  A detailed investigation of a superconducting spin-triplet valve is
presented. This spin-valve consists of a superconducting film covering a metal
with an intrinsic spiral magnetic order, which could result from competing
isotropic exchanges or, if the crystal lattice breaks central symmetry, from
asymmetric Dzyaloshinskii-Moriya exchange. Depending on the anisotropy, such a
metal may change its magnetization either from a spiral to uniform order, as
seen in Ho and Er, or in the direction of the spiral itself, as in crystals of
the B20-type structure (such as MnSi, (Fe,Co)Si, FeGe, etc.). The nonuniform
magnetic order controls the appearance of long-range triplet superconducting
correlations at strong exchange fields, affecting the detailed character of the
proximity effect. We show that the magnetic control of the spin-valve behavior
can also be obtained from moderately low exchange fields (typically associated
to negligible long-range triplet correlations), thanks to an
orientation-dependent averaging mechanism of the magnetic inhomogeneity on the
scale of the Cooper pairs. Our numerical calculations reveal that the
spin-valve effect is in fact magnified at moderately low exchange fields, when
the exchange splitting in the spiral magnet is comparable to the
superconducting gap, and the spiral period is less than or equal to the
superconducting coherence length in the magnet multiplied by $2\pi$.
