Robust pinning of magnetic moments in pyrochlore iridates

TL;DR

This paper investigates a unique magnetic phenomenon in pyrochlore iridates, where non-magnetic Ir defects induce ferromagnetic droplets and a hysteresis loop shift, explained by an exchange-bias-like effect involving complex magnetic interactions.

Contribution

It introduces a model explaining the cooling-field induced hysteresis shift in pyrochlore iridates with Ir defects, highlighting the role of exchange interactions and magnetic octupolar order.

Findings

Hysteresis loop shifts are observed in pyrochlore iridates with Ir defects.

The shift is explained by ferromagnetic droplets pinned by the AIAO antiferromagnetic background.

The magnetic pinning remains stable up to 35 T, indicating robust magnetic order.

Abstract

Pyrochlore iridates A2Ir2O7 (A = rare earth elements, Y or Bi) hold great promise for realizing novel electronic and magnetic states owing to the interplay of spin-orbit coupling, electron correlation and geometrical frustration. A prominent example is the formation of all-in/all-out (AIAO)antiferromagnetic order in the Ir4+ sublattice that comprises of corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon, namely a cooling-field induced shift of magnetic hysteresis loop along magnetization axis, and its possible origin in pyrochlore iridates with non-magnetic Ir defects (e.g. Ir3+). In a simple model, we attribute the magnetic hysteresis loop to the formation of ferromagnetic droplets in the AIAO antiferromagnetic background. The weak ferromagnetism originates from canted antiferromagnetic order of the Ir4+ moments surrounding each non-magnetic Ir defect. The shift of hysteresis loop can be understood quantitatively based on an exchange-bias like effect in which the moments at the shell of the FM droplets are pinned by the AIAO AFM background via mainly the Heisenberg (J) and Dzyaloshinsky-Moriya (D) interactions. The magnetic pinning is stable and robust against the sweeping cycle and sweeping field up to 35 T, which is possibly related to the magnetic octupolar nature of the AIAO order.

Figures

Equations

$$H=\sum _{<i,j>}\left[J{\vec{S}}_i\cdot {\vec{S}}_j+{\vec{D}}_{ij}\cdot {\vec{S}}_i\times {\vec{S}}_j+{\vec{S}}_i\Gamma {\vec{S}}_j\right].$$

$$E=\frac{3}{2}J{\sin }^2\theta -\frac{\sqrt{3}}{2}D\sin (2\theta +{\theta }_1)-Jf(H_{\mathrm{cool}})\cos (\theta -{\theta }_0).$$

$$|{\vec{S}}_0|:=f(H)=c_1\tanh (\frac{H}{H_0})+c_{2}H,$$

$$M_{\mathrm{sh}}\propto \frac{\sin {\theta }_p{e}^{-\beta E_p}+\sin {\theta }_{ap}{e}^{-\beta E_{ap}}}{e^{-\beta E_p}+e^{-\beta E_{ap}}},$$

Full text

I Introduction

Spin-orbit coupling (SOC), a relativistic interaction between the spin and the orbit of an electron, is a key ingredient for topologically non-trivial states in condensed matter.  TIRev1 ; TIRev2 Adding strong electron interaction into a spin-orbit coupled system is believed to foster novel magnetic and topological phases, Rev1 ; Rev2 ; Rev3 such as chiral spin liquid, Weyl semimetals, Wan1 topological Mott insulators Pesin1 and topological crystalline insulators. Fiete1 Among various transition-metal oxides, iridates represent such a unique system with electron correlation and SOC of comparable energy scales. Cao1 ; ArimaScience ; Jackeli1 ; Wan1 ; Pesin1 ; Cao2 ; Cao3 ; SOC1 In particular, the pyrochlore compounds A2Ir2O7 (A = rare earth elements, Bi or Y) are theoretically predicted to host topological Weyl semimetal (WSM) phases that are characterized by a linear energy dispersion in bulk and open Fermi arcs on surface. Wan1 While photoemission studies of these electronic characteristics remain elusive, Nakayama optical conductivity measurements have revealed signatures of WSMs in Rh-doped Nd2Ir2O7 RhDoped and undoped Eu2Ir2O7. Optical1

