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LaMnO3] multilayers
Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx
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Research articles
Thickness dependent interfacial magnetic coupling in [La2NiMnO6/ LaMnO3] multilayers Amit Kumar Singh, Ramesh Chandra ⇑ Institute Instrumentation Centre, Indian Institute of Technology Roorkee, 247667, India
a r t i c l e
i n f o
Article history: Received 24 January 2017 Received in revised form 14 August 2017 Accepted 27 August 2017 Available online xxxx Keywords: Epitaxial Multilayers SQUID magnetometer Exchange bias Training effect
a b s t r a c t In the present work, interfacial magnetic exchange coupling at FM/AFM interface has been studied by varying the thickness of AFM layer (LaMnO3) in (La2NiMnO6/LaMnO3)15 multilayer thin film based system. In multilayer thin films, the thickness of LMO was varied from 30 to 50 Å, while the thickness of LNMO was kept constant at 100 Å. Thin films of LNMO, LMO and LNMO/LMO multilayers have been deposited by pulsed laser deposition technique on (0 0 1) LaAlO3 substrate. The thin films have been studied for their structural and magnetic properties. XRD analysis reveals the c-axis epitaxial growth of LNMO, LMO and their multilayer thin films. Exchange bias (EB) effect has been observed in the field cooled hysteresis loops of multilayer thin films and the interaction between FM and AFM spins at the interface is responsible for the observed effect. EB measurements reveal that the thickness variation influences the interfacial interaction between two layers. The EB increases with increasing AFM film thickness in multilayer thin film samples and maximum EB (740 Oe) is observed for sample with 50 Å thickness of AFM layer. Temperature dependence of EB and training effect measurements have also been performed to confirm the EB in the sample. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The interfaces between dissimilar complex oxide materials have been widely studied over the past few decades due to the coupling of the charge, orbital, lattice and spin degrees of freedom of electrons [1,2]. The advancement of techniques for fabricating and characterizing oxide thin films has given a direction for the study of the interfacial effect between perovskite oxides [3,4]. Perovskite based heterostructures composed of magnetically active materials are considering much more attention with discoveries of ferromagnetism at interfaces between two antiferromagnets or even between an antiferromagnet and a paramagnet [5–8]. Thus, it can be realized that epitaxial heterostructures of different complex oxides may offer excellent opportunities to study the rich and fascinating magnetic phenomena at the interface due to the competing interactions. One of the most interesting interfacial phenomenon is the exchange bias (EB) in the heterostructures of ferromagnetic (FM) and antiferromagnetic (AFM) layers [9,10]. When a FM/AFM interface based system is cooled through the Neel temperature (TN) of antiferromagnetic material (TN < TC, TC is the curie temperature of the ferromagnetic material), the hysteresis
⇑ Corresponding author.
loop is now shifted away from the origin [11]. The EB effect manifests itself as a shift in field cooled hysteresis loop along the magnetic field axis and the additional unidirectional anisotropy is attributed to the exchange interaction between the FM and AFM spins at the interface [12,13]. A unidirectional pinning or anisotropy of the magnetization of ferromagnet is observed at the FM/AFM interface due to the effect of EB and can be utilized to control the magnetization in a magnetic reference layer in a spin valve structure or a magnetic tunnel junction (MTJ), which consists of two FM layers separated by a nonmagnetic material (spin valve) or an insulator (MTJ)[14,15]. Among perovskites, ABO3 type LaMnO3 (LMO) compound has attracted much more attention for being a building block in some heterostructures exhibiting fascinating phenomena such as EB in LaNiO3-LaMnO3 superlattices, LaMnO3/SrMnO3 heterostructures and observation of ferromagnetism (FM) at the interface of LaMnO3/SrTiO3 [16–19]. Additionally, bulk LMO is a Mott insulator and an antiferromagnet with a Neel temperature of 140 K. Nevertheless, when grow as a thin film, the magnetic behaviour of LMO is rather conflicting as LMO exhibits ferromagnetic behaviour with a curie temperature TC 200 K [20] as well as it shows antiferromagnetic behaviour with a Neel temperature of TN 131 K, depending on the synthesis conditions [21]. It has been found that in oxygen environment, the epitaxial strain stabilizes the
E-mail address: [email protected] (R. Chandra). http://dx.doi.org/10.1016/j.jmmm.2017.08.080 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
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A.K. Singh, R. Chandra / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx
