Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4spinel

Abstract

Nuclear magnetic resonance (NMR), neutron diffraction (ND), x-ray diffraction, magnetic susceptibility χ and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below TN = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057 (20) in (Co1−xAlx)[Al2−xCox]O4, shows a broad maximum around 15 K in χ(T ) and a sharp peak at TN in heat capacity Cp. The average ordered magnetic moment of Co 2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) µB from NMR and 1.9(5) µB from ND which are smaller than the expected value of 3 µB for S = 3/2 and g = 2. Antiferromagnetic spin fluctuations and correlations in the paramagnetic state are revealed from the χ, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice TN.

Low-temperature nuclear magnetic resonance investigation of systems frustrated by competing exchange interactions

“…From the intensities of the Co signals occupying the A and the B sites, corrected by their respective longitudinal and transverse relaxation times (T 1 and T 2 , respectively), we have evaluated the inversion parameter to be x ≈ 0.06 (2). This value is in good agreement with that obtained from the x-ray analysis [191]. We notice that the T dependence and the magnitude of The hyperfine coupling constant of Co at the A-site is estimated from the K − χ plot in respectively.…”

“…The Co NMR spectrum under zero magnetic field was obtained by plotting the spin-echo intensity point by point at different fixed frequencies. The CoAl 2 O 4 exhibits an antiferromagnetic ordering below the Néel temperature of 9.8(2) K confirmed by the neutron diffraction study [191] which indicates a collinear antiferromagnetic ordering below T N . The ground state properties do not change much with application of pressure.…”

“…Different magnetic properties at low temperatures originating from different heat treatments in the preparation of the sample were studied by 75 As NMR and NQR. From the NMR and NQR spectra, the Knight shift and the 1/T 1 data, we could conclude that while at high temperatures both the "as-grown" and the "annealed" crystals of CaFe 2 As 2 are in The ambient pressure NMR data on CoAl 2 O 4 mentioned in this chapter is published in [191].…”

“…The polycrystalline sample was synthesized by the solid-state reaction method using Co 3 O 4 (99.9985%) and Al 2 O 3 (99.995%) from Alfa Aesar as the starting materials, the details of which can be found in Ref. [191]. Nuclear magnetic resonance (NMR) measurements were conducted by probing the 27 Al nuclei (I = 5/2, γ/2π = 11.09375 MHz/T, Q = 0.149 barns) and the 59 Co nuclei (I = 7/2, γ/2π = 10.03 MHz/T, Q = 0.4 barns) by using a phase-coherent spin-echo pulse spectrometer.…”

“…The details of the structural, the neutron diffraction and the magnetization properties of the sample used for our NMR study is in Ref. [191]. Thus we utilize the tool of nuclear magnetic resonance (NMR) to investigate the ground state of CoAl 2 O 4 .…”