Escalated Phase Separation Driven Enhanced Magnetoresistance in Manganite/Iridate Epitaxial Heterostructures

Abstract

Phase separation in manganites leads to unique magnetic and electronic properties. 50% Ca‐doped LaMnO₃ (LCMO), at the boundary of ferromagnetic (FM) and antiferromagnetic (AFM) states in La_1‐xCa_xMnO₃ (0 ≤ x ≤ 1), is an ideal system to study phase separation behavior. The investigation reveals the effect of a 5d‐metal perovskite SrIrO₃ (SIO) on the phase separation, magnetic, and magnetoresistance (MR) properties of LCMO. Single‐layer and bilayer LCMO films, both appear purely ferromagnetic along the in‐plane (IP) magnetic field direction, but show the tendency of temperature‐dependent ferromagnetic and antiferromagnetic or charge‐ordered (CO) phase separation with the out‐of‐plane (OOP) applied field. The MR, and colossal magnetoresistance (CMR), observed in LCMO/SIO bilayers are two orders and an order of magnitude (in %) larger, respectively than that in the single‐layer film. The coexistence of FM and AFM/CO phases is responsible for the CMR and MR enhancement in the LCMO/SIO bilayer, pointing toward the importance of the phase separation and competition of both the individual materials in enhancing their magnetic and electronic properties.

Introduction

High-performance spintronic devices require tuneable magnetic and magneto-transport properties. Transition metal oxides (TMOs) could be the candidates for this purpose where their electronic and magnetic properties can be regulated by the correlated spin, charge, orbital, and lattice degrees of freedom. Strain engineering, defect engineering and interface engineering are the most used approaches to tune functional properties in TMOs.^[1–6] Among various possible approaches, interface engineering in heterostructures and superlattices is an accessible and efficient way to explore these degrees of freedom.^[7] Moreover, the strong interfacial coupling among TMOs can also give rise to exotic electronic and magnetic properties.^[8,9]

Among 3d oxides, manganites show fascinating phenomena such as double exchange, spin/charge/orbital ordering, Jahn-Teller distortion, and electronic phase separation. The control of these electronic phases is crucial for the application of manganite-based devices. Among 5d oxides, iridates are promising due to their large spin-orbit coupling (SOC) and tuneable Coulombic correlations. Moreover, iridates with high SOC break the orbital degeneracy of t_2g orbital into filled J_eff = 3/2 and half-filled J_eff = ½ levels. Depending on the interaction strength U, it can exhibit an antiferromagnetic insulating state or metallic/semi-metallic state.^[10–12] Therefore, manganite/iridate heterostructures may provide a platform to achieve systematic control of spin/charge/orbital ordering and Coulomb interaction. Recent discoveries of magnetic order and Topological Hall effect (THE) in Mn (3d)/ Ir (5d) heterostructures have demonstrated intriguing interfacial couplings to tune these parameters.^[13–19] The 3d-5d TMO heterostructures have generated huge interest due to the possibility of achieving highly tuneable quantum phenomena and functionalities.^[20–23]

Magnetic and electronic studies of manganite/iridate heterostructures have been limited to manganite compositions exhibiting either ferromagnetic or antiferromagnetic^{[18,19,24,25]} states, but heterostructures with phase competition are rare. When the manganite is in a critical composition regime (e.g., at the boundary between a ferromagnetic metal and an antiferromagnetic insulator), it can exhibit phase coexistence. Moreover, depending on the type of anti-ferromagnetism, the spins on the (111) plane of a cubic manganite perovskite can be in frustrated magnetic states. La_0.5Ca_0.5MnO₃ (LCMO) is such a material, in a ferromagnetic state with T_c = 220 K that transitions into a charge-ordered antiferromagnetic phase at T_CO = 150 K. The complex nature and properties of this compound still drives numerous experimental and theoretical investigations. Colossal magnetoresistance (CMR) is significantly higher than optimally doped manganites due to the inherent phase competition in the system.^[26] In this work, we investigated the effect of SrIrO₃ on the magnetic and electronic properties of LCMO grown on SrTiO₃ (111) substrates. The low temperature magnetic measurements reveal that LCMO in both the single-layer and bilayer systems shows magnetic anisotropy, and the LCMO is ferromagnetic along the in-plane direction with the applied field. However, with the field applied along the out-of-plane direction, multiple phase transitions with phase coexistence are more prevalent in the bilayer as compared to pure LCMO film. The CMR and MR in bilayer have been increased by an order and two orders of magnitude (in %), respectively, by SIO proximity.

