Absence of zero-field-cooled exchange bias effect in single crystalline La2−xAx CoMnO6

C. Macchiutti

¹

,

J.R. Jesus

²

,

F. B. Carneiro

³

et al.

Abstract

Magnetic properties of A 2 BB O 6 (A = rare or alkaline-earth ions; B, B = transition-metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero-field-cooled exchange bias effect observed for the hole-doped La 2−x A x CoMnO 6 polycrystalline samples. In this paper we synthesize La 2 CoMnO 6 , La 1.5 Ca 0.5 CoMnO 6 , and La 1.5 Sr 0.5 CoMnO 6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca-and Sr-doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin-glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples.

The spontaneous exchange bias effect in La2-xCaxCoMnO6 series

“…Such pronounced changes in the SEB effect from one sample to another can be attributed to different proportions of Co 2+ /Co 3+ and to the spin state of Co 3+ , that affect the uncompensated antiferromagnetic (AFM) Co-Mn coupling [17,18,19]. Interestingly, single-crystalline La 2−x A x CoMnO 6 (A = Ca, Sr) samples are not reentrant SG and showed no trace of SEB effect, giving further evidence that an SG-like phase is necessary for the emergence of SEB [20] Very recently, a thorough investigation of the La 1.5 (Sr 0.5−x Ba x )CoMnO 6 series has evidenced how the crystal structure affects the SEB effect by indirectly altering the systems' electronic structure [21]. The lattice expansion due to Sr 2+ to Ba 2+ partial substitution increases the portion of HS Co 3+ , which by one hand strengthens the Co 3+ -Mn 4+ interaction, and by the other, it decreases the portion of uncompensated spins ate the magnetic interfaces.…”