Abstract
A new class of materials termed topological insulators have been intensively investigated due to their unique Dirac surface state carrying dissipationless edge spin currents. Recently, it has been theoretically proposed that the three dimensional analogue of this type of band structure, the Weyl Semimetal phase, is materialized in pyrochlore oxides with strong spinorbit coupling, accompanied by allinallout spin ordering. Here, we report on the fabrication and magnetotransport of Eu_{2}Ir_{2}O_{7} single crystalline thin films. We reveal that one of the two degenerate allinallout domain structures, which are connected by timereversal operation, can be selectively formed by the polarity of the cooling magnetic field. Once formed, the domain is robust against an oppositely polarised magnetic field, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring the predicted novel quantum transport phenomenon at the surfaces/interfaces or magnetic domain walls of pyrochlore iridates.
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Introduction
A zerogap surface state protected by timereversal symmetry is formed at the surface of topological insulators, where electrons with up or down spins counterpropagate^{1,2,3,4,5,6}. When a topological insulator is doped with magnetic elements^{5,6,7}, these surface states may become gapped, resulting in the dissipationless flow of a single chiral edge mode which effectively exhibits the ν = 1 quantum Hall state in the absence of an external magnetic field^{8,9}. A similar zerogap state in three dimensional momentum space, referred to as Weyl Semimetal (WSM) phase^{10,11,12,13}, has recently been predicted to emerge in a certain class of pyrochlore oxide with broken timereversal symmetry^{14,15}. As WSM possesses a topologically nontrivial band structures, provided momentum is conserved, a unique surface state accompanied with edge currents is predicted at interfaces with trivial insulators as in the case of topological insulators^{15,16,17}.
Pyrochlore oxides, formulated as A_{2}B_{2}O_{7}, are comprised of cation sublattices forming a cornersharing tetrahedron network (Fig. 1(a); Ref. 18) where antiferromagnetically coupled spins at the cation sites cannot determine their configurations uniquely due to geometrical frustration. In the case of lanthanides (Ln) iridate pyrochlore (Ln_{2}Ir_{2}O_{7}; A = Ln, B = Ir), DzyaloshinskiMoriya interaction enhanced by strong spinorbit coupling can stabilize an allinallout antiferromagnetic spin order for each cation sublattice^{15,19,20}, in which tetrahedra with all four spins at the vertices pointing inward or outward are alternatingly arranged along <111> direction, as shown in Fig. 1(b). A notable feature of this spin structure is while characterised as antiferromagnetic, two distinct spin arrangements exist, of which are interchangeable by timereversal operation. We hereafter refer to these two spin arrangements as Adomain and Bdomain. This characteristic antiferromagnetic ordering has begun to attract attention as a key ingredient to substantiate WSM and to explore the surface states at the boundaries between A and Bdomains^{21}.
To investigate such intriguing phenomena through electrical measurements, it is necessary to control the magnetic domains and the carrier density of a single crystal. A promising approach to realising this is the growth of thin films, as it may enable the control of artificial magnetic domain boundaries and the exploration of carrier transport at such interfaces, as well as tunability of the Fermi energy at Weyl point by pursuing fieldeffect transistor devices. Nevertheless, obtaining highquality Ln_{2}Ir_{2}O_{7} single crystal has remained challenging even in bulk because of the low reactivity of metallic iridium and the volatility of iridium peroxides^{22}. Here, we report the fabrication of single crystal pyrochlore iridate thin films, Eu_{2}Ir_{2}O_{7} (Ln = Eu) and their magnetotransport properties mediated by allinallout spin ordering. We chose Ln = Eu because of the total magnetic moment J = 0 of Eu^{3+} for among Ln_{2}Ir_{2}O_{7} series, this compound is a sole and ideal platform for studying carrier transport in the background of allinallout spin structure composed of the Ir^{4+} moment in J_{eff} = 1/2 state^{23}. The results indicate that the magnetic domain structure is detected via peculiar asymmetric term in the magnetoresistance (MR) and zerofield offset in Hall resistance. These observations provide controllability of the exotic electronic phase in this compound towards accessing WSM.
