Interfacial reconstruction in La0.7Sr0.3MnO3 thin films: giant low-field magnetoresistance

Herein, interfacial reconstruction in a series of La0.7Sr0.3MnO3 (LSMO) films grown on a (001) oriented LaAlO3 (LAO) substrate using the pulsed plasma sputtering technique is demonstrated. X-ray diffraction studies suggested that the LSMO film on LAO was stabilized in a tetragonal structure, which was relaxed in-plane and strained along the out-of-plane direction. The interfacial reconstruction of the LSMO–LAO interface due to the reorientation of the Mn ion spin induced spin-glass behavior due to the presence of non-collinear Mn ion spins. Consequently, the interface effect was observed on the Curie temperature, temperature-dependent resistivity, metal-to-semiconductor transition temperature, and magnetoresistance (MR). At a magnetic field of 7 T, MR decreased from 99.8% to 7.69% as the LSMO film thickness increased from 200 Å to 500 Å. A unique characteristic of the LSMO films is the large low-field MR after a decrease in the field from the maximum field. The observed temperature-dependent magnetization and low-temperature resistivity upturn of the LSMO films grown on LAO provide direct evidence that the low-field MR is due to the non-collinear interfacial spins of Mn. The present work demonstrates the great potential of interface and large low-field MR, which might advance the fundamental applications of orbital physics and spintronics.

La 0.7 Sr 0.3 MnO 3 (LSMO) exhibits metal-like electronic transport with striking properties such as ferromagnetic ordering at room temperature and nearly 100% spin polarization; 1,2 it is oen reported as the most itinerant electron material with the highest Curie temperature (T C $ 369 K) among the known manganites. 1 These unique properties make LSMO a fascinating material for technological applications. 3 However, the growth of high-quality ultrathin LSMO lms is very challenging because the bulk rhombohedral crystal structure of LSMO stabilizes in the form of a pseudocubic structure, which signicantly modies the bulk-like properties in LSMO lms. [4][5][6][7][8][9] Interestingly, structural reconstruction intermittently results in unique or improved functionalities in addition to control over the magnetic moment, T C 5,6 and metal-to-insulator transition temperature (T P ) of the LSMO lms. [6][7][8] The magnetic moment, 5 T C and T P of the LSMO lms decrease as the LSMO lm thickness decreases and below a certain lm thickness, which is called the critical thickness, the metal-like conduction is suppressed by semiconducting behavior, where decoupling between ferromagnetism and metallicity occurs. [6][7][8] The critical thickness of the LSMO lms strongly depends on the substrate-induced stress. 8 However, understanding the deviations in the physical properties of the lms compared to that of bulk LSMO remains elusive. Nevertheless, many other factors have been found to account for this behavior, for example, the oxygen vacancies and charge transfer at the interfaces.
Extensive efforts are being devoted to exploring the electronic transport properties of LSMO lms grown on a wide variety of substrates with or without a buffer layer. 8,9 It has been demonstrated that the magnetoresistance (MR) behavior in LSMO lms can be obtained at much lower magnetic elds by naturally or articially modifying the Mn-O-Mn bond angle and bond length. 10 Large low-eld MR (LFMR) was observed by incorporating articial grain boundaries in ferromagnetic manganite/spacer/ferromagnetic manganite trilayers. The highest LFMR value of 45-50% was obtained below a 0.02 T eld at 4.2 K for La 0.67 Sr 0.33 MnO 3 /SrTiO 3 /La 0.67 Sr 0.33 MnO 3 thin lm junctions. 11,12 The LFMR value of the composite lms such as La 0.67 Sr 0.33 MnO 3 /ZnO was 23.9% at 0.5 T eld and 10 K. 13 The contribution of the grain boundary towards 23-27% LFMR at 0.02-0.1 T eld and 77 K was explained by designing a single SrTiO 3 bicrystal substrate grain boundary into an epitaxial lm of La 0.7 Ca 0.3 MnO 3 . 14 However, the growth of high-quality epitaxial manganite thin lms with controlled and atomically smooth articial grain boundaries is a very challenging task. Recently, we reported a unique pulsed plasma deposition (PPD) method, where the thin lms of LSMO were stabilized epitaxially, even on the native SiO 2 surface of the (001) oriented Si substrate by using a home-built RF magnetron sputtering system. 4,15 Herein, we report the study of the spin dynamics in LSMO grown on an LaAlO 3 (LAO) substrate, which exhibited giant LFMR ($99%) at low temperatures. This LFMR strongly depends on the LSMO thickness (t LSMO ), which suggests the interfacial origin of MR due to the variation in the bond length and bond angle at the interface.
