Effect of chemical pressure on the electronic phase transition in Ca 1 − x Sr x Mn 7 O 12 films

Effect of chemical pressure on the electronic phase transition in Ca1−xSrxMn7O12 films A. Huon,1 D. Lee,2 A. Herklotz,2 M. R. Fitzsimmons,3,4 H. N. Lee,2 and S. J. May1,a 1Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 4Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

We demonstrate how chemical pressure affects the structural and electronic phase transitions of the quadruple perovskite CaMn 7 O 12 by Sr doping, a compound that exhibits a charge-ordering transition above room temperature making it a candidate for oxide electronics.We have synthesized Ca 1x Sr x Mn 7 O 12 (0 ≤ x ≤ 0.6) thin films by oxide molecular beam epitaxy on (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates.The substitution of Sr for Ca results in a linear expansion of the lattice, as revealed by X-ray diffraction.Temperature-dependent resistivity and X-ray diffraction measurements are used to demonstrate that the coupled charge-ordering and structural phase transitions can be tuned with Sr doping.An increase in Sr concentration acts to decrease the phase transition temperature (T*) from 426 K at x = 0 to 385 K at x = 0.6.The presence of a tunable electronic phase transition, above room temperature, points to the potential applicability of Ca 1x Sr x Mn 7 O 12 in sensors or oxide electronics, for example, via charge doping.© 2017 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4994089]There is growing interest in quadruple perovskites, (AA 3 )B 4 O 12 , 1-3 due to the wide range of physical properties found in this material class.3][14] The variety of functionalities present in (AA 3 )B 4 O 12 compounds is enabled by their structural framework that can accommodate transition metals on both the A and B positions allowing both A-site and B-site ordering. 3,15Similar to ABO 3 perovskites, the quadruple perovskites accommodate substitution on the cation sites to form solid solutions, allowing for a large variety of material systems, with unique and customizable properties for various applications.
Of the AMn 7 O 12 compounds, CaMn 7 O 12 has attracted considerable attention surrounding the four distinct phase transitions it exhibits.At T * = 440 K, a simultaneous charge-ordering 13 and structural phase transition occurs from the distorted cubic (Im 3) to rhombohedral (R 3) structure. 1,13The charge-ordering occurs on the B-site Mn cations with a 3:1 ratio of Mn 3+ and Mn 4+ , respectively, as evidenced by local changes in the Mn-O bond lengths determined from powder diffraction measurements. 13,16An orbital-ordering transition is observed at ∼250 K (T OO ) through scattering and Raman spectroscopy measurements. 17,18Below 90 K (T N1 ), non-collinear helical magnetic and ferroelectric ordering is observed, 12,14,19 with a second helical magnetic transition at ∼45 K (T N2 ). 1,14,16,20,21Author to whom correspondence should be addressed: smay@coe.drexel.edu2166-532X/2017/5(9)/096105/7 Similar transitions are also observed in bulk SrMn 7 O 12 , with little change in the magnetic transition temperatures but a decrease of ∼60 K reported in T *. 22,23 These intriguing properties have motivated many experimental and theoretical studies exploring the nature of multiferroicity in CaMn 7 O 12 , in which the ferroelectricity and helical magnetism are believed to be intimately coupled.12,14,19,[23][24][25][26][27][28][29] Most previous studies have focused on the properties of ternary CaMn 7 O 12 and SrMn 7 O 12 ; in contrast, the effect of chemical substitutions on functional behavior in quaternary compounds is not yet fully understood.7,30 A few examples have been reported, where substitution of La for Ca on the A-site was shown to enhance the electric polarization 31 and doping Fe 3+ , Al 3+ , and Cr 3+ on the Mn sites suppresses charge-ordering and leaves only the cubic structure.30 Finally, insertion of Cu on the B-site yields significant magnetoresistance.7,[32][33][34][35][36] Of particular relevance to this report, polycrystalline Ca 1x Sr x Mn 7 O 12 ceramics with 0.05 ≤ x ≤ 0.2 exhibit an enhanced low temperature magnetization compared to the undoped (x = 0) compound.37 Most recently, a second study focused on Ca 1x Sr x Mn 7 O 12 ceramics with x less than 0.05 that showed enhanced polarization.38 However, the effect of Sr substitution on the electronic properties remains largely unexplored, specifically at larger Sr concentrations (x > 0.2) and in thin films.Given the growing interest in electronics that utilize the correlated electron behavior and/or abrupt phase transitions of complex oxides, [39][40][41][42][43] a detailed understanding of how A-site substitution can be used to tune the chargeordering transition is needed to engineer quadruple perovskites for such applications.We have synthesized a set of Ca 1x Sr x Mn 7 O 12 thin films with various concentrations of Sr up to x = 0.6 on (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates.Sr is isovalent to Ca but has a larger ionic radius (1.31 Å and 0.99 Å for Sr 2+ and Ca 2+ , respectively) and thus allows for the effect of chemical pressure to be studied without changing the nominal Mn valence.In conventional AMnO 3 manganites, substitution of Sr for Ca on the A-site acts to increase the tolerance factor and reduce rotations of the corner-connected MnO 6 octahedra.44 This is known to have significant effects on physical properties, including a greater propensity for charge-ordering as the magnitude of rotations (tolerance factor) is increased (decreased).