Coercivity Research Articles

Exploring the impact of non-magnetic ion Mg2+ into M- type Ba0.5Ca0.5Fe12-xO19 (x = 0.0, 0.1 and 0.2) hexaferrites on structural, magnetic and magnetocaloric effect properties

The Mg substituted Ba-Ca Hexaferrites Ba0.5Ca0.5Fe12-xMgxO19 (BCFMO) (x = 0.0, 0.1 and 0.2) were prepared through the conventional ceramic technology. The microstructure, morphology, magnetic and magnetocaloric Effect (MCE) of all samples were investigated by XRD, SEM, EDX and BS1 and BS2 magnetometer. XRD analysis revealed the formation of Magnetoplumbite structure with P63/mmc space group and minor secondary phase R-3 m and having crystallites in the range of 121–108 nm. The XRD refinement indicates a preference for Mg2+ ions to occupy the octahedral 12k site. SEM analysis showed the presence of the hexagonal shape for all products. The saturation magnetization Ms decreases from 69.77 to 65.55 emu/g and the coercive field Hc increases from 1.29 to 3.54 kOe with the increase in Mg concentration. The increase of Hc with increasing Mg concentration is attributed to the reduction in crystallite sizes. Additionally, the magnetocrystalline anisotropy parameters (obtained by the "Law of Approach to Saturation method") such as the anisotropy field (Ha) and the effective magnetic anisotropy constant (Keff) were found to be 16.59–24.18 kOe and 5.15–6.94 105 emu/cm for x = 0.0–0.2, respectively.The MFC-MZFC curves for all compounds show a paramagnetic (PM) to ferromagnetic (FM) phase transition at the Curie Temperature TC. Doping with Mg2+ in BCFO lowers the Curie temperature TC down to T ∼700 K. The maximum entropy (-ΔSmmax) (x = 0.1) and the corresponding RCPmax (x = 0.2) at 5 T are found to be 1.80 J.kg-1.K-1 and 114.29 J.kg-1 respectively. The above results demontrate that the samples have great potential for permanent magnets and guide for the Magnetocaloric Effect (MCE) applications.

Giant coercive-field (Hc @ 2 K > 17 T) by freezing of magnetic domains in BaFe2(PO4)2

In the 2D-Ising BaFe2(PO4)2 ferromagnet (FM), the competition between giant magneto-crystalline anisotropy and strong FM couplings creates remarkably narrow magnetic domain walls. These domains freeze below TF ≈ 15 K which is accompanied by progressive “soft-magnet” → “super-hard magnet” transition at both sides of TF. In the hard-regime, the coercive force is above 17 T at 2 K. At 5 K, we calculate a B.Hmax figure of merit (i.e., energy product) of 6.5 MG Oe superior to what observed in most permanent magnet oxides. This allows for the printing above TF and robust locking below TF of any magnetization between the saturation limits. The outstanding Hc value surpasses the standards by far, and brings questions about origin of such unusually strong pinning effects. They have to be surely attributed to narrow domain walls, estimated to be only ∼16 Å thick. Besides the observation of the domain wall structure by magnetic-force microscopy in various conditions, their thermodynamic signature appears in the specific heat data.

Magnetic Behaviour, and initial permeability of green synthesized Co2+ substituted Ni-Zn ferrite

This study investigates the magnetic properties of Ni0.55−xZn0.45CoxFe2O4 (x = 0.0, 0.15, 0.25, 0.35, 0.45, 0.55) ferrite nanoparticles synthesized via the green synthesis route using orange juice as a green reductant. The magnetocrystalline anisotropy constant (Ms) and the Bohr magneton number (nB) of the Ni-Zn ferrites increase linearly with an increase in the content of Co2+ ions. The magnetic properties such as Saturation magnetization, Coercive force, and remanence ratio (Mr / Ms) are determined. The Curie temperature for the studied ferrites is found to be reduced upon doping with cobalt ions and is considered as the weakening of the A-B exchange interaction.

