What metals are not magnetic?
There is not a simple answer. For a metal (or any other substance)
to be magnetic, it must have electron spin. This gives the substance
an electronic angular momentum to interact with the magnetic field.
Some metals, like the lanthanides, consistently have unpaired
electrons due to the Pauli Exclusion Principle, and so are typically
strongly magnetic. But other metals may be magnetic or not magnetic
depending upon what substance they are found. Alloys made of
nominally magnetic metals such as Fe and Ni may become non-magnetic
in certain alloys grouped together as "stainless steel". In
addition, the term "magnetic" is not very precise. Some substances
become "magnetic" in the presence of a magnetic field, but are not
magnetic in the absence of a magnetic field. These are called
"paramagnetic". Other substances form "permanent" magnets and have
their own intrinsic magnetic field. These are called "ferromagnetic"
materials because iron metal is the "typical" example. Yet other
substances have a structure in which some of the electrons point in
one direction and another layer of domain point in the opposite
direction. These more complex structures are called
"antiferromagnetic". A further complication is that the magnetic
behavior depends upon the temperature. So at low temperature a
substance may have one kind of magnetic properties but at a higher
temperature may have another type of magnetic behavior.
The bottom line is that the magnetic properties of a substance
is complicated, and it is hard to assign metals as being strictly
magnetic and others to be strictly non-magnetic.
You might find the attached article from the 16 May edition of the
New York Times (on-line) interesting:
Numerous metals are not ferromagnetic. Common examples are copper,
silver, aluminum, lead, magnesium, platinum and tungsten. All
materials, however, react to magnetic fields in one of three ways:
Ferromagnetic materials, such as iron, cobalt, and nickel, have
small domains in which all the atoms line up with their permanent
magnetic moments pointing in the same direction. When an external
magnetic field is applied, the domains pointing in the direction of
the field grow at the expense of other domains producing a very
strong magnetic field in the direction of the external field. When
the external field is removed, the domains remain aligned, producing
a permanent magnet.
In paramagnetic materials, such as aluminum, magnesium, and
platinum, the atoms have permanent magnetic moments, but do not form
domains. An external magnetic field tends to line the atoms up
parallel to the external field, but the effect is much smaller and
is proportional to the external field, so when the external field is
removed, the atoms point randomly and no permanent effect remains.
In diamagnetic materials, such as copper, lead, and silver, the
atoms have no permanent magnetic moment. The effect of an external
magnetic field is small (similar to paramagnetic materials) and in
the opposite direction.
As you can see, the subject is complicated, but these are the main ideas.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Virtually anything that we commonly think of as matter can be
influenced by a magnetic field,
provided certain conditions are met. However, for the most part
there are only 3 elements (and then
compounds made that include those elements) which are commonly
thought of as magnetic: Iron, Nickel, and
Cobalt. While most any matter can be influenced by a magnetic
field, these 3 elements are thousands of times more
susceptible to the effects of a magnetic field. Their interaction
is so strong, their effects so dramatic, and other
things so weak or subtle, that we generally just think of other
things as not "magnetic".
The reason these three metals are so strongly magnetic is not
completely understood. Their
outer-most electrons tend to possess spins that line-up. This
alignment, whatever the reason, is what
gives cobalt, iron and nickel their very strong magnetic properties
compared to most other materials.
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Update: June 2012