Phases competition in half-doped La_0.5-xDy_xCa_0.5-ySr_yMnO₃ films

Perovskite-type manganese are RE_1-xAE_xMnO₃ (RAMO, where RE=rare earth element, AE=divalent alkaline cation) is a typical strongly correlated electronic system.^1–3 Due to the coexistence of charge, orbit, spin, and lattice, strong and complex interactions have always been a research focus in the field of condensed matter physics. RAMO has rich phase diagrams including ferromagnetic metal phases and insulating phases, as well as interesting magnetic transmission characteristics, such as the huge magnetoresistance effect.⁴ Both the Mn³⁺-O-Mn⁴⁺ double exchange theory and the Jahn-Teller distortion are related to these properties. RAMO has become one of the most suitable candidates for basic research and technology applications.^5,6 In recent years, magnetoresistive transducers, spintronic devices, magnetic recording devices, and some sensitive devices, such as radiometers or infrared detectors have been widely developed and applied.^7–11 

Half-doped manganese is at the boundary of phase transition from ferromagnetic (FM) metal to charge ordered (CO) antiferromagnetic (AFM) insulator. The FM double exchange and AFM super exchange interactions maintain a delicate balance in these materials, which makes their physical properties very sensitive to external disturbances. Below the charge ordering temperature, the structure changes from orthorhombic to monoclinic. L_a0.5Ca_0.5MnO₃ and shows a paramagnetic to ferromagnetic transition at ∼225 K, followed by an orderly transition of charge at ∼155 K. Relevant studies have shown that the order state of the antiferromagnetic insulating charge can be adjusted to the ferromagnetic (FM) metal phase by external disturbances such as electric field, magnetic field, pressure and doping.^12,13 In addition to the distortion of the crystal structure caused by the mismatch of the A-site ion radius, in the case of thin films, the strain caused by the lattice-substrate mismatch will also cause additional interface lattice distortion. Therefore, the strain dependence of thin films is worthy of further study.

The strain resulted from the mismatch between the substrate and the film causes the magnetic and transport characteristics of the film to deviate from the bulk performance. Materials of AMnO₃ with Dy³⁺ at A-site shows complex and fascinating magnetic properties because of the interaction between Dy³⁺ and Mn³⁺/Mn⁴⁺ ions.^14–21 There are two magnetic transition temperatures in these systems, with the first-order transition from paramagnetic (PM) to ferromagnetic (FM) at ∼180 K, and the second-order transition from AF to ferromagnetic (FM) at ∼90 K. Tejaswini et al. reported that the Curie temperature T_C of Pr_0.8-xDy_xSr_0.2MnO₃ was decreased by Dy³⁺-doping for the structural distortion resulted from the smaller ionic radium of Dy³⁺ compared with that of Pr³⁺.²² In Dy_0.5(Sr_1-xCa_x)_0.5MnO₃, the spins of Dy³⁺ kept antiferromagnetic at low temperatures and no spin-glass state was found in these systems.²³ The ferromagnetism of Dy-doped La_0.7Sr_0.3CoO₃ system was strengthened by Dy³⁺ doping originated from the coupling of spin-phonon.²⁴ In the Sr³⁺-doped La_0.7Sr_xCa_0.3-xMnO₃ sample, it was found that ferromagnetic transition was related to the abnormality of the elastic modulus.²⁵ In our previous work, we have reported that the disorder at the A-site increases the Jahn-Teller distortion in co-doped La_0.5-xDy_xCa_0.5-ySr_yMnO₃ (0≤x≤0.2) polycrystalline samples and thereby suppresses the metal behavior and MR effect.²⁶ 

In this paper, La_0.5-xDy_xCa_0.5-ySr_yMnO₃ (LDCSMO) thin films with x = 0.1 and y = 0.102 were prepared on STO substrates. The dopant content of Dy and Sr is selected to keep the Mn³⁺/Mn⁴⁺ ratio in the LDCSMO unit cell and the average A-site ion radius being the same as the La_0.5Ca_0.5MnO₃ (LCMO) unit cell. The random and disordered distribution of RE³⁺ and AE²⁺ cations at A-site in LDCSMO is defined as σ²=∑y_ir_i² − ⟨r_A⟩². The random disorder parameters of σ² are 0 (LCMO) and 0.0025 Å (LDCSMO), respectively. The effects of strain on the structure, magnetic and electrical properties of LDCSMO films with different thicknesses are studied.

