Nuclear magnetic resonance investigation of the heavy fermion system Ce${}_2$CoAl${}_7$Ge${}_4$

Abstract

We present nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements performed on single crystalline \ccag{}, a member of a recently discovered family of heavy fermion materials Ce${}_{2}M$Al${}_7$Ge${}_4$ ($M$ = Co, Ir, Ni, or Pd). Previous measurements indicated a strong Kondo interaction as well as magnetic order below $T_M=1.8$ K. Our NMR spectral measurements show that the Knight shift $K$ is proportional to the bulk magnetic susceptibility $\chi$ at high temperatures. A clear Knight shift anomaly ($K/\propto \chi$) is observed at coherence temperatures $T^{\ast }\sim 17.5$ K for $H_0\parallel \hat{c}$ and 10 K for $H_0\parallel \hat{a}$ at the ${}^{59}$Co site, and $T^{\ast }\sim 12.5$ K at the ${}^{27}$Al(3) site for $H_0\parallel \hat{a}$ characteristic of the heavy fermion nature of this compound. At high temperatures the ${}^{59}$Co NMR spin-lattice relaxation rate $T_1^{-1}$ is dominated by spin fluctuations of the 4$f$ local moments with a weak metallic background. The spin fluctuations probed by ${}^{59}$Co NMR are anisotropic and larger in the basal plane than in the $c$ direction. Furthermore, we find $(T_{1}TK{)}^{-1}\propto T^{-1/2}$ at the ${}^{59}$Co site as expected for a Kondo system for $T>T^{\ast }$ and $T>T_K$. ${}^{59}$Co NQR \slrr{} measurements at low temperatures indicate slowing down of spin fluctuations above the magnetic ordering temperature $T_M\sim 1.8$ K. A weak ferromagnetic character of fluctuations around $\mathbf{q}=0$ is evidenced by an increase of $\chi T$ versus $T$ above the magnetic ordering temperature. We also find good agreement between the observed and calculated electric field gradients at all observed sites.

References in corpus (11)

• Quantum Criticality in Heavy Fermion Metals
• Heavy Fermions and Quantum Phase Transitions
• CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations
• Universal Behavior in Heavy Electron Materials
• Multiple quantum phase transitions and superconductivity in Ce-based heavy fermions
• Long range order and two-fluid behavior in heavy electron materials
• Nuclear magnetic resonance in the heavy fermion superconductors
• Superconductivity in Ce- and U-based "122" heavy-fermion compounds
• Electronic structure and electric-field gradients analysis in $CeI{n}_3$
• Hyperfine Interactions in the Heavy Fermion CeMIn_5 Systems
• Investigation of the physical properties of the tetragonal CeMAl4Si2 (M = Rh, Ir, Pt) compounds