Highly Tunable Magnetic Phases in Transition-Metal Dichalcogenide Fe_{1/3+δ}NbS_{2}

Layered transition-metal dichalcogenides (TMDCs) host a plethora of interesting physical phenomena ranging from charge order to superconductivity. By introducing magnetic ions into 2H-TA_{2} (T=Nb, Ta; A=S, Se), the material forms a family of magnetic intercalated TMDCs M_{x}TA_{2} (M=3d transition metal). Recently, Fe_{1/3+δ}NbS_{2} has been found to possess intriguing resistance switching and magnetic memory effects coupled to the Néel temperature of T_{N}∼45 K [Maniv et al., Nat. Phys. 17, 525 (2021)NPAHAX1745-247310.1038/s41567-020-01123-w; Sci. Adv. 7, eabd8452 (2021)SACDAF2375-254810.1126/sciadv.abd8452]. We present comprehensive single crystal neutron diffraction measurements on underintercalated (δ∼-0.01), stoichiometric, and overintercalated (δ∼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 underintercalated to overintercalated, that is, from Fe vacancies to Fe interstitials. The under- and overintercalated samples reveal distinct antiferromagnetic stripe and zigzag 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 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+δ}NbS_{2} provides a possible new avenue to implement controllable antiferromagnetic spintronic devices.