Enhanced magnetic moment and conductive behavior inNiFe2O4spinel ultrathin films

Abstract

Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that -under appropriate growth conditions -epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (MS) -about 250% larger -and a metallic character. A systematic study of the thickness dependence of MS allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the A and B sites of the spinel structure resulting from the offequilibrium growth conditions and to interface effects. The relevance of these findings for spinel-and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.

Origin of the giant magnetic moment in epitaxialFe3O4thin films

“…Similar results have been found for the spinel ferrite NiFe 2 O 4 grown on SrTiO 3 substrates, 28 showing for 3-nmthick films a magnetization enhancement of almost four times the bulk value, as measured by means of SQUID magnetometry. This effect was ascribed to the cation inversion: the presence of Ni 2+ in A sites and Fe 3+ in B sites would increase the magnetic moment.…”

Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures

“…Interestingly enough, however, the saturation magnetization of ultrathin films of NiFe 2 O 4 has been found to be substantially larger than bulk magnetization [7]. This unexpected result has been attributed to the out-of-equilibrium atomic distribution resulting from growth conditions [7,8] although definite explanation remains to be settled.…”

“…As an explanation for the higher M s observed in the thinner films, we consider the possibility for the structure to experience a transition to the normal spinel structure, where trivalent ions are only found in [B] [8] and Lüders et al [7] invoked the same argument to account for the observation of an enhanced magnetization of ultrathin NFO films. It is worth however to mention that, so far, no other evidence has been reported.…”

“…It has been found, for instance, that the saturation magnetization of films relatively thick can be substantially lower than the bulk value and this anomalous behavior has been attributed to a large lattice film/substrate mismatch or to the presence of anti-phase boundaries (APB) [5,6]. Interestingly enough, however, the saturation magnetization of ultrathin films of NiFe 2 O 4 has been found to be substantially larger than bulk magnetization [7]. This unexpected result has been attributed to the out-of-equilibrium atomic distribution resulting from growth conditions [7,8] although definite explanation remains to be settled.…”

The magnetization of epitaxial nanometric CoFe2O4(001) layers

“…Similar behavior has also been reported for NiFe 2 O 4 and attributed to partial inverse cationic distribution ͑Fe 1−x Ni x ͒ ϫ͓Fe 1+x Ni 1−x ͔O 4 over octahedral and tetrahedral sites of the spinel structure. 17 In the context of the present manuscript, of major interest is to elucidate the origin of the shrinking of the M͑H͒ loops, particularly noticeable for the thinnest films in Fig. 2.…”