Isovalent Substitution Effect of P to as on Magnetic

Physics Procedia Volume 75, 2015, Pages 192–199 20th International Conference on Magnetism

Isovalent substitution effect of P to As on magnetic characteristics of EuFe2(As1−xPx)2 single crystals Takanari Kashiwagi1,2 , Yuki Arakawa1 , Takuya Ishikawa1 , and Kazuo Kadowaki1,2 1 2

Graduate School of Pure & Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Division of Materials Science, Faculty of Pure & Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan

Abstract We have studied transport properties and magnetization curves of EuFe2 (As1−x Px )2 single crystals with x = 0, 0.08, and 0.2. EuFe2 (As0.8 P0.2 )2 shows superconductivity at ∼10 K. The magnetoresistance strongly reflects the magnetization process of the magnetic moment of Eu. The anisotropy with respect to the case for the magnetoresistance and the magnetization curves between Bab-plane and Bc is reduced as the doping P is increased. The experimental results suggest that an A-type of antiferromagnet changes to a canted antiferromagnet or a ferromagnetic ordering with increasing P doping. Superconductivity of this system seems to coexist with a component of magnetic moment of Eu-ions along the c axis. Keywords: FeAs superconductor, magnetization, transport properties

1

Introduction

After the discovery of a new class of superconductors with the iron arsenide layered structure a great number of both experimental and theoretical studies have been performed for the elucidation of high temperature superconductivity up to 57 K[16, 4, 3, 26]. So far, they are classified to five types with different crystal structures. Among them the AFe2 As2 -systems (A = Ba, Sr, Ca, and Eu) are most intensively studied because they have a rather simple crystal structure with the well-known ThCr2 Si2 -type and they are obtained easier in a single crystal form. The parent compound of the AFe2 As2 -system shows the antiferromagnetic spin density wave (SDW) ordering of Fe moments in the FeAs layer. In addition to this SDW transition, the structural transition from tetragonal to orthorhombic symmetry occurs side by side almost simultaneously[23, 32]. Superconductivity of the AFe2 As2 -system seems to appear as the SDW state is suppressed. This was commonly observed by the substitution of the A-site by K[31], the Fe-site by Co[24], Ni[17], and the As-site by P[14]. Superconductivity of the mother AFe2 As2 -system is also found by applying high pressures[1, 5, 8, 7, 18]. 192

Selection and peer-review under responsibility of the Scientific Programme Committee of ICM 2015 c The Authors. Published by Elsevier B.V.

doi:10.1016/j.phpro.2015.12.194

Isovalent substitution effect of P to As on magnetic characteristics of EuFe2 (As1−x Px )2 single crystals Kashiwagi, Arakawa, Ishikawa and Kadowaki

