Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite of BiFeO${}_3$-Bi${}_2$Fe${}_4$O${}_9$

Abstract

We observe an enormous $\text{spontaneous}$ exchange bias ($\sim$300-600 Oe) - measured in an unmagnetized state following zero-field cooling - in a nanocomposite of BiFeO${}_3$ ($\sim$94%)-Bi${}_2$Fe${}_4$O${}_9$ ($\sim$6%) over a temperature range 5-300 K. Depending on the path followed in tracing the hysteresis loop - positive (p) or negative (n) - as well as the maximum field applied, the exchange bias ($H_E$) varies significantly with $\mid -H_{Ep}\mid$ $>$ $\mid H_{En}\mid$. The temperature dependence of $H_E$ is nonmonotonic. It increases, initially, till $\sim$150 K and then decreases as the blocking temperature $T_B$ is approched. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by "superinteraction bias coupling" between ferromagnetic core of Bi${}_2$Fe${}_4$O${}_9$ (of average size $\sim$19 nm) and canted antiferromagnetic structure of BiFeO${}_3$ (of average size $\sim$112 nm) via superspin glass moments at the shell.

5 pages with 4 figures; published in Phys. Rev. Lett

References in corpus (5)

• Aging and Memory Effects in Superparamagnets and Superspin Glasses
• Large exchange bias after zero-field cooling from an unmagnetized state
• Nanoscale control of exchange bias with BiFeO3 thin films
• Training Induced Positive Exchange Bias in NiFe/IrMn Bilayers
• Particle size dependence of magnetization and non-centrosymmetry in nanoscale BiFeO3

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