Evolution of magnetic properties of CaMn_1-xNb_xO₃ with Nb-doping

Magnetic and structural properties of Nb-doped CaMnO₃ have been studied and the effect of doping with 0.02 x 0.1 has been investigated. Substitution of Nb⁵⁺ ion for the Mn⁴⁺ site of the parent matrix causes one-electron doping with the chemical formula , accompanied by a monotonous increase of the lattice parameters, unit-cell volume, average Mn-O bond distance and a decrease in Mn-O-Mn bond angle, with increasing x. Low temperature magnetic ground state of CaMn_1-xNb_xO₃ has been found to be dependent on niobium doping level. The ground magnetic state evolves from mostly antiferromagnetic, with a weak ferromagnetic component for x = 0.02-0.08, to charge ordered C-type antiferromagnetic state at x = 0.1. Spontaneous magnetization increases sharply with increasing doping level, approaches a maximal value of 4.1 emu g^-1 at T = 10 K for x = 0.08, and then decreases rapidly to reach a very small value of 0.2 emu g^-1 for x = 0.1. Anomalous negative magnetization behavior below the magnetic transition temperature has been observed for the compound with x = 0.04 in the field cooled magnetization and remanent dc magnetization measurements. Vertical and horizontal shifts of the hysteresis loop of the field cooled sample have been observed for CaMn_0.9Nb_0.1O₃ as possible signatures of the exchange bias effect. The effect of hydrostatic pressure on dc magnetization for the sample with x > 0.02 revealed a significant increase of the ferromagnetic phase volume under pressure, linked to progressive suppression of a negative magnetization in x = 0.04 sample.