High Coercive Field Research Articles

Study the influence of Nd and Co/Cr co-substitutions on structural, electrical and magnetic properties of BiFeO3 nanoparticles

Multiferroic BFO nanoparticles of the compositions Bi1−xNdxFeO3 (x=0.05 and 0.1) and Bi0.9Nd0.1Fe0.95TM0.05O3 (Transition Metals, TM=Co and Cr) have been synthesized by low temperature citric acid assisted auto-combustion route. Subsequently, the co-doping effects on structural, dielectric and ferroelectro-magnetic properties of BFO have been investigated. X-ray diffraction analysis hints phase transformation from rhombohedral to orthorhombic for 10% Nd-substituted BFO. TM addition further modifies structure and co-doped samples possess pseudo-cubic symmetry. Magnetic measurement reveals gradual enhancement in weak ferromagnetic nature with increasing doping. A well-developed magnetic hysteresis loop with high magnetization (Mr~0.35emu/g) and coercive field (Hc~1255Oe) is observed for Nd–Co co-doped sample, whereas other samples exhibit rarely-observed ‘wasp-waisted’ type loops. Dielectric behavior is improved significantly on doping. Temperature-dependent dielectric constant presents a broad peak in the vicinity of Neel׳s temperature (TN) for all the samples, signifying strong magnetoelectric coupling. An increment in TN is also observed as a result of Nd-doping and further on adding TM. Leakage current decreases significantly as a result of reduced oxygen vacancies on substitutions. Therefore, co-substitutions of rare-earth and magnetically active transition ions remarkably enrich electric and magnetic characteristics of BFO, and boost multiferroic feasibility.

A single-chain magnet with a very high blocking temperature and a strong coercive field.

Two isostructural 1D complexes, [M(hfac)2NaphNN]n [M = Mn(II) (1) or Co(II) (2); NaphNN = 1-naphthyl nitronylnitroxide], were synthesized and exhibit very strong antiferromagnetic metal-radical exchange coupling. Compound 2 shows slow magnetic relaxation behavior with a high blocking temperature (TB ≈ 13.2 K) and a very high coercive field of 49 kOe at 4.0 K.

Influence of global and local distortion on magnetic properties of cubic La0.6Ba0.4–xCaxCoO3

The magnetic and structural study of the La0.6Ba0.4–xCaxCoO3 (x=0.0, 0.1, 0.2, 0.3, and 0.4) compounds with the lowest global or local distortion are studied. The compounds with x=0, 0.1, 0.2 and 0.3 is crystallized in the structure with the space group Pm-3m, and that with x=0.4 is Pnma. A ferromagnetic-like transition is observed and the Curie temperature, ranging from 235K to 220K, decreases slightly with the increasing Ca2+ content for x≤0.3, and the transition temperature is as low as 175K with x=0.4. A hump, with the hump temperature slightly increase with the Ca2+ content, is observed in the thermal magnetization curves of all of the compounds at the ZFC state, and it is owing to the magnetic frustration because of the coexistence of the FM and the AFM interaction. Above the transition temperature, the magnetic susceptibility versus the temperature is fitted with the ferromagnetic Curie–Weiss law for the compounds with x≤0.3, and that with x=0.4 coincides with the ferrimagnetic Weiss-mean-field model. The absolute values of the exchange constants J1 in the compounds with x≤0.3 and those of JCO3+CO3+,JCO3+CO4+,JCO4+CO4+ of La0.6Ca0.4CoO3 are deduced from the fitting. The results indicate that (i) the ferromagnetic exchange constants J1 increases with the Ca2+ content x≤0.3; (ii) the ferromagnetic interaction, JCo3+Co4+, plays a main role in the magnetic properties of La0.6Ca0.4CoO3; (iii) the antiferromagnetic interactions, JCo3+Co3+, JCo4+Co4+, are not negligible in the compound x=0.4. The unsaturated magnetization at 70kOe and the high coercive field in the hysteretic magnetization curve supports the existence of the antiferromagnetic interaction, and the percentage of the antiferromagnetic domain is calculated.

The role of pH on the particle size and magnetic consequence of cobalt ferrite

Cobalt ferrite (CoFe2O4) nanoparticles with various size distributions were prepared by a chemical co-precipitation method at different pH condition from 8 to 13. The structural characterizations of the prepared samples were carried out using powder X-ray diffraction, Fourier transform infrared spectroscopy and field emission scanning electron microscope. The XRD results revealed that a single cubic CoFe2O4 phase with the average crystallite sizes of about 5–24nm were formed. Cation distribution occupancy in tetrahedral and octahedral sites were estimated by employing Rietveld refinement technique. The results showed that the whole series of samples contain a partial inverse spinel structure. FTIR measurements between 370 and 4000cm−1 confirmed the intrinsic cation vibrations of spinel structure of the samples. The room temperature magnetic properties of the samples have been examined using vibrating sample magnetometer. It is found that with increasing the pH of reaction, the magnetization and coercive field could be increased. The sample synthesized at pH~8 and 9 showed superparamagnetic behavior and highest coercive field up to 650Oe is attributed to the sample synthesized with pH~13.