orthorhombic phase with slightly less Mn4+ in LMO and leaves the film insulating but on the AFM side of the AFM/FM boundary [21]. In recent years, double perovskite (A2BB’O6, A = alkaline earth and B,B’ = transition metal oxides) R2NiMnO6 (RNMO, R = Rare earth) compounds are of considerable interest due to their rich physics and possible application in spintronics [12,22–27]. Among the double perovskites, La2NiMnO6 (LNMO) is well studied due to its near room temperature ferromagnetic transition (TC 280 K) accompanied by magnetoresistance and magnetodielectric effects [22,28,29]. A lot of work on epitaxial growth of LNMO thin film and its magnetic properties is already reported [23,30–32]. Both the LNMO and LMO exhibit perovskite structure with lattice parameters of about 3.88 Å and 3.98 Å. The close lattice parameters of FM (LNMO) and AFM (LMO) layers allow the growth of epitaxial heterostructures with almost atomically perfect interfaces. It has been observed that the EB is inversely proportional to the thickness of FM layer in all the studied systems [11]. However, many discrepancies have been observed in EB behaviour with antiferromagnetic thickness variation. Generally, EB is independent of thickness for thick antiferromagnetic layers and reduces with decreasing antiferromagnetic layer thickness and becomes zero for thin enough AFM layer (a few Å) [11,33]. In some cases, EB decreases with larger AFM thicknesses [34,35]. On the other hand, EB also shows a peak like behaviour with increasing AFM layer thickness in some systems [36,37]. Additionally, the presence of nonmagnetic defects in the AFM layer can enhance the EB [38]. It has also been noticed that AFM domain structure may also affect EB, if the thickness becomes comparable to the AFM domain wall size [11]. Thus, it would be interesting to see the trend and magnitude of EB with thickness variation of antiferromagnetic LaMnO3 layer. In the present study, we have fabricated the high quality LMO, LNMO and their [LNMO/LMO]15 multilayer thin films. We have studied the EB effect in [LNMO/LMO]15 multilayers with varying LMO thickness, while the thickness of LNMO was kept constant. This effect is also confirmed by performing the temperature dependence of EB and training effect measurements. To the best of our knowledge, EB effect in LNMO/LMO multilayer thin films have not been reported yet.
ter (Bruker D8 advance) of Cu Ka (1.54 °A) radiation. Magnetic properties of the samples were characterized using superconducting quantum interference device magnetometer (MPMS XL Evercool, Quantum Design). For all the magnetic measurements, the samples were mounted parallel to the direction of applied magnetic field. 3. Results and discussion 3.1. Structural properties We have deposited the two single layer thin film samples (LMO and LNMO) and three multilayer samples [LNMO (100 Å)/LMO (x Å)]15 with varying thickness of AFM layer LMO (x = 30, 40 and 50 Å). The three multilayer samples with different LMO layer thickness x = 30, 40 and 50 Å, are represented as samples S1, S2, and S3, respectively. For each deposition, the number of laser shots was varied to obtain the LMO layer with different thickness. The cross-sectional FE-SEM images of both single layer thin films of LMO (190 nm) and LNMO (190 nm) are shown in Fig. 1 (a) and (b), respectively. Using the obtained thickness and the applied number of laser shots, we have determined the growth rate of both the materials and then deposited the multilayer of these materials based on the same deposition parameters. In order to confirm the growth of LNMO on LMO layer, we have taken the cross-sectional FE-SEM image (Fig. 1(c)) for a bilayer (LNMO/ LMO) sample, which shows that the LNMO can be grown easily on LMO layer. X-ray diffraction patterns of LMO, LNMO and multilayer thin film S3 [LNMO (100 Å)/LMO (50 Å)]15 deposited on LAO substrate at fixed deposition temperature 700 °C and pressure of 10 mTorr are shown in Fig. 2. It is clear from Fig. 2 that all the samples show only the diffraction peaks corresponding to (0 0 l) reflections. No other peaks belonging to any impurity phase are present in the XRD pattern which reveals that the films are epitaxial and grown coherently in the pseudocubic 0 0 l-direction. This implies that films are highly oriented along c-axis. 3.2. Magnetic properties
2. Experimental LNMO, LMO and [LNMO/LMO]15 multilayer thin films were fabricated on single crystalline (0 0 1) LaAlO3 (LAO) substrate using multitarget pulsed laser deposition (PLD) technique. To ablate sintered pellet targets, a pulsed laser beam generated by a KrF excimer laser at a wavelength of 248 nm and pulse duration of 25 ns was introduced into the deposition chamber and focused onto the target surface. Prior to each deposition, the targets were preablated for 1 min in order to ascertain the same state of the target in every deposition. The substrate temperature, oxygen pressure, laser fluence, repetition rate and target-substrate distance were held constant for all the deposited films. Epitaxial thin films were grown at fixed substrate temperature and pressure of 700 °C and 10 mTorr, respectively. The laser repetition rate of 5 Hz was used for the ablation. The target to substrate distance was fixed at 50 mm for all the samples. The laser energy density on the target surface was 2 J/cm2. The thickness of the films was measured using cross-sectional FE-SEM and was kept constant for LNMO and LMO thin films at approx. 190 nm. In case of multilayer, first layer of LMO with different thickness (30, 40 and 50 Å) was deposited on the substrate followed by LNMO layer with constant thickness (100 Å). The fifteen such bi-layers were deposited so the total thickness of multilayers were approx. 195 nm, 210 nm and 225 nm, respectively. The crystallinity and epitaxial quality of thin films were investigated in h-2h geometry using X-ray diffractome-