The polycrystalline target of LDCSMO was prepared by standard solid-state reaction with stoichiometric amounts of La₂O₃, Dy₂O₃, CaCO₃, Sr₂O₃, and MnO₂. The LDCSMO pure phase target was obtained by synthesizing raw materials under intermediate grinding conditions in air at 1200°C for 6h, 1300°C for 16h, and 1350°C for 24h. LDCSMO films were deposited on single crystal STO (001) substrates by pulsed laser deposition (PLD) technology. A 248 nm KrF excimer laser was used for deposition at 2 Hz, the substrate temperature was 700°C, and the oxygen pressure was maintained at 15 Pa. The deposited films were annealed in situ for 40 minutes in 15 Pa of flowing oxygen. The thicknesses of the films were regulated by controlling the depositing time. The thickness of the film varies from 52.66 nm to 157.98 nm. The crystal structure and epitaxial orientation were characterized by conventional X-ray diffraction (XRD) techniques. Raman spectroscopy is measured in a backscattering geometry using an Ar⁺ excitation source (λ=633nm). Zero field cooling (ZFC) and field cooling (FC) DC magnetic properties are measured parallel to the sample surface using a superconducting quantum interferometer (SQUID) magnetometer under 500 Oe magnetic field in the temperature range of 5–300 K. The M-H hysteresis loop of the LDCSMO film is measured at 5 K and 50 K. The physical property measurement system (PPMS) was used to measure the temperature dependence of resistivity in the temperature range of 5-300 K with an applied magnetic field of 0, 2, 4, and 6T.

Figure 1 (a)-(d) shows the θ-2θ XRD pattern of the LDCSMO film deposited on the (001)-oriented STO substrate. It can be clearly seen that in addition to the diffraction peaks of the substrate STO, only the peaks of the sample (00l) are observed. This shows that the sample grows co-epitaxially according to the lattice of the STO substrate. By increasing the film thickness from 52.66nm to 157.98nm, the intensity, position and width of the (004) peak of the LDCSMO film have undergone some obvious changes. It is expected that the peak intensity will increase because the amount of diffractive material is increased by increasing the film thickness. The (004) LDCSMO peak moves to a smaller angle, which corresponds to the increase of the out-of-plane parameter c. The out-of-plane lattice parameters of the LDCSMO film are calculated based on the position of its (004) diffraction peak, and are calibrated by the STO diffraction peak (Figure 1 (e)). Amount of strain was calculated from ε_zz= (c_f − c_b)/c_b relation, where c_f and c_b are the out-of-plane lattice constants of the film and the substrate, respectively. In addition, the in-plane strain can be converted by the Poisson relationship ε_xx =-(1-ν)ε_zz/2ν, where ν is the Poisson constant of the LCMO structure, which is about 0.35.²⁷ These parameters are listed in Table I. The out-of-plane lattice parameter c of the films increases from 3.832 Å (52.66 nm) to 3.896 Å (157.98 nm), with the increase of the film thickness, indicating that the LDCSMO film has in-plane tensile strain. The in-plane tensile strain is relaxed gradually as the film thickness increases. Since the in-plane lattice parameter of LDCSMO is smaller than that of STO ∼3.905Å, the deposition of LDCSMO on the STO substrate produces in-plane tensile strain. The effects of tensile strain on structural distortion, the magnetic and transport properties of semi-doped manganese films were studied in the following.

