Nuclear magnetic resonance (NMR) measurements have been performed on the heavily Cu-doped ($x = 0.13$, 0.18, 0.24, 0.39, 0.48), and underdoped (x = 0.01 and 0.012) \NaFeCuAs~ single crystals, to reveal the nature of the magnetism, and to investigate how the electronic and magnetic properties of the system evolves with changing Cu concentration. We have used $^{23}$Na NMR spectra and spin-lattice relaxation measurements to investigate the development of magnetism in the \NaFeCuAs~ single crystals ($x$ = 0.13, 0.18, 0.24, and 0.39). We find multiple inequivalent Na sites, each of which is associated with a different number of nearest neighbor Fe sites occupied by a Cu dopant. We show that the distribution of Cu substituted for Fe is random in-plane for low concentrations ($x = 0.13$ and 0.18), but deviates from this with increasing Cu doping owing to the Fe-Cu stripe formation. A spin pseudogap behavior in an Arrhenius form is shown by the spin-lattice relaxation and frequency shift data, and is associated with a gap energy that increases with dopant concentration. There is an increase in orbital NMR frequency shift indicating a change in valence from the magnetic Cu$^{2+}$ to the nonmagnetic Cu$^{1+}$ state as $x$ exceeds 0.18, concomitant with the change of Fe$^{2+}$ to Fe$^{3+}$ resulting in the cluster formation of Fe spins segregated by nonmagnetic Cu dopants. We conclude that the antiferromagnetic (AFM) spin fluctuations are at the origin of the Curie-Weiss behavior in the $1/T_{1}T$ results of the $x = 0.13$ and 0.18 compounds for $T < 50$ K. The evolution of structural and magnetic ordering using $^{23}$Na and $^{75}$As NMR for \NaFeCuAs~ single crystals ($x$ = 0.39 and 0.48) has been investigated, leading to a confirmation of cluster spin-glass transition in both compounds. For $x = 0.48$, a long-range AFM order emerges below the N\'eel temperature, $T_{N}\approx200$ K, and coexists with the spin-glass phase at low temperatures. The two magnetic transitions are from distinct regions of the Fe-Cu plane. We confirm that the Fe and Cu atoms form magnetic stripes in $x = 0.39$ and 0.48 with end-chain defects which are responsible for the magnetic frustration, a necessary ingredient for the spin-glass transition. Aided by our numerical simulation of the $^{75}$As spectra of the $x = 0.48$ compound, we show that a staggered magnetization at the Fe sites induced by non-magnetic Cu dopants, combined with end-chain defects, gives rise to the splitting in the $^{75}$As spectra of $x = 0.48$ for $T\gtrsim T_{N}$. We have also investigated coexistence of localized and itinerant magnetism in the underdoped NaFe$_{1-x}$Cu$_{x}$As single crystals ($x$ = 0.01 and 0.012). Our computer simulation of the $^{23}$Na NMR spectrum indicates magnetic coupling of local moments on the Cu dopants and conduction electrons, in the form of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. From this simulation, a reasonable value is obtained for the Fermi wave vector $k_{F}$. Both compounds undergo antiferromagnetic transition with glassy behavior at low temperatures. This glassy behavior originates from the Cu-induced magnetic perturbation to the AFM background, as shown by our simulation. Evidence for the itinerant magnetism can be found in the Curie-Weiss behavior of the spin-lattice relaxation data with temperature approaching the antiferromagnetic transition. A comparison of the critical exponent associated with the Curie-Weiss behavior and that for the heavily Cu-doped samples is consistent with th e metal-to-insulator crossover in the doping range $x\gtrsim0.2$.

Creator

• Xin, Yizhou

DOI

• https://doi.org/10.21985/n2-wj9e-qj69

Subject

• Physics

Language

• en

Alternate Identifier

• etdadmin_upload_795405
• http://dissertations.umi.com/northwestern:15465

Keyword

• iron pnictides
• nuclear magnetic resonance
• magnetism

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright