Constricted Hysteresis Loops Research Articles

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

On the anhysteretic magnetization of soft magnetic materials

Electrical steels are still the materials of choice for large-scale transformers and most electric motors. Yet, they may present a nonhomogeneous magnetic nature which prevents describing accurately their anhysteretic magnetization with the Langevin-Weiss model. Although interpolation and extrapolation methods may be used to model any anhysteretic curve, a simple and physically-based model would be of great value for fundamental and applied research. Inspired in the law of partial volumes for gas mixtures, we proposed a law of partial magnetizations for magnetic mixtures. In a two-component system, the model leads to the double Langevin-Weiss function. We also introduced a graphical method and a fitting approach to analyze and model anhysteretic magnetization curves. A semi-log magnetization derivative plot is central to this end. We validated our strategy through well-motivated examples using published data on soft magnets. The single Langevin-Weiss function provided an accurate description of the magnetization of isotropic and anisotropic magnetically homogeneous materials: a soft ferrite and a nanocrystalline alloy, respectively. For modelling a magnetization transverse to the material’s preferred direction, the key is to allow a negative molecular field constant. The double Langevin-Weiss function was suitable for less homogeneous materials, such as a grain-oriented electrical steel magnetized along the rolling direction and a non-oriented electrical steel. Moreover, a highly-grain-oriented electrical steel magnetized transverse to the rolling direction, which exhibits a constricted hysteresis loop, could be modeled in excellent agreement with data. The key for the latter, has been to allow an antiparallel arrangement of the mean magnetization of both components.

Improved energy storage capacity in strontium and manganese co-doped 0.925(Bi1/2Na1/2)TiO3-0.075BaTiO3 ceramics

Inducing slimmer P-E loops while maintaining high ferroelectric polarization is a major challenge in dielectric capacitor applications. In this work, strontium (Sr) doped and strontium-manganese (Sr, Mn) co-doped 0.925(Bi1/2Na1/2)TiO3-0.075BaTiO3 (BNT7.5BT) ceramics were prepared to study energy storage performance. Sr-doped BNT7.5BT samples showed enhanced relaxor ferroelectric behavior and reduced remanent polarization (Pr) and coercive field (Ec) which can be attributed to the disruption of ferroelectric long-range order by the presence of non-polar cubic Pm3-m phase. (Sr, Mn) co-doped BNT7.5BT samples revealed improved energy storage density of 0.90 J/cm3 and thermal stability, affirming the vital role of Mn ions. The presence of MnTi″-VOoo defect-dipoles can create a dipole moment (PD) which can result in an internal field causing a constricted ferroelectric hysteresis loop with low Pr and Ec. Surface screening of polarization bound charges by Mn-induced defects affects the overall Pmax contribution to energy storage density.

Magnetic hysteresis of 0.6–110 μm magnetites across the Verwey transition

We report saturation magnetization, Ms, saturation remanence, Mrs, coercive force, Hc, and remanence coercivity, Hcr, as a function of grain size, d, and temperature, T, for 0.6–135 μm magnetites. Five annealed and four unannealed samples were measured at 5–10 K intervals from 300 to 20 K. Mrs and Hc increase by factors of 1.5–4 in cooling through the Verwey transition (TV ≈ 120 K) and by smaller amounts around 50 K. Hysteresis properties change continuously over ≈20 K below TV or for annealed 0.6, 3, and 6 μm grains, within ≈10 K below TV. Hc(d) changes for annealed magnetites from ∼d–0.5 at 300 K to ∼d–0.6–d–0.7 at 120–130 K to ∼d–0.3 at 80–100 K. Day plots of Mrs(T)/Ms(T) versus Hcr(T)/Hc(T) indicate major domain structure changes with T, e.g., 6 μm grains change from large pseudo-single-domain (PSD) at 300 K to multidomain (MD) just above TV and return to PSD below TV, evolving to higher Mrs and Hc down to 20 K. Hysteresis loops change from normal at 300 K to slightly constricted near TV to severely constricted below 50 K. We interpret these results in the light of electron microscopic observations by Kasama et al. (2010 , 2012) . Hardening of magnetic hysteresis below TV and the evolution from MD to PSD, and even to single-domain in the finest grains, results from subdivision of grains by monoclinic twinning, reduced magnetic domain sizes in monoclinic magnetite, and confinement of magnetic domains within twin domains. Constricted hysteresis loops indicate coexisting magnetically hard and soft phases, initially growing monoclinic regions and residual cubic magnetite.

