  The angular-dependent magnetoresistance (AMR) of the ab plane is measured on
the single crystals of FeSe1-xSx (x = 0, 0.07, 0.13 and 1) and FeSe1-yTey (y =
0.06, 0.61 and 1) at various temperatures under fields up to 9 T. A pronounced
twofold-anisotropic carrier-scattering effect is identified by AMR, and
attributed to a magnetic-field-induced spin nematicity that emerges from the
tetragonal normal-state regime below a characteristic temperature Tsn. This
magnetically polarized spin nematicity is found to be ubiquitous in the
isoelectronic FeSe1-xSx and FeSe1-yTey systems, no matter whether the sample
shows an electronic nematic order at Ts < Tsn, or an antiferromagnetic order at
TN < Tsn, or neither order. Importantly, we find that the isoelectronic
substitution with sulfur does not suppress but even enhances the characteristic
Tsn of the induced spin nematicity in FeSe1-xSx samples. This contrasts sharply
with their rapidly suppressed Ts, the transition temperature of the spontaneous
electronic nematicity. Furthermore, we find that the superconductivity is
significantly suppressed with the enhancement of the induced spin nematicity in
both FeSe1-xSx and FeSe1-yTey samples.
