The basic system used for the production of films in Sheffield is the Nordiko RF magnetron sputtering machine.
The system is a large non magnetic chamber containing cleaned substrates and up to three targets. This is evacuated to a high vacuum. This is to remove any impurities. Next Argon gas is pumped into the system, providing an inert environment. Then the Argon is ionised giving a plasma. A potential is applied between the target and the substrate. The ionised Argon then knocks out atoms from the targets, which are attracted towards the substrate to produce a thin film. The materials which we can sputter using the machine include Iron-Cobalt, Metglas, Copper, Silver and Aluminium. Currently a second Sputter coating machine is being constucted which will be used to produce films such as NdFeB.
The induction magnetometer identifies a materials response to an applied magnetic field.
The current in the large solenoid induces a voltage in two field coils. If the induced voltage is known, it is possible to determine the magnetisation and produce an MH loop.
The Magneto Optic Kerr Effect (MOKE) looks at the effect of reflecting plane polarised light from a magnetic material through an analysing polariser.
It relates the changes in internsity to the magnetisation within the surface region of the sample (10 - 20 nm). This can be applied using a He-Ne laser to produce MH loops or using a white light source to give domain imaging .
Small Angle Magnetisation Rotation (SAMR) is used to measure the saturation magnetostriction especially in thin elongated films.
The experimental set-up of the equipment used in SAMR involves the application of a dc field along the ribbon axis to saturate the sample. A perpendicular ac field rotates the magnetisation vector out of the ribbon axis inducing an emf, which can be measured in secondary pick-up coils. The magnetisation’s deflection is then helped or hindered by the application of a tensile stress on the ribbon axis. The dc field and the stress are simultaneously varied to allow a constant value of emf induced in the secondary coils. This then allows the magnetostriction constant to be evaluated.
Annealing is the application of a heat to relieve stress in the system. The facilities allow the control of the temperature to a high accuracy. It is possible to perform annealing in air, an inert gas or a vacuum. Also there is the opportunity to field anneal the samples up to 0.4 Tesla.
One method for magnetostriction measurement is the bending of a cantilever, which has been coated with a magnetic film. Various methods have been examined for obtaining measurements. When a cantilever is firmly clamped at one end and is assumed to have been coated with a magnetostrictive material that is firmly bonded, then changing the magnetisation will cause the film to strain. This film may then be relieved of strain by bending of the cantilever. Then it is possible to examine the bending to determine the magnetostriction.
Sheffield Centre for Advanced Magnetic Materials and Devices
Michael Cooke ([email protected])