Exchange Bias Effect in LaFeO3: La0.7Ca0.3MnO3 Composite Thin Films

Composite thin films arouse great interests owing to the multifunctionalities and heterointerface induced physical property tailoring. The exchange bias effect aroused from the ferromagnetic (FM)–antiferromagnetic (AFM) heterointerface is applicable in various applications such as magnetic storage. In this work, (LaFeO3)x:(La0.7Ca0.3MnO3)1−x composite thin films have been deposited via pulsed laser deposition (PLD) and the exchange bias effect was investigated. In such system, LaFeO3 (LFO) is an antiferromagnet while La0.7Ca0.3MnO3 (LCMO) is a ferromagnet, which results in the exchange bias interfacial coupling at the FM/AFM interface. The composition variation of the two phases could lead to the exchange bias field (HEB) tuning in the composite system. This work demonstrates a new composite thin film system with FM-AFM interfacial exchange coupling, which could be applied in various spintronic applications.


Introduction
Exchange bias (EB) describes the pinning of the magnetic dipoles due to the interfacial exchange coupling between ferromagnetic (ferrimagnetic, FM) and antiferromagnetic (AFM) components after cooling through Néel temperature (T N ) of the antiferromagnets, which results in the shift of the hysteresis loop. The EB field (H EB ) is used to evaluate the EB effect, which could be defined as H EB = |H + + H − |/2, where H + and H − are the positive and negative coercivity in the hysteresis loop, respectively. The EB phenomenon was first observed in the AFM CoO nanoparticle wrapped by an FM Co shell, reported by Meiklejohn and Bean in 1956, and the Meiklejohn-Bean model has been widely accepted to explain the mechanism of EB effect [1]. The EB effect could be employed in a lot of applications, such as magnetoresistive read heads, magnetic sensors, spin valve devices, as well as high-density data storage [2][3][4][5]. Thus far, the EB effect has been realized in various material systems and geometries, such as granular composites, core-shell nanoparticles, nanocluster arrays with capping layer and thin films [6][7][8][9].
Among all the different nanostructures with EB effect, the thin film form might be the most promising for practical applications, and a tremendous research effort has been devoted to exploring the EB effect in thin films. Typically, the FM/AFM exchange coupling in thin film could be achieved in the following forms: multilayer thin film, vertically aligned nanocomposite thin film, solid solution (or composite) thin film or even a singlephase thin film [10][11][12][13]. Multilayer is the most conventional approach to obtain the EB effect, which alternatively grows FM and AFM layers to create a transverse FM/AFM interface. The multilayers could be stacked by different material combinations, such as metal/alloy, metal/oxide and oxide/oxide, for example. Co/CuMn bilayer, Fe/FeO bilayer and La 0.7 Sr 0.3 MnO 3 (LSMO)/BiFeO 3 (BFO) bilayer have been designed to investigate their EB effect [14][15][16]. The H EB could be tailored by tuning the number of layer and the thickness of each layer, for example, the relationship between H EB and the thickness of the FM layer (t FM ) is derived as H EB ∝ 1/t FM [17]. The composite thin film evolving FM and AFM components is another platform to achieve EB, which forms a unique nanostructure of one phase exhibited as nanopillars embedding into another matrix phase, termed as vertically aligned nanocomposite (VAN) thin film. Different from the transverse FM/AFM interface in multilayers, VAN thin film presents FM/AFM coupling along the vertical direction, which is beneficial to realize perpendicular EB effect. Various oxide systems have been explored, such as LSMO:NiO, LSMO:LaFeO 3 (LFO), NiO:NiFe 2 O 4 (NFO), BFO:Fe 3 O 4, etc. [18][19][20][21][22]. Similar lattice parameters/crystal structures and phase immiscibility are required to form such VAN structure, otherwise it is highly possible to be phase mixed as a solid solution or composite thin film. Solid solution thin films with FM and AFM phases can also induce the EB effect by the FM/AFM exchange coupling in nanoscale interface [12]. Lastly, the EB effect has been observed even in single-phase thin films, which is induced by either defects [23], strain [24] or some unusual interfaces [13,25]. Overall, although the EB effect has been obtained in variety of thin films with different nanostructures, there are still plenty of material systems and geometries needing further investigation.
In this work, a new FM-AFM system has been developed for the EB effect, e.g., LFO and La 0.7 Ca 0.3 MnO 3 (LCMO) are selected as the AFM phase and FM phase, respectively. We deposited LFO x :LCMO 1−x (x = 0.33, 0.5, 0.67) composite thin films by pulsed laser deposition (PLD) and investigated their EB effect; the thickness of the films was controlled at~200 nm. Both LFO (a = 3.940 Å, pseudo-cubic) and LCMO (a = 3.867 Å, pseudocubic) present perovskite structures with lattice parameters close to the selected STO (001) substrate (a = 3.905 Å, cubic). LFO is a typical AFM material (T N = 710 K) and has been applied in various systems with an EB effect [19,26,27], while LCMO is a widely studied FM material (Curie temperature T C = 225 K). [28] Therefore, the EB effect is expected in the LFO:LCMO composite thin film, and the H EB value can be tailored by varying the composition of the two phases.