The Weyl semimetal states in pyrochlore iridates are predicted to be accompanied with a non-collinear antiferromagnetic (AFM) order in the Ir4+ sublattice that comprises of corner-sharing tetrahedra. Wan1 ; YBKim2 The four Ir4+ moments at the vertices of each tetrahedron point either into or outward from its center. This peculiar all-in/all-out (AIAO) magnetic order results from the competition between Heisenberg interaction ($J$), Dzyaloshinsky-Moriya (DM) interaction ($D$), and single-ion anisotropy: with only Heisenberg antiferromagnetic interaction, the Ir4+ moments are geometrically frustrated; the magnetic frustration is removed by the DM interaction and the single-ion anisotropy that are enhanced by SOC. Wan1 ; YBKim2 The AIAO AFM order of Ir moments has been experimentally evidenced by a variety of magnetic probe techniques, including resonant X-ray scattering, Xray1 ; Xray2 ; SmIr muon spin relaxation studies, muSR1 ; muSR2 and neutron diffraction. NdIr

A prominent feature of the AIAO order is that the four moments in a tetrahedron can be treated as a magnetic octupole whose susceptibility is a third-rank tensor. Arima ; NatPhysFu Besides being a weak coupling to magnetic field, the order is unique in the sense that an out-of-plane magnetization can be induced by an in-plane magnetic field. NatPhysFu Another notable characteristic is the existence of two interchangeable magnetic configurations that are linked by a time-reversal operation. PRX1 Nontrivial metallic interface states are predicted to exist on the wall of these two time-reversal-related magnetic domains. PRX1 Magneto-transport studies have provided strong evidences for the metallic domain walls in Nd2Ir2O7. Nd1 ; Nd2 Such domain walls of low sheet resistance have been visualized in spatially-resolved microwave impedance microscopy measurements. Science2015

In spite of remarkable electronic and magnetic properties, the synthesis of phase-pure, stoichiometric pyrochlore iridates is often challenging. Indeed, minor impurity phases such as A2O3, Ir and IrO2 were often detected in polycrystalline samples that were grown by conventional solid-state reaction method. exp_method1 ; muSR1 ; Iridate227Zhu ; NdIr ; JPCM ; NdIr2 ; NdIr3 ; anisotropy Those impurities are either paramagnetic or diamagnetic and hence do not contribute significantly to the observed magnetic phenomena; nevertheless, their presence indicates possible non-stoichiometry of the samples as a result of incomplete reaction in the synthesis process. LnIr Non-stoichiometry has also been reported in single crystalscrystal1 ; Xray2 and in epitaxial thin films. thinfilm1 A remarkable consequence of the non-stoichiometry is the deviation of oxidation states of the elements from their nominal values, Iridate227Zhu ; IrRu ; YIr ; thinfilm1 which could have important impact on the magnetic properties of the compounds. In particular, the Ir4+ (5d5) has an electronic configuration of $J_{\rm eff}=1/2$ with a magnetic moment of 1 $\mu_{B}$ in the atomic limit. Cao1 Other oxidation states such as Ir3+ (5d6) is nonmagnetic because the $t_{2g}$ states (or $J_{\rm eff}=1/2$ and 3/2) are completely occupied. In this work, we report on an unusual phenomenon, namely a cooling-field induced shift of magnetic hysteresis loops along the magnetization axis, which may be related to the existence of nonmagnetic defects (e.g. Ir3+) in the networks of Ir4+ tetrahedra. The shift of magnetic hysteresis loop is robust against the sweeping cycle and the sweeping field up to 35 T. We propose a simple exchange-bias like model in the framework of the AIAO magnetic order to understand the origin of this unusual magnetic behavior. We attribute the robust exchange bias in the magnetization loop to the magnetic octupolar nature of the AIAO order.

II EXPERIMENTAL METHODS

Due to the difficulty in synthesizing single crystal Lu2Ir2O7, we focused on polycrystalline sample which was prepared by a standard solid-state reaction method, similar to our earlier reports. Iridate227Zhu ; IridateDPZhu ; IridateDPCuIr High purity Lu2O3 (99.99%) and IrO2 (99.99%) powder was mixed at a stoichiometric ratio and was heated in air at 900 *∘*C and then 1000 *∘*C for about four days, with intermediate grinding. The growth condition of Y2Ir2O7 sample was reported earlier. Iridate227Zhu X-ray powder diffraction (XRD) measurements were performed using a PANalytical EMPYREAN diffractometer (Cu K $\alpha$ radiation). Both samples are composed of a major pyrochlore phase along with some minor impurities of IrO2 and Lu2O3 (Y2O3) due to incomplete reaction and Ir metal due to decomposition of IrO2 [Supplemental Material Figure S1 and Ref. Iridate227Zhu, ]. Low field magnetization measurements were carried out in Quantum Design Magnetic Property Measurement Systems (MPMS). High field measurement up to 35 T was carried out in the National High Magnetic Field Laboratory in Tallahassee. X-ray photoelectron spectroscopy (XPS) measurements were conducted in a PHI Versa Probe II system. The XPS spectra were fitted using a standard software package CasaXPS provided by Casa Software, Ltd.

