ORIGINAL PAPER Superparamagnetic Behavior of Zn and Al Substituted Cobalt Nanoferrites J. Z. Msomi 1 & S. J. Masuku 1 & T. A. Nhlapo 2 & T. Moyo 2 & A. M. Strydom 3 & D. Britz 3 & T. P. Jili 1 Received: 20 June 2018 /Accepted: 18 January 2019 # Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Nanostructured Zn- and Al-doped cobalt spinel ferrites Zn x Co 1-x Fe 2-x Al x O 4 (0 ≤ x ≤ 1.0) with particle size in the range of 7– 13 nm were synthesized by glycol-thermal reaction. XRD analysis confirmed single-phase cubic spinel structure with no impurity phases. The Mössbauer spectra for Zn- and Al-substituted fine powders could be resolved into two quadrupole doublets associated with 57 Fe nuclei in paramagnetic spin phase. An unusual reducing magnetization with increasing particle size has been observed. The field cooled and zero field cooled magnetization measurements show superparamagnetic nanoparticles in the compounds investigated. The blocking temperatures (T B ) are sensitive to particle size. Larger particles are blocked at higher temperatures compared to smaller particles at the same field. Keywords Ferrites . Nanoparticle . Magnetization . Mössbauer spectra 1 Introduction Ferrites nanoparticles are a novel group of nanomaterials with interesting properties making them suitable for important technologies such as drug delivery, medical diagnostic, mag- netic recording, ferrofluid technology, sensors, high-density information storage media, as catalyst, and as antibacterial agents [1, 2]. The crystal structure is that of a mineral spinel MgAl 2 O 4 . In ferrites, Mg and Al ions are replaced by divalent metal ions and Fe ions respectively. Oxygen ions form face- centered cubic lattice with metal ions distributed between two lattice sites, tetrahedral (A) and octahedral (B) sites. A and B site ions are surrounded by 4 and 6 oxygen ions respectively [3]. Magnetic and electrical properties are dependent on the distribution of metal cations at different sites, which is sensi- tive towards the synthesis route, microstructure, heat treatments, and type and amount of divalent metal cations [3–6]. The ferrite crystal structure has vacant A and B inter- stitial sites, which allow filling with suitable divalent cations in order to tune the properties for practical application [3–10]. This has resulted in several articles associated with doping effects on the magnetic properties of ferrite materials appearing in literature recently [1–25]. In a preliminary study, bulk Zn x Co 1-x Fe 2-x Al x O 4 com- pounds produced by standard ceramic process showed more rapid transformation from ordered to paramagnetic spin phase as compared to single site dilution [13, 14]. In this communi- cation, Zn x Co 1-x Fe 2-x Al x O 4 (0 ≤ x ≤ 0.9) fine powders were produced by glycol-thermal reaction and characterized by Mössbauer spectroscopy and magnetization measurements. Of interest are crystal size effects on the magnetic properties of Zn and Al double substituted cobalt ferrite nanoparticles. 2 Experimental Nanocrystalline Zn x Co 1-x Fe 2-x Al x O 4 (x = 0, 0.1, 0.2, 0.4, 0.5, 0.7, 0.8, and 0.9) ferrite nanoparticles were prepared by glycol-thermal technique, following procedures discussed previously [15]. The XRD patterns of the as prepared samples were obtained using a Phillips powder diffractometer (type: PW1710) with Co-K α radiation (λ = 1.7903 Å) or Rigaku powder diffractometer with Cu-K α (λ = 1.54059 Å) radiation. * J. Z. Msomi msomij@unizulu.ac.za 1 Department of Physics and Engineering, University of Zululand, P/ bag X1001, KwaDlangezwa, Richards Bay 3886, South Africa 2 School of Chemistry and Physics, University of KwaZulu-Natal, P/ Bag X54001, Durban 4000, South Africa 3 Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, P. O. Box 524, Auckland Park 2006, South Africa Journal of Superconductivity and Novel Magnetism https://doi.org/10.1007/s10948-019-5036-1