Influence of K0.5Bi0.5TiO3 on the structure, dielectric and ferroelectric properties of (Ba,Ca)(Zr,Ti)O3 ceramics

A new lead-free ferroelectric solid solution between (Ba,Ca)(Zr,Ti)O3 (BCZT) and K0.5Bi0.5TiO3 (KBT) has been systematically investigated in terms of its phase transformations, microstructure, dielectric and ferroelectric properties. The incorporation of KBT into BCZT was found to enhance the sintering behavior, although secondary phases of K4Ti3O8 and BaBi4Ti4O15 were detected at high KBT contents. Chemical heterogeneity was also observed in the form of core-shell grain structures comprising tetragonal ferroelectric BCZT-rich cores with pseudo-cubic relaxor ferroelectric KBT-rich shell regions. Temperature-dependent dielectric property measurements revealed that the BCZT-KBT ceramics exhibited both normal and relaxor ferroelectric behaviour simultaneously, associated directly with the core-shell structure. Ferroelectric hysteresis measurements indicated that the remanent polarisation and coercive field were strongly dependent on KBT content. In common with other lead-free relaxor ferroelectrics, increasing temperature led to the formation of constricted polarisation-electric field hysteresis loops, indicating the occurrence of a reversible electric field-induced nanopolar to long-range ordered ferroelectric state.

Effect of Mn-doping on the structure and electric properties of 0.64Pb(In0.5Nb0.5)O3-0.36PbTiO3 ceramics

In order to study the effect of defect dipoles on relaxor-based ferroelectrics, 0.64Pb(In0.5Nb0.5)O3-0.36Pb(Ti1−xMnx)O3 (x=0−0.15) ferroelectric ceramics were synthesized using the two-step solid state reaction method. The phase structure, ferroelectric, piezoelectric and dielectric properties were investigated. Upon increasing the Mn content, the phase of the ceramics transforms from tetragonal to rhombohedral symmetry, the magnitude of remnant polarization and piezoelectric coefficient decreasing because of the hardening effect of the defect dipoles and the variation of composition. The coercive field, however, decreases from 22kV/cm to 12.5kV/cm with x≤0.04 initially owing to a phase transition, and then increasing since the defect dipoles provided a driving and stabilizing force for domain stability. The stabilizing force also resulted in constricted hysteresis loops for samples with composition of x≥0.04 after aging. The temperature dependence of dielectric properties showed that the Curie temperature decreased from 312°C (x=0) to 237°C (x=0.15), and the degree of relaxor behavior was enhanced with increase of Mn content. It indicates that the defect dipoles, confirmed by the existence of oxygen vacancies, play an important role in the electrical properties of Mn-doping PIN-PT ceramics.

Magnetic properties and magnetic domain structure of grain-oriented Fe-3%Si steel under compression

The influence of an applied compressive stress on the magnetic properties and domain structure in Goss-textured (110) [001] Fe-3%Si steel is studied. The magnetic domains and magnetization processes were observed by longitudinal Kerr microscopy at different levels of compressive stress. With stress increase the domain structure without applied field evolves from 180° slab-like domains along the surface-parallel easy axis first into stress pattern I, then into the checkerboard pattern and finally into stress pattern II, in which all internal domains are oriented along the transverse axes. The magnetization process under compression is realized by surface closure [001] domains that grow into the bulk at the expense of transverse domains. The domain evolution by these three stress patterns is not practically noticeable in hysteresis curves above 10 MPa—they change continuously with the same effective field being valid for curves from 10 to 67 MPa. The comparison with previous measurements under different stress/cutting angle combinations shows that for the prediction of a constricted hysteresis loop it is sufficient to consider the energy difference between surface-parallel and transverse easy axes neglecting details of the spatial organization of transverse domains.

