High Coercive Field Research Articles

Electric field induced polarization and strain of (Bi1/2Na1/2)TiO3–BaTiO3 ceramics

This study investigated the whole composition range of the electric field induced polarization and strain of (1−x)(Bi0.5Na0.5)TiO3–xBaTiO3 ceramics fabricated by a conventional solid-state reaction. In most of the composition regions (x<0.7) except for the MPB region (0.06≤x<0.08), saturated square-shape P–E loops and large negative strains in the S–E curves, along with a high coercive field and a large remnant polarization, were obtained. In these composition ranges, the remnant polarization and maximum positive strain decreased gradually with an increasing BaTiO3 content and the coercive field reached the maximum at x=0.4. The high coercive field was intrinsically affected by (Bi0.5Na0.5)TiO3, possessing a doubling of the unit cell owing to octahedron tilting and compositional ordering. At 0.7≤x, the crystal structure became a simple tetragonal structure, and the remnant polarization and the coercive field significantly decreased with an increasing BaTiO3 content.

Structural and magnetic properties of granular Co-Pt multilayers with perpendicular magnetic anisotropy

We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt $L{1}_{0}$ alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.

Bit patterned media optimization at 1 Tdot/in2 by post-annealing

We report on the fabrication of 1 Tdot/in2 bit patterned media with high coercivity (HC) and narrow intrinsic switching field distribution (iSFD) based on nanoimprint from a master pattern formed by e-beam guided block copolymer assembly onto a carbon hard mask and subsequent pattern transfer via etching into a thin CoCrPt perpendicular anisotropy recording layer. We demonstrate that an additional vacuum annealing step after pattern transfer into the CoCrPt layer and after Carbon hard mask removal not only yields recovery from undesired damage of the island edges, but actually transforms the islands into a magnetically more favorable compositional phase with higher HC, lower iSFD/HC, and three-fold increased thermal stability. Energy filtered transmission electron microscopy analysis reveals that the diffusion of Cr from the island cores to the periphery of the islands during post-annealing is responsible for the transformation of the magnetic bits into a more stable state.

A Novel Design of Fluxgate Magnetometer: An Insect like Configuration

In the present work, we propose a novel design of the fluxgate magnetometer which utilizes the detection of ultra-low DC fields down to (even less) 10 μOe. Measurement point has four eyes, like insects, with the diameter of each having 100 μm. It means that the magnetometer can be used for the magnetization analysis of local areas as low as 8,000 μ m 2. Sensing elements used in the sensor are in the form of metallic glass with high relative permeability (∼10,000) and low coercive field (<1 Oe). In addition to high sensitivity, it also has quite a large dynamical measurement range. The 2 nd harmonic voltage used as the sensor signal shows linear dependence on the applied field till almost 0.4 Oe. Such a large dynamical range facilitates its use in spacecraft applications, mine detection, earth skills, etc.

Effects of CuO additive on structure and electrical properties of low-temperature sintered Ba0.98Ca0.02Zr0.02Ti0.98O3 lead-free ceramics

Lead-free Ba0.98Ca0.02Zr0.02Ti0.98O3–xmol% (x=0–2.0) CuO ceramics (BCZT–xCu) were fabricated by the traditional solid state reaction technique, and their structure and electrical properties were studied systematically. With the introduction of Cu, the orthorhombic–tetragonal (TO–T) phase transition shifted towards lower temperature, while Curie temperature (Tc) remained at relatively high value of about 120°C. The sintering temperature was reduced by 150°C than that for pure BCZT ceramics while showing equivalent properties. 0.8mol% Cu modified ceramics showed excellent piezoelectric properties with d33=315pC/N and kp=44%. Besides these, a relatively high dielectric coefficient (εr=2280), a low dielectric loss (tanδ=1.14%), a high remnant polarization (Pr=11.6μC/cm2) and low coercive field (Ec=0.37kV/mm) were also obtained on Ba0.98Ca0.02Zr0.02Ti0.98O3–0.8mol% Cu lead-free ceramics. Moreover, the influence of sintering temperature on the microstructure development and electrical properties of BCZT–0.8mol% Cu ceramics has also been studied.