[45][46][47] Herein, we demonstrate that similar trends are operative in quadruple manganites. We cofirm a lattice expansion with increasing Sr concentration and show that T * decreases systematically with increasing x.Our work provides insight into how to tune electronic phase transitions in quadruple perovskite thin films, which is important to engineer materials with a specific electronic switching functionality for devices or sensors.70 nm thick Ca 1x Sr x Mn 7 O 12 films were synthesized using oxide molecular beam epitaxy (MBE).Single crystal (001)-oriented LSAT substrates were used.The calculated lattice mismatch between the LSAT (a = 3.868 Å) substrates and CaMn 7 O 12 (a = 3.682 Å in a pseudocubic perovskite lattice) is 5.1%.The large mismatch leads to strain relaxation of the films at the thicknesses studied here.25 Growth conditions and post-growth anneals were set similar to the CaMn 7 O 12 film growth 25 with an O 2 chamber pressure of 2 × 10 6 Torr and a substrate temperature of ∼650 • C. The Ca and Sr content was controlled by tuning the effusion cell temperatures and shutter times. The films werennealed ex-situ in a tube furnace with flowing oxygen at 850 • C for 3 h and then at 200 • C for 1 h in a flowing mixture of O 2 :O 3 (∼95:5).
The films were characterized by the Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), reciprocal space mapping (RSM), and high temperature resistivity measurements.For the RBS measurements, as-grown films deposited on the MgO (001) substrates directly adjacent to the LSAT substrates were used.The SIMNRA software package was used to simulate and analyze the RBS data. 48XRD and X-ray reflectivity (XRR) measurements were carried out using a four-circle high-resolution X-ray diffractometer (X'Pert Pro, Panalytical) with Cu-Kα 1 radiation.Reciprocal space maps were used to determine the in-plane lattice parameters in the thin films.To investigate thermal expansion and the structural phase transition, we performed temperature dependent XRD equipped with a controlled temperature stage (DHS 1100, Anton Paar) in air.We used silver paste to adhere the sample to the heating plate.Data were collected at nine different temperatures above room temperature with a step size of 25 • C. Sample realignment was conducted at each temperature to maximize the XRD intensities.We note that the temperature of the hot stage has an estimated error of ∼10 • C. We performed high temperature in-plane DC resistivity measurements with a custom-built setup using a Keithley 6220 current source, a Keithley 2148 nanovoltmeter, and a Lakeshore 211 temperature monitor by first heating the sample to 600 K and collecting data while the sample is cooled slowly at a rate of approximately 5 K/min to room temperature.For these measurements, the temperature sensor was placed adjacent to the sample and silver paint was used to make a van der Pauw geometry electrical contact.In addition, resistivity data were collected below room temperature using a Quantum Design Physical Property Measurement System (PPMS).
To confirm the Sr-induced lattice expansion, XRD was measured as a function of x.These results are consistent with previous work on bulk Ca 1x Sr x Mn 7 O 12 37 over the range of x = 0 -0.2 and comparable to bulk SrMn 7 O 12 , which has a pseudocubic lattice parameter of 3.702 Å. 23 Figure 1(c) shows reciprocal space maps for x = 0 and x = 0.6 samples to determine the in-plane pseudocubic lattice parameter of the samples.The obtained c/a ratios are 0.999 (x = 0) and 1.001 (x = 0.6), consistent with the films being unstrained.Based on this, we believe that the out-of-plane parameter obtained from the 002 peak is a good measure of the general pseudocubic lattice parameter of the films.We note that there is a subtle splitting of the 103 peak along L for the CaMn 7 O 12 film, the origin of which is unknown and merits future study.