The highly dense ceramic target of LSMO prepared by the solid-state reaction method was used to grow thin lms on the (001) oriented LAO by adopting the PPD growth sputtering process. The sputtering system was designed in such a way that it (i) generates a horizontally orientated plasma, (ii) offers a larger cathode surface in front of the plasma, (iii) places the substrate heater inside the specied port, (iv) maintains a uniform temperature of the port wall using cooled water, and (v) provides an option to customize the target-to-substrate distance. 15 The thin lms were grown at 700 C under 7 Â 10 À3 mbar of an argon and oxygen gas mixture. The chamber pressure was maintained by owing the argon and oxygen gas at a 1 : 4 proportion during the deposition. The deposition was performed in pulsed mode by keeping the sputtering power density at 2.96 W cm À2 . All the thin lms were grown for different durations by altering the opening and closing of the shutter for a period of 10 s and 50 s, respectively. Aer the deposition of the desired thickness, the deposition chamber was lled with 50 mbar O 2 followed by post-annealing for 45 min at 700 C, and then, the lm was cooled down to room temperature. LSMO was deposited on the (001) oriented LAO for six different growth periods. The LSMO lms were found to be 100, 150, 200, 250, 375, and 500 A thick, which was conrmed by optical measurements.
A four-cycle X-ray diffractometer was used to record the outof-plane X-ray diffraction (XRD) patterns and reciprocal space mappings (RSMs) of the thin lms. The temperature dependence of magnetization (M(T)) measurements for the out-ofplane orientation of the magnetic eld were performed using a superconducting quantum interference device-based vibrating sample magnetometer. The electronic transport properties of the thin lms were measured in the presence of a magnetic eld using a physical property measurement system. The resistivity of the thin lms was measured using the twoprobe technique in the presence of an out-of-plane-oriented magnetic eld. The temperature-dependent resistance of the lms was measured while warming the lms from the lowest temperature. Fig. 1a shows the q-2q XRD patterns of the LSMO lms with different thickness grown on (001) oriented LAO. The XRD patterns show only the (00l) oriented Bragg's peaks of LSMO and LAO without any signature of the other phase of LSMO. The out-of-plane pseudocubic lattice parameter (c pc ) of the different LSMO lms calculated from the (00l) peak positions of the XRD patterns is plotted in Fig. 1b. The c pc of the LSMO lms is independent of thickness and the average value is 3.99 A, which is larger than the pseudo-cubic lattice parameter of the bulk LSMO (c pc ¼ 3.88 A), as shown in Fig. 1. 16 The larger c pc value of the LSMO lm compared to that of its bulk suggests that LAO provides out-of-plane tensile strain for the epitaxial growth of LSMO. The 100 A thick LSMO lm shows À2.68% tensile strain along the [001] orientation. As the LSMO lm thickness increased to 150 A, the strain also increased to À3.06%. However, the strain uctuated between À2.65% to À3.06% as the LSMO lm thickness varied from 100 to 500 A [ Fig  close to each other, which conrm the presence of the same inplane lattice parameters irrespective of h101i. The c pc extracted from the RSM is very close to that measured from the symmetric scan. The in-plane lattice parameter of LSMO is a pc ¼ b pc ¼ 3.87 A. As the LSMO lm thickness decreased below 500 A, the inplane lattice parameter remained the same up to 200 A and then decreased to 3.86 A for the 150 A thick lm. However, the (101) peak intensity of the 100 A thick LSMO lms was not noticeable to calculate the in-plane lattice parameter. Overall the in-plane lattice parameter of LSMO is close to the pseudocubic lattice parameter of the bulk LSMO with a negligible strain of $0.2% [ Fig. S2 †]. The substrate-induced in-plane compressive strain relaxed to $0.2%, while the out-of-plane tensile strain remained at around À2.8% [ Fig. S2 †] in the LSMO lms grown on the (001) oriented LAO substrate. The presence of anisotropic strain may be due to the substrate-induced strain, twin boundaries, and oxygen vacancies.
