Magnetoelectric coupling in MnTiO3

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 TN that appears in an applied magnetic field. The measurement of magnetodielectric response thus provides a definitive and experimentally accessiblemethod 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 MnTiO3, which we show to exhibit the linear magnetoelectric effect. No dielectric anomaly is observed at TN 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 TN increases with the square of the magnetic field.

The CARMENES search for exoplanets around M dwarfs: Two planets on opposite sides of the radius gap transiting the nearby M dwarf LTT 3780

We present the discovery and characterisation of two transiting planets observed by the Transiting Exoplanet Survey Satellite (TESS) orbiting the nearby (d∗ ≈ 22 pc), bright (J ≈ 9 mag) M3.5 dwarf LTT 3780 (TOI-732). We confirm both planets and their association with LTT 3780 via ground-based photometry and determine their masses using precise radial velocities measured with the CARMENES spectrograph. Precise stellar parameters determined from CARMENES high-resolution spectra confirm that LTT 3780 is a mid-M dwarf with an effective temperature of Teff = 3360 ± 51 K, a surface gravity of log g∗ = 4.81 ± 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 ± 0.16 dex, with an inferred mass of M∗ = 0.379 ± 0.016M· and a radius of R∗ = 0.382 ± 0.012R·. The ultra-short-period planet LTT 3780 b (Pb = 0.77 d) with a radius of 1.35-0.06+0.06 R·, a mass of 2.34-0.23+0.24 M·, and a bulk density of 5.24-0.81+0.94 g cm-3 joins the population of Earth-size planets with rocky, terrestrial composition. The outer planet, LTT 3780 c, with an orbital period of 12.25 d, radius of 2.42-0.10+0.10 R·, mass of 6.29-0.61+0.63 M·, and mean density of 2.45-0.37+0.44 g cm-3 belongs to the population of dense sub-Neptunes. With the two planets located on opposite sides of the radius gap, this planetary system is anexcellent target for testing planetary formation, evolution, and atmospheric models. In particular, LTT 3780 c is an ideal object for atmospheric studies with the James Webb Space Telescope (JWST)

Magnetodielectric coupling in MnCr2O4 spinel

We have investigated the magnetic and dielectric properties of polycrystalline samples of the spinel MnCr2O4. Below the ferrimagnetic ordering temperature at TN~43 K, both magnetization and dielectric measurements show signatures of the onset of a conical structure at Ts~17 K and a lock-in temperature at Tf~14 K. These values are similar to those previously reported for single-crystal samples, where the spiral structure is short-range ordered (SRO) at low temperatures. The application of magnetic field suppresses the dielectric anomaly at Tf indicating that the coherence length of the ordering increases. MnCr2O4 exhibits a symmetrical magnetodielectric response between Tf and Ts that scales with the square of the magnetization. This suggests that the magnetodielectric coupling originates from the P2M2 term in the free energy expansion. The magnetodielectric response becomes asymmetric with respect to field below Tf.

Magnetodielectric coupling in frustrated spin systems: the spinels MCr2O4 (M = Mn, Co and Ni)

We have studied the magnetodieletric coupling of polycrystalline samples of the spinels MCr2O4 (M = Mn, Co and Ni). Dielectric anomalies are clearly observed at the onset of the magnetic spiral structure (Ts) and at the ‘lock-in’ transition (Tf) in MnCr2O4 and CoCr2O4, and also at the onset of the canted structure (Ts) in NiCr2O4. The strength of the magnetodielectric coupling in this system can be explained by spin–orbit coupling. Moreover, the dielectric response in an applied magnetic field scales with the square of the magnetization for all three samples. Thus, the magnetodielectric coupling in this state appears to originate from the P2M2 term in the free energy.

Relaxor ferroelectric behavior in Ca-doped TbMnO3

We have studied the effect of Ca-doping in single-crystal Tb1−xCaxMnO3(x≤0.1) on the crystal and magnetic structure, magnetocapacitance, and electric polarization. For low doping (x=0.05), the presence of Mn4+ ions gives rise to a state with behavior resembling that of a relaxor ferroelectric. The coherence length of the Mn magnetic spin spiral is reduced, while the Mn-modulation wave vector is unchanged. For doping larger than 5%, the ferroelectric state is suppressed, which we ascribe to breakdown of the spiral magnetic structure.

Spin–lattice coupling in iron jarosite

We have studied the magnetoelectric coupling of the frustrated triangular antiferromagnet iron jarosite using Raman spectroscopy, dielectric measurements and specific heat. Temperature dependent capacitance measurements show an anomaly in the dielectric constant at TN. Specific heat data indicate the presence of a low frequency Einstein mode at low temperature. Raman spectroscopy confirms the presence of a new mode below TN that can be attributed to folding of the Brillouin zone. This mode shifts and sharpens below TN. We evaluate the strength of the magnetoelectric coupling using the symmetry unrestricted biquadratic magnetoelectric terms in the free energy.