Effect of Ferrimagnetic Cr₂S₃ on AC Susceptibility and Superconducting Properties of YBa₂Cu₃O_7−δ

Abstract

Superconductivity and magnetism are mutually exclusive phenomena. This work was carried out to investigate the effect of ferrimagnetic Cr₂S₃ addition on YBa₂Cu₃O_7−δ superconductor. The solid-state reaction method was used to prepare YBa₂Cu₃O_7−δ (Cr₂S₃)_x (x = 0 to 0.3 wt%). The samples were characterized by using the four-probe method to measure the electrical resistivity and AC susceptibility to measure the critical temperature, T_cχ and critical current density at the peak temperature T_p of the complex susceptibility, J_c(T_p). The x-ray diffraction (XRD) method was used to determine the phase and structure of the sample. Field emission electron microscopy (FESEM) was used to determine the microstructure. The x = 0.05 sample showed the highest onset transition temperature, T_c−onset (93 K), zero-resistance temperature, T_c−zero (89 K), T_cχ (93 K) and T_p (86 K). Addition of Cr₂S₃ did not affect the orthorhombic structure. However, the addition of Cr₂S₃ for x ≥ 0.10 showed some impurity peaks in the XRD patterns. The grain size decreased with Cr₂S₃ addition. Cr₂S₃ improved the intergrain coupling and flux pinning as indicated by the increase in J_c(T_p) and T_p. These results are compared with other magnetic and non-magnetic materials addition to YBa₂Cu₃O_7−δ.

1 Introduction

The interaction between magnetism and superconductivity is an interesting topic to study. There are several reports on the effect of magnetic impurities on the copper oxide-based high-temperature superconductor (HTSC). Nano-magnetic materials are reported to enhance the critical current density of high-temperature superconductors with a strong interaction between flux line network and magnetic texture [1, 2]. Addition of γ-Fe₂O₃, Fe₃O₄ and ZnO in Bi–Sr–Ca–Cu–O enhanced the critical current density [3,4,5]. GdBa₂Cu₃O_7−δ containing α-Fe₂O₃ also exhibited enhanced critical current density [6]. Nano-sized PbO showed improvements in the superconducting properties of YBa₂Cu₃O_7−δ [7]. However, addition of ferromagnetic Co₃O₄ decreased the transition temperature, T_c of YBa₂Cu₃O_7−δ [8].

Apart from metal oxides, metal sulphides are an interesting substitution in YBa₂Cu₃O_7−δ. Moreover, oxygen and sulphur belong to the same group (VI) in the periodic table. Metal fluorides such as antiferromagnetic FeF₂ addition in YBa₂Cu₃O_7−δ(FeF₂)_x showed the highest transition temperature and critical current density for x = 0.03 wt% [9]. Other compounds including Co₃O₄, CdTe, CoFe₂O₄, graphene and graphene oxide showed different effects on YBa₂Cu₃O_7−δ depending on the magnetic property [8, 10,11,12,13].

Chromium sulphide (Cr₂S₃) is a ferrimagnetic semiconductor with Curie temperature of 120 K and energy gap of 1.1 eV [14, 15]. The melting point of Cr₂S₃ is 1350 ^∘C and this is much higher than the formation temperature of YBa₂Cu₃O_7−δ (∼ 900 ^∘C). The transition temperature of YBa₂Cu₃O_7−δ is around 92 K and this is well below the Curie temperature of Cr₂S₃. Hence, it is interesting to study the interaction of YBa₂Cu₃O_7−δ superconductor with ferrimagnetic Cr₂S₃.

The objectives of this work were to study the properties of YBa₂Cu₃O_7−δ as the amount of ferrimagnetic Cr₂S₃ is increased. The AC susceptibility and electrical resistivity of YBa₂Cu₃O_7−δ(Cr₂S₃)_x with x = 0 to 0.3 wt% as a function of temperature were measured. The structure, microstructure, and the chemical composition of the composite are also reported in this paper. The result from this work was compared with previous reports. Table 1 shows the effect of various compounds on the transition temperature of YBa₂Cu₃O_7−δ from previous works as well as this work. In Table 1, samples with x = 0 showing the maximum T_c indicated that the transition temperature was suppressed for the respective additions. Antiferromagnetic material such as FeF₂ [9], ferrimagnetic such as Cr₂S₃, and CdTe [10] which is diamagnetic [16, 17], enhanced T_c of YBa₂Cu₃O_7−δ while ferromagnetic materials such as Co₃O₄ [8] and CoFe₂O₄ [11] suppressed the transition temperature.

Table 1 The highest transition temperature of YBa₂Cu₃O_7−δ added with various magnetic compounds

2 Materials and Methods

The solid-state reaction method was used to prepare the samples. High purity (> 99.9 %) Y₂O₃, BaO or BaCO₃ and CuO was mixed with starting chemical formula YBa₂Cu₃O_7−δ and heated at 900 ^∘C for over 24 h with intermittent grindings. Cr₂S₃ was then added to the resultant powders with chemical formula YBa₂Cu₃O_7−δ(Cr₂S₃)_x for x = 0, 0.05, 0.15, 0.2, and 0.3 wt%. The powders were pressed into pellets of about 12-mm diameter and 2-mm thickness and heated at 900 ^∘C for 24 h.

