  Layered transition metal dichalcogenides (TMDCs) host a plethora of
interesting physical phenomena ranging from charge order to superconductivity.
By introducing magnetic ions into 2H-NbS$_2$, the material forms a family of
magnetic intercalated TMDCs T$_x$NbS$_2$ (T = 3d transition metal). Recently,
Fe$_{1/3+\delta}$NbS$_2$ has been found to possess intriguing resistance
switching and magnetic memory effects coupled to the N\'{e}el temperature of
T$_N \sim 45$ K [1,2]. We present comprehensive single crystal neutron
diffraction measurements on under-intercalated ($\delta \sim -0.01$),
stoichiometric, and over-intercalated ($\delta \sim 0.01$) samples. Magnetic
defects are usually considered to suppress magnetic correlations and,
concomitantly, transition temperatures. Instead, we observe highly tunable
magnetic long-ranged states as the Fe concentration is varied from
under-intercalated to over-intercalated, that is from Fe vacancies to Fe
interstitials. The under- and over- intercalated samples reveal distinct
antiferromagnetic stripe and zig-zag orders, associated with wave vectors $k_1$
= (0.5, 0, 0) and $k_2$ = (0.25, 0.5, 0), respectively. The stoichiometric
sample shows two successive magnetic phase transitions for these two wave
vectors with an unusual rise-and-fall feature in the intensities connected to
$k_1$. We ascribe this sensitive tunability to the competing next nearest
neighbor exchange interactions and the oscillatory nature of the
Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. We discuss experimental
observations that relate to the observed intriguing switching resistance
behaviors. Our discovery of a magnetic defect tuning of the magnetic structure
in bulk crystals Fe$_{1/3+\delta}$NbS$_2$ provides a possible new avenue to
implement controllable antiferromagnetic spintronic devices.
