Field-cooled Conditions Research Articles

Large exchange bias in polycrystalline ribbons of Ni56Mn21Al22Si1

Large exchange bias (EB) effect is demonstrated in a Ni56Mn21Al22Si1 polycrystalline ribbon that shows spin-glass behaviour below 68K and superparamagnetic (SPM) behaviour above this temperature. The average magnetic moment μ of the SPM clusters is estimated to be considerably smaller than that reported for other Heusler alloy systems that demonstrate spin glass behaviour and EB effect. The M–H loops measured in field-cooled (FC) conditions show significant asymmetry that is observed to be strongly field and temperature dependant. The maximum EB field (HEB ~2.6kOe at 20kOe and 2K) is much larger than that reported for any metallic alloy system with conventional EB effect. The dependence of HEB on temperature shows an exponential decrease. An approach towards enhancement of the EB effect in this conventional EB system can be realized via manipulation of SPM or superferromagnetic (SFM) cluster size with variation of cooling magnetic field.

Dynamic Force Characteristics in a Superconducting Interface Module

Bulk high-temperature superconductors (HTSs) with good flux trapping provide a feasible application to passive docking and assembling of space module systems. To learn dynamic process and stability of a superconducting interface module consisted of the flux-trapped HTS and an interfacing magnet, an experimental setup is designed to simulate the horizontal approaching process in the interface module and measure the dynamic force properties. Momentum difference in the module is given by changing the swinging angle of the interfacing magnet. Displacement of the interfacing magnet relative to the magnetized superconductor is recorded by a charge-coupled device to state dynamic behavior. Effects of different field-cooling (FC) magnetization conditions on the dynamic properties are demonstrated by analyzing the displacement dissipation. The FC magnetization can provide a stable interfacing process, although a long damping time is required. Influence of additional aluminum metal structure on the damping properties is also investigated, and the damping ability is increased by adding the metal thickness. Therefore, the aluminum structure producing eddy current loss is proposed to be applied in the superconducting interface module to improve the interface stability.

Phase coexistence and transformations in field-cooled ternary piezoelectric single crystals near the morphotropic phase boundary

Structural phase transformations in (100)-oriented Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals have been investigated by X-ray diffraction. A cubic (C) → tetragonal (T) → monoclinic-C (MC) transformation sequence was observed in the field-cooled condition. Two phase coexistence regions of C + T and T + MC were found. In addition to an increase in the C → T phase transition temperature and a decrease of the T → MC one, a broadening of the coexistence regions was also found with increasing field. This broadening can be explained by the presence of polar nano regions within the C, T, and MC phase regions.

Temperature-induced and electric-field-induced phase transitions in rhombohedralPb(In1/2Nb1/2)O3−Pb(Mg1/3Nb2/3)O3−PbTiO3ternary single crystals

Temperature and electric field effects on rhombohedral $(R)$ ternary ${\mathrm{Pb}(\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{Pb}(\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{)\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{PbTiO}}_{3}$ (PIN-PMN-PT) ferroelectric single crystals were comprehensively studied by x-ray diffraction. We have focused on how the individual phase transitions as well as the phase transition sequences depend on thermal and electrical history. Electric field-temperature phase diagrams have been constructed under [001] field-cooling and field-heating conditions. As happens to the $R$ phase of binary PMN-PT crystals [H. Cao, J. F. Li, D. Viehland, and G. Y. Xu, Phys. Rev. B 73, 184110 (2006)], the $R$ phase of the zero-field-cooled (ZFC) state is replaced by a monoclinic A $(M$) phase in the field-cooled (FC) diagram. In particular, reciprocal-space mesh scans demonstrated that the $M$ phase was stable for crystals poled along the [001] crystallographic direction rather than the initial $R$ phase of the ZFC state. Furthermore, an $E$-field-induced phase transformational sequence of $R\ensuremath{\rightarrow}M$\ensuremath{\rightarrow} tetragonal $(T)$ was observed at constant temperature, revealing a gradual increase in the $c$ lattice parameter. These findings demonstrate that the ternary PIN-PMN-PT crystals exhibit common phase transition features with binary PMN-PT and ${\mathrm{Pb}(\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}{)\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{PbTiO}}_{3}$ (PZN-PT) ones for compositions in the low PT side of the morphotropic phase boundary.

