Interim project report

Introduction

This interim report provides information regarding the progress of the project "Magnetostriction and Magnetisation of NiFe/FeCo Multilayers" which aims to study how nanometer thick NiFe layers change the anisotropy, magnetostriction and texture of thin FeCo films. This report mainly emphasises on the experiments that are carried out during the course of the project i.e., Radio Frequency (RF) Sputtering for growing the thin films and MOKE magnetometer to investigate the properties of the thin films. Data obtained from the MOKE is further analysed and a detailed report is provided. Basing on these results further project plan is prepared by modifying the original project plan, which includes addition of oxide layer to the thin films and investigating their properties. Work plan is modified according to the project plan, which is shown in gantt chart.

Experiments

Sputtering:

Nordiko NM2000 sputtering system is used to sputter deposit the NiFe and FeCo thin films by radio frequency (RF) magnetron sputtering. This process includes formation of plasma by bombarding the target with inert gas ions, which causes ejection of atoms from the target material. The atoms that are ejected from the target are made to sputter on the substrate, which in turn forms the thin film. Sputtering of atoms on to the substrate depends on the sputtering parameters i.e., pressure and power. Sputter-up mode is used i.e., substrate is placed above the target, which are separated by a distance of 6cm. Three target electrodes can be used at a time to sputter multi-layered films in Nordiko NM2000. Growth rate of thin films is high as magnetron source is used, and mainly depends on the sputtering parameters namely sputtering power and pressure. Growth rate can be increased by increasing the sputtering power, which in turn increases the temperature. Growth rate can also be increased by increasing the pressure at low pressures.

The properties of the resulting thin films are sensitive to sputtering power, temperature and pressure. Two types of substrates are used namely silicon-based and glass-based substrates, which are cleaned by rinsing in acetone and isopropanol. Clean-room gloves and non-magnetic tweezers are used to handle the substrates.

The composition of iron (Fe) in NiFe target is 19% and 50% in FeCo target. In order to investigate the properties of NiFe and FeCo thin films a range of films are grown with different thicknesses. This project mainly aims at the properties of NiFe and FeCo multilayered thin films, but during the course of training a wide range of monolayer films are grown. Five thin films are grown of which four are monolayers and one multilayer, which are grown on silicon substrate.

MOKE:

MOKE (Magneto Optic Kerr Effect) magnetometer is used to study the properties of thin films that were grown. Optical anisotropy is observed in magnetic materials when external magnetic field is applied, this is due to the magnetisation of surface domains. Magneto optic effects will arise due to the presence of optical anisotropy, which is also known as magneto-optic kerr effect. This effect is used to obtain hysteresis loops of the thin films.

He-Ne laser is used with wavelength (?=633nm) which is made to incident on the sample at an angle of 450. Before the light is made to incident on the sample, light is p-polarized by a glan taylor polariser and passed through a lens of focal length 30cm. The reflected light from the thin film is made to pass through analyser onto a photo- detector. The intensity of the light was measured by the photo-detector and by using a computer based program a hysteresis loop is obtained. The sample holder can be rotated freely in the plane of the magnetic field through 360o.

For each thin film data is obtained by rotating the sample holder from 0o to 180o at an interval of 30o. For all the graphs 256 data points are taken with a shape factor of 0.4 and analyser angle 30o degrees. To get the accurate data three averages are taken. As the experiment is conducted in open environment, noise is included in the data. Hence drift and symmetry of the loop should be corrected; this can be done by normalising the data. This includes a series of steps to be performed using Microsoft excel program. The data obtained is utilised to determine coercivity, remanence and saturation point of each thin film.

By externally straining the thin film we can determine the magnetostriction of the film. These films are strained under bending radii of 300mm, 400mm and 500mm, and are investigated by using MOKE. Eight average readings are taken with 256 data points and shape factor 0.4. Data is then normalized in order to reduce the noise. Magnetostriction can be calculated from the formula ?s= dHk2µoMs(1-d1Rv2)3tY Hk is the anisotropy field, t is thickness of the substrate, Y is young's modulus of the substrate, R is the bend radius, v is Poisson ratio of the substrate.

Results

In this section, the normalized data obtained from MOKE for each thin film is plotted as a hysteresis loop, and it is analysed. As the thin films are sputtered they form polycrystalline thin films. Graphs are plotted against normalised data and the external magnetic field applied. Magnetic field has units in mT (millitesla) while the normalised data (Mr/Ms) have no units. From these graphs, coercivity, remanence and saturation field for each thin film is obtained, which is shown in table 3. The below hysteresis loops show easy axis and hard axis for each film, which is due to the uniaxial anisotropy. Uniaxial anisotropy can be due to the stray field caused by magnetron gun during sputtering, stress and shape anisotropy mechanisms can also be a cause.Some of the films may be isotropic i.e., showing no change with rotation angle. Easy axis is the axis of the film in which it can be easily magnetised, where as hard axes cannot be easily magnetised. The field needed to magnetize iron to saturation is smaller in the easy axis than in the others. In the absence of an external magnetic field, the magnetization prefers to lie along the easy directions.

