Appearance Of Exchange Bias Research Articles

Defect-Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites.

The crystal site occupancy of different divalent ions and the induction of lattice defects represent an additional tool for modifying the intrinsic magnetic properties of spinel ferrites nanoparticles. Here, the relevance of the lattice defects is demonstrated in the appearance of exchange-bias and in the improvement of the magnetic properties of doped ferrites of 20nm, obtained from the mild oxidation of core@shell (wüstite@ferrite) nanoparticles. Three types of nanoparticles (Fe0.95 O@Fe3 O4 , Co0.3 Fe0.7 O@Co0.8 Fe2.2 O4 and Ni0.17 Co0.21 Fe0.62 O@Ni0.4 Co0.3 Fe2.3 O4 ) are oxidized. As a result, the core@shell morphology is removed and transformed in a spinel-like nanoparticle, through a topotactic transformation. This study shows that most of the induced defects in these nanoparticles and their magnetic properties are driven by the inability of the Co(II) ions at the octahedral sites to migrate to tetrahedral sites, at the chosen mild oxidation temperature. In addition, the appearance of crystal defects and antiphase boundaries improves the magnetic properties of the starting compounds and leads to the appearance of exchange bias at room temperature. These results highlight the validity of the proposed method to impose novel magnetic characteristics in the technologically relevant class of nanomaterials such as spinel ferrites, expanding their potential exploitation in several application fields.

Investigation of magnetic ordering and origin of exchange-bias effect in doped manganite, Sm0.4Ca0.6MnO3

We have carried out detailed investigations of magnetic ordering and the origin of exchange-bias phenomenology in polycrystalline 154Sm0.4Ca0.6MnO3 perovskite compound by employing dc and ac magnetizations, neutron depolarization, and neutron diffraction techniques. The neutron diffraction study reveals a broad charge ordering transition in the temperature range of 275–150 K, manifested by gradually varying structural distortions. DC magnetization and neutron diffraction studies infer a magnetic ordering around 110 K. Below ∼40 K, this compound exhibits a large enhancement of the magnetization and an exchange-bias effect (∼18 kOe at 5 K). The exchange-bias effect shows a characteristic temperature and cooling field dependence and a training effect. The compound undergoes a CE-type (checker-board type) antiferromagnetic (AFM) ordering of Mn3+/Mn4+ spins below 110 K. The large magnetization below ∼40 K arises from the ferrimagnetic (FIM) ordering of a minority (∼5 vol. %) non-perovskite phase of Mn3O4. The appearance of exchange bias is ascribed due to the presence of an interface, formed between the FIM (Mn3O4) phase and the main AFM perovskite phase. Our study has implications in giving new directions in the search of large exchange-bias effects in mixed FIM/AFM bulk systems.

Influence of the shape on exchange bias in core/shell nanoparticles

Abstract We evaluate the influence of the shape on hysteretic properties in ferromagnetic/antiferromagnetic core/shell nanoparticles using Monte Carlo simulations. Exchange bias and coercive fields were estimated in nanoparticles with shapes ranging from octahedra to spheres to cubes. The net magnetic moment in the shell interface, produced by uncompensated spins responsible for the appearance of exchange bias, was found to vary significantly with the nanoparticle shape. Strong and intermediate exchange bias fields were found in octahedral and spherical nanoparticles, respectively, while a nearly zero exchange bias field was found in cubical nanoparticles. In general, decreasing exchange bias and increasing coercive fields were observed as the nanoparticle shape varied from nanoparticles with many to few uncompensated spins in the shell interface. A faster increase (decrease) of the exchange bias field (coercive field) as function of the magnitude of the interfacial exchange interaction was observed in nanoparticles with higher uncompensated interfaces. Furthermore, a predominance of shape effects over size effects on the hysteretic behavior of the nanoparticles was observed in the results.

Exchange bias in defect-induced ferromagnetic SiC

Abstract In this paper, the observation of exchange bias in defective ferromagnetic SiC is presented. Two typical features of exchange bias are clearly observed: the shift of magnetic hysteresis upon field cooling and the training effect. A core-shell morphology particle model is employed to quantitatively analyze the exchange bias field dependence on the cooling field. Based on the model, the calculated anisotropy constant is 2.5 × 105 erg/cm3. The training effect can be described by the Kolmogorov-Avrami model, which is used for the classical exchange bias system. The appearance of exchange bias is the signature of the existence of an interface between ferromagnetic and antiferromagnetic layers or cores/shells.

