Presence Of Exchange Bias Field Research Articles

Magnetism at the Interface of Magnetic Oxide and Nonmagnetic Semiconductor Quantum Dots.

Engineering interfaces specifically in quantum dot (QD) heterostructures provide several prospects for developing multifunctional building block materials. Precise control over internal structure by chemical synthesis offers a combination of different properties in QDs and allows us to study their fundamental properties, depending on their structure. Herein, we studied the interface of magnetic/nonmagnetic Fe3O4/CdS QD heterostructures. In this work, we demonstrate the decrease in the size of the magnetic core due to annealing at high temperature by the decrease in saturation magnetization and blocking temperature. Furthermore, surprisingly, in a prominently optically active and magnetically inactive material such as CdS, we observe the presence of substantial exchange bias in spite of the nonmagnetic nature of CdS QDs. The presence of exchange bias was proven by the increase in magnetic anisotropy as well as the presence of exchange bias field (HE) during the field-cooled magnetic measurements. This exchange coupling was eventually traced to the in situ formation of a thin antiferromagnetic FeS layer at the interface. This is verified by the study of Fe local structure using X-ray absorption fine structure spectroscopy, demonstrating the importance of interface engineering in QDs.

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.

Crossover of spin glass characteristics as a function of field in an NiMnSnAl alloy

The magnetic state in martensitic phase of Ni50Mn34Sn6Al10 shows interesting glassy behaviour. The irreversibility was observed in d.c. magnetization measurement below martensitic transformation temperature. Further measurements at lower temperature showed the presence of exchange bias field in sample. H-T phase diagram of the irreversibility showed that the system was in Heisenberg like spin glass state at low field, which changed its type when the external field was above the exchange bias field. In order to determine the cause of cross over, a.c. susceptibility measurements were performed in zero and d.c. biasing fields. This switching of glassy nature of the system is a novel phenomenon.

Exchange bias and suppression of superparamagnetism of α-Fe nanoparticles in NiO matrix

Nanocomposites of Fe–NiO are synthesized by the mechanical milling technique. The phase purity of the sample was checked by X-ray diffraction (XRD), which shows only lines of α-Fe and NiO. Transmission electron microscopy (TEM) measurements confirmed a homogeneous dispersion of α-Fe nanoparticles in NiO matrix. Magnetic hysteresis measurements showed an enhancement in coercivity at room temperature with increasing milling durations and presence of exchange bias field. With increasing milling durations, the exchange field is found to increase while blocking temperature decreased. Presence of defects in Fe–NiO nanocomposites ball-milled for prolonged durations is confirmed by positron annihilation lifetime spectroscopy (PALS) measurements. Mössbauer spectroscopic measurements reveal that Fe nanoparticles are in the blocked state though their size is less than the critical size for becoming single domain and superparamagnetic. This is attributed to strong exchange coupling between the ferromagnetic and antiferromagnetic moments.

Exchange bias effect in ball milled Co–Cr 2O 3 FM–AFM nanocomposites

Exchange coupled nanocomposite powders (Co) x (Cr 2O 3) 1− x consisting of mixture of ferromagnetic (FM) Co and antiferromagnetic (AFM) Cr 2O 3 have been prepared by mechanical ball milling. Upon field cooling, going through the Neel temperature, T N ∼ 307 K, of Cr 2O 3, a hysteresis loop shift, coercivity, and remanence enhancement were observed; effects indicative of the presence of exchange bias field ( H E) between the FM–AFM phases of the composite. The temperature dependent exchange bias field measurement shows that the exchange bias field reduces to zero at temperature T < ∼200 K < T N (307 K) of Cr 2O 3. However, since FM nanoparticles remain magnetic above T N (Cr 2O 3), the coercivity remains finite up to the room temperature. The role of interparticle interaction over surface spin disorder effects is also assessed by varying the FM/AFM ratio. The surface-spin disorders and dipolar interaction influence the magnetic properties of the composite at low and high FM content of the composite, respectively. The present study has potential implication of improving magnetic properties of hard-magnetic materials upon milling with antiferromagnetic materials.