Studies On Frustrated Systems And Kondo Semiconductors

Muon spin relaxation (μSR), which uses the spin-polarized muon as a local probe, is one of the valuable techniques to investigate the magnetism of many kinds of compounds. The muon is injected into the sample and the decayed positron is detected.μSR has several advantages com-pared with other techniques like NMR and neutron scattering:i) It is very sensitive to magnetic field as small as about0.1G, therefore, even magnetic fields from neighboring nuclear moments can be detected;ii) The muon spins are100%polarized, no external field which may perturb the investigated com-pound is needed;iii) The muon spin is1/2, thus no interaction between the muon spin and the electric quadrupole needs to be considered compared with some isotopes used for NMR experiments;iv) Due to the local probe character, phase separation can be easily detected by μSR. By careful analyses, the volume fraction of different phases can be obtained.Taking advantage of these features, we investigate the magnetism of several compounds which are hot topics recently. One is the pyrochlore iridate Nd2Ir2O7, another is Nd2Ti2O7showing cooperative behavior, and the Kondo semiconductor CeT2Al10(T=Ru, Os).Ⅰ) Our μSR results reveal the magnetic ordering in the vicinity of MIT, indicating the close relationship between these two processes. Meanwhile, our results suggest that the Ir4+moments are reduced with a decrease in the MIT transitional temperature.R2Ir2O7(R=rare earth elements) is geometrically frustrated due to the cubic pyrochlore structure. The Ir4+ion has the electronic configurations5d5. A naive speculation is that these samples should be conductive since the5d electronic wave functions are more extensive than the3d electrons. However, resistivity measurements show that these compounds show semiconduc-tor or semimetal behavior at high temperatures and undergo metal-insulator transitions (MITs) at temperature TMI.Magnetic anomalies are always observed to be accompanied with the MIT. No thermal hysteresis is observed so that the transition is continuous. We are interested in the rela- tionship between the MIT and the magnetic transition. We focus on Nd2Ir2O7which resides just on the boundary of the ionic radius-T phase diagram, and is expected to be a candidate of topo-logical semimetal from theoretical calculations, in which the magnetic ordering is essential. Muon spin precession is observed in the vicinity of TMI, which is a strong indication of a long-range magnetic ordering. The precession frequency seems to follow a scaling relationship with TMI, i.e., the higher the TMI, the faster the frequency. Therefore, we conclude that the magnetic moment of Ir4+decreases with reducing TMI-The magnetic ordering temperature is close to TMI, we specu-late that the strong spin-orbit coupling inherent in Ir4+5d electrons and the specific all-in/all-out magnetic structure may shift the valence and conduction bands downwards and upwards, respec-tively, and thus induce the MIT. Such a process can be termed as a Lifshitz-like transition. The internal field is observed to increase again below about9K after showing a saturation behavior below about20K, indicating the ordering of Nd3+moments, consistent with a previous neutron scattering experiment.II) Nd2Ti2O7crystallizes in the monoclinic structure instead of the cubic pyrochlore structure although with the same formula as the pyrochlore titanate. Geometrical frustration is absent in NTO. However,μSR experiments on NTO exhibit properties resembling that of the frustrated ma-terials, therefore, this system may provide us new understanding of the cooperative paramagnetic behaviors.Our previous measurement suggested that its frustration index is as large as70. Muon spin relaxation measurement on NTO shows that the muon spin relaxation rate becomes temperature independent below about10K. Such a behavior has been observed in many frustrated materials like Dy2Ti2O7in the spin ice state, Tb2Ti2O7in the spin liquid state, Tb2Sn2O7and Gd2Ti2O7in the magnetically ordered state, and the kagome-like volborthite. Since NTO is free from frus-tration, we therefore propose that the strong spin correlations between Nd3+moments induces sufficient density of low energy magnetic excitations at low temperatures and responsible for the muon spin relaxation rate behavior. Another important observation is that the muon spin relax-ation rate is enhanced by a longitudinal field in the paramagnetic state, which is in contrast to the general expectation that the muon spin polarization will be recovered by an external longitudinal field. This result directly gives the evidence that the spectral density at the muon Larmor frequen-cy is enhanced by the magnetic field even in the paramagnetic state, which challenges the usual assumption that the magnetic field effect is negligible in the paramagnetic state. Ⅲ) The internal fields in the electron-doped Kondo semiconductor0,0.03,0.05,0.10) is largely enhanced. Compared with the dipolar field calculation, it is suggested that the magnetic structure in the undoped sample is unstable, and can be tuned easily by external perturbations.The Kondo semiconductor CeT2Al10(T=Ru, Os) shows unusual magnetic properties which are summarized as follows:i) The magnetic transition temperature To～30K is quite high considering that the distance be-tween the nearest-neighbor Ce ions is about5A; usually the ordering temperature is lower than2K;ii) The magnetic moment below To is along the c axis from neutron scattering experiment, while the magnetic easy axis is the a axis;iii) The transition temperature for T=Ru and Os is27and29K, respectively, while the magnetic moment is about0.42and0.34μB for the former and latter, respectively;iv) By applying a physical pressure on T=Ru, To increases firstly and reaches a maximum at P-2GPa, then decreases with increasing pressure, and eventually disappears as a first-order-like transition at P～4GPa.These properties cannot be explained by conventional exchange-interaction mechanism, there-fore, the hybridization between the conduction-and f-electron(c-f hybridization) should play a key role. We investigate the c-f hybridization via the substitution of Rh for Ru in a series of materials Ce(Ru1-xRhx)2Al10(x=0,0.03,0.05, and0.10). The internal field is drastically changed by just a small dopant:i) Two frequencies are observed in the x=0sample, which are denoted as Hsmall and Hlarge, re-spectively, while only one is observed in the x=0.05and0.10samples;ii) Hlarge is about180G in the x=0sample, while it is about800G in the doped sample at about2K;iii) Hlarge follows the mean-field-like behavior in the x=0sample, while showing non-mean-field-like behavior in the doped samples.Compared with simple dipolar field calculations, it is indicated that the magnetic moment lies along the a direction in the doped samples instead of the c axis as in the x=0sample, and the boundary is around x=0.03. Therefore, we conclude that the magnetic structure in the undoped sample is unstable to perturbations, and the c-f hybridization resides in the critical region so that the ground state can be tuned easily. On the other hand, combined with previous NMR, Neutron scattering, thermoelectric power, and μSR experimental results, we suggest that the non-mean-field-like behavior of Hsmall in the undoped sample arises from the Fermi-contact field as a result of the hyperfine interaction between the muon and conduction electrons. The non-mean-field-like behaviour of Hlarge in the doped samples needs to be clarified by future neutron scattering experiment.