Claims:We claim:
1. A method of processing Co-Zn bilayers to achieve high magnetization, the method comprising the steps of:
preparing ferrite targets of cobalt and zinc, namely CoFe2O4 and ZnFe2O4 from respective binary oxides;
enclosing the ferrite target and a fused quartz substrate within a vacuum chamber evacuated to a base vacuum of 4.6* 10-6 mbar;
depositing the target ferrites on the fused quartz substrate by means of a pulsed layer deposition technique to form a bilayer, wherein a ratio of 3:2 is maintained in the bilayer thickness of CoFe2O4 and ZnFe2O4; and
annealing the bilayer at a temperature of 650o C for a duration of 2 hours in air.
2. The method as claimed in claim 1, wherein the target ferrites are deposited at an oxygen pressure of 0.16 mbar.
3. The method as claimed in claim 1, wherein the fused quartz substrate is disposed at a distance of 4.5 cm from the ferrite target.
4. The method as claimed in claim 1, wherein the depositing by means of a pulsed layer deposition technique further includes a ND: YAG laser.
5. The method as claimed in claim 3, wherein the ND: YAG laser uses a third harmonic (355 nm) with 10n Hz repetition rate and 5–6 ns pulse width.
6. The method as claimed in claim 4, wherein the ND: YAG laser focuses a laser beam fluence of ~ 2.5 J /cm2 at the target ferrite at a power of 1.7 watt.
7. The method as claimed in claim 1, wherein the thickness of ZnFe2O4 is varied in the bilayer.
8. The method as claimed in claim 1, wherein the thickness of CoFe2O4 is maintained at 60 nm in the bilayer.
, Description:TITLE
Method of Processing Co-Zn ferrite bilayers to achieve high magnetization
FIELD OF INVENTION
The present invention generally relates to magnetization of ferrite materials, and more specifically to processing of Co-Zn ferrite bilayers into thin films to achieve a magnetization value hitherto unattained in such class of materials.
BACKGROUND OF INVENTION
Magnetization plays a pivotal role in modern day communications, digital storage drive and various microwave communications circuits; these being only a few of the many applications. The abundance of applications which require the property of magnetization has created a huge market rendering it a highly researched subject matter in the past few decades.
Ferrites are iron containing oxides with spinel, garnet, magnetoplumbite and orthoferrite structures with low conductivity, low eddy current loss and reasonable magnetic properties.
The magnetization of the ferrites is not as large as the well-known ferromagnetic metallic materials due to the antiparallel alignment of the moments in the sub lattices. There is also the nonmagnetic element oxygen occupying the lattice positions and diluting the overall magnatic property. Most of the ferrimagnetic ferrites are electrical insulators and hence can be used in an oscillating magnetic field. The rapidly changing ac magnetic field then induces a voltage and causes a current to flow through the magnetic cores. This current flow causes large heating in metal (ferromagnet) cores, which results in energy loss and thus mutual cores are not suitable for such applications. However solid ferrite cores with much smaller current flow being insulators, can be used in many high frequency applications including aerials and transformers requiring high permeability and low energy loss, as well as in microwave components.
Processing of ferrites for higher magnetization involved, in recent times, attempts to increase the surface area of the ferrite powders, leading to sizes of a few nanometers in ferrites. The magnetic properties of these particles could be radically different from their bulk counterparts. The synthesis of nanocrystalline ferrite systems necessitated use of certain solution techniques, which make it difficult to implement on commercially established bulk ferrite compositions owing to difficulties in attaining desired stoichiometric control.
With the materialization of physical vapour depositions techniques with relatively easier deposition processes for making nanocrystalline grain sized thin films, several ferrite systems could be studied anew. Past few decades have seen immense studies carried out in line with the physical vapour deposition techniques on nanocrystalline ferrite systems. Magnetization in thin film ferrite systems may be taken to be governed by the well-established Neel Model as the investigated systems could be characterized in a similar manner as their bulk counterpart.
In the quest for higher magnetization, numerous ferrite systems, made possible owing to the permutations and combinations of several cations were ventured upon. However the reliability of the systems were obstructed because of a process called canting. Other intrinsic concerns faced while operating with ferrite systems could be issues like inversion of zinc ferrite in nanocrystalline form as opposed to a normal ferrite in bulk, the need of extensively high temperatures for deposition techniques to attain reasonable magnetic properties, two step hysteresis loops at higher substrate temperature.
