The half-metallic ferromagnetic characters of (001)-oriented thin films of the double perovskite Pb2FeMoO6
Introduction
Double perovskites, firstly discovered by Ward et al. in 1961 [1], [2], are a broad class of compounds with a general chemical formula of A2BB'O6 [3]. Where A is usually an alkaline-earth metal atom (Ca, Sr, Ba) or a rare-earth metal atom (La, Ce, Nd), B a 3d (Cr, Mn, Fe, Co, Ni, Zn) and B′ a 4d (Mo, Te, Ru) or 5d (W, Re, Os) transition-metal (TM) atom. A recent finding of the intrinsic tunneling magnetoresistance (TMR) effect at room temperature in Sr2FeMoO6 [4] revives the extensive investigates on the double perovskites. The band structures reveal that they are half-metallic ferromagnetic (HM-FM) with completely (100%) spin-polarized transport properties at the Fermi level [3], [4]. The Curie temperature TC of the Sr2FeMoO6 is found to be 415 K much higher than room temperature, making it a potential application in magnetoresistive [5] and spintronics [6] devices.
Besides experimental works [3], [4], [7], various first-principles calculations [3], [4], [8], [9], [10], [11], [12], [13] have been performed to investigate the structural, electronic and magnetic properties of the bulk Sr2FeMoO6. It has been found that, the crystalloid structure of the Sr2FeMoO6 is a body-centered tetragonal (BCT) with a space group of I4/mmm (No. 139). The single perovskite units SrFeO3 and SrMoO3 alternate along three crystallographical axes. The corners of each single perovskite unit are in turn occupied by the Fe and Mo with oxygen atoms located in between, forming alternate FeO6 and MoO6 octahedra along the three cubic axes. The Sr atoms occupy the hollow formed by the corners of FeO6 and MoO6 octahedra at the body-centered positions. The Fe3 + (3d5) ion is lowly ionized at a high-spin state of S = 5/2, while the Mo5 + (4d1) ion is highly ionized with a low-spin state of S = 1/2. Each of the two TM sublattices is in an FM arrangement, while the two sublattices are in an antiferromagnetic (AFM) coupling, giving rise to the total magnetic moment of 4 μB per formula unit (f.u.). The smaller saturation magnetization of 3 μB/f.u. at 4.2 K [4], is attributed to the mis-site-type disorder on the TM sites [14], [15], [16].
Because the physical and chemical properties of the double perovskites A2BB'O6 are not only depended on the TM elements B and B′ but also depended on the A element [17], [18]. Considering the comparable crystal radius of Pb2 + (1.63 Å) with that of Sr2 + (1.58 Å) each ion with a 12-fold oxygen coordination [19], in our previous paper [20], we proposed to substitute Sr2 + ion with Pb2 + ion in Sr2FeMoO6 and the structural, electronic and magnetic properties of the bulk Pb2FeMoO6 have been studied in detail. The HM-FM nature implies a potential application of this new compound in magnetoresistive and spintronics devices. One year later, the magnetic properties and magnetoresistive effects of the Pb2FeMoO6 were systematically studied in experiment [21]. Recently, the HM-FM behavior has also been observed in numerous double perovskites, such as Sr2CoFeO6 [22], Ba2CrMoO6 and Ba2FeMoO6 [23], and which also present completely spin polarization of the conduction electrons crossing the Fermi level and thus offer potential technological applications in the devices of single-spin electron source and high-efficiency magnetic sensor.
However, these devices are usually constructed by two HM-FM films (e.g. Pb2FeMoO6) sandwiching a thin nonmagnetic film (e.g. Cu film in spin valve device) or a very thin insulating layer (e.g. Al2O3 film in magnetic tunnel junction device) [6]. So maintaining the HM-FM nature in Pb2FeMoO6 film is a key issue for its technological applications in magnetoresistive and spintronics devices and just the aim of this work to check the electronic and magnetic properties of a thin Pb2FeMoO6 film. It is found that the three possible terminations of the (001)-oriented thin films of the double perovskite Pb2FeMoO6 maintain the HM-FM character and thus can be potentially utilized and applied to the technological applications in magnetoresistive and spintronics devices. The rest of the paper is organized as follows. In Section 2, the calculation methods and models are described in detail. The results and discussions are given in Section 3 and the last section is devoted to the conclusions.
Section snippets
Calculation methods and models
The calculations are performed using the Vienna ab-initio simulation package (VASP) based on the density function theory (DFT) [24], [25], [26], [27]. The interactions between electron and ionic core are represented by the projector augmented wave (PAW) potentials [28]. The exchange and correlation of the electrons are treated by the Perdew-Burke-Ernzerhof (PBE) [29] formulation of the generalized gradient approximation (GGA) taking into account on-site Coulomb repulsive energy U (GGA + U) (U = 2.0
Optimized structures
The optimized structures of the 10-L FeMoO4 and PbO terminated, 9-L FeMoO4 terminated and 11-L PbO terminated thin films of the Pb2FeMoO6 are shown in left panels of Fig. 1(a), (b) and (c), respectively. The corresponding bond lengths of the FeO and MoO bonds along FeOMoOFe chain or MoOFeOMo chain parallel to and perpendicular to the film surface are shown in right panels of Fig. 1(a), (b) and (c) within (110) plane. The detail fractional coordinates x, y and z along a, b and c axes for
Conclusions
The structural, electronic and magnetic properties of the three possible terminations of the (001)-oriented thin films of the double perovskite Pb2FeMoO6, the 10-L FeMoO4 and PbO terminated, 9-L FeMoO4 terminated and 11-L PbO terminated, have been studied by using the first-principles PAW potential within the GGA taking into account the on-site Coulomb repulsive interactions. An outwards relaxation is observed for several layers near surface and the relaxed fractional rumpling s of the PbO
Acknowledgments
The authors would like to acknowledge the National Natural Science Foundation of China (Grant No. 51501017) for providing financial support for this research.
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