The EuFe2 As2 -system is a unique system where the A-site ions show magnetism among others. This unique system is expected to show many interesting behaviors related to the magnetism of the Eu moments, the SDW state and superconductivity. The parent compound of EuFe2 As2 shows the SDW transition of Fe at ∼ 190 K and the magnetic ordering of Eu2+ (J = S = 7/2) at around 20 K[30, 9]. The ordered state of Eu2+ is known to exhibit an A-type antiferromagnetic(AFM) structure[12, 34]. Superconductivity of this compound is also realized by suppressing the SDW state. For example, superconductivity of EuFe2 As2 -system was observed by substitution for the Eu-site by K (Tsc ∼ 32 K)[10] and by Na (Tsc ∼ 35 K)[27, 28], the Fe-site by Co (Tsc ∼ 21 K)[13] and by Ru (Tsc ∼ 23 K)[15], and the As-site by P (Tsc ∼ 28 K)[29, 11, 2, 25]. The parent compound of EuFe2 As2 also undergoes the superconducting state under physical pressures (Tsc ∼ 25-30 K)[20, 33, 19]. It is known that this pressure induced superconductivity can be regarded as the equivalent phenomenon to the chemical pressure in this system, which induces superconductivity in the case of isovalent P substitution for the As-site. The most interesting characteristic phenomenon in the EuFe2 (As1−x Px )2 system is the coexistence phenomenon of magnetic ordering of Eu2+ magnetic moment with superconductivity. According to the phase diagram[11], the magnetic ordering temperature of Eu2+ moments first decreases with increasing x, then it increases again slightly by further increasing x. Superconductivity was observed to coexist with the magnetic order of Eu2+ in a narrow region of 0.15 < x < 0.22. For x > 0.22, superconductivity disappears, perhaps due to the ferromagnetic ordering of Eu2+ moment[11, 2]. It is noted that EuFe2 P2 (x = 1) does not show superconductivity, however it becomes ferromagnetic at 29 K and it turns to a helimagnetic ordering at 26 K. In addition, it is noted that there is no structural transition in this compound at low temperatures[6]. The peculiarity of this EuFe2 (As1−x Px )2 compound consists in the co-existence phenomenon of the ferromagnetic Eu2+ moments and superconductivity which occurs in a very narrow range of x (0.15 < x < 0.22)[15, 29, 2, 25, 22, 21] with a reentrant behavior of ferromagnetism at lower temperatures[13, 29, 20]. The existence of a ferromagnetic component of Eu2+ in the superconducting phase in this compound has become a common understanding[2, 22, 21, 36]. It is interesting to point out that the crystal structure in the superconducting state was suggested to be orthorhombic not tetragonal as found in the superconducting state with lower x concentration[11]. The structural and magnetic characteristics seem to be intimately related, which may give an important clue for the understanding of the occurrence of superconductivity in this system. In this paper, in order to understand the magnetic characteristics of Eu ions, we have studied the transport properties and the magnetization curves of EuFe2 (As1−x Px )2 single crystals for x = 0.0, 0.08, and 0.20.

2

Material and methods

Single crystals of EuFe2 (As1−x Px )2 were grown by the self-flux method. All the starting materials used for the present crystal growth were better than 99.99 %. The stoichiometric amounts of the starting materials of Eu metal, and pre-synthesized compounds FeAs and FeP were mixed well in a glove box in order to prevent from oxidation, then placed in an Al2 O3 crucible. The crucible was put into an evacuated quartz tube and it was sealed off, then the quartz tube was placed into a welded stainless tube for the safety. The stainless tube were heated according to the following procedure: firstly it was heated at a rate of 50 ◦ C/hour up to 900 ◦ C and was kept for 24 hours. After this reaction it was heated up to 1200∼1300 ◦ C to melt the sample 193


Figure 1: Temperature dependences of the ab-plane resistivity for the single crystals of EuFe2 (As1−x Px )2 with x=0.0, 0.08, and 0.20. The arrows indicate the characteristic temperatures. and was kept for 24 hours, and then slowly cooled to 900 ◦ C with 3 ◦ C/hour. The obtained crystals were characterized by X-ray diffraction and electron probe microanalysis (EPMA) to elucidate x of P and the homogeneity of the crystals. The transport properties of the samples were measured by using an AC resistance bridge (LakeShore Model:370), a 16 T superconducting magnet (Oxford Instruments) and a Physical Property Measurement System (PPMS: Quantum design). The temperature of the sample was controlled by using a variable temperature insert. The magnetization curves were measured by using a Magnet Property Measurement System (MPMS: Quantum design). It is noted that the magnetoresistance shown here were mainly obtained with the transverse configuration of I⊥B.

3

Results and Discussion

We synthesized EuFe2 (As1−x Px )2 single crystals with nominal compositions of x = 0, 0.08 and 0.2. The compositions of each crystal were analyzed by using the EPMA and these were estimated to be (Eu : Fe : As : P = 1 : 1.99 : 1.90 : 0) for x = 0, (Eu : Fe : As : P = 1 : 2.00 : 1.70 : 0.16) for x = 0.08, and (Eu : Fe : As : P = 1 : 1.99 : 1.53 : 0.39) for x = 0.2, respectively. The composition was normalized by the concentration of Eu. Figure 1 shows the temperature dependences of the ab-plane resistivity for the single crystals of EuFe2 (As1−x Px )2 with x=0.0, 0.08, and 0.20. The resistivity data of EuFe2 As2 clearly shows two kinks at 185 K and 20 K. These kinks indicate the SDW transition of Fe magnetic moments and the AFM transition of Eu magnetic moments, respectively[30, 9]. The SDW transition temperature is clearly suppressed with increasing x as shown in the resistivity data of EuFe2 (As0.92 P0.08 )2 . With further increasing x, the clear kink indicative of the SDW transition is lost in the data of EuFe2 (As0.8 P0.2 )2 , then it shows the superconducting transition at around 10 K. These characteristic features for the P doping effects agree well with the previous studies except for the superconducting transition temperature of EuFe2 (As0.8 P0.2 )2 [11, 2]. A possible reason for the discrepancy of the superconducting transition temperature between our sample and the previous studies is not clear at this moment. Figures 2(a) and 2(d) show the magnetization curve and the magnetoresistance(MR) for 194


