# Amorphization Effect for Kondo Semiconductor CeRu2Al10

Authors: Yusuke Amakai; Yasuhiro Shiojiri; Kei Ishihara; Hiroto Hitotsukabuto; Shigeyuki Murayama; Naoki Momono; Hideaki Takano
Journal: Advances in Condensed Matter Physics
 (2017)

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## Abstract

We measured the magnetic susceptibilityχ, electrical resistivity ρ, and specific heat Cp of a sputtered amorphous (a-)CeRu2Al10 alloy. χ value fora-CeRu2Al10 alloy follows a Curie-Weiss paramagnetic behavior in the high-temperature region, and magnetic transition was not observed down to 2 K. The effective paramagnetic moment peff is 1.19 μB/Ce-atom. The resistivity shows a typical disordered alloy behavior, that is, small temperature dependence for the whole temperature range. We observed an enhancement of ρ and Cp/T in the low-temperature region of T<10 K. The enhancement in ρ is suppressed by applying a magnetic field. It is suggested that this behavior is caused by the Kondo effect.


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## Body

## 1. Introduction

The ternary rare-earth compound, CeRu2Al10, exhibits an unusual antiferromagnetic phase transition at T0≈27 K [1, 2]. The resistivity for CeRu2Al10 exhibits a semiconducting behavior in the paramagnetic phase. However, this behavior is suppressed by substituting La in Ce-site of CeRu2Al10. In addition, T0 decreases rapidly with an increase in La concentration and disappears when about half of Ce is substituted by La. The resistivity for Ce dilute region of La-substituted CeRu2Al10 exhibits a metallic behavior at high temperatures, and the resistivity exhibits a minimum in the low-temperature region. Tanida et al. proposed that the long-range magnetic order was suppressed randomly in Ce dilute region for La substitution of CeRu2Al10. The authors pointed out that the resistivity minimum can be explained by a typical impurity Kondo effect, where Ce exists as a magnetic impurity [3, 4]. Moreover, the semiconducting band gap of CeRu2Al10 is broken by La substitution.However, several studies on the Kondo effect of structural-disordered Ce-alloys such as bulk metallic glasses and amorphous alloys, where Ce-atom is arranged randomly, have been recently performed. For example, in Ce-Al bulk metallic glasses, the tunable competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction with the variation in Ce-concentration and the magnetic field [5] is suggested. We studied the low-temperature properties of several binary amorphous Ce-alloys. Amorphous (a-)Ce-Mn anda-Ce-Ru alloys in Ce high-concentration region exhibit a large electronic specific heat coefficient γ (>200 mJ/molK2) and T2 law with a large coefficientA (>0.02 μΩcm/K2) in the low-temperature resistivity [6–10]. From these results, we show that an itinerant heavy-fermion state occurs at low temperatures as a Fermi-liquid ground state in the structure-disordered system after the formation of a dense Kondo state.In this study, to investigate the influence of the structural-disordered effect on the electrical resistivity and magnetic properties for CeRu2Al10, we prepareda-CeRu2Al10 alloy and measured its magnetic susceptibility, resistivity, and specific heat. In addition, we prepareda-LaRu2Al10 alloy that does not have 4f-electrons with a rare-earth element for comparison.


## 2. Experimental

Bulk ingots of CeRu2Al10 and LaRu2Al10 were prepared by using the arc-melting method with a stoichiometric composition of Ce 99.9%, La 99.9% (Nippon Yttrium Co., Ltd.), Ru 99.95% (Rare Metallic Co., Ltd.), and Al 99.99% (Mitsuwa Chemicals Co., Ltd.), in Ar atmosphere. The amorphous alloy was prepared using a dc high-rate sputtering method with arc-melt ingots on a water-cooled Cu substrate (30 mmϕ). The sample thickness was ~200 μm. The structure of the obtained samples was confirmed using X-ray diffraction measurements. Measurements were performed with an as-sputtered film. The chemical compositions of the present amorphous alloys were determined using scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS) to be Ce10Ru15Al75 and La8Ru17Al75 (suffixes represent at%). Thus, we will use the notationsa-CeRu2Al10 anda-LaRu2Al10 for the samples, hereafter. The magnetic susceptibility was measured using a commercial SQUID magnetometer (Quantum Design MPMS) from 2 to 300 K. The electrical resistivity was measured by using a typical four-terminal method (Quantum Design PPMS) from 2 to 300 K. The resistivity measurements ofa-CeRu2Al10 alloy were performed in a magnetic field (0, 20 kOe, 40 kOe, and 60 kOe) and the temperature range was 2–60 K. The specific heat was measured by PPMS from 2 to 300 K.


