Changes In Magnetic Structure Research Articles

Fe and Co NMR studies of magnetoelectric Co2 Y-type hexaferrite BSCFAO

The Fe3+ and Co2+ NMR spectra for Ba0.3Sr1.7Co2(Fe0.96Al0.04)12O22 (BSCFAO) and Ba0.3Sr1.7Co2Fe12O22 (BSCFO) were obtained in a zero magnetic field at a low temperature. We observed change in the enhancement effect of the NMR signals depending on the setting field, which was varied when applied along the b-axis and then turned off before the measurement was taken. The experimental results indicate that the magnetic structure changes from an alternating longitudinal cone to a transverse cone when the setting field is 250 mT. They also show that the spins of Co2+ ions together with those of Fe3+ ions constitute a part of the overall magnetic structure and that the substitution of Al3+ for Fe3+ weakens the magnetic anisotropy within the easy plane. From a comparison of the enhancement factors of the Fe3+ NMR obtained with the RF pulse applied along the a-axis and the c-axis, we found that the magnetic easy plane anisotropy is approximately 16 times greater than the anisotropy within the easy plane. No changes of the NMR spectra were observed under an electric field of 1.2 MV m−1.

Effect of cooling rate on magnetic domain structure and magnetic properties of Tb0.27Dy0.73Fe1.95 alloys solidified in high magnetic field

In this work, Tb0.27Dy0.73Fe1.95 alloys were solidified in a high magnetic field of 4.4 T at various cooling rates. Changes in the magnetostriction, crystal orientation, magnetization, and magnetic domain of the solidified alloys were investigated. The application of the magnetic field can induce <111> orientation of (Tb, Dy)Fe2 phase. However, the effect of the magnetic field is strongly dependent on the cooling rate. The alloy solidified at 5 °C/min shows the highest magnetostriction, strongest <111> orientation, best contrast of light and dark in the domain image, and fastest magnetization, and followed in descending order by the alloys solidified at 1.5 °C/min and 60 °C/min. The change in the magnetostriction of the alloys can be attributed to the changes in crystal orientation and magnetic domain structure caused by both the magnetic field and cooling rate.

Large positive magnetoresistance in intermetallic compound NdCo2Si2

The magnetic, magneto-transport and magnetocaloric properties of antiferromagnetic intermetallic compound NdCo2Si2(TN=32K) have been studied. The compound yields a positive magnetoresistance (MR) of about ∼123% at ∼5K in 8 T magnetic field. The MR value is significantly large vis – a – vis earlier reports of large MR in intermetallic compounds, and possibly associated with the changes in magnetic structure of the compound. The large MR value can be explained in terms of field induced pseudo-gaps on Fermi surface.

Change in Magnetic Domain Structure of Nd‐Fe‐B Sintered Magnets by Compressive Stress

SUMMARYTo clarify the relationship between compressive stresses and demagnetization of Nd‐Fe‐B sintered magnets, we have examined the change in domain configuration by compressive stresses using a Kerr microscope. The magnetic domains of five kinds of Nd‐Fe‐B sintered magnets have been observed. The magnets have a coercivity of 0.8 MA/m to 1.4 MA/m and residual magnetic flux density of 1.3 T to 1.5 T. Irreversible demagnetization of Nd‐Fe‐B magnets with a low coercivity of 0.875 MA/m and high residual magnetic flux density of 1.41 T to 1.47 T have occurred from applying a compressive stress of 100 MPa. The compression‐affected area is approximately 0.14%. The stress more than 50 MPa is needed to demagnetize Nd‐Fe‐B magnets. The amount of irreversible demagnetization depends upon the intensity of the compressive stress as well as the residual magnetic flux and coercive force of the magnets.