Study of the sign change of exchange bias across the spin reorientation transition in Co(Cr1−xFex)2O4 (x = 0.00–0.125)

We present the evolution of novel phenomena of magnetic compensation effect, exchange bias (EB) effect and the field induced anomalies in ‘’ substituted multiferroic compound . A few percent of ‘’ substitution for ‘’ in results in the reversal of field cooled magnetization under low applied fields below compensation temperature Tcomp. Further, increase in the field leads to the spin reorientation transition (TSR). Signature of EB in a narrow temperature window in the vicinity of TSR and its sign change across TSR is observed. Magnitude of EB depends on the amount of compensation and rigidity of the spin reorientation. We also notice the appearance of positive EB below the lock-in transition (TL). Presence of unidirectional anisotropy developed in the commensurate spin-spiral below TL could be responsible for the appearance of EB below TL.

Thickness dependent exchange bias in co-sputter deposited Ni–Mn–Al Heusler alloy hard nanostructured thin films

In the present work thin films of off-stoichiometric Ni–Mn–Al thin films have been successfully deposited on Si substrates. The films have been deposited by DC/RF magnetron sputtering by co-sputtering of the targets of Ni, Mn and Al. The films have been studied for their structural and magnetic properties. Nanoindentation has also been done for film hardness and resistant to crack properties. It has been observed that Martensitic transformation occurs in the film exhibiting large thickness (1.5μm). All the films exhibit exchange bias at low temperature with the exchange bias field depending strongly on the thickness of the film. ac susceptibility result shows the existence of spin glass state at low temperature. The exchange bias appearing in the sample may be associated with the interface containing spin glass SG/FM phase. The appearance of exchange bias in all the samples shows that in Ni–Mn–Al system, exchange bias is independent of the occurrence of martensitic transition in the system. The existence of mixed phase (L21 and B2) is an intrinsic property of Ni–Mn–Al system and the formation of L21 phase can be controlled by using different deposition and annealing parameters. In this respect, exchange bias properties of Ni–Mn–Al may be tuned by controlling the formation of ferromagnetic L21 phase. Nanoindentation results reveal that the film exhibits high hardness (H) ~21GPa and is resistant to scratch.

Exchange bias induced by the Fe3O4Verwey transition

We present a study of exchange bias in different configurations of V${}_{2}$O${}_{3}$ thin films with ferromagnetic layers. The exchange bias is accompanied by a large vertical shift in magnetization. These effects are only observed when V${}_{2}$O${}_{3}$ is grown on top of Ni${}_{80}$Fe${}_{20}$ permalloy. The magnitude of the vertical shift is as large as 60$%$ of the total magnetization, which has never been reported in any system. X-ray diffraction studies show that the growth conditions promote the formation of a ferrimagnetic Fe${}_{3}$O${}_{4}$ interlayer. The change in the easy magnetization axis of Fe${}_{3}$O${}_{4}$ across the Verwey transition at 120 K is correlated with the appearance of exchange bias and vertical shift in magnetization. Both phenomena disappear above 120 K, indicating a direct relationship between the magnetic signature of the Verwey transition and the exchange bias.

Modeling of exchange bias in the antiferromagnetic (core)/ferromagnetic (shell) nanoparticles with specialized shapes

Zero-field-cooled (ZFC) and field-cooled (FC) hysteresis loops of egg- and ellipsoid-shaped nanoparticles with inverted ferromagnetic (FM)-antiferromagnetic (AFM) core-shell morphologies are simulated using a modified Monte Carlo method, which takes into account both the thermal fluctuations and energy barriers during the rotation of spin. Pronounced exchange bias (EB) fields and reduced coercivities are obtained in the FC hysteresis loops. The analysis of the microscopic spin configurations allows us to conclude that the magnetization reversal occurs by means of the nucleation process during both the ZFC and FC hysteresis branches. The nucleation takes place in the form of “sparks” resulting from the energy competition and the morphology of the nanoparticle. The appearance of EB in the FC hysteresis loops is only dependent on that the movements of “sparks” driven by magnetic field at both branches of hysteresis loops are not along the same axis, which is independent of the strength of AFM anisotropy. The tilt of “spark” movement with respect to the symmetric axis implies the existence of additional unidirectional anisotropy at the AFM/FM interfaces as a consequence of the surplus magnetization in the AFM core, which is the commonly accepted origin of EB. Our simulations allow us to clarify the microscopic mechanisms of the observed EB behavior, not accessible in experiments.

