⁷⁵As NMR Studies of Magnetic Properties of the Magnetic Superconductor CaK(Fe_0.967Ni_0.033)₄As₄

Abstract

We have carried out75As nuclear magnetic resonance (NMR) measurements to investigate a new antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC) in CaK(Fe0.967Ni0.033)4As4. The hedgehog SVC order is clearly demonstrated by the direct observation of internal magnetic induction along the c axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature of TN~ 45 K. In the superconducting (SC) state, the NMR signal intensity decreases suddenly just below Tc~ 20 K due to Meissner effect, evidencing the coexistence of the hedgehog SVC AFM and SC states from a microscopic point of view.

Antiferromagnetic order and its interplay with superconductivity in CaK(Fe1−x Mn x )4As4

“…The two sets of I = 3 2 quadrupole split lines observed in 75 As NMR spectra measurements shown in figure 3 correspond to the two inequivalent As sites in the paramagnetic state previously observed in pure and Ni-doped CaKFe 4 As 4 [7,[21][22][23][24]. The NMR lines with a larger ν Q of ∼13 MHz (shown by blue lines in figure 3) can be assigned to the As2 site closer to the K layers, and the other lines with a smaller ν Q of ∼11 MHz (red lines) are attributed to the As1 site close to the Ca layers in CaK(Fe 0.964 Mn 0.036 ) 4 As 4 , based on similar analysis used in pure CaKFe 4 As 4 [21].…”

“…When T is lowered below T N , for H ∥ c axis, each line of the NMR spectra becomes broad and the observed spectra displays a more complicated shape as shown in the bottom panel of figure 3(a), which is similar to the NMR spectra in the magnetically ordered state for the Ni-doped CaKFe 4 As 4 [7,22,24]. The observed spectra in the AFM state can be well explained by the superposition of NMR spectra from two As sites: with the splitting of each of the NMR lines due to an internal magnetic induction B int for the As1 site (six lines) and no splitting of the NMR lines for the As2 site (three lines).…”