稀有金属(英文版) 2015,34(02),133-136

Electrical switching in cadmium ferrite with different rare-earth ions(Sm³⁺, Y³⁺, and La³⁺)

Department of Electronics, Shivaji University

Department of Physics, Gopal Krishna Gokhale College

摘 要：

The cadmium ferrite and 5 % rare-earth ions(Sm3+, Y3+, and La3+) added Cd ferrites were synthesized by oxalate co-precipitation method and characterized by X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FT-IR), and scanning electron microscope(SEM) techniques. All ferrite samples under investigation exhibit current-controlled negative resistance type I–E characteristics at room temperature. The required electrical-switching field in cadmium ferrite is higher than that for 5 % Sm3+, Y3+, and La3+added cadmium ferrites. The5% addition of Sm3+, Y3+, and La3+in cadmium ferrite is found to decrease the grain size in this ferrite. This decrement in the grain size makes the required switching field to decrease in cadmium ferrite. No aging effect for electrical switching is observed in these ferrites.

收稿日期：9 June 2013

基金：financially supported by the Major Research Project of University Grants Commission, New Delhi, India (No. F.No. 36-212/2008);

Electrical switching in cadmium ferrite with different rare-earth ions(Sm³⁺, Y³⁺, and La³⁺)

Sanjeev P.Dalawai Ashok B.Gadkari Pramod N.Vasambekar

Department of Electronics, Shivaji University

Department of Physics, Gopal Krishna Gokhale College

Abstract：

The cadmium ferrite and 5 % rare-earth ions(Sm3+, Y3+, and La3+) added Cd ferrites were synthesized by oxalate co-precipitation method and characterized by X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FT-IR), and scanning electron microscope(SEM) techniques. All ferrite samples under investigation exhibit current-controlled negative resistance type I–E characteristics at room temperature. The required electrical-switching field in cadmium ferrite is higher than that for 5 % Sm3+, Y3+, and La3+added cadmium ferrites. The5% addition of Sm3+, Y3+, and La3+ in cadmium ferrite is found to decrease the grain size in this ferrite. This decrement in the grain size makes the required switching field to decrease in cadmium ferrite. No aging effect for electrical switching is observed in these ferrites.

Keyword：

Rare-earth ions; Current-controlled negative resistance type electrical switching; Grain size; Cd ferrite;

Author： Sanjeev P.Dalawa e-mail: sanjeevdalawai@gmail.com; Pramod N.Vasambekar e-mail: pnv_eln@unishivaji.ac.in; Ashok B.Gadkari,e-mail: ashokgadkari88@yahoo.com;

Received： 9 June 2013

1 Introduction

The cadmium ferrite has important applications like structural and gas sensor [1], electrical and humidity sensor[2], electrical switching [3] and magnetic [4], etc. The techniques such as ceramic [3], sol–gel [5], hydrothermal[6], spray pyrolysis [7], and co-precipitation [8] are used for synthesis of ferrites.

Electrical-switching phenomenon in Cu Fe₂O₄was first reported by Yamashiro [9]. Vaingankar et al. [10] studied thermally treated bulk Cu Fe₂O₄and reported structural transformation during electrical switching. The electricalswitching characteristics in various ferrites were studied by many researchers [3, 9–18]. The electrical and dielectric behavior of Zn substituted cobalt ferrialuminates was reported by Vasoya et al. [13].

The switching properties were observed to show linearity in I–V characteristic at low-electric field. This is due to the presence of thermally generated carriers [14]. The switching behavior of ferrite substrate due to magnetostatic and spin wave exchange was reported by Saxena et al. [17].Recently, Hu and Chen et al. [18, 19] studied switching properties of Pt/Ni Fe₂O₄/Pt and Pt/Zn O/Co O_x/Zn O/Pt for nonvolatile memory device, respectively.

In this work, we report the electrical switching in cadmium ferrite, and 5 % rare-earth (RE) ions (Sm³⁺, Y³⁺,and La³⁺) added cadmium ferrites were prepared by coprecipitation method.

2 Experimental

Cd Fe₂O₄and 5 % RE ions (Sm³⁺, Y³⁺, and La³⁺) added Cd Fe₂O₄systems were prepared by the oxalate co-precipitation method using AR-grade Cd SO₄?7H₂O (Sd Fine Chemicals ltd.), Sm₂(SO₄)₃?8H₂O (Alfa Aesar), Y₂(SO₄)₃?8H₂O (Alfa Aesar), La₂(SO₄)₃?8H₂O (Alfa Aesar), and Fe SO₄?7H₂O (Thomas Baker Chemicals). The detailed synthesis was explained elsewhere [8]. The co-precipitated powders were dried and sintered at 700 °C for 6 h to get final product. The sintered powders were pressed under hydraulic press at 5.3 N?m^-2pressure to form the pellets of13 mm in diameter. These pellets were finally sintered at1,050 °C for 5 h.

Fig. 1 Typical XRD pattern of 5 % Sm3+added Cd Fe2O4

The ferrite phase formation was confirmed by X-ray diffractometer (XRD, Philips PW-3710) using Cu Ka radiation(λ = 0.15424 nm) in the range of 2θ = 20°–80° with a step size of 0.02°. The surface morphology of fractured pellets was examined under scanning electron microscope (SEM,JEOL-JSM 6360 SEM model, Japan). Fourier transform infrared spectroscopy (FT-IR) absorption spectra of powdered samples were recorded on a Perkin Elmer spectrum one spectrometer in the range of 350–800 cm^-1. I–E characteristics of the samples under investigation were measured at room temperature in the range of 0–600 V using highvoltage regulated DC power supply (Aplab make).

