Microstructure and morphology

The top view images of the disk-3 type shown in Figure 7.2 by using OM and FESEM were found consisting of sub-10 m CFO discs embedded in PTO matrix rather than nanometer sized CFO rods in PTO matrix [2]. It reveals the similar morphology as illustrated in Figure 7.1(c). Forming the CFO discs instead of CFO rods may be due to the cohesion of CFO gel is stronger than the adhesive force between CFO and PTO gels. According to the mechanism of synthesis, we may infer that the CFO and PTO multilayered structure (2-2 type) and the CFO particles

embedded in PTO matrix (0-3 type) as illustrated in Figure 7.1(a) and Figure 7.1(b), respectively.

Figure 7.2: Top view of FESEM and OM images of the disk-3 type showing the CoFe2O4 disks in the PbTiO3 matrix.

Figure 7.1: Schematic illustration of three thin films with different connectivity schemes: the 0-3 type with CFO particles embedded in PTO matrix, the 2-2 type with CFO and PTO nanolayers, and the disk-3 type with CFO disc aligned in PTO matrix.

20 25 30 35 45 50 55 60

The XRD patterns of the pure CFO and pure PTO powders, and three multiferroic films on Pt/Si substrates taken at RT as shown in Figure 7.3 reveal the correct phases with various planes without obvious secondary phases, and “*” and “^O” represent Si and Pt signals from the substrates, respectively. By using the refinement analysis of XRD data, the refined structure parameters were listed in Table 7-1 and Table 7-2.

We also define strain as variation in lattice constant in this study.

Figure 7.3: X-ray diffraction patterns of the 2-2, 0-3, and disk-3 multiferroics together with those of, pure CFO powder, pure PTO powder, and PTO on Pt/Si film for comparison.

Table 7-1 The refined lattice parameters of PTO for the pure PTO powder and the disk-3/Pt/Si 2.9032 3.8871 0.6604 3.9814 -0.9331 1.0269 0-3/Pt/Si 3.1619 3.8872 0.6629 3.9470 -1.7890 1.0120 2-2/Pt/Si 3.6929 3.9078 1.1964 3.9422 -1.9085 1.0088

Table 7-2 The refined lattice parameters of CFO for the films with different types CFO

According to Table 7-1, the result of pure PTO powder agrees with the JCPDS-International Center for Diffraction Data No. 78-0298; and its intensity ratio of diffraction peaks (100) and (001) is close to 2, indicating the random orientation.  

On the other hand, prefer-oriented vertical a-axis growth with the c-axis lying on the substrate surface is obvious in the 2-2 type and the 0-3 type films but is less in the disk-3 type film and pure PTO/Pt/Si film. We also found that the a-axis of PTO matrix is lengthening with the compression of c-axis for all type films and PTO/Pt/Si film. The compression of c-axis is the most obvious in 2-2 type and is the least in PTO/Pt/Si film. The lattice constant of Pt is about 3.9240 Å and that of CFO is

about 8.3873 Å. For PTO/Pt/Si film, the tensile stress of lattice a and the compressed one of lattice c in PTO matrix arise from mismatch of the Pt lattice, whose lattice constant lies in between them. The stain/stress of PTO matrix is due to the interface of PTO and Pt substrate only. However, for three multiferroic samples, the stress in PTO matrix arises from both the interface of PTO and Pt substrate and that of PTO and CFO matrices.

There are more than twice as many lattice constant of CFO (8.3873 Å) to those of PTO (a: 3.8616 Å, c: 4.0189 Å).   Having the larger mismatch along a of PTO with CFO than along c, CFO exerts the larger tensile stress on the a-lattice of PTO.

Therefore, the PTO matrices in all the CFO-embedded PTO samples are strongly elongated in a-axis that leads to compress in the c-axis for preserving the unit cell volume. Consequently, the decreasing trend of c/a for PTO matrix is not difficult to comprehend as a result of the tensile stress induces a-lattice elongation with the compressive c-axis.

From the information of lattice parameters of PTO matrices in Table 7-1, it is difficult to differentiate the stress/strain due to the interface of PTO and CFO matrices from that of PTO and Pt layer. We therefore predict the lattice parameters of the CFO matrices for all types of samples in Table 7-2 and find that the lattices of CFO matrices are compressed for all types. Because in three multiferroic films the CFO

matrices only bond to the PTO ones, the stresses exerted in CFO matrices should be only on the interfaces of PTO and CFO matrices. As mentioned previously, more than twice as many lattice constant of CFO to the PTO ones, PTO exerts compressive stress on the lattice of CFO. From Table 7-2, we found the most obvious compression is in the disk-3 type and the least compression in the 0-3 type for these three types of samples. Therefore, the train/stress caused by the lattice mismatch between the CFO and PTO matrices is the most pronounced in the disk-3 type and is the least in the 0-3 type that will be further confirmed by micro-Raman spectroscopy later on..

The ME coupling effect resulting from the elastic bonding at the interface [1, 2, 3]

should also be transmitted through the stress/strain between the interface of CFO and PTO matrices. The magnetic properties in our CFO/PTO multiferrics should be influenced by the stress/strain, thus it is more important to directly observe the interfacial stress/strain using another appropriate probe of local behavior besides the peak shifts of XRD diffractions of CFO which covers mm² area and sub-m depth.

In the following study, we used SQUID to investigate the ferromagnetic properties in our multiferroics and the micro-Raman measurement system to probe the stress dependence of behavior of interfacial phonon, which is sensitive to the interfacial stress/strain.