## 3. Results and Discussion

Figure1 shows the X-ray diffraction patterns fora-CeRu2Al10 anda-LaRu2Al10 alloys. The diffraction patterns for both alloys exhibit two broad peaks at the center at about 25° and 42°, and definite Bragg peaks are not observed. Therefore, the samples are identified as amorphous materials.Figure 1
X-ray diffraction pattern ofa-CeRu2Al10 anda-LaRu2Al10 alloys.Figure2 shows the temperature dependence of the magnetic susceptibility χ (left axis) and the inverse susceptibility 1/(χ-χ0) (right axis) fora-CeRu2Al10 anda-LaRu2Al10 alloys atH = 10 kOe. We calculated χ using the composition ratio of Ce10Ru15Al75 obtained by SEM-EDS as 1 mol. χ value fora-CeRu2Al10 alloy increases monotonically with decreasing temperature, and a magnetic transition is not observed in the measurement temperature region. However, χ value fora-LaRu2Al10 alloy is almost independent of the temperature and is very small value (<10−6 emu/mol). 1/(χ-χ0) value fora-CeRu2Al10 alloy exhibits a linear behavior in the high-temperature region of T>30 K following the Curie-Weiss law,(1)1χ-χ0=1CT-θ,where χ0 is a constant for the independence of temperature, C is the Curie constant, and θ is the Weiss temperature. χ0 value fora-CeRu2Al10 alloy obtained from (1) was ~1.0 × 10−6 emu/mol. The value of θ is −20 K. C is obtained as (2)C=NCeμeff23kB,where NCe is the number of Ce atoms, μeff is the paramagnetic effective magnetic moment, and kB is the Boltzmann constant. Here, μeff is calculated from Ce-concentration (10 at%) of composition ratio Ce10Ru15Al75 obtained from SEM-EDS. The estimated μeff obtained using (2) is 1.19 μB/Ce-atom. Since the effective magnetic moment expected for trivalent Ce (J = 5/2) is 2.54 μB, the obtained value of 1.19 μB/Ce is about half this value. Therefore, it is expected that about half of Ce ofa-CeRu2Al10 alloy exists as nonmagnetic tetravalent Ce and the remaining half exists as magnetic trivalent Ce in the alloy.Figure 2
Temperature dependence of the magnetic susceptibilityχ and inverse susceptibility 1/(χ-χ0) fora-CeRu2Al10 alloy. The green points and line are the temperature dependence of the magnetic susceptibility χ fora-LaRu2Al10 alloy.Figure3 shows the temperature dependence of the resistivity ρ fora-CeRu2Al10 anda-LaRu2Al10 alloys. The value of ρ at 300 K fora-CeRu2Al10 alloy is about 2 times greater than that ofa-LaRu2Al10 alloy. ρ value for both alloys exhibits small temperature dependence less than 10% in the whole temperature region, and it increases with decreasing temperature. Such temperature dependence is one of the characteristics of disordered alloys. In contrast to ρ value ofa-LaRu2Al10 alloy, which increases monotonically with decreasing temperature, ρ value ofa-CeRu2Al10 alloy exhibits an increase in the low-temperature region. Generally, in the case of disordered alloys, the resistivity is larger than that for the crystalline counterparts owing to the random arrangement of atoms. However, the absolute value of the low-temperature ρ ina-CeRu2Al10 alloy is on about the same order as that for the polycrystalline CeRu2Al10. In addition, although ρ value ofa-CeRu2Al10 alloy increases with decreasing temperature, a small temperature dependence similar to that for the typical disordered alloy was observed. Therefore, it is considered that the semiconductor band for crystalline CeRu2Al10 exhibited a metallic behavior as a result of amorphization.Figure 3
Temperature dependence of the resistivityρ for a-CeRu2Al10 anda-LaRu2Al10 alloys.Figure4 shows the low-temperature specific heat Cp over T versus T2 plots fora-CeRu2Al10 anda-LaRu2Al10 alloys. The inset shows the temperature dependence of Cp for the alloys. Cp values of both alloys are almost in agreement in the whole temperature region, as shown in the inset. The low-temperature specific heat of a usual metal can be expressed by (3)Cp=γT+βT3,where γT is the electronic specific heat term and βT3 is the phonon specific heat term. This relation can also be applied to amorphous alloys [11]. As shown in Figure 