Dc Field Dependence Research Articles

Phase transition and energy storage performance in Ba-doped PLZST antiferroelectric ceramics

(Pb0.97−xBaxLa0.02)(Zr0.7Sn0.27Ti0.03) (0 < x < 0.08) antiferroelectric (AFE) ceramics were successfully fabricated by a solid state reaction, and the effect of barium (Ba) additions and temperature on the dielectric properties and energy storage performance were investigated. The ceramics with lower Ba content undergo two phase transitions during heating from room temperature to 300 °C: orthorhombic (O)-rhombohedral (R)-cubic (C). With the increase of Ba content, dielectric constants increased and transition temperature decreased obviously. The ferroelectric phase was induced as the composition x increased from 0 to 0.08, however, which was not stable and transformed into AFE state upon heating, and then paraelectric phase, which was confirmed by DC field dependence of dielectric constant. The polarization sharply increased from 9.7 μC/cm2 at 20 °C to 24. 6μC/cm2 at 100 °C in (Pb0.89Ba0.08La0.02)(Zr0.7Sn0.27Ti0.03) ceramic. As a result, the maximum recovered energy density of 2.1 J/cm3 was obtained at 80 °C, and the corresponding energy-storage efficiency was 76.5 %, which made this material a promising potential application in capacitors for pulsed power systems.

Weak ferromagnetism and temperature dependent dielectric properties of Zn0.9Ni0.1O diluted magnetic semiconductor

In this study the room temperature ferromagnetic behaviour and dielectric properties of ZnO based diluted magnetic semiconductor (DMS) have been investigated using nominal chemical composition Zn0.9Ni0.1O. The X-ray diffraction analysis confirmed formation of single phase hexagonal wurtzite structure. An increase in grain size with increasing sintering temperature was observed from scanning electron microscopy. Field dependent DC magnetization values indicated dominant paramagnetic ordering along with a slight ferromagnetic behaviour at room temperature. Frequency dependent complex initial permeability showed some positive values around 12 at room temperature. In dielectric measurement, an increasing trend of complex permittivity, loss tangent and ac conductivity with increasing temperature were observed. The temperature dependent dispersion curves of dielectric properties revealed clear relaxation at higher temperature. Frequency dependent ac conductivity was found to increase with frequency whereas complex permittivity and loss tangent showed an opposite trend.

Controlling the magnetic-field sensitivity of atomic-clock states by microwave dressing

We demonstrate control of the differential Zeeman shift between clock states of ultracold rubidium atoms by means of non-resonant microwave dressing. Using the dc-field dependence of the microwave detuning, we suppress the first and second order differential Zeeman shift in magnetically trapped $^{87}$Rb atoms. By dressing the state pair 5S$_{1/2} F= 1, m_F = -1$ and $F= 2, m_F = 1$, a residual frequency spread of <0.1 Hz in a range of 100 mG around a chosen magnetic offset field can be achieved. This is one order of magnitude smaller than the shift of the bare states at the magic field of the Breit-Rabi parabola. We further identify double magic points, around which the clock frequency is insensitive to fluctuations both in the magnetic field and the dressing Rabi frequency. The technique is compatible with chip-based cold atom systems and allows the creation of clock and qubit states with reduced sensitivity to magnetic field noise.

Design and performance analysis of three superconducting magnetic sensors for the measurement of small fields

We analyse here the sensing performance of a magnetometer which is based on the linear dc-field dependence of the second harmonic signal generated by high-Tc polycrystalline superconductors. Experiments realized in a series of polycrystalline REBa2Cu3O7−δ (RE = Y, Sm and Nd) superconductors have shown that some parameters (e.g. Hc1J and H*) peculiar to the studied superconductors change depending on the RE ions, and those determine the sensitivity limit of the magnetometer. The lowest measurable magnetic fields are 0.1, 5 and 38 nT for RE = Sm, Y and Nd, respectively at the functioning temperature of 77 K. With an upper limit above the Earth's field the magnetometer has attracted the attention of manufacturers in the industry.

Effect of 200 MeV Ag+15 ion irradiation on magnetic and dielectric properties of nanocrystalline Zn–Cr ferrite

Nanocrystalline samples of ZnCr0.4Fe1.6O4 ferrite were synthesized by the advanced sol–gel method to investigate the effect of 200 MeV Ag+15 ion irradiation on the cation distribution, magnetic and dielectric properties. Rietveld profile refinement of the X-ray diffraction (XRD) patterns confirms the single-phase cubic spinel structure of the specimens. The irradiated sample retains the cubic spinel structure with a slight increase in the lattice parameters and the average crystallite size. Temperature- and field-dependent dc magnetization studies show an appreciable enhancement in the saturation magnetization and blocking temperature of the irradiated samples, which could be attributed to the slight increase in the particle size due to the heat evolved during irradiation. Subsequently, the rearrangement of cations in the lattice structure and the ion-induced modifications on the surface states of the nanoparticles could be accountable. The room temperature dielectric constant and the loss tangent in the frequency range 75 kHz–10 MHz revealed the normal frequency dispersion. The increase in ϵr and tan δ on irradiation could be attributed to the slight crystal growth and hence the availability of the sufficient number of Fe2+ and/or Zn3+ ions particularly at the octahedral site on the grain boundaries, showing a fair agreement with the magnetization results.

