Study on magnetic properties of nanocrystalline La-, Nd-, or Gd-substituted Ni–Mn ferrite at low temperatures

Abstract

The effects of rare-earth ions with different radii and magnetic moments on the magnetic properties of Ni–Mn ferrite are investigated. X-ray diffraction pattern has shown the presence of cubic structure of spinel ferrite for all samples. The values of M_s and H_c are decreasing with increasing of testing temperatures for all samples. The H_c value of Ni_0.7Mn_0.3La_0.1Fe_1.9O₄ reaches 1082 Oe at 2 K. Mössbauer spectra tested at 273 K indicate the presence of superparamagnetism for samples calcined at 873 K.

Introduction

In ferrite MFe₂O₄, the choice of rare-earth ions allows a relative tenability of the magnetic properties such as magnetization or anisotropy. The presence of rare-earth ions influences mainly the magnetic anisotropy of the system. The octahedral and tetrahedral sublattice magnetizations are antiparallel and therefore a noncompensated magnetic moment occurs. This structure is called ferrimagnetic. Various properties such as superparamagnetism [1], [2] and spin canting [3], [4] are observed when the particle size is much reduced compared to the bulk materials. Spinel-type ferrites powders can be prepared by ceramic technique [5], evaporative decomposition of metal organic solution [6], co-precipitation [7], sol–gel [8], [9], [10], microemution [11], micelle and hydrothermal methods [12], [13]. Emulsion is a novel method with the features of convenient operation and well-distributed particle sizes.

In recent years, magnetic nanoparticles have been a subject of intense research due to their unique magnetic properties. Hulova et al. reported coercivity of 20 kOe at 2 K, for CoFe₂O₄ particles of a size of 12 nm in amorphous SiO₂ prepared by the sol–gel method and annealed at 1273 K [14]. An et al. reported that spin canting for Zn_0.4Cu_0.6Fe_1.2Cr_0.8O₄ ferrite emerges around 25 K and increased with decreasing temperature [15]. In this paper, we investigated the effect of rare-earth ions substituted for the iron ions in Ni–Mn ferrite on the magnetic properties at low temperature. It is known that the rare-earth ions play an important role in determining the magnetocrystalline anisotropy in 4f–3d intermetallic compounds [16], [17], [18].

The crystalline sizes are calculated using Scherrer's relationship $D=k\lambda /B\cos \theta$, where ‘D’ is the average diameter in nm, ‘k’ is the shape factor, B are the half intensity width of the relevant diffraction peak and instrumental broadening, respectively. ‘λ’ is the X-ray wavelength and θ is the Bragg's diffraction angle. The broadening of the (3 1 1) diffraction line of the ferrite materials was considered after computer fit of the X-ray data using the Gaussian line shape. The broadening of the diffraction line due to reduction of crystallite dimensions, i.e. B, was estimated by the relation, $B^2=B_{\mathrm{m}}^2-B_{\mathrm{s}}^2$, where B_m is the measured width of the diffraction line at its half maximum and B_s is the measured breadth of the line for the standard at its maximum.