Structural and magnetic properties of granular CoPd multilayers

Multilayers of bimetallic CoPd alloyed and assembled nanoparticles, prepared by room temperature sequential sputtering deposition on amorphous alumina, were studied by means of high-resolution transmission electron microscopy, x-ray diffraction, SQUID-based magnetometry and x-ray magnetic circular dichroism. Alloying between Co and Pd in these nanoparticles gives rise to a high perpendicular magnetic anisotropy. Their magnetic properties are temperature dependent: at low temperature, the multilayers are ferromagnetic with a high coercive field; at intermediate temperature the behavior is of a soft-ferromagnet, and at higher temperature, the perpendicular magnetic anisotropy in the nanoparticles disappears. The magnetic orbital moment to spin moment ratio is enhanced compared with Co bare nanoparticles and Co fcc bulk.

Synthesis and characterization of CoFe2O4 magnetic nanotubes, nanorods and nanowires. Formation of magnetic structured elastomers by magnetic field-induced alignment of CoFe2O4 nanorods

Magnetic CoFe2O4 nanotubes, nanorods and nanowires were synthesized by the template method. The materials are highly crystalline and formed by compactly packed ceramic particles whose equivalent size diameter depends on the nanostructure type. Nanotubes and nanorods present the remarkable characteristic of having very large coercive fields (1000–1100 Oe) in comparison with nanoparticles of the same crystallite size (400 Oe) while keeping similar saturation magnetization (53–55 emu/g). Nanorods were used as filler material in polydimethylsiloxane elastomer composites, which were structured by curing in the presence of uniform magnetic field, H curing. In that way the nanorods agglomerate in the cured elastomer, forming needles-like structures (pseudo-chains) oriented in the direction of H curing. SEM analysis show that pseudo-chains are formed by bunches of nanorods oriented in that direction. At the considered filler concentration (1 % w/w), the structured elastomers conserve the magnetic properties of the fillers, that is, high coercive fields without observing magnetic anisotropy. The elastomer composites present strong elastic anisotropy, with compression constants about ten times larger in the direction parallel to the pseudo-chains than in the perpendicular direction, as determined by compression stress–strain curves. That anisotropic factor is about three-four times higher than that observed when using spherical CoFe2O4 nanoparticles or elongated Ni nanochains. Hence, the use of morphological anisotropic structures (nanorods) results in composites with enhanced elastic anisotropy. It is also remarkable that the large elastic anisotropy was obtained at lower filler concentration compared with the above-mentioned systems (1 % w/w vs. 5–10 % w/w).

Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe7−yMyX8 chalcogenides (X = S, Se; M = Ti, Co)

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe7−yMyX8 (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe7−yMyX8 compounds having a ferrimagnetic order at , the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around –1.5 in all the Fe7−yMyX8 families except Fe7−yCoySe8. Suppression of a magnetically ordered state with substitutions in Fe7−yMyX8 is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe7−yTiyX8 and Fe7−yCoyX8 is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20–24 kOe) observed at low temperatures in the Ti-containing compounds Fe7−yTiyX8 with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution.

Structural and magnetic properties of CoxFe3−xO4 versus Co/Fe molar ratio

CoxFe3−xO4 (x=0.5–2.5) magnetic nanoparticles were synthesized via redox reaction between cobalt nitrate, iron nitrate and 1-4-butanediol using five Co/Fe molar ratios, followed by calcination at 1000°C. Single phase nanoscaled cobalt ferrite was obtained at x=1.0 and at slight Co excess (x=1.5), while at high Co/Fe molar ratios (x=2.0 and x=2.5) the prevailing phase was CoO accompanied by CoFe2O4 traces. The highest values of coercive field and saturation magnetization were obtained for the sample at x=1.0, while the lowest values were obtained in the sample with the highest Co excess (x=2.5). The results indicated that the used synthesis route was suitable for the synthesis of cobalt ferrite with moderate saturation magnetization and high coercive field values.

