ence of butterfly loop, which is expected as measurements performed are already in low temperature regime of PMN-PT(001). It becomes a bit difficult to study the influence of voltage on such a thin film of LSMO as it consists of defects as well as magnetically inhomogeneous regions. There is a plausibility of some magnetization relaxation as a function of time.
Figure 7.6: Magnetization vs electric field curves of 110Å LSMO thin film (a) at 80 K and (b) at 200 K. Green circles are showing small variation in magnetization as a function of electric field at 200 K.
7.4 Conclusion
The growth of 110 Å LSMO layer was successful with good crystalline quality. Re-duction in TC was observed compared to 300 Å LSMO layer. With decreasing thick-ness and presence of defects, leads to reduction of magnetization in the LSMO layer.
ZFC curve demonstrates the pinning of magnetic moments which further confirms the presence of defects in the film. However, the negative remanence effect is still visible at 200 K. This indicates the existence of magnetically inhomogeneous regions
in the LSMO film. The magnetoelectric measurements demonstrate increase in the magnetization, although the variation of magnetization as a function of an applied voltage is not clearly visible, probably due to higher concentration of the defects in the film which suppresses the magnetic signal. Ultra-thin films having low magnetic signal, makes the study of magnetoelectric properties difficult.
8 Experimental Results III: Magnetic field mapping of
LSMO/PMN-PT(001) by off-axis electron holography
In the previous two chapters the magnetic properties of LSMO were investigated using macroscopic measurements and scattering studies. In macroscopic measure-ments, magnetic properties are averaged over the whole sample volume, whereas in scattering, the magnetic depth profile is accessible by model fitting of the mea-sured signal. In order to directly visualize and measure the magnetic field in LSMO, off-axis electron holography (EH) in TEM was carried out using cross-sectional spec-imens of the LSMO deposited on PMN-PT. The main advantage of using off-axis EH is that the magnetic information can be recorded alongside the microscopic struc-ture and composition information enabling a direct comparison between magneti-zation profile and real-space structure. Off-axis EH in conjunction with polarized neutron reflectometry gives a holistic information on the magnetization profile of LSMO/PMN-PT(001) heterostructures.
This chapter discusses the results of off-axis EH measurements performed on LSMO/
PMN-PT(001) heterostructure as a function of temperature. The off-axis EH exper-iments were performed with the help of Dr. Qianqian Lan from ER-C-1. A relatively thick LSMO layer of about 500 Å was deposited, so that the magnetic signal from LSMO layer is strong enough to be recorded. The FIB specimen for the experiment was prepared by Lidia Kibkalo from ER-C-1.
8.1 Structural and magnetic characterizations
Figure 8.1 shows the LEED and RHEED patterns recorded after growth in OMBE.
One observes sharp LEED spots and absence of any surface reconstruction. The presence of Laue ring with sharp RHEED spots indicates smooth surface and good crystalline quality of the LSMO layer. The heterostructure was then characterized using XRD (fig. 8.2) confirming a single phase growth of LSMO oriented along [001]
direction. The zoom-in on (001) peak shows present of Laue oscillations indicating good crystalline quality of the as-grown LSMO layer. The crystalline thickness cal-culated from the Laue oscillations shows that 92% of the film is single crystalline.
Figure 8.1: In-situ structural characterizations (a) LEED and (b)RHEED pat-tern of 500 Å LSMO layer on PMN-PT(001) substrate.
It was not possible to perform XRR scan on this sample as the substrate had some surface corrugation due to broad rocking curve that hinder any reflectivity mea-surement. Although the reflectivity measurements were not successful, the surface corrugation of the substrate didn’t affect the growth and magnetic properties of the LSMO film. The quality of the as-grown LSMO is good that was further confirmed by STEM measurements on this sample.
The magnetic properties of this sample were probed by recording the magnetization as a function of temperature, as shown in fig. 8.3 (a). The M-T curve gives a Curie temperature of TC = 358 K. One still observes jump in magnetization due to Mn3O4 particles near 43 K, however, this effect is not as sharp as observed for 300 Å samples. This probably indicates that the ratio of Mn3O4 particles is reduced compared to 300 Å samples. This can arise due to stoichiometric fluctuation from sample to sample. Also, Mn3O4 has a good chemical compatibility with the whole perovskite manganite family, and therefore, it can nucleate in the LSMO matrix.
Fig.8.3 (b) shows the magnetization hysteresis measured at different temperatures.
The M-H curve shows enhanced magnetization at 300 K from 0.92µB/u.c. for 300 Å to 1.55µB/u.c. for 500 Å LSMO layer.
Interestingly, in an inspection of the half-cycle in M-H loops where the applied mag-netic field varies from +2.5 T to -2.5 T, the presence of negative remanence (NRM) effect at 300 K was observed (fig. 8.4). The presence of NRM effect in LSMO film at room temperature has been consistent in this work. This effect was found in the LSMO/BTO/Nb:STO (001) heterostructure as well. Until now the only speculation for this behaviour has been attributed the presence of defects/oxygen vacancies in the system [93,94]. A systematic set of experiments are required to understand the presence of this NRM effect. Nevertheless the bulk magnetic measurements confirm the stable ferromagnetic properties of the LSMO film that can be locally mapped and measured with off-axis EH in TEM.