Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo2 –xO5.5 – δ

Abstract

The magnetic properties of polycrystalline EuBaCo2 –xO5.5 – δ (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo1.90O5.36, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co3+ ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co3+ ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co3+ ions in the low-spin state can also act as defects.