We have measured the zero field cooled M H loop of LNMO thin film at 5 K within the range of 50 kOe to +50 kOe applied magnetic field in a direction parallel to the surface of the film. The M H curve of LNMO is shown in Fig. 3 after subtracting the substrate magnetization. A clear hysteresis curve suggests the ferromagnetic behaviour of LNMO thin film. LNMO film shows a saturation magnetization of 4.8 lB/f.u., which approaches the theoretical value of 5 lB/f.u. as expected for completely ordered Ni2+/Mn4+ ferromagnetic configuration [30,31]. As the cation ordering in the double perovskites is the B-site ordering. If a double perovskite is not properly B-site ordered, it means that antisite disorders are present in the system. Experimentally, the presence of the antisite disorders is well evidenced by the fact that the saturation magnetization (MS) of the LNMO synthesized by different methods is found to be always smaller than the expected MS of 5 lB/f.u. [39–41]. However, in the present case, we found MS (4.8 lB/f.u.) close to the MS of B-site ordered LNMO [30,31]. Additionally, we observed that M H loop shows a saturation behaviour even at below ±50 kOe. Such type of behaviour of M H loop in the parallel direction of magnetic field clearly shows the presence of in-plane magnetic anisotropy in LNMO thin film [27]. The temperature dependent magnetization (M T) measurements under zero field cooled (ZFC) and field cooled (FC) conditions were performed in a temperature range of 5–300 K in presence of applied magnetic field of 100 Oe. Prior to the measurements, the sample was cooled from 300 K to 5 K in the absence of magnetic field. Then, the exter-
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
A.K. Singh, R. Chandra / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx
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Fig. 1. (a,b) The cross-section image of single layer thin film of (a) LaMnO3 and (b) La2NiMnO6. (c) The cross section image of a bilayer (La2NiMnO6/LaMnO3) sample.
Fig. 3. Zero field cooled Magnetic hysteresis (M H) loop of La2NiMnO6 thin film at 5 K.
Fig. 2. X-ray diffraction pattern in h/2h geometry of LMO, LNMO and [LNMO (100 Å)/LMO(50 Å)]15 multilayer (Sample S3) thin films deposited on LAO substrate.
nal field was applied and the data was recorded during heating cycle, which is the ZFC curve. For FC curve, the sample was again cooled but now in presence of field and data was recorded during heating cycle. In order to find the correct magnetization of the film, the diamagnetic contribution of LaAlO3 substrate is subtracted from the total magnetization data of the sample. The ZFC and FC curves of LMO and LNMO are shown in Fig. 4(a) and (b) respectively. A cusp can be clearly seen at 129 K in ZFC curve (Inset of
Fig. 4(a)) of LMO thin film, which corresponds to its antiferromagnetic transition temperature (TN) [21]. Fig. 4(b) shows a ferromagnetic transition in M T curves of LNMO around its Curie temperature (TC 280 K) [30,42]. As all the multilayer samples (S1, S2 and S3) exhibit the almost similar M T curves, then only the temperature dependent ZFC and FC curves of sample S3 ([LNMO(100 Å)/LMO(50 Å)]15) are shown in Fig. 5. ZFC curve of multilayer sample S3 shows the two transitions as indicated by T and T’. The transition T is observed around 278 K, which is close to the Curie temperature of LNMO, while the another transition at 128 K, is close to the Neel temperature of LMO. A splitting below 130 K can be clearly seen from the ZFC and FC curves of S3. The temperature can be considered to be the magnetic blocking temperature TBM of the FM/AFM systems, above which the two magnetization curves (ZFC and FC) overlap [9].
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
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Fig. 4. Temperature dependent magnetization (ZFC and FC) curves at 100 Oe for (a) LMO thin film (Inset shows the enlarged view of ZFC curve) (b) LNMO thin film.
Fig. 5. Magnetization as a function of temperature (M T curves) in ZFC and FC mode at 100 Oe of [LNMO(100 Å)/LMO(50 Å)]15 multilayer (sample S3) thin film.
Fig. 6. Field cooled M H loops at 5 K of [LNMO/LMO]15 multilayer thin films with varying thickness of LMO layer [S1 (30 Å), S2 (40 Å) and S3 (50 Å)]. Inset shows the variation of EB (HEB) of [LNMO/LMO]15 multilayer thin films with thickness of LMO layer.
