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AT-LAD growth of Barium Hexagonal Ferrite

            Laser ablation deposition (LAD) has long been established as an effective technique for the growth of thin films of various materials, including ferrites of spinel, garnet, and hexagonal crystal structures. Various process parameters, including substrate temperature, pressure, ablation yields, and deposition rate, influence the chemical composition and the crystal structure of the deposited films. The composition and the structure of the films are also highly dependent on the composition, structure, and ionic distribution of the target. Epitaxial growth imposes an additional requirement that the lattice constant and the thermal expansion coefficient mismatches be minimal.

            It has been previously demonstrated that high quality hexagonal barium ferrite films can be deposited by the single target LAD technique. The fundamental question raised in this work is the following: Is it possible to grow high quality hexagonal ferrite films with good crystallographic, magnetic, and microwave properties while relaxing the restrictions imposed on these properties by the chemical composition and the ionic distribution of a single target? To address this question we have developed a deposition process that utilizes multiple targets of different compositions and ionic distributions to grow hexagonal ferrites. As a result, this process allowed specific ions to be introduced in any order and at any time during film growth while maintaining the crystal structure coordination within the unit cell. We have referred to this deposition method as alternating target laser ablation deposition (ATLAD).

            Hexagonal M-type barium ferrite (BaFe12O19) films were deposited on c-axis oriented sapphire (Al2O3) substrates from BaFe2O4 and Fe2O3 targets. The crystal structure of the films was characterized by θ-2θ x-ray diffractometry using a Cu kα source, rocking curve measurements of the <008> barium ferrite peak, and pole figure measurements of the highest intensity <107> barium ferrite peaks as shown in figure 1.
XRD
Figure 1 θ-2θ  x-ray diffraction spectrum, rocking curve measurement of the <008> barium ferrite peak (inset a), pole figure measurement of the <107> barium ferrite peaks (inset b).

            Magnetic properties were determined by vibrating sample magnetometer (VSM) measurements with the magnetic field applied perpendicular and parallel to the film plane and ferromagnetic resonance (FMR) measurements with the dc magnetic field applied perpendicular to the film plane. Typical VSM hysteresis loops are shown in figure 2.
VSM
Figure 2 Magnetic hysteresis loops of the barium ferrite films measured by VSM with the magnetic field applied perpendicular (┴) and parallel (//) to the film plane.

            Microwave properties of the films were investigated by FMR measurements using a shorted waveguide technique with magnetic field applied perpendicular to the film plane between 40 and 60 GHz. A peak-to-peak resonance linewidth of 42 Oe was measured at 52 GHz as shown in the inset of figure 3.
FMR
Figure 3 Spinwave resonance field versus spinwave order number squared. Main ferromagnetic resonance mode fitted with derivative Lorentzian power absorption profile (inset).

            In Table 1 a comparison of magnetic and microwave properties of high quality barium ferrite thin films and single crystals prepared by different techniques is presented. It is evident that the properties of the films deposited by the ATLAD technique are comparable with those prepared by single target LAD, LPE, and flux-melt growth methods.

Table 1 Comparison of properties of barium ferrite thin films deposited by different techniques


Technique

Δω
(Degrees)

4πMS
(kG)

HA
(kOe)

HC,perp
(Oe)

ΔH
(Oe)

A
(erg/cm)

g

ATLAD

0.259

4.6±0.2

16.5±0.2

154±10

42

0.6±0.1∙10-6

1.996

LAD9,14

0.15

4.2±0.12

16.4±1.0

250

23

0.64∙10-6

2

LPE10

0.08

4.4

16.4

<10

27

∙∙∙

1.991

Flux-melt12

∙∙∙

4.7

16.3

∙∙∙

25

∙∙∙

2.01

            From this comparison we conclude that the ATLAD technique is capable of producing high quality hexagonal barium ferrite thin films as indicated by the low coercive field, narrow main mode linewidth, and multiple spin wave mode excitations in the SWR spectra. Having developed the ATLAD technique for the growth of hexagonal ferrites we are now well positioned to explore the unique possibilities the technique offers to study the effects of various lattice substitutions on the resulting magnetic and microwave properties of this important class of ferrite materials.

 

 

 

 
 
         
Maintained by Aria Yang, Last updated on Nov. 3, 2008