Occurrence Of Exchange Bias Research Articles

Characterization of the kinetic arrest of martensitic transformation in Ni45Co5Mn36.8In13.2 melt-spun ribbons

The kinetic arrest (KA) of martensitic transformation (MT) observed in Ni45Co5Mn36.8In13.2 melt-spun ribbons has been studied. These alloy ribbons show an ordered columnar-like grain microstructure with the longer grain axis growing perpendicular to ribbon plane and transform martensitically from a single austenitic (AST) parent phase with the L21-type crystal structure to a monoclinic incommensurate 6M modulated martensite (MST). Results show that the volume fraction of austenite frozen into the martensitic matrix is proportional to the applied magnetic field. A fully arrest of the structural transition is found for a magnetic field of 7T. The metastable character of the non-equilibrium field-cooled glassy state was characterized by introducing thermal and magnetic field fluctuations or measuring the relaxation of magnetization. The relaxation of magnetization from a field-cooled kinetically arrested state at 5 and 7T follows the Kohlrausch–Williams–Watts (KWW) stretched exponential function with a β exponent around 0.95 indicating the weak metastable nature of the system under the strong magnetic fields. The relationship between the occurrence of exchange bias and the frozen fraction of AST into the MST matrix was studied.

Effect of Fe doping in the structural, electronic and magnetic properties of CoCr2O4: insights from ab initio calculations

CoCr2O4 has attracted significant attention recently due to several interesting properties such as magnetostriction, magnetoelectricity etc. More recent experiments on Fe substituted CoCr2O4 observed a variety of novel phenomena such as the magnetic compensation accompanied by the occurrence of exchange bias, which reverses its sign. Understanding of such phenomena may lead to control the properties of these material in an efficient way to enhance its potential for multifunctional applications. In this paper, we study the fundamental understanding of Fe doping in modifying the structural and magnetic properties of CoCr2O4 with varying composition and substitution of Fe at different sublattices by first-principles density functional calculations. We have analysed in detail the effect of Fe substitution on crystal field and exchange splittings, magnetic moments and interatomic exchange parameters. It is also observed that with increasing concentration of Fe impurity, the system has a tendency towards forming an ‘inverse spinel’ structure as observed in experiments. Such tendencies are crucial to understand this system as it would lead to modifications in the magnetic exchange interactions associated with sites with different symmetry finally affecting the magnetic structure and the multiferrocity in turn.

Negative rotatable anisotropy in IrMn/Cr/Co thin films

This work presents modifications of the magnetic properties of polycrystalline IrMn/Cr(${t}_{\mathrm{Cr}}$)/Co films where the Cr spacer thickness ${t}_{\mathrm{Cr}}$ was varied between 0.25 and 3 nm. High-resolution transmission electron microscopy, x-ray diffractometry and reflectivity, as well as x-ray absorption near-edge structure were used for the structural characterization of the films; static magnetization curves as well as ferromagnetic resonance measurements were employed for the magnetic characterization. Decrease of the exchange-bias field and considerable and nonmonotonous enhancement of the coercivity with ${t}_{\mathrm{Cr}}$ were observed. The films' anisotropy parameters were extracted from the experimental angular variations of the resonance field and from the hard-axis magnetization curves via numerical simulations. It was obtained that the two phenomena studied, namely, the exchange bias and rotatable anisotropy, have different origins. The rotatable anisotropy was ascribed to Cr interface coupled antiferromagnetically with the Co atoms. On the other hand, the occurrence of exchange bias, even for the film with the thickest Cr layer, was attributed to uncompensated spins at the topmost IrMn interface. Together with the significant increase with ${t}_{\mathrm{Cr}}$ of the rotatable anisotropy field, near saturation it is antiparallel to the external magnetic field.

