The effect of stress on the magnetic properties of low bandwidth manganites

The low bandwidth Pr 0.9 Ca 0.1 MnO 3 (PCMO) thin lms prepared by sol-gel method were suffered tensile and compressive stress grown on SrTiO 3 and LaAlO 3 substrates. The hysteresis loops at different temperatures show that the coercivity eld with tensile stress is larger due to the much stronger pinning potential of ferromagnetism motion. The temperature dependence of the ZFC and FC magnetizations indicates that the stress signicantly affects the ferromagnetic (FM) and antiferromagnetic (AFM) transition temperature of PCMO, and the Curie temperature (T C ) decreases with tensile stress. The lms show strong anisotropy properties that the magnetization increases much faster with the magnetic eld when H ⊥ c, but the coercive eld and saturation magnetization do not change signicantly. In addition, the persistent photoinduced magnetization is investigated, and signicant improvement of the ferromagnetic ordering was observed in low temperature.


Introduction
The perovskite manganese oxide with the general formula R 1 − x A x MnO 3 (R = La, Pr, Nd, etc., A = Ca, Sr, Ba, etc.) has a high correlation between charge, spin, lattice, and orbital degrees of freedom, which leads to a variety of exotic behavior, such as colossal magnetoresistance (CMR), photoinduced change of magnetization and resistivity, etc [1][2][3][4]. It is not only interesting from the application point of view but also broadens the scienti c understanding about the material properties. The low bandwidth Pr 1 − x Ca x MnO 3 (PCMO) is especially interesting due to the multiple magnetic phases and sensitive magnetic properties [5][6][7][8]. At the low-hole doping content x = 0.1, the valence states of Mn are mixed, and both the ferromagnetic (FM) and antiferromagnetic (AFM) phases coexist in the ground state [9]. The double exchange interaction between these Mn 3+ and Mn 4+ leads to FM, whereas the superexchange interaction between Mn 3+ -Mn 3+ and Mn 4+ -Mn 4+ leads to AFM ordering in manganites oxides [10]. The structure, both the Mn-O-Mn bond length and angle, will strongly affect the electronic transition between Mn 3+ and Mn 4+ , thereby changing its magnetic properties. Meanwhile, stress is an effective means to control the crystal structure of manganese oxide [11][12][13]. The epitaxial lm on different substrates can control the transition from compressive stress to tensile stress, affect its Mn-O bond and regulate magnetic properties. The changes in magnetic properties caused by the structure will inevitably be re ected in the anisotropic physical properties [14][15]. The low doped PCMO lms also have a great magnetization change under light [10,16,17]. Light can change the FM domain motion or volume fraction in AFM matrix, which leads to the change of magnetization signal. Although relevant researches have been done, there is few reports on the stress effect systemically on magnetic and magneto-optical properties.
In this paper, in order to attain a detailed understanding of the substrate-induced strain effects in c-axisoriented Pr 1 − x Ca x MnO 3 (PCMO) lms, we used SrTiO 3 (STO) and LaAlO 3 (LAO) single crystals as substrates and studied the effects of substrate-induced strain on the magnetic properties, anisotropic and photoinduced magnetization of PCMO lms. The experimental results indicate that, the in-plane tensile strain was 0.60% for PCMO/STO lms and the in-plane compressive strain in the lm was 0.21% for PCMO/LAO lms respectively. It is giving rise to the distortion of MnO 6 octahedra and an increase of T C by 3 K. At the same time, the magnetization of FM state is strengthened by the compressive strain while photoinduced magnetization in the AFM state is enhanced. The strain effects are found to be closely related to the Jahn-Teller electron-lattice coupling linked to magnetic properties. deionized water and acetylacetonate to prepare the precursor solution. The rotation speed and the spin time were at 5000 rpm and 30 s, respectively. After coating each layer, the lm was red at 200 o C for 180 s, then pyrolyzed at 400 o C for 180 s, and nally annealed at 850 o C for 600 s. The cycle was repeated several times in order to get lms with a nal thickness around 120 nm. Detailed information about material preparation and the structural properties of the lm has been reported in our previous correspondence [18].The phase purity and structure of the lms were measured q -2q and reciprocal space mapping by high-resolution X-ray diffraction (HRXRD, Bruker D8 Discover, Germany).

