Hidden magnetic order in Sr2VO4clarified withμ+SR

Abstract

In order to elucidate the magnetic ground state of Sr 2 VO 4 , we have measured muon spin rotation and relaxation (μ + SR) spectra of a powder sample in the temperature range between 1.8 and 140 K. As a result, we have clarified that the transition at 105 K is not magnetic but structural and/or electric in origin and found the appearance of static antiferromagnetic (AF) order below 8 K. Moreover, the distribution of the internal AF field was found to be very broad, even at the lowest temperature measured. These results are consistent with the formation of an orbital-stripe order with collinear AF order for the magnetic ground state of Sr 2 VO 4 .Tetragonal Sr 2 VO 4 with a K 2 NiF 4 -type structure has been considered as an analog of a parent compound of the first superconducting cuprate, i.e., La 2 CuO 4 , since the electron configuration of the V 4+ ion is t 1 2g with S = 1/2 in a tetragonal crystal field of a VO 6 octahedron [1-7]. Although susceptibility (χ m ) measurements clearly show a magnetic anomaly at 105 K (=T c ) [6], a past neutron diffraction study reported the absence of magnetic peaks even at 5 K [2]. Since x-ray diffraction studies revealed a sudden enhancement of the c/a ratio below T c [6], it was proposed that an orbital-ordering transition occurs at T c . In fact, first principles calculations predicted an antiferromagnetic (AF) and orbital-ordered state with a nontrivial and large unit cell structure in the ground state, because of the coexistence and competition of ferromagnetic and AF exchange interactions [8,9].Recently, a more exciting possibility was proposed, namely, that a hidden magnetic order is induced by spin-orbit coupling for a t 1 2g electron system [10]. Such work also predicted an unconventional magnetic octupolar ordering at low T for Sr 2 VO 4 . Furthermore, alternating spin-orbital order in the ab plane was proposed by considering the effects of spin-orbit coupling, crystal field, and superexchange on the energy levels of the V 4+ ions [11].Despite the above attractive predictions, there are few reports on Sr 2 VO 4 utilizing microscopic magnetic techniques [12]. Macroscopic magnetic measurements such as χ m and neutron scattering usually give us significant insight into the ground state of magnetically ordered solids. However, such techniques are sometimes not suitable, particularly for the materials exhibiting order with a broad field distribution, i.e., when short-range order, random, or nearly random order appears in a material, due to the absence of periodic structure and/or the presence of rapid fluctuations. In contrast, the muon spin rotation and relaxation (μ + SR) technique is very sensitive * e0589@mosk.tytlabs.co.jp to local magnetic environments with a different time window compared to neutron scattering; thus μ + SR has provided crucial information on the magnetic ground state of materials [13,14]. We have therefore performed a μ + SR experiment on Sr 2 VO 4 and found the appearance of magnetic order not below T c but below 8 K.A powder sample of Sr 2 VO 4 w...

Structural anomalies and short-range magnetic correlations in the orbitally degenerate systemSr2VO4

“…11,12 These results suggest that the spin-orbital ordering state, which was proposed by first-principles calculation studies by Imai et al 14,15 , appeared below T c0 . On the basis of the theoretical studies, we can expect a severe competition between many types of spin-orbital ordering patterns due to the coexistence of ferromagnetic and antiferromagnetic correlations and the associated effect of frustration.…”

Reversed Crystal-Field Splitting and Spin–Orbital Ordering in α-Sr2CrO4

“…We find here that indeed the crystal-field levels are reversed in α-Sr 2 CrO 4 , regardless of the elongation of the CrO 6 octahedron examined, which is in sharp contrast to the case of α-Sr 2 VO 4 . [18][19][20] We also examined the electronic state of LaSrVO 4 , which has the same crystal structure with the 3d 2 electron configuration; 21,22) only the antiferromagnetic phase transition was observed experimentally. 8,23) Our DFT calculation shows that the reversal of the crystal-field levels does not occur in this material.…”

Phase Transitions of α-Sr2VO4 with K2NiF4-type Structure

“…Indeed, magnetic properties, such as the magnitude of the drop and absolute value of magnetic susceptibility even at high temperatures, depended strongly on samples as seen in reference [8]. We recently succeeded in establishing the synthesis method of the compound, and found that magnetic dipole order appears only below T N ∼8 K [9]. This fact naively seems to support the nontrivial order, but the magnetic octupole order has not been ruled out, because inelastic neutron scattering measurements have revealed the excitation between the Kramers doublets expected by the spin-orbit coupling [10].…”

“…This is a part of proceedings of ICM 2015, where magnetic properties of isostructural Sr 2 CrO 4 will be presented together with those of the V compound. In this paper, the characterization of the phase transitions of the latter compound is focused because the other parts of the presentation have already been reported elsewhere [9,11].…”