We found from M T curves of our samples that the Neel temperature (TN) of AFM layer (LMO) is less than the Curie temperature (TC) of FM layer (LNMO) i.e. TN < TC. Thus, we could expect the EB at the interface of FM (LNMO) and AFM (LMO) layer in our multilayer thin films, after field cooling of the samples below the TN of the AFM material. To investigate the EB behaviour in our samples, we measured the M H loops of sample S1, S2 and S3 at 5 K, after field cooling (HFC = 10 kOe) the sample from 300 K to 5 K. The field cooled M H loops were traced between ±50 kOe, as shown in Fig. 6. A clear shift along the magnetic field axis in the FC hysteresis loops confirms the presence of EB in the samples. The magnitude of EB field (HEB) was determined from the field cooled M H loops as |HEB| = |(H+ + H )/2|; where, H+and H are the positive and negative coercivity, respectively. The AFM layer (LMO) thickness dependence of EB is shown in the inset of Fig. 6. We observe a small value of EB for multilayer sample S1 with thin (20 Å) AFM layer, which increases with increasing AFM layer thickness from 30 Å to 50 Å. A maximum exchange bias (HEB 740 Oe) is observed for sample S3. This is well known behaviour of FM/AFM based EB systems with increasing AFM layer thickness [11,33]. The observation of EB in FM/AFM systems, requires the condition KAFM. tAFM JINT to be fulfilled, where KAFM is the anisotropy of the AFM
layer, tAFM is the thickness of AFM layer and JINT is the interface coupling constant [11]. Thus as tAFM is decreased, this condition is violated. The enhancement of EB in FM/AFM systems with increasing AFM layer thickness can be understood as follows: The rotation of uncompensated moments in the AFM layer plays a key role in the establishment of EB. When the thickness of LMO is very small (20 Å), then the anisotropy energy of LMO leads to a small number of uncompensated spins in the LMO layer. In this condition, when the magnetic field is reversed and the FM spins start to rotate, as a result AFM spins can be easily dragged by them resulting into decrease in HEB. As the AFM thickness increases (up to 50 Å), the increases in anisotropy energy of AFM (LMO) layer leading to a large number of uncompensated and pinned AFM spins in LMO layer. In this condition, FM spins needed large value of magnetic field to rotate in opposite direction, thus EB field increases [36]. The exact thickness at which the different stages in the above described process occur, depends on the particular system, its microstructure and the measurement temperature [11,33–35]. As the maximum EB is observed in sample S3, so we have performed the field cooled M H loops of sample S3 as a function of temperature. The enlarged view (for sake of clarity) of field cooled M H loops at different temperatures are shown in Fig. 7. The variation of EB (HEB) with temperature is shown in the inset
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
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4. Conclusion In summary, high quality epitaxial LMO, LNMO and [LNMO/ LMO]15 multilayer thin films were fabricated using pulsed laser deposition on single-crystalline (0 0 1) LaAlO3 substrate. Exchange bias effect in LNMO/LMO multilayers was systematically studied as a function of thickness of AFM (LMO) layer. We observed that the EB increases with increasing thickness of AFM layer (LMO) from 30 to 50 Å, which can be attributed to the large number of uncompensated and pinned AFM spins at FM AFM interface due to large anisotropy energy of AFM layer. We observed the maximum EB (740 Oe) in multilayer thin film [LNMO/LMO]15 with LMO film of thickness 50 Å. We have also studied the temperature dependence of EB and found that EB decreases with increasing temperature and finally disappears. Training effect measurements were also performed to confirm that EB is genuine in our sample. The observation of tunable exchange bias with AFM layer thickness at FM/AFM interface is helpful in the development of spintronic devices. Fig. 7. The magnified view of field cooled M H loops at different temperatures for sample S3 [LNMO(100 Å)/LMO(50 Å)]15. Inset shows the variation of EB (sample S3) with temperature.
Acknowledgements One of the authors, A. K. Singh acknowledges the University Grant Commission, India (Grant No. 7412-32-044) for financial support through fellowship.
References
Fig. 8. Enlarged view of loops measured in training effect experiment at 5 K for sample S3. Inset shows the variation of HEB (sample S3) with loop index number (n).
of Fig. 7. HEB decreases with increase in temperature and approaches towards zero above 90 K (EB blocking temperature, TB). The decrease of HEB with increasing temperature can be attributed to the fact that the AFM anisotropy decreases and FM interaction begins to dominate with increasing temperature. This results in weakening of FM/AFM interface coupling which is responsible for reduction of EB with temperature. In order to confirm that the EB behaviour is intrinsic in our sample, we have performed training effect measurements. A gradual reduction of anisotropy interaction occurs in EB systems, upon the subsequent field dependent magnetization cycles of the material, which is the so called training effect [12,43]. As a result, EB decreases with increasing loop index number (n). Field cooled M H loops for 11 continuous cycles have been performed at 5 K after a cooling field of 10 kOe. Fig. 8 shows the magnified view (for sake of clarity) of the low field region of loops. The variation of HEB with increase in loop index number is depicted in inset of Fig. 8, which shows that HEB decreases with increase in loop index number, which is a typical response of EB based systems.