Structural and magnetic properties of cadmium oxides with different annealing temperatures

A series of cadmium oxide samples (CdOs) were synthesized by thermal treatment method with various annealing temperatures (TA = 500 ℃ - 900 ℃). All samples crystallize in a pure cubic structure with Fm3̅m symmetry. The crystallite size, lattice parameter and volume change slightly with TA, and show an unexpected inflection point for the sample with TA = 800 ℃ (CdO-800), while the average particle size increases monotonically from ∼0.4 µm to ∼3.0 µm. Distinct ferromagnetic features accompanied by diamagnetic and/or paramagnetic phases were detected. The XPS data indicate the valence of cadmium and oxygen vacancy concentration varies with TA, which are responsible for the ferromagnetic origin in CdOs. CdO-800 shows the maximum magnetization, significant coercive force and residual magnetization. Oxygen vacancy plays a crucial role in the ferromagnetic performance, which can be effectively controlled by changing the sintering temperature. This work claims new characteristics for CdO besides optical and semi-conducting, which may be important in the preparation and possible applications for CdO dilute magnetic semiconductors.

Domain Switching Characteristics in Ga-Doped HfO2 Ferroelectric Thin Films with Low Coercive Field.

The gallium-doped hafnium oxide (Ga-HfO2) films with different Ga doping concentrations were prepared by adjusting the HfO2/Ga2O3 atomic layer deposition cycle ratio for high-speed and low-voltage operation in HfO2-based ferroelectric memory. The Ga-HfO2 ferroelectric films reveal a finely modulated coercive field (Ec) from 1.1 (HfO2/Ga2O3 = 32:1) to an exceptionally low 0.6 MV/cm (HfO2/Ga2O3 = 11:1). This modulation arises from the competition between domain nucleation and propagation speed during polarization switching, influenced by the intrinsic domain density and phase dispersion in the film with specific Ga doping concentrations. Higher Ec samples exhibit a nucleation-dominant switching mechanism, while lower Ec samples undergo a transition from a nucleation-dominant to a propagation-dominant reversal mechanism as the electric field increases. This work introduces Ga as a viable dopant for low Ec and offers insights into material design strategies for HfO2-based ferroelectric memory applications.

Ferroelectric behavior arising from polar topologies in epitaxially strained SrTiO3-δ ultrathin films

In this work, we show that epitaxially strained SrTiO3-δ thin films, grown by ion-beam sputtering onto (001)Nb:SrTiO3 substrates, exhibit a ferroelectric behavior. At the atomic-scale, through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, it was possible to identify the presence of polar nanoregions with non-trivial polar topological structures in the SrTiO3-δ film, which are induced through oxygen vacancies. To further confirm the presence of strained regions with a polar structure, Raman spectroscopy and high-resolution X-ray diffraction were employed and it was possible to confirm the presence of a tetragonal structure in the SrTiO3-δ film, with a tetragonality ratio (c/a) of 1.005. Scanning probe microscopy and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior with maximum polarization of ∼1.5 μC/cm2, remnant polarization of ∼0.4 μC/cm2 and coercive field of ∼0.3 MV/cm. This work opens a window for exploring novel polar topological effects in sub-10 nm thin film materials for non-volatile memory application.

Electric field control of 180° magnetization reversal in a spin-valve multiferroic heterostructure

Electrical tuning of magnetism, rather than a spin current or magnetic field, has attracted much attention due to its great potential for designing energy-efficient spintronic devices. However, pure electric field (E-field) control of 180° magnetization reversal is still challenging. Thus, we report an E-field-controlled 180° magnetization reversal in a spin valve (SV)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) multiferroic heterostructure. Via the converse magnetoelectric coupling effect in CoFe/PMN-PT heterostructures, the magnetic anisotropy and coercive field of the CoFe film can be tremendously modulated by an E-field. We fabricated an optimized SV grown on the PMN-PT substrate, in which the magnetic moments of the free and pinned layers are parallel to each other at the initial state. By applying an E-field, the coercive field of the free layer was enhanced, exhibiting an antiparallel configuration between the free and pinned layers. Based on the theoretical and experimental results, a complete 180° magnetization reversal of the free layer can be obtained without a magnetic field. Accordingly, an E-field-controlled giant, reversible and repeatable magnetoresistance modulation can be achieved. This work proposes a feasible strategy to achieve E-field-controlled 180° magnetization reversal, which is critical for exploring ultralow power consumption magnetic memory devices.