The bulk LSMO possesses a rhombohedral unit cell with the space group R 3C and lattice constants 5.471 A and a r ¼ 60.43 . 17 In this unit cell, the octahedron rotations can be described by Glazer's tilt system a À b À c À , which consists of equivalent out-ofphase rotations (see ESI †) about the h100i cubic axes. 18 Thus, a rhombohedral crystal structure for the LSMO thin lms on LAO is expected. However, the X-ray diffraction studies of the LSMO lms on LAO in Fig. 1e indicate that LSMO stabilizes in tetragonal structures with the lattice parameters a ¼ 5.47 A (i.e., a pc ¼ 3.87 A) and c ¼ 7.98 A (i.e., c pc ¼ 3.99 A). Thus, the tetragonal structure has the space group I4/mcm with Glazer's tilt system a 0 b 0 c À . 19 The synthesis of the LSMO lm on LAO introduced structural reconstruction of LSMO from a À b À c À type octahedral tilt to a 0 b 0 c À similar to PrMnO 3 /SrRuO 3 . 20 Interestingly, the tetragonal LSMO thin lm does not conserve the unit cell volume, in contrast to the LSMO lm on LAO with a conserved volume. 21 As shown in Fig. 2, the zero-eld-cooled (ZFC) magnetization of the 200 A thick LSMO lm is higher with a relatively sharper peak compared to that of the 500 A thick LSMO lm. The ZFC M(T) indicates the presence of randomly oriented non-collinear Mn ion spins in the LSMO lm. The difference in the ZFC magnetization of these two LSMO lms suggests that the 500 A thick lm has comparatively more non-collinear Mn ion spin domains with a coupling energy stronger than the Zeeman energy at 0.01 T eld. The difference in ZFC and eld-cooled (FC) magnetization at a temperature below the irreversible temperature and the decrease in peak temperature in the ZFC curve with an increase in applied eld observed for these LSMO lms [ Fig. S3 †] are the characteristics of the spin-glass state. The 0.01 T FC M(T) of the 200 A thick LSMO indicates that the paramagnetic phase transforms into a ferromagnetic phase at a temperature of T C z 196 K, which is the T C of LSMO. The monotonic increase in the 0.01 T FC magnetization at a temperature below 112 K indicates the existence of uniform exchange coupling of the non-collinear Mn ion spin domains. The distinct change in 0.01 T FC magnetization with the temperature of around 112 K was suppressed aer increasing the cooling eld to 0.5 T [ Fig. S3 †] due to the increase in the ferromagnetic domain. The T C of the LSMO lms decreased from 262 K to 102 K as the lm thickness decreased from 500 A to 100 A [ Fig. S3 †]. The increase in the 0.01 T FC magnetization of the 500 A thick LSMO lm at a temperature below 60 K suggests the existence of non-uniform exchange coupling of the non-collinear Mn ion spin domains with ferromagnetic Mn ion spin domains. The T C values of the LSMO lms are signicantly lower than that of the bulk LSMO. However, the observed T C values of these lms are very close to that of the reported LSMO/ LAO. 22 The T C of the LSMO lms depends on the spin orientation of Mn, which is determined by MnO 6 . As seen in Fig. 1b, MnO 6 in the LSMO lm on LAO is elongated along the 'c' direction compared to its bulk. Thus, the reduced T C of the LSMO lm on LAO compared to its bulk is attributed to possible sources of MnO 6 rotation and tilting near the interfaces such as surface twining, substrate-induced strain, 8 and oxygen nonstoichiometry. 23 The FC M(T) of the LSMO lms with a thickness of <200 A did not show a clear paramagnetic to ferromagnetic transition, suggesting the richness of the non-collinear Mn ion spin domains [ Fig. S3 †].