SUMMARY OF INVENTION
This summary is provided to introduce concepts related to method of processing ferrite bilayers to achieve high magnetization and the concepts are further described below in the detail. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
In accordance with the invention, the present disclosure describes a method of processing Co-Zn ferrite bilayers to achieve higher magnetization than the already existing attainable magnetization. The method of processing the ferrite bilayers of Co-Zn comprises the steps of depositing the ferrite bilayers from binary oxides in a combination with a specific value of ZnFe2O4/CoFe2O4 in order to achieve highest optimum levels of magnetization.
BRIEF DESCRIPTION OF DRAWINGS
The detailed description is described with reference to the accompanying figures.
Figure 1 is representative of the magnetization value as a function of annealing temperatures of ZnFe2O4 films.
Figure 2 is representative of the magnetization value of CoFe2O4 films as a function of annealing temperatures and substrate temperatures.
Figure 3 is representative of a schematic diagram of the deposited bilayer sample as per the invention.
Figure 4 is representative of the XRD patterns of the ZnFe2O4 thickness variation on Co-Zn ferrite bilayer annealed at TA=650 °C.
Figure 5a is representative of M-H loops at 300 K of the ZnFe2O4 thickness variation on Co-Zn ferrite bilayer annealed at TA=650 °C
Figure 5b is representative of M-H loops at 10 K of the ZnFe2O4 thickness variation on Co-Zn ferrite bilayer annealed at TA=650 ° C
Figure 6a and 6b is representative of the cross-sectional FEG-TEM images of the CoFe2O4 and ZnFe2O4 bilayers sample.
DETAILED DESCRIPTION
The present disclosure relates to a method for processing a Co-Zn ferrite bilayer to achieve exceptionally high magnetization. Magnetization which is a result of antiferromagnetic interactions based on Neel’s phenomena of magnetization is often the central driving force for a plethora of applications.
In the present invention, larger magnetization than that achieved by the governing Neel model has been obtained by employing conducive deposition parameters for processing the Co-Zn bilayer.
Ferrites are ferrimagnetic materials wherein the magnetic moments of two sub-lattices are aligned anti-parallel to each other. The two ferrites in contention here are CoFe2O4 and ZnFe2O4. These ferrites are ferrimagnetic materials with magnetic sub-lattices and unequal magnetic moments of the sub-lattices which are aligned antiparallel to each other. In the Neel’s model of magnetization, one sub-lattice (A-site) CoFe2O4 contains Fe3+ ions having net magnetic moments of 5 µB, while the other sub-lattice (B-site) contains both Co2+ and Fe3+ ions having net magnetic moments of 8 µB, with the resultant total magnetic moments becoming (8-5=3) µB per formula unit of CoFe2O4. When Zn2+ is substituted in CoFe2O4, this ion occupies ‘A’ site, which results in a decrease of moments on that site resulting in an increase of the net magnetic moments. If Zn concentration goes beyond 0.3, the antiparallel arrangement between A-site and B-site gets disturbed in a way that there is no further increase in the magnetic moment, through a process defined as canting.
The present invention provides a substituted ternary ferrite evolving from two binary ferrite layers of Co and Zn undergoing a processing technique which results in high magnetization, thereby overcoming the process of canting.
The present invention describes a method to achieve magnetization higher than that achieved by equivalent bulk compositions comprises of Co-Zn bilayers deposited under controlled conditions of atmosphere and thicknesses. The method of processing Co-Zn ferrite bilayers includes the steps described herein.
CoFe2O4 and ZnFe2O4, essentially constituting the bilayer, are ferrites prepared from the respective binary oxides, the ferrites being referred to as the target ferrites. The target ferrites along with the fused quartz substrate on which the bilayer is deposited are enclosed in vacuum chamber evacuated to a base vacuum of 4.6* 10-6 mbar. The target ferrites are deposited on the fused quartz substrate by means of a pulsed layer deposition technique (PLD) using a Nd:YAG laser. The structural alignment of the bilayer on the substrate is as shown in Figure 3.