Room temperature coercivity and interaction effects in L10 FePt nanoparticles

FePt alloy systems in the L10 phase are characterized by high anisotropy values that lead to exceptionally high coercive fields, making them a primary candidate for high density data storage. In this paper, we report the synthesis of L10 FePt nanoparticles by thermal annealing of chemically disordered FePt nanoparticles. The high degree of ordering and high coercivity achieved in these systems is further studied using Monte Carlo simulations by incorporating the effects of anisotropy, interactions, and disorder, with a view to understand the role of these interactions. A comparison of the simulation results with the experiment results also helps us to estimate an anisotropy value of ∼2.25–2.5 MJ m−3.

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal–insulator–metal structures is reported. After field cycling a remanent polarization up to 40 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (Sr) was evaluated. In all cases, an improved polarization–voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling.

Structural and Magnetoelectric Properties of a New W-Type Hexaferrite (Sr&lt;sub&gt;0.85&lt;/sub&gt;Ce&lt;sub&gt;0.15&lt;/sub&gt;Co&lt;sub&gt;2&lt;/sub&gt;Fe&lt;sub&gt;16&lt;/sub&gt;O&lt;sub&gt;27-δ&lt;/sub&gt;)

A new W-type hexaferrite (Sr0.85Ce0.15Co2Fe16O27-δ) was synthesized through proteic sol-gel using coconut water as a solvent for the precursor salts. Structural analysis was carried out using an X-ray diffractometer (XRD). The XRD results showed that the resulting ceramic exhibited preferential orientations along the crystal planes in addition to lattice displacements. The magnetic properties evaluated using a vibrating sample magnetometer indicated a high saturation magnetization value and low coercive field and remnant magnetization values. The Curie temperature determined through differential scanning calorimetry was 473 oC. The electric resistivity value determined through complex impedance was similar to those already reported in the literature, thus promising the use of this hexaferrite in electronic and microwave device applications.

Effects of cobalt and sintering temperature on electrical properties of Ba0.98Ca0.02Zr0.02Ti0.98O3 lead-free ceramics

Lead-free (Ba0.98Ca0.02)(Zr0.02Ti0.98)O3-xmol% (x = 0–1.6) cobalt ceramics (BCZT-xCo) have been fabricated by the traditional solid-state reaction technique and the effects of Co and sintering temperature on ferroelectric, dielectric and piezoelectric properties of (Ba0.98Ca0.02)(Zr0.02Ti0.98)O3 lead-free ceramics have been studied systematically. The orthorhombic–tetragonal (T O–T) transition shift towards lower temperature with increasing Co addition, while Curie temperature (T c) remained at relatively high value of 107 °C. And the Main piezoelectric parameters are optimized at x = 0.8 mol% with a high piezoelectric coefficient (d 33 = 330 pC/N), a planar mode electromechanical coupling factor (k p = 46.7 %), a high dielectric constant (e r = 2,675) and a low dielectric loss (tanδ = 0.90 %) at 1kHZ. Besides these, high remnant polarization (P r) and low coercive field (E c) of 11.5 μC/cm2, 0.31 kV/cm are also obtained at (Ba0.98Ca0.02)(Zr0.02Ti0.98)O3-0.8 mol%Co lead-free ceramics. Furthermore, greatly enhanced temperature stability of the piezoelectric properties was obtained in the temperature range from 20 to 90 °C. The above results indicate that BCZT-Co ceramics are promising lead-free materials for practical applications.

MRI induced torque and demagnetization in retention magnets for a bone conduction implant.