The effect of the bipolar field on the aging behavior and the associated properties of the Mn-doped BaTiO3 ceramics

In this work, the influence of the typical bipolar electric field on the aging behavior, rather than the normally time evolution aging, of the Mn-doped BaTiO3 ceramics and the associated properties were studied. Our results show the obviously different influences of the bipolar filed when applied in the different periods of the time-aged process: for an unaged sample, a 100ms bipolar pre-field assists to develop a displaced hysteresis loop and a high d33 of 170pC/N which is nearly 65% d33 of the sample with the dc poling in the same electric amplitude of 1.0kV/mm for 30min; more importantly, it assists the sample shortly aged 10h to develop a well-constricted hysteresis loop and a large recoverable electro-strain of nearly 0.23% under 3.0kV/mm field, which is about 20% larger strain than the sample aged 3days under the same test condition. Contrarily, when the sample well aged 3days, the pre-field shows only the de-aging effect, it released the constricted hysteresis loop. The above results thus indicate a possible way to accelerate aging process and enhance the associated properties by pre-load bipolar field. A microscopic understanding was discussed from the viewpoint of the influence of a pre-load bipolar field on the redistribution of the defects and thus the stabilization of domain configuration in different aging periods.

Defect dipoles-driven ferroelectric behavior in potassium sodium niobate ceramics

K0.48Na0.52NbO3-0.05mol% Re2O3 (Re=La, Nd, and Yb) ceramics were prepared by a conventional solid-state sintering method, and defect dipoles-induced ferroelectric behaviors have been investigated. All ceramics show an orthorhombic phase at room temperature. The tolerance factor, X-ray diffraction and Raman spectra analysis show that La3+ and Nd3+ predominantly dissolve in (K, Na) sites and induces the unit cell shrinkage, while Yb3+ occupies both (K, Na) and Nb cationic sites and causes the unit cell expansion. Due to defect dipoles polarization, PDA and PDAB, KNN–Re2O3 ceramics display different ferroelectric properties. Ferroelectric order of KNN–Yb2O3 ceramics were stabilized by PDAB, so their ferroelectric properties (Pr~32.7μC/cm2, Ec~7.6kV/cm) and thermal stability were improved. A constricted hysteresis loop in KNN–Yb2O3 ceramic at a high measurement temperature is formed because of the domain pinning from Nb5+, K+ and Na+ ions.

Recent Progress on Defect Dipoles Characteristics in Piezoelectric Materials

Defect dipoles which have pronounced effects on properties of piezoelectric materials can be introduced either by partially modifying with acceptor impurities or volatizing of cation elements in perovskite lattice during sintering. The formation mechanism of defect dipoles and responses under external fields were reviewed in the present paper.Besides,the main physic mechanisms of ferroelectric aging,hysteresis loop abnormalities and electroshapememory effects in piezoelectric materials were discussed in detail from the defect dipoles viewpoints.Based on a defect mediated reversible domain switching mechanism,large recoverable electro-strain,as well as constricted hysteresis loops were obtained in unpoled aged ferroelectric crystals and/or ceramics,which indicated that aging was not always a detrimental effect,and may be promising for future novel applications.Furthermore,the defect dipole relaxation process was briefly described and the development trends of the high reliability piezoelectric actuators were proposed.

Constricted double loop hysteresis and exchange bias attributed to the surface anisotropy in nanocrystalline La1/3Sr2/3Fe1−xCrxO3

We investigate magnetic properties of nanocrystalline ferrites with composition La1/3Sr2/3Fe1−xCrxO3 (LSFCO) for x=0, 0.02, 0.04 and 0.06. Thermal variation of zero field cooled magnetization displays a weak signature close to the charge (TCO) as well as antiferromagnetic ordering for x=0, 0.02 and 0.04. In addition, another disordered glassy magnetic transition (Tg) is noticed for all the compositions. Tg is predominantly observed, although signature of charge ordering almost disappears for x=0.06. Interestingly, a constricted double loop type magnetic hysteresis loop is observed for x=0.02. This structure in the hysteresis loop disappears with further increase in Cr substitution. A systematic shift in the magnetic hysteresis loop is observed for all the compositions, while samples are cooled in a static magnetic field, which is the manifestation of exchange bias (EB) effect. The EB field (HE) and magnetization (ME) decrease remarkably due to minor Cr substitution at x=0.02 and then it reveals a sluggish increase with increasing x. For x=0.04 the EB effect emerges below ∼TCO, increases sluggishly with lowering of temperature and below ∼Tg, it increases rapidly. The cooling field dependence exhibits significant increase of HE and ME for x=0.04, which is associated with the considerable increase of coercivity. Possible origin of EB correlated with the magnetic phase coexistence has been argued for nanocrystalline LSFCO.

