Single-axis control of manganite films by helium doping via He-co-sputtering

We have studied the effect of the out-of-plane lattice on tensile strained (001) La0.7Ca0.3MnO3 thin films. The films were deposited on SrTiO3 substrates through magnetron sputtering technique under different Ar/O2/He gas flow ratios, varying the out-of-plane lattice from 3.823 A to 3.845A, which corresponds to an increase in the metal-insulator transition temperature. These changes are reversible after high-temperature anneal due to a massive helium release from the films occurring at temperatures around 540°C. The dependence of the transition temperature on lattice distortion is in good agreement with the prediction proposed by Millis et al. [J. Appl. Phys. 83, 1588 (1998)]. Considering the Jahn-Teller distortion enhanced by the in-plane biaxial strain in the films, we attribute the elevated transition temperature to the distortion relaxation due to He doping in the tensile strained films. The effective He doping by magnetron sputtering technique provides a simple strategy for manipulating functionality of oxide films.We have studied the effect of the out-of-plane lattice on tensile strained (001) La0.7Ca0.3MnO3 thin films. The films were deposited on SrTiO3 substrates through magnetron sputtering technique under different Ar/O2/He gas flow ratios, varying the out-of-plane lattice from 3.823 A to 3.845A, which corresponds to an increase in the metal-insulator transition temperature. These changes are reversible after high-temperature anneal due to a massive helium release from the films occurring at temperatures around 540°C. The dependence of the transition temperature on lattice distortion is in good agreement with the prediction proposed by Millis et al. [J. Appl. Phys. 83, 1588 (1998)]. Considering the Jahn-Teller distortion enhanced by the in-plane biaxial strain in the films, we attribute the elevated transition temperature to the distortion relaxation due to He doping in the tensile strained films. The effective He doping by magnetron sputtering technique provides a simple strategy for manipulating functionality...


I. INTRODUCTION
As a critical aspect of epitaxial oxide films, strain engineering has been intensively studied since the discovery of high-temperature superconducting cuprates. 1,2So far, most previous work on the strain effects has been carried out by choosing various single crystal substrates, such as SrTiO 3 and LaAlO 3 , etc, to tune the in-plane biaxial strain in films. 3However, the in-plane strain always accompanies by an opposite out-of-plane strain due to positive Poisson's ratios for oxides, resulting in an obscure phenomenon behind multiple dimensions.
Take colossal magnetoresistance (CMR) manganites R 0.7 A 0.3 MnO 3 for example, 4,5 where R is La or other rare earth elements and A is a divalent element, such as Sr and Ca.With increasing the average radius of the R/A-site cation, according to the phase diagram reported by H. Y. Hwang et al., 6 the ferromagnetic Curie temperature (T C ) increases from 265 K for La 0.7 Ca 0.3 MnO 3 (LCMO) to 370 K for La 0.7 Sr 0.3 MnO 3 .Therefore, it was naturally expected that LCMO films with tensile strain should have an elevated T C .In fact, the opposite is true -suppression of T C in tensile strained LCMO films has been commonly reported. 3,7,8Compared to isovalent substitution, biaxial strain in the CMR manganite films has an additional effect because of the Jahn-Teller (JT) active Mn 3+ cation. 9The in-plane biaxial strain and corresponding opposite out-of-plane strain inevitably enhance the Jahn -Teller distortion of MnO 6 octrahedral, 10 which is an important localization mechanism for the e g electrons in the CMR manganites, 11 and suppresses the double exchange (DE) interaction in consequence.To compensate such a negative impact due to the biaxial strain, single-axis control of the outof-plane lattice is an emergent task in the strain engineering.Because of its small size and chemical stability, He doping in oxide films was recently proposed as a promising method for expanding the out-of-plane lattice.By using helium ion implantation technique and vacuum annealing, H. W. Guo et al. 12 reported reversible control for the out-of-plane lattice in La 0.7 Sr 0.3 MnO 3 films epitaxially grown on SrTiO 3 substrates, which in turn altered the JT distortion in the films.Later, they demonstrated that implanted He atoms modulate the out-of-plane strain in SrRuO 3 films on SrTiO3 substrates. 13Considering that ion implantation needs special facility and it may introduce defect and/or disorder in films during the relatively high energy process, in this work we demonstrate He doping in tensile strained LCMO films by using a simple and popular magnetron sputtering technique.With increasing the He gas flow rate during sputtering, the out-of-plane lattice parameter increases markedly, corresponding to an increase in the transition temperature.The evolution is reversible by high-temperature anneal.