4, Cp/T ofa-LaRu2Al10 alloy follows a linear relation as a function of T2 below 50 K2. However, Cp/T ofa-CeRu2Al10 alloy increases rapidly with decreasing T2 below 100 K2. The value of Cp/T at the lowest temperature is ~54 mJ/molK2. The temperature region where the enhancement in Cp/T occurs agrees with the temperature region of the upturn for ρ. Such enhancement in Cp/T and ρ at low-temperature does not occur fora-LaRu2Al10. Therefore, it is considered to be due to the magnetic contribution of the 4f-electron of Ce. In order to clarify the magnetic contribution of the 4f-electron of Ce fora-CeRu2Al10, we measured the magnetoresistance ofa-CeRu2Al10.Figure 4
C
p
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T versus T2 plot fora-CeRu2Al10 anda-LaRu2Al10 alloys. The inset shows the temperature dependence of the specific heat Cp fora-CeRu2Al10 anda-LaRu2Al10 alloys.Figure5 shows the magnetic field dependence of the transverse magnetoresistance Δρ/ρ at 2 K, 10 K, and 60 K fora-CeRu2Al10 alloy. We calculated Δρ/ρ as follows:(4)Δρρ=ρH-ρ0ρ0,where ρ(H) is the resistivity in the magnetic field and ρ(0) is the resistivity at zero field. Δρ/ρ increases negative with increasing magnetic field strength. Δρ/ρ at 2 K exhibits a large negative enhancement compared with that at other temperatures. Figure 6 shows the temperature dependence of Δρ/ρ at 20 kOe, 40 kOe, and 60 kOe fora-CeRu2Al10 alloy. The Δρ/ρ in the high-temperature region (T>10 K) is almost independent of the temperature for all the magnetic fields. However, Δρ/ρ in the low-temperature region increases rapidly with decreasing temperature at T<10 K for all the fields. Thus, the enhancement of ρ at low temperatures is suppressed by the applied magnetic field.Figure 5
Field dependence of the magnetoresistanceΔρ/ρ at various temperatures fora-CeRu2Al10 alloy.Figure 6
Temperature dependence of the magnetoresistanceΔρ/ρ under various magnetic fields fora-CeRu2Al10 alloy.The presenta-CeRu2Al10 alloy seems reasonable that the magnetic order of the crystalline counterpart have disappeared because they lose a long-range crystallographic order by amorphization. However, the local structure of the amorphous alloy is generally considered to be similar to that of the crystalline counterpart. Even ina-CeRu2Al10 alloy, a local structure similar to that of the crystalline CeRu2Al10 is expected to be realized. In addition, based on μeff, we have shown that half of Ce ina-CeRu2Al10 alloy is in the trivalent state. Ce3+ ions ina-CeRu2Al10 alloy are distributed randomly in the amorphous sample. In this case, it is considered that the random distribution of Ce3+ in the disordered structure realizes the impurity Kondo effect at low temperatures. Thus, the enhancement of ρ and Cp/T is observed in the low-temperature region fora-CeRu2Al10 alloy. Since the Kondo scattering is suppressed by the applying a magnetic field, Δρ/ρ increases negatively. Therefore, the negative increase of Δρ/ρ fora-CeRu2Al10 alloy at low temperatures and high magnetic fields indicates that the scattering center of the resistivity is due to Kondo scattering.


## 4. Conclusion

We prepareda-CeRu2Al10 anda-LaRu2Al10 alloys and measured their susceptibility, resistivity, and specific heat. χ value fora-CeRu2Al10 alloy follows the Curie-Weiss law in the high-temperature region of T>30 K with no magnetic transition. The effective paramagnetic moment and the Weiss temperature are 1.19 μB/Ce-atom and –20 K, respectively. ρ value fora-CeRu2Al10 anda-LaRu2Al10 alloys shows a small temperature dependence. However, an enhancement of ρ was observed in the low-temperature region fora-CeRu2Al10 alloy. Cp/T value ofa-CeRu2Al10 alloy increases rapidly with decreasing T2 below 100 K2. The magnetoresistance Δρ/ρ fora-CeRu2Al10 alloy increases rapidly with decreasing temperature for T<10 K. Therefore, it is considered thata-CeRu2Al10 alloy is formed the impurity Kondo state in the low-temperature region.



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*Source: 9848151-2017-05-24.xml*