Magnetization-steps in Y2CoMnO6 double perovskite: The role of antisite disorder

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at Tc ≈ 75 K. At 2 K, it displays a strong ferromagnetic hysteresis with a significant coercive field of Hc ≈ 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8 K. In the temperature range 2 K ≤ T ≤ 5 K, the hysteresis loops are anomalous as the virgin curve lies outside the main loop. The field-cooling conditions as well as the rate of field-sweep are found to influence the steps. Quantitative analysis of the neutron diffraction data shows that at room temperature, Y2CoMnO6 consists of 62% of monoclinic P21/n with nearly 70% antisite disorder and 38% Pnma. The bond valence sums indicate the presence of other valence states for Co and Mn which arise from disorder. We explain the origin of steps by using a model for pinning of magnetization at the antiphase boundaries created by antisite disorder. The steps in magnetization closely resemble the martensitic transformations found in intermetallics and display first-order characteristics as revealed in the Arrott's plots.

Cooperative spin freezing and the pinning assisted thermoremanent magnetization in Ni2.04Mn1.36Sn0.6 alloy

Detailed investigation on the ground-state magnetic properties of Ni2.04Mn1.36Sn0.6 alloy is reported. The sample undergoes martensitic type structural transformation from a cubic austenite phase to an orthorhombic martensite phase on cooling around TM = 220 K. It orders ferromagnetically just above room temperature with TC = 310 K. The phases above and below TM are predominantly ferromagnetic, although incipient antiferromagnetic correlations have been mooted between the Mn atoms sitting at two different crystallographic sites. The zero-field-cooled magnetic state shows a step like anomaly around TB = 100 K, and the sample is found to show clear signature of spin glass like behaviour below this point. It is also associated with considerable exchange bias effect below TB, where horizontal shift of isothermal magnetization loop is observed in the field-cooled state. Apart from exchange bias, there exists large thermoremanent magnetization. Interestingly, the thermoremanent magnetization obtained by cooling the sample from above and just below the first order martensitic transition failed to show any sizable change, which rules out any major role of field induced arrest across the first order transition. The observed thermoremanent magnetization is presumably due to the effect of pinning of uncompensated spins in this magnetically inhomogeneous system with random ferromagnetic as well as antiferromagnetic bonds on field cooling through the onset point (TB) of the spin-glass like state.

Anomalous magnetic behavior in nanocomposite materials of reduced graphene oxide-Ni/NiFe2O4

Magnetic Reduced Graphene Oxide-Nickel/NiFe2O4 (RGO-Ni/NF) nanocomposite has been synthesized by one pot solvothermal method. Respective phase formations and their purities in the composite are confirmed by High Resolution Transmission Electron Microscope and X Ray Diffraction, respectively. For the RGO-Ni/NF composite material finite-size effects lead to the anomalous magnetic behavior, which is corroborated in temperature and field dependent magnetization curves. Here, we are reporting the behavior of higher magnetization values for Zero Field Cooled condition to that of Field Cooled for the RGO-Ni/NF nanocomposite. Also, the observed negative and positive moments in Hysteresis loops at relatively smaller applied fields (100 Oe and 200 Oe) are explained on the basis of surface spin disorder.

Enhanced Maglev Performance by Field Cooling for HTS Maglev System in Curve Negotiation

In the practical operation of high-temperature superconducting (HTS) maglev system, the problem of curve negotiation cannot be neglected. During the process of going through curve path, the maglev vehicle would laterally deviate from the center of the permanent magnet guideway (PMG) because of centrifugal forces. To explore the performance variation of the maglev system in this process, the electromagnetic forces (levitation force and guidance force) of a HTS bulk levitation unit were experimentally investigated by different eccentric distances (EDs). The ED is emulated by laterally moving the levitation unit relative to the PMG. Experimental result shows that in field-cooling (FC) conditions, the levitation force generally shows a increase tendency with the growing EDs, while it keeps decreasing in zero-field-cooling (ZFC) conditions. And, the levitation force with large EDs in the FC condition is larger than that in the ZFC condition. The guidance force is enhanced in both conditions within ED of 15 mm, and in FC condition, the force is larger than the case of ZFC. Comparing the above two important magnetic force results, the FC condition is recommended for the safe operation of the HTSn system in curve negotiation.