III RESULTS AND DISCUSSION

Temperature dependent dc magnetic susceptibility of Lu2Ir2O7 suggests a magnetic transition at TN $\sim$ 135 K, below which the zero-field cooled (ZFC) and field-cooled (FC) susceptibilities exhibit a clear difference [Fig. 1(a)]. A similar behavior was reported in other pyrochlore iridates of intermediate/strong electron interactions (e.g. Y2Ir2O7 exp_method1 ; muSR1 ; Iridate227Zhu and Eu2Ir2O7 Xray1 ) and the magnetic transition was attributed to the onset of long-range AIAO magnetic order. muSR2 ; Xray1 ; Nakatsuji ; anisotropy ; Werner Magnetic-field dependence of magnetization was measured at 5 K, after the sample was cooled down in zero field (i.e. ZFC), in a magnetic field of +1 k Oe and -1 k Oe (i.e. FC). A small magnetic hysteresis loop was observed in all three cases [Fig. 1(b)], indicating a weak ferromagnetism coexisting with the AFM background. Unlike the ZFC loop, the FC loops show a shift along the magnetization axis with its sign being determined by the polarity of the magnetic field, i.e. +1 kOe gives rise to a positive shift while -1 kOe leads to a negative shift. A cooling-field induced shift of magnetic hysteresis loop was observed earlier in polycrystalline Y2Ir2O7 Iridate227Zhu and Sm2Ir2O7 JPCM as well. It is also noted that the Hall resistivity versus magnetic field of single crystalline Eu2Ir2O7 thin films also exhibits a vertical shift after the films are cooled down in a magnetic field. SciRepJpn Since the anomalous component of the Hall resistivity in a magnetic material is generally proportional to its magnetization, a shift of magnetic hysteresis loop along the magnetization axis is anticipated in Eu2Ir2O7. Therefore, we argue that such a cooling-field induced shift of hysteresis loop may be a common phenomenon in magnetic pyrochlore iridates.

To understand the origin of the shift, we measured magnetization versus magnetic field at different temperatures (10 - 170 K) after the sample was cooled down in a magnetic field of 1 kOe. The shift of hysteresis loop in its magnetization is quantitatively defined as $M_{\rm sh}\equiv\frac{M_{1}+M_{2}}{2}$, where $M_{1}$ and $M_{2}$ are the two magnetization values at zero field in a hysteresis loop [inset of Fig. 1(c)]. The $M_{\rm sh}$ decreases monotonically upon warming and reaches zero around $T_{N}$ [Fig. 1(c)]. To illustrate the relation between $M_{\rm sh}$ and the magnetic moments that are pinned as a result of field cooling, we carried out another temperature dependence of magnetization measurement in the following protocol: the sample was first cooled down in a field of 1 kOe; then the magnetic field was set to zero using an oscillating mode in the MPMS; finally the magnetization was measured in zero magnetic field upon warming. In such a process, the measured magnetization reveals the magnetic moments that are pinned after the cooling process. As shown in Fig. 1(c), the magnetization agrees well with $M_{\rm sh}$ at all temperatures, indicating that the vertical shift is clearly due to the pinned magnetic moments. Another parameter $M_{\rm h}\equiv\frac{M_{1}-M_{2}}{2}$ is defined to characterize the weak ferromagnetic component. As shown in Fig. 1(d), the $M_{\rm h}$ has a non-monotonic dependence on the temperature: it increases first and then decreases, yielding a peak around $T_{N}$. Magnetic hysteresis loops were also recorded after zero field cooling and the corresponding $M_{\rm h}$ shows nearly the same trend as the FC data. It is worth noting that the $M_{\rm h}$ shows some variations among different polycrystalline powder from the same batch, which likely results from stoichiometric inhomogeneity. However, the fact that both the $M_{\rm sh}$ and $M_{\rm h}$ show strong temperature dependence around $T_{N}$ confirms that the vertical shift of hysteresis loops is an intrinsic feature of the pyrochlore phase, in contrast to the paramagnetic or diamagnetic nature of the impurity phases (i.e. Lu2O3, IrO2, and Ir). impurity1 ; impurity2 ; impurity3 We note that the enhancement of magnetic hysteresis loop (or coercive field) around $T_{N}$ was reported in exchange-bias systems in which FM domain/layer is pinned by AFM. Leighton ; Scholten The maximum $M_{\rm h}$ is hence an indication of interfacial coupling between the FM droplets and the AFM background which will be discussed in details below.