Figure 2: (a),(b), and (c) The magnetization curve (black) and the magnetoresistance (red) for Bab-plane are displayed for x = 0, 0.08, and 0.2 at 5 K. (d),(e), and (f) The magnetization curve(black) and the magnetoresistance (red) for Bc are plotted for x = 0, 0.08, and 0.2 at 5 K. It is noted that the magnetoresistance for x = 0.2 is plotted in the unit of resistance(mΩ). EuFe2 As2 at 5 K. MR is defined as MR(%)=((R(B)-R(B=0))/R(B=0)) × 100. The magnetization curves show the saturation behavior around ∼1 T for Bab-plane and ∼1.6 T for Bc. In addition, a sharp change of the slope of the magnetization curve was observed at ∼0.5 T for Bab-plane. This is explained by considering a spin flop transition of the Eu-moments for Beasy-axis of the A-type AFM as discussed in the previous study[30]. The magnetization curves follow the behavior as expected by the A-type antiferromagnetic structure[30, 9]. This indicates that there must be the biaxial-type of magnetic anisotropy, where the magnetic hard axis is along the c-axis of the single crystal of EuFe2 As2 . It is noted that the detailed shape of the in-plane magnetization curve strongly depends on the direction of the applied magnetic fields in the ab-plane. Roughly speaking, the slopes of the MR curves for EuFe2 As2 in both directions change from negative to positive at the saturation fields of the Eu-moments. The qualitative characteristics of the observed MR are well consistent with previous studies[12, 35]. In a low field below 0.5 T for Bab, a clear hysteresis was observed. It is noted that the MR in Fig. 2(a) was measured in the case of the angle between I and B of 45 ◦ , because the clear hysteresis was observed in this configuration. The data of I⊥B at 5 K is displayed in Fig. 3(a). This hysteresis behavior may be due to the effect of the twining and the magnetic domain. The in-plane MR seems to be very sensitive to the direction of the applied magnetic fields[35]. Figures 2 (b) and 2(e) show the magnetization curve and the MR for EuFe2 (As0.92 P0.08 )2 at 5 K. The estimated saturation fields are ∼0.6 T for Bab-plane and ∼1.0 T for Bc. The decrease of the saturation fields as compared to that of EuFe2 As2 indicates a reduction of the biaxial-type of magnetic anisotropy by doping of P. The MR curves for EuFe2 (As0.92 P0.08 )2 in both directions also show a change of the slope 195


