Muon Spin Relaxation and Neutron Diffraction Studies of Cluster-Glass States in Sr_1−xLa_xRuO₃

Abstract

To clarify the magnetic properties of cluster-glass states in Sr 1−x La x RuO 3 (0.3 ≤ x ≤ 0.5), we report herein the results of muon spin relaxation (µSR) and neutron powder diffraction measurements. The µSR experiments showed that magnetic clusters start developing well above the peak temperature T * in the ac susceptibility. The volume fraction of the magnetically ordered region increases continuously with decreasing temperature, showing no anomaly at T * , and reaches nearly 100% at the lowest temperature. The temperature variation of the volume fraction is essentially independent of the La concentration in the x range presently investigated, although the dc magnetization is significantly suppressed with increasing x. Neutron powder diffraction experiments revealed that the ground state for x = 0.3 is a long-range ferromagnetic ordered state. These results indicate that, with decreasing temperature, cluster-glass states in Sr 1−x La x RuO 3 gradually develop into long-range ferromagnetic ordered states with decreasing temperature, and that the magnetic ordering process differs strikingly from that expected for a conventional second-order ferromagnetic transition. IntroductionIn general, the 4d orbitals in 4d transition-metal oxides are more delocalized than the 3d orbitals in 3d transition-metal oxides; therefore, a conventional band picture is considered a suitable starting point for understanding such 4d electronic states. However, 4d transition-metal oxides exhibit a variety of intriguing electronic properties, such as quantum criticality, 1, 2) non-Fermi liquid behavior, 3) and unconventional superconductivity.4) These intriguing electronic properties imply that the correlation effect of 4d electrons is important, and that the electronic states are not trivial.Although 4d transition-metal oxides rarely show magnetic ordering, SrRuO 3 exhibits ferromagnetic order below T C = 160 K, and the ordered moment is approximately 1.1 µ B .

Avoided quantum criticality and cluster-glass formation in itinerant ferromagnet Sr$_{1-x}$(La$_{0.5}$K$_{0.5}$)$_x$RuO$_3$

“…In Sr 1−x (La 0.5 K 0.5 ) x RuO 3 , we observed the emergence of the cluster-glass state in the vicinity of the FM critical concentration: x = 0.5. This feature is similar to the spontaneous phase separation in Sr 1−x Ca x RuO 3 revealed by the µSR and magneto-optical experiments [24,25] and the cluster-glass formation in Sr 1−x La x RuO 3 [29,30], but different from the non-Fermi-liquid state originating from the quantum critical fluctuations in Sr 1−x Ca x RuO 3 suggested by the NMR and thermodynamic investigations [21][22][23]. Furthermore, we found that Ru 4d electrons have itinerant and localized characteristics simultaneously in Sr 1−x (La 0.5 K 0.5 ) x RuO 3 .…”

“…In Sr 1−x La x RuO 3 , the FM ordered state is rapidly suppressed by La doping [26][27][28], and is then replaced by a cluster-glass state for 0.3 ≤ x ≤ 0.5, shown by the experimental result that ac magnetic susceptibility shows frequency dependence near the cluster-glass freezing temperature [29]. The development of inhomogeneous FM clusters was also confirmed with µSR measurement [30]. Furthermore, photoemisson experiments revealed that most of the coherent component of Rh 4d electrons remains at the Fermi level in the photoemission spectra for 0.3 ≤ x ≤ 0.5, although the spectral weight transfers from the coherent to incoherent parts with increasing x [31].…”

Correlation effect in Sr1−xLaxRuO3 studied by soft x-ray photoemission spectroscopy

“…At this temperature, the samples with x ≤ 0.2 were in ferromagnetic ordered states, and samples with x ≥ 0.3, which exhibit cluster-glass states at low temperatures, were in paramagnetic states. 29,30 However, it should be mentioned here that the valence spectra hardy depend on the temperature in the temperature and x range investigated here, as shown later in this section. The incident photon energy was 700 eV.…”

“…Although the cluster-glass ordering temperatures for x ≥ 0.3 are lower than 25 K, the magnetic clusters already start developing at this temperature according to our recent µSR study. 30 The PES spectra exhibit no discernible dependence on the temperate over the investigated x range.…”

“…Furthermore, µSR investigations for x ≥ 0.3 demonstrated that magnetic clusters start developing well above T * and that the magnetic ordering process of the cluster-glass state is strikingly different from that expected for a conventional long-range ferromagnetic order. 30 Note that the emergence of a clusterglass state is an indication of the localized character of the Ru 4d states because it reflects the presence of spatial inhomogeneities in the magnetism. We have also revealed that the electronic specific-heat coefficient increases significantly with La doping, indicating that La doping may enhance the effective mass of the conduction electrons due to the electron correlation effect.…”