Dipole spring ferroelectrics in superlattice SrTiO3/BaTiO3 thin films exhibiting constricted hysteresis loops

Ferroelectric superlattice heterostructures have recently been explored for potential applications in electronic devices. In this letter, we employed the phase-field approach to simulate the domain structure and switching of a (BaTiO3)8/(SrTiO3)3 superlattice film constrained by a GdScO3 substrate. A constricted ferroelectric hysteresis loop was observed with a high saturation polarization but a small coercive field. The shape of the hysteresis loop is understood by analyzing the ferroelectric polarization distributions during switching. It is demonstrated that the multilayers stack behaves as dipole spring ferroelectric, named in analogy to exchange spring magnets in magnetic multilayers that show similar loops.

Glassy ferromagnetism and phase separation in Pr0.5Ca0.5CoO3

The dc and ac susceptibilities, magnetization curves and long-time relaxation were measured on a stoichiometric polycrystalline sample of Pr0.5Ca0.5CoO3. The compound shows a metal-insulator transition at TM-I=76 K, associated with combined spin-state and valency transformation of cobalt ions. Below 10 K the onset of a glassy ferromagnetism (Tf≈6.5 K) is observed, characterized by a frequency dependent susceptibility, S-type character of the virgin curve and ageing effect. An anomalously constricted hysteresis loop at T=Tf accompanied by a minimum in the temperature dependence of the coercivity was ascribed to an interaction between the glassy ferromagnetic and additional minor FM phase.

Origin of the constricted hysteresis loop in cobalt ferrites revisited

A series of Co ferrites (Co x Fe 3− x O 4 ( x=0−1)) were prepared using solid-state method in this work. The aging effect of their structures and constrictions of hysteresis loops under low magnetic field were investigated. It was found that during the aging process, the migration of trivalent (bivalent) ions between tetrahedral (A-site) and octahedral (B-site) coordination induced a shrinking of the lattice, which would expand again due to the precipitation of Fe 3+ after a much longer aging time. The first process caused a pronounced constriction of the loops, due to the uniaxial anisotropy led by this migration. The depression of constriction could attribute to both the expansion of lattice and the change of ionic ratios as a result of the second-phase-precipitation. The impacts of Co content, aging time and temperature upon the constriction were also discussed.

Double hysteresis loop induced by defect dipoles in ferroelectric Pb(Zr0.8Ti0.2)O3 thin films

Pb(Zr0.8Ti0.2)O3 (PZT80/20) thin films were deposited on the Pt(111)/Ti/SiO2/Si(100) substrates by RF magnetron sputtering. Mainly perovskite crystalline phase with highly (202)-preferred orientation, determined by x-ray diffraction, was formed in the lead zirconate titanate (PZT)(80/20) thin films. Polarization measurements of the unannealed and aged films showed a clear double hysteresis loop. However, the double hysteresis loop phenomenon was greatly suppressed in the PZT thin films annealed under pure oxygen, and thus they exhibited larger remnant polarization (Pr = 6.3 μC/cm2). The related mechanism for the appearance of constricted and double hysteresis loops was investigated to be associated with the realignment and disassociation of defect dipoles via oxygen octahedral rotations or oxygen vacancy diffusion. The butterfly-shaped C-V characteristic curve with a valley gave further evidence for double hysteresis loop characteristic in the unannealed and aged PZT thin films.