Figure 3: (a),(b), and (c) Temperature dependence of the ab-plane magnetoresistance for x = 0, 0.08, and 0.2. (d),(e), and (f) Temperature dependence of the c-axis magnetoresistance for x = 0, 0.08, and 0.2. from negative to positive at the saturation fields of the Eu moments. A similar behavior is observed in EuFe2 As2 . It is noted that a clear hysteresis was not observed in EuFe2 (As0.92 P0.08 )2 . In Figs. 2 (c) and 2(f) the magnetization curves and the MR for EuFe2 (As0.8 P0.2 )2 at 5 K are presented. As shown in Fig. 1, EuFe2 (As0.8 P0.2 )2 shows superconductivity at ∼10 K. The saturation fields are estimated to be ∼0.5 T for Bab-plane and ∼0.7 T for Bc, which are smaller than those of EuFe2 (As0.92 P0.08 )2 . In the case of antiferromagnetic ordering, the biaxial-type of magnetic anisotropy becomes smaller and smaller by doping of P. In the low magnetic fields, the MR for EuFe2 (As0.8 P0.2 )2 is zero because the sample is in the superconducting state. With increasing magnetic fields, the resistance starts to increase at ∼3.5 T for Bab-plane and at ∼0.5 T for Bc. The difference of these magnetic fields is related to the anisotropy in the superconducting characteristics of the Eu122-system. Figure 3 (a) shows the temperature dependences of the MR of EuFe2 As2 for Bab-plane. The MR shows a weak peak but it turns to positive above about 1 T which corresponds to the saturation field of the magnetization curve. The hysteresis loop is also observed at lower temperatures for this direction only. With increasing the sample temperatures, the weak peak is lost and the hysteresis loop becomes small. The MR turns the negative to positive slope at about 1.5 T. The temperature dependences of the MR for both directions are clearly correlated with the magnetic ordering of the Eu moments and its magnetization curves. The experimental results indicate that the induced magnetic moment of Eu along the direction of the applied magnetic fields causes a clear negative MR and these are consistent with previous studies[12, 35]. The temperature dependences of the MR for EuFe2 (As0.92 P0.08 )2 for Bab-plane and Bc 196


are displayed in Fig. 3(b) and (e). These are almost same as those of EuFe2 As2 . A peak of the MR was seen blow 1 T for Bc. This result suggests that the change of the magnetic ordering structure of the Eu-moments from the A-type AFM to similar but slightly modulated structure. According to the previous studies[13, 29, 11, 2, 20, 22, 21, 36], it is expected that the Eu moments tilt along the c-axis by doping P. As mentioned above, the alignment of the Eu magnetic moment along the direction of the applied magnetic fields causes a decrease of the MR. The temperature dependences of the MR for the superconducting sample of EuFe2 (As0.8 P0.2 )2 with the transition temperature Tc = 10 K (onset) are shown in Fig. 3(c) and 3(f). The MR observed below Tc indicates the anisotropy in the superconducting characteristics of EuFe2 (As0.8 P0.2 )2 . The MR in EuFe2 (As1−x Px )2 can be written by the three major terms: negative MR due to magnetic moment of Eu below T N ∼ 20 K, a broad negative contribution due to spin fluctuations of Eu moments, and a positive MR from conduction electrons due to Landau quantization. The MR of EuFe2 (As0.8 P0.2 )2 at 10 K shows a kink behavior at 0.5 T for Bab-plane and at 0.7 T for Bc, these fields are almost consistent with the saturation fields of the magnetization curves of EuFe2 (As0.8 P0.2 )2 shown in Fig. 2(c) and (f). It is noted that the MR of non superconducting EuFe2 (As0.85 P0.15 )2 (not shown) is similar to EuFe2 (As0.8 P0.2 )2 . According to the experimental results of the MR for EuFe2 As2 and EuFe2 (As0.92 P0.08 )2 , the induced magnetic moment of Eu along the direction of the applied magnetic field causes a clearly negative MR. The experimental results for EuFe2 (As0.8 P0.2 )2 suggest that the Eu magnetic moments have a large magnetic component along the crystallographic c axis. Therefore, the MR of EuFe2 (As0.8 P0.2 )2 for Bc does not show a negative slope, however the MR for Babplane shows a negative slope because the direction of the Eu-moments changes from the c axis to the ab-plane. In addition to the pioneer works[29, 11, 2, 25], Nandi et al.[22, 21] reported the ferromagnetic ordering of the Eu moments in the superconducting sample of Eu122 by using neutron and X-ray experiments recently. Our experimental result seems to be explained well by considering a ferromagnetic structure or a canted antiferromagnetic structure of Eu moments along the c-axis. It is difficult to determine the ordering structure of Eu moments by using only the data of the MR experiments. In conclusion, we have studied transport properties and magnetization curves of EuFe2 (As1−x Px )2 single crystals for x = 0, 0.08, and 0.2. The anisotropy between Bab-plane and Bc observed in the MR and the magnetization curve becomes small with increasing doping level of P. The experimental results suggest that the magnetic ordering of the Eu-ion is changed from the Atype AFM to a canted antiferromagnet or a ferromagnetic ordering. Superconductivity of this system seems to coexist with a large c axis component of magnetic moment of Eu-ions.

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