The weak ferromagnetic component observed earlier in Y2Ir2O7 was attributed to the presence of Ir5+ as a result of non-stoichiometry. Iridate227Zhu We carried out X-ray photoelectron spectroscopy to study the oxidation state of Ir in Lu2Ir2O7. As shown in Supplemental Material Figure S2, the iridium spectrum has three components which are attributed to: Ir4+, Ir3+, and Ir0. The Ir0 is consistent with the presence of iridium metal impurity, resulting from the decomposition of some IrO2 in the solid state reaction process. The Ir3+ which may arise from non-stoichiometry of the sample has an electronic configuration of 5d6. The six 5d electrons completely fill the $t_{2g}$ states (or the $J_{\rm eff}=1/2$ and $3/2$ states), leading to a nonmagnetic state. We show below that the presence of non-magnetic defects in the network of Ir tetrahedra gives rise to ferromagnetic (FM) droplets and the exchange coupling between the FM droplets and the AFM background yields a vertical shift of magnetic hysteresis loop.

The Hamiltonian for the magnetic moments in a standard magnetic system with strong SOC has the following form,

[TABLE]

The first term with $J>0$ represents the antiferromagnetic Heisenberg exchange coupling between the nearest magnetic moments ($\vec{S}_{i}$), while the second term is the antisymmetric DM interaction. With the convention for the site indices in Ref. DM_AIAO, , the direction of $\vec{D}$ determines whether the interaction is “direct” ($D>0$) or “indirect” ($D<0$). The third term stands for the anisotropic coupling with a symmetric traceless matrix $\Gamma$. In pyrochlore iridates, the DM interaction is comparable with $J$, while the anisotropic term is much smaller Pesin1 and is hence neglected in our calculations. The angular momentum $\vec{S}$ is treated classically.

In a perfect pyrochlore iridate with intermediate or strong electron interactions (e.g. stoichiometric Y2Ir2O7), the direct DM interaction stablizes a long-range antiferromagnetic order with an AIAO magnetic configuration. DM_AIAO In the presence of a nonmagnetic defect, the six Ir4+ moments surrounding the defect can be considered as an FM (or canted AFM) droplet [Fig. 2(a)]. Each droplet shall correspond to a magnetic moment of 1 $\mu_{\rm B}$ if the remaining spins are not re-oriented. However, the presence of non-magnetic defect influences the orientations of the remaining spins. Namely, a coplanar configuration (i.e. $\theta=0$) is expected in the limit $D/J\rightarrow 0$, whereas a configuration with $\sin\theta>\frac{1}{3}$ shall occur in the large $D/J$ limit (details will be elaborated in next paragraph). In a field cooling process, the Ir4+ moments in a FM droplet [denoted by red arrows in Fig. 2(b)] are aligned by the cooling-field $H_{\rm cool}$ via Zeeman coupling. The remaining Ir4+ moments form either AIAO or its time-reversal counterpart AOAI order [denoted by green arrows in Fig. 2(b)] and their orientations are pre-determined by the moments in the FM droplet. On the other hand, the Heisenberg and the DM interactions between the Ir4+ moments at the interface of the FM droplets and the AFM background give rise to an additional exchange field that acts on the FM moments. This exchange field pins the FM moments, giving rise to an exchange-bias like behavior.

