MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
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26 Chapter 3<br />
3.1.2.3 Contacts<br />
Bad contacts are often the cause of problems in a transport experiment.<br />
Making good contacts is the closest IÕve come to producing fine art in the<br />
laboratory. Contacts need to be strong and durable yet small. The thermal<br />
stresses of temperature cycling can weaken or break a contact. A weak contact<br />
is worse than a broken one because a weak contact always seems to work at<br />
room temperature, give unusual results at some other temperature and then<br />
return to normal at room temperature. A good contact is not delicate, it<br />
should be able to withstand some mechanical stress at room temperature.<br />
Contacts need to be small when it is assumed that they are point contacts<br />
or if the sample is physically small. If the size or shape of the contact affects<br />
the geometry of the measurement (section 3.1.2.2) any sintering, melting,<br />
cracking, etc. of the contact will affect the measurement. Unfortunately, the<br />
smaller the contact, the weaker it usually is.<br />
The contact wires can also cause stress in the contact since they also will<br />
expand and contract with temperature. Contact wires should have bends to<br />
minimize the strain on the contacts. Contact wires should, if possible, also be<br />
plastically deformed so that they will stay at the desired position without any<br />
mechanical force, before the contact material is applied.<br />
Indium metal makes good contacts for measurements at temperatures less<br />
than 400K. Indium is quite ductile and therefore can withstand considerable<br />
strain before breaking. Freshly cut indium can be pressed onto most flat<br />
sample surfaces. The contact wire can then be sandwiched between a second<br />
piece of indium. In this way, contacts a fraction of a millimeter can be made.<br />
For surfaces which are difficult to adhere to, an ultrasonic soldering iron<br />
usually helps. Shiny, ultrasonically vibrating, liquid indium will stick to<br />
almost anything and can be drawn out into long wires. Any yellow, oxidized<br />
surface of the molten indium will hinder the adhesion to a sample.
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