Magnetoelectric coupling in MnTiO3

Abstract

We give general arguments that show that the linear magnetoelectric effect in antiferromagnetic materials gives rise to a magnetocapacitance anomaly-a divergence of the dielectric constant at the magnetic ordering temperature T N that appears in an applied magnetic field. The measurement of magnetodielectric response thus provides a definitive and experimentally accessible method to recognize antiferromagnetic linear magnetoelectric materials, circumventing the experimental difficulties often involved in measuring electric polarization. We confirm this result experimentally using the example of MnTiO 3 , which we show to exhibit the linear magnetoelectric effect. No dielectric anomaly is observed at T N in the absence of an applied magnetic field. However, a sharp peak in the dielectric constant appears here when a magnetic field is applied along the c axis, reflecting a linear coupling of the polarization P with the antiferromagnetic order parameter L. In accordance with our theoretical analysis, the dielectric constant close to T N increases with the square of the magnetic field.

Electronic structure and magnetic symmetry in MnTiO3analyzed by second harmonic generation

“…In the spin-flop phase of Cr 2 O 3 , its ME tensor acquires non-zero antisymmetric off-diagonal terms, and this is indicative of non-zero toroidic moments above the critical spin-flop magnetic field 34 . The linear magnetoelectric effect in MnTiO 3 has been experimentally verified by Mufti et al 14 . Therefore, MnTiO 3 has also been proposed as a possible ferrotoroidic candidate.…”

“…Specific heat measurements ( Fig. 1(e)) provided evidence for a continuous phase transition around 64 K, and prior magnetocapacitance measurements revealed no dielectric anomaly in zero magnetic field at T N (also confirmed in our samples) 14 . Therefore, no change in lattice symmetry was observed at T N and so the MD process was related to the crystalline structure, while the ED process was related to the magnetic structure 31 .…”

“…This was remarkable since the rotation of the spins into the ab-plane removed the trigonal axis of the zero magnetic field AFM phase, thereby lowering the magnetic symmetry group. Although the zero magnetic field AFM symmetry of MnTiO 3 has been reported as 3 14,30,31,35 , it is possible that the magnetic cation sublattice still retains the twofold axes and mirror planes of the higher symmetry corundum structure (i.e. 3 2/m).…”

“…However, below T N , the point group becomes 3 14,30,31,35 , due to the antiferromagnetic ordering of spins breaking both spatial and temporal symmetry. However, the combined space-time-reversal operation, 1, remains a symmetry element.…”

“…It has also been reported that these materials are important for technological applications such as memory devices and solar cells [3][4][5][6][7][8][9][10][11] . Antiferromagnetic MnTiO 3 (T N = 64 K), exhibiting ilmenite structure, possesses a magnetic field induced spin-flop phase (H c = 6.5 T), and it further exhibits a linear magnetoelectric (ME) effect [12][13][14] . It is thought that the spin-flop phase might also display ferrotoroidic order 12 , which is a novel state characterized by a spontaneously ordered vortex of spins.…”