Magnetic behavior of nanocrystalline CoFe 2O 4

Magnetic nanoparticles of CoFe 2O 4 have been synthesized under an applied magnetic field through a co-precipitation method followed by thermal treatments at different temperatures, producing nanoparticles of varying size. The magnetic behavior of these nanoparticles was investigated. As-grown nanoparticles demonstrate superparamagnetism above the blocking temperature, which is dependent on the particle size. One of the nanoparticles demonstrated a constricted magnetic hysteresis loop with no or small coercivity and remanence at low magnetic field. However, the loop opens up at high magnetic field. This magnetic behavior is attributed to the preferred Co ions and vacancies arrangements when the CoFe 2O 4 nanoparticles were synthesized under an applied magnetic field. Furthermore, this magnetic property is strongly dependent on the high temperature heat treatments that produce Co ions and vacancies disorder.

Constricted hysteresis loops in Fe and Ni single crystals

Magnetic hysteresis loops reflect the variety of magnetic domain structures and have been considered to have normal rectangular or leaf-like shapes in standard ferromagnets such as Fe and Ni metals. We report on observations of constricted hysteresis loops in Fe and Ni single crystals with very low defect densities. The constricted loops were observed below T=150 K and in a medium temperature range from 150 to 430 K in Fe and Ni single crystals, respectively. These constricted loops disappear by weak plastic deformation for both single crystals. The origin of constricted hysteresis loops was explained by eddy current effects under less domain wall pinning due to dislocations.

Physical Properties and Electrical Aging Effects in Fe3+/Nb5+ Hybrid-Doped Barium Titanate Ceramics

It is well known that acceptor and donor doping results in opposite ferroelectric aging effects; however, the aging effects in hybrid-doped (acceptor+donor) materials has remained mysterious. Thus, the aging behavior of dielectric and ferroelectric properties in Fe 3+ /Nb 5+ hybrid-doped BaTiO 3 ceramics was investigated in this study. The concentration of acceptor dopant (Fe 3 + ) was fixed at 1 mol%, while that of donor dopant (Nb 5+ ) was varied from 0.5–2.0 mol%. The SEM and XRD techniques were used to study the microstructure and phase formation, respectively. The results showed that the dielectric properties of all de-aged samples decreased exponentially with time. The aged samples with 0.5 and 1.0 mol% Nb 5+ obviously showed constricted hysteresis loops, while those with higher concentration of Nb 5+ (1.5–2.0 mol%) showed normal hysteresis loops. The explanations based on the effects of defect dipole were discussed.

Dipolar chains formed by chemically synthesized cobalt nanocubes

Cobalt nanocubes with an average edge length of 50 nm and epsilon crystalline structure were synthesized via thermo-decomposition in 1,2-dichlorobenzene at 120 °C in the presence of surfactants. These nanocubes form dipolar chains upon drying in zero applied field and bundles of chains along the direction of an applied magnetic field. The magnetic measurements reveal strong interparticle couplings among the nanocubes in their dried magnetic-field-induced assemblies. The constricted hysteresis loops and near-zero coercivity indicate the existence of vortex states in the assemblies. Exposure to electron beam heats up the nanocubes and turns the dipolar chains into nanowires.

Synthesis and Magnetic Properties of Silica-Coated FePt Nanocrystals

Colloidal FePt nanocrystals, 6 nm in diameter, were synthesized and then coated with silica (SiO2) shells. The silica shell thickness could be varied from 10 to 25 nm. As-made FePt@SiO2 nanocrystals have low magnetocrystalline anisotropy due to a compositionally disordered FePt core. When films of FePt@SiO2 particles are annealed under hydrogen at 650 degrees C or above, the FePt core transforms to the compositionally ordered L1(0) phase, and superparamagnetic blocking temperatures exceeding room temperature are obtained. The SiO2 shell prevents FePt coalescence at annealing temperatures up to approximately 850 degrees C. Annealing under air or nitrogen does not induce the FePt phase transition. The silica shell limits magnetic dipole coupling between the FePt nanocrystals; however, low temperature (5 K) and room temperature magnetization scans show slightly constricted hysteresis loops with coercivities that decrease systematically with decreased shell thickness, possibly resulting from differences in magnetic dipole coupling between particles.