Results
Sample fabrication and structural property
In this study (111)oriented Eu_{2}Ir_{2}O_{7} thin films were epitaxially grown on Ystabilized ZrO_{2} (YSZ) (111) single crystal substrates by pulsed laser deposition (see Methods and Supplementary Fig. S1). Transmission electron microscopy (Figs. 1(c) and 1(d)) confirms the formation of a singlecrystal Eu_{2}Ir_{2}O_{7} film free from crystalline domain boundaries. The macroscopic crystal structure was examined by Xray diffraction (XRD), with the high quality of the films supported by the observation of a typical rocking curve of 0.09° for the Eu_{2}Ir_{2}O_{7} (222) peak (Fig. 1(e) and Supplementary Fig. S2). In Fig. 1(f), the reciprocal space mapping is shown, indicating that the lattice of the film is elongated along [111] direction by 0.7% with respect to inplane lattices. The surface morphology was measured by atomic force microscope, which showed root mean square roughness of ~1 nm before and after postgrowth annealing (Supplementary Fig. S3). The film thickness is fixed at about 70 nm.
Longitudinal resistivity
Figure 2(a) displays the temperature dependence of the longitudinal resistivity (ρ_{xx}). For electrical measurements, the sample was defined into a Hallbar geometry (inset of Fig. 2(b)) to reduce mixing of the longitudinal and Hall resistances. A metalinsulator transition (MIT) is observed around transition temperature (T_{M}) of 105 K, which is close to the reported value for bulk (120 K, Ref. 24). The absolute values of ρ_{xx} and the strength of the MIT sensitively depend on the growth conditions (data for typical films are shown in Fig. S4(a)), likely as a result of filmdependent Eu/Ir nonstoichiometry. Since analytical determination on the composition of thin films is challenging, we estimate the composition of films by comparing the resistivity ratio ρ_{xx}(2 K)/ρ_{xx}(300 K) with those of previously reported polycrystalline bulk data^{25}. The result indicates that the films are Irrich by 1–4% (Fig. S4(b)). The reduction of T_{M} compared with the bulk may also be explained by the cation nonstoichiometry. Irrespective of quantitative variations in ρ_{xx}, we qualitatively obtain the same magnetotransport properties discussed later for all the thin films. Hence, we focus on the most conducting film (sample 3 in Supplementary Fig. S4) in the structural and transport data presented. As also noted in Ref. 25 for polycrystalline bulk samples, the temperature dependence of Hall coefficient (R_{H}) estimated by Hall measurement (see Methods) does not show significant anomaly across T_{M}, in contrast to the sharp kink at MIT in ρ_{xx} as shown in Fig. 2(b). Although the origin of this behaviour remains controversial^{26,27}, we simply refer to this transition as the MIT.
Magnetoresistance
Magnetotransport experiments provide versatile information reflecting the orbital motion and spin states of electrons, as exemplified by the spin liquid recently detected by the zerofield anomalous Hall effect in Pr_{2}Ir_{2}O_{7} (Ref. 28). Figure 3(a) shows the outofplane MR at 2 K. The MR for zerofield cooling (left panel) shows a symmetric shape with respect to zero field with a roughly 0.5% decrease in resistance at high magnetic field  a typical response in magnetic materials. The sample cooled with an applied magnetic field, however, unexpectedly exhibits an asymmetric term in MR in addition to the symmetric term (right panel). Although the symmetric term is found to be independent of cooling field (Fig. S5), the sign of the asymmetric term is changed by inverting the polarity of the cooling field. We emphasize that this asymmetric term is an intrinsic property of ρ_{xx} and is not caused by intermixing from Hall resistivity (ρ_{yx}), as evidenced by the fact that the twoterminal MR shows qualitatively the same asymmetric term (Supplementary Fig. S6).
This observation appears peculiar as MR usually does not include an asymmetric term and is not dependent on the cooling field. Although magnetic metals or semiconductors show field asymmetric MR around zero field with a hysteretic behaviour due to a finite coercive field, it is symmetric when spins are flipped at high magnetic field. The MR observed in the data presented here can phenomenologically be understood by taking into account the allinallout spin structure, based on the double exchange model as in the case of perovskite manganites. Here, the angle between the neighbouring spins dramatically affects the hopping probability, giving rise to MR^{29}. For example, if Adomain is stabilized with cooling under +9 T, the spins on basal planes are canted towards the inplane direction with positive magnetic field resulting in an increase in ρ_{xx} (top right inset). Conversely, a negative magnetic field tends to align those spins, giving rise to a lower ρ_{xx} (bottom left inset). The opposite response arises in the case of Bdomain with −9 T field cooling. We have additionally checked the angle dependence of MR, which would reflect the crystallographic symmetry (Supplementary Figs. S9(a) and S9(b)). It turns out that the outofplane anisotropy is rather strong probably due to the structural shape anisotropy of the film and/or lattice distortion by epitaxial strain as is often the case of magnetic thin films (Supplementary Figs. S9(c) and S9(d)). Thus we utilize other methods for further verification as described in the following section.