Magnetic properties of Co doped MgTiO3 ceramics

Structural, magnetic and dielectric properties of (Mg1−xCox)TiO3 (x=0–0.07) ceramics prepared by the semi-alkoxide precursor method are investigated. All the sintered samples exhibit paramagnetic nature at room temperature, but magnetic susceptibility increases with increasing Co content up to 0.03 and decreases at higher Co content. Temperature dependent magnetization data obtained under zero-field-cooled (ZFC) and field-cooled (FC) conditions reveal a ferromagnetic to paramagnetic phase transition around 228K for x=0.03 and bifurcation between the ZFC and FC data shifts to lower temperature from 170 to 150K with increasing the applied field from 100 to 500Oe. The broadband dielectric properties are studied in the temperature range 133–300K and in the frequency range of 1MHz to 1GHz. It is observed that both ε′ and ε′′ of (Mg0.97Co0.03)TiO3 ceramic increase with increasing temperature. Interestingly, the temperature dependent dielectric properties of (Mg0.97Co0.03)TiO3 ceramic measured at 1GHz exhibit anomalies at temperatures where the magnetic phase transitions were observed.

Negative magnetization and the tunable exchange bias field in LaCr0.8Mn0.2O3

Manganese substituted Lanthanum chromite LaCr0.8Mn0.2O3 exhibits negative magnetization with decrease in temperature under the field cooled (FC) condition for the applied field H≤2000Oe. The maximum magnetic compensation temperature, (Tcomp) was 147K. A reentrant positive magnetization was observed at T≤50K due to low temperature transition. The negative magnetization is explained by considering the paramagnetic moment of Mn ions under the influence of negative internal field. Measurement of magnetic hysteresis loops under FC condition shows the presence of exchange bias field at T<Tcomp and its magnitude can be tuned by varying the temperature and cooling field.

The Effect of Single Crystalline Substrates and Ion-Beam Bombardment on Exchange Bias in Nanocrystalline ${\hbox{NiO}}/{\hbox{Ni}}_{80}{\hbox{Fe}}_{20}$ Bilayers

Methods to modify the magnetic coercivity and exchange bias field of nanocrystalline antiferromagnetic/ferromagnetic NiO/Ni80Fe20 thin films were investigated for bilayers grown using ion-assisted deposition onto different single crystalline substrates. An enhanced coercivity was found at 298 K for the films deposited on single crystalline MgO (100) and Al2O3 (11-20) substrates. After field cooling the films to 50 K, the NiO/NiFe bilayer grown on Al2O3 (11-20) exhibited the largest exchange bias ( - 25 Oe). The second part of the study investigated ion-beam modification of the ferromagnetic surface prior to the deposition of the NiO layer. A range of ion-beam bombardment energies (VEH) were used to modify in situ the NiFe surface during the deposition of NiO/NiFe/SiO2 films. Cross-sectional transmission electron microscopy showed a systematic reduction in the thickness of the NiFe layers with increasing Ar+ bombardment energies attributed to etching of the surface. In addition, the bombardment procedure modified the magnetic exchange bias of the composite structure in both the as-prepared and field-cooled state.

Influence of Off-Centre Operation on the Performance of HTS Maglev

Owing to instinctive self-stable levitation characteristics, high-temperature superconducting (HTS) maglev using bulk high-temperature superconductors attracts more and more attention from scientists and engineers around the world. In this paper, the levitation force relaxation and guidance force characteristics of a Y–Ba–Cu–O levitation unit with different eccentric distances (EDs) off the center of the permanent magnet guideway were experimentally investigated under field-cooling (FC) conditions. Experimental results indicate that the levitation force slightly increases at small EDs firstly, but degrades with further increasing of EDs. However, the maximum guidance force and its stiffness exhibit enhancement in moderate ED range. The results demonstrate that a properly designed initial FC eccentric distance is important for the practical applications of HTS maglev according to specific requirements like running in curve lines.