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Research articles
Thickness dependent interfacial magnetic coupling in [La2NiMnO6/ LaMnO3] multilayers Amit Kumar Singh, Ramesh Chandra ⇑ Institute Instrumentation Centre, Indian Institute of Technology Roorkee, 247667, India
a r t i c l e
i n f o
Article history: Received 24 January 2017 Received in revised form 14 August 2017 Accepted 27 August 2017 Available online xxxx Keywords: Epitaxial Multilayers SQUID magnetometer Exchange bias Training effect
a b s t r a c t In the present work, interfacial magnetic exchange coupling at FM/AFM interface has been studied by varying the thickness of AFM layer (LaMnO3) in (La2NiMnO6/LaMnO3)15 multilayer thin film based system. In multilayer thin films, the thickness of LMO was varied from 30 to 50 Å, while the thickness of LNMO was kept constant at 100 Å. Thin films of LNMO, LMO and LNMO/LMO multilayers have been deposited by pulsed laser deposition technique on (0 0 1) LaAlO3 substrate. The thin films have been studied for their structural and magnetic properties. XRD analysis reveals the c-axis epitaxial growth of LNMO, LMO and their multilayer thin films. Exchange bias (EB) effect has been observed in the field cooled hysteresis loops of multilayer thin films and the interaction between FM and AFM spins at the interface is responsible for the observed effect. EB measurements reveal that the thickness variation influences the interfacial interaction between two layers. The EB increases with increasing AFM film thickness in multilayer thin film samples and maximum EB (740 Oe) is observed for sample with 50 Å thickness of AFM layer. Temperature dependence of EB and training effect measurements have also been performed to confirm the EB in the sample. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The interfaces between dissimilar complex oxide materials have been widely studied over the past few decades due to the coupling of the charge, orbital, lattice and spin degrees of freedom of electrons [1,2]. The advancement of techniques for fabricating and characterizing oxide thin films has given a direction for the study of the interfacial effect between perovskite oxides [3,4]. Perovskite based heterostructures composed of magnetically active materials are considering much more attention with discoveries of ferromagnetism at interfaces between two antiferromagnets or even between an antiferromagnet and a paramagnet [5–8]. Thus, it can be realized that epitaxial heterostructures of different complex oxides may offer excellent opportunities to study the rich and fascinating magnetic phenomena at the interface due to the competing interactions. One of the most interesting interfacial phenomenon is the exchange bias (EB) in the heterostructures of ferromagnetic (FM) and antiferromagnetic (AFM) layers [9,10]. When a FM/AFM interface based system is cooled through the Neel temperature (TN) of antiferromagnetic material (TN < TC, TC is the curie temperature of the ferromagnetic material), the hysteresis
⇑ Corresponding author.
loop is now shifted away from the origin [11]. The EB effect manifests itself as a shift in field cooled hysteresis loop along the magnetic field axis and the additional unidirectional anisotropy is attributed to the exchange interaction between the FM and AFM spins at the interface [12,13]. A unidirectional pinning or anisotropy of the magnetization of ferromagnet is observed at the FM/AFM interface due to the effect of EB and can be utilized to control the magnetization in a magnetic reference layer in a spin valve structure or a magnetic tunnel junction (MTJ), which consists of two FM layers separated by a nonmagnetic material (spin valve) or an insulator (MTJ)[14,15]. Among perovskites, ABO3 type LaMnO3 (LMO) compound has attracted much more attention for being a building block in some heterostructures exhibiting fascinating phenomena such as EB in LaNiO3-LaMnO3 superlattices, LaMnO3/SrMnO3 heterostructures and observation of ferromagnetism (FM) at the interface of LaMnO3/SrTiO3 [16–19]. Additionally, bulk LMO is a Mott insulator and an antiferromagnet with a Neel temperature of 140 K. Nevertheless, when grow as a thin film, the magnetic behaviour of LMO is rather conflicting as LMO exhibits ferromagnetic behaviour with a curie temperature TC 200 K [20] as well as it shows antiferromagnetic behaviour with a Neel temperature of TN 131 K, depending on the synthesis conditions [21]. It has been found that in oxygen environment, the epitaxial strain stabilizes the
E-mail address: [email protected] (R. Chandra). http://dx.doi.org/10.1016/j.jmmm.2017.08.080 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