Thin film field-effect transistor with ZnO:Li ferroelectric channel

An n-type channel transparent thin film field-effect transistor (FET) using a top-gate configuration on a sapphire substrate is presented. ZnO:Li film was used as a channel, and MgF2 film as a gate insulator. Measurements showed that ZnO:Li films are ferroelectrics with spontaneous polarization [Formula: see text]–5[Formula: see text][Formula: see text]C/cm2 and coercive field [Formula: see text]–10[Formula: see text]kV/cm. The dependences of drain–source current on drain–source voltage at various gate–source voltages in two antiparallel [Formula: see text] states were measured and the values of field-effect mobility and threshold voltage were determined for two [Formula: see text] states are as follows: (a) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V; (b) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V. Thus, [Formula: see text] switching leads to a change in FET channel parameters. Results can be used to create a bistable or, more precisely, digital FET.

Effect of Sm2O3 additive on the microstructure, magnetic and electrical properties of LiTiZn ferrite ceramics

LiTiZn ferrites were prepared by a conventional two-steps solid sintering method. According to X-ray diffraction (XRD) results, the primary phase of all specimens was ferrite spinel. It is shown that Sm2O3 additive 1 wt% slightly reduces the lattice parameter of the spinel phase and leads to the appearance of an additional phase SmFeO3. For the first time for LiTiZn ferrite received, that Sm2O3 additive leads to significant increase of the electrical resistivity and the Curie temperature by 25°. Thus, the problem with dielectric loss reduction in the microwave and thermal stability increasing of ferrite ceramics was solved. At the same time, there is a 6 % decrease in saturation magnetization and 11 % in density while coercive force increases by the 60 %. In addition, scanning electron microscope (SEM) data show significant influence of Sm2O3 additive on grain morphology. The energy dispersive spectroscopy (EDS) patterns confirm SmFeO3 phase formation. The results of the study can be used for improvement of sintering ferrite products in a factory.

Optimizing the Sintering Conditions of (Fe,Co)1.95(P,Si) Compounds for Permanent Magnet Applications.

(Fe,Co)2(P,Si) quaternary compounds combine large uniaxial magnetocrystalline anisotropy, significant saturation magnetization and tunable Curie temperature, making them attractive for permanent magnet applications. Single crystals or conventionally prepared bulk polycrystalline (Fe,Co)2(P,Si) samples do not, however, show a significant coercivity. Here, after a ball-milling stage of elemental precursors, we optimize the sintering temperature and duration during the solid-state synthesis of bulk Fe1.85Co0.1P0.8Si0.2 compounds so as to obtain coercivity in bulk samples. We pay special attention to shortening the heat treatment in order to limit grain growth. Powder X-ray diffraction experiments demonstrate that a sintering of a few minutes is sufficient to form the desired Fe2P-type hexagonal structure with limited secondary-phase content (~5 wt.%). Coercivity is achieved in bulk Fe1.85Co0.1P0.8Si0.2 quaternary compounds by shortening the heat treatment. Surprisingly, the largest coercivities are observed in the samples presenting large amounts of secondary-phase content (>5 wt.%). In addition to the shape of the virgin magnetization curve, this may indicate a dominant wall-pining coercivity mechanism. Despite a tenfold improvement of the coercive fields for bulk samples, the achieved performances remain modest (HC ≈ 0.6 kOe at room temperature). These results nonetheless establish a benchmark for future developments of (Fe,Co)2(P,Si) compounds as permanent magnets.