The samples were characterized by x-ray diffraction (XRD) method to study the phase and structure using Bruker model D8 Advance diffractometer with CuK_α source. The field emission scanning electron micrographs (FESEM) were recorded using a Merlin Gemini scanning electron microscope. The chemical composition of the samples was analysed by energy-dispersive x-ray analysis (EDX) using an Oxford Instrument analyser.

The dc electrical resistivity was measured using the four-probe method with silver paste contacts. A closed cycle refrigerator from CTI Cryogenics Model CSA 22 and a temperature controller from Lake Shore model 303 was used for low-temperature measurements. The onset transition temperature, T_c−onset, is the temperature where the resistivity starts to drop abruptly. The zero-resistance temperature, T_c−zero, is the temperature where the resistivity drops to zero.

An AC susceptometer from Cryo Industry model number REF-1808-AS was used to measure the complex susceptibility χ = χ^′ + χ^″. The frequency used was 295 Hz and applied magnetic field was H = 5 Oe. The Bean model was used to calculate the critical current density at the peak temperature T_p of χ” with formula J_c(T_p) = H/(lw)^1/2, where H is the applied field and l and w are the dimensions of the cross section of the bar-shaped sample.

3 Results and Discussion

The XRD patterns of x = 0 and 0.05 wt% samples showed a single YBa₂Cu₃O_7−δ phase with orthorhombic structure (Pmmm space group). The x = 0.1, 0.2 and 0.3 wt% samples showed the YBa₂Cu₃O_7−δ phase as well as impurity peaks belonging to the Y₂BaCuO₅ (Y211) and Cr₂S₃ as indicated by (*) and (#), respectively in Fig. 1. Other unmarked peaks observed in the x = 0.1, 0.2 and 0.3 patterns (Fig. 1) may be due to unreacted starting materials. At least 15 peaks were used to calculate the lattice parameters. The lattice parameters a, b andc and the unit cell volume V did not show systematic change (Table 1). The lattice parameters of the non-added YBa₂Cu₃O_7−δ are a = 3.829(1) Å, b = 3.881(1) Å and c = 11.696(2) Å. The ratio of the lattice parameter c/a and c/b indicates the internal lattice strain. The ratio of c/a ∼ 3.06 and c/b ∼ 3 for all samples. Hence, no changes in the internal lattice strain is expected in these samples.

Fig. 1

X-ray powder diffraction patterns of YBa₂Cu₃O_7−δ(Cr₂S₃)_x. x = 0, 0.05, 0.1, 0.2 and 0.3 wt%. Asterisk and number sign indicate Y₂BaCuO₅ and Cr₂S₃, respectively

The grain size of the samples decreased as Cr₂S₃ was added. Figure 2a shows the SEM micrograph of the non-added sample and Fig. 2b shows the micrograph for x = 0.05 sample. The x = 0 sample showed slightly elongated grains of more than 2 μ m and the x = 0.05 sample showed irregular grains of around 1 μ m with vivid grain boundaries. Hence, Cr₂S₃ reduced the grain size of YBa₂Cu₃O_7−δ. Figure 3 shows the EDX spectra of the x = 0 and 0.05 samples with approximate elemental weight percent of the related elements. The presence of Cr and S with the approximate weight percent are shown in the EDX spectrum (Fig. 3b).

Fig. 2

Scanning electron micrographs of YBa₂Cu₃O_7−δ(Cr₂S₃)_x. ax = 0 and bx = 0.05 wt%

Fig. 3

EDX spectra of YBa₂Cu₃O_7−δ(Cr₂S₃)_x for ax = 0 and bx = 0.05 wt%. Inset shows the weight percent of each element

The electrical resistivity versus temperature curves of all samples are shown in Fig. 4a, b. The x = 0 and 0.05 samples showed metallic normal state behaviour with T_c−onset = 92 and 93 K, respectively. The x = 0.1, 0.2 and 0.3 samples showed a semiconducting-like normal-state behaviour. T_c−onset for the x = 0.1, 0.2 and 0.3 samples was 93, 92 and 89 K, respectively. Among all samples, the x = 0.05 showed the highest T_c−onset (93 K) and T_c−zero (89 K). T_c−zero does not show systematic change; however, all samples showed a consistent and systematic change in T_c−onset with Cr₂S₃ content. The transition width ΔT_c is 4 K for x = 0.05 sample and 32 K for x = 0.2 sample. The large ΔT_c in the x = 0.2 sample was probably specific to this sample and may be due to the large variation of the transition temperature of individual superconducting grains as Cr₂S₃ was increased. The room-temperature resistivity decreased for x = 0.05 sample and then increased with further Cr₂S₃ additions. Sample with x = 0.3 wt% showed the highest room-temperature resistivity which was 8.1 × 10^− 5 Ω m. The increase in electrical resistivity indicated a decrease in the carrier concentration in the samples. The x = 0.05 sample showed the lowest electrical resistivity (i.e., the highest carrier concentration) resulting in the highest transition temperature which is probably due to the optimization of the oxygen content in this sample. The electrical resistivity at room temperature, T_c−onset and T_c−zero are shown in Table 2.