3 Results and discussion

3.1 Structural properties

The typical XRD pattern of 5 % Sm³⁺added Cd Fe₂O₄is presented in Fig. 1. It shows the spinel structure with secondary phase (RFe O₃). The structural properties of 5 %RE ions added Cd Fe₂O₄are already reported [8]. The presence of secondary phase suggests a solubility limit of Sm³⁺, Y³⁺, and La³⁺in the spinel lattice due to their radii(0.083, 0.095, and 0.115 nm), respectively. From Table 1,it can be noticed that the lattice constant for RE ions added cadmium ferrites is smaller than that for cadmium ferrite and it decreases with the increase of radii of RE ions. From Table 1, it is also seen that the mean crystallite size of the samples lies in the range of 28.86–30.40 nm.

The typical SEM image is presented in Fig. 2. The grain size was determined from SEM image of fractured surfaces of pellets. The average grain size was calculated by linear intercept method and is listed in Table 1. The grain size lies in the range of 0.66–2.20 lm. It is the highest for cadmium ferrite and smaller for RE ions added cadmium ferrites. It decreases with the increase of radii of RE ions. It is attributed to the formation of secondary phase on the grain boundaries which suppress abnormal grain growth in RE added cadmium ferrites [8].

The IR spectrum of 5 % La³⁺added cadmium ferrite presented in Fig. 3 shows two major absorption bands in the range of 350–800 cm^-1. It shows further the broadening of the t₂band. The broadening of the t₂band in RE ions added cadmium ferrites suggests the occupancy of RE ions on the B-sites [8].

3.2 Electrical switching

Yamashiro [9] reported the I–V characteristics measured at room temperature in Cu Fe₂O₄, quenched from 800 to600 °C. He found the increment in the first breakdown voltage for decreased quenching temperature with almost constant current, while almost independent second breakdown voltage with quenching temperature. No switching phenomenon was observed for samples quenched below510 °C.

Kaplan et al. [15] reported the current-controlled negative resistance (CCNR) in Si-doped YIG. They observed an excellent agreement between the theoretical and experimental I–V characteristics of this material.

Hisatake et al. [16] reported strongly time-dependant I–V characteristics in Li-ferrite polycrystal. They observed the breakdown process above the threshold current. From the temperature change of the sample with time, for applied voltage, they found that the process is based on the Joule self-heating effect. For the sample kept in distilled water bath with higher resistance than the sample, no breakdown process even over the threshold current was observed.

Table 1 Structural parameters and electrical-switching field for Cd Fe₂O₄and 5 % Sm³⁺, Y³⁺, and La³⁺added Cd Fe₂O₄  下载原图

Table 1 Structural parameters and electrical-switching field for Cd Fe₂O₄and 5 % Sm³⁺, Y³⁺, and La³⁺added Cd Fe₂O₄

Fig. 2 Typical SEM image of 5 % Y3+added Cd Fe2O4

Fig. 3 Typical IR spectra of 5 % La3+added Cd Fe2O4

Fig. 4 I–E characteristics of Cd Fe2O4and 5 % Sm3+, Y3+, and La3+added Cd Fe2O4

Sagare et al. [11] reported the electric field-dependent switching in Li–Cd ferrites at room temperature. Pandya et al. [14] also reported this phenomenon in Li ferrite. They explained the phenomenon on the basis of space charge limited currents in the system [12].

Fig. 5 Variation of grain size and switching field with ionic radii for Cd Fe2O4and 5 % Sm3+, Y3+, and La3+added Cd Fe2O4

Vasambekar et al. [3] reported CCNR type electrical switching in Cd–Co–Cr ferrites at room temperature. They attributed the phenomenon to cation distribution, structure,and Joule self-heating [16].

The I–E characteristics of the cadmium ferrite and RE ions added cadmium ferrites under investigation are presented in Fig. 4. From Fig. 4, it can be seen that the current increases with the increase of field (0 to about1,750 V?cm^-1), reaches threshold, and suddenly increases further with the rapid decrease of the voltage across the sample indicating the switching. Thus, all samples under investigation show CCNR-type electrical switching.

The variation of grain size and switching field with radii of RE ions in cadmium ferrite is presented in Fig. 5. From Fig. 5, it can be noticed that the grain size in cadmium ferrite is the highest, also this ferrite requires highest switching field, and for 5 % RE ions (Sm³⁺, Y³⁺, and La³⁺) added cadmium ferrites, the required switching field becomes smaller than that for cadmium ferrite. The dependence of the grain size on radii of RE ions in these ferrites was reported earlier [8].

In Sm³⁺added cadmium ferrite, the required switching field decreases with the decreases of grain size. In Y³⁺added cadmium ferrite, the required switching field is further lower than that for Sm³⁺added cadmium ferrite as the grain size in this ferrite decreases. In La³⁺added cadmium ferrite, the required switching field seems to increase with the increase of radii of RE ions in this ferrite.However, it is less than that required in Sm³⁺added cadmium ferrite. Thus, the radii of RE ions and the grain size have significant effect on the required switching field in RE ions (Sm³⁺, Y³⁺, and La³⁺) added cadmium ferrites.

The I–E characteristics in the samples under investigation were again measured after 15 days. No aging effect is observed in the repeated testing cycle.

4 Conclusion

The CCNR-type electrical switching is observed in the cadmium ferrite and 5 % RE ions (Sm³⁺, Y³⁺. and La³⁺)added cadmium ferrites at room temperature. The required electrical-switching field in cadmium ferrite is higher than that for 5 % Sm³⁺, Y³⁺, and La³⁺added cadmium ferrites.The 5 % addition of Sm³⁺, Y³⁺, and La³⁺in cadmium ferrite is found to decrease the grain size in this ferrite. This decrement in the grain size makes the required switching field to decrease in cadmium ferrite. No aging effect for electrical switching is observed in these ferrites.