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

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

μSR study of Fe-substitution eﬀects on ferromagnetic ﬂuctuations innonsuperconducting heavily overdoped Bi-2201 cuprates
T. Adachi,∗1,∗2 Y. Komiyama,∗2 D. P. Sari,∗1,∗3 and I. Watanabe∗1

∗1
∗2
∗3

RIKEN Nishina Center
Department of Engineering and Applied Sciences, Sophia
University
College of Engineering, Shibaura Institute of Technology

Bi1.74Pb0.38Sr1.86Cu1-yFeyO6+G22Raw Asymmetry (%)

Hole-doped high-Tc cuprate superconductivity arises
by hole doping into a parent antiferromagnetic (AF)Mott insulator, suggesting that superconductivity ismediated by AF spin ﬂuctuations in the underdoped
and optimally doped regimes. For the overdoped
regime, where superconductivity weakens and eventually disappears with hole doping, it has been theoretically suggested that a ferromagnetic (FM) phase exists
in the nonsuperconducting heavily overdoped regime
and competes with superconductivity.1) In fact, zeroﬁeld (ZF) μSR and transport measurements have revealed an FM order/FM ﬂuctuations in nonsuperconducting heavily overdoped La2−x Srx CuO4 2) and Bi2201.3) To understand the details of the FM ﬂuctuations, we previously performed ZF-μSR at RIKENRAL in 5% Fe-substituted Bi-2201 single crystals of
Bi1.74 Pb0.38 Sr1.88 Cu1−y Fey O6+δ . It was found that therelaxation rate of the muon spins was signiﬁcantly enhanced at low temperatures and a peak was observed
at approximately 6 K, which coincided with the onset
temperature of the hysteresis between the ZF-cooled
and ﬁeld-cooled magnetic susceptibility, TSG .4) Theseresults suggest the occurrence of an FM cluster spinglass state in which FM spin clusters are formed aroundFe and the random orientation of the spins between theclusters results in the formation of a spin-glass state.4)
However, the details have not yet been clariﬁed.
To further understand the Fe-substitution eﬀects on
FM ﬂuctuations, we performed ZF and longitudinalﬁeld μSR of 9.6% Fe-substituted single crystals of
Bi1.74 Pb0.38 Sr1.88 Cu1−y Fey O6+δ with y = 0.096 using
the HELIOX and VARIOX cryostats at the RIKENRAL.
Figure 1 shows ZF-μSR time spectra of the 9.6% Fesubstituted
singlecrystalsofBi1.74 Pb0.38 Sr1.86 Cu1−y Fey O6+δ with y = 0.096. At
high temperatures above 57 K, the spectra show
Gaussian-like relaxations due to nuclear dipole ﬁelds
randomly oriented at the muon site. Below 57 K, themuon spin relaxation becomes fast and an exponentiallike behavior is caused by the development of spin correlation. At 4.1 K, a muon spin precession in a short-time
region and slow relaxation in a long-time region are observed, suggesting the coexistence of a long-range magnetic order and slowly ﬂuctuating spins in a sample.
The spectra were analyzed using the following function:
A(t) = A0 exp(−λ0 t)GZ (Δ, t) +A1 exp(−λ1 t) cos(ωt + φ) + ABG . The ﬁrst, second,y = 0.096
Heavily
overdoped

2018T = 203 K
157 K
57 K
14 K
12 K
4.1 K

16140246

8

1012

14

Time (Psec)
Fig. 1. ZF μSR time spectra of 9.6% Fe-substituted single crystals of Bi1.74 Pb0.38 Sr1.86 Cu1−y Fey O6+δ with y =0.096.

and third terms represent slowly ﬂuctuating component, muon spin precession component, and backgroundasymmetry, respectively. λ0 was found to exhibit a peakat approximately 12 K, which almost coincides with
TSG determined from magnetic susceptibility.4) The internal ﬁeld at the muon site, Bint , was estimated from ωas ∼92 G, which is comparable to Bint ∼100 G obtainedfor overdoped Bi-2201, in which an incommensurate AForder is formed.5) The magnetic volume fraction was estimated from the A0 at the base temperature as ∼33%,suggesting the coexistence of long-range ordered andslowly ﬂuctuating regions.
The present results suggest that 9.6% Fe substitution forms a long-rage magnetic order, which is not observed in a 5% Fe-substituted sample. Accordingly, theFM ﬂuctuations are probably stabilized by the Fe substitution. Concurrently, considering the similar valuesof Bint of the heavily overdoped and overdoped5) samples, an incommensurate AF order might also occur inthe heavily overdoped Bi-2201. How the AF and FM
order/ﬂuctuations coexist in the heavily overdoped Bi2201 is to be clariﬁed in future.
References
1) A. Kopp et al., Proc. Natl. Acad. Sci. U.S.A. 104, 6123
(2007).
2) J. E. Sonier et al., Proc. Natl. Acad. Sci. U.S.A. 107,17131 (2010).
3) K. Kurashima et al., Phys. Rev. Lett. 121, 057002(2018).
4) Y. Komiyama et al., J. Phys. Soc. Jpn. 90, 084701(2021).
5) H. Hiraka et al., Phys. Rev. B 81, 144501 (2010).

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