In order to study the microstructural distortion of the LDCSMO film, the Raman spectroscopy of thin films were investigated. The laser uses the Ar⁺ excitation source and the laser wavelength is 633 nm. Raman spectroscopy was performed on samples with backscattering geometry at room temperature. Figure 2 shows the Raman spectrum of LDCSMO films with different thickness. As seen from Figure 2, it is found that the Raman spectrum of the LDCSMO film on the STO substrate is mainly dominated by two peaks, located at approximately 245 cm^-1 (ν₁) and 620 cm^-1 (ν₂), respectively. The ν₁ mode corresponds to the rotational vibration mode of the MnO₆ octahedron (A_g (2)). The frequency of the v₁ mode decreases with the decrease of thickness. These shows that the rotation of MnO₆ may be suppressed in thinner films. The LDCSMO film grown on STO is subjected to tensile strain, the in-plane lattice expands due to the tensile strain, while the out-of-plane lattice shrinks. This anisotropic deformation of the crystal lattice may cause the tilt of the MnO₆ octahedron. According to related reports, the Raman spectra of LCMO films have characteristic peaks near 450 cm^-1 and are more obvious at low temperatures.²⁸ However, this vibration mode does not exist near room temperature in the figure, which may be caused by the increase of octahedral bending of MnO₆. The ν₂ mode at 620 cm^-1 has B_2g symmetry, which is assigned to the stretching vibration of the in-plane oxygen atoms around the Mn atoms in the ab plane. It can be seen from the figure that as the thickness decreases, the v₂ mode position shifts to low frequency. The weakened frequency with decreasing thickness of the films may mean the in-plane Mn-O bond stretching and MnO₆ distortion were suppressed.²⁹ 

Figure 3(a)-(c) shows the temperature (T) dependence of the ZFC and FC magnetization (M) of the LDCSMO films with three different thickness samples at 500 Oe. The value of ferromagnetic (FM)-paramagnetic (PM) transition temperatures T_C is defined as the beginning of the FM transition in FC-M(T) curves. The T_C values of LDCSMO films with thicknesses of 157.98nm, 78.99nm and 52.66nm are 235k, 255k and 273k. T_C increases with the decrease of film thickness. Raman analysis has showed that the in-plane Mn-O stretching and MnO₆ rotation were suppressed by the in-plane tensile strain, which will increase the electron bandwidth, and the double exchange (DE) interaction of Mn³⁺-O-Mn⁴⁺ will be enhanced. The increased DE interaction is beneficial to enhance the FM transition temperature T_C. With temperature decreasing further, the separation between the FC and ZFC curves is found below T_C. This spin-glass effect may be related to the degree of magnetic randomness and the short-range order of spins in FM and AFM coexisting system.³⁰ As the film thickness decreases, the temperature T_G at which the ZFC and FC curves begin to bifurcate increases. Figure 3(d) gives the difference of FC-ZFC curve varying with temperature. As seen from Figure 3(d), the difference of FC-ZFC at low temperature increases from 2 emu/cm³ to 8 emu/cm³ with the tensile strain increasing (thickness decreasing), which indicates spin-glass effect becomes more and more obvious. This behavior shows the fierce competition between FM and AFM order and/or spin disorder becomes much stronger. As seen from the ZFC curves of all the films with different thickness. It can be clearly seen that the charge-ordering temperature (Tco) is obvious at about 50 K. This is a sign of the beginning of a tilted antiferromagnetic sorting with orderly correlation of charges. Compared with the FC curve, the ZFC curve always has a peak lower than Tc. The obvious decrease in magnetization of the ZFC curve in the low temperature section can be explained as follows. In the zero-field cooling, the FM clusters are frozen into a random orientation, at the low temperature and 500 Oe external magnetic field cannot overcome the local anisotropy field. However, as the temperature increases, the spin freezing effect decreases, and the magnetization begins to show an upward trend. Therefore, the ZFC-FC curves of the three pictures show an interesting “λ” shape. It is worth noting that Tco showed a slight downward trend in thinner film, which is consistent with the previous increase of Tc. Therefore, the FM ordering is enhanced while the AFM is suppressed with the thickness of film decreasing, and the AFM order FM order coexists in all the films. The improved FM ordering may be resulted from the enhancement of DE interaction because of the MnO₆ distortion suppressed in thinner film under the in-plane tensile strain.

