SrCo_xZr_xFe_12−2xO₁₉ and SrNi_xZr_xFe_12−2xO₁₉ hexaferrites: A Comparison Study of AC Susceptibility, FC-ZFC and hyperfine interactions

Highlights

• The hexaferrites (HFs) manufactured via ultrasonic route.

• Mössbauer showed that Co²⁺ and Zr⁴⁺ ions located octahedral.

• Mössbauer presented Ni²⁺ and Zr⁴⁺ ions located at octahedral 12k and 4f₂ sites.

• Magnetization properties displayed ferrimagnetic behavior.

• Super-spin glass-like behavior was noticed at lower temperatures.

Abstract

This study compares the AC susceptibility, FC-ZFC and hyperfine interactions of Sr(Co_xZr_x)Fe_12−2xO₁₉ and Sr(Ni_xZr_x)Fe_12−2xO₁₉ hexaferrites (HFs) manufactured via ultrasonic route. The formation of M-type hexaferrites have been confirmed by X-ray powder diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and high-resolution transmission electron microscopy (HR-TEM) techniques. Scanning electron microscopy (SEM) presented the hexagonal-platelet morphology of products. The variation in isomer shift, line width, quadrupole splitting and hyperfine magnetic field values of them have been governed by ⁵⁷Fe Mössbauer spectroscopy which showed that Co²⁺ and Zr⁴⁺ ions located at generally octahedral and 2b sites, while Ni²⁺ and Zr⁴⁺ ions located at octahedral 12k and 4f₂ sites. Measurements of magnetization versus temperature (M-T) and AC susceptibility versus temperature were carried out. The various synthesized HFs displayed ferrimagnetic behavior in the temperature interval of 10–325 K. Super-spin glass-like behavior was noticed at lower temperatures. Neel-Arrhenius and Vogel-Fulcher models were used to explore the experimental AC susceptibility. It was showed that a lower Co-Zr substitution content leads to strengthen the magnetic exchange interactions, however even low Ni-Zr substitution content provoke a reduction in magnetic exchange interactions.

Introduction

Nanosized ferrites (NSF) are magnetic nanoparticles (MNPs) which have found considerable interest and novel uses in recent years. These applications vary from technological uses such as magnetic recording and electromagnetic radiation (EMR) shielding, biomedical uses such as drug delivery and contrast agents, environmental uses including catalysts and adsorbants to use in polymers, batteries, fuel cells, sensors, etc. [1], [2], [3], [4], [5], [6], [7], [8]. Strontium hexaferrites have been discovered in late 1950s in Philips Laboratories in Eindhoven, Netherlands [9] and have been used in many technological applications as hard magnets since then. Strontium hexaferrite (SrM) has a magnetoplumbite structure with a high coercive field (H_c), high anisotropy, and its space group is P6₃ /mmc. The crystallographic sites of hexaferrites include five sites 12k, 4f₁, 4f₂, 2a, 2b containing 24 iron ions in two-unit cells in the structure of SrM. Three of those are located on the octahedral (Oh) sites with symmetries 4f₂, 2a, 12k whereas the tetrahedral (Td) site, 4f₂, is occupied by one iron ion and the 2b symmetry site in the trigonal bipyramidal site is occupied by the other iron ion [10]. Therefore, the structural, electric, and magnetic characteristics of SrM can be tuned by the introduction of different cations into the structure. Different groups have studied the substitution of the cations by various metallic ions. For instance, Abdellahi et al. have introduced both Zn and Zr ions simultaneously to SrM by a mechanical milling method and sintering to decrease its high anisotropy for hyperthermia applications [4]. Iqbal and co-workers synthesized Sr(Zr_xCu_x)Fe_12-2xO₁₉ (x = 0.0–0.8) nanoparticles to obtain a lower sintering temperature with optimum size for a use in magnetic recording. The saturation magnetization (M_s) increased, the coercivity (Hr) decreased (137 to 49 KAm⁻¹), and the remanence (M_r) stayed constant up to x=0.04 [11]. R.K. Mudsainiyana et al. [12] prepared Co–Zr doped BaCo_xZr_xFe_(12−2x)O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) under tow different conditions sol–gel and citrate precursor sol–gel. They found that the coercivity for samples synthesized by sol–gel was decreased in compassion with those prepared via citrate precursor sol–gel. The saturation magnetization and Mössbauer analysis were varied in both conditions. Prabhjyot Kaur at et al. [13] investigated the effect of Co²⁺ and Zr⁴⁺ substitution on magnetic, microwave and electromagnetic characteristics of Sr hexaferrites synthesized by citrate precursor sol-gel method. They noticed that the Co²⁺ and Zr⁴⁺ enhanced the coercivity and preserved high saturation magnetization. Moreover, the microwave absorption features are adjusted with the thickness and the amount of Co²⁺ and Zr⁴⁺.

