Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La1/3_{1/3}Sr2/3_{2/3}FeO3−δ_{3-\delta}: Magnetization, Mossbauer, and polarised neutron studies

The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M\"{o}ssbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the M\"{o}ssbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe$^{3+}$ and Fe$^{5+}$ are about 3.15$\mu_B$ and 1.57$\mu_B$ for bulk, and 2.7$\mu_B$ and 0.53$\mu_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\sim$ 70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass like transition around $\sim$ 65 K, below which EB appears. Overall results propose that finite size effect directs the complex glassy magnetic behavior driven by unconventional short range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.Comment: 10 pages, 9 figures. Fig. 1 available upon request or in http://www.ffn.ub.es/oscar/Articles.html. Accepted in Phys. Rev.

Exchange bias effect in alloys and compounds

The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, cluster-glass and disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus on improvement of fundamental issues of the exchange bias effects rather than on their technological importance

Size effect on magnetic phase coexistence in Pr_0.5Sr_0.5Mn_1-xCr_xO₃

Magnetic and transport properties are investigated in Pr_0.5Sr_0.5Mn_1-xCr_xO₃ (0 ⩽ x ⩽ 0.05) focusing on the effect of Cr substitution and particle size dependence. The studies demonstrate that magnetic and transport properties are significantly different for nanocrystalline compound from the bulk. Neutron depolarization and diffraction studies confirm that different fractions of magnetic phase separation between antiferromagnetic and ferromagnetic components, and ferromagnetic domain size give rise to the different magnetic and transport properties. Intriguingly, nanocrystalline compound with x = 0.05 shows apparent signature of arrested state in magnetization and magnetoresistance both in thermal and magnetic field variations which is absent in the bulk counterpart. A spin-glass like state is identified for nano at low temperature, which is argued to be correlated with the arrested or pinning mechanism

Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La1/3Sr2/3FeO3-δ: Magnetization, Mössbauer, and polarized neutron studies

The charge ordered La1/3Sr2/3FeO3−δ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M¨ossbauer, and polarized neutron studies. A complex scenario of short-range charge and magnetic ordering is realized from the polarized neutron studies in nanocrystalline specimen. This short-range ordering does not involve any change in spin state and modification in the charge disproportion between Fe3+ and Fe5+ compared to bulk counterpart as evident in the M¨ossbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe3+ and Fe5+ are about 3.15 μB and 1.57 μB for bulk, and 2.7 μB and 0.53 μB for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size ∼70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass-like transition around ∼65 K, below which EB appears. Overall results propose that finite-size effect directs the complex glassy magnetic behavior driven by unconventional short-range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO