supplementary material
Low-temperature synthesis of K0.5FeF3 with tunable exchange bias
Qiao-Ru Xu1, Yang Liu1, Yu-Di Zheng1, Wenbing Rui 2, Yan Sheng1, Xuan
Shen3, Jun Du2,*, Mingxiang Xu1, Shuai Dong1, Di Wu 3, and Qingyu Xu1,*
1
Department of Physics, Southeast University, Nanjing 211189, & Key
Laboratory of MEMS of the Ministry of Education, Southeast University,
Nanjing 210096, China
2)
National Laboratory of Solid State Microstructures and Department of
Physics, Nanjing University, Nanjing 210093, China
3)
Department of Materials Science and Engineering, Nanjing University,
Nanjing 210008, China
* Corresponding author: jdu@nju.edu.cn; xuqingyu@seu.edu.cn
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1. Confirmation of the reaction at 150 oC
To confirm the formation of K0.5FeF3 even at the starting sintering temperature of 150
o
C, the XRD patterns of the sample synthesized at 150 oC, and the reagents KF, FeF2 and
FeF3 for comparison, are shown in Fig. S1. The main experimental XRD peaks marked
by the arrows, cannot be found in the XRD patterns of each reagent, which is a direct
proof for the already reaction at sintering temperature as low as 150 oC.
Fig. S1 XRD pattern of sample synthesized at 150 oC, and mixed XRD patterns of KF
(PDF#36-1458), FeF2 (PDF#45-1062) and FeF3 (PDF#33-0647, 34-1188 and
38-1305).
2. Confirmation of the assistance of crystal water
From the basic knowledge of thermodynamics, the diffusion of ions between the
particles of reagents depends strongly on the reaction temperature. Thus the formation of
K0.5FeF3 at such low temperatures cannot be explained by the mechanism of conventional
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high temperature solid state reaction. In fact, in the present low temperature synthesis, the
crystal water in the reagents is the key factor, which facilitates the interdiffusion of ions
through the surface solvent layer during the reaction process and thus effectively decrease
the reaction temperature [1]. Here, we prove the crystal water in reagent FeF33H2O have
a critical impact on the formation of the K0.5FeF3 phase. An additional experiment was
done as the former preparation procedure except for that the crystal water in FeF33H2O
was eliminated before the mixing of the reagents and the mixture was grinded in a
glovebox filled with pure Ar.
Fig. S2
XRD patterns for the samples synthesized at 230 oC with FeF33H2O and
FeF3, respectively.
The XRD patterns of the sample prepared at 230 oC with and without the crystal water
in FeF3 are shown in Fig. S2. Obviously, with dehydrated FeF3, the XRD pattern is quite
different from the K0.5FeF3 phase. Therefore, the crystal water in FeF33H2O plays an
important role in the reaction. Generally, at low temperature, the interdiffusion of the ions
of each reagent is rather limited, leading to the worse reaction between the reagents, as in
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the case with dehydrated reagents. With the crystal water in the reagents during the
grinding and sintering process, a thin solution film might be formed at the surface of the
reagents, which will accelerate the reaction and the reaction temperature might be
significantly decreased [1-4].
3. Confirmation of the spin glass behavior by the dc magnetic relaxation
measurement
The magnetic relaxation behavior has been studied to confirm the spin glass (SG)
behavior. The thermo-remnant magnetization (TRM) depending on time was measured at
10 K (shown in Fig. S3) by cooling the sample in a field of 10 kOe from 300 K to the
final temperature, decreasing the field to zero and observing the decay of remnant
magnetization. By switching off the field, the interface spins will remain in the field
direction due to the pinning from the AFM core, and the AFM interfacial coupling forced
the spins in SG shell to be aligned antiparallel. It should be noted that as the field cannot
be switched off abruptly, the relaxation of the magnetization in the SG shell happened
already during the switching off of the field, leading to the negative remnant
magnetization. Since the AFM interfacial coupling from the pinning interface spins
behaves as an effective magnetic field on the SG spins, we have fitted the time dependent
remnant magnetization data with a stretched exponential function
𝑡 1−𝑛
𝑀(𝑡) = 𝑀0 − 𝑀r exp [− (𝜏)
]
(1)
where M0 the intrinsic ferromagnetic component and Mr the glassy component. The time
constant τ and exponent n are related to the relaxation rate of the SG phase. The value of
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n is 0.61, which is close to 0.5, confirming the SG behavior [5, 6]. In addition, τ is fitted
to be 1370 s.
Fig. S3 Measured (open symbol) and fitted (solid curve) time dependent remnant
magnetization at 10 K.
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