Experimental Section

Epitaxial LCMO/SIO and LCMO thin films on SrTiO₃ (STO) substrates with (111) orientation were grown by pulsed laser deposition with a KrF excimer laser (λ = 248 nm). The as-received STO substrates were annealed at 1000 °C for 2 h in air. The base pressure of the chamber was pumped down to the range of ≈1×10⁻⁷ Torr. An optimized growth temperature of 700 °C, an oxygen pressure of 100 mTorr, a frequency of 2 Hz, and a 1.8 J cm⁻² laser fluence were used for the film deposition. The thickness of LCMO lies between 30–36 nm and SIO is ≈5 nm thick. To get uniform laser fluence, an imaged rectangle laser beam was focused onto the target.^[27] After deposition, the samples were cooled at a rate of 10 °C min⁻¹. The structural properties of all the films were characterized using a Panalytical X'pert PRO X-ray diffractometer with Cu-Kα radiation. Magnetic and magneto-transport properties were investigated using the vibrating sample magnetometer (VSM) and DC resistance setup in Physical Property Measurement System (Quantum Design). The magnetoresistance measurements were carried out in a magnetic field up to 4 T using four-probe geometry. The out-of-plane and in-plane M-H measurements were carried out at different temperatures within a field range from −3 to 3 T. The M – T data were collected at 0.05 T while warming the samples from 10 to 350 K under field-cooled conditions.

Results and Discussion

The structural properties of the films were characterized by X-ray diffraction (XRD). Figure 1a shows the specular XRD of LCMO/SIO and LCMO films along the substrate [111] direction. As can be seen from this figure, the LCMO is (111)_pc oriented with respect to the SrTiO₃ (111) substrate and both films were single-phase. The small peak near 33° for single-layer LCMO film corresponds to a minor amount of (121) orientation. The epitaxial nature and crystallinity of both samples were determined by RSM around the (112) reflection of the STO substrate (Figure 1b,c). The narrow FWHM and vertical alignment of reflection suggests good quality strained LCMO epitaxial thin films on top of STO (111) substrates.

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Magnetoresistance (MR) is a well-known phenomenon for LCMO, where the resistance of a material can be changed using a magnetic field.^[28] The MR is defined as: 1 $$$\begin{equation}MR\ = \ \frac{{R\left( H \right) - R\left( 0 \right)}}{{R\left( H \right)}}\end{equation}$$$where R(H) and R(0) are the resistance of a material under a magnetic field and without a field, respectively.

The effect of SrIrO₃ on LCMO has been investigated by measuring transport and magnetic property. As shown in Figure 2a, the zero field R-T measurements of the LCMO/SIO bilayer exhibit an insulator-like behavior from room temperature to 97 K, switch to metallic behavior below 97 K, and then take an upturn at ≈33 K. The insulator-to-metal transition temperature (T_MI) shifted toward higher temperature with a magnetic field and the maximum T_MI of 217 K was observed at 7 T for the bilayer (Figure 2a). The temperature derivative of resistance (dR/dT) at zero-field shows a kink near 137 K, suggesting a charge order (CO) transition. The disappearance of such an anomaly in resistance at higher magnetic fields indicates the CO phase is suppressed by magnetic fields (Figure 2c). Similar measurements for pure LCMO film exhibit insulator-to-metal transition at the same applied field condition but with a much narrow temperature range. The T_MI was 201 K at 0 T and 288 K at 7 T respectively for pure LCMO film (Figure 2d). The T_MI has been defined where the slope of dR/dT curve changes from negative (insulating behavior) to positive (metallic behavior). The calculated CMR using Equation (1) for the single-layer and bilayer is shown in Figure 2g, where we can see that the percentage of CMR in the bilayer film is one order of magnitude larger than that in the single-layer LCMO film. The room temperature resistivity of the bilayer and single layer LCMO films was 6.63 and 5.76 mΩ cm, respectively.

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The magnetic field-dependent MR at different temperatures of both the samples is shown in Figure 2b,e, respectively. The suppressed resistance due to the magnetic field was observed for both samples but the MR was not linear with temperature. Maximum MR was achieved at an intermediate temperature near T_MI. A comparison of observed maximum MR near the T_MI for both samples shows that interfacing SIO with LCMO has increased the MR of the bilayer by two orders of magnitude. Since MR is an electron spin-dependent scattering phenomenon, the enhanced MR in the bilayer SIO/LCMO suggests that SIO has changed the magnetic structure of LCMO.