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orthorhombic phase with slightly less Mn4+ in LMO and leaves the film insulating but on the AFM side of the AFM/FM boundary [21]. In recent years, double perovskite (A2BB’O6, A = alkaline earth and B,B’ = transition metal oxides) R2NiMnO6 (RNMO, R = Rare earth) compounds are of considerable interest due to their rich physics and possible application in spintronics [12,22–27]. Among the double perovskites, La2NiMnO6 (LNMO) is well studied due to its near room temperature ferromagnetic transition (TC 280 K) accompanied by magnetoresistance and magnetodielectric effects [22,28,29]. A lot of work on epitaxial growth of LNMO thin film and its magnetic properties is already reported [23,30–32]. Both the LNMO and LMO exhibit perovskite structure with lattice parameters of about 3.88 Å and 3.98 Å. The close lattice parameters of FM (LNMO) and AFM (LMO) layers allow the growth of epitaxial heterostructures with almost atomically perfect interfaces. It has been observed that the EB is inversely proportional to the thickness of FM layer in all the studied systems [11]. However, many discrepancies have been observed in EB behaviour with antiferromagnetic thickness variation. Generally, EB is independent of thickness for thick antiferromagnetic layers and reduces with decreasing antiferromagnetic layer thickness and becomes zero for thin enough AFM layer (a few Å) [11,33]. In some cases, EB decreases with larger AFM thicknesses [34,35]. On the other hand, EB also shows a peak like behaviour with increasing AFM layer thickness in some systems [36,37]. Additionally, the presence of nonmagnetic defects in the AFM layer can enhance the EB [38]. It has also been noticed that AFM domain structure may also affect EB, if the thickness becomes comparable to the AFM domain wall size [11]. Thus, it would be interesting to see the trend and magnitude of EB with thickness variation of antiferromagnetic LaMnO3 layer. In the present study, we have fabricated the high quality LMO, LNMO and their [LNMO/LMO]15 multilayer thin films. We have studied the EB effect in [LNMO/LMO]15 multilayers with varying LMO thickness, while the thickness of LNMO was kept constant. This effect is also confirmed by performing the temperature dependence of EB and training effect measurements. To the best of our knowledge, EB effect in LNMO/LMO multilayer thin films have not been reported yet.
ter (Bruker D8 advance) of Cu Ka (1.54 °A) radiation. Magnetic properties of the samples were characterized using superconducting quantum interference device magnetometer (MPMS XL Evercool, Quantum Design). For all the magnetic measurements, the samples were mounted parallel to the direction of applied magnetic field. 3. Results and discussion 3.1. Structural properties We have deposited the two single layer thin film samples (LMO and LNMO) and three multilayer samples [LNMO (100 Å)/LMO (x Å)]15 with varying thickness of AFM layer LMO (x = 30, 40 and 50 Å). The three multilayer samples with different LMO layer thickness x = 30, 40 and 50 Å, are represented as samples S1, S2, and S3, respectively. For each deposition, the number of laser shots was varied to obtain the LMO layer with different thickness. The cross-sectional FE-SEM images of both single layer thin films of LMO (190 nm) and LNMO (190 nm) are shown in Fig. 1 (a) and (b), respectively. Using the obtained thickness and the applied number of laser shots, we have determined the growth rate of both the materials and then deposited the multilayer of these materials based on the same deposition parameters. In order to confirm the growth of LNMO on LMO layer, we have taken the cross-sectional FE-SEM image (Fig. 1(c)) for a bilayer (LNMO/ LMO) sample, which shows that the LNMO can be grown easily on LMO layer. X-ray diffraction patterns of LMO, LNMO and multilayer thin film S3 [LNMO (100 Å)/LMO (50 Å)]15 deposited on LAO substrate at fixed deposition temperature 700 °C and pressure of 10 mTorr are shown in Fig. 2. It is clear from Fig. 2 that all the samples show only the diffraction peaks corresponding to (0 0 l) reflections. No other peaks belonging to any impurity phase are present in the XRD pattern which reveals that the films are epitaxial and grown coherently in the pseudocubic 0 0 l-direction. This implies that films are highly oriented along c-axis. 3.2. Magnetic properties
2. Experimental LNMO, LMO and [LNMO/LMO]15 multilayer thin films were fabricated on single crystalline (0 0 1) LaAlO3 (LAO) substrate using multitarget pulsed laser deposition (PLD) technique. To ablate sintered pellet targets, a pulsed laser beam generated by a KrF excimer laser at a wavelength of 248 nm and pulse duration of 25 ns was introduced into the deposition chamber and focused onto the target surface. Prior to each deposition, the targets were preablated for 1 min in order to ascertain the same state of the target in every deposition. The substrate temperature, oxygen pressure, laser fluence, repetition rate and target-substrate distance were held constant for all the deposited films. Epitaxial thin films were grown at fixed substrate temperature and pressure of 700 °C and 10 mTorr, respectively. The laser repetition rate of 5 Hz was used for the ablation. The target to substrate distance was fixed at 50 mm for all the samples. The laser energy density on the target surface was 2 J/cm2. The thickness of the films was measured using cross-sectional FE-SEM and was kept constant for LNMO and LMO thin films at approx. 190 nm. In case of multilayer, first layer of LMO with different thickness (30, 40 and 50 Å) was deposited on the substrate followed by LNMO layer with constant thickness (100 Å). The fifteen such bi-layers were deposited so the total thickness of multilayers were approx. 195 nm, 210 nm and 225 nm, respectively. The crystallinity and epitaxial quality of thin films were investigated in h-2h geometry using X-ray diffractome-