Prediction of giant anomalous Nernst effect in Sm(Co,Ni)5

Sm-Co bulk alloys are well-known permanent magnets having large remanent magnetizations and coercive forces and are widely used in many industrial products. Recently, a large transverse thermoelectric conversion was observed for SmCo5 over a wide temperature range in the absence of magnetic fields. The large thermoelectric conductivity tensors (αxy) was also confirmed by the first-principles density functional theory (DFT) calculations. In this study, we predicted further enhancement of the αxy by including Ni in Co site of SmCo5. We showed that the αxy of Sm(Co1−xNix)5 increases with increasing the Ni ratio and takes the maximum value αxy = 11.3 A K−1 m−1 around x = 0.08 at 300 K, which is about 77% enhancement of αxy = 6.4 A K−1 m−1 in SmCo5. We clarified that the band proximity points near the nodal line of Sm(Co0.92Ni0.08)5 are the main contributing factor to the large Berry curvature, providing the steep slope of the energy dependence in the anomalous Hall conductivity around the Fermi energy.

Antioxidant activity and cytotoxicity of greigite nanoparticles synthesized by hydrothermal technique.

The effects of 180, 210, and 230°C reaction temperatures on the structural and magnetic properties of synthesized iron sulfide nanoparticles were studied. The Rietveld refinement analysis result of the X-ray diffraction data indicated that greigite was the dominant phase in all samples. The sample was prepared at 210°C for 18h and had a greater wt% ratio of the greigite phase. The crystallite and particle sizes increased with increasing reaction temperatures. Scanning electron microscope images confirmed the presence of aggregation of synthesized rod-shaped nanoparticles. The magnetic hysteresis curves of all samples showed ferromagnetic behavior at room temperature. The magnetic saturation of three samples increases with increased reaction temperature, but the coercive force has the opposite behavior. Antioxidant activity and cytotoxicity of the sample synthesized at 210°C were investigated. This sample killed cancer cells at relatively moderate and high concentrations with high viability of normal cells, demonstrating the sample's suitability for use in killing cancer cells while avoiding normal cells.

Understanding the depolarization phenomena in (1−x) Na0.5Bi0.5TiO3. (x) Ba0.85Ca0.15Ti0.90Zr0.10O3 solid solutions using in-situ temperature dependent Raman spectroscopy

A decrease in depolarization temperature (T d) from 456 K to 352 K was observed with an increase in BCZT substitution in the NBT for the (1−x) Na0.5Bi0.5TiO3. (x) Ba0.85Ca0.15Ti0.90Zr0.10O3 solid solutions. A transition towards a higher ergodic state was elucidated with an increase in BCZT content that helped to reduce the free energy barrier, hence lesser thermal energy was required to depolarize the modified systems. Furthermore, a decrease in remnant polarization and coercive field, coupled with an increase in energy storage (W stored) and efficiency (η%) with higher BCZT content. In-situ temperature-dependent Raman spectra provide additional insights, highlighting the faster changes in phonon shifts and lifetimes corresponding to the A–O, B–O, and BO6 vibrations around the depolarization temperature (T d). The observed phase transformation to a P4bm phase at temperatures significantly higher than T d is substantiated by Raman shift and phonon lifetime variations in the modes associated with the A–O and B–O vibrations. The transitions can be understood as: at T ∼ T d the polar nano regions (PNRs) start to appear due to weakening of bonds, T > T d all the long-range ferroelectric domains transform to PNRs converting the material to a fully ergodic state, and at much higher temperatures (T ≫ T d) the R3c PNRs vanish and P4bm PNRs appear.