To understand the interplay between the transport and magnetic properties, we performed the temperature-dependent magnetization measurement for both single-layer LCMO and LCMO/SIO bilayer along the out-of-plane (OOP) and in-plane (IP) directions, as shown in Figure 3a,c. The temperature-dependent magnetization provides the Curie temperature (T_c) ≈260 K and the Neel temperature (T_N) ≈195 K for LCMO/SIO bilayer. Whereas T_c and T_N or T_CO for the single-layer film were found to be ≈260 and 140 K, respectively. The out-of-plane magnetization of both samples exhibits multiple magnetic phase transitions, and such phase transitions are more prominent in the bilayer. The magnetization drop from room temperature to 267 K in the bilayer suggests the negation of ferromagnetically aligned moments, possibly from AFM alignment. The sudden rise in magnetization from 267 K to 196 K indicates the existence of ferromagnetism in that range. Below 196 K, AFM and FM transitions are observed again. We also investigated the in-plane magnetization behavior and found both single-layer and bilayer LCMO films are in the ferromagnetic ground state down to 10 K without the presence of any additional phases. Such an anomaly between temperature-dependent OOP and IP magnetization has not been reported before and can be understood from the possible G-type antiferromagnet formation, where spins on (111) plane are parallel with each other but aligned antiparallell with the spins on ($$\bar{1}\bar{1}\bar{1}$$) planes (Figure 2h). The magnetic behavior has been further investigated by M versus H measurements at 10 K, shown in Figure 3b,d. Both single-layer and bilayer LCMO films exhibit well-defined ferromagnetic hysteresis in the IP measurement, whereas they are weakly ferromagnetic in the OOP direction. In addition, the low magnetic field required to saturate the magnetic moment along the IP direction suggests the IP direction is much easier to magnetize these samples. Therefore, when the magnetic field is applied along the in-plane direction, spins are easier to align and ferromagnetic behavior was observed, whereas mixed magnetic phases were observed due to the incomplete spin alignment for the out-of-plane field direction. It is evident from the comparison of magnetic properties between these two films that interfacing SIO with LCMO could enhance the mixed-phase coexistence in LCMO. Since all our magnetotransport measurements were performed under the out-of-plane field direction, we expect MR to have a contribution from all phases presents.

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CMR in the manganite's is associated with a paramagnetic (PM) to ferromagnetic (FM) transition that results in a unique insulator-metal transition.^[29,30] Since manganite's are strongly correlated electron systems, the ground states of these material systems tend to be intrinsically inhomogeneous. It shows strong tendencies toward phase separation, typically involving FM metallic and antiferromagnetic (AFM) charge and orbital-ordered insulating domains. To date, almost all of the research on CMR of manganite's focuses on the FM regime,^[31,32] and CMR in the AFM phase has been rarely studied. Previously, Gong et al. have shown that CMR values of 10⁸% at 57 K could be achieved in the AFM phase of La_0.5Ca_0.5MnO₃ (LCMO).^[26,33] Such a large CMR has been attributed to the field-induced AFM-FM transition in LCMO. Zarifi et al.^[34,35] has also observed enhanced magnetoresistance when FM and AFM phases coexist with a frozen glassy state at low temperatures. Therefore, our results also indicate that PM-AFM and AFM-FM transitions are as equally important as PM-FM transition for the CMR effect.^[36–38] Orders of magnitude CMR in our bilayer sample also supports the phase transition theory. Therefore, we found that manganite-iridate heterostructures are an ideal platform to explore the effect of such phase coexistence on the CMR.

Previously, the CMR in manganites was well-defined with the phenomenological model in paramagnet/ferromagnet phase-separated systems.^[39] The phenomenological model describes the resistance depending on the temperature-dependent volume fraction of FM (f_FM) and PM (f_PM) phases, where f_FM+f_PM = 1 and the resistance of coexisting phases were considered as,^[40–42]2 $$$\begin{equation} R={f}_{\textit{FM}}.{R}_{\textit{FM}}+{f}_{\textit{PM}}.{R}_{\textit{PM}} \end{equation}$$$

The temperature dependence of volume fraction f_FM obeys the two-energy level Boltzmann distribution as: 3 $$$\begin{equation}{f}_{FM} = \ \frac{1}{{1 + \exp \left( {\frac{{\Delta U}}{{{k}_BT}}} \right)}}\end{equation}$$$4 $$$\begin{equation}\Delta U\ = \ - {U}_0\left( {1 - \frac{T}{{T_{MI}^{\mathrm{*}}}}} \right)\end{equation}$$$where ΔU is the energy difference between FM and PM phases, U₀ is the same energy difference but at 0 K, and T^*_MI is the insulator to metal transition temperature obtained from fitting the phenomenological model.