High-cadence, High-resolution Spectroscopic Observations of Herbig Stars HD 98922 and V1295 Aquila

Abstract Recent observational work has indicated that mechanisms for accretion and outflow in Herbig Ae/Be star–disk systems may differ from magnetospheric accretion (MA) as it is thought to occur in T Tauri star–disk systems. In this work, we assess the temporal evolution of spectral lines probing accretion and mass loss in Herbig Ae/Be systems and test for consistency with the MA paradigm. For two Herbig Ae/Be stars, HD 98922 (B9e) and V1295 Aql (A2e), we have gathered multi-epoch (∼years) and high-cadence (∼minutes) high-resolution optical spectra to probe a wide range of kinematic processes. Employing a line equivalent width evolution correlation metric introduced here, we identify species co-evolving (indicative of common line origin) via novel visualization. We interferometrically constrain often problematically degenerate parameters, inclination and inner-disk radius, allowing us to focus on the structure of the wind, magnetosphere, and inner gaseous disk in radiative transfer models. Over all timescales sampled, the strongest variability occurs within the blueshifted absorption components of the Balmer series lines; the strength of variability increases with the cadence of the observations. Finally, high-resolution spectra allow us to probe substructure within the Balmer series’ blueshifted absorption components: we observe static, low-velocity features and time-evolving features at higher velocities. Overall, we find the observed line morphologies and variability are inconsistent with a scaled-up T Tauri MA scenario. We suggest that as magnetic field structure and strength change dramatically with increasing stellar mass from T Tauri to Herbig Ae/Be stars, so too may accretion and outflow processes.

Change of magnetic domain structure by mechanically induced twin boundary motion in Ni-Mn-Ga single crystal

The single variant state exhibits usual labyrinth and band magnetic domains depending on orientation of easy magnetization axis. By the passage of single twin boundary induced by mechanical stress the rake and granular domain patterns are formed. These domain patterns are further modified by repeated passage of the twin boundary resulting in similar domain patterns in the sample even though the orientation of the magnetization is different.

GdPtPb: A noncollinear antiferromagnet with distorted kagome lattice

In the spirit of searching for Gd-based, frustrated, rare earth magnets, we have found antiferomagnetism (AF) in GdPtPb which crystallizes in the ZrNiAl-type structure that has a distorted Kagom\'e lattice of Gd-triangles. Single crystals were grown and investigated using structural, magnetic, transport and thermodynamic measurements. GdPtPb orders antiferromagnetically at 15.5 K arguably with a planar, non-collinear structure. The high temperature magnetic susceptibility data reveal an "anti-frustration" behavior having a frustration parameter, $|f|$ = $|\Theta|$/ $T_N$ = 0.25, which can be explained by mean field theory (MFT) within a two sub-lattice model. Study of the magnetic phase diagram down to $T$ = 1.8 K reveals a change of magnetic structure through a metamagnetic transition at around 20 kOe and the disappearance of the AF ordering near 140 kOe. In total, our work indicates that, GdPtPb can serve as an example of a planar, non collinear, AF with a distorted Kagom\'e magnetic sub-lattice.

Effect of the magnetic core size of amino-functionalized Fe3O4-mesoporous SiO2 core-shell nanoparticles on the removal of heavy metal ions

Magnetite (Fe3O4)-mesoporous silica (mSiO2) core-shell nanoparticles are attractive heavy metal ion adsorbents. However, most studies have focused on the use of superparamagnetic Fe3O4 nanoparticles as core materials, resulting in low magnetic field responses due to their low susceptibility and saturation magnetization (Ms) values. Here, we report the synthesis, microstructure, and properties of ferrimagnetic Fe3O4-mSiO2 core-shell nanoparticles, focusing on the effects of the magnetic core size on their removal efficiency. We analyzed the magnetic properties and structural changes of the surface according to the magnetic core size and elucidated the correlation with the removal efficiency of heavy metal ions. Fe3O4 cores with diameters of 103, 123, or 207nm were synthesized by a modified polyol method, while the silica layer with a porous structure was coated using a sol-gel reaction. Amino-functionalized ferromagnetic Fe3O4-mSiO2 nanoparticles with different core sizes exhibited a faster and more efficient removal behavior of heavy metal ions than other reported superparamagnetic nanoparticles. The highest removal capacity of 84.4 mgg−1 for Cu2+ ions was observed with the nanoparticles having the largest specific surface area of 483.78m2g−1.

Foolproof Method for Fast and Reversible Switching of Water-Droplet Adhesion by Magnetic Gradients.

Reversible switching of water-droplet adhesion on solid surfaces is of great significance for smart devices, such as microfluidics. In this work, we designed a foolproof method for fast and reversible magnet-controlled switching of water-droplet adhesion surfaces by doping iron powders in soft poly(dimethylsiloxane). The water adhesion is adjusted by magnetic field-induced structure changes, avoiding complex chemical or physical surface design. The regulation process is so convenient that only tens of milliseconds are needed. The on-site responsive mechanism extends its use to unusual curved surfaces. Moreover, the excellent reversibility and stability make the film an ideal candidate for real-time applications.

