Research articles
Half-metallic ferromagnetism and metal–insulator transition in Sn-doped SrRuO3 perovskite oxides

https://doi.org/10.1016/j.jmmm.2018.03.034Get rights and content

Abstract

We investigate the electronic and magnetic properties of SrRu1-xSnxO3 by carrying out density-functional-theory calculations to show that a half-metallic ferromagnetic ground state emerges for the Sn doping of x0.5. To examine the effect of on-site Coulomb interactions for the Ru d orbitals, which was suggested to enhance the half-metallicity in SrRuO3, we employed both the local spin-density approximation (LSDA) as well as the LSDA + U method. For all the possible configurations of Sn doping for x=1/8,1/4,1/2,5/8,3/4, and 7/8 within the 2×2×2 unit cell, we monitor the Ru t2g bandwidth as well as the valence band maximum in the majority-spin channel and demonstrate that the Ru d electron hopping is blocked by the Sn-substituted sites so that the Ru t2g bandwidth becomes reduced as the doping x increases. For x<0.5, the valence band maximum still touches the Fermi level so that SrRu1-xSnxO3 remains as a usual ferromagnetic metal. A further reduction of the Ru t2g bandwidth for the range of 0.5x0.7 turns it into a half-metallic ferromagnet. As for x>0.7, the Ru t2g bandwidth gets so narrow that even a small on-site Coulomb interaction, e.g., Ueff=1.0 eV induces a band-gap, which indeed corresponds to a gap of the Ru impurity bands in the SrSnO3 oxide semiconductor.

Introduction

Since the pioneering work on the Mn-based Heusler alloys by de Groot and co-wokers [1], half-metallic ferromagnets (HMFs) have attracted considerable interest as a new class of materials with potential applications for its possible memory and spintronic device applications. An ideal half-metal has only one spin channel for conduction, that is, metallic behavior in one spin channel but an insulating gap in the other spin channel. However, while many different HMFs have been suggested, its fabrication has been challenging. Since the experimental verification of half-metallicity remains controversial, there are not many HMF materials other than NiMnSb with enough evidence for its half-metallic ferromagnetism [2]. Despite the controversies and challenges, there have been many theoretical attempts in searching for new half-metallic materials [2], [3].

Besides Heusler alloys, perovskite oxides have been considered as a promising class of materials for HMFs. Recent advances in the growth technique of oxide heterostructures have made it possible to manipulate complex electronic structures having both metallic and insulating characters. One of the first realizations of HMF was in doped manganites like La0.66Ca0.33MnO3 and La0.7Sr0.3MnO3, where the double-exchange mechanism for the doped carrier leads to the half-metallic state and eventually to the colossal magnetoresistance behavior [3], [4]. Later, the idea of double-exchange ferromagnetism in manganites was applied to a series of double perovskite structures as a new form of HMFs. The first series of double perovskites like Sr2FeMoO6, Sr2FeReO6, Sr2CrWO6, Sr2CrFeO3, and La2VMnO6 were predicted where the B-site magnetic ion is substituted by another magnetic ion so that an ordered double perovskite structure can be stabilized [5], [6], [7], [8], [9]. Also suggested were a double doping of both A- and B-sites in La(Sr,Ca,Ba)VRuO6 [10] as well as the doping of the B-site magnetic ion with non-magnetic ions in Sr2Fe(Sn,Ti,Zr)O6 [11]. Initially, Sr2FeMoO6 was reported to be half-metallic exhibiting intrinsic tunneling-type magnetoresistance at room temperature. However, the half-metallicity of such double perovskite systems turns out to be sensitive to the ordering of anti-ferromagnetic ordered B-site ions, e.g., Fe3+ and Mo5+ ions. Even a small amount of disorder can be crucial to the half-metallicity in such double perovskite systems [5].