Improving the Sensitivity of Superconductor-Based DC Magnetometer

In this study we report on some critical factors affecting the sensitivity of a magnetic field sensor whose working principle is based on a linear DC field dependence of the second harmonic of the AC response in polycrystalline type-II superconductors. DC-fields down to 2 nT in magnitude could be detected by finding optimal conditions. The optimal sensor design was determined by studying superconductor cores having different geometries and coil configurations. The optimal AC-field frequency, which is required for excitation of the specimen, was found to be 50 kHz. The second harmonic signal changes quite linearly with the DC-field up to 0.1 Oe.

Size Dependence of Inter- and Intracluster Interactions in Core–Shell Iron–Iron Oxide Nanoclusters

The room-temperature magnetic properties of core–shell iron–iron oxide nanoclusters (NCs) synthesized by a cluster deposition system were investigated, and their dependence on mean cluster size is discussed. In this study, the surface/boundary spins of clusters were not frozen and were thermally activated during the measurements. The intercluster interactions between clusters and intracluster interactions between the iron core (ferromagnetic) and iron oxide shell (ferrimagnetic) were investigated by field-dependent isothermal remanent magnetization and dc demagnetization measurements at room temperature. The Henkel and ΔM plots support the existence of dipolar intercluster interactions that become stronger with the growth of the clusters. The derivative of the initial magnetization curve implies that smaller clusters require lower fields and less time than larger ones to overcome various energy barriers before saturating the moments along the field direction. Coercive field and magnetization were also correlated with the interaction parameters. To compare the room-temperature magnetic results, one system was studied at low temperature, where exchange coupling at the interface between the oxide and metallic phases was observed through bias effect and anisotropy enhancement.

Magnetic properties and magnetocaloric effect in intermetallic compounds NdMn2−xCoxSi2

We report magnetic properties and magnetocaloric effect in intermetallic compounds NdMn2−xCoxSi2 (x = 0.2, 0.4, 0.6, 0.8, and 1). dc magnetization study shows that these compounds undergo a ferromagnetic-like transition at around 45 K. Neutron diffraction study at 5 K for the x = 0.2 sample shows a canted-ferromagnetic state at lower temperature (5 K), and a collinear antiferromagnetic state at higher temperature (above ∼50 K). At 5 K, the Nd moments are aligned along the crystallographic c-axis and the Mn moments are canted to the c-axis. At higher temperatures (50, 100, 200, and 300 K), Nd sublattice does not order but Mn sublattice orders antiferromagnetically. A magnetocaloric effect (MCE) is found with a magnetic entropy change of 14.4 and 12.4 J kg−1 K−1 for the x = 0.2 and 0.4 samples, respectively, at 47.5 K under a field variation of 50 kOe. Various interesting phenomena such as metamagnetic transitions and domain wall pinning have been observed, and their role in obtaining a large MCE and an inverse MCE, respectively, has been brought out. The hysteresis (in magnetic field dependent dc magnetization study) reduces significantly at temperatures near and above the magnetic transition temperature (TC), which makes these materials important for their practical applications in magnetic refrigeration around TC.

Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11nm and nanodiscs of dimensions ∼2.5×15nm2 were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.

Relaxor characteristics of highly tunable lead-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 thin film grown on LaNiO3/Si by pulsed laser deposition

We report a comprehensive study on relaxor-like characteristics exhibited by (l00) oriented 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 thin film grown on LaNiO3 coated Si substrate by pulsed laser deposition. The diffuseness coefficient value close to 2 and excellent fit of Vogel–Fulcher curve to the experimental data, suggest its relaxor characteristics. The ‘butterfly’ shaped dc field dependence of permittivity and evolution of P–E hysteresis loop, at different temperatures, clearly indicate the occurrence of polar nanoregions in this material. The high tunability nr ∼ 70 and low temperature coefficient TCC = 5.18 × 10−4 K−1 show that the BCZT50 ceramics could be a promising candidate for tunable capacitor applications.

