Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 51 (2018)

Anomalous peak eﬀect in 122-type iron-based superconductors
T. Tamegai,∗1 N. Ito,∗1 S. Pyon,∗1 A. Ichinose,∗2 A. Yoshida,∗3 and T. Kambara∗3
Iron-based superconductors (IBSs) have attracted
considerable attention due to their potential for highﬁeld applications. In such applications, the critical current density, Jc , has to be reasonably large even understrong magnetic ﬁeld. Introduction of artiﬁcial pinning
centers in terms of heavy-ion irradiation, which createscolumnar defects (CDs), is one of the promising ways toenhance Jc .1) The eﬀects of heavy-ion irradiation havebeen studied in 122-type IBSs.2–4) The ﬁrst attempt tocreate CDs in Ba(Fe,Co)2 As2 made its Jc more than ﬁvetimes larger compared with unirradiated crystals.2) Jc
has been enhanced to ∼15 MA/cm2 in (Ba,K)Fe2 As2 byirradiating various kinds of heavy ions.3) Theoretically,
it is predicted that further enhancement of Jc is possible
by dispersing the direction of CDs, thereby suppressing
the motion of kinks and promoting ﬂux entanglements.
In fact, we have conﬁrmed that Jc in (Ba,K)Fe2 As2 canbe enhanced by ∼30% by dispersing the direction ofCDs.5) In the course of such studies, we discovered ananomalous peak eﬀect in (Ba,K)Fe2 As2 when CDs areintroduced at angles of θCD = 15◦ or more.5) The peakof Jc as a function of magnetic ﬁeld appears at about
1/3 of the matching ﬁeld BΦ (= nΦ0 , n: density of CDs,
Φ0 : ﬂux quantum).
In the present experiment, we studied how theanomalous peak eﬀect shows up in another IBSs,
Ba(Fe0.93 Co0.07 )2 As2 (Tc ∼ 24 K). U ion irradiation of
2.6 GeV has been performed at the RI Beam Factoryat RIKEN Nishina Center at a total dose of BΦ = 8 T.
U ions are irradiated from two directions at ±θCD with
θCD = 0◦ to 30◦ . Jc is evaluated by measuring the magnetization of the sample with the help of the extended
Beam model.
Figure 1(a) shows the magnetic ﬁeld dependence ofJc at 25 K in (Ba0.6 K0.4 )Fe2 As2 (θCD = ±15◦ ) for theﬁeld angle from the c-axis, θH , from 0◦ to 20.6◦ . As wehave mentioned above, an anomalous peak eﬀect shows
up at around H ∼ 1/3BΦ . It should be noted thatthe anomalous peak is strongly suppressed when the direction of the magnetic ﬁeld is away from the average
direction of CDs. The magnetic ﬁeld dependence of Jc
in Ba(Fe0.93 Co0.07 )2 As2 (θCD = ±15◦ ) from T = 2 K
to 20 K with the ﬁeld parallel to the c-axis is shownin Fig. 1(b). Unlike the case of (Ba0.6 K0.4 )Fe2 As2 , noanomalous peaks are observed at H ∼ 1/3BΦ at anytemperature. The weak anomalies observed below 5 kOe
are due to the self-ﬁeld eﬀect as we have discussed inRef. 4). In order to reveal the origin of the diﬀerencein the Jc behavior between the two materials, scanningtransmission electron microscopy (STEM) observations
∗1
∗2
∗3

Department of Applied Physics, The University of Tokyo
Central Research Institute of Electric Power Industry, ElectricPower Engineering Research Laboratory
RIKEN Nishina CenterFig. 1. Magnetic field dependence of Jc in (a) (Ba0.6 K0.4 )Fe2 As2(T = 25 K, BΦ = 8 T, θCD = ±15◦ ) and (b)Ba(Fe0.93 Co0.07 )2 As2 (BΦ = 8 T, θH = 0◦ , θCD = ±20◦ ).

Fig. 2. STEM images of CDs created by 2.6 GeV U irradiation (BΦ = 8 T) in (a) (Ba0.6 K0.4 )Fe2 As2 (θCD = ±20◦ )and in (b) Ba(Fe0.93 Co0.07 )2 As2 (θCD = ±30◦ ).

have been made. Figures 2(a) and (b) show STEM
images for (Ba0.6 K0.4 )Fe2 As2 and Ba(Fe0.93 Co0.07 )2 As2 .
The defects created by 2.6 GeV U irradiation are almost
continuous CDs in the case of (Ba0.6 K0.4 )Fe2 As2 , while
they are strongly discontinuous in Ba(Fe0.93 Co0.07 )2 As2 .
Such discontinuity of CDs are believed to make the eﬀectof splay insigniﬁcant in Ba(Fe0.93 Co0.07 )2 As2 , leading tothe suppression of the anomalous peak eﬀect.
References
1) L. Civale et al., Phys. Rev. Lett. 81, 45 (1991).
2) Y. Nakajima et al., Phy. Rev. B 80, 012510 (2009).
3) F. Ohtake et al., Physica (Utrecht) 518, 47 (2015).
4) T. Tamegai et al., Supercond. Sci. Technol. 25, 084008(2012).
5) A. Park et al., Phy. Rev. B 97, 064516 (2018).

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