Performing magnetic resonance imaging (MRI) examinations in patients who use implantable medical devices involve safety risks both for the patient and the implant. Hearing implants often use two permanent magnets, one implanted and one external, for the retention of the external transmitter coil to the implanted receiver coil to achieve an optimal signal transmission. The implanted magnet is subjected to both demagnetization and torque, magnetically induced by the MRI scanner. In this paper, demagnetization and a comparison between measured and simulated induced torque is studied for the retention magnet used in a bone conduction implant (BCI) system. The torque was measured and simulated in a uniform static magnetic field of 1.5 T. The magnetic field was generated by a dipole electromagnet and permanent magnets with two different types of coercive fields were tested. Demagnetization and maximum torque for the high coercive field magnets was 7.7% ± 2.5% and 0.20 ± 0.01 Nm, respectively and 71.4% ± 19.1% and 0.18 ± 0.01 Nm for the low coercive field magnets, respectively. The simulated maximum torque was 0.34 Nm, deviating from the measured torque in terms of amplitude, mainly related to an insufficient magnet model. The BCI implant with high coercive field magnets is believed to be magnetic resonance (MR) conditional up to 1.5 T if a compression band is used around the skull to fix the implant. This is not approved and requires further investigations, and if removal of the implant is needed, the surgical operation is expected to be simple.

Effect of Thickness and Deposition Rate on the Structural and Magnetic Properties of Evaporated Fe/Al Thin Films

We report experimental results on the structural and magnetic properties of Fe thin films deposited by thermal evaporation onto polycrystalline Al substrates. The effect of thickness t (in the 76 to 431 nm range) and deposition rate are investigated. The texture, the strain, and the grain size values were derived from X-ray diffraction (XRD) experiments. The thinner film (76 nm) has no texture, while all other samples have a texture. The strain values e are small and negative (compressive stress) and equal to −0.1 % for films with t less than 100 nm, it decreases (in absolute value) to −0.07 % and then remains constant for t greater than 100 nm. The grain size values D are found to be between 44 and 73 nm. The grain size decreases with increasing deposition rate regardless of t. Hysteresis curves, inferred from the vibrating sample magnetometer (VSM), show an in-plane magnetic anisotropy for all samples. The squareness S increases and the coercive field H C decreases with increasing D up to D = 55 nm, then they remain constant beyond this grain size value. Higher deposition rates led to smaller grain, smaller remnant magnetization, and higher coercive field. For t = 99 nm, the decrease of the temperature T from room temperature to −130 °C led to 20 and 5 % increases in H C and S, respectively.

Growth and Characterization of a New Lead Lutetium Niobate with Lead Ti-tanate Ferroelectric Crystal

A series of new lead lutetium niobate with lead titanate (1-x)Pb(Lu1/2Nb1/2)O3-xPbTiO3 (PLN-xPT) ferroelec- tric crystals were grown successfully by the top-seed solution growth technique. Structural analysis and electrical proper- ties measurement showed that the crystals have high Curie temperatures and each composition had different performances. The temperature dependences of permittivity showed typical relaxor behavior in the rhombohedral phase region. Based on the measurement result, a solid state phase diagram in the low temperature range was established, which revealed a mor- photropic phase boundary (MPB) in the composition of x = 0.49 to 0.51. Electric domains on (001) surface were observed by the polarizing microscope, in agreement with the X-ray diffraction data. Those compositions within MPB exhibited good piezoelectric properties, such as high coercive field Ec (>10 kV/cm) and high rhombohedral-tetragonal phase transition temperature TRT (> 200 ℃). For PLN-0.49PT, it had a high Curie temperature Tc = 360 ℃ and a piezoelectric coefficient of d33 > 1600 pC/N. High Curie temperature and excellent piezoelectric property make this material a promising candidate

3D distribution of magnetic CoNi alloy nanoparticles electrodeposited on vertically aligned MWCNT showing exceptional coercive field

A novel, fast and simple method to achieve a 3D distribution of CoNi nanoparticles (NPs) by electrochemical deposition on vertically aligned multiwall carbon nanotubes is described. Alignment is maintained during the electrodeposition. This method allows to easily obtain magnetic anchored, no aggregation, nanoparticles spatially distributed almost homogenously in a thick 3D structure (10µm). Potential and charge of deposition condition size, composition and crystalline structure of the CoNi NPs and then the magnetic properties of the NPs/CNTs structures. The structures with hcp CoNi NPs of around 70nm obtained at low deposition potentials present an exceptional high coercive magnetic field (higher than 1kOe), never previously found for CoNi NPs. Structures with hcp+fcc CoNi NPs present lower coercivity, which corresponds to the lower magnetic anisotropy of the fcc crystalline phase.