Biferroic LuCrO3: Structural characterization, magnetic and dielectric properties

Multiferroic LuCrO3 perovskite-type structure (Pbnm) obtained via auto-ignition synthesis was characterized by a combination of X-ray diffraction (XRD) and thermogravimetric (TG) techniques, and through magnetization and permittivity measurements. Results showed that amorphous combustion powders were fully transformed to orthorhombic LuCrO3 structure at 1200 K after the first LuCrO4 crystallization at 700 K. The magnetic response displays thermal irreversibility between zero-field-cooling and field-cooling condition which is due to spin canted AF switching at 116 K. Accordingly, a hysteresis loop in the M(H) data confirms weak ferromagnetism in LuCrO3. On the other hand, the permittivity measurement shows a broad peak transition typical of relaxor-type ferroelectrics transition at ∼450 K. Electrical conductivity increases as temperature increases showing an anomaly around the diffuse phase transition. The diffuse phase transition and the formation of the charge carriers are discussed in terms of a local distortion around the Lu Site.

A Study of the Origin of Weak Ferromagnetism in Zn1−x Co x O

To study the origin of ferromagnetism in Zn1−xCoxO thin films, its thermal diffusivity, in addition to its magnetization measurements, were analyzed. Thin films of Zn1−xCoxO (x = 0.03) were deposited on Si (100) substrates through ultrasonic spray pyrolysis. Magnetization M(B) measurements at low temperature showed a hysteresis loop that indicated the existence of ferromagnetic ordering in Zn0.97Co0.03O. However, the magnetic moment per Co ion was much lower than expected. A comparison of M(T) measured at zero-field-cooled and field-cooled conditions showed a superparamagnetic-like behavior, with a blocking temperature of about 130 K. Temperature dependence on the thermal diffusivity of Zn0.97Co0.03O showed a pronounced lambda-shaped minimum at 130 K, which indicated the existence of a second-order phase transition at this temperature. The weak ferromagnetism in the Zn0.97Co0.03O with the Curie temperature of 130 K was ascribed to the uncompensated magnetic moment at the surface of CoO nanoclusters.

Spacer layer and temperature driven magnetic properties in multilayer structured FeTaC thin films

We report the effects of thickness of spacer layers and temperature on the magnetic properties of multilayer structured [FeTaC(50 nm)/Ta(x nm)]n=3/FeTaC(50 nm)/substrate amorphous thin films. A transcritical loop with a large coercivity (HC) of 25 Oe was observed for x = 0 film, but the loop shape was changed to flat loop along with a rapid decrease in HC (<0.25 Oe) by introducing Ta spacer layers. This is attributed to loss of perpendicular anisotropy causing a transition from stripe domain structure to in-plane orientation of spins. Magnetic hysteresis loops measured at different temperatures exhibited a spacer layers' thickness dependent multistep magnetization reversal processes for temperature below 80 K. Thermo- magnetization curves obtained under zero-field-cooled and field-cooled conditions displayed a bifurcation between them for x = 0 film. However, the bifurcation point was shifted to lower temperatures with increasing x and disappeared eventually for films with x ⩾ 4. High-temperature magnetization data revealed no significant changes in the magnetic properties up to Curie temperature. The observed results are elucidated on the basis of change in magnetic structure with the thickness of the spacer layers, pinhole effects at the interface, and temperature, instigating an effective reduction in perpendicular anisotropy and magnetic disorder, and thereby enhancing magnetic properties in multilayer thin films.

Role of magnetic chirality in polarization flip upon a commensurate-incommensurate magnetic phase transition in YMn2O5

We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b-axis of single crystals of YMn$^{4+}$(Mn$_{1-x}$Ga$_{x}$)$^{3+}$O$_{5}$ with $x=0.047$ and 0.12, in which nonmagnetic Ga-ions dilute Mn$^{3+}$ spins. The $x=0.047$ sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the $x=0.12$ sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak-ferroelectric, respectively. Measurements conducted under zero field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: the magnetic chirality of Mn$^{4+}$ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn$^{4+}$-Mn$^{3+}$ chain plays a dominant role in the CM phase. The polarization of YMn$_{2}$O$_{5}$ flips upon CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.

Anomalous magnetization behaviour in a single crystal of vanadium spinel FeV2O4

Spinel oxide FeV2O4, having the orbital degrees of freedom at Fe2+ and V3+ ions, exhibits multi-step magnetic phase transitions and successive structural phase transitions at low temperatures. In order to clarify the magnetic properties of FeV2O4, we have measured the temperature dependence of magnetization, isothermal magnetization curves and specific heat using a single crystal of FeV2O4. Temperature-induced magnetization jumps below the 110 K were observed in the zero-field-cooled magnetization curves. Furthermore, we found that the behaviours of the isothermal magnetization curves were quite different between the zero-field-cooled and field-cooled conditions. We suggest that the change of the magnetic domain structure under the magnetic field associated with the orbital states of Fe2+ ions is the possible origin of these intriguing and anomalous magnetic properties in a single crystal of FeV2O4.

