Evolution of spatial spin-modulated structure with La doping in Bi1-yLayFeO3 multiferroics

• The presence of spatial spin-modulated structure (SSMS) in Bi 1-y La y FeO 3 is confirmed. • The SSMS is characterised by a positive magnetic anisotropy constant Κ u > 0 at 4.2 K. • At 300 K Κ u > 0 for y = 0 – 0.03 while Κ u < 0 for higher La content. • At 300 K for y = 0.03 – 0.05 the SSMS is close to harmonic. • A high-frequency peak proportional to La content was observed in 57 Fe NMR spectra . In this paper we present study of the magnetic structure of multiferroic series Bi 1-y La y FeO 3 ( y = 0, 0.015, 0.03, 0.05 and 0.10) using nuclear magnetic resonance (NMR) at 4.2 K and Mössbauer spectroscopy at room temperature on 57 Fe nuclei. We observed the presence of cycloid type spatial spin-modulated structure (SSMS) in the whole range of compositions. The study of the concentration dependence of the anharmonicity parameter m revealed that at the temperature of liquid helium in the composition region y = 0 – 0.10 as well as at room temperature for y = 0.03 the magnetic state of multiferroics is described by cycloid type SSMS with a positive effective uniaxial constant of magnetic anisotropy Κ u > 0 (“easy axis” model). The study at room temperature in turn revealed that magnetic state of the multiferroics with y = 0 – 0.03 is described by cycloid type SSMS with a positive effective uniaxial constant of magnetic anisotropy Κ u > 0 (“easy axis” model). In contrast, the magnetic state of multiferroics with y = 0.05 and 0.10 is described by a negative effective uniaxial constant of magnetic anisotropy Κ u < 0 (“easy plane” model). Anharmonicity parameter m at room temperature for y = 0.03 – 0.05 is close to zero, which means that SSMS in this range of compositions is close to harmonic.