  We present SNSPDs from NbN nanowires shaped after square-spiral that allows
an increase not only in critical currents but also an extension of spectral
detection efficiencies by just applying an external magnetic field. Using
negative electron-beam lithography with the positive resist for shaping
nanowires, made it possible to reduce the inner bend radius. Consequently, the
effect of critical-current enhancement in the magnetic field becomes stronger
than it was demonstrated earlier. Here we achieved a 13% increase of the
critical current in the magnetic field. We measured spectra of the
single-photon detection efficiency in the wavelength range from 400 to 1100 nm
in the magnetic field. At zero field, the square spiral has the spectrum
similar to that of a meander. At the field providing the maximum of the
critical current, the detection efficiency and the cut-off wavelength in the
spectrum increase by 20% and by 54%, correspondingly. The magnetic-field
dependence of dark count rate is well described by proposed analytical model.