The resistance in the FM phases is usually described as a sum of temperature-independent residual resistance R₀, resistance due to electron-electron scattering AT² and resistance due to electron-phonon and electron-magnon interaction BT⁵,^[43–46] i.e., R_FM = R₀+AT²+BT⁵. Whereas the insulating behavior in the paramagnetic regime is described based on the thermal activation model, R_PI = D exp(E_a/k_BT), where D is a constant and E_a is the activation energy required to hop the electron from the valence band to the conduction band. Our resistance data for the bilayer also shows an increase below ≈33 K and such an increase in resistance has been observed in various manganites.^[46–48] It is often believed that the Kondo effect has a significant role in low-temperature resistance upturn.^[49] In our investigation, we found the Kondo effect describes the resistance very well at low temperatures. Nevertheless, it is insufficient to describe the resistance over the entire temperature range when the Kondo effect is considered in the phenomenological model. It is believed that electron-electron correlation interaction can also give rise to resistance upturn that includes an additional term ∞ T^1/2 in the resistance expression.^[47,50,51] Therefore, we modified the equation for resistance in the metallic region as R_FM = R₀+AT²+BT⁵ – CT^1/2 and fitted our resistance in the whole temperature range. Here, C is the electron-electron correlation coefficient. Since the bilayer sample has the PM to FM transition along with FM-AFM transition, we adopted a similar approach by considering the AFM phases as the inseparable part of the FM phase, i.e., (f_FM+f_AFM)+f_PM = 1, and fitted our resistance data. The phenomenological model fits reasonably well with our data in the entire temperature range (shown in Figure 2a,d), and the fitting parameters for LCMO bilayer and single-layer films are shown in Tables S1 and S2 (Supporting Information), respectively.

First, we would like to discuss two fit parameters, U₀, which is the energy difference between the FM+AFM phase and the PM phase at 0 K, and E_a which is the activation energy required for electrons to hop in the lattice (shown in Figure 4a,b). The magnetic field promotes PM to FM transition by decreasing the energy difference between these two magnetic phases. The decrease in the energy difference U₀ has been observed in various manganites undergoing PM to FM transition.^{[41,42,46,52]} We also observed similar behavior in the LCMO single-layer film but the field dependence of U₀ was opposite in the case of LCMO/SIO bilayer. The decrease in U₀ with the field for LCMO suggests the magnetic phase transition is PM to FM dominated. Alternatively, the increase in U₀ in the case of the LCMO/SIO bilayer with magnetic field suggests the phase transition is not as simple as a PM to FM transition. An increase in U₀ for the bilayer is an indication that the PM to FM transition is suppressed. The E_a for electron hopping provides further information about the nature of magnetic phases in the LCMO/SIO bilayer. The E_a decreases for both the single-layer and bilayer with an applied magnetic field. A rapid decrease in E_a is an indication of charge delocalization due to the applied magnetic field in the single-layer LCMO film. However, the smaller magnitude (at low field) and insignificant change in E_a for the LCMO/SIO bilayer compared to single-layer LCMO suggests electronic charges are easier to delocalize but the magnetic field has a marginal effect on the delocalization of charge carriers.

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Double exchange (DE) and super exchange (SE) mechanisms are often used to describe ferromagnetism and antiferromagnetism in manganite's, respectively.^[53,54] Applied magnetic field-driven decrease in E_a suggests DE mechanism is dominant in single-layer LCMO. The low activation energy in the bilayer indicates DE mechanism is present, but it's relative insensitivity to the magnetic field suggests a different mechanism in the heterostructure. A localized charge carrier state can be well understood by the SE mechanism that is usually observed in antiferromagnets. Our temperature-dependent magnetic measurement has verified the presence of ferro and antiferromagnetic phase coexistence in the out-of-plane measurements for the bilayer. Moreover, parameters (E_a and U₀) obtained from fitting also suggest the phase transition in the bilayer is not as simple as that in single-layer LCMO film. These results indicate contributions from multiple magnetic phases are simultaneously present in the LCMO/SIO bilayer over a certain range of temperatures. Such a phase coexistence has enhanced the CMR by an order of magnitude and MR by two orders of magnitude in the LCMO/SIO bilayer. In 50% Ca doped LaMnO₃, the presence of Mn³⁺ and Mn⁴⁺ states have equal possibility. Depending on the periodic arrangement of Mn³⁺ and Mn⁴⁺ ionic states in the lattice, both superexchange and double exchange mechanisms can be present in such a compound. When the magnetic field is applied along the out-of-plane direction these exchange interactions can give rise to FM and AFM orders over a short range, as shown in Figure 5.