Defect dynamics and strain coupling to magnetization in the cubic helimagnet Cu2OSeO3

Small but significant static and dynamic strain coupling effects have been detected in $\mathrm{C}{\mathrm{u}}_{2}\mathrm{OSe}{\mathrm{O}}_{3}$ through elastic and anelastic anomalies associated with magnetic phase transitions observed as a function of temperature (1.5--150 K) and magnetic field (0--300 mT). The magnetic transition near 60 K is accompanied by a small increase in single-crystal elastic constants which can be understood in terms of biquadratic coupling between shear strain and the magnetic order parameter, even though the shear strain itself is almost negligibly small. The conical-collinear transition is associated with distinct minima in the elastic properties, while weaker anomalies at lower fields may be related to changes in the configuration of magnetic domains. A distinctive acoustic loss peak at $\ensuremath{\sim}42\phantom{\rule{4.pt}{0ex}}\text{K}$, independent of magnetic field, is attributed to freezing of a defect which is coupled with shear strain, has an associated activation energy of $\ensuremath{\sim}5\phantom{\rule{4.pt}{0ex}}\text{kJ}\phantom{\rule{4.pt}{0ex}}\text{mo}{\mathrm{l}}^{\ensuremath{-}1}$, and may play a role in pinning the magnetic microstructures. Anomalies below $\ensuremath{\sim}10\phantom{\rule{4.pt}{0ex}}\text{K}$ indicate the presence of some additional relaxation process which could signify a change in magnetic structure.

Structural, magnetic, and dielectric properties of solid solutions between BiMnO3 and YMnO3

Bi1−xYxMnO3 (0.1≤x≤0.9) solid solutions were prepared by the high-pressure high-temperature method at 6GPa and 1573K. They crystallize in the GdFeO3-type perovskite structure with the Pnma symmetry. Crystal structures of Bi0.9Y0.1MnO3 and Bi0.5Y0.5MnO3 are studied by synchrotron X-ray powder diffraction at room temperature. Only one Néel temperature, TN, is found in samples with 0.1≤x≤0.9 in comparison with two Néel temperatures observed in YMnO3 (TN=29 and 39K). Samples with 0.5≤x≤0.9 have almost constant TN=44K, while TN starts to increase linearly for other compositions: TN=46K for x=0.3, TN=58K for x=0.2, and TN=68K for x=0.1. Field-induced transitions from canted-antiferromagnetic states to antiferromagnetic states are detected at about 30kOe for x=0.2 and 70kOe for x=0.1. Dielectric constant increases below TN in samples with 0.5≤x≤1, while it decreases below TN in samples with 0.1≤x≤0.3. Our data suggest that a magnetic structure changes near x=0.4. By extrapolation, we could estimate lattice parameters (a=5.9221Å, b=7.5738Å, and c=5.4157Å) and TN=79K for a hypothetical Pnma modification of BiMnO3.

Effects of composition, thickness and temperature on the magnetic properties of amorphous CoFeB thin films

We report composition, thickness and temperature dependent magnetic properties of amorphous Co80-yFeyB20 (x nm) films with y = 40,60 and x = 7–200 nm prepared directly on thermally oxidized Si substrate using magnetron sputtering technique. All the as-deposited films exhibit amorphous structure. For both the compositions, the shape of magnetic hysteresis (M-H) loops strongly depends on their thicknesses, i.e., the films with x ≤ 20 exhibit either rectangular shape or flat type loops, which change into transcritical type beyond a critical thickness (x > 50). Room temperature coercivity (HC) and field required for saturation (HS) increase almost linearly with increasing x between 7 and 20, but exhibit a rapid increase for the films with x up to 100 and then decrease slightly for x = 200 film. Thickness dependence of HC and HS varies also with composition. This is attributed to the development of effective magnetic anisotropy (Keff) with increasing film thickness, which causes a change in magnetic domain structure from in-plane magnetization to stripe domains and degrades the magnetic properties above critical thickness. Temperature dependent M-H loops reveal that the loop shapes do not change with temperature for a given film. However, the values of HC and HS increase with decreasing temperature and the rate of increase depends on the composition and thickness. Similarly, the variation of Keff with temperature also displays a considerable dependency on the composition. The observed results are discussed on the basis of development of compositional and thickness dependent effective magnetic anisotropy, which changes the magnetic domain structure with increasing CoFeB film thickness.