The pulsed laser deposition technique which has been employed in view of thin film deposition required in this method, uses a third harmonic (355 nm) of Nd: YAG laser with 10 Hz repetition rate and 5–6 ns pulse width. A typical fluence of the high energy laser beam of ~ 2.5 J /cm2 from the Nd: YAG laser was focused on the target ferrites. All the bilayers were deposited at RT by varying the ZnFe2O4 thickness, whereas the CoFe2O4 thickness (~60 nm) was nearly same for all the samples. Following the deposition of the target ferrites on the quartz substrate, an important step of annealing the deposited film at a temperature of 650 °C for duration of 2 hours occurs. The thickness of ZnFe2O4 is varied in the bilayer to achieve the increase in the magnetization.
After processing the bilayer of Co-Zn ferrite as per the procedure described above, X-ray diffraction (XRD) is carried out at room temperatures for structural analysis. The XRD studies of the films was taken by a PANalyticalX’Pert PRO X-ray diffractometer using CuKa radiation of wavelength 1.5406 Å. Fig.4 is representative of the XRD patterns of the annealed bilayers. From XRD patterns of the films it is found that only spinel ferrite peaks are observed with significant line broadening. Therefore, it is ensured that the films are single phase spinel ferrites with nanocrystalline grain sizes. From the cross-sectional FEG-TEM studies as represented in Figure 6a and 6b, it is observed that the individual ferrite layers are not identifiable after annealing at 650 °C.
The magnetic properties of these bilayer thin films were measured using a vibrating sample magnetometer (VSM) of Physical Property Measurement System (PPMS). The magnetization (M) vs. applied magnetic field (H) hysteresis is plotted giving rise to the (M–H) loops. The variation of magnetization at 10 K and 300 K with annealing temperature of ZnFe2O4 thin films are shown in Figure 1. Figure 2 shows the variation of the magnetization with the annealing temperature and substrate temperature of CoFe2O4 thin films. Figure 1 is indicative of the trends of the magnetization value of ZnFe2O4 thin films wherein it initially increases with the annealing temperatures up to 350 °C and then decreases with further increase of annealing temperatures. It is seen from the Fig. 2 that the magnetization value of CoFe2O4 thin films gradually increases with the TA. On other hand the magnetization value larger than the bulk is found for the sample deposited at low substrate temperature and annealed at temperatures greater than 650 °C.
Furthermore, Figure 5a is representative of the M-H loops at 300 K and Figure 5b shows the M-H loops at 10 K of the ZnFe2O4 thickness variation on Co-Zn ferrite bilayer. It is found that the magnetization increases with the increase of ZnFe2O4 thickness up to a ~3:2 combination of CoFe2O4 and ZnFe2O4 thickness and then decreases with further increase in the ZnFe2O4 layer thickness. The obtained 4pMS value at 10 K and 300 K for the ~3:2 combination of CoFe2O4 and ZnFe2O4 thickness is (15±1) kG and (7.0±0.5) kG respectively. This magnetization value is much higher than that of the equivalent established ternary chemical composition in the bulk. The calculated spontaneous magnetization (4pMS) and coercivity (HC) value of the Co-Zn ferrite bilayer samples are listed in Table 1. The CoFe2O4 thickness is same for all samples in the calculations below.
ZnFe2O4 thickness on the bilayers 4pMS at 300 K (G) HC at
300 K (Oe) 4pMS at
10 K (G) HC at
10 K (Oe)
30 nm 5870 1030 9640 6130
40 nm 7200 255 15070 2370
55 nm 4615 60 11530 2300
70 nm 2590 25 6515 2480
120 nm 1000 20 5010 3130
150 nm 400 --- 3260 2620

The present invention is advantageous over the use of the well know ferromagnetic metallic materials in spite of magnetization values in ferrites being not as large as ferromagnetic materials. These ferromagnetic metallic being electrical insulators can be used in an oscillating magnetic field and the rapidly changing ac magnetic field induces a voltage and causes a current flow through the magnetic cores. This current in turn causes large heating in the metal cores, which results in energy loss thereby rendering it unsuitable for applications at high frequencies. However, solid cores being insulators can be employed in many high frequency applications and when incorporated in the processing technique of the present invention, a high magnetization value hitherto unattained in such class would be of importance.
One skilled in the art will realize the disclosure may be embodied in other specific forms without departing from the disclosure or essential characteristics thereof. Scope of the invention is thus indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.