Production of High-Coercive nanostructured Nd-Fe-B alloy by chemical method

For the first time, a nanostructured alloy powder with high coercive force was obtained by a chemical method without the addition of Dy, which can compete with physical methods of production. The work carried out a study of the influence of stoichiometric composition on the magnetic properties of nanostructured Nd-Fe-B alloys. Alloys with different contents of Nd, Fe, and B were synthesized: Nd12Fe84B6, Nd14Fe80B6, Nd16Fe76B8, and Nd16Fe72B8. With a decrease in the iron content in the first two compositions, a decrease in the iron content in the soft magnetic phase α-Fe was observed, which led to an increase in the coercive force Hc and a decrease in the values of Ms and Mr. Experiments have shown that an increase in the content of Nd and B promotes the formation of the hard magnetic phase Nd2Fe14B and an increase in the coercive force. For example, the nanostructured Nd16Fe76B8 alloy demonstrated a maximum coercive force of Hc = 8439 Oe, while compensating for the values of Ms (109 emu*g−1) and Mr (78 emu*g−1). A comprehensive study of the nanostructured Nd16Fe76B8 alloy was carried out, including structural analysis. XRD using FullProff and Vesta software revealed the tetragonal crystal structure of the Nd2Fe14B phase, providing details of the atomic arrangement. The unit cell contains four layers with Nd and Fe atoms, and boron atoms are embedded in the center of the octahedron of Nd and Fe atoms, playing an important role in the magnetic exchange interaction. The results of Mössbauer spectroscopy confirmed the presence of the Nd2Fe14B phase and revealed details of the state of Fe atoms in the structure of six sextets.

Perpendicular magnetic anisotropy of Pd/Co2MnSi/AlN/Pd multilayer films

In this paper, the perpendicular magnetic anisotropy（PMA）of Pd/Co2MnSi (CMS)/AlN/Pd multilayer film is studied, which is designed based on the two-interface interaction, the orbital hybridization effect of Pd5d-Co3d electrons at the Pd/CMS interface and the orbital hybridization effect of Co3d-N2p electrons at the CMS/AlN interface. Compared with the Pd/CMS/Pd multilayer film, the PMA of the Pd/CMS/AlN/Pd structure shows significant enhancement with the insertion of the AlN layer. The Hall resistance (RHall) of the Pd/CMS/AlN/Pd structure reached 0.02 Ω (increased by 300 %) and the coercive force (Hc) is 388 Oe (increased by about 76 %). The magnetic anisotropic energy density (Keff) is 0.61Merg/cm3 (increased by about 135 %). The PMA was optimized with the film factor (layer thickness) of Pd(6 nm)/CMS(5 nm)/AlN(6 nm)/Pd(6 nm). The X-Ray Diffraction（XRD）and High Resolution Transmission Electron Microscope（HRTEM）analysis showed that the PMA is affected by the crystallinity and interface quality of multilayer films.

Structure and physical properties of multicomponent films based on the Fe-Si-B system

The influence of modernized three-electrode ion-plasma sputtering of a complex Fe–Si–B– (Cu, Nb)/(Ni, Mo) target on the structure and properties of sputtered films was studied. The formation of amorphous and nanocrystalline phases in Fe73Si15.8B7.2-(Cu, Nb)4 and Fe78.5Si6B14-(Ni, Mo)1.5 films with a coherent scattering region size of 1.6 nm and 12 nm, respectively, was established as a result of a modernized three-electrode ion - plasma sputtering. The thermal stability of metastable states of films as well as electrical and magnetic properties of freshly prepared and heat-treated films has been investigated. The conditions for obtaining films with low values of the temperature coefficient of electrical resistance (−0.9∙10−5 K−1) and coercive force (HC ∼11 A/m) have been determined.

