Jurnal Elektronika dan Telekomunikasi (JET), Vol. 21, No. 1, August 2021, pp. 27-34 Accredited by RISTEKDIKTI, Decree No: 32a/E/KPT/2017 doi: 10.14203/jet.v21.27-34 Structural, Magnetic, and X-Band Microwave Absorbing Properties of Ni-Ferrites Prepared Using Oxidized Mill Scales Ardita Septiani * , Novrita Idayanti, Tony Kristiantoro, Dedi, Nadya Larasati Kartika, Dadang Mulyadi, Asep Rusmana, Pepen Sumpena Research Center for Electronics and Telecommunication Indonesian Institute of Sciences Jl. Sangkuriang 154 D - Kampus LIPI, Gedung 20, Lt. 4 Bandung, Indonesia Abstract This study aims to evaluate the structural, magnetic, and microwave absorbing properties at the X-band region of oxidized mill scales as by-product derived from a steel making process by means of a facile solid-state reaction. The oxidized mill scales were heated at 600 °C for 4 h followed by mixing with NiO. A calcination process took place at 900 °C and sintering process were conducted at 1260 °C with a milling process conducted in between the heating process. X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) were employed to evaluate the structural properties of the Ni-ferrites samples. Remacomp measurement were conducted to evaluate the magnetic properties and vector network analyzer (VNA) to measure its microwave properties. A single phase of NiFe2O4 was confirmed by XRD data. The site occupancies derived from the Rietveld refinement shows that the Ni:Fe:O ratio deviates from the 1:2:4 ratio as that suggests vacancies formed in the Ni 2+ and Fe 3+ that lowers the unit cell density to 5.08 g/cm 3 that further confirmed by EDS measurement. The coercivity of 11 kOe is also higher than the bulk NiFe2O4 prepared by the chemical grade raw materials. The reflection data of the microwave properties at X-band of 8-12 GHz do not shows significant absorptions. This study suggests that the selected preparation method yields a single phase, however with the significant crystallographic defects and has less ‘soft’ magnetic properties compared to NiFe2O4 prepared using chemical grade by previous study. Keywords: Spinel ferrites, mill scales, soft magnets, magnetic materials I. INTRODUCTION The iron and steel making industry are estimated to generate more than ten million tons of scales every year globally [1]. The scales have ranging levels of oxidation depending on the type of the milling process it derives from. Recovery of such scales are usually includes the re-melting and reduction process, which are energy- exhaustive [1]-[2], even more so for scales that are derived from milling process at elevated temperatures. The scales from this process have higher oxidation states, therefore, to yield the metal form, it requires similar amount energy to the extraction from its raw mineral. The high content of the iron in the scales allows it to be considered secondary raw materials. The greener alternative is to utilize the scales to produce material in the ferrites form. Compared to steel and irons that are dominantly utilized as structural materials, ferrites have the application in the electrical and electronics that partly owing to their particular permittivity and permeability values. Several studies attempted to utilize mill scales to produce ferrites such as Zn-ferrites [3], Ba-hexaferrites [3]-[4], Mg-ferrites [5], and NiZn-ferrites [6]. Ni- ferrites has considerably lower magnetic crystalline anisotropy constant of 0.07 × 10 6 erg/cm 3 [7] in comparison to other ferrites, such as, NiZn-ferrites at 2.63 × 10 5 to 5.94 × 10 5 erg/cm 3 [8], Co-ferrites at 2.96 × 10 6 erg/cm 3 [7] and Mg-ferrites 6.1 × 10 6 erg/cm 3 [9]. Lower values of the magnetocrystalline anisotropy is associated with lower loss of energy for each magnetization and demagnetization cycle, thus suitable to be classified as a soft magnetic material. NiFe2O4 as ferrimagnetic materials are closely related with the resonance behaviour, which is strongly associated to magnetic materials. Ferromagnetic resonance, electron paramagnetic resonance, among others, are few phenomenons occur at varying frequencies [10], which compose the resonance behaviour of materials. X-band (8-12 GHz) is widely used in terrestrial wireless communication. Combined with its low electrical conductivity that results in preventing eddy currents and its high permeability, ferrites can be used in telecommunication application in both resonance and non-resonance states. Significant resonance effect in X- band results in the possibility of utilizing Ni-ferrites prepared using mill scale as electromagnetic shielding by absorbing the incoming electromagnetic waves, whereas the non-existence of resonance behaviour in Ni-ferrites makes it potential to use as circulators. In the previous study, we have tried to prepare the hexagonal barium ferrites using the mill scale with the * Corresponding Author. Email: ardi012@lipi.go.id Received: November 15, 2020 ; Revised: February 21, 2021 Accepted: March 15, 2021 ; Published: August 31, 2021 Open access under CC-BY-NC-SA  2021 PPET - LIPI 25-32