Sawtooth lattice multiferroic BeCr2 O4: Noncollinear magnetic structure and multiple magnetic transitions

Noncollinear magnetic structures and multiple magnetic phase transitions in a sawtooth lattice antiferromagnet consisting of Cr3+ are experimentally identified in this work, thereby proposing the scenario of magnetism-driven ferroelectricity in a sawtooth lattice. The title compound, BeCr2O4, displays three magnetic phase transitions at low temperatures - at TN1≈7.5 K, at TN2≈25 K, and at TN3≈26 K - revealed through magnetic susceptibility, specific heat, and neutron diffraction in this work. These magnetic phase transitions are found to be influenced by externally applied magnetic fields. Isothermal magnetization curves at low temperatures below the magnetic transitions indicate the antiferromagnetic nature of BeCr2O4 with two spin-flop-like transitions occurring at Hc1≈29 kOe and Hc2≈47 kOe. Our high-resolution x-ray and neutron diffraction studies, performed on single crystal and powder samples, unambiguously determined the crystal structure as orthorhombic Pbnm. By performing the magnetic superspace group analysis of the neutron diffraction data at low temperatures, the magnetic structure in the temperature range TN3,N2<T<TN1 is determined to be the polar magnetic space group P21nm.1′(00g)0s0s with a cycloidal magnetic propagation vector k1=(0,0,0.090(1)). The magnetic structure in the newly identified phase below TN1 is determined as P21/b.1′[b](00g)00s with the magnetic propagation vector k2=(0,0,0.908(1)). The cycloidal spin structure determined in our work is usually associated with electric polarization, thereby making BeCr2O4 a promising multiferroic belonging to the sparsely populated family of sawtooth lattice antiferromagnets.