The temperature-dependent resistivity (r(T)) of the $2 mm wide LSMO lms was measured using the two-probe method [ Fig. 3a]. The resistance of the 100 A thick LSMO lm at room temperature was $170 kU, which on cooling below room temperature increased monotonically and became outside the measurable range of the voltmeter at $140 K [ Fig. 3b]. Qualitatively, a similar r(T) was observed with an extended measurable temperature range aer the application of a magnetic eld along the out-of-plane direction of the 100 A thick LSMO lm. The (r(T,H)) of the 150 A thick LSMO lm [ Fig. 3b] is similar to that of the 100 A thick LSMO lm. The r(T) of the LSMO lms with a thickness of #150 A did not exhibit metal-like behavior but semiconductor-like behavior. The r(T) data of these lms ts well with rðTÞ ¼ r 0 e ðT 0 =TÞ 0:5 [ Fig. 3b and S4 †], which indicates that their transport behavior is consistent with the Efros-Shklovskii variable range hopping (ES-VRH) mechanism. 24 The r 0 may be either independent of temperature or a slowly varying function of temperature, while T 0 is a constant of the material, and is related to the rate at which the wavefunction decreases with hopping distance. Thus, the T 0 of the ES-VRH is correlated inversely with the localization The resistivity of the 200 A thick LSMO lm on cooling below room temperature increased monotonically and became maximum at a temperature of T P $ 95 K, which we marked as the metal-insulator transition temperature. Upon further cooling the 200 A thick LSMO lm below T P , the resistivity decreased gradually and became minimum at a temperature of T Min $ 45 K, followed by a sharp increase down to the lowest temperature [ Fig. 4a]. As t LSMO increased above 200 A, a qualitatively similar r(T,0) was observed with a lower resistivity value, higher T P , and lower T Min [Fig. 4]. The resistivity of the 200 A thick LSMO lm decreased with the application of a magnetic eld, which indicates the existence of a negative MR character in the lm. As the magnetic eld increased, the T P of the LSMO lm increased, while the T Min decreased [ Fig. 5a], which is consistent with the formation of a ferromagnetic metallic domain with the magnetic eld. As the thickness of the LSMO increased above 200 A, the resistivity and T Min decreased, while the T P increased [Fig. 5b]. The increase in the T P of the LSMO lm grown on LAO with an increase in LSMO lm thickness has been reported previously. 25 The variation in resistivity at temperatures T P > T > T Min can be explained by the double exchange 26 and phase separation 27 mechanisms in the LSMO lms. The observed variation in resistivity with t LSMO is attributed to the expanded 'c pc ', which induces a Jahn-Teller distortion along the 'c' direction and controls the spin reorientations and electron localization. 28 The resistivity at T < T Min for the LSMO lms [ Fig. S5 †] was tted using the following expression: 29 where r 0 is the residual resistivity, the second term is due to the contribution of electron-electron scattering, the third term is the spin-dependent Kondo-like effect, and the last term is related to the inelastic scattering. The tting result as denoted by the solid lines in Fig. 5c [Fig. S5 †] agrees well with our experimental data, which indicates that the contribution to the resistivity of LSMO from the inelastic scattering is negligible [ Fig. S6 †]. As the LSMO lm thickness increased, the coefficients r1 2 and r 1 decreased [ Fig. 5d-g], suggesting a decrease in electron-electron scattering and spin-dependent Kondo-like scattering, consistent with the observed thickness-dependent M(T). The coefficients r1 2 and r 1 also decreased with an increase in the magnetic eld [ Fig. 5d-g]. The coefficient r1 2 is smaller than the coefficient r 1 irrespective of the thickness of the LSMO lm or magnetic eld value, and thus the electron-electron scattering is smaller compared to the spin-dependent Kondolike effect. The Kondo effect is attributed to the interaction of conduction electrons with the localized spin at the noncollinear Mn ion spins domain boundary. As the temperature decreases below T Min , the lattice distortion due to structural  Nanoscale Advances reconstruction and magnetic disorder effect becomes dominant, and the resistance upturn emerges. 