We have measured the zero field cooled M H loop of LNMO thin film at 5 K within the range of 50 kOe to +50 kOe applied magnetic field in a direction parallel to the surface of the film. The M H curve of LNMO is shown in Fig. 3 after subtracting the substrate magnetization. A clear hysteresis curve suggests the ferromagnetic behaviour of LNMO thin film. LNMO film shows a saturation magnetization of 4.8 lB/f.u., which approaches the theoretical value of 5 lB/f.u. as expected for completely ordered Ni2+/Mn4+ ferromagnetic configuration [30,31]. As the cation ordering in the double perovskites is the B-site ordering. If a double perovskite is not properly B-site ordered, it means that antisite disorders are present in the system. Experimentally, the presence of the antisite disorders is well evidenced by the fact that the saturation magnetization (MS) of the LNMO synthesized by different methods is found to be always smaller than the expected MS of 5 lB/f.u. [39–41]. However, in the present case, we found MS (4.8 lB/f.u.) close to the MS of B-site ordered LNMO [30,31]. Additionally, we observed that M H loop shows a saturation behaviour even at below ±50 kOe. Such type of behaviour of M H loop in the parallel direction of magnetic field clearly shows the presence of in-plane magnetic anisotropy in LNMO thin film [27]. The temperature dependent magnetization (M T) measurements under zero field cooled (ZFC) and field cooled (FC) conditions were performed in a temperature range of 5–300 K in presence of applied magnetic field of 100 Oe. Prior to the measurements, the sample was cooled from 300 K to 5 K in the absence of magnetic field. Then, the exter-
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
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Fig. 1. (a,b) The cross-section image of single layer thin film of (a) LaMnO3 and (b) La2NiMnO6. (c) The cross section image of a bilayer (La2NiMnO6/LaMnO3) sample.
Fig. 3. Zero field cooled Magnetic hysteresis (M H) loop of La2NiMnO6 thin film at 5 K.
Fig. 2. X-ray diffraction pattern in h/2h geometry of LMO, LNMO and [LNMO (100 Å)/LMO(50 Å)]15 multilayer (Sample S3) thin films deposited on LAO substrate.
nal field was applied and the data was recorded during heating cycle, which is the ZFC curve. For FC curve, the sample was again cooled but now in presence of field and data was recorded during heating cycle. In order to find the correct magnetization of the film, the diamagnetic contribution of LaAlO3 substrate is subtracted from the total magnetization data of the sample. The ZFC and FC curves of LMO and LNMO are shown in Fig. 4(a) and (b) respectively. A cusp can be clearly seen at 129 K in ZFC curve (Inset of
Fig. 4(a)) of LMO thin film, which corresponds to its antiferromagnetic transition temperature (TN) [21]. Fig. 4(b) shows a ferromagnetic transition in M T curves of LNMO around its Curie temperature (TC 280 K) [30,42]. As all the multilayer samples (S1, S2 and S3) exhibit the almost similar M T curves, then only the temperature dependent ZFC and FC curves of sample S3 ([LNMO(100 Å)/LMO(50 Å)]15) are shown in Fig. 5. ZFC curve of multilayer sample S3 shows the two transitions as indicated by T and T’. The transition T is observed around 278 K, which is close to the Curie temperature of LNMO, while the another transition at 128 K, is close to the Neel temperature of LMO. A splitting below 130 K can be clearly seen from the ZFC and FC curves of S3. The temperature can be considered to be the magnetic blocking temperature TBM of the FM/AFM systems, above which the two magnetization curves (ZFC and FC) overlap [9].
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Fig. 4. Temperature dependent magnetization (ZFC and FC) curves at 100 Oe for (a) LMO thin film (Inset shows the enlarged view of ZFC curve) (b) LNMO thin film.
Fig. 5. Magnetization as a function of temperature (M T curves) in ZFC and FC mode at 100 Oe of [LNMO(100 Å)/LMO(50 Å)]15 multilayer (sample S3) thin film.
Fig. 6. Field cooled M H loops at 5 K of [LNMO/LMO]15 multilayer thin films with varying thickness of LMO layer [S1 (30 Å), S2 (40 Å) and S3 (50 Å)]. Inset shows the variation of EB (HEB) of [LNMO/LMO]15 multilayer thin films with thickness of LMO layer.
