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There are only few substances like iron, nickel, cobalt with magnetic property. But in chemistry, when we take account of molecular orbital theory, we can find magnetic properties of every molecules. In general, presence of unpaired electrons indicates paramagnetic and otherwise, it is diamagnetic. That means every molecules and thus every substance have some magnetic property. But, we don't consider all to be magnetic material (even not all paramagnetic materials). What's the reason?

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    $\begingroup$ I think you have a false assumption. Many elements, and a multitude of compounds, have measured magnetic properties. The fact that only a few are the best, and thus used in applications, is beside the point. $\endgroup$ – Jon Custer Mar 1 '16 at 13:27
  • $\begingroup$ But, often we find to read that most materials in our surrounding are non-magnetic. We're also taught in books that some objects like blade or rods can be magnetized by rubbing with magnets; but not taught as we can increase their magnetic property. $\endgroup$ – Priyankush Deka Mar 1 '16 at 13:37
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    $\begingroup$ Or, that most materials are not highly magnetic. Remember, magnetism is a pretty weak force compared with electrostatics, for example. Ferromagnets are strong, but there aren't many of them (well, Fe, Cr, a number of RE compounds,...). The rest are pretty weak and it is hard to measure their magnetic moment without special equipment. As for how to change their properties, well, there is a lot in (some) solid state physics courses. $\endgroup$ – Jon Custer Mar 1 '16 at 14:13
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Strong forces are more important than weak ones. The strongly magnetic materials you mention can crystallize with all-atoms-aligned magnetic moments, which gives a relatively strong external field for a large object. That's called ferromagnetism, but as you point out, there's also diamagnetism and paramagnetism, which are NOT strongly-coupled across all atoms in a solid. There is also a state called antiferromagnetism (half the spins cancel the other half). So, one can deflect a stream of water (slightly) by holding a magnet alongside, because water is diamagnetic. It isn't very strongly diamagnetic, though. In gasses and most liquids or mixed materials, the small dipole moments of atoms don't (at room temperature) interact strongly enough to be ferromagnetic. Even ferromagnetic materials are prone to lose their magnetization at elevated temperatures (like melting, loss of magnetic ordering is a kind of phase change).

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  • $\begingroup$ Please elaborate the case of water. How much have to be the size of the magnet for deflecting a stream of water? $\endgroup$ – Priyankush Deka Mar 2 '16 at 6:52
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    $\begingroup$ I can't make it happen with a small magnet and my (aerator-equipped) home faucets. Maybe in a lab sink with a nozzle, the stream would be uniform enough to see the effet (it's certainly frictionless). A memorable demonstration of water diamagnetism used a polyethylene bag of water, suspended by a couple of meters of monofilament, and it took a 5 lb magnet (big) to shift it pendulum-like a centimeter or two. $\endgroup$ – Whit3rd Mar 9 '16 at 8:04

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