Temperature and cooling field dependence of the linear component in ρ_{xx}
To further explore this idea, the same measurements are systematically performed as a function of temperature and cooling field (see Methods). Here, we define a parameter α to describe the sign and the magnitude of the asymmetric term as [ρ_{xx}(B) − ρ_{xx}(0)]/ρ_{xx}(0) = (even term) + αB, since the odd terms equal to or higher than the third order are not significant (less than 1% of the linear term by fitting at 2 K). Figure 3(b) shows α as a function of cooling field at 2 K, indicating that α saturates above the cooling field of ±3 T. This bistability suggests that Adomain or Bdomain may be selectively stabilized by applying a cooling field above ±3 T. The temperature dependence of α shown in Fig. 3(c) is also consistent with the above scenario as α disappears above T_{M}. Although small hysteresis is observed around 60 K, the asymmetric term is clearly observed even in such a temperature region (Supplementary Fig. S7), indicating that one magnetic domain is dominant over the other.
Hall measurements
So far we have demonstrated that the MR contains a linear term reflecting the formation of a domain structure depending on the cooling field. We now show that the Hall resistance additionally captures the lattice distortion through the anomalous terms. ρ_{yx} is generally expressed as
with R_{H} the ordinary Hall coefficient, R_{A} the Hall coefficient proportional to the macroscopic magnetization M as a result of spinorbit interaction and ρ^{T} the topological term induced by spin chirality χ = S_{i}·(S_{j} × S_{k}) (Ref. 30). Finite M and χ may remain in the presence of uniaxial lattice distortion induced by epitaxial strain, while they should be cancelled out in the case of ideal allinallout spin structure with cubic symmetry. Figure 4(a) shows the raw data of the Hall measurement at 2 K for the cooling magnetic fields of ±9 T. In addition to the ordinary Blinear component common in both measurements, the Hall resistance clearly exhibits a nonzero value at 0 T, as measured by the difference in ρ_{yx} between the opposite cooling magnetic fields (Δρ_{yx}). The overall linear term is reasonably assigned to the ordinary Hall effect (the first term in Eq. (1)), since no anomaly is present across T_{M} as shown in Fig. 2(b). We can therefore ascribe Δρ_{yx} to the sum of the second and third terms in Eq. (1) as Δρ_{yx} disappears above T_{M} (Fig. 4(b)).
Discussion
As opposed to the standard fieldsymmetric MR typically observed in prevalent conductors, the magnetoresponse in the presence of allinallout spin ordering is described in terms of a third rank tensor. Therefore, odd terms should characteristically be observed in response functions due to timereversal symmetry breaking as proposed by Arima^{31}, which is a clear fingerprint of the allinallout spin ordering. Here, conduction electrons must be sensitive to the local spin structure of the compound owing to spinorbit coupling, known as the Kondo lattice model. Hence, our observation substantiates that MR reflects not only the magnetic spin structure but also the magnetic domain structure. From these considerations, the magnetic domain structure in Eu_{2}Ir_{2}O_{7} is robust against a magnetic field of at least 9 T below T_{M}. This is in marked contrast to the case of Nd_{2}Ir_{2}O_{7} where Adomain and Bdomain are thought to be switchable through the Ising character of Nd ions by sweeping magnetic field^{32}. Therefore, it is plausible to attribute the robust domain structure to the J = 0 character of Eu^{3+}.
