The first-excited state g factor of ^26Mg has been measured relative to the g factor of the ^24Mg(2^+_1) state using the high-velocity transient-field technique, giving g=+0.86±0.10. This new measurement is in strong disagreement with the currently adopted value, but in agreement with the sd-shell model using the USDB interaction. The newly measured g factor, along with E(2^+_1) and B(E2) systematics, signal the closure of the ν d_5/2 subshell at N=14. The possibility that precise g-factor

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... rements may indicate the onset of neutron pf admixtures in first-excited state even-even magnesium isotopes below ^32Mg is discussed and the importance of precise excited-state g-factor measurements on sd shell nuclei with N≠ Z to test shell-model wavefunctions is noted.