We found from M T curves of our samples that the Neel temperature (TN) of AFM layer (LMO) is less than the Curie temperature (TC) of FM layer (LNMO) i.e. TN < TC. Thus, we could expect the EB at the interface of FM (LNMO) and AFM (LMO) layer in our multilayer thin films, after field cooling of the samples below the TN of the AFM material. To investigate the EB behaviour in our samples, we measured the M H loops of sample S1, S2 and S3 at 5 K, after field cooling (HFC = 10 kOe) the sample from 300 K to 5 K. The field cooled M H loops were traced between ±50 kOe, as shown in Fig. 6. A clear shift along the magnetic field axis in the FC hysteresis loops confirms the presence of EB in the samples. The magnitude of EB field (HEB) was determined from the field cooled M H loops as |HEB| = |(H+ + H )/2|; where, H+and H are the positive and negative coercivity, respectively. The AFM layer (LMO) thickness dependence of EB is shown in the inset of Fig. 6. We observe a small value of EB for multilayer sample S1 with thin (20 Å) AFM layer, which increases with increasing AFM layer thickness from 30 Å to 50 Å. A maximum exchange bias (HEB 740 Oe) is observed for sample S3. This is well known behaviour of FM/AFM based EB systems with increasing AFM layer thickness [11,33]. The observation of EB in FM/AFM systems, requires the condition KAFM. tAFM JINT to be fulfilled, where KAFM is the anisotropy of the AFM
layer, tAFM is the thickness of AFM layer and JINT is the interface coupling constant [11]. Thus as tAFM is decreased, this condition is violated. The enhancement of EB in FM/AFM systems with increasing AFM layer thickness can be understood as follows: The rotation of uncompensated moments in the AFM layer plays a key role in the establishment of EB. When the thickness of LMO is very small (20 Å), then the anisotropy energy of LMO leads to a small number of uncompensated spins in the LMO layer. In this condition, when the magnetic field is reversed and the FM spins start to rotate, as a result AFM spins can be easily dragged by them resulting into decrease in HEB. As the AFM thickness increases (up to 50 Å), the increases in anisotropy energy of AFM (LMO) layer leading to a large number of uncompensated and pinned AFM spins in LMO layer. In this condition, FM spins needed large value of magnetic field to rotate in opposite direction, thus EB field increases [36]. The exact thickness at which the different stages in the above described process occur, depends on the particular system, its microstructure and the measurement temperature [11,33–35]. As the maximum EB is observed in sample S3, so we have performed the field cooled M H loops of sample S3 as a function of temperature. The enlarged view (for sake of clarity) of field cooled M H loops at different temperatures are shown in Fig. 7. The variation of EB (HEB) with temperature is shown in the inset
Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080
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4. Conclusion In summary, high quality epitaxial LMO, LNMO and [LNMO/ LMO]15 multilayer thin films were fabricated using pulsed laser deposition on single-crystalline (0 0 1) LaAlO3 substrate. Exchange bias effect in LNMO/LMO multilayers was systematically studied as a function of thickness of AFM (LMO) layer. We observed that the EB increases with increasing thickness of AFM layer (LMO) from 30 to 50 Å, which can be attributed to the large number of uncompensated and pinned AFM spins at FM AFM interface due to large anisotropy energy of AFM layer. We observed the maximum EB (740 Oe) in multilayer thin film [LNMO/LMO]15 with LMO film of thickness 50 Å. We have also studied the temperature dependence of EB and found that EB decreases with increasing temperature and finally disappears. Training effect measurements were also performed to confirm that EB is genuine in our sample. The observation of tunable exchange bias with AFM layer thickness at FM/AFM interface is helpful in the development of spintronic devices. Fig. 7. The magnified view of field cooled M H loops at different temperatures for sample S3 [LNMO(100 Å)/LMO(50 Å)]15. Inset shows the variation of EB (sample S3) with temperature.
Acknowledgements One of the authors, A. K. Singh acknowledges the University Grant Commission, India (Grant No. 7412-32-044) for financial support through fellowship.
References
Fig. 8. Enlarged view of loops measured in training effect experiment at 5 K for sample S3. Inset shows the variation of HEB (sample S3) with loop index number (n).
of Fig. 7. HEB decreases with increase in temperature and approaches towards zero above 90 K (EB blocking temperature, TB). The decrease of HEB with increasing temperature can be attributed to the fact that the AFM anisotropy decreases and FM interaction begins to dominate with increasing temperature. This results in weakening of FM/AFM interface coupling which is responsible for reduction of EB with temperature. In order to confirm that the EB behaviour is intrinsic in our sample, we have performed training effect measurements. A gradual reduction of anisotropy interaction occurs in EB systems, upon the subsequent field dependent magnetization cycles of the material, which is the so called training effect [12,43]. As a result, EB decreases with increasing loop index number (n). Field cooled M H loops for 11 continuous cycles have been performed at 5 K after a cooling field of 10 kOe. Fig. 8 shows the magnified view (for sake of clarity) of the low field region of loops. The variation of HEB with increase in loop index number is depicted in inset of Fig. 8, which shows that HEB decreases with increase in loop index number, which is a typical response of EB based systems.
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Please cite this article in press as: A.K. Singh, R. Chandra, Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers, Journal of Magnetism and Magnetic Materials (2017), http://dx.doi.org/10.1016/j.jmmm.2017.08.080