Magnetic transitions in LaFe_(13-x-y)Co_ySi_x compounds

摘要

The magnetic properties of a set of LaFe_(13-x-y)Co_ySi_x compounds (x = 1.6 - 2.6; y = 0, y = 1.0) have been investigated using magnetic measurements, thermal expansion, ~(57)Fe M?ssbauer spectroscopy and high resolution neutron powder diffraction methods over the temperature range 10-300 K. The natures of the magnetic transitions in these LaFe_(13-x-y)Co_ySi_x compounds have been determined. The Curie temperatures of LaFe_(13-x)Si_x were found to increase with Si content from T_C = 219(5) K for Si content x = 1.6 to T_C = 250(5) K for x = 2.6. Substitution of Co for Fe in LaFe_(10.4)Si_(2.6) resulted in a further enhancement of the magnetic ordering temperature to T_C = 281(5) K for the LaFe_(9.4)CoSi_(2.6) compound. The nature of the magnetic transition at the Curie temperature changes from first order for LaFe_(11.4)Si_(1.6) to second order for LaFe_(10.4)Si_(2.6) and LaFe_(9.4)CoSi_(2.6). The temperature dependences of the mean magnetic hyperfine field values lead to TC values in good agreement with analyses of the magnetic measurements. The magnetic entropy change, -?S_M, has been determined from the magnetization curves as functions of temperature and magnetic field (?B = 0 - 5 T) by applying the standard Maxwell relation. In the case of LaFe_(12.4)Si_(1.6) for example, the magnetic entropy change around T_C is determined to be -?S_M ~ 14.5 J kg~(-1) K~(-1) for a magnetic field change * B= 0 - 5 T.