In order to further study the coexistence and competition of FM and AFM phases, the M-H hysteresis loops of films with different thicknesses were measured at low temperatures of 5 K and 50 K, as shown in Figure 4. It is found that there is obvious hysteresis phenomenon and no saturation magnetization for all films even under the highest applied magnetic field of 40 kOe. This observation indicates AFM and FM phases may coexist in all films. The M-H curve measured at 5K clearly shows an S shape, and there is an open loop under a high magnetic field. There is a small hysteresis near the origin, which clearly shows the AFM ordering is the dominant phase, and the whole S shape indicate weak FM ordering exists. The value of the coercive force field increased from 507 Oe (157.98nm) to 704 Oe (52.66nm). This clearly shows that the strain-enhanced DE interaction leads to a stronger pinning center of the FM domain, which leads to an increase in coercivity. The M-H curve measured at 50 k, as shown from Figure 4(b), shows the hysteresis loop under low magnetic field, which show week FM ordering. The coercivity field is about 125 Oe, which is independent of thickness. With the further increase of the magnetic field (≥1.5 T), the magnetization increases linearly under a higher magnetic field, which implies the existence of AFM. Therefore, the week FM at low field may be resulted from the canted CO-AFM at temperature of 50 K. Furthermore, the magnetization at high magnetic field is increased obviously with decreasing the thickness of the film at both 5 K and 50 K, which is also related to the enhanced FM phase imbedded in AFM matrix induced by the in-plane tensile strain. The magnetic ordering could be controlled by the structural distortion, which indicates spin-phonon coupling in strained LDCSMO films.

In order to understand the strain dependence of the electric and magnetic ground state of the films more deeply, the electrical transport properties of the films were measured under an applied magnetic field. Figure 5 shows the temperature dependence of the resistivity curves of 52.66nm and 78.99nm films under different applied magnetic fields (0, 2, 4, and 6 T). By increasing the magnetic field, the resistivity drops sharply. The reason is that the magnetopolaron fluctuates with an external magnetic field applied, as is found in other manganese oxides.^31,32 The resistivity of film with 52.66nm is smaller than that of thick film with 78.99nm. Figure 5 (a) shows that the resistivity temperature curves of the film with 52.66nm show insulator behavior in the whole temperature range. And there is only a slight change in the range of 2T-6T for the thinner film. For films with thickness of 78.99nm, however, the effect of the external magnetic field on the resistivity of the film is more obvious. It can be seen from Fig. 5(b) that when a small external magnetic field is applied, the film sample is still in an insulated state over the entire temperature range. When the applied magnetic field is increased to 4T and 6T, there is a metal insulator transition (T_MI) at about 100 k. It is found that T_C (∼250 k) is higher than T_MI. This is consistent with the fact that in the theoretical framework of double exchange interaction,³³ the Curie temperature T_C is higher than T_MI.³⁴ Therefore, the conductivity is weakened with thickness decreasing, while the FM ordering was enhanced at low temperature. These unconventional behaviors may be related to the disordering state resulted from the stronger FM and AFM competition and the dominated “dead layer” in thin film.³⁵ At the temperature of T_MI, the magnetoresistance (MR) values of film with 78.99 nm are increased from 42% (2T) to 128% (6T) with magnetic field increasing, which is consistent with the reports in literature.³⁶ Because the resistivity of the film of 52.66 nm exceeds the measurement limit of the instrument for temperature below 120 K. The MR value of the two samples at the temperature of 150 K under 4T was calculated typically for comparation. The MR values of the films are 27% (78.99 nm) and 62% (52.66 nm), respectively. As the film thickness decreases, the MR increases accordingly. This can be attributed to the fact that in-plane tensile strain will increase the Mn³⁺-O-Mn⁴⁺ double exchange in the system by suppressing the MnO₆ distortion, thereby weakening the magnetic scattering effect and enhancing the MR effect.

Effects of strain on the electrical and magnetic properties of LDCSMO films epitaxially grown on STO substrates were studied in details. Films were well deposited along [001] direction. As the thickness of the LDCSMO film decreases, the in-plane tensile strain of the film increases. X-ray diffraction and Raman scattering analysis show that the MnO₆ distortion is suppressed, which induces the improvement of Mn³⁺-O-Mn⁴⁺ double exchange with tensile strain increasing (thickness decreasing). It was found CO-AFM insulating phase and FM metal phase coexist at low temperature in all films. The FM ordering was enhanced and spin-glass effect becomes more obviously, which is attributed to improved double-exchange interaction of Mn³⁺-O-Mn⁴⁺ in thinner films. The weakened conductivity of thinner film may be related to the stronger FM and AFM competition and the disorder caused by the dominant “dead layer” in the film.

The financial support for this project was from the National Natural Science Foundation of China (Grant No. 11404091), Natural Science Foundation of Jiangsu Province (Grant No. BK20140839) and the Fundamental Research Funds for the Central Universities (Grant No. B200203027).

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