Pressure Induced Crossover from Non-Fermi-Liquid Behavior to Fermi-Behavior in a Heavy-Fermion Antiferromagnet Ce_7Ni_3

The pressure dependence of transport, magnetic and thermal properties of a heavy-fermion antiferromagnet Ce7Ni3 has been investigated. At ambient pressure, this compound orders antiferromagnetically below TN = 1.9 K and exhibits a large Sommerfeld coefficient of 9 J/K2mol f.u. The Ce ions in the three nonequivalent sites are found to be very close to trivalent from LIII-XANES (X-ray absorption near edge structure) spectra and magnetic susceptibility measurements. The resistivity and magnetization are strongly anisotropic at temperatures below TN = 1.9 K. The combination of Kondo effect and crystal-field effect substantially reduces the specific heat, magnetic entropy and magnetic moment in the antiferromagnetic state. The Kondo temperature TK is estimated to be ~ 5 K from the values of the specific heat jump and of the magnetic entropy at TN. With increasing pressure, TN of Ce7Ni3 is substantially suppressed and vanishes near Pc ≈ 0.32 GPa. Non-Fermi-liquid (NFL) behavior appears around 0.4 GPa in both the specific heat and AC magnetic susceptibility; Cm/T ∝ -lnT and χAC ∝ (1- αT1/2). Thus, Ce7Ni3 is found to be the example of the ordered compound showing the NFL behavior near the magnetic instability. Above 0.62 GPa, the normal Fermi-liquid state recovers as indicated by the T-independence of Cm/T and the T2-dependence of magnetic resistivity. The observed crossover in Cm(T) with pressure is analyzed in terms of two models, the impurity Kondo model with three Kondo temperatures and the SCR theory of spin fluctuations (SF). It is found that the NFL behavior in Cm/T is better described by the SCR theory. The characteristic SF temperature T0 increases by a factor of 20 for 0.33 ≤ P ≤ 0.75 GPa, yielding a large Grüneisen parameter Γe = 220 around 0.4 GPa.contents Abstract Chapter1 Introduction / p1 1.1. Phase diagram of Kondo lattice system / p1 1.2. Non-Fermi-liquid behavior / p2 1.3. Structural and magnetic properties of Ce₇Ni₃ / p4 1.4. Purpose of the present study / p6 Chapter2 Experimental techniques / p7 2.1. Sample preparation / p7 2.2. Magnetic and transport measurements at ambient pressure / p9 2.3. Measurements at high pressures / p9 Chapter3 Experimental Results / p13 3.1. Physical properties of Ce₇Ni₃ at ambient pressure / p13 3.2. Physical properties of Ce₇Ni₃ at high pressures / p22 Chapter4 Discussion / p30 4.1. Electronic state of Ce₇Ni₃ at ambient pressure / p30 4.2. Pressure dependence of antiferromagnetic transition temperature / p31 4.3. Pressure dependence of Kondo temperature in the Fermi-liquid region / p32 4.4. Crossover from non-Fermi-liquid to Fermi liquid / p35 4.5. Pressure dependence of Kondo effect and spin fluctuations / p44 Chapter5 Conclusion / p48 Acknowledgments / p50 References / p51博士(理学)Physical Science広島大