From the above table we can determine the properties of the thin films, here FeCo has high coercivity where as NiFe has the smallest. Using NiFe of 5nm as a under layer for FeCo didn't change the coercivity. Saturation field is high in NiFe compared to FeCo. Change in coercivity depends on the grain size, and on stress and shape anisotropy which can be determined by investigating the thin films by using XRD and SEM. Data is not obtained for NiFe film with 5nm thickness, as MOKE works efficiently from 10nm, as its penetration depth is 20nm. This means that the 5nm NiFe film is too thin to be measured. Fig 7 and 8 shows the change in coercivity and remanence in FeCo (20nm) thin film. From the graphs it is clear that coercivity and remanence is not the same in all directions, this can be due to the presence of weak uniaxial anisotropy.

Magnetostriction

By using the data obtained from MOKE and the magnetostriction constant formula, the magnetostriction was calculated. Magnetostriction constant of FeCo with 20nm is 6.84 ppm ± 1ppm and for multilayer NiFe (5nm)/FeCo(20nm) it is 9.05 ppm ± 1 ppm. The change magnetostriction can be due to the change in interfacial magnetostriction, which is explained in neel's two-component description. Below figure shows the graphs of magnetostriction for bending radii 500mm, 400mm and 300mm.

Multilayer

Achievements

During the training period, I have gained experience with Sputtering technique and MOKE. As per the project work plan first set of multilayer thin film is to be investigated by using MOKE, XRD and SEM, where we investigated with MOKE. As the sputtering system is down since two weeks it is not possible to grown more multilayer thin films. Progress of the project is running according to the work plan submitted during planning report, except the SEM and XRD investigation.

Further work

In order to achieve low coercivity and high magnetostriction properties for thin films, some modifications are done to the original work plan.

We would like to grow thin films with oxide layers on the surface which should change the coercivity and magnetostriction properties. A maximum of 2nm of oxide layer can be grown on the thin films when it is oxidised in open air. This can be done by exposing the thin film to dry air for a certain time at room temperature.

Residual stress also impacts the magnetic anisotropy field and coercivity, which depends on the argon pressure, by varying the argon pressure we can control the induced stress in the thin films. We would like to change the argon pressure while growing the thin films and investigate the properties. Substrate can also change the properties of thin films, possibility of using silicon and glass substrates.

The thickness of NiFe layer should be more than 7nm as below 7nm it shows negative magnetostriction. Further we need to grow another three multilayer thin films with varying thickness and with and without oxide layer, and these films are investigated using MOKE, XRD and SEM

Sample 2 NiFe (15nm)/FeCo (15nm)

Sample 3 NiFe (10nm)/FeCo (10nm)/NiFe (10nm)/FeCo (10nm)

Sample 4 NiFe (15nm)/FeCo (10nm)/NiFe (10nm)/FeCo (20nm)

XRD and SEM are used to study the texture and structure of the thin films. Films which exhibit desired properties of coercivity and magnetostriction are investigated under XRD and SEM.Modifications are done to the original work plan according to the changes in project planning; the revised work plan is represented as a gantt chart as shown in fig.

Conclusion

This report emphasises on the experiments carried out during the training period of the project. It includes working on NORDIKO NM2000 sputtering system and MOKE magnetometer. It also includes accomplishments during the training period. Results obtained from these experiments are analysed and provided in this report, based on which modifications are done to the original project plan. More thin films are to grown and investigated in order to achieve the desired results. The progress in the project until now mainly concentrated on training and investigating the thin films, desired results are expected if much time invested on investigating the thin films, which is further part of the project.

References

  1. Mannan ali, "Growth and study of magnetostrictive FeSIBC thin films, for device applications", Thesis (PhD), 1999, Uni. of Sheffield.
  2. N. A. Morley, M. R. J. Gibbs, E. Ahmad, I. Will, and Y. B. Xu,"MOKE hysteresis loop method of determine the anisotropy constants of ferromagnetic thin film: Fe on GaAs(1 0 0) with over layers of Au and Cr," Journal of magnetism and magnetic materials, vol. 300, pp. 436-444, 2006.
  3. C. Y. Hung, M. Mao, S. Funada, T. Schneider, M. Miloslavsky, M. Miller, C. Qian, and H. C. Tong, "Magnetic properties of ultrathin NiFe CoFe films," Journal of Applied Physics, vol. 87, pp. 6618-6620,2000.
  4. N. A. Morley, S. Rigby, and M. R. J. Gibbs, "Anisotropy and magnetostriction constants of nanostructures FeCo films," Journal Optoelectronics and Advanced Materials, vol. 1, pp. 109 - 113, 2009.
  5. O'Handely, R.C., Modern Magnetic Materials. 2000: John Wiley & son.
  6. Y. K. Kim et al, "Magnetostriction characteristics of ultrathin permalloy films", Appl phy. Let., 68, 1996, 2885-2886.
  7. M. M. Yang et al, "rf-diode sputtered permalloy film," J. Appl. Phy, 68,1989, 3734-3740.
  8. Masahiro K, "Magnetic properties of peralloy/permalloy-oxide multilayer thin films," Journal of material science, 26, 1991, 4150-4154.

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