Ferromagnetic Cluster-Glass State in Itinerant Electron System Sr_1−xLa_xRuO₃

Abstract

The magnetic and electronic properties of Sr 1−x La x RuO 3 were studied by means of dc-magnetization, acsusceptibility, specific heat, and electrical resistivity measurements. The dc-magnetization and ac-susceptibility measurements have revealed that the transition temperature and the ordered moment of the ferromagnetic order are strongly suppressed as La is substituted for Sr. The ac-susceptibility exhibits a peak at T * due to the occurrence of spontaneous spin polarization. Furthermore, we observed that T * shows clear frequency variations for x ≥ 0.3. The magnitude of the frequency shifts of T * is comparable to that of cluster-glass systems, and the frequency dependence is well described in terms of the Vogel-Fulcher law. On the other hand, it is found that the linear specific heat coefficient γ enhances with the suppression of the ferromagnetic order. The relatively large γ values reflect the presence of the Ru 4d state at Fermi level, and hence, the magnetism of this system is considered to be tightly coupled with the itinerant characteristics of the Ru 4d electrons. The present experimental results and analyses suggest that the intrinsic coexistence of the spatially inhomogeneous magnetic state and the itinerant nature of the Ru 4d electrons is realized in this system, and such a feature may be commonly involved in La-and Ca-doped SrRuO 3 .

Muon Spin Relaxation and Neutron Diffraction Studies of Cluster-Glass States in Sr1−xLaxRuO3

“…23) We obtained the following results: (1) T * varies clearly with frequency, and the initial frequency shift δ = ∆T * /(T * ∆log 10 ω) is estimated to range from 0.040 to 0.084 for 0.3 ≤ x ≤ 0.5. This result for δ is comparable to that of cluster-glass systems.…”

“…14,22) Recently, we have studied the electronic and magnetic properties of Sr 1−x La x RuO 3 by means of macroscopic measurements of the specific heat, electrical resistivity, ac susceptibility, and dc magnetization. 23) We showed that the ferromagnetic order is strongly suppressed with increasing La concentration x and that the ordered state varies from ferromagnetic states to cluster-glass states for x ≥ 0.3, demonstrating that spatially inhomogeneous magnetic states, which are indicative of the localized character of Ru 4d states, occur for x ≥ 0.3. On the other hand, we also revealed that this system is metallic and that the density of states for conduction electrons at the Fermi level is dominated by the Ru 4d states over the entire range of La concentration investigated (0 ≤ x ≤ 0.5).…”

“…Thus, although critical-like magnetic fluctuations are detected by the LF-µSR experiments near T * , they cannot simply be understood within the framework of the SCR theory for itinerant ferromagnets. 31) In the previous paper, 23) we discussed the properties of the cluster-glass states in Sr 1−x La x RuO 3 based on the optimal fluctuation theory combined with a finite-size scaling technique. 32) This theory assumes that the magnetic property of a local region is characterized by the local La concentration and the size of the local region.…”

“…A pronounced change in the ZF-µSR spectra is observed for x = 0.3 above and below 160 K, which is the transition temperature of the impurity ferromagnetic phase. 23) However, the initial asymmetry does not decrease across this temperature; therefore, the volume fraction of the impurity ferromagnetic phase should be negligible. The λ value at 143 K for x = 0.3 is estimated to be extremely small (1.7 × 10 −2 µs −1 ), which shows that the internal field is nearly static.…”

“…Details of the sample preparation and characterization are given in Ref. 23. To estimate the ordering temperature and to characterize the magnetic properties of the present samples, we measured the ac susceptibility and dc magnetization using a commercial superconducting quantum interference device magnetometer in the temperature range of 2 to 300 K. The µSR experiments were performed at the RIKEN-RAL Muon Facility in the UK, where an intense pulsed muon beam is available.…”