Fig. 4

Electrical resistivity versus temperature of YBa₂Cu₃O_7−δ(Cr₂S₃)_x for x = 0, 0.05, 0.1, 0.2 and 0.3 wt%. a 50–220 K and b 50–100 K range

Table 2 Lattice parameters, transition temperature and critical current density for all samples

The AC susceptibility versus temperature curves for the x = 0, 0.05 and 0.1 samples are shown in Fig. 5a, b. The sudden decrease in the real part χ^′ of the complex susceptibility (χ = χ^′ + iχ^″) is the transition temperature, T_cχ’ which is due to diamagnetic shielding and can be identified as the onset of bulk superconductivity. The peak temperature (T_p) in the imaginary part of the susceptibility χ” represents AC losses. The weaker the pinning, the larger T_p shift to lower temperature. Below T_p the amplitude of χ^″ falls due to the decrease in flux penetration. The x = 0 and 0.05 samples showed the highest $T_{\mathrm {c}\chi ^{\prime }}$ which was 93 K. The x = 0.1, 0.2 and 0.3 samples showed $T_{\mathrm {c}\chi ^{\prime }}= 90$, 84 and 86 K, respectively. T_p was highest for x = 0.05 indicating that Cr₂S₃ improved the flux pinning for lower value of x. Higher Cr₂S₃ content decreased T_p severely to 53–58 K in the x = 0.1, 0.2 and 0.3 samples indicating flux penetration only occurred at lower temperatures The transport critical current density at the peak temperature J_c(T_p) was around 18–22 A/cm². Table 2 shows the values for $T_{\mathrm {c}\chi ^{\prime }}$, T_p and J_c(T_p).

Fig. 5

AC susceptibility (χ = $\chi ^{\prime }+$$i\chi ^{\prime \prime }$) versus temperature of YBa₂Cu₃O_7−δ(Cr₂S₃)_x for x = 0, 0.05 and 0.10 wt%. a 40–95 K and b 80–95 K range

Figure 6 shows T_c−onset, T_p and room-temperature electrical resistivity versus Cr₂S₃ content. This graph shows that the optimal superconducting properties in terms of T_c−onset,T_p and room temperature resistivity were observed for the x = 0.05 sample. This sample also showed the lowest electrical resistivity and narrowest transition width (ΔT_c = 4 K) compared to other samples. This work showed that a small amount of ferrimagnetic Cr₂S₃ addition improved the superconducting properties. This result is similar to the work reported on YBa₂Cu₃O_7−δ added with antiferromagnetic FeF₂ where the optimal addition is for x = 0.03 wt%. Changes in the transition temperature of our samples are not due to variation in the internal lattice strain because the lattice ratio (c/a ∼ 3.06 and c/b ∼ 3) did not change with Cr₂S₃ addition.

Fig. 6

T_c−onset, T_p and electrical resistivity at room temperature versus Cr₂S₃ content of YBa₂Cu₃O_7−δ(Cr₂S₃)_x for x = 0–0.30 wt%. The lines (solid and dash) are for eye guide

In Table 1, the suppression of transition temperature with ferromagnetic-added sample was due to the exclusive nature of superconductivity and magnetism. Ferromagnetic materials such as CoFe₂O₄ [11] and Co₃O₄ [8] may act as effective pair breaking centre leading to the suppression of the transition temperature. However in ferrimagnetic (this work), antiferromagnetic [9] and diamagnetic [10] material-added samples, the enhancement of T_c−onset and J_c was due to the improved grain coupling and carrier density and these materials may not be as effective pair breaking centre as ferromagnetic materials.

In conclusion, the effects of Cr₂S₃ on YBa₂Cu₃O_7−δ have been investigated. Ferrimagnetic Cr₂S₃ enhanced the transition temperature for low addition level (x = 0.05). The peak temperature of χ” (T_p) and critical current density J_c(T_p) increased as Cr₂S₃ was added (x = 0.05) indicating improved flux pinning and enhanced intergrain coupling. The grain size was reduced as Cr₂S₃ was added. The presence of Cr₂S₃ peaks in the XRD patterns showed that it exists as a separate phase and did not enter the YBa₂Cu₃O_7−δ lattice. By comparing with previous works, in general, ferromagnetic materials suppressed T_c but ferrimagnetic, antiferromagnetic and diamagnetic materials enhanced T_c for low-level addition. These results can be useful in determining the appropriate addition to improve the superconducting properties of YBa₂Cu₃O_7−δ. Further works on other materials can be useful in determining the effects of magnetism on HTSC in general.