Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 49 (2016)

Antiferromagnetic ordering in organic π − d hybrids [Pd(tmdt)2 ]R. Takagi,∗1 I. Watanabe,∗2 S. S. Mohd-Tajudin,∗2,∗3 R. Asih,∗2,∗4
K. Kanoda,∗5 S. Ogura,∗6 B. Zhou,∗6 and A. Kobayashi∗6
Emergent phenomena in the systems of interacting
electrons are expected to be more diverse when electrons are accommodated by multi-orbitals with distinctive characters. [M (tmdt)2 ], a family of organic
π − d hybrids, are multi-orbital correlated electron systems, where a transition metal ion, M , is coordinatedwith organic ligands, tmdt, from both sides.1–3) The
orbitals near the Fermi level are a d-orbital in M andπ-orbitals in tmdt.4,5) The energy-level difference between the d and π orbitals depends on M , and leads
to the diverse ground states; paramagnetic metals withappreciable electron correlations for M = Ni and Pt,
an antiferromagnetic (AF) metal (TN = 110 K) for M= Au, and a one-dimensional AF Mott insulator (TN= 13 K) for M = Cu.6–11)
For the M = Ni, Pt, and Pd compounds, the π andd orbitals are well separated in terms of energy; theirconduction bands are composed of only π orbitals. Although the Ni and Pt systems are paramagnetic metals as expected, we recently found that the Pd system was exceptional. It showed a decrease in ESR
signal intensity and broadening of the NMR spectra
below 100 K.12,13) A broad peak appeared around 50
K in the temperature dependence of the NMR relaxation rate.13) These results suggest that an AF ordering inhomogeneously appears in the sample. The
inhomogeneously-ordered state might be a long-rangeordered state, which cannot be explained by the bandcalculation and implies the importance of the electron
correlation of the π-orbitals and/or spin-orbit couplingin this system.
In order to achieve full confirmation of the appearance of magnetic ordering and to investigate the magnetic ground state, we carried out zero-field (ZF) µSR
measurements on [Pd(tmdt)2 ] polycrystalline sample
in the DOLLY area at Paul Scherrer Institut. Figure1(a) shows the ZF-µSR time spectra of [Pd(tmdt)2 ] atvarious temperatures. It can clearly be seen that therelaxation becomes faster at lower temperatures andthat the shape of the relaxation function changes withtemperature. At 50 K and 10 K, muon-spin precession
signals were observed. This provides clear evidence forthe appearance of a long-range-ordered magnetic statein [Pd(tmdt)2 ] at least below 50 K. The slow relaxationof the non-oscillating signal can originate from tempo∗1
∗2
∗3
∗4
∗5
∗6RIKEN Center for Emergent Matter Science (CEMS)
RIKEN Nishina CenterSchool of Distance Education, Universiti Sains Malaysia
Department of Physics, Osaka University
Department of Applied Physics, University of Tokyo
Department of Chemistry, College of Humanities and Sciences, Nihon University- 223 -Fig. 1. (a)ZF-µSR time spectra of [Pd(tmdt)2 ] observed
at 10, 50, 100, and 150 K. (b) LF-µSR time spectra
observed at 10 K.

rally fluctuating fields and/or small static local fields.
To determine whether the spin state in the groundstate is static or not and to find the residual spin dynamics, we performed µSR measurements under a longitudinal field (LF). As shown in Fig. 1(b), the slowly
relaxing signal was suppressed and became nearly flaton applying a field of 20 G. This indicates the existenceof a small static field, thus ruling out the temporally
fluctuating fields that are slower than the µSR timescale (10−6 −10−11 sec). The small static field might
originate from the nuclear dipolar fields in a possible
paramagnetic volume within the sample. This suggeststhat the magnetically-ordered state of [Pd(tmdt)2 ] isa inhomogeneously-ordered state, which is consistent
with the preceding NMR and ESR results.
References
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12) S. Ogura et al ., unpublished.
13) R. Takagi et al ., unpublished.