The idea of double-exchange ferromagnetism for the half-metallicity in double perovskite systems was quite intriguing, but its realization is still hampered by the requirement of the strict B-site ordering. As an alternative to the double-exchange idea, here we attempt to find a way of transforming a simple perovskite ferromagnetic metal into half-metal with the substitution of B-site ions. We draw special attention to SrRuO3 because it is known to be the only ferromagnetic metal (Tc160 K) among the 4d transition-metal oxides [12], [13], [14]. Early on, Katsnelson and co-workers [2] has pointed out that SrRuO3 has all the ingredients for HMF, where a low-spin state of Ru occurs with the almost full t2g majority-spin band and the partially filled t2g minority-spin band. Even there was a theoretical claim of a half-metallic ground state with Jahn–Teller distortion based on the LDA + U calculation with a large value of U, [15] but, unfortunately, there has been no experimental report of either Jahn–Teller distortion or half-metallicity [12]. Rondinelli and co-workers made an extensive theoretical investigation on the ground state properties of SrRuO3 and concluded that the on-site Coulomb interaction parameter Ueff needs to be set to be larger than 2 eV to achieve the half-metallic ground state in SrRuO3, which is inconsistent with the absence of orbital ordering in experiment as well as the theoretical estimation of Ueff by the self-interaction-correction calculations [16].

On the other hand, there has been an interesting suggestion of inducing the half-metallicity by doping Ti atoms in SrRuO3. Following up the experimental observation of a metal–insulator transition in SrRu1-xTixO3 (x0.3), which was attributed to both the disorder and on-site Coulomb correlations at the Ru-site, [17], [18] a GGA + U calculation for SrRu1-xTixO3 has suggested the presence of half-metallic state for x>0.1 with a metal–insulator transition near x0.65 [19]. Although the Ueff parameter used in the calculation was a bit too large to be realistic, [16] it may be considered as a meaningful suggestion for a possible route of doping SrRuO3 into HMF.

Here we propose the Sn-doped SrRuO3 perovskite oxide as a candidate material for HMF. SrRuO3 is one of the most frequently used oxide electrode materials. It has high chemical stability and good thermal properties, which are advantageous for integration with other oxide materials into heterostructures [20]. Because of the similar ionic sizes of Ru4+ and Sn4+ ions, the substitution of Ru by Sn atoms in SrRuO3 is quite feasible. Indeed, it has been demonstrated that the epitaxial Sr(Ru0.48Sn0.52) O3 thin film exhibits an excellent lattice match with the (100)-oriented KTaO3 [21]. At the endpoints of SrRu1-xSnxO3, SrRuO3 is a ferromagnetic metal while SrSnO3 is a large gap semiconductor with the optical gap of around 4 eV [22], [23]. Since the band gap of SrSnO3 is larger than the Ru t2g bandwidth of less than 3 eV in SrRuO3, the Sn atom at the Ru site is expected to block the hopping of Ru d electrons effectively. As results, by controlling the amount of doping x in SrRu1-xSnxO3, one can change the Ru t2g bandwidth which results in a ferromagnetic-metal-to–half-metal transition near x0.5 and a metal-to-insulator transition as well for x>0.7.

Section snippets

Methods and computational details

To investigate the electronic structure and properties of Sr(Ru1-xSnx) O3 as a function of x, we carried out density functional theory (DFT) calculations by using the full-potential projected augmented wave method [24] as implemented in the VASP package [25], [26] with the local spin-density approximation (LSDA) [27] and the LSDA + U scheme [28] as well. We adopted the spherically averaged form of the rotationally invariant LSDA + U introduced by Dudarev and co-workers, [29] where only one

LSDA band structures of Sr(Ru1-xSnx) O3 (x=0.0,0.25,0.5, and 0.75)

Among various 4d transition metal oxides, bulk SrRuO3 is the only 4d ferromagnetic metal-oxide. Since the 4d electrons are less localized than 3d, SrRuO3 is considered to be a candidate of Stoner-type band ferromagnetism [14]. The structural optimization of pristine SrRuO3 within LDSA calculations leads to an orthorhombic GdFeO3-type structure with four formula units per cell as shown in Fig. 1. The orthorhombic distortion and the internal positions of atoms are found to be consistent with the

Discussions

In SrRuO3, the Ru ion is in a low-spin state with an almost full t2g majority-spin band and a partially filled t2g minority-spin band. But the strength of exchange split is not large enough to drive the system into a half-metallic ferromagnet. Here we propose the modulation of the Ru t2g bandwidth by doping Sn into the Ru site as a mechanism for the half-metallic ferromagnetic state in SrRu1-xSnxO3. The mixing of B-site atoms with Ru and Sn will drive the localized Ru t2g orbitals near the

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) (No. 2013R1A2A2A01067950).

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