Experimental
Superconducting quantum interference device (SQUID, Quantum Design, USA) magnetometer was used for the magnetization characterization. The magnetic eld is usually perpendicular to the out-of-plane of the thin lm besides both perpendicular and parallel direction in magnetic anisotropy part. The magnetooptical magnetization was made for the sample in the dark and illumination with the magnetic eld parallel to the out-of-plane of the thin lm and laser working at l = 500 nm by the FOSH option of the SQUID.

Results And Discussion
Figure 1 (a) shows the q -2q patterns of PCMO lms grown on both substrates. It is observed only the (00l) diffraction peaks of the PCMO lms and the substrates, which means the phase purity and the epitaxial grown of the PCMO lms on both STO and LAO substrates. Compared with the sample PCMO/LAO, the (002) diffraction peak of the PCMO/STO shifts to a higher angle, which indicates that the out-of-plane lattice parameters of PCMO/LAO is larger. The lattice parameter of PCMO ceramics (0.3854nm) is between the STO substrate (0.3905nm) and the LAO substrate (0.3792nm), which makes the PCMO subject to different stresses on these two substrates. The presence of stress will cause distortion of the lattice structure, affect the bond length and bond angle of Mn-O-Mn, and then change the magnetic properties of PCMO. The stress state of the lm can also be seen through reciprocal space mapping in Fig. 1 (b) and (c). The abscissa of PCMO/STO lm is almost consistent with that of substrate, indicating that the sample is in stress bound state, while that of PCMO/LAO has a certain deviation, which indicates that the structure relaxation of the lm. The calculated values of the lattice parameters are the following, a = 0.3846nm, c = 0.3859nm (PCMO/LAO), and a = 0.3877nm, c = 0.3845nm while PCMO/STO is under tensile strain. The strain will result in the distortion of the lattice structure and change the lattice constant of the MnO 6 octahedra. Hence, the magnetic properties will be discussed in detail due to the different stress states of PCMO grown on LAO and STO substrates.
The M-H curves of both PCMO/LAO and PCMO/STO at different temperatures are shown in Fig. 2. The saturation magnetization is almost the same at different temperatures, while the coercive eld and remanent magnetization decrease with the increase of temperature. The temperature dependence of coercive eld (Fig. 2c) shows that the coercive eld of PCMO/LAO is smaller than PCMO/STO, but the change trend is similar. At 5K, the coercive eld is 760 Oe and 870 Oe for PCMO/LAO and PCMO/STO, respectively. However, the coercive eld of the two substrates is similar at 50K, about 300Oe. The temperature dependence of the coercivity is related to the competition between AFM and FM in the spinglass (SG) state [21]. In the ground state, the AFM and FM phases are coexisted in our low doped manganites. As PCMO/STO is subjected to tensile from the substrate, thereby static lattice stress increases the Mn-O bond length while decreasing the Mn-O-Mn bond angle [22,23]. Tensile strain suppresses ferromagnetism by a strain induced distortion of MnO 6 octahedra. This leads to the enhancement of the pinning effect of the AFM phase fraction on the FM domain movement in the PCMO/STO lm, thereby observing a higher coercive eld at low temperatures [24,25]. As the temperature gradually approaches the Neel temperature (T N ), the volume fraction of AFM begins to decrease, the pinning effect weakens, and the coil begins to become sharper. On the other hand, the coercive eld of PCMO/LAO is smaller, which may be due to the weaker pinning effect caused by the inplane compressive stress. When the temperature rises, the SG state gradually disappears, and the gap between the lms on two kinds of substrate is no longer obvious.
To observe the relationship between stress and AFM-FM phase transition, we have measured the zero eld cooling (ZFC) and eld cooling (FC) magnetization of PCMO/STO and PCMO/LAO under series applied magnetic elds in Fig. 3(a) and Fig. 3(b). It can be observed that the magnetization increases rapidly with the decrease of temperature below 120K, which indicates that PCMO lms begin to transform from paramagnetic state to FM state. As the temperature decreases more, the ZFC magnetization begins to decrease. The result shows that AFM phase and FM phase coexist in PCMO lms at low temperature [21,25]. Comparing the M-T curves of PCMO/STO and PCMO/LAO (Fig. 3(c)), the magnetization of PCMO/STO is much smaller