Exchange Bias Effect in NdFeO_3 System of Nanoparticles

Abstract

We study the effect of nanometric size on the crystal structure, magnetic environment of iron and magnetization in NdFeO3 system of nanoparticles. The average particle size of NdFeO3 nanoparticles increases with annealing at 600• C from about 15 nm to 40 nm. The smallest particles on annealed sample have size approximately 30 nm and typically have character of single crystalline samples. All samples adopt orthorhombic crystal structure, space group Pnma with lattice parameters a = 5.5817 Å, b = 7.7663 Å and c = 5.456 Å for as prepared sample. The presence of superparamagnetic particles was indicated by the Mössbauer measurements. The reduction of dimensionality induces a decrease of TN1 from 691 K to 544 K. The shift of magnetic hysteresis loop in vertical and horizontal direction was observed at low temperatures after cooling in magnetic field. We attribute such behaviour to exchange bias effect and discuss in the frame of core-shell model. IntroductionThe exchange bias (EB) was discovered more than 55 years ago, by Meiklejohn and Bean on Co/CoO coreshell nanoparticles [1], and its characteristic signature is the horizontal shift of the centre of magnetic hysteresis loop from its normal position at H = 0 to another one at H e = 0 and vertical shift which can be characterised by remnant asymmetry µ e . EB usually occurs in systems which are composed by an antiferromagnet (AFM) that is in atomic contact with a ferromagnet (FM) after the system is cooled, below the respective Néel and Curie temperatures T N and T C , in an external cooling field H cf . EB phenomena were observed in various materials like the Laves phases, intermetallic compounds and alloys, binary alloys, the Heusler alloys [2] or on layered bulk fluorometallocomplex [3], where different aspects of magnetism were focused from the EB effect. The first evidence of the EB effect in mixed-valent manganites having perovskite structure was reported in a spontaneously phase separated system Pr 1/3 Ca 2/3 MnO 3 [4] which stimulated new interest for study of the EB effect in structurally single-phase compounds. In the case of a nanoparticle (NAP) system the surface to volume ratio becomes significantly large compared to the bulk counterpart. In such a case the surface effect dominates over and core-shell model can provide good interpretation of observed phenomena. Both concepts were frequently used for interpretation of EB effects in the La 1−x Ca x MnO 3 [5, 6], Nd 0.5 Ca 0.5 MnO 3 [7] and Pr 0.5 Ca

Evaluation of the optical and magnetic properties of novel Nd0.9Zn0.1FeO3 perovskite nanoparticles and their adsorption of Pb2+ ions from water

“…The shift of the hysteresis loop to the positive direction is due to the exchange bias (EB) field 34 . The EB originates from the coupling between the ferromagnetic spins and AFM spins in the sample.…”

Exchange Bias in Nanostructures: An Update

“…In Zn-substituted nickel ferrite (Zn 0.3 Ni 0.7 Fe 2 O 4 ) nanoparticles with diameters of 5-33 nm, an EB shift of 1.2 kOe was observed, which was quite large for a single-phase system and was attributed to the interaction between disordered surface spins and highly ordered core spins [156]. NdFeO 3 nanoparticle systems also showed an EB at low temperatures with only a weak training effect, which was not investigated further [157]. Pure nickel ferrite was used as a shell around an antiferromagnetic BiFeO 3 core and not only caused an EB, as expected due to the exchange coupling at the interface, but also showed reduced coercivity after field cooling, as compared to zero-field cooling, which was the opposite to the usual behavior of EB systems [158].…”

Modeling the Training Effect in Exchange‐Biased Bilayers for Large Numbers of Magnetization Reversal Cycles

“…[5,6] Another often reported feature, associated with the EB phenomenon, is the training effect (TE), i.e., a reduction of the EB shift upon repeated measurements of hysteresis loops without new field cooling. [7][8][9][10] While this effect can be very strong in some systems, there are other material combinations where it is quite weak [11] or even completely absent. [12] Depending on the system, varying explanations for the TE have been proposed.…”