Nanoscale imaging of magnetization reversal driven by spin-orbit torque.

We use scanning electron microscopy with polarization analysis to image deterministic, spin-orbit torque-driven magnetization reversal of in-plane magnetized CoFeB rectangles in zero applied magnetic field. The spin-orbit torque is generated by running a current through heavy metal microstrips, either Pt or Ta, upon which the CoFeB rectangles are deposited. We image the CoFeB magnetization before and after a current pulse to see the effect of spin-orbit torque on the magnetic nanostructure. The observed changes in magnetic structure can be complex, deviating significantly from a simple macrospin approximation, especially in larger elements. Overall, however, the directions of the magnetization reversal in the Pt and Ta devices are opposite, consistent with the opposite signs of the spin Hall angles of these materials. Our results elucidate the effects of current density, geometry, and magnetic domain structure on magnetization switching driven by spin-orbit torque.

The Conducting Spin-Crossover Compound Combining Fe(II) Cation Complex with TCNQ in a Fractional Reduction State.

The radical anion salt [Fe{HC(pz)3}2](TCNQ)3 demonstrates conductivity and spin-crossover (SCO) transition associated with Fe(II) complex cation subsystem. It was synthesized and structurally characterized at temperatures 100, 300, 400, and 450 K. The compound demonstrates unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional conductivity. Pronounced changes of the in-plane direct-current resistivity and intensity of the electron paramagnetic resonance (EPR) signal, originated from TCNQ subsystem, precede the SCO transition at the midpoint T* = 445 K. The boltzmannian growth of the total magnetic response and structural changes in the vicinity of T* uniquely show that half [Fe{HC(pz)3}2] cations exist in high-spin state. Robust broadening of the EPR signal triggered by the SCO transition is interpreted in terms of cross relaxation between the TCNQ and Fe(II) spin subsystems.

Hydrostatic pressure effect on magnetic hysteresis parameters of pseudo‐single‐domain magnetite

AbstractThis paper reports the first in situ magnetic hysteresis measurements of pseudo‐single‐domain (PSD) magnetite under high pressure up to 1 GPa. The magnetic hysteresis measurements of stoichiometric PSD magnetite samples under hydrostatic pressure were carried out using a piston‐cylinder high‐pressure cell, and the pressure dependence of the hysteresis parameters of PSD magnetite was calculated from the hysteresis curves. It was found that coercivity (Bc) increases with increasing pressure as a quadratic function up to 1 GPa by ∼90%, which is different from the pressure dependences of Bc of multidomain and single‐domain magnetites. Coercivity of remanence also increases as a quadratic function, and saturation remanence (Mrs) increases with pressure up to 0.5 GPa by ∼20% until reaching saturation. In contrast, saturation magnetization is constant up to 1 GPa. The approximate demagnetizing factor calculated from the ratio Bc/Mrs increases with increasing pressure, suggesting that the number of lamellar domains increases with increasing pressure. The number of lamellar domains and domain wall width are theoretically estimated to increase under high pressure due to the changes in magnetostriction, elastic, and magnetocrystalline anisotropy constants, and these changes in magnetic domain structure should relate to the changes in the magnetic properties of PSD magnetite.

Temperature effect on the magnetic property and ferroelectricity in hexaferrite SrFe12O19

We studied the temperature effect on magnetic and electrical properties in bulk SrFe12O19 prepared by conventional ceramic technique. The jumping behavior of magnetization has been observed under the zero-field-cooling mode, but disappeared under the field-cooled cooling mode. The spin moment of iron ions reorients below 50 K leading to the magnetic structure changes. Magnetic parameters, saturation magnetization (Ms) and coercivity field (Hc), show opposite tendency with temperature throughout the measuring range, which is mainly ascribed to the Fe3+ ions situated at 4f2 and 2b sites. The curves of electrical polarization P vs temperature T under different external magnetic field indicate the existence of ferroelectricity and magnetoelectric coupling effect at low temperature, and the transition temperature T P is about 120 K.