Differences Observed in the Phase Structure, Grain Size–Shape, and Coercivity Field of Electrochemically Deposited Ni–Co Thin Films with Different Co Contents

Nanostructured Ni–Co thin films were produced using galvanostatic electrodeposition on indium tin oxide (ITO)-coated glass substrates from an aqueous electrolyte solution without stirring at ambient temperature. Different compositions of the films were achieved depending on the Co ion concentration within the electrolyte. From the compositional analysis performed using energy dispersive X-ray (EDX) spectroscopy, the anomalous codeposition was found for all films and an increase in the Co content of the films was observed with increasing Co ion concentration within the electrolyte. The X-ray diffraction (XRD) analyses revealed that the phase structure of the electrodeposited Ni–Co thin films changes from single face-centered cubic (FCC) to a mixture of FCC and hexagonal close-packed (HCP) with the Co content within the films. It was also observed that the mean crystallite size estimated by applying the Scherrer method gradually decreases as the Co content within the films increases. The scanning electron microscopy (SEM) employed for the surface morphology showed that the size and shape of the grains formed on the film surface are strongly dependent on Co content within the films. Magnetic measurements performed by vibrating sample magnetometer (VSM) revealed that the films with different Co contents exhibit different magnetic properties; the Co-rich film has much higher coercivity field when compared with the Ni-rich film.

Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods

Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co80Ni20 nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co80Ni20 alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature.

Unraveling the roles of thermal annealing and off-time duration in magnetic properties of pulsed electrodeposited NiCu nanowire arrays

Magnetic alloy nanowires (ANWs) have long been studied owing to both their fundamental aspects and possible applications in magnetic storage media and magnetoresistance devices. Here, we report on the roles of thermal annealing and duration of off-time between pulses (toff) in crystalline characteristics and magnetic properties of arrays of pulsed electrodeposited NiCu ANWs (35 nm in diameter and a length of 1.2 μm), embedded in porous anodic alumina template. Increasing toff enabled us to increase the Cu content thereby fabricating NiCu ANWs with different crystallinity and alloy compositions. Although major hysteresis curve measurements showed no considerable change in magnetic properties before and after annealing, the first-order reversal curve (FORC) analysis provided new insights into the roles of thermal annealing and toff. In other words, FORC diagrams indicated the presence of low and high coercive field regions in annealed Ni-rich ANWs, coinciding with the increase in toff in as-deposited ANWs. The former has a small coercivity with strong demagnetizing magnetostatic interactions from neighboring NWs and may correspond to a soft magnetic phase. The latter has a greater coercivity with weak interactions, corresponding to a hard magnetic phase. On the other hand, for as-deposited and annealed Cu-rich NiCu ANWs, a mixed phase of the soft and hard segments could be found. Furthermore, a transition from the interacting Ni-rich to non-interacting Cu-rich ANWs took place with a magnetic field applied parallel to the NW axis. Thus, these arrays of ANWs with tunable magnetic phases and interactions may have potential applications in the nanoscale devices.

Influence of Co and Dy Doping on the Optical and Magnetic Properties of ZnO Nanoparticles for DMS Application

Dysprosium-(Dy) and cobalt (Co)-doped zinc oxide (ZnO) nanoparticles with various concentrations have been synthesized by simple chemical precipitation method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDAX) measurements were conducted respectively for the structural, morphological, and compositional investigation of the sample. XRD analysis indicated that diffraction peaks of all the samples can be indexed to the hexagonal wurtzite structure of ZnO. No secondary phases including Co and Dy ions were detected in the samples even for the highest doping concentrations of Dy. Average particle size is found to be 25–34 nm. Scanning electron microscope and transmission electron microscope analysis revealed that the dopant atoms of Co and Dy are homogeneously distributed in ZnO wurtzite structure. The samples were characterized by EDAX to confirm the expected stoichiometry. In the photoluminescence spectra, Dy co-doped samples show a violet emission along with a broad yellow emission due to the4F9/2–6H15/2 inner shell transitions of Dy3+ ion. The photoluminescence (PL) enhancement process in Dy co-doped ZnO:Co is mainly due to Dy as a sensibilizer which could effectively enhance the luminescence intensity. Absorption spectra of dysprosium- (Dy) and cobalt (Co)-doped ZnO nanoparticles exhibited enhanced optical absorption in visible region. Magnetization measurements have shown that the particles have room temperature ferromagnetic behavior with relatively high coercive fields which are decreasing with the increase of doping concentration. The analysis of optical and magnetic properties shows that Co- and Dy-doped ZnO nanoparticles are a promising material for DMS application and have potential applications in optoelectronic devices.

