The linear magnetoelectric effect in LiCoPO₄Revisited

Abstract

With a view to verifying the symmetry of the antiferromagnetic phase of LiCoPO₄ crystals, quasi-static and dynamic magnetoelectric (ME) experiments have been undertaken by means of magnetic field induced polarization (ME_H effect). After magnetoelectric annealing all nine components of the ME tensor have been tested at 4.2 K. Two of them are non-zero, thus consistent with mmm' symmetry, confirming earlier work. In the present study, these coefficients have been found to have very large values, i.e. |α _xy (4.2 K)| = 18.4 [ps/m] or 5.5·10⁻³ Gaussian units and |α _yx (4.2 K)| = 30.6 [ps/m] or 9.2·10⁻³ Gaussian units. They have been also measured as a function of temperature, giving a mean Néel temperature T_N = 21.90 K (±0.05 K). Coefficient α _xy (T) follows a power law from about 64% T_N to the last measured point at about 99.9% T_N , with a critical exponent very close to one third (β = 0.338, measured quasi-statically and β = 0.3344, measured dynamically). The maximum value of the coefficients always appears at the lowest measured temperature, i.e. no maximum occurs at higher temperature as found earlier. No spontaneous polarization was measurable. Very close to T_N , ME “butterfly loops” have been observed but are interpreted as a manifestation of antiferromagnetic domain reversal, with the probable presence of an internal (electric) bias field.

Key Words:

• LiCoPO₄
• linear magnetoelectric effect
• large magnetoelectric coefficients
• α_xy(T)
• α_yx(T)
• critical exponent
• antiferromagnetism
• spin flop
• butterfly loops