Interface magnetism inFe2O3/FeTiO3heterostructures

Abstract

To resolve the microscopic origin of magnetism in the Fe2O3/FeTiO3-system, we have performed density functional theory calculations taking into account on-site Coulomb repulsion. By varying systematically the concentration, distribution and charge state of Ti in a hematite host, we compile a phase diagram of the stability with respect to the end members and find a clear preference to form layered arrangements as opposed to solid solutions. The charge mismatch at the interface is accommodated through Ti 4+ and a disproportionation in the Fe contact layer into Fe 2+ , Fe 3+ , leading to uncompensated moments in the contact layer and giving first theoretical evidence for the lamellar magnetism hypothesis. This interface magnetism is associated with impurity levels in the band gap showing halfmetallic behavior and making Fe2O3/FeTiO3 heterostructures prospective materials for spintronics applications.A challenge of today's materials science is to design ferromagnetic semiconductors operating at room-temperature (RT) for spintronics devices. Most of the efforts concentrate on homogeneous doping of semiconductors with magnetic impurities [1,2,3,4], but the interfaces in complex oxides prove to be another source of novel behavior [5,6,7]. The unique magnetic properties of the hematiteilmenite system [8, 9, 10] (a canted antiferromagnet and a RT paramagnet, respectively) currently receive revived interest as a possible cause of magnetic anomalies in the Earth's deep crust and on other planets [11] as well as for future device applications [3,12,13].Both hematite (a = 5.035Å, c = 13.751Å [14]) and ilmenite (a = 5.177Å, c = 14.265Å [15]) crystallize in a corundum(-derived) structure shown in Fig. 1 where the oxygen ions form a distorted hexagonal close packed lattice and the cations occupy 2/3 of the octahedral sites. In α-Fe 2 O 3 (space group R3c) there is a natural modulation of electronic density along the [0001]-direction where negatively charged 3O 2− layers alternate with positively charged 2Fe 3+ layers. At RT the magnetic moments of subsequent iron layers couple antiferromagnetically (AFM) in-plane with a small spin-canting, attributed to spin-orbit coupling [16,17,18]. In ilmenite, FeTiO 3 , Fe-and Ti-layers alternate, reducing the symmetry to R3 and the corresponding sequence is 3O 2− /2Fe 2+ /3O 2− /2Ti 4+ with AFM coupling between the Fe layers and T N =56-59 K [19].At the interfaces (IFs) in hematite-ilmenite exsolutions, charge neutrality is disrupted. One way to balance the excess charge at the interface is by a disproportionation in the Fe layer, now becom- * Electronic address: pentcheva@lrz.uni-muenchen.de ing mixed Fe 2+ and Fe 3+ . This lamellar magnetism hypothesis (LMH) was proposed by Robinson et al. [20] based on bond valence models and kinetic Monte Carlo simulations (kMC) with empirical chemical and magnetic interaction parameters. The increased technological interest in this system calls for an atomistic material specific understanding that can only be obtained from first principles calculatio...

Insights into the structural, electronic, and magnetic properties of Fe2−x Ti x O3/Fe2O3 thin films with x = 0.44 grown on Al2O3 (0001)

“…Under the assumption that the magnetite content was similar in each sample, the enhanced magnetization of sample 2 can be ascribed to the presence of Ti, which would induce an enrichment of Fe 2? at the bilayer interface consistent with theoretical calculations [17]. The results of the ELNES investigations shown above were made on sample 3 and demonstrate the enrichment of Fe 2?…”

“…Ti-doped hematite (x * 0.1) was suggested as a ferrimagnetic semiconductor with magnetic transition temperature up to 1000 K, provided that Ti atoms order on alternate cation layers [15]. DFT calculations showed that ordered arrangements (in one spin lattice) are preferred compared to randomly arranged Ti impurities [16,17], while experiments reveal smaller effects [18]. The thermodynamically stable state might not be realized in experiments due to e.g., fast cooling.…”

Electronic structure of CaFe₂O₄ with antiferromagnetic spin ordering

“…In the GGA+U calculation, the strong correlation between localized d electrons was taken into account through the screened effective interaction U eff = U ¹ J with Coulomb (U) and exchange (J) parameters, which are mean values for Coulomb and exchange interactions in the considered shell. 15) Each self-consistency was judged from the convergence criterions of charge, total energy, and force. We have also checked the sensitivity of the total energy on the number of k-points in each self-consistent calculation and found the total energy decrease of only 7.94 © 10 ¹3 mRy/atom when k-points increased from 24 to 54.…”

Magnetic, dielectric and photo-absorption study of a ferromagnetic semiconductor α-Fe1.4Ga0.6O3

“…However, distorted (pinched shaped) ferromagnetic loop of Ga doped samples suggests the coexistence of different type magnetic layers; one from Fe rich and other from Ga rich layers in rhombohedral structure. [12][13][14] These distorted loops indicate typical signature of lamellar ferromagnetism, exchange bias effect, and exchange spring magnet due to exchange coupling between two different magnetic layers in metal doped α-Fe 2 O 3 system. 12-14, 30, 36 Our recent work on Ti doped α-Fe 2 O 3 system 38 also showed some of these magnetic features.…”

“…10,11 This is important for developing magnetoelectronic materials for spintronics devices. In the last few years attention was paid mostly on α-Fe 2-x Ti x O 3 series to understand the lamellar magnetism with large magnetic coercivity, [12][13][14] 24 Their results showed α phase stability only below 400 0 C. However, preliminary results of α-Fe 2-x Ga x O 3 series are encouraging to achieve non-conventional ferromagnetic semiconductor for spintronic applications, including band gap tailoring, photoconductivity, ferromagnetism, multiferroic domain switching and ferroelectric polarization. [25][26][27][28] Although thin films of α-Fe 2-x Ga x O 3 series for different contents of Ga exhibited ferrimagnetism with wide range of T C (150-350 K) and showed possible magnetoelectric memories applications, but most of the reports are confined mainly for the perovskite structured FeGaO 3 .…”