Absence of polar order in LuFe2O4

Intrinsic electrical properties of LuFe2O4

“…Measurements with silver paint and indium metal in the same single-crystal and polycrystalline samples interpreted with only one RC (resistor and capacitor)-parallel element yielded very different values for the dielectric constant thus implying a main nonintrinsic contribution coming from the contacts (electrode/sample interface) as previously indicated. 17 As can be checked in Fig. 4, our samples measured with silver paint electrodes show the expected frequency dependence for relaxation processes with high-temperature and low-frequency colossal dielectric constants in agreement with other works.…”

“…This clearly demonstrates that LuFe 2 O 4 is not ferroelectric and that there is not any effect of a magnetic field on the intrinsic electrical polarization. Our frequency-and temperature-dependent complex impedance data confirm the results and conclusions previously reported, [15][16][17] i.e., the giant dielectric constant is not intrinsic of the material. The relatively large electrical resistivity at temperatures near the charge ordering transition 24 joined to the high electrical capacity of the electrodes yield an apparent colossal dielectric constant when the electrical resistance and capacity of the contacts are not considered.…”

“…4 On the contrary, a P (E) curve obtained in another recent work for a polycrystal sample at 140 K and rather high frequency of 80 kHz (Ref. 17) shows a nearly linear response suggesting the lack of ferroelectricity. Regrettably the applied electric field of 0.2 kV/cm in the latter work is by far below the typical values of coercive fields for the ferroelectric materials [usually about tens of kV/cm (Ref.…”

“…We recall here that although the evidenced noncolossal dielectric permittivity values [15][16][17] from impedance spectroscopy point to a normal dielectric character for LuFe 2 O 4 , this fact does not necessarily imply a nonferroelectric character [e.g., the dielectric constant values of about 30 for the ferroelectric oxide TbMnO 3 (Ref. 23)].…”

“…4-6) were accepted as originating from the motion of the ferroelectric domain boundary due to electron exchange between Fe 2+ and Fe 3+ . However, it has been lately pointed out that the large dielectric permittivity values may be misleading due to extrinsic effects related to its relative high electrical conductivity and the charge-depleted interface at the electrode contact or grain boundaries, [15][16][17] suggesting that the electronic ferroelectric order in LuFe 2 O 4 below 320 K has to be questioned. A definite proof to check for ferroelectric order is measuring the electric-field-dependent polarization [P (E)].…”