Materials and Methods
Target Preparation and Thin Film Deposition: LFO x :LCMO 1−x (x = 0.33, 0.5, 0.67) targets were made by a conventional solid-state mixing of the LFO and LCMO powders with the designed ratio, high-pressure pressing into a 1-inch pellet and followed by a sintering process at 1200 • C for 10 h. Then, the thin films were deposited using a pulsed laser deposition (PLD) system with a KrF excimer laser (Lambda Physik, λ = 248 nm). The detailed deposition parameters are as follows: base pressure was below 1 × 10 −6 Torr, 45 Pa of high-purity O 2 was inflowed into the chamber during deposition, the deposition temperature was 750 • C, the deposition frequency was 5 Hz, target-substrate distance was 4.5 cm and laser energy was 1 mJ/cm 2 ; after deposition, 30 kPa O 2 was inflowed into the chamber and the samples were cooled down at 10 • C/min. Microstructure Characterizations: The crystal structure of the films was characterized by X-ray diffraction (XRD) (Panalytical X'Pert X-ray diffractometer). The surface morphology of the films was characterized by atomic force microscopy (AFM, Bruker Icon AFM).
Physical Property Characterizations: Temperature dependence of ZFC and FC magnetization (M-T, 5-380 K) and magnetic hysteresis curves (M-H, field along the direction perpendicular to the film surface) were carried out by using a vibrating sample magnetometer (VSM) in the physical property measurement system (PPMS: Quantum Design).

Results and Discussion
The main aspect of this work is to grow a new FM-AFM system with a tunable EB effect by composition variation of the two phases. First, standard θ-2θ XRD scans were characterized on the LFO:LCMO thin films with different compositions, as shown in Figure 1a-c for LFO 0.33 LCMO 0.67 , LFO 0.5 LCMO 0.5 and LFO 0.67 LCMO 0.33 , respectively. Only LFO (00l) and LCMO (00l) peaks can be observed, which indicates the textured growth of both phases and that no impurity is formed in the composite films (note that the peak at~42 • is from the instrument, not from the samples). To obtain the actual 2θ values of the LFO (002) and LCMO (002) peaks, the local area (45-48 • ) has been enlarged and shown in the right panels of An atomic force microscope (AFM) has been used to investigate the surface morphology of the composite thin films, and the AFM images with 2 µm × 2 µm squares are shown in Figure 2a-       To further explore the magnetic properties of the composite thin film system, temperature dependence of zero-field cooling (ZFC) and field cooling (FC, 1000 Oe) magnetization (M-T) measurements of all the samples with different compositions have been carried out and compared in Figure 5. All the samples show a similar trend. For the FC condition, the magnetization decreases with increasing temperature (10−350 K) monotonically. However, for ZFC condition, magnetization firstly increases to a maximum value (blocking temperature: T B ) and then decreases with increasing temperature. Another feature is the bifurcation between the ZFC and FC curves, which could be used to define the irreversibility temperature (T irr ). The T B value is slightly lower than T irr , which has been observed in many other magnetic systems [29,30]. Overall, the LFO:LCMO composite thin films presents an interesting magnetic response; specifically, the large H EB values demonstrate a strong exchange interfacial coupling between the AFM LFO phase and FM LCMO phase. This new FM-AFM design provides more insights into how to develop composite thin films with the EB effect, and how to tune and optimize the H EB values via deposition condition optimization and composition variation to generate different nanostructures. To further explore the magnetic properties of the composite thin film system, temperature dependence of zero-field cooling (ZFC) and field cooling (FC, 1000 Oe) magnetization (M-T) measurements of all the samples with different compositions have been carried out and compared in Figure 5. All the samples show a similar trend. For the FC condition, the magnetization decreases with increasing temperature (10−350 K) monotonically. However, for ZFC condition, magnetization firstly increases to a maximum value (blocking temperature: T B ) and then decreases with increasing temperature. Another feature is the bifurcation between the ZFC and FC curves, which could be used to define the irreversibility temperature (T irr ). The T B value is slightly lower than T irr , which has been observed in many other magnetic systems [29,30]. Overall, the LFO:LCMO composite thin films presents an interesting magnetic response; specifically, the large H EB values demonstrate a strong exchange interfacial coupling between the AFM LFO phase and FM LCMO phase. This new FM-AFM design provides more insights into how to develop composite thin films with the EB effect, and how to tune and optimize the H EB values via deposition condition optimization and composition variation to generate different nanostructures.

Conclusions
A new composite thin film system of LFO:LCMO with different compositions has been grown and characterized in order to achieve a tailorable exchange bias effect by composition variation. The composite thin films show excellent crystal quality with c-axis growth for both the LFO and LCMO phases. Furthermore, an obvious shift of the hysteresis loops has been observed for all the films, while the LFO 0 . 67 LCMO 0 . 33 sample obtains the strongest FM-AFM interfacial exchange coupling. The H EB values of such film are estimated as 891 Oe, 459 Oe, 162 Oe and 28 Oe at 10 K, 50 K, 150 K and 250 K, respectively, which is relatively large compared to other oxide-oxide composite thin films. Furthermore, both blocking temperature and irreversibility temperature can be determined from the FC and ZFC M-T measurements.

Conclusions
A new composite thin film system of LFO:LCMO with different compositions has been grown and characterized in order to achieve a tailorable exchange bias effect by composition variation. The composite thin films show excellent crystal quality with caxis growth for both the LFO and LCMO phases. Furthermore, an obvious shift of the hysteresis loops has been observed for all the films, while the LFO 0.67 LCMO 0.33 sample obtains the strongest FM-AFM interfacial exchange coupling. The H EB values of such film are estimated as 891 Oe, 459 Oe, 162 Oe and 28 Oe at 10 K, 50 K, 150 K and 250 K, respectively, which is relatively large compared to other oxide-oxide composite thin films. Furthermore, both blocking temperature and irreversibility temperature can be determined from the FC and ZFC M-T measurements.