Three-dimensional molecular-based magnet of octacyanometalate-based tungstate(V)-cobalt(II) bimetallic assembly with TC = 26 K

A three-dimensional (3D) octacyanotungstate(V)-based molecular based magnet containing Co(II) ions, {[CoII4(H2O)6(C2O4)][WV(CN)8]2·6H2O}n (1), has been synthesized in acidic solution and structurally characterized. X-ray determination indicates that in 1 each [W(CN)8]3− structural unit is linked by seven neighboring Co(II) ions forming 2D Co–W–CN inorganic connectivity layers. Further short conjugated oxalate bridges join these neighboring 2D layers ultimately forming the 3D framework. Magnetic investigation of complex 1 reveals that long-range ferromagnetic ordering occurs below the Curie temperature of 26K. The beautiful butterfly-shaped magnetic hysteresis loop at 10K between −70KOe and 70KOe exhibits a rather high coercive field (HC=3010Oe). The remnant magnetization Mr is equal to 3055cm3mol−1. To our best knowledge, complex 1 is the first example of 3D cyano-based magnets bridged by C2O42− anions.

PEG-assisted hydrothermal synthesis and characterization of Co0.1Zn0.9O DMS nanoparticles

Abstract Nanoparticles of Co 0.1 Zn 0.9 O were successfully synthesized by a simple polyethylene-glycol (PEG)-assisted hydrothermal method. A systematic investigation was done to determine the structural, morphological and magnetic properties of the as synthesized sample and XRD, FE-SEM and EDX measurements were conducted respectively for the structural, morphological and compositional investigation of the product. For the magnetic property investigations VSM was used. Additionally FMR was further used to confirm the room temperature ferromagnetism of the sample. Average particle size of the nanoparticles was estimated using Debye-Scherer׳s equation and found as 23.5 nm. SEM image of the nanoparticles confirms that the size of the nanoparticles vary between 20 nm and 100 nm with small aggregation. Magnetization measurements have shown that the particles have room temperature ferromagnetic behavior with relatively high coercive fields which are decreasing with the temperature as expected. FMR spectrum also confirms the room temperature ferromagnetism of the product without any metallic precipitates.

Investigation of phase formation of (Zn, Mg)0.5Co0.5Fe2O4 nanoferrites

Zn 0.5 Co 0.5 Fe 2 O 4 and Mg 0.5 Co 0.5 Fe 2 O 4 nanoparticles have been prepared through high energy ball milling of single phase mixtures of ZnFe 2 O 4 and CoFe 2 O 4 or MgFe 2 O 4 and CoFe 2 O 4 nanosized oxides. The Mössbauer spectra of the milled products ZnFe 2 O 4 /CoFe 2 O 4 and MgFe 2 O 4 /CoFe 2 O 4 reveal the presence of Zn 0.5 Co 0.5 Fe 2 O 4 and Mg 0.5 Co 0.5 Fe 2 O 4 , respectively. In an alternative route, Zn 0.5 Co 0.5 Fe 2 O 4 and Mg 0.5 Co 0.5 Fe 2 O 4 compounds were prepared directly from metal chloride solutions through glycol-thermal reaction. The coercive fields of the Zn 0.5 Co 0.5 Fe 2 O 4 and Mg 0.5 Co 0.5 Fe 2 O 4 oxides produced by glycol-thermal reaction were found to be about 100 Oe and 161 Oe, respectively. Larger coercive fields of about 184 Oe and 255 Oe have been observed for the milled ZnFe 2 O 4 /CoFe 2 O 4 and MgFe 2 O 4 /CoFe 2 O 4 oxides. • Nanosized Zn 0.5 Co 0.5 Fe 2 O 4 and Mg 0.5 Co 0.5 Fe 2 O 4 compounds have been successfully synthesized from single phase mixtures of ZnFe 2 O 4 /CoFe 2 O 4 and MgFe 2 O 4 /CoFe 2 O 4 respectively. • The Mössbauer results for samples prepared in this technique and glycol-thermal reaction are comparable. • Higher coercive fields have been observed for compounds prepared by ball milling technique.