than that of PCMO/LAO. The phenomenon may be due to tensile strain suppresses ferromagnetism in CMR thin lms [23]. Figure 3(d) shows the dependence of T C and T N on magnetic eld for the two samples. The tensile strain reduces the T C of PCMO by 3K compared with the compressive strain. It is generally caused by the strain induced distortion of MnO 6 octahedra [26]. For PCMO thin lms in both stress states, the T N shifts towards lower temperature for higher applied magnetic eld. As the magnetic eld increases to 1000Oe, T N of PCMO/LAO and PCMO/STO is close (about 45K), which is consistent with the phenomenon that the coercive eld is equal at 50K observed in Fig. 2. When the magnetic eld is lower than the coercive eld, the T N is affected by the stress as the same trend with T C . However, when the magnetic eld is higher than the coercive eld, the T N of PCMO with tensile stress is higher than that with compressive stress. Besides the phase transition temperature both T C and T N , the magnetic moments below T C of PCMO with tensile stress is smaller than that with compressive stress due to the distortion of MnO 6 octahedron.
In order to better understand the magnetic properties of PCMO, we studied the magnetic anisotropy of PCMO lms. The hysteresis loops of PCMO/LAO at two directions are shown in Fig. 4(a). The magnetization curves show clear anisotropy behavior at lower elds and isotropic at higher elds. Along H⊥c direction, the magnetization increases steeply at low elds (< 20 kOe), and gradually reaches saturation with the increase of magnetic eld. It is clear that the in-plane magnetization (H⊥c ) is now much easier to be saturated than the perpendicular magnetization [27]. For PCMO/LAO, the in-plane lattice is under compressive strain which will intensify the distortion of MnO 6 octahedra, and the [00l] lattice direction is the easy axis for the magnetocrystalline anisotropy of PCMO/LAO lms [28,29]. Although the magnetization of two directions changed obviously, we observed that the coercive eld H C was almost the same, indicating that magnetic anisotropy has a correspondingly negligible effect on pinning potential of FM domain. Figure 4(b) shows the temperature dependence of the magnetization under 500 Oe magnetic eld. In addition, T N of PCMO/LAO along easy axis is slight lower than that in hard axis.  Fig. 5(a) shows that the coercivity eld decreases under illumination which was explained by the improvement of domain shift of FM clusters [25]. AFM and FM coexist in PCMO thin lms at low temperatures, resulting in a SG state at low temperature [21]. The lattice parameters of these coexisting AFM and FM domains are slightly different which create a blocking strain.
The blocking strain makes FM domain face a lot of pinning potential. When we add external light to the system, the light can provide energy to the system to overcome the pinning potential [21,25]. To observe the photoinduced magnetization with temperature, the ZFC and FC curves were characterized in the dark and during illumination of PCMO/LAO under 1000 Oe and 1500 Oe magnetic eld as shown in Fig. 5 (b) and (c), respectively. The magnetization of ZFC was lager under light at low temperature, but the magnetization of FC was inhibited under light. In order to better analyze this phenomenon, we compared the magnetization of ZFC and FC under light and dark conditions. Figure 5d shows plots of as functions of temperature. The magnetic moment during the ZFC process increases signi cantly under illumination, showing a good photoinduced magnetization effect. However, the magnetization of FC was inhibited under light. This clearly signi es that light can provide energy to overcome the blocking strain produced by AFM and FM domain and improve the FM interaction. However, due to the large JT distortion of PCMO, the 2p(O)→3d(Mn) charge transfer energy can be increased signi cantly, and the energy provided by light can not induce the charge transfer process. The content of Mn 3+ in PCMO will increase slightly, and a small amount of electrons may be transferred to Mn 4+ [30]. The increase of Mn 3+ ions and the decrease of hole concentration under illumination actually increases AFM Mn 3+ -Mn 3+ super-exchange interaction and weakens the FM-DE interaction which reduces the saturation moment of FC [25].

Conclusion
In conclusion, the low-bandwidth manganite Pr 1 − x Ca x MnO 3 (x = 0.1) thin lms with both tensile and compressive stress were prepared by sol-gel method on STO and LAO single crystal substrates. Stress