II. EXPERIMENTAL
La 0.7 Ca 0.3 MnO 3 films were deposited on SrTiO3(001) (STO) substrates by using dc magnetron sputtering technique in an Ar-O 2 -He mixed atmosphere.The used normal La 0.7 Ca 0.3 MnO 3 target with 2-inch diameter was prepared by using solid state reaction.The sputtering process was carried out for 9 min in an Ar/O 2 gas ratio of 28 sccm/4 sccm with a pressure of 3.5 Pa at a dc power of 35 W. To dope He element in the films, helium gas was added to the sputtering atmosphere with various flow rates up to 6sccm.At the beginning of this work, the LCMO films were deposited at high substrate temperatures ∼ 700 • C, as the usual manner for epitaxial growth of oxide films.But we found that the LCMO films changed little with varying the He gas flow rate, implying a very low He-doping level during the high temperature deposition.After that we adopted a two-step procedure for the epitaxial growth of the He-doped LCMO films.The LCMO films were first deposited on STO substrates at room temperature in Ar-O 2 -He gas mixtures to ensure He doping in the films.After the deposition, the films were post annealed at 900 • C under 1 atm flowing O 2 for 3 h to obtain epitaxial films.The cross-section image of scanning electron microscope (SEM) revealed the film thickness to be 30 nm.The orientation and quality of the films were examined by high resolution Cu K α X-ray diffraction (XRD).Particularly, evolution of the out-of-plane lattice is a direct reflection of the He-content.Electrical resistivity of the films was measured by using a standard four pointprobe technique over a temperature range from 20 to 300 K. Magnetization was measured in a VSM magnetometer (Quantum Design MPMS).