Temperature dependent XRD data were also measured to identify the effect of Sr doping on the structural phase transition temperature.Figure 2 shows the temperature dependence of the c-axis lattice parameter of the Ca 1x Sr x Mn 7 O 12 films grown on LSAT substrates as obtained from the (002) diffraction peak.We observe a discontinuity of the c-axis parameter in the films near 400-440 K; consistent with the presence of a phase transition, the c-axis increases abruptly above the transition.While our measurements do not provide direct evidence that the phase transition is due to chargeordering, we denote this as a charge-ordering transition based on previous experimental evidence and density functional theory calculations that revealed the state below phase transition temperature (T *) to be charge-ordered. 13,25Also consistent with bulk CaMn 7 O 12 , 49 we observe an increase in the lattice parameter on warming across the phase transition.The temperature of the discontinuity in lattice expansion decreases with increasing x, indicating that Sr-doping decreases the structural phase transition temperature.Linear fits above and below the discontinuity were performed to determine the rate of change of the c-axis parameter on both sides of the phase transition with a 3σ error.In the charge-ordered phase (below T *), the c-axis parameter expands at a rate of 1.5(1), 1.1(1), and 1.1(1) × 10 5 Å/K for the x = 0, 0.21, and 0.6 samples, respectively.The high temperature data (above T *) have significantly more variation; the x = 0 and 0.6 samples expand at 2.0(1) and 2.2(1) × 10 5 Å/K, while the x = 0.21 sample expands at 0.9(1) × 10 5 Å/K.We believe that the high value [∼2.0(1) × 10 5 Å/K] is more accurate, as this was found in more of our samples and in the x = 0.6 film, which has the most data points above T*.In previous work on bulk CaMn 7 O 12 , the non-charge-ordered phase was found to expand at a greater rate with increasing temperature than the charge-ordered phase. 49In contrast, the LSAT substrates exhibit simple linear expansion [∼0.9(1) × 10 5 Å/K] over the measured temperature range, as shown in the bottom panel of Fig. 2.
Temperature dependent resistivity (ρ) measurements were performed to determine how the Sr content alters the charge-ordering transition and to determine the conduction mechanisms.As can be seen in Fig. 3, Sr doping does not appear to systematically alter the magnitude of the resistivity and all samples exhibit insulating (dρ/dT < 0) behavior.While differences in resistivity at 300 K and 500 K are observed across the samples, we speculate that these variations occur due to differences in oxygen vacancy concentration or to varying defect densities and microstructural quality within the films.We note that similar variability in the resistivity magnitude was observed in undoped CaMn 7 O 12 films, but T* was relatively insensitive to cation and/or oxygen off-stoichiometry with measured T* values varying from 420 to 429 K. To determine T * as a function of Sr doping, we fit d(ln ρ)/dT as a function of temperature 50 to a bi-gaussian function as shown in Fig. 3(c); the center position is taken as T *. Figure 3(d) shows the charge-ordering transition temperature as a function of x.Increasing the Sr concentration leads to a continuous decrease in the charge-ordering transition temperature.The T * values determined from the fits are 426, 405, 401, and 385 K for the x = 0, 0.21, 0.41, and 0.6 samples, respectively.The calculated 3σ error for all the T * values obtained from the resistivity data is ±1 K.The results are in qualitative agreement with the high temperature XRD, but there is some discrepancy in the obtained T * values between the two measurements.We believe these relate to the experimental conditions and likely do not represent the decoupling of the electronic and structural transitions in our films.We first note that our method of extracting T * from the electrical measurements yields T * values that are lower than the onset of the resistivity changes by 5-10 K. Additionally, the resistivity measurements are taken while cooling the sample, thus if the cooling rate of the film is slower than that of the thermocouple, this will also act to depress the T * value obtained   4(b) also presents the activation energies from these fits, which are between 180 and 210 meV.The error from the fits is on order of 1-2 meV, while the experimental error is estimated to be on order of 10 meV, which was the standard deviation obtained from measuring two CaMn 7 O 12 films.These results indicate that while Sr doping yields a systematic reduction in T *, it does not have a significant effect on either the dominant conduction mechanism above and below T * or the relevant activation energies.
We have carried out an experimental study on the effect of Sr doping in CaMn 7 O 12 films.The structural phase transition temperature from the rhombohedral to the cubic phase is shown to decrease with Sr doping.This structural change is attributed to the increase in the tolerance factor by substitution of Ca 2+ with Sr 2+ .Furthermore, we relate these changes in structure to the chargeordering transition temperature as indicated by the temperature dependent resistivity measurements, which confirmed the presence of an electronic phase transition ranging from 385 K < T* < 426 K for the Ca 1x Sr x Mn 7 O 12 films.An increase in Sr doping leads to a decrease in T* but does not significantly alter electronic conduction on either side of the transition.Our results suggest structural control by chemical doping as a route to tune T * in AMn 7 O 12 compounds, an important parameter in future device structures based on control of phase transitions above room temperature.