Effect of Electrical Field on the Phase Transformation of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Single Crystals

The structural phase transformations of <TEX>$0.7Pb(Mg_{1/3}Nb_{2/3})O_3-0.3PbTiO_3$</TEX> (PMN-0.3PT) were studied using high resolution x-ray diffraction (HRXRD) as a function of temperature and electric field. A phase transformational sequence of cubic (C)<TEX>${\rightarrow}$</TEX>tetragonal (T)<TEX>${\rightarrow}$</TEX>rhombohedral (R) phase was observed in zero-field-cooled conditions; and a <TEX>$C{\rightarrow}T{\rightarrow}$</TEX>monoclinic <TEX>$(M_C){\rightarrow}$</TEX> monoclinic (<TEX>$M_A$</TEX>) phase was observed in the field-cooled conditions. The transformation of T to <TEX>$M_A$</TEX> phase was realized through an intermediate <TEX>$M_C$</TEX> phase. The results also represent conclusive and direct evidence of a <TEX>$M_C$</TEX> to <TEX>$M_A$</TEX> phase transformation in field-cooled conditions. Beginning from the zero-field-cooled condition, a <TEX>$R{\rightarrow}M_A{\rightarrow}M_C{\rightarrow}T$</TEX> phase transformational sequence was found with an increasing electric field at a fixed temperature. Upon removal of the field, the <TEX>$M_A$</TEX> phase was stable at room temperature. With increasing the field, the transformation temperature from T to <TEX>$M_C$</TEX> and from <TEX>$M_C$</TEX> to <TEX>$M_A$</TEX> phase decreased, and the phase stability ranges of both T and <TEX>$M_C$</TEX> phases increased. Upon removal of the field, the phase transformation from R to <TEX>$M_A$</TEX> phase was irreversible, but from <TEX>$M_A$</TEX> to <TEX>$M_C$</TEX> was reversible, which means that <TEX>$M_A$</TEX> is the dominant phase under the electric field. In the M phase region, the results confirmed that lattice parameters and tilt angles were weakly temperature dependent over the range of investigated temperatures.

Vortex matter in the type-II superconductor La3Ni2B2N3−x in the light of NMR

A quantitative analysis of spin-echo NMR measurements of the local field distribution and the spin–spin relaxation time T2 in the field-cooled superconducting state of the type-II superconductor La3Ni2B2N3−x shows evidence for two fluctuation modes of the weakly pinned vortex matter dominating T2 at different temperatures: long wavelength diffusive modes in the vortex solid at low temperature, and plastic deformation of the viscous liquid close to the critical temperature Tc, with a broad crossover region in between where a more complex theory is required. We determine the melting temperature Tm = 11.5 K of the vortex matter as the temperature where the characteristic time of the plastic deformation slows down to the time scale set by the spin-echo experiment. This is preferred to the conventional estimates from a Lindemann criterion for the melting vortex solid since the quantitative description of T2 near Tm by the viscous vortex liquid appears to be more consistent. It indicates that the plastic deformation mode might be important for T2 in the mixed state of other type-II superconductors in the weak pinning regime near Tc as well.

Spontaneous magnetization and magnetic susceptibility of LaVO3

Five characteristic temperatures of TM = 148 K, TN = 142 K, Tt = 138 K, Tf ~ 125 K and Tg ~ 50 K were found by the measurements of the magnetization curves at various temperature. The spontaneous magnetization appears below TM. It increases up to Ms ≃ 2 × 10−4 μB at Tt and then decreases steeply below Tt, which qualitatively agrees with the temperature dependence of magnetization obtained under field-cooling (FC) condition. On the other hand, the slope of the magnetization curve, namely the magnetic susceptibility, drops below TN, which coincides with the temperature dependence of magnetization obtained under zero-FC condition, although the magnetization curves were obtained under FC condition. The temperature dependence of the spontaneous magnetization shows a minimum at Tf and a drop at Tg although there is no anomaly in the temperature dependence of FC or ZFC magnetization.