Studies on magnetoelectric effects in Fe3O4 nanoparticles and S = 1/2 frustrated spin systems

Abstract

Magnetoelectric (ME) effect is the phenomenon in which the electric polarization (P) is controlled by magnetic field (H) or the magnetization is controlled by electric field. From early 2000s, the magneto-electric properties have attracted tremendous interest due to fundamental interests on the physics of ferroelectricity induced by spin order and great application potential for numerous low-power electronic devices. Therefore, thousands of reports on extensive experimental results and theoretical models have well established the understandings on the ME effect. However, there has been lack of extensive and quantitative investigations on the ME effect in nano-sized particles and spin S = 1/2 systems in which the high quantum effects are expected. In this thesis, I focus on the nanoparticles of Fe3O4 and two new discovered ME materials with Cu2+ (S = 1/2) ions, PbCu3TeO7 and Cu3TeO6. First, we investigate quantitatively the magnetoelectric coupling of spherical Fe3O4 nanoparticles with uniform diameters from 3 to 15 nm embedded in an insulating host, using a sensitive ME susceptometer. The intrinsic ME susceptibility is measured, exhibiting a maximum value of ∼0.6 ps/m at 5 K for d = 15 nm. We found that the ME susceptibility is reduced with reduced d but remains finite until d = ∼5 nm, which is close to the critical thickness for observing the Verwey transition. Moreover, with reduced diameter the critical temperature below which the ME susceptibility becomes conspicuous increased systematically from 9.8 K in the bulk to 19.7 K in the nanoparticles with d = 7 nm, reflecting the core−shell effect on the ME properties. In addition, we report the observation of H induced P in a Cu2+ (S=1/2) based staircase kagome compound PbCu3TeO7, in which anisotropic magnetic exchange interaction and spin frustration result in two Neel temperatures at TN1=35 K and TN2=24 K. Below TN2, both pyroelectric and ME current measurements reveal that a finite P//a up to 15 µC/m2 develops under H//c of ~ 8.3 T at which the field induced spin flop transition occurs as verified by the magnetization curve. Furthermore, measurements under H//a uncover that the P =14 µC/m2 appears at ~16 T and disappears at ~38 T. Monte-Carlo simulations reveals that the two antiferromagnetic spin ordering exhibit a sinusoidal modulation below TN1 and an incommensurate proper screw type spin rotation below TN2. The simulation results uncover an ab plane type spiral order under H//a and H//c, while the successive transition to form spiral spin order rotating in bc-plane for H//a. In combination with the experimental and theoretical results, we propose field-induced phase diagram along the two applied H directions. Finally, we study the ME properties in cubic Cu3TeO6 which is first discovered linear ME compound with Cu2+ (S=1/2). The P increases linearly under H below TN=62 K and the sign of P does remains same upon changing the H direction. The spin structure without spatial inversion symmetry support the linear ME effect. These results are meaningful since they opened new possibility of successive researches on the ME effect by providing new technique of ME measurement and discovering new ME materials.