Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe 2−x Cr x O 4 Nanoparticles Synthesized by Chemical Combustion Igor S. Lyubutin, † Chun-Rong Lin,* ,‡ Sergey S. Starchikov, † Arseniy O. Baskakov, † Natalia E. Gervits, † Konstantin O. Funtov, † Yaw-Teng Tseng, ‡ Wen-Jen Lee, ‡ Kun-Yauh Shih, § and Jiann-Shing Lee ‡ † Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, Moscow 119333, Russia ‡ Department of Applied Physics, National Pingtung University, Pingtung County 90003, Taiwan § Department of Applied Chemistry, National Pingtung University, Pingtung County 90003, Taiwan ABSTRACT: A series of nickel−chromium−ferrite NiFe 2−x Cr x O 4 (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mö ssbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (T N is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mö ssbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe 0.75 Ni 0.25 )[Ni 0.75 Cr 1.25 ]O 4 . The saturation magnetization M S in the largest NPs is about (0.98−0.95) μ B , which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O 4 . At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about T com = 360−365 K. This value significantly exceeds the point T com in bulk ferrites NiFe x Cr 2−x O 4 (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe 0.75 Cr 1.25 O 4 with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure. 1. INTRODUCTION Due to interesting electromagnetic characteristics, ferrites can be used in various fields, 1 such as in moisture sensors, 2 in microwave electronic devices, 3 in power devices, 4 and also as hypothermic agents in biomedicine. 5 High resistance, the possibility of tuning of magnetic and electric properties, and simple and cheap production makes them very useful for magnetic recording, ferro-liquids, biosensors, and other applications. 6 The location of magnetic ions in octahedral [B] and tetrahedral (A) crystal sites of ferrites can be varied by introducing different metal ions, 7 size reduction, 8 and changing the condition of synthesis. 9 Thus, the information on such compounds tends to be vital and highly appreciated for theoretical and practical studies. Nickel ferrite NiFe 2 O 4 , possessing inverse spinel structure (Fe)[FeNi]O 4 , 10 demonstrates ferrimagnetism, produced by antiferromagnetic (AFM) ordering of magnetic moments of Fe 3+ in A-sites and Ni 2+ and Fe 3+ moments in octahedral B- sites. The system has a collinear magnetic structure and obeys the Neel model. 11 Nickel ferrites are characterized by superior magnetic permeability at high frequencies, high electric resistance, 12,13 chemical stability, mechanical hardness, and low cost, and they can find wide application in the high- frequency region of electronics. Magnetic parameters, such as saturation magnetization and coercive force, which are of extreme technological importance, can be changed and tuned by chromium doping. The introduction of Cr 3+ ions leads to significant variations of cation distribution over the A- and B- sites depending on the method of synthesis and Cr concentration. The structure of NiFe (2−x) Cr x O 4 (0 ≤ x ≤ 1) is a cubic spinel, and the lattice parameter decreases slightly due to Cr-substitution. The bulk sample with chromium content of x = 1.0 is advised to be used as a permanent magnet, since it exhibits high coercivity. 14−16 Another interesting property of the system NiFe (2−x) Cr x O 4 is the occurrence of the compensation point T comp at some Cr concentrations. 17−19 At this point the temperature-dependent magnetization decreases to zero even below the Neel temperature due to compensation of the magnetic moments of the A- and B-sublattices, ordered antiferromagnetically in the ground state at low temperatures. According to the Neel approach, T comp is not a point of the magnetic phase transition; however, very many anomalous effects were observed at this point, such as magnetoresistance, the magneto-optical Faraday effect, the Hall effect, coercitivity, and the magnetocalorimetric effect. 20−22 The aim of the work is to synthesize and study nanoscale materials based on nickel ferrites, which have anomalous Received: July 29, 2017 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b01935 Inorg. Chem. XXXX, XXX, XXX−XXX