The electron correlation in the 3d heavy-electron compound with linear spin chains, LaMn 4 Al 8 , has been investigated. Like a similar compound, YMn 4 Al 8 , we found a broad maximum in the temperature dependence of the susceptibility. This is interpreted by assuming the presence of a narrow pseudogap with the width Á ' 280 K in the spin excitation spectrum, which leads to the quenching of spin at low temperatures. Anomalies in susceptibility, resistivity and thermal expansion data, observed below approximately 50 K in spite of the absence of any phase transition, suggest the recovery of spin and the development of short-range magnetic correlation in this low-temperature range. This is in contrast to the case of YMn 4 Al 8 . No apparent long-range ordering in spite of the markedly developed magnetic correlation is probably due to the one-dimensional nature of the Mn spin arrangement.
Electronic properties of the narrow-gap semiconductor FeSb 2 have been studied by the magnetization and 121/123 Sb nuclear quadrupole resonance measurements. In addition to the susceptibility, the spin-lattice relaxation rate has revealed a fully opened gap at the Fermi level. The relaxation process is dominated by the magnetic contribution at high temperature T, while the quadrupole contribution becomes dominant below 70 K. Electronic field gradient at Sb sites shows anisotropic T dependence, reflecting most probably the anisotropic thermal expansion.
La 1Àx Y x )Mn 4 Al 8 (0 x 1) is a very unique itinerant electron system, in which the spin pseudogap can be controlled continuously and nearly uniformly in a wide range of the gap width from Á ' 250 to 500 K by the anisotropic volume shrinkage induced by the chemical pressure. The hydrostatic pressure on LaMn 4 Al 8 also increases Á but tends to fill the gap. The strong and anisotropic volume dependence of Á strongly supports the idea that the origin of the gap formation is associated with the one-dimensional geometry of the Mn spin arrangement.The effect of geometry is now known to be important in describing magnetism even in itinerant electron magnets, provided that electron correlations are sufficiently strong. For example, geometric frustration plays a nonnegligible role in determining the physical properties of strongly correlated itinerant electron magnets such as the Laves phase compounds RMn 2 (R ¼ rare earth) 1) and one of allotropes of manganese -Mn. 2) On the other hand, spin-gap formation in correlated electron systems is one of the extensively investigated topics in a wide variety of fields in the magnetism. Among itinerant electron magnets, a unique correlated-electron gap system, FeSi, attracts much attention, and the origin of the spin compensation at the ground state is still a matter of debate. 3) Recently, we have confirmed a susceptibility maximum at T max ' 500 and 200 K for the itinerant electron compounds YMn 4 Al 8 and LaMn 4 Al 8 , respectively, and interpreted this as due to the presence of a narrow pseudogap in the spin excitation spectrum, 4,5) but the origin of the gap remains open for question. The crystal structure of YMn 4 Al 8 and LaMn 4 Al 8 is of the tetragonal CeMn 4 Al 8 type (space group I4=mmm) derived from the ThMn 12 type, in which Mn atoms at the 8 f site form linear chains along the c axis. The large difference in intrachain and interchain Mn-Mn interatomic distances (d intra ' 2:6 and d inter ' 4:4 # A 6,7) ) suggests a quasi-onedimensional (1D) nature of magnetic interaction. YMn 4 Al 8 and LaMn 4 Al 8 show relatively large electronic specific heat coefficients, . 8) Therefore, RMn 4 Al 8 has appeared as another candidate for strongly correlated itinerant electron systems in which the effect of geometry plays a leading role in determining the magnetism. Thus far, no indication of phase transitions has been reported, at least, for YMn 4 Al 8 . For example, the C=T vs T 2 relation (C: specific heat) shows a good linearity down to 1.5 K for both compounds. 8) Although Yamasaki et al. 9) did not find any appreciable evidence of a phase transition in an 27 Al NMR experiment of LaMn 4 Al 8 down to 4.2 K, our recent experiments suggested that magnetic correlation develops below $ 50 K. 5) A recent zero-field SR experiment detected the appearance of static internal fields in LaMn 4 Al 8 at low temperatures, and suggested an antiferromagnetic order below 4.5 K. 10) In this paper, we report the concentration dependences of lattice volume, susceptibility and resistivity for poly...
The temperature dependence of the magnetic susceptibility shows a broad maximum at ∼550 and 630 K for LuMn(4)Al(8) and ScMn(4)Al(8), respectively, which can be interpreted as due to the presence of a pseudogap in the effective bands as in LaMn(4)Al(8) and YMn(4)Al(8). The anisotropic thermal expansion observed for RMn(4)Al(8) (R = La, Y, Lu and Sc) and the sensitive volume dependence of the gap width throughout the RMn(4)Al(8) system suggest dominant magnetic coupling in Mn spin chains along the c axis.
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