To quantitatively study the dependence of $M_{\rm sh}$ on cooling-field $H_{\rm cool}$, we model the magnetic energy $E$ of an FM droplet [i.e. the top tetrahedron in Fig. 2(b)] as a function of canting angle $\theta$ (i.e. the angle made by the spin $\vec{S}$ with the bottom face of the same tetrahedron). In the absence of non-magnetic defect, the three canted moments at the bottom face collectively balance the moment at the top vertex, so the canting angle has $\sin\theta=1/3$. Because of the three-fold rotational symmetry, it suffices to consider the lower-left moment $\vec{S}$ as a representative for the three moments. Based on Eq. 1, we have obtained the following expression in which all terms irrelevant to dynamics are neglected,

[TABLE]

The first term proportional to $J$ results from rewriting the sum of inner products $J\sum\vec{S}_{i}\cdot\vec{S}_{j}=(1/2)J|\sum\vec{S}|^{2}$. This term favours a coplannar configuration corresponding to $\sin\theta=0$. The second term along with the constant angle $\sin\theta_{1}=1/\sqrt{3}$ results from employing the explicit expressions for the normalized $\vec{D}$ vectors specified in Ref. DM_AIAO, . This term alone gives rise to a canting angle of $\sin\theta\approx 0.46$ in the FM droplet, larger than the canting angle with $\sin\theta_{0}=1/3$ in a perfect tetrahedron. The last term stands for the antiferromagnetic coupling between $\vec{S}$ and the rest three moments (denoted by green arrows) in the lower-left tetrahedron. We denote the sum of those three moments by $\vec{S}_{0}$. To simplify the calculation, we adopt the mean-field picture: given the direction of $H_{\rm cool}$ shown in Fig. 2 (b), the “averaged” magnitude $|S_{0}|$ monotonically increases from zero to a saturated value as $H_{\rm cool}$ is increased from zero (ZFC) to some critical value $H_{0}$. Thus, it suffices to consider the effect of cooling-field through the fitting functional, SciRepJpn ; PRB05

[TABLE]

where $c_{1}$ and $c_{2}$ are positive parameters to be determined by matching with measurement data. Qualitatively, $H_{0}$ is the critical field beyond which the FM droplets are aligned and the “averaged” $S_{0}$ stops increasing rapidly, while its value is to be determined by data, too. In the absence of $H_{\rm cool}$, i.e. ZFC, the third term in Eq. 2 vanishes and $E_{\rm tot}$ has two identical local minima corresponding to two $\theta$ values that are separate by $\pi$ [dashed curve in Fig. 2(c)]. This can be seen from the symmetry of the first two terms in Eq. 2 under $\theta\rightarrow\theta+\pi$. As such, the moments in the FM droplet have an equal chance to point at one direction or at its opposite, and such situation is independent of the ratio $D/J$. On the other hand, in the presence of $H_{\rm cool}$ (i.e. FC), the energy has two different local minima ($E_{p}$ and $E_{ap}$) that are separated by an energy barrier (solid curve in Fig. 2(c)). The difference between $E_{p}$ and $E_{ap}$ is significant in determining the magnetization in the sweeping process. Following the discussion in Ref. PRB05, , when the sweeping field changes from a very large and positive (negative) value to zero, the FM droplets initially occupying the state of $E_{p}$ ($E_{ap}$) will partially switch to the state of $E_{ap}$ ($E_{p}$) to reach an equilibrium state. Although the details depend on the tunneling mechanism and the barrier between two states, the average of the two intersections, $M_{1}$ and $M_{2}$, from the two paths of varying the $H$ only depends on the energy difference between $E_{p}$ and $E_{ap}$. Namely, we can write  PRB05

[TABLE]

with $\theta_{p(ap)}$ representing the canting angle corresponding to the lower (higher) energy minimum. $\beta=k_{B}T_{0}$ is associated with the measurement temperature $T_{0}$.

