Electronic phenomena studied in the nordic countries

Magnetic phases in a mixed anisotropy magnetoelectric system

Not scheduled

15m

Max IV Lund

Speaker

Dr Rasmus Toft-Petersen (DTU, Denmark)

Description

In many classes of isostructural magnetic systems with a replaceable transition metal ion, the exchange constants are very similar, but the single ion anisotropy will inevitably vary greatly with the flavor of the magnetic ion. A textbook example is the inherently and system-independent different nature of the Heisenberg case of Mn2+ (L = 0), and the Kramer’s doublet Ising-like case of Co2+ (S = 3/2). The family lithium orthophosphate family of magnetoelectrics – LiMPO4 (M = Mn, Ni, Fe and Co) – have very similar exchange interaction but unusually strong single ion anisotropy terms [1, 2, 3], due to the very low symmetry of the crystal field. In addition, the nature of the single ion anisotropy differ greatly from system to system, from Heisenberg over XY-like to Ising, which is the underlying factor determining the non-zero magnetoelectric coupling constants of the family. In mixed anisotropy magnets [4], the ion-dependent anisotropy in is competition with the exchange terms and a peculiar form of frustration ensues. We have investigated the consequence of that frustration in the compound LiNi0.8Fe0.2PO4. The two mother compounds both have XY-like anisotropy but of almost reciprocal nature. LiNiPO4 is XY-like with an easy ac-plane and hard b-axis, while LiFePO4 has an easy ab-plane and a hard c-axis. We used XYZ-polarized neutron diffraction to find two new phases in LiNi0.8Fe0.2PO4. At 26 K, only the Fe-ions seem to order along b in a mean field like transition with β = ½. Below 20 K, collective ordering of all magnetic ions occur along the intermediate a-axis for both system, suppressing the b-component of the ordered moment. In zero field, the spin-waves are broadened, suggesting a finite lifetime of the excitations. At 8 T longitudinal field, we find a re-orientation of the ordered moments followed by a sharpening of the spin waves to the resolution limit. Just recently, Monte Carlo simulations produced the two observed zero field phases from a Hamiltonian of minimal complexity. An interesting consequence of this permutable orientation of the ordered moments, is an equally permutable magnetoelectric effect. From pyroelectric current measurements we show that most of the magnetoelectric tensor components are non-zero in one phase or another, demonstrating the possibility of tailoring the magnetoelectric effect by mixing anisotropies. [1] T. B. S. Jensen, et. al., Phys Rev B, 79, 092413 (2009) [2] R. Toft-Petersen, et. al., Phys Rev B, 84, 054408 (2011) [3] R. Toft-Petersen, et. al. Phys Rev B, 92, 024404 (2015) [4] Q. J. Harris, et. al. Phys. Rev. Lett. 78, 346 (1997)