The resistive switching memory of CoFe2O4 thin film using nanoporous alumina template

Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti1-x Mn x Co2O4 spinel

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti_1-x Mn _x Co₂O₄ in the temperature interval 1.6 K [Formula: see text] T [Formula: see text] 300 K. Our neutron diffraction study reveals cooperative distortions of the TO₆ octahedra in the Ti_1-x Mn _x Co₂O₄ system for all the Jahn-Teller active ions T  =  Mn³⁺ , Ti³⁺ and Co³⁺ , having the electronic configurations 3d ¹, 3d ⁴ and 3d ⁶, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x  =  0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a  <  1, in which the apical bond length d _c (T _B -O) is longer than the equatorial bond length d _ab (T _B -O) due to the splitting of the e _g level of Mn³⁺ ions into [Formula: see text] and [Formula: see text]. For the composition x  =  0.4, the distortion in the T _B O₆ octahedra is stronger as compared to x  =  0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti_0.6Mn_0.4Co₂O₄ and Ti_0.8Mn_0.2Co₂O₄ sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti³⁺ , Mn³⁺ , and Co³⁺ occupy the octahedral, whereas, Co²⁺ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T _COMP  =  25.4 K in Ti_0.8Mn_0.2Co₂O₄ and a reentrant spin-glass behaviour in Ti_0.6Mn_0.4Co₂O₄ with a freezing temperature of  ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T _COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

Electric Field Control of Magnetism in Iron Oxide Nanoporous Thin Films

Voltage control of the magnetic properties of oxide thin films is highly appealing to enhance energy efficiency in miniaturized spintronic and magnetoelectric devices. Herein, magnetoelectric effects in electrolyte-gated nanoporous iron oxide films are investigated. Highly porous films were prepared by the evaporation-induced self-assembly of sol-gel precursors with a sacrificial block-copolymer template. For comparison, films with less porosity but analogous crystallographic structure were also prepared using an identical procedure except without the polymer template. The films were found to be 70-85 nm in thickness as measured by scanning electron microscopy and primarily hematite as determined by Raman spectroscopy. The templated (highly porous) films showed a very large magnetoelectric response with a maximum increase in magnetic moment at saturation of a factor of 13 and a noticeable (2-fold) increase of coercivity (after applying -50 V). The nontemplated films also exhibited a pronounced increase of magnetic moment at saturation of a factor of 4, although the coercivity remained unaffected over the same voltage range. Magnetoelectric effects in these latter films were found to be fully reversible in the voltage window ±10 V. The observed changes in magnetic properties are concluded to be magneto-ionically driven with oxygen ion exchange between the iron oxide and the liquid electrolyte, as evidenced from Raman and X-ray photoelectron spectroscopy experiments.

Regulation of the forming process and the set voltage distribution of unipolar resistance switching in spin-coated CoFe2O4 thin films

We report the preparation of (111) preferentially oriented CoFe2O4 thin films on Pt(111)/TiO2/SiO2/Si substrates using a spin-coating process. The post-annealing conditions and film thickness were varied for cobalt ferrite (CFO) thin films, and Pt/CFO/Pt structures were prepared to investigate the resistance switching behaviors. Our results showed that resistance switching without a forming process is preferred to obtain less fluctuation in the set voltage, which can be regulated directly from the preparation conditions of the CFO thin films. Therefore, instead of thicker film, CFO thin films deposited by two times spin-coating with a thickness about 100 nm gave stable resistance switching with the most stable set voltage. Since the forming process and the large variation in set voltage have been considered as serious obstacles for the practical application of resistance switching for non-volatile memory devices, our results could provide meaningful insights in improving the performance of ferrite material-based resistance switching memory devices.