Effect of Fe-doping concentration on microstructure, electrical, and magnetic properties of Pb(Zr0.5Ti0.5)O3 thin films prepared by chemical solution deposition

The highly (l00) oriented Pb(Zr0.5Ti0.5)O3 thin films with different Fe3+ doping concentrations were fabricated on LaNiO3-coated silicon substrates by chemical solution deposition. And the microstructure, ferroelectric, leakage, and magnetic properties were investigated. The results indicate that incorporation of Fe3+ into PZT thin films can promote the degree of the lattice distortion and greatly improve the surface roughness. In comparison with the pure PZT sample, the ferroelectric hysteresis loops of Fe-doped PZT samples demonstrate larger and larger polarizations and coercive fields with the increase in Fe3+ doping amount. Moreover, leakage mechanism of present films evolves from the space charge limited conduction to the “modified” space charge limited conduction, and then returns to the space charge limited conduction with increasing Fe3+ doping concentration. The occurrence of exchange bias in these Fe-doped PZT samples implies that the magnetic exchange interaction can be explained by the bound magnetic polaron model.

Controlling exchange bias in Co–CoOx nanoparticles by oxygen content

We report on the occurrence of exchange bias on laser-ablated granular thin filmscomposed of Co nanoparticles embedded in an amorphous zirconia matrix.The deposition method allows one to control the degree of oxidation of theCo particles by tuning the oxygen pressure at the vacuum chamber (from2 × 10−5 to10−1 mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix ismonitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM)CoOx, with a FM/AFM intermediate regime for which the percentage of the AFM phasecan be increased at the expense of the FM phase, leading to the occurrence ofexchange bias in particles of about 2 nm in size. For an oxygen pressure of about10−3 mbar the ratio between the FM and AFM phases is optimum with an exchange bias field ofabout 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also atthe origin of the induced exchange anisotropy on the FM leading to high irreversiblehysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.

The dynamics of magnetic ordering in a natural hemo-ilmenite solid solution

SUMMARY We investigated the micromagnetic properties of hemo-ilmenite particles in an alluvial soil. All magnetic accessory minerals except the weathering resistant hemo-ilmenite grains were removed from the soil matrix by chemical treatment with concentrated acid followed by magnetic separation. X-ray diffraction revealed hemo-ilmenite grains with single crystal properties and an ilmenite mole fraction of y = 0.86. Magnetization versus applied magnetic field plots in a temperature range between 6 and 300 K were recorded in order to study the hysteresis and the exchange properties. In addition, field and frequency-dependent AC susceptibility measurements were performed with and without a DC bias field in order to analyse the dynamic magnetization of the sample down to 3 K. The hemo-ilmenite particles are considered as a mixed system with nano-sized cationordered areas (COA) and cation-disordered areas (CDA), which differ in their local Fe (III) concentration. Ferrimagnetic single-domain order in the Fe (III) -enriched CDA started at about 220 K. Upon cooling gradual transdomain transformation generates multidomain order. A maximum in the blocking distribution was reached at 44 K, followed by the onset of spin-glass dynamics. At lower temperature, blocking of superparamagnetic clusters in the COA created antiferromagnetic (AFM) ordering, which became more prominent with decreasing temperature. The interaction between the spin-glass like CDA and the AFM areas was documented by the onset of exchange bias at T < 20 K. The occurrence of exchange bias as well as spin-glass dynamics in the hemo-ilmenite grains is probably an effect of the inhomogeneity of the local Fe (III) concentration. This effect leaves a magnetically competitive regime with areas showing ilmenite-like magnetic properties, and Fe (III) rich disordered areas with magnetic long-range ordering up to 220 K and frustration near the ordering temperature of ilmenite.

Exchange Bias Phenomenology and Models of Core/Shell Nanoparticles

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Coercivity and exchange bias of ferromagnetic/antiferromagnetic multilayers

For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic layer with a compensated interface, detailed mean-field-type calculations are performed. Both the coercive field and the exchange bias field are calculated. For the coercive field, a rather broad enhancement around the N\'eel temperature ${T}_{N}$ of the antiferromagnetic layer is found irrespective of whether the antiferromagnetic layer is structurally disordered or not, while exchange bias is only found for disordered systems. We show that the observed enhancement of the coercivity around ${T}_{N}$ also found experimentally and the occurrence of exchange bias are of different origin.