Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite ofBiFeO3−Bi2Fe4O9

Abstract

We observe an enormous spontaneous exchange bias (~300-600 Oe)--measured in an unmagnetized state following zero-field cooling--in a nanocomposite of BiFeO(3) (~94%)-Bi(2)Fe(4)O(9) (~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 | - H(Ep) | > | H(En) |. The temperature dependence of H(E) is nonmonotonic. It increases, initially, till ~150 K and then decreases as the blocking temperature T(B) is approached. 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 the ferromagnetic core of Bi(2)Fe(4)O(9) (of average size ~19 nm) and the canted antiferromagnetic structure of BiFeO(3) (of average size ~112 nm) via superspin glass moments at the shell.

Spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9

“…The coarser BiFeO 3 particles, on the other hand, are antiferromagnetic with uncompensated local spins. It has also been observed that there are superparamagnetic domains with a blocking temperature T B > 350 K. 4 The memory effect, on the other hand, signifies the presence of superspin glass moments. As the interparticle distance reduces and the exchange interaction increases, the superparamagnetic domains give way, initially, to superspin glass phase and then even superferromagnetic phase as well.…”

“…In recent times, the spontaneous exchange bias (SEB) has been reported for different alloys and nanoparticle composite systems. [1][2][3][4] The origin of this appears to be lying in the biaxiality of AFM grains and variation in the FM-AFM bias coupling among an ensemble of grains. We report here that we have observed an even more interesting feature of the SEBvariation in the magnitude of the bias depending on the path followed in tracing the hysteresis loop-in a nanocomposite of BiFeO 3 -Bi 2 Fe 4 O 9 .…”

“…From the detailed analyses of the microstructure and crystallographic data of the nanocomposite, 4 it has been found out that the BiFeO 3 particles are bigger ($112 nm) while the Bi 2 Fe 4 O 9 particles are finer ($19 nm). It has already been reported by others 8 that finer Bi 2 Fe 4 O 9 particles exhibit FM order.…”

“…% Bi 2 Fe 4 O 9 which exhibits maximum exchange bias. While in an earlier work 4 we focussed primarily on the extent of spontaneous and conventional exchange bias observed across a temperature range of 5-300 K and path dependency at a particular temperature, we lay more emphasis here on how the path dependency of the SEB and CEB evolves as a function of temperature dependence. In Fig.…”

“…The details of the microstructural and crystallographic data and their analyses are available elsewhere. 4 The magnetic measurements have been carried out primarily in a SQUID magnetometer (MPMS; Quantum Design) and also in a vibrating sample magnetometer (VSM) of LakeShore Cryotronics, Inc. The SEB has been measured following zero-field cooling from $350 K. In order to ensure that the sample is not biased even by the trapped flux of the superconducting magnet, we have discharged the magnet following appropriate demagnetization protocol where the field is decreased in an oscillation mode.…”