Enhancement of ferromagnetic and ferroelectric properties in calcium doped BiFeO3 by chemical synthesis

Calcium (Ca)-doped bismuth ferrite (BiFeO3) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), polarization and magnetic measurements. Structural studies by XRD and Rietveld refinement reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO3 (BFO) where enhanced ferroelectric and magnetic properties are produced by internal strain. A high coercive field in the hysteresis loop is observed for the BiFeO3 film. Fatigue and retention free characteristics are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain.

The nano-microfibrous R11Ni4In9 intermetallics: New compounds and extraordinary anisotropy in Tb11Ni4In9 and Dy11Ni4In9

R11Ni4In9 (R=rare earth) compounds exhibit an unusual self-assembled nano/microfibrous morphology that results in anisotropic structural and magnetic behaviors. The existence of new compounds for R=Dy, Ho, Er, Tm and Lu, has been established (orthorhombic Nd11Pd4In9-type, oC48, Cmmm, Z=2), showing that the formation of these phases, previously known for R=La–Nd, Sm, Gd, Tb and Y, extends to all of the rare earth elements, except Sc, Eu and Yb. The results of physical property measurements performed on oriented fibers of Tb11Ni4In9, Dy11Ni4In9 and Y11Ni4In9 are presented. Multiple magnetic transitions are observed in Tb11Ni4In9 and Dy11Ni4In9 with the highest ordering temperature, TC, of 112 and 88K, respectively. Y11Ni4In9 is a Pauli paramagnet down to 2K. The fibrous microstructure of these compounds leads to a strong anisotropy in their electrical resistivity and magnetization behaviors. The c-axis of the orthorhombic cell is the easy magnetization and high electrical-conductivity direction. Ferrimagnetic-like behavior, with extremely high coercive fields (HC=6.6T for Tb11Ni4In9 at 5K and HC=5.7T for Dy11Ni4In9 at 2K), is found when the fibers (and the c-axis) are oriented parallel to the magnetic field direction; antiferromagnetic-like ground state is observed with the fibers oriented orthogonal (i.e., in the a–b plane). Appearance of a Griffiths phase regime is observed in both compounds before entering the ordered magnetic states. This is more evident for fibers orthogonal to the magnetic field and is even preserved at 1T. Field induced spin-flop magnetic transitions are also observed in Tb11Ni4In9 and Dy11Ni4In9 with fibers orthogonal and parallel to the field, respectively. First principles calculations have been performed for several representative compounds to explain the underlying phase stability and their magnetism.

High magnetic hardness for the canted antiferromagnetic, ferroelectric, and ferroelastic layered perovskite-like (C2H5NH3)2[Fe(II)Cl4

An unusual high magnetic hardness for the layered perovskite-like (C2H5NH3)2[Fe(II)Cl4], in addition to its already found canted antiferromagnetism, ferroelasticity, and ferroelectricity, which are absent for (CH3NH3)2[Fe(II)Cl4], has been observed. The additional CH2 in the ethylammonium compared to methylammonium allows more degrees of freedom and therefore numerous phase transitions which have been characterized by single-crystal structure determinations from 383 to 10 K giving the sequence from tetragonal to orthorhombic to monoclinic (I4/mmm ↔ P42/ncm ↔ Pccn ↔ Pcab ↔ C2/c) accompanied by both tilting and rotation of the FeCl6 octahedra. The magnetic properties on single crystal and powder samples at high temperature are similar for both compounds, but at TN (C2H5NH3)2[Fe(II)Cl4] is a proper canted antiferromagnet unlike the hidden canting observed for (CH3NH3)2[Fe(II)Cl4]. The canting angle is 0.6° toward the c-axis, and thus the moments lie in the easy plane of the iron-chloride layer defined by a critical exponent β = 0.18. The isothermal magnetizations for the field along the three orthogonal crystallographic axes show wider hysteresis for H ∥ c and is present at all temperature below 98 K. The coercive field increases as the temperature is lowered, and at T < 20 K it is difficult to reverse all the moments with the available 50 kOe of the SQUID for both single crystal and powder samples. The shape of the virgin magnetization after zero-field-cool (ZFC) indicates that the high coercive field is due to domain wall pinning. Thus, there are unusual associated anomalies such as asymmetric hysteresis and history dependence. The difference in magnetic hardness of the two compounds suggests that magnetic, electric, and elastic domains are intricately manifested and therefore raise the key question of how the different domains interact.

