, forming Sr(1-x)LaxFe12O19. The saturation magnetization (σs) shows its maximum at x=0.04 while the peak value of the intrinsic coercive field (Hjc) presented at x=0.06 as the La substituted content increases. The obtained results also indicated that the shape of the formed SrLaxFe(12−x)O19 is oblong with 70nm length and 40nm width.