Giant magnetic coercivity in YNi4B-type SmNi3TB (T=Mn–Cu) solid solutions

The effects of transition metal substitution for Ni on the magnetic properties of the YNi4B-type SmNi4B via SmNi3TB (T=Mn, Fe, Co, Cu) solid solutions have been investigated. SmNi4B, SmNi3MnB, SmNi3FeB, SmNi3CoB and SmNi3CuB show ferromagnetic ordering at 40K, 210K, 322K, 90K and 57K and field sensitive metamagnetic-like transitions at 15K, 100K, 185K, 55K and 15K in a magnetic field of 10kOe, respectively. The magnetocaloric effects of SmNi3TB (T=Mn–Cu) were calculated in terms of isothermal magnetic entropy change (ΔSm). The magnetic entropy ΔSm reaches value of –0.94J/kgK at 40K for SmNi4B, –1.5J/kgK at 205K for SmNi3MnB, –0.54J/kgK at 320K for SmNi3FeB, –0.49J/kgK at 90K for SmNi3CoB and –0.54J/kgK at 60K for SmNi3CuB in field change of 0–50kOe around the Curie temperature. They show positive ΔSm of +0.71J/kgK at ~10K for SmNi4B, +1.69J/kgK at 30K for SmNi3MnB, +0.89J/kgK at 110K for SmNi3FeB, +1.08J/kgK at 25K for SmNi3CoB and +1.12J/kgK at 10K for SmNi3CuB in field change of 0–50kOe around the low temperature metamagnetic-like transition. Below the field induced transition temperature (change of magnetic structure), SmNi3TB (T=Mn–Cu) exhibits giant magnetic coercivity of 74kOe at 5K for SmNi4B, 69kOe at 20K (90kOe at 10K) for SmNi3MnB, 77kOe at 60K for SmNi3FeB, 88kOe at 20K for SmNi3CoB and 52kOe at 5K for SmNi3CuB.

Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition.

We present a new type of colossal magnetoresistance (CMR) arising from an anomalous collapse of the Mott insulating state via a modest magnetic field in a bilayer ruthenate, Ti-doped Ca_{3}Ru_{2}O_{7}. Such an insulator-metal transition is accompanied by changes in both lattice and magnetic structures. Our findings have important implications because a magnetic field usually stabilizes the insulating ground state in a Mott-Hubbard system, thus calling for a deeper theoretical study to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities and spin-lattice-charge coupling. This study further provides a model approach to search for CMR systems other than manganites, such as Mott insulators in the vicinity of the boundary between competing phases.

Pressure induced antiferromagnetism in the manganite La0.7Sr0.3Mn0.83Nb0.17O3

The crystal and magnetic structure of a manganite La0.7Sr0.3Mn0.83Nb0.17O3 have been studied by means of a neutron diffraction method at pressures up to 4.5 GPa and in temperature range 4–290 K. At ambient pressure and temperature below TC = 150 K a ferromagnetic FM state is stabilized in this compound. At pressures P ≥ 1.5 GPa and temperature below TN∼110 K appearance of a new A-type antiferromagnetic AFM phase in La0.7Sr0.3Mn0.83Nb0.17O3 has been observed. A further pressure increasing leads to an increase in the volume fraction of the A-type antiferromagnetic AFM phase, which coexists with the suppressed ferromagnetic phase in the whole experimental pressure range. The Curie and Néel temperature increase upon lattice contraction with comparable pressure coefficients dTC/dP = 2.1 K/GPa and dTN/dP = 1.6 K/GPa. The structural mechanisms of the changes in magnetic structure of La0.7Sr0.3Mn0.83Nb0.17O3 manganite are discussed.

RAPID PENUMBRA AND LORENTZ FORCE CHANGES IN AN X1.0 SOLAR FLARE

ABSTRACT We present observations of the violent changes in photospheric magnetic structures associated with an X1.1 flare, which occurred in a compact δ-configuration region in the following part of AR 11890 on 2013 November 8. In both central and peripheral penumbra regions of the small δ sunspot, these changes took place abruptly and permanently in the reverse direction during the flare: the inner/outer penumbra darkened/disappeared, where the magnetic fields became more horizontal/vertical. Particularly, the Lorentz force (LF) changes in the central/peripheral region had a downward/upward and inward direction, meaning that the local pressure from the upper atmosphere was enhanced/released. It indicates that the LF changes might be responsible for the penumbra changes. These observations can be well explained as the photospheric response to the coronal field reconstruction within the framework of the magnetic implosion theory and the back reaction model of flares.