Argon Ion Milling.
D. C Sputter deposition.
4.3 Device characterisation.
4.3.1 Electrical characterisation.
Resistivity values of thin film materials tend to be larger than the bulk due to increased surface scattering. The problem of electron scattering from thin film metal surfaces has been of interest for more than a century, and the subject is still not completely free from controversy. In 1901 Thomson used classical physics to provide a simple way to visualise the increase in resistivity as the film thickness decreases compared to the bulk value [11]. He derived the expression given in Eqn. (4.1), which shows that as the thickness is decreased, the mean free path of the thin film is reduced and the resistivity rises; a size effect is exhibited.
.
2
3
1
ln
2
0 0
+
=
=
κ
κ
ρ
ρ
σ
σ
f f(4.1)
Where σf and λf are the resistivity and mean free path of the thin film, σ0 and λ0 are the
resistivity and mean free path of the bulk material, and κ= d/λ0, where d is the thickness of the
film. A more accurate quantum theory of thin-film conductivity was developed by Fuchs in 1938 [12] and elaborated upon in the ensuing half-century by other investigators, most notably Sondheimer [13]. The Fuchs-Sondheimer (F-S) theory is in close agreement with the Thomson equation.
A standard four-point measurement technique was used to measure the resistance of the thin films and trilayers. A constant current is applied across the film and the resistance is measured by detecting the voltage drop through separate connections. Copper contact pads were used to make electrical contact and distribute the current flow evenly across the ends of the thin film. The effect of the increased current density on the resistance of the nanopatterned samples was investigated by applying a constant current and studying the change in resistance with time in zero applied field. It was found that the resistance of the sample remained constant, which was a direct indication that the increased current density was not detrimental to the GMR response.
CHAPTER 4
Magnetoresistance measurements provide an effective method for analysing the magnetisation reversal processes of ferromagnetic thin films and trilayers. The change in resistance with applied field can give a direct indication of the orientation of the magnetisation in a thin film with respect to the current, and an indication of the relative orientation of two ferromagnetic layers with respect to each other. Coercivity, switching fields and magnetic saturation values can be extracted from the measurements. Two types of magnetoresistance measurements were carried out: in-situ during milling and ex-situ for a range of temperatures from liquid nitrogen to 493K. The set-up for in-situ magnetoresistance measurements was purpose built and is described in detail in Chapter 6. For the ex-situ measurements, the sample was wire bonded to the sample holder using an ultrasonic wire bonder. The sample holder was placed in a probe, which was positioned between the pole pieces of an electromagnet. Typically 0.5mA constant current was supplied to the sample, and the field was swept between ±100Oe. The magnetoresistance of the sample was extracted from the change in voltage as the field was swept, and the current remained constant. The percentage MR of the sample is determined by Eqn. (4.2),
( ) ( )
( )
− ×100 s s H R H R HR(4.2)
where R(H) is the resistance in an applied field H and R(Hs) is the resistance at the saturation
field.
For low temperature measurements the probe was placed in liquid nitrogen and left for a couple of hours before measuring to allow the temperature to stabilise. High temperature measurements were carried out by placing the probe in the centre of a quartz tube, which was sealed at one end to contain the heat. The quartz tube was resistively heated using a steel coil, which was wrapped in both directions to avoid any unwanted magnetic fields. The temperature was carefully monitored using a thermo-couple attached to the sample holder in close proximity to the sample. The furnace was placed between the pole pieces of the magnet.
4.3.2 Magnetic characterisation.
Magnetic characterisation was carried out using a Vibrating Sample Magnetometer (VSM), which was first described by Foner [14], and is basically a comparator, which measures the difference in induction between a region of space with and without the specimen. It therefore gives a direct measure of the magnetisation M. Fig. 4.10 is a schematic of the mechanism of a VSM. The sample is oscillated vertically in a region of uniform field, and if the sample if driven by a loudspeaker mechanism, the frequency is usually near 80Hz and the amplitude is 0.1-0.2mm. The AC signal induced in the pick-up coils (according to the Faraday law of electromagnetic induction) by the magnetic field of the sample is compared with the signal
CHAPTER 4
from a reference specimen, and is converted to a number proportional to the magnetic moment. The advantages of this technique are that it is non-destructive, has a high sensitivity, and is easy to operate.
Pick-up Reference specimen Loudspeaker/ vibration mechanism Reference Sample electromagnet
Fig. 4.10 Schematic diagram of a Vibrating Sample Magnetometer (VSM). (After Ref [15]).
The system is designed to measure small samples with high accuracy (less than 4 by 4mm). For larger samples a calibrated correction is required in order to determine the magnetisation values. However, a comparative analysis of other magnetic properties including coercivity and the magnetisation reversal process is always possible.