Tailoring coercive fields and the Curie temperature via proximity coupling in WSe2/Fe3GeTe2 van der Waals heterostructures

Hybrid structures consisting of two-dimensional (2D) magnets and semiconductors have exhibited extensive functionalities in spintronics and opto-spintronics. In this work, we have fabricated WSe2/Fe3GeTe2 van der Waals heterostructures and investigated proximity effects on 2D magnetism. Through reflective magnetic circular dichroism, we have observed a temperature-dependent modulation of magnetic order in the heterostructure. For temperatures above 40K , WSe2-covered Fe3GeTe2 exhibits a larger coercive field than that observed in bare Fe3GeTe2, accompanied by a noticeable enhancement of the Curie temperature by 21K . This strengthening suggests an increase in magnetic anisotropy in the interfacial Fe3GeTe2 layer, which can be attributed to the spin–orbit coupling (SOC) proximity effect induced by the adjacent WSe2 layers. However, at much lower temperatures ( T<20K ), a non-monotonic modification of the coercive field is observed, showing both reduction and enhancement, which depends on the thickness of the WSe2 and Fe3GeTe2 layers. Moreover, an unconventional two-step magnetization process emerges in the heterostructure, indicating the short-range nature of SOC proximity effects. Our findings on proximity coupling may shed light on the design of future spintronic and memory devices based on 2D magnetic heterostructures.

Towards wake-up free ferroelectrics and scaling: Al-doped HZO and its crystallographic texture

Ferroelectric (FE) hafnium zirconium oxide (HZO) is an excellent candidate for data storage applications. However, it has some reliability limitations such as imprint and retention. Herein, we explore Al doping of HZO to overcome these limitations. FE behavior is tuned by the aluminum (Al) concentrations in the films and by annealing temperature. A correlation is done between electrical behavior, crystallographic texture, and FE phases determined by grazing-incidence X-ray diffraction (GIXRD) measurements. Reduced coercive field (2Ec) values and wake-up free HZO-based ferroelectrics are explored. We show the tunability of remanent polarization (2Pr) and 2Ec with respect to Al-doping concentration and anneal temperature, hence crystallographic texture.

Phase transition and polar cluster behavior above Curie temperature in ferroelectric BaTi0.8Zr0.2O3

We study the phase transition behavior of the ferroelectric BaTi0.8Zr0.2O3 in the paraelectric region. The temperature dependencies of thermal, polar, elastic, and dielectric properties indicate the presence of local structures above the paraelectric–ferroelectric transition temperature Tc = 292 K. The non-zero remnant polarization is measured up to a characteristic temperature T* ∼ 350 K, which coincides with the temperature where the dielectric constant deviates from the Curie–Weiss law. Resonant piezoelectric spectroscopy shows that DC field cooling above Tc using fields smaller than the coercive field leads to an elastic response and remnant piezoelectricity below T*, which likely corresponds to the coherence temperature associated with polar nanostructures in ferroelectrics. The observed remnant effect is attributed to the reorientation of polar nanostructures above Tc.

Molecular beam epitaxy and characterization of ferroelectric quaternary alloy Sc0.2Al0.45Ga0.35N

In this study, we demonstrate ferroelectricity in high-quality monocrystalline quaternary alloy ScAlGaN. Sc0.2Al0.45Ga0.35N films are grown by plasma-assisted molecular beam epitaxy and exhibit a surface roughness of 0.5 nm, limited by the roughness of the underlying molybdenum template. Polarization-electric field and positive-up-negative-down measurements reveal unambiguous ferroelectric switching with a coercive field of ∼5.5 MV cm−1 at 10 kHz and high remanent polarization of ∼150 μC cm−2. Time-dependent measurements suggest that the polarization reversal behavior adheres to the Kolmogorov–Avrami–Ishibashi model and follows a scheme of domain nucleation and growth. Detailed piezoresponse force microscopy studies further elucidate the evolution of polarity reversal domains in wurtzite nitride ferroelectrics and support the notion that the growth of inversion domains occurs via an in-plane motion of the domain walls. The realization of functional ferroelectric quaternary alloys in the wurtzite nitride family extends beyond being a technical demonstration. The additional degree of bandgap, band alignment, lattice parameter, and piezoelectric constant tunability achievable through quaternary alloys unveils a vast dimension through which wurtzite nitride ferroelectrics can be optimally engineered for a broad variety of high-performance electronic, optoelectronic, and acoustic devices and systems.