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We further investigated the fitting parameters (as shown in Figure 6) obtained from R/R(300) versus T plot for both samples to understand the effect of such coexisting magnetic phases on the scattering mechanisms. The residual resistance term (R₀) arises due to the scattering of electrons by defects. The R₀ term for the bilayer was found to be greater than that of the single layer LCMO (Figure 6a), which suggests more defect scattering in the bilayer as compared to the single layer. The electron-electron scattering (A), the scattering due to electron-phonon and electron-magnon interaction (B), and the electron–electron correlation constant (C), are also more prominent in the bilayer as compared to single layer LCMO (Figure 6b–d). Interestingly, once the magnetic field is applied, all these resistance terms decrease rapidly for the bilayer. A comparison of these terms, although the magnetic field suppresses these scattering events for both films, suggests that multiple-phase coexistence provides an additional factor in effectively reducing resistance as a result of the magnetic field. The coefficient of Boltzmann factor, D, is also larger in the bilayer as compared to the single layer and both of them tend to saturate at a high magnetic field (Figure 6e). This kind of behavior indicates that thermally activated transport in the bilayer is more prominent. As a result, the bilayer has an enhanced tendency of behaving as an insulator with decreasing temperature. Whereas the terms of R₀, A, B, and C discussed above suggest that the bilayer becomes more metallic due to reduced scattering with the applied magnetic field. The difference in metallic and insulating behavior is more pronounced in the bilayer. The rapid increase in metal-insulator transition temperature (T^*_MI) in the bilayer also supports this enhanced phenomenon (Figure 6f) and may provide an explanation to the CMR value which is one order of magnitude larger for the bilayer than the single layer LCMO.

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In order to verify our findings, we traced the field and temperature-dependent phase fraction, calculated using the Boltzmann distribution function in Equation 2, shown in Figure 4c,d, respectively. The field-dependent phase coexistence exists over a wider range of temperatures for the bilayer than the single-layer film. It is also interesting to note the existence of temperature-dependent phase fractions at different magnetic fields (shown in Figure 4e,f) for both samples. The change in FM+AFM phase fraction for the bilayer starts at ≈80 K, below which there is no change in phase fraction up to 7 T magnetic field, (Figure 4e). A similar change in phase fraction for the single-layer LCMO was observed beyond 120 K (Figure 4f). When we project these temperatures on the M-T plot (Figure 3a,c) and compare the MR at different temperatures it helps us to figure out the link between phase coexistence and MR. In the M-T plot we can see the AFM phase transition existed between 195 to 76 K for the bilayer and maximum MR in the bilayer was observed at 80 K. Maximum MR in the AFM transition regime suggests the coexistence of FM, AFM, and PM phases are contributing to the enhancement of MR. Whereas, for the single-layer film, the AFM or CO phase existed below 140 K but the maximum MR was found at ≈180 K, which lies in the PM to FM phase transition regime. Therefore, we conclude that MR in the bilayer is governed by the FM, AFM, and PM phase coexistence. Whereas, for the single-layer even though multiple phases coexist, the MR is mostly governed by the FM and PM phases.

In summary, we have investigated the magnetic and electronic properties of manganite/iridate heterostructures grown on STO (111) substrates. The percentage of normal MR and CMR in the LCMO/SIO bilayer was found to be two orders and one order of magnitude, respectively larger than that in the LCMO single-layer film. Magnetic measurements suggest that MR and CMR of the LCMO/SIO bilayer were maximum when FM, PM, and a significant amount of AFM/CO phases coexist. On the other hand, the maximum MR and CMR for single-layer LCMO were observed in the PM to FM transition regime. Our findings indicate that phase separation in manganite's can be enhanced by interfacing LCMO with a heavy metal compound such as SrIrO₃. In addition, the coexistence of the FM+AFM/CO phase with PM phase can amplify the MR and CMR to a large extent. The escalated phase separation and resultant magneto-transport properties in manganite/iridate heterostructures lay a foundation to modulate the functional properties of manganite's for spintronic applications.

Acknowledgements

The work at the University at Buffalo (UB) was partially supported by the U.S. National Science Foundation (ECCS-1902623). P. R. and Q. X. J. acknowledge the CINT Users Program. The work at Los Alamos National Laboratory was supported by the NNSA's Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies (CINT), an Office of Science User Facility operated for the U.S. Department of Energy Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, was managed by Triad National Security, LLC for the U.S. Department of Energy's NNSA, under contract 89233218CNA000001. P. R. and A. C. acknowledge the support from LANL Seaborg Institute.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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