Intercluster Interaction and Magnetic Interaction between Iron Core and Iron Oxide Shell in Core-Shell Nanoclusters

ABSTRACTThe intercluster interactions of iron/iron oxide core shell nanoclusters have been investigated, and their dependence on cluster size (d) has been discussed. A cluster deposition system is used to prepare core-shell nanoclusters with different d, varying from 9 to 14 nm.Transmission Electron Microscopy has been used for physical characterization, and Vibrating Sample Magnetometer for magnetic study. The cluster – cluster interactions have been investigated by field dependent isothermal remanent magnetization (IRM) and dc demagnetization (DCD) measurements at room temperature. Henkel plot shows more negative deviation from non- interacting case for bigger size nanoclusters than smaller size clusters.

Synthesis of Ag–CoFe2O4 dimer colloidal nanoparticles and enhancement of their magnetic response

This paper reports the structural and magnetic properties of Ag–CoFe2O4 colloidal dimer nanoparticles (NPs) synthesized using a two-step solution-phase route. Ag NPs were used as seeds to grow Ag–CoFe2O4 dimer NPs using thermal decomposition of metallic precursor. By means of temperature and field dependent dc magnetization measurements, it is found that the silver due to its interface with CoFe2O4 particles leads to thermal stabilization of the dimer NPs superior as compared to CoFe2O4 alone. Our results show enhancement of the magnetic anisotropy and a large coercivity at 2 K for dimer NPs, which could be ascribed to interface effect between Ag and CoFe2O4 components and the related structural defects.

Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter 〈ϕ〉 = 15 nm have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ′) and out of phase (χ″) ac-susceptibility. The studies were performed in non-interacting and interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 and 17.6%. The ac-susceptibility of the non-interacting system, measured with HDC = 0 Oe, presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)–ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system shows two anomalies. One frequency independent peak associated to TC and a second low temperature maximum was observed in χ″, located at TP. The position of the TP maximum shifts to higher temperature when the frequency increases. The relation between the relaxation time and TP was well described by the Vogel–Fulcher law. When the susceptibility was measured with HDC = 20 kOe, the PM–FiM transition was observed in both systems. Remarkably, in the non-interacting system, the low temperature anomaly is evidenced by the magnetic field. This anomaly is present as a well-defined maximum, which shifts to higher temperature when the frequency increases.

Magnetic properties of geometrically frustrated polymorphic crystals of Cu4-xMgx(OH)6Cl2

Geometric frustration can suppress magnetic long-range order (LRO) in either ferromagnetic (FM) [1-3] or antiferromagnetic (AFM) systems with Kagomé structure. Crystals of Cu4-xMgx(OH)6Cl2, magnesium-containing paratacamite and herbertsmithite (x < 1 and x ~ 1, R3̄m) and haydeeite (x ~ 1, P3̄m1), have been grown by the hydrothermal technique and characterized by single crystal X-ray diffraction. The 2D S =1/2 polymorphs can be classified into the two types of magnets based on the susceptibility at high temperature. Temperature and dc field-dependence of magnetization of the magnesium-containing phase are quite similar to those of isostructural single crystals of Cu4-xZnx(OH)6Cl2 (R3̄m), zinc-containing paratacamite (or herbertsmithite). However, for haydeeite, though a ferromagnetic long range ordering at 20.6 K can be predicted by the Curie-Weiss law, the magnetic phase transition is absent until 4.3 K, indicating a high degree of frustration (|θ|/Tc ~ 5). Its ac frequency and dc field-dependence of spin freezing temperature are analogous to those of spin ice and/or spin glass.

Structural characteristics and dielectric properties of glass-ceramic nanocomposites of (Pb,Sr)Nb 2O 6-NaNbO 3-SiO 2

The structure and dielectric properties of (Pb,Sr)Nb 2O 6-NaNbO 3-SiO 2 glass-ceramics with different Pb and Sr contents were investigated. The XRD pattern of glass-ceramics without Sr substitution is different from that with Sr substitution, which indicates the existence of orthorhombic phase in the latter ones. TEM bright field observation shows nanosized microstructures, while for samples with Sr, typical eutectic microstructure with separated crystallized bands is found in the glass matrix. Dielectric properties measurement of the samples indicates an obvious improvement of dielectric constant, dielectric loss, DC field and temperature dependence of dielectric constant when the molar ratio of Sr to Pb is 4?6.