We computed $M_{\rm sh}$ vs $H_{\rm cool}$ curves with different parameters in Supplemental Material Figure S3. While the $M_{\rm sh}$ is dependent on other parameters (mostly on $k_{\rm B}T_{0}/J$), it is nearly insensitive to the $D$ value. This behavior can be understood based on Eq. 2. In brief, the magnetic energy of the FM droplet always has a pair of degenerate local minima and their canting angles differ by 180 degrees when the third term is neglected (i.e. in the case of ZFC). Thus, different values in $D$ only shift the canting angles corresponding to the local minima but do not affect the energy difference between the two minima when $H_{\rm cool}$ is present. Figure 3 shows a good agreement between the measured $M_{\rm sh}$ and the computed data with parameters of $c_{1}=$ 0.3, $H_{0}=$ 9 kOe (or 0.9 T), $c_{2}=$ 0.025, $k_{\rm B}T_{0}$ = 0.75 $J$ and $D/J$=0.2 (the least sensitive parameter). The coefficient $c_{1}$ = 0.3 reflects roughly the polycrystalline nature of the sample in which the crystals orient randomly with respect to $H_{\rm cool}$. The $H_{0}=0.9$ T is close to the critical field ($\approx$ 1 T) observed in the measurement of linear magnetoresistance coefficient $\alpha$ for Eu2Ir2O7 where $\alpha$ reaches $tanh(1)\approx$0.76 of its saturated value at the field of 1 T. SciRepJpn The $c_{2}$ = 0.025 is indeed small so $M_{\rm sh}$ may saturate at a large cooling-field. The ratio of temperature to antiferromagnetic coupling $k_{\rm B}T_{0}$ = 0.75 $J$ corresponds to $J\approx$ 1.1 meV at a measurement temperature of 10 K. This value is lower than the exchange coupling in Sm2Ir2O7 ( 27 meV) that was determined by resonant magnetic x-ray scattering. SmIr . The $J$ and $D$ values in pyrochlore iridates, according to Ref. DM_ABINITO, , are very sensitive to the Ir-O-Ir bonding angle, so the magnetic interaction values vary among the pyrochlore iridate family. It is worth mentioning that while magnetic domain walls are expected in pyrochlore iridates, the $M_{\rm sh}$ is unlikely to be contributed from defect pinning of the domain walls. Indeed, increasing cooling field tends to reduce the number of domains Science2015 and hence decreases $M_{\rm sh}$ if the domain wall pinning picture holds, which contradicts with the monotonic increase of $M_{\rm sh}$ shown in Figure 3.

Lastly, we note that the magnetic hysteresis loops in a conventional exchange-bias system are horizontally shifted. EB1 ; EB2 The vertical shift observed here suggests that the magnetic pinning is very robust in the pyrochlore iridate system. We demonstrate next the robustness of magnetic pinning against sweeping cycle and sweeping field. Figure 4(a) presents four magnetic hysteresis loops that were measured in a consecutive manner, after the sample was cooled down to 10 K in a magnetic field of 1 kOe. The M-H loops overlap with each other and no training effect is observed, indicating the pinning of magnetic moment is indeed stable. We further carried out a high field measurement on a Y2Ir2O7 sample up to 35 T. As shown in Figure 4(b), the vertical shift of magnetic hysteresis loop persists even after the sample was subject to a field swept from 35 to -35 T. The robustness of magnetic pinning may be related to the octupolar nature of the AIAO order. Indeed, both the dipole and quadrapole terms vanish in an AIAO state and the resulted octupolar term has a susceptibility that is described by a third-rank tensor. Arima ; NatPhysFu The third-order tensor coupling with the magnetic field is presumably weaker than the exchange couplings $J$ and $D$ between dipole moments, which is possibly why the pinned moments are not switchable by an external magnetic field.

IV CONCLUSIONS

In summary, we report on a cooling-field induced vertical shift of magnetic hysteresis loop and its possible origin in pyrochlore iridates that contain non-magnetic Ir defects. In the presence of nonmagnetic defects, FM droplets are formed out of the background AIAO/AOAI spin ordering. The vertical shift can be understood quantitatively based on an exchange-bias like model in which the moments at the shell of the FM droplets are pinned by the AIAO AFM background. The exchange bias in the magnetization loop is stable and robust against sweeping cycle and sweeping field, which is possibly associated with the magnetic octupolar nature of the AIAO order.

ACKNOWLEDGMENTS

We thank Professors L. Hozoi, W. Tong, and G. Chen for helpful discussions, and Professors L. Pi and J. Long for experimental assistance. S.X.Z. acknowledges Indiana University (IU) College of Arts and Sciences for startup support. C.K.L. was supported by the Taiwan Ministry of Science and Technology through Grant No. 03-2112-M-003-012-MY3. H.D.Z. thanks the support from NSF-DMR through Award DMR-1350002. The work at NHMFL is supported by NSF-DMR-1157490 and the State of Florida. We are grateful to the Indiana University Molecular Structure Center for the use of XRD facility (NSF CHE-1048613) and the Nanoscale Characterization Facility for XPS instrument (NSF DMR 1126394). We also thank the High Magnetic Field Laboratory, Chinese Academy of Sciences for partial use of MPMS.