Large structure-dependent room temperature exchange bias in self-assembled BiFeO3 nanoparticles

Abstract

We studied the magnetic properties of self-assembled aggregates of BiFeO3 nanoparticles (~ 20-40 nm). The aggregates formed two different structures - one with limited and another with massive cross-linking - via `drying-mediated self-assembly' process following dispersion of the nanoparticles within different organic solvents. They exhibit large coercivity H_C (>1000 Oe) and exchange bias field H_E (~ 350-900 Oe) in comparison to what is observed in isolated nanoparticles (H_C ~ 250 Oe; H_E ~ 0). The H_E turns out to be switching from negative to positive depending on the structure of the aggregates with |H_E| being larger. The magnetic force microscopy reveals the magnetic domains (extending across 7-10 nanoparticles) as well as the domain switching characteristics and corroborate the results of magnetic measurements. Numerical simulation of the `drying-mediated-self-assembly' process shows that the nanoparticle-solvent interaction plays an important role in forming the `nanoparticle aggregate structures' observed experimentally. Numerical simulation of the magnetic hysteresis loops, on the other hand, points out the importance of spin pinning at the surface of nanoparticles as a result of surface functionalization of the particles in different suspension media. Depending on the concentration of pinned spins at the surface pointing preferably along the easy-axis direction - from greater than 50\% to less than 50% - H_E switches from negative to positive. Quite aside from bulk sample and isolated nanoparticle, nanoparticle aggregates - resulting from surface functionalization - therefore, offer remarkable tunability of properties depending on structures.

14 pages, 33 pdf figures. Contact authors for the supplementary data and movies

References in corpus (9)

• Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite
• Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides
• Electric-field-induced spin-flop in BiFeO3 single crystals at room-temperature
• Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias
• Particle size dependence of magnetization and non-centrosymmetry in nanoscale BiFeO3
• Large magnetoelectric coupling in nanoscale BiFeO${}_3$ from direct electrical measurements
• Origin of the asymmetric exchange bias in BiFeO${}_3$-Bi${}_2$Fe${}_4$O${}_9$ nanocomposite
• Effect of surface pinning on magnetic nanostuctures
• Reply to the Comment on "Superspin Glass Mediated Giant Spontaneous Exchange Bias in BiFeO${}_3$-Bi${}_2$Fe${}_4$O${}_9$ Nanocomposite [Phys. Rev. Lett. 110, 107201 (2013)]"