Antiferromagnetic order in Li(Ni1−xFex)PO4(

Abstract

The investigation of Li(Ni 1−x Fe x )PO 4 by optical second harmonic generation yields the competition of the antiferromagnetic (AFM) structures and 180 • domain patterns found in the end compounds LiNiPO 4 (point group mm ′ m, spins along z) and LiFePO 4 (point group mmm ′ , spins along y). While AFM order and the distribution of the AFM domains of LiNiPO 4 are unaffected by the ion substitution at x = 0.06, striking changes are observed at x = 0.20. We find that Fe 2+ is dominant in establishing the magnetic order. For x = 0.20 we find that the magnetic order of the solid solution interpolates the magnetic order of its end compounds by exhibiting an orientation of the spins in the (100) plane which include an angle of 40 • ± 3 • with respect to the y axis toward 0 K (point group m). In contrast to the two end members, the associated AFM domains form rods of a few mm length and ∼ 10 µm width occurring in neither of the end compounds. Mechanisms responsible for the magnetic order and domain pattern (180 • domains still being one of the least explored aspects of AFM materials in spite of their omnipresence) are discussed.

Tuning magnetoelectricity in a mixed-anisotropy antiferromagnet

“…These two transitions were previously reported and we compare our findings with those of the authors of ref. 27 later in the Results section. Note that overall the susceptibility of the mixed system is higher than for the parent compounds.…”

“…Grey curves in d , e show optical second harmonic generation measurements reprinted with permission from ref. 27 . Copyright (2023) by the American Physical Society.…”

“…Simultaneous fits to all data sets yield transition temperatures T 2 = 25.7(2) K and T 1 = 20.8(1) K respectively, in good agreement with refs. 27 and 28 . We note that the critical exponents for the two order parameters are clearly different.…”

“…2 d, e represent the intensity of the second harmonic generation (SHG) susceptibility tensor element, χ z x x , from ref. 27 . Here the first subscript signifies the component of the non-linear polarization induced by an electric field with components denoted by the last two subscripts.…”

“…For x > 0.6, the spins order along b like in LiFePO 4 . For x = 0.2, two magnetic phases appear upon cooling 27 . Neutron diffraction reveals ordered moments predominantly along the crystallographic b -axis below T 2 = 25 K, while below T 1 = 21 K, the moments partially reorient towards the a -axis in a low-temperature oblique phase.…”