Nanocomposites and nanomaterials
Piezomagnetic response in FeSe
Gaydamak T.N.1, Fil V.D., Chareev D.A.2

1B. Verkin Institute for Low Temperature Physics and Engineering, 
47 Lenin Ave., 61103 Kharkov, Ukraine
E-mail: gaydamak@ilt.kharkov.ua
2Institute of Experimental Mineralogy, Chernogolovka, Moscow Region, 142432, Russia

The results of experiments on acoustic-electric transformation (AET) in high-quality single crystalsFe0.963 ± 0.005Se indicating the existences of the piezomagnetic effect in them are presented.
In spite of this in the absence of an external magnetic field were recorded signals of AET, in FeSe single crystals, observed in the Hall-polarization. The signals appear below the temperature of solidification of the silicon oil (T ~ 110K), used as bonded material. 
The explanation of our results in terms of the PZM effect immediately raises the question on the type of the magnetic structure in FeSe single crystals. It is obviously not an AFM in the common sense. It was shown in [1] that for the usual AFM the dependence of the PZM modulus on temperature reproduces the temperature dependence of the sublattice magnetization. But in our case the amplitude in the normal state increases almost linearly under decrease in temperature, and it does not show any tendency to the saturation. It is most likely that in case of FeSe we deal with a short range magnetic order. This conclusion correlates with an increase in AFM fluctuations in FeSe under approaching TC [2]. 
The signal decreases below the temperature of the superconducting transition, but it does not disappear completely and saturates at the value easily accessible for the measurements. We believe the electromagnetic signal decreases with lowering of temperature due to the change in the London penetration depth. So we can measure the temperature dependence λ-2L (T) and the value λL (0) = 1.65 μm. Our estimate for λL (0) corresponds to the carrier density n~1019 cm-3.

1.	A.S. Borovik-Romanov, G.L. Alexanjan, E.G. Rudashevskij, Int. Conf. on Magnetism and Cryogenics, Kyoto, (1961).

2.	T. Imai, K. Ahilan, F. L. Ning, T. M. McQueen, and R.J. Cava, Phys. Rev. Lett. 102, 177005 (2009).