Magnetic properties of La and (La, Zr) doped BiFeO3 ceramics

Bi0.9La0.1FeO3 (BLF) and Bi0.9La0.1Fe0.98Zr0.02O3 (BLFZ) ceramics were synthesized by conventional solid state reaction method. X-ray diffraction studies and Rietveld refinements indicated that both of the samples possessed rhombohedrally distorted perovskite structure. The average grain size of BLFZ (~700 nm) was greatly reduced in comparison with BLF (~6 μm). The remanent magnetization of BLFZ was greatly enhanced compared with BLF. The enhancement of remanent magnetization was attributed to the collapse of the spiral spin structure. Both samples showed exchange bias. The possible reason for the appearance of exchange bias was attributed to the inhomogeneity in doping. The measurements of magnetization as a function of temperature indicated that BLF and BLFZ had almost the same magnetic transition temperature (~355 °C).

Spin-glass-like freezing and enhanced magnetization in ultra-smallCoFe2O4 nanoparticles

The magnetic properties of ultra-small (3 nm)CoFe2O4 nanoparticles have been investigated by DC magnetization measurements as a function oftemperature and magnetic field. The main features of the magnetic behaviour areblocking of non-interacting particle moments (zero-field-cooled magnetizationTmax≈40 K),a rapid increase of saturation magnetization (up to values higher than for the bulk material) at lowT and an increase in anisotropy below 30 K due to the appearance of exchange bias. The lowtemperature behaviour is determined by a random freezing of surface spins. Localizedspin-canting and cation distribution between the two sublattices of the spinel structureaccount quantitatively for the observed increase in saturation magnetization.

Growth and Characterization of BiFeO3 Film for Novel Device Applications

Among multiferroic materials, BiFeO3 (BFO) is much attractive owing to its high Curie temperature (T c ) and Neel temperature (T N ), which presents opportunities for potential device applications at room temperature. Single phase and epitaxial BFO thin films were deposited on SrRuO3 /SrTiO3 substrates by pulsed-laser deposition method. BFO film showed the remnant polarization of as large as 45 μ C/cm2 and strong immunity for the fatigue cycles up to 109. In addition, the antiferromagnetic nature of BFO film was confirmed by clear appearance of exchange bias of 50 Oe in spin valve system. And, based on the observed multiferroic properties of BFO film, we proposed a novel multi-bit memory cell.

New Trends in Magnetic Exchange Bias

New Trends in Magnetic Exchange Bias

Theoretical study of carbon-coated iron nanowires

Several properties of hybrid systems made of iron nanowires coated with carbon are computed from first principles. In particular, we focus on how the presence of carbon determines the magnetic ordering. A quasi-one-dimensional fcc (or hcp) Fe structure favors ferromagnetic ordering, but when encapsulated into a C tube, antiferromagnetic ordering can become favorable. The spin polarization at the Fermi level is large for the bare nanowires, but it decreases due to hybridization with the carbon coating. Implications of these results for the fabrication of nanodevices, as well as for the appearance of exchange bias, are discussed.

High-frequency electron magnetic resonance and magnetic studies of ferrihydrite nanoparticles and evidence of a phase transition

Temperature variations (300–4 K) of the electron magnetic resonance (EMR) spectra of ferrihydrite (FeOOH· nH 2O) nanoparticles of size ≃5 nm are reported at the frequencies of ν = 190.62 and 285.93 GHz. The EMR line at 300 K, occurring near g ≃ 2 , broadens and shifts to lower fields with decreasing T for T > T s = 30 K . For T < T s , the trend reverses in that the line narrows and shifts to higher fields. In magnetic studies, the coercivity H c is maximum at T s (approaching zero at the blocking temperature T B ≃ 65 K ) with the appearance of exchange bias H E for T < T s . The magnetic viscosity is also maximum near T s , approaching zero above T B . These observations lead to the suggestion that T s represents a phase transition to a new magnetically ordered state, with spin-glass-like ordering of the uncompensated Fe 3+ spins.

Exchange bias of antiferromagnets with random anisotropies and perfectly compensated interfaces

An alternative mechanism for exchange bias for ferromagnet/antiferromagnet bilayers with completely compensated interfaces is proposed and analyzed within the biquadratic coupling model. We show that a distribution of anisotropies in the antiferromagnet can lead to the appearance of exchange bias in a bilayer with a perfectly compensated interface without defects. The energy associated with the unidirectional anisotropy that gives rise to the shifted hysteresis curve is stored in antiferromagnetic domain walls between regions in the antiferromagnet with different anisotropy. This mechanism also leads naturally to an enhanced coercivity of the ferromagnet which is caused by the anisotropy in the antiferromagnet.

Variation of domain formation in a 15 nm NiFe layer exchange coupled with NiO layers of different thicknesses

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.