30 Fig. 6 shows the temperature-dependent MR ¼ RðHÞ À Rð0Þ Rð0Þ of the LSMO lms with different thicknesses. The MR of the 100 A thick LSMO lm was positive for a 1 T eld. As the eld increased to 3 T, the MR of this lm remained positive at room temperature; however, on cooling below room temperature, the MR decreased and became negative at $175 K. A similar temperature-dependent MR (MR(T,H)) was observed at a higher magnetic eld with an enhanced MR and expanded temperature range for the negative MR. At a 1 T eld, the MR of the 150 A thick LSMO was positive in the temperature range of 360 K to 207 K, while it was negative at a temperature lower than 207 K [ Fig. 6b]. Upon increasing the magnetic eld, although the MR(T) curve was similar, the positive MR of the 150 A thick LSMO decreased, while its negative MR increased. The MR of the 200 A thick LSMO lm was negative at a 1 T eld. As the temperature decreased below room temperature, the MR at 1 T increased up to 80 K and remained at $96% until 35 K and then started decreasing down to the lowest temperature [ Fig. 6c]. Upon increasing the magnetic eld, the negative MR of the 200 A thick LSMO increased, and the plateau became wider. At a 7 T eld, the MR of the 200 A thick LSMO was $99% in the temperature range of 3 K to 120 K. As the LSMO lm thickness increased, the MR(T,H) was qualitatively similar to that of the 200 A thick LSMO with (i) a decrease in MR, (ii) the plateau became a peak, and (iii) the peak became narrow [ Fig. 6d-f]. The peak observed in the MR(T,H) shied towards a higher  temperature with an increase in its sharpness upon increasing the lm thickness as the T C and ferromagnetic domain size increased [ Fig. S3 †]. The variation in MR at around 360 K in the MR(T,H) was negligible since the lms were in the paramagnetic state. The constant MR temperature window near 360 K became wider as the lm thickness decreased from 500 A because of the decrease in T C , i.e., expansion of the paramagnetic state. Fig. 7a shows the isothermal MR(H) of the 200 A thick LSMO lm. Aer applying a current, the voltage was measured as a function of the magnetic eld at 10 K from 0 T to 7 T, which is the virgin curve. The virgin curve of the 200 A thick LSMO, as the magnetic eld increased from zero to 7 T, shows a colossal drop in resistance to 99.8%. The MR(H) of this lm exhibited several unique features such as (i) sharp drop in MR up to 60% under a 300 mT eld in the virgin curve, (ii) aer decreasing the eld from 7 T, the MR remained $99% for the entire range of eld variation of AE7 T, and (iii) the buttery shape MR loop is asymmetric [inset of Fig. 7a]. A qualitatively similar MR(H) with a lower value of MR was observed at 10 K for the LSMO lms of higher thicknesses [ Fig. 7b-d]. The variation in the MR at a 300 mT eld during the rst eld increasing branch, and aer decreasing the eld from 7 T, with the LSMO lm thickness, is shown in the inset of Fig. 7e. The asymmetry in the MR(H) curve may be due to the exchange biasing between the Mn ion non-collinear spin and ferromagnetic spin in LSMO. The sharp drop in the MR with a low eld (<300 mT) indicates that a fraction of the lm has non-collinear Mn ion spins with weak magnetic coupling. The gradual drop in MR with an increase in the magnetic eld above 300 mT demonstrates the presence of weak magnetic coupling between the Mn ions. The negligible variation in MR aer applying AE7 T even at a low eld attests the achievement of complete ferromagnetic ordering in the LSMO lms. The virgin curve in the MR(H) of the 200 A thick LSMO lm at a 300 mT eld shows MR 60%, which decreases to 0.7% for the 500 A thick LSMO lm. Similarly, at 7 T eld, the 99.8% MR in the MR(H) of the 200 A thick LSMO lm decreased to 7.69% for the 500 A thick LSMO lm [Fig. 7]. The observed decrease in MR at 10 K with t LSMO is attributed to the decrease in the non-collinear Mn ion spins.