Comparison of inelastic and quasielastic scattering effects on nonlinear electron transport in quantum wires

When impurity and phonon scattering coexist, the Boltzmann equation has been solved accurately for nonlinear electron transport in a quantum wire. Based on the calculated nonequilibrium distribution of electrons in momentum space, the scattering effects on both the nondifferential (for a fixed dc field) and differential (for a fixed temperature) mobilities of electrons as functions of temperature and dc field have been demonstrated. The nondifferential mobility of electrons is switched from a linearly increasing function of temperature to a paraboliclike temperature dependence as the quantum wire is tuned from an impurity-dominated system to a phonon-dominated one, as described by Fang et al. [Phys. Rev. B 78, 205403 (2008)]. In addition, a maximum has been obtained in the dc field dependence of the differential mobility of electrons. The low-field differential mobility is dominated by the impurity scattering, whereas the high-field differential mobility is limited by the phonon scattering as described by Hauser et al. [Semicond. Sci. Technol. 9, 951 (1994)]. Once a quantum wire is dominated by quasielastic scattering, the peak of the momentum-space distribution function becomes sharpened and both tails of the equilibrium electron distribution centered at the Fermi edges are raised by the dc field after a redistribution of the electrons is fulfilled in a symmetric way in the low-field regime. If a quantum wire is dominated by inelastic scattering, on the other hand, the peak of the momentum-space distribution function is unchanged while both shoulders centered at the Fermi edges shift leftward correspondingly with increasing dc field through an asymmetric redistribution of the electrons even in low-field regime as described by Wirner et al. [Phys. Rev. Lett. 70, 2609 (1993)].

Slow Magnetic Relaxation in a High-Spin Iron(II) Complex

Slow magnetic relaxation is observed for [(tpa(Mes))Fe](-), a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = -39.6 cm(-1)) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of U(eff) = 42 cm(-1) and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at approximately 1500 Oe.

Surface Spin-glass Freezing and Blocking in NiFe2O4 Nanoparticles

AbstractWe prepared single-phase nickel ferrite nanoparticles separated by silicon dioxide using sol-gel method with tetraethyl orthosilicate (TEOS) as a precursor for SiO2. The magnetic properties are investigated by using SQUID-magnetometry over a broad temperature range (4.2 – 350 K), magnetic field (2–70,000 Oe) and frequency (0.1 – 1000 Hz) range. The particle size is in the range 8 – 12 nm. Exchange bias and spin disorder appear at the core-shell interface due to broken bonds on the surface. Disorder and core-shell interaction induces spin-glass freezing which is manifested by a low temperature peak in the AC susceptibility well separated from magnetic blocking peak. This low temperature peak is assigned to spin-glass freezing. The proof of spin-glass freezing is managed by zero field cooled/field cooled (ZFC/FC), frequency and DC field dependence of AC susceptibility, low temperature hysteresis loop and time dependent thermoremanent magnetization at different temperatures. All the measurements stated above signify blocking/unblocking at higher temperatures and surface spin-glass freezing at low temperatures. The aim of our work is to contribute to a better understanding of “spin-frozen” magnetic ferrite nanoparticles at diameters 8 – 12 nm which could be important in future for stabilizing the magnetic state of “core-shell”-structured nanomagnets.

AC loss study by AC susceptibility measurement in a commercial Bi:2223/Ag tapeprepared by the CTFF method: axis and short specimen

The effect of temperature, field amplitude, frequency and orientation of the field on AClosses in a commercial Bi:2223/Ag tape prepared by the continuous filling and forming(CTFF) method has been investigated. The measurements were performed using an ACsusceptometer. The experimental results show that the losses depend on field amplitude,temperature, DC field, orientation of the sample and strongly on frequency. Due to thestrong dependence of AC losses on frequency, measurements were pursued at severalfrequencies between 20 Hz and 10 kHz at various temperatures (10, 20, 30 and 77 K). Thestrong frequency dependence at these low temperatures is attributed to eddycurrent losses and/or inter-filamentary coupling losses. At higher temperatures andlow frequencies, the main contribution to AC losses comes from hysteresis lossesdue to the flux entry into and exit from the superconducting tape during the upand down swings of the magnetic field. We have calculated coupling losses as afunction of frequency and temperature-dependent time constant. Experimentaldata are analyzed by comparing them with the calculated curves. From the DCfield dependence of the AC susceptibility, we obtained the irreversibility line.

Slow Magnetic Relaxation in a Trigonal Prismatic Uranium(III) Complex

Results of ac magnetic susceptibility measurements performed on the trigonal prismatic complex U(Ph(2)BPz(2))(3) demonstrate the presence of slow magnetic relaxation under zero applied dc field. Analysis of both the temperature and frequency dependence of the ac susceptibility indicate a temperature regime (T > approximately 3 K) where Arrhenius behavior dominates the relaxation processes, leading to a spin relaxation barrier of U(eff) = 20 cm(-1). The dc-field dependence of the relaxation time was studied to reveal evidence of quantum tunneling processes occurring at lower temperatures. The results suggest a general strategy for identifying further uranium(III)-based single-molecule magnets by concentrating the ligand-field contributions above and below the equatorial plane of an axially symmetric coordination complex.