On the other hand, the emerging trend on strontium hexaferrites is the synthesis and characterization of novel materials where strontium or iron is or both of those are substituted with different ions, especially rare earth metal ions, for the search of unique magnetic properties. For example, samarium doped SrM, Sr_1– Sm_xFe_{12– x}Zn_xO₁₉ (x = 0.0 - 0.4) were produced by ball-milling followed by a solid-state synthesis at 1473 K by Wu et. al. [14]. The authors found out that the sample where x=0.2 displayed the single phase and the highest saturation magnetization at both 5 K and 300 K, Curie temperature (T_c) decreased with the elevation in the composition. and the coercive force increased along with the temperature and decreased with increasing “x” ratio up to x=0.3 [14]. To understand the exact positions of those dopant ions and hence their effect on the magnetic properties, Mossbauer spectroscopy stays as the correct technique. The high energy (10-12 eV) of Mossbauer spectroscopy enables researchers to obtain detailed information regarding site occupancy and hyperfine interactions in ferrites [5]. Bercoff and co-workers investigated the Co-Nd co-doped SrM nanoparticles (Sr_1-xNd_xFe_12(1-x)Co_xO₁₉ (x = 0.0 – 0.4)) by sol-gel auto-combustion followed by calcination [15]. The decrease in the particle size while a very slight change in the cell parameters confirmed the introduction of those dopant ions into the structure easily. M_s values slightly increased from 72 to 76 emu/g as well as the coercivity (26.40 to 58.70 A/m for x = 0.0 - 0.3). Mossbauer analysis showed that the vacancies of irons were not evenly distributed over the lattice. On the other hand, the site for Co/Fe substitution was mainly in the 4f₂ site. The migration of ions was promoted by iron and oxygen vacancies so that the substitution of Nd was successful and did not have a significant magnetic difference up to x=0.3 whereas the co-substitution with Nd and Co enhanced their magnetic behavior in the iron-deficient SrM. Many studies in the literature focus just on the room-temperature magnetization behavior of SrM however, to truly understand the thermal effect on the magnetic characteristics of SrM, measurements over a wide range of temperature should be conducted [16].

A comparison study of Sr(Co_xZr_x)Fe_12−2xO₁₉ and Sr(Ni_xZr_x)Fe_12−2xO₁₉ HFs (x ≤ 0.10) have been presented in previous publication to investigate the structural and magnetic characteristics. It has been found that both compositions showed hard ferrimagnetic (FM) nature at 300 and 10K. Moreover, the M_s, M_r and n_B were enhanced for Sr(Co_xZr_x)Fe_12−2xO₁₉ whereas these values are reduced for Sr(Ni_xZr_x)Fe_12−2xO₁₉ at measured temperatures [17]. As a continuation to our previous paper, we report a comparison study of magnetic properties versus low temperature, ZFC-FC, and AC susceptibility along with Mossbauer spectroscopy for Sr(Co_xZr_x)Fe_12−2xO₁₉ and Sr(Ni_xZr_x)Fe_12−2xO₁₉ HFs.