Particular Transport Properties of NiFe 2 O 4 Thin Films at High Temperatures Cecilia Solís, † Simona Somacescu, ‡ Elena Palafox, † María Balaguer, †,§ and Jose ́ M. Serra* ,† † Instituto de Tecnología Química, Universidad Polite ́ cnica de Valencia - Consejo Superior de Investigaciones Científicas, Av. Naranjos s/n, E-46022 Valencia, Spain ‡ “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania § Forschungszentrum Jü lich GmbH, Institute of Energy and Climate Research IEK-1, D-52425 Jü lich, Germany * S Supporting Information ABSTRACT: NiFe 2 O 4 (NFO) thin films were deposited on quartz substrates by rf magnetron sputtering, and the influence of the deposition conditions on their physic- chemical properties was studied. The films structure and the high temperature transport properties were analyzed as a function of the deposition temperature. The analysis of the total conductivity up to 800 °C in different pO 2 containing atmospheres showed a distinct electronic behavior of the films with regard to the bulk NFO material. Indeed, the thin films exhibit p-type electronic conductivity, while the bulk material is known to be a prevailing n- type electronic conductor. This difference is ascribed to the dissimilar concentration of Ni 3+ in the thin films, as revealed by XPS analysis at room temperature. The bulk material with a low concentration of Ni 3+ (Ni 3+ /Ni 2+ ratio of 0.20) shows the expected n-type electronic conduction via electron hopping between Fe 3+ -Fe 2+ . On the other hand, the NFO thin films annealed at 800 °C exhibit a Ni 3+ /Ni 2+ ratio of 0.42 and show p-type conduction via hole hopping between Ni 3+ -Ni 2+ . ■ INTRODUCTION Nanostructured thin films of NiFe 2 O 4 (NFO) are important candidates for magnetic components such as resonators, phase shifters, tunable signal filters, and, more recently, for spintronics applications. 1-3 Along with their diverse applications, ferrites have properties of fundamental interest. These include their structure, which can be inverse, normal, or mixed spinel, and the dependence of their physical properties on size and synthesis techniques. Spinel ferrites have the general chemical formula (M 1-δ 2+ Fe δ 3+ )[M δ 2+ Fe 1-δ 3+ ]O 4 2- , where the divalent ions M 2+ can occupy either tetrahedral (A) or octahedral (B) sites or both. Bulk NiFe 2 O 4 (NFO) is a well-known inverse spinel with all Ni 2+ ions occupying only the B sites. However, NFO in nanocrystalline form has been reported to exhibit a mixed spinel structure with Ni 2+ ions occupying both A and B sites. 4 In addition, NFO has shown n-type or p-type electronic conductivity depending on the different synthesis techniques and crystal sizes. The n-type behavior has been attributed to the presence of Fe 2+ , which enables the electron hopping from Fe 2+ to Fe 3+ . The p-type behavior has been attributed to the presence of Ni 3+ and hole hopping from Ni 3+ to Ni 2+ . The last can be assigned to the deficiency or excess in Ni, because a Ni excess corresponds to Fe 3+ deficiency compensated with Ni 3+ (p-type) while a Ni deficiency corresponds to a Fe 2+ compensation (n-type). 4 This type of spinel is a potential candidate for solid oxide fuel cell component due to the tunability and the appropriate magnitude of the total conductivity at high temperature. Specifically, some doped nickel ferrites have been recently studied as possible cathodes for SOFC applications due to their mixed ionic -electronic conduction; for example, Ni- Fe 1.5 Co 0.5 O 4 presents, at 700 °C in air, 0.24 S/cm and 9.6 × 10 -4 S/cm of electronic and ionic conductivity, respectively. 5 Because the ionic conductivity is very low as compared to the electronic transport and limits the oxygen transport, NFO was also proposed to be combined in a composite with a predominant ionic conducting material to work as dual-phase oxygen separation membrane. 6,7 This work reports the fabrication of NFO spinel thin films by rf sputtering at different temperatures. The study transport properties at high temperature of thin films combined with XPS analysis make it possible to obtain deeper insight into electronic conduction mechanisms. ■ EXPERIMENTAL SECTION NiFe 2 O 4 (NFO) powders were prepared by the sol-gel route (Pechini method), 7 and the powder was calcined in air at 700 °C. One inch ceramic target was prepared by uniaxially pressing into pellets at 30 kN for 3 min and subsequently sintered in air at 1400 °C for 5 h. X-ray diffraction (XRD) of the sintered target confirmed the complete formation of the corresponding spinel structures. 5 The thin films were deposited with a radio frequency (13.56 MHz) Pfeiffer Classic 250 deposition system. NiFe 2 O 4 target was used for sputtering on a rotating substrate Received: July 11, 2014 Revised: October 1, 2014 Published: October 2, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 24266 dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266-24273