The glassy behaviour of poorly crystalline Fe2O3 nanorods obtained by thermal decomposition of ferrous oxalate

Nanorod ferrous oxalate dihydrate (FeC2O4 × 2H2O) which had been synthesized by the microemulsion method, was used as a precursor in the thermal decomposition process performed in air atmosphere. The formation of nanocrystalline hematite as the final product was preceded by the appearence of an intermediate product. Comprehensive study comprising several complementary techniques (x-ray diffraction, transmission electron microscopy, selected area electron diffraction, thermogravimetric/differential thermal analyses and SQUID magnetometry) confirmed that the intermediate product corresponds to the poorly crystalline Fe2O3. Due to the specific nanorod shape and poorly crystalline structure, the investigated Fe2O3 showed high coercive field value of at Special attention in this study was devoted to the peculiar magnetic properties of poorly crystalline Fe2O3, which were thoroughly investigated by employing sophisticated experimental procedures such as relaxation of thermoremanent magnetization for different cooling fields, zero field and field cooled memory effects as well as aging experiments for different waiting times. At low temperatures and weak applied magnetic fields, the investigated system behaves similarly to spin glasses, manifesting slow, collective relaxation dynamics of magnetic moments through memory, rejuvenation and aging effects.

Room temperature ferromagnetism in conducting α-(In1−xFex)2O3 alloy films

We have studied electronic transport and magnetic properties of α-(In1−xFex)2O3 alloy films. Temperature dependence of resistivity of the films showed semiconducting behavior of conductivity. Room temperature ferromagnetism was observed. Relatively high coercive fields indicated that observed ferromagnetism in α-(In1−xFex)2O3 films were not arisen from magnetic metallic iron nano-precipitates. Remanence measurement revealed the Curie temperature of 520 K and 620 K for α-(In0.52Fe0.48)2O3 and α-(In0.23Fe0.77)2O3 films, corresponding to a weakening of superexchange interactions in these alloys (with less magnetic iron cations) with respect to canted antiferromagnet α-Fe2O3. Nevertheless, Curie temperatures remain much higher than 300 K, and semiconducting behavior with low activation energy in resistivity for middle composition alloy, exhibiting combined multi-functionality of room ferromagnetism and semiconducting properties, in corundum alloys such as α-(Ga1−xFex)2O3 or α-Fe2−xTixO3.

AC conductivity and dielectric properties of (Bi,Na)TiO3–BaTiO3 lead free ceramics

(Bi,Na)TiO3 (BNT) based ceramics have been extensively investigated for electronic device applications. BNT is ferroelectric material with ABO3 perovskite structure. BNT ceramics have high Curie temperature (Tc=320°C) and remanent polarization (Pr=38µC/cm2). Despite its relatively high coercive field (Ec=7.3kV/mm) and low piezoelectric constant (d33=83pC/N). Temperature and frequency dependent relative dielectric permittivity and loss tangent can play an important role in the device performance. To analyze the dielectric characteristics across wide temperature (303–403K) and frequency range (1kHz to 3MHz), AC conductivities of (1−x)(Bi0.5Na0.5TiO3)−x(BaTiO3) (x=0, 0.02, 0.04, 0.06, 0.08, or 0.10) were measured and analyzed. The conduction mechanism was evaluated through simulations and calculations based on an Arrhenius-type equation.

Electrical conductivity and thermopower of (1 - x) BiFeO(3) - xBi(0.5)K(0.5)TiO3 (x = 0.1, 0.2) ceramics near the ferroelectric to paraelectric phase transition.

Ferroelectric BiFeO3 has attractive properties such as high strain and polarization, but a wide range of applications of bulk BiFeO3 are hindered due to high leakage currents and a high coercive electric field. Here, we report on the thermal behaviour of the electrical conductivity and thermopower of BiFeO3 substituted with 10 and 20 mol% Bi0.5K0.5TiO3. A change from p-type to n-type conductivity in these semi-conducting materials was demonstrated by the change in the sign of the Seebeck coefficient and the change in the slope of the isothermal conductivity versus partial pressure of O. A minimum in the isothermal conductivity was observed at ∼10(-2) bar O2 partial pressure for both solid solutions. The strong dependence of the conductivity on the partial pressure of O2 was rationalized by a point defect model describing qualitatively the conductivity involving oxidation/reduction of Fe(3+), the dominating oxidation state of Fe in stoichiometric BiFeO3. The ferroelectric to paraelectric phase transition of 80 and 90 mol% BiFeO3 was observed at 648 ± 15 and 723 ± 15 °C respectively by differential thermal analysis and confirmed by dielectric spectroscopy and high temperature powder X-ray diffraction.

2E3-4 Room Temperature Poling of Sol-Gel Composite Materials with High Coercive Field Piezoelectric Powder Phase

2E3-4 Room Temperature Poling of Sol-Gel Composite Materials with High Coercive Field Piezoelectric Powder Phase