Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

Abstract

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 A, b = 7.833 A, and c = 7.551 A. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.