III. RESULTS AND DISCUSSION
XRD patterns (not shown here) for all the annealed films display only (00n) peaks, indicating epitaxial growth of the films.Figure 1(a) shows the enlarged θ-2θXRD patterns around the (002) peak of the LCMO films deposited under different He gas flow ratios with identical Ar/O 2 gas flow ratio of 7:1.Considering that the bulk LCMO has a shorter lattice constant a = 3.859 Å compared to that of STO substrate (a = 3.905 Å), the epitaxial LCMO films on STO substrates were supposed to suffer an in-plane biaxial tensile strain, which in turn results in a shorter out-of-plane lattice for the LCMO films due to the Poisson effect.Figure S1 (supplementary material) shows the x-ray reciprocal space map (RSM) on ( 103) reflection for a He-doped LCMO film, which confirms the fully strained inplane lattice even after He-doping.In Fig. 1(a), the (002) peak for the LCMO film deposited without He gas locates at the highest angle as expected, corresponding to the shortest out-of-plane lattice c = 3.823 Å.With increasing the He gas flow ratio in the mixed gases, the (002) peak shifts to the lower angle side, accordingly the c-axis increases gradually through 3.834 Å under the He gas flow ratio of 0.5, to 3.845 Å under that of 1.5.It is demonstrated that the out-of-plane lattice of the tensile strained LCMO films is stretched by 0.28% and 0.57% respectively by using the He-co-sputtering method.According to the literature, 12 we estimate the He-doping-content for the LCMO film with the longest c axis to be of ∼ 1 × 10 21 He/cm 3 , corresponding to approximately 10% LCMO unit cells doped with the He atoms.The effect of the He doping on transport property has been investigated by means of resistivity measurements shown in Fig. 1(b).The He-undoped film presents a metal-insulator transition temperature(T MI ) of 260 K, a little bit lower than 265 K for the LCMO bulk sample. 14With increasing the He-content, T MI rises to 266 K and 280 K for the films with the 0.28% and 0.57% stretched c-axes, respectively.Meanwhile, the peak resistivity decreases.Figure 1(c) shows the temperature dependence of magnetization after field-cooling under H a = 200 Oe for the LCMO films.It can be seen that the ferromagnetic transition temperature T C increases correspondingly with increasing the He gas flow ratio.The results clearly indicate that the He-co-sputtering process indeed enhances the e g electron hopping probability in the films, which in turn leads to a stronger DE interaction and higher T C .As commonly reported, 15,16 the transition temperatures and resistivity of the CMR manganites are strongly dependent on the oxygen content.However, in the present work, the LCMO films were post annealed in the same batch in pure oxygen to guarantee a complete oxidation.Therefore, the oxygen content is not responsible for the elevated transition temperatures in the He-co-sputtered LCMO films.In the LCMO/STO films suffering in-plane tensile strain, we argue that elongation of the shorter out-of-plane lattice (c-axis) by He-doping partially relieves the strain-enhanced JT distortion, which is the key factor for the higher T MI in the He-doped films.
Considering that He as an inert element is difficult of bonding with other ions, the He atoms should be interstitially doped in the LCMO films.This draws into an issue of thermal stability for the doped He atoms.To investigate this question, we further annealed a set of the maximum Hedoped films (Ar:O 2 :He = 7:1:1.5)in a vacuum of 3.5 Pa at various annealing temperatures T a .The films were heated to the set value of T a at a rate of 10 • C/min, and after annealing for 3 h, cooled to room temperature at the same rate.to 273 and 264K for T a = 200 and 350 • C, respectively, accompanied by an overall increase of the resistivity.Considering the possibility of oxygen release during the further anneal, which also leads to the decrease of T MI , we heated the max-He-doped LCMO film in 3.5Pa oxygen atmosphere at 650 • C for comparison.As shown in Fig. 2(a), after the O 2 /650 • C anneal T MI decreases further to 260K, same as that of the He-undoped film.It implies that the reduction of T MI after the further anneal is due to the helium release rather than the oxygen deficiency.The corresponding XRD patterns are shown in Fig. 2(b).With increasing T a from the as-prepared film to 650 • C, it can be seen that the out-of-plane lattice decreases from 3.845Å to 3.823Å, which is also in agreement with that of He-undoped films.Referring to the reported results on vacuum annealed LCMO/STO films without He-doping, 17 we note that oxygen release leads to an elongation of the out-of-plane lattice, similar to lattice expansion in other oxygen-deficient manganites. 18Therefore, the remarkable reduction of the c-axis lattice for the He-doped films is definitely caused by the helium release during the further anneal.It is an explicit evidence for the helium release governing the reduction of T MI in the furtherannealed-films.These results also demonstrate that the helium doping effects in the LCMO films are reversible by high-temperature anneal.
To shed more light on the helium release process, we investigated the real-time change in resistivity for the He-doped films at different annealing temperatures.After the max-He-doped films were heated to T a in vacuum, the resistivity measurement was started and carried out for 2 h. Figure 3(a respect to time, implying a continuous helium release from the film.It is quite clear that the helium release behavior of the He-doped LCMO films is dependent on the annealing temperature, and we speculate that a large helium release might occur at a certain temperature between 350 and 650 • C. To verify this point, we measured high-temperature-dependent resistivity of the max-He-doped LCMO film over a temperature range from 50 to 600 • C at a heating rate of 10 • C/min in air atmosphere. For comparison, the measurement was also carried out for the He-undoped film.Figure 3(b) displays the high-temperature-dependent resistivity for the two films.It can be seen that the resistivities for both the samples decrease with increasing temperature, mainly due to the insulator-like transport behavior at high temperatures.It is noteworthy that the He-doped film displays an apparent peak at 545 • C in the ρ -T curve, while the He-undoped film shows a continued decline of resistivity as normal.Such a resistivity peak in the He-doped film clearly reveals that the helium atoms have a massive release at temperatures around 545 • C. With increasing temperature, the resistivity upturn at 521 • C implies the beginning of the massive helium release, while the peak temperature 545 • C corresponds to the helium release drawing to an end.Ultimately at 560 • C, recovery of the normal decrease in resistivity indicates the finality of the helium release.Therefore, we conclude that the helium release of the He-doped LCMO films occurs primarily in a narrow temperature window between 521 and 560 • C.
To understand the He-doping effect on the transition temperature T MI , we consider the doubleexchange model, which is strongly dependent on the e g electron delocalization, along with the Jahn-Teller distortion for the Mn 3+ ions. 19The magnetic and transport properties of the CMR manganites are dominated by the one-electron bandwidth W, which can be effectively tuned by the average R/Asite ionic radius or high external pressure via changing the Mn-O bond length and Mn-O-Mn bond angle. 20Apparently in strained films, changed lattice volume also induces such a change in W. On the other hand, in-plane biaxial strain in films enhances the JT distortion of MnO 6 octrahedral, leading to a larger energy splitting(∆) between the d x2-y2 and d 3z2-r2 orbitals and increasing localization tendency of the e g electrons. 21Millis et al. 22  The negative a value indicates that the tensile strain in the LCMO films tends to increase the oneelectron bandwidth W and the e g electron hopping probability, which is in agreement with the H. Y. Hwang's phase diagram. 6We note that the reported a value for the tensile-strained La 0.7 Sr 0.3 MnO 3 films is positive, 23 which also follows the tendency because La 0.7 Sr 0.3 MnO 3 is at the peak of the phase diagram.Furthermore, the absolute value of a for the LCMO films is one order of magnitude larger than that of reported La 0.7 Sr 0.3 MnO 3 , implying that LCMO is more sensitive to the change in lattice volume.On the other hand, the positive b value makes sure that the biaxial strain in the CMR manganite films inevitably plays a negative role to the Mn e g electron hopping due to the enhanced JT distortion.Therefore, the results unambiguously illustrate that while the biaxial tensile strain in LCMO films is a two-edged sword to the e g electron delocalization, He doping in the films by the He-co-sputtering method can effectively offset the negative impact of the biaxial strain enhanced JT distortion on the e g electron hopping.