The change in magnetoconductance (MC) at a eld below 0.4 T for the LSMO lms was tted using the Hikami-Larkin-Nagaoka (HLN) theory [Fig. 7f]. According to the HLN theory, the change in MC in the two-dimensions (2D) is: 31 where j is the digamma function and a is a coefficient reecting the strength of the spin-orbit coupling and magnetic scattering. The value of a is 1 if the spin-orbit interaction and the magnetic scattering are weak, a ¼ 0 (unitary) for strong magnetic scattering, and a ¼ À0.5 for the symplectic case. The a value obtained from the t of Ds(H) for the 200 A thick LSMO lm is 0.08, which increased to 0.54 for the 500 A thick LSMO. Thus, the LSMO lms exhibit weak localization, and the magnetic scattering decreases with an increase in thickness, consistent with the observed reduction in MR with an increase in thickness. The phase coherence length (l f ) describes the quantum correction to the conductivity in 2D systems. The l f obtained from the t of Ds(H) of the different LSMO lms is plotted in Fig. 7g. The l f values of the LSMO lms are larger than the lm thickness, which indicates that the charge carriers are conned in 2D. In addition, these l f values are consistent with that for the previously reported LSMO lms grown on SrTiO 3 . 32 The increase in l f with the LSMO lm thickness is consistent with the formation of ferromagnetic and conducting domains with an increase in the LSMO lm thickness on LAO.
According to Hund's rule, the e g 1 electron of Mn 3+ in manganites couples ferromagnetically to the local spin of the This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 2792-2799 | 2797 Paper Nanoscale Advances t 2g 3 orbital. 1 However, the thermal energy induces the spin reorientation of the e g 1 electrons, which increases the magnetic disorder and requires a high magnetic eld to reduce the spindependent scattering, i.e., colossal magnetoresistance. 33,34 The spin-dependent scattering, i.e., the resistivity of LSMO, was reduced to a signicant fraction by the application of a < 1000 mT magnetic eld, which is associated with a large number of grain boundaries having a non-collinear Mn ion spin structure. 35,36 Thus, the low eld MR of the LSMO lms can be attributed to the non-collinear spins of Mn near the LSMO-LAO interface. Further, the observed LFMR in LSMO/LAO is considerably larger than that in the ferromagnetic manganite/ spacer/ferromagnetic manganite trilayers, which consist of articial grain boundaries at the interface. [11][12][13] The strain analysis indicates that the effect of strain on the LSMO lms with a thickness of 100 to 500 A is the same. However, the variation in M(T), r(T,H), and MR(T,H) with lm thickness conrms that the orientation of the Mn ion spin plays a vital role in the physical properties of these lms. The spin state of the Mn ions in LSMO on LAO is controlled by MnO 6 , which in general can be inuenced by the twin boundaries, strain, and oxygen vacancies.
In conclusion, a series of La 0.7 Sr 0.3 MnO 3 thin lms of different thicknesses were grown on (001) oriented LaAlO 3 by adopting the pulsed plasma deposition method in a sputtering system. The reciprocal space mapping indicated that the in-plane growth of LSMO is relaxed; however, the symmetric XRD scan conrmed the strained growth along the out-ofplane direction with tetragonal structure. The consequences of the LSMO-LAO interface effect appear in the T C , temperature-dependent resistivity, metal-to-semiconductor transition temperature, and MR. At a magnetic eld of 7 T, the MR decreased from 99.8% to 7.69% as the LSMO thickness increased from 200 A to 500 A. A unique characteristic of the LSMO lms is the large low-eld MR aer decreasing the eld from the maximum eld. The observed ZFC and FC M(T) of the LSMO thin lms grown on LAO provide direct evidence that the low eld MR in LSMO lms on LAO is associated with a large number of grain boundaries having non-collinear Mn ion spins. Thus, the present work demonstrates the great potential of interface and large low-eld MR, which may advance the fundamental applications of orbital physics and spintronics.

Conflicts of interest
The authors declare no competing nancial interest.