Figure 1
(a) displays room temperature XRD data from Ca 1x Sr x Mn 7 O 12 films grown on LSAT substrates.All samples exhibit a single diffraction peak indexed to the pseudocubic 002 reflection, without any additional peaks over the 2θ range of 46 • -52 • .The black line and arrow show the peak shifting towards smaller 2θ values with increasing x.

Figure 1 (
b) presents the calculated c-axis lattice parameter from the 002 pseudocubic reflection as a function of Sr concentration in the films on LSAT substrates.We observe a linear lattice expansion with increasing Sr concentration, given by c = 3.683 + 0.021x Å.

FIG. 1 .
FIG. 1.(a) XRD 2θ-θ scans of Ca 1x Sr x Mn 7 O 12 films synthesized on LSAT substrates display clear 002 reflections of the pseudocubic structure with no indications of secondary phases or crystallographic orientations.Black arrow highlights the shift towards smaller 2θ in the Ca 1x Sr x Mn 7 O 12 film peaks with increasing x.(b) Plot of the c-axis lattice parameter as a function of Sr doping.The open black diamond is from data in Ref. 22. (c) XRD-RSMs show the 103 Bragg reflection of the Ca 1x Sr x Mn 7 O 12 (x = 0 and x = 0.6) thin films grown on LSAT, confirming that the in-plane strain is relaxed.

FIG. 3 .
FIG. 3. (a) Plots of ρ as a function of T for the four Ca 1x Sr x Mn 7 O 12 films on LSAT substrates.(b) Schematics of the crystal structures showing the charge-ordered and disordered arrangements of the B-site Mn below and above T *.(c) Closer view of the resistivity near the electronic phase transition and corresponding plots of d(ln ρ)/dT as a function of T with the accompanying fits to a bi-gaussian function.The bi-gaussian fits are represented by solid lines.(d) T * as a function of x as calculated by taking the peak center of the bi-gaussian fits to the resistivity data (T * ρ ).

FIG. 4 .
FIG. 4. (a) Non-adiabatic polaron model fit above T *; the fits are shown as solid black lines.(b) The calculated activation energies from the polaron model (above T *, represented as open red squares) and the activated conduction model (below T *, represented as solid black squares) are plotted as a function of Sr concentration.