The anomalies found in ρ_{yx} are consistent with the interpretation of MR data in which conducting carrier reflects the magnetic domain structure in another way and additionally indicate that the lattice distortion actually induces macroscopic M and χ. An attempt to directly measure M by magnetrometer is not successful since M of this compound is still too small to compare with the signal from the substrate, yet ρ_{yx} is sufficiently sensitive to detect it without involving the substrate. In addition to Δρ_{yx}, there exists a lowfield nonlinear component (ρ_{NL}), which is deduced by subtracting the linear term and the zerofield offset (Δρ_{yx}) as shown in Fig. S8. Remarkably, ρ_{NL} shows sign inversion between the two domains. If we assume that M is proportional to B as in the case of the bulk^{25}, ρ_{NL} may be therefore attributed to ρ^{T} possibly originating from spins at the surface or the interface. Confirming thickness dependence of magnetotransport properties would be useful comparison to clarify this issue. However, clear dependence is not obtained probably due to slight fluctuation of the stoichiometry from sample to sample as mentioned above, while the qualitative features of MR and Hall effect are maintained. Although this sign inversion in ρ_{NL} is an intriguing character of this system, the details remain to be investigated including the possibility of band crossing as in the case of EuTiO_{3} (Ref. 33) or SrRuO_{3} (Ref. 34).
In conclusion, we have fabricated single crystal Eu_{2}Ir_{2}O_{7} thin films via solid phase epitaxy and systematically explored the magnetotransport properties originating from the allinallout spin order. The MR contains a linear field dependent term while the Hall resistance has zero field offset and anomalous bending, both of which inherently depend on the type of domain formed. Our findings indicate that the allinallout spin structure is controllable by the polarity of the cooling field and once formed is robust below T_{M}. The ability to realize this spin structure in thin film form will become the cornerstone to realizing WSM phases via tailored pyrochlore iridate heterostructures and/or utilizing electrostatic gating.
Methods
The Eu_{2}Ir_{2}O_{7} (111) films were prepared on commercial YSZ (111) single crystal substrates by pulsed laser deposition. Phasemixed ceramic targets fabricated by hotpress method at 950°C under 25 MPa pressure were used with a prescribed ratio of Eu/Ir = 1/3. The films were deposited at a substrate temperature of 500°C in an atmosphere of 100 mTorr Ar gas containing 1% O_{2}. A KrF eximer laser (λ = 248 nm) was used for ablating the target, with a fluence and frequency of 6 J/cm^{2} and 10 Hz. The films were in an amorphous phase after the deposition, with the pure Eu_{2}Ir_{2}O_{7} phase appearing after annealing in an electrical muffle furnace at 1000°C for 1.5 hours in air.
The Hall bar structure was processed by conventional photolithography and Ar ion milling. Ohmic Ni (10 nm)/Au (50 nm) contacts were deposited by an electronbeam evaporator. Magnetotransport measurements were performed in a liquid He cryostat equipped with a 9 T superconducting magnet (PPMS, Quantum Design Co.). The ordinary Hall coefficient (R_{H}) is calculated by linear term in Hall resistance above ±5 T region. Temperature dependence measurements for α (Fig. 3(c)) were carried out by the following sequence. Magnetic field was applied at 150 K along [111] direction of YSZ substrate. Then the samples were cooled from 150 K down to 2 K under the fixed magnetic field. At 2 K, magnetotransport was measured as follows. In the case of positive (negative) field cooling, magnetic field was first set to −9 T (+9 T), then returned to +9 T (−9 T) and finally back to the initial cooling field. For example, in the case of +0.5 T cooling, magnetic field was swept as +0.5 T →−9 T → +9 T →+0.5 T. After this measurement, the sample was warmed to the next measurement temperature with +0.5 T of magnetic field.
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Acknowledgements
We thank J. Fujioka, K. Ueda, Y. Tokura, N. Nagaosa and Y. Motome for helpful discussions. We thank J. Falson for editing of the manuscript. This work was partly supported by GrantinAids for Scientific Research (S) No. 24226002 and No. 24224010, for JSPS Fellows No. 26·10112 (TCF), for Challenging Exploratory Research No. 26610098 (MU) from MEXT, Japan and by “Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST)” Program from the Japan Society for the Promotion of Science (JSPS) initiated by the Council for Science and Technology Policy.
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T.C.F. performed the sample fabrication, measurements and analysis. Y.K. assisted with the sample fabrication, measurements and analysis. M.U., A.T. and T.A. assisted with the planning and analysis. M.K. directed the project.
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Fujita, T., Kozuka, Y., Uchida, M. et al. Oddparity magnetoresistance in pyrochlore iridate thin films with broken timereversal symmetry. Sci Rep 5, 9711 (2015). https://doi.org/10.1038/srep09711
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DOI: https://doi.org/10.1038/srep09711
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