Experimental evidence of enhanced ferroelectricity in Ca doped BiFeO3

Calcium (Ca)-doped bismuth ferrite (BiFeO3) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), polarization and piezoelectric measurements. Structural studies by XRD and TEM reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO3 where enhanced ferroelectric and piezoelectric properties are produced by internal strain. Resistive switching is observed in BFO and Ca-doped BFO which are affected by the barrier contact and work function of multiferroic materials and Pt electrodes. A high coercive field in the hysteresis loop is observed for the BiFeO3 film. Piezoelectric properties 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. This observation introduces magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom which are already present in the multiferroic BiFeO3.

Switching fields of high-resolution magnetic force microscope tips coated with Co, Co75Pt10Cr15, Co75Pt25, and Co50Pt50films

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 4 nm radius with Co, Co75Pt10Cr15, Co75Pt25, and Co50Pt50 (at. %) films of 20 nm thickness at 300 °C. The effects of coating film material on the spatial resolution and the switching field are investigated. Higher resolutions are observed in the order of Co75Pt10Cr15 < (Co50Pt50, Co75Pt25) < Co. The Co-coated tip shows the highest resolution of 7.3 nm, which seems to be depending on a high detection sensitivity related with the magnetic moment of Co material. The saturation magnetization increases in the order of Co75Pt10Cr15 < Co50Pt50 < Co75Pt25 < Co. Higher switching fields are observed in the order of Co < Co75Pt10Cr15 < Co75Pt25 < Co50Pt50. The Co50Pt50-coated tip shows the highest switching field of 1.675±0.025 kOe, which is due to a high coercive field of the magnetic film involving L11 ordered phase with high magnetocrystalline anisotropy energy. The coercive field is recognized in the order of Co < Co75Pt10Cr15 < Co75Pt25 < Co50Pt50. A tip prepared by coating Co50Pt50 film which has high resolution and high switching field is useful for MFM observations of high-density recording media and permanent magnets.

The growth status of relaxor-based ferroelectric crystals and their development trends

Our progress in the growth of ternary system Pb ( In 1/2 Nb 1/2) O 3– Pb ( Mg 1/3 Nb 2/3) O 3– PbTiO 3 (PIMNT) crystals and reviews on the possible developing trends in the piezoelectric crystals are presented. Large PIMNT crystals with 3 inches in diameter have been grown by using a modified Bridgman method. Meanwhile, Mn -doped PIMNT crystals have also been obtained. PIMNT crystals exhibit Curie temperature Tc ~ 190° C , rhombohedral–tetragonal phase transition temperature T rt &gt; 100° C and coercive field Ec ~ 5.0–7.0 kV/cm on (001) cuts. Meanwhile, higher piezoelectric constant d32 ~ -1500–2200 pC/N , d15 ~ 4000–6000 pC/N and coercive field Ec ~ 7.0–9.0 kV/cm have been achieved on (110) cuts. The development trends of relaxor-based ferroelectric crystals are suggested to grow novel crystals with high rhombohedral–tetragonal phase transition temperature T rt &gt; 150 ~ 160° C and/or high coercive field Ec and mechanical quality factor Qm, to promote growth efficiency of PIMNT crystals and to create new growth methods fitting for the growth of metastable or instable crystals.

Effects of Heat Treatment after Poling on Dielectric and Piezoelectric Properties in Li0.06Na0.94NbO3 Ceramics

The Influence of the heat treatment on the electrical properties in Li0.06Na0.94NbO3 (LNN6) lead-free ceramics was investigated. LNN6 poled samples showed an electromechanical coupling factor, kp, of 0.19–0.20 and a mechanical quality factor, Qm, of 800–900. By heat treatment at 320 °C for 10 h, kp hardly changed, but Qm markedly increased from 850 to 2890. The increase in Qm that depended on the heat treatment temperature and a higher Qm were obtained at 300 and 320 °C, which are near phase transition temperatures. The dielectric loss tan δ at a low frequency decreased with heat treatment. The poling field dependence of piezoelectric properties showed that the heat-treated samples have higher coercive field and internal bias. These results revealed that the heat treatment affects piezoelectric hardening for LNN6 ceramics.