IV. CONCLUSIONS
In conclusion, we have demonstrated an elongation of the out-of-plane lattice in the tensile strained LCMO films by using the He-co-sputtering method, accompanied by an increase in the transition temperature.The evolutions in structure and transport properties are reversible after hightemperature annealing process, due to a centralized helium release in the He-doped LCMO films at temperatures around 540 • C. Based upon the Millis model, the fitting results reveal that the elongated out-of-plane lattice not only increases the one-electron bandwidth W, but also relieve the enhanced JT distortion in the tensile strained LCMO films.As a consequence, the e g electron delocalization as well as the double exchange interaction are enhanced through the He doping, which in turn results in the elevated transition temperatures.These results show that He-co-sputtering can provide a new manipulation tool for functionality of oxide films in view of practical applications.

SUPPLEMENTARY MATERIAL
See supplementary material for the reciprocal space map (RSM) on ( 103) reflection from the max-He-doped LCMO film grown on STO(001) substrate.

125201- 3 Wang
FIG. 1.(a) θ-2θX-ray diffraction patterns for the (002) peak of the LCMO films grown on STO substrates at different Ar-O 2 -Heratios.(b) ρ(T ) for the LCMO films with respect to the helium flow ratio.(c) Field-cold M(T ) of the LCMO films under applied magnetic field of 200Oe.
Figure 2(a) illustrates the temperature-dependent resistivity of the max-He-doped LCMO films after further anneal.It is found that T MI of the films decreases

FIG. 2 .
FIG. 2. (a) ρ(T ) after different annealing processes for the max-He-doped LCMO films.(b) θ-2θX-ray diffraction patterns and the out-of-plane lattice parameters for the corresponding films.
FIG. 3. (a) Real-time change in resistivity for the max-He-doped LMCO films at different further annealing temperatures within 2 hours.(b) High-temperature-dependent resistivity for the LCMO films with and without helium doping, respectively.
FIG. 4. The Curie temperature T C vs the bulk strain ε B and the square of the biaxial strain ε * 2 .Red scatter points represent experimental data.The black plane is the fitting surface based on the Millis model.