J Supercond Nov Magn DOI 10.1007/s10948-016-3965-5 ORIGINAL PAPER High-Temperature Magnetic Behaviour of 10 % Aluminium-Substituted Cobalt Ferrite Lawrence Kumar 1 · Pawan Kumar 2 · Mukesh Kumar Zope 3 · Manoranjan Kar 4 Received: 7 November 2016 / Accepted: 23 December 2016 © Springer Science+Business Media New York 2017 Abstract Magnetic studies on aluminum-substituted cobalt ferrite nanoparticles (CoFe 1.9 Al 0.1 O 4 ) of average particle size ∼20 nm in the broad range of temperature varying from room temperature (300 K) to 900 K has been pre- sented. The temperature dependence of DC magnetization curve exhibits paramagnetic to ferrimagnetic transition at ∼736 K. Zero field cooling and field cooling magnetiza- tion measurement shows that the blocking temperature is far above the room temperature. The presence of ferrimagnetic order in the sample has been analyzed by Arrott plot tech- nique for M-H data measured over the temperature range 300–900 K in the magnetic field of ±9 T. The magnetocrys- talline anisotropy constant was determined by employing the “law of approach (LA) to saturation” theory. The satura- tion magnetization, magnetocrystalline anisotropy constant and coercivity are found to decrease with the increase in temperature. Keywords Ferrimagnetic ordering · Blocking temperature · Magnetocrystalline anisotropy · Coercivity  Manoranjan Kar mano@iitp.ac.in 1 Centre for Nanotechnology, Central University of Jharkhand, Ranchi 835205, India 2 Department of Physics, Mahatma Gandhi Central University, District - East Champaran, Motihari, Bihar 845401, India 3 Indira Gandhi Institute of Medical Sciences, Sheikhpura, Patna 800014, India 4 Department of Physics, Indian Institute of Technology, Patna 800013, India 1 Introduction Cobalt ferrite (CoFe 2 O 4 ) is a centrosymmetric mag- netic material [1]. It crystallizes to Fd - 3m space group with mixed spinel structure represented as (Co 2+ δ Fe 3+ 1-δ )[Co 2+ 1-δ Fe 3+ 1+δ ]O 4 , where cations in the round and square brackets occupy tetrahedral sites (A sites) and octahedral sites (B sites) respectively, and δ depends on thermal history and preparation condition [2]. The spin coupling between the unpaired 3d electrons of Co 2+ and Fe 3+ cations present at the A and B sites governs the mag- netic behavior. Its magnetic ordering temperature is around 793 K, which is far above the room temperature. It exhibits large magnetocrystalline anisotropy, high coercivity, and moderate saturation magnetization at room temperature [3]. These magnetic parameters of cobalt ferrite could be tuned in a controlled way by tailoring its chemical composition with the help of the suitable dopant elements at Fe and Co sites [4–8]. One could extract detailed information about the magnetic behavior of this cobalt ferrite and its derivative compound by carrying out temperature-dependent magneti- zation measurement. The temperature-dependent magnetic properties of cobalt ferrite and its derivative compounds have already been studied extensively in a broad range of temperature varying from room temperature to low as 5 K[9–16]. However, very limited effort has been made to study the temperature-dependent magnetic properties of nanocrystalline cobalt ferrite and its derivative compounds in the broad range of temperature varying from room tem- perature to higher as 900 K [3, 17–19]. Al substitution at Fe site in cobalt ferrite modifies its magnetic properties [5]. Mostly, the Al substitution modifies the hard ferrimagnetic cobalt ferrite to the soft ferromagnetic material. Recently, Zaki et al. have reported the possibility of Al-substituted cobalt ferrite for the magnetic memory device applications