Magnets seem mysterious. Unseen forces pull magnetic materials together or, with the flip of one magnet, push them apart. The stronger the magnets, the stronger the attraction or repulsion. And, of course, the Earth itself is a magnet. While some magnets are made of steel, other types of magnets exist.
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Magnetite is a natural magnetic mineral. The spinning Earth core generates a magnetic field. Alnico magnets are made of aluminum, nickel and cobalt with smaller amounts of aluminum, copper and titanium. Ceramic or ferrite magnets are made of either barium oxide or strontium oxide alloyed with iron oxide. Two rare-earth magnets are samarium cobalt, which contains an alloy of samarium-cobalt with trace elements (iron, copper, zircon), and neodymium iron boron magnets.
Defining Magnets and Magnetism
Any object that produces a magnetic field and interacts with other magnetic fields is a magnet. Magnets have a positive end or pole and a negative end or pole. Lines of the magnet field move from the positive pole (also called the north pole) to the negative (south) pole. Magnetism refers to the interaction between two magnets. Opposites attract, so the positive pole of a magnet and the negative pole of another magnet attract each other.
Types of Magnets
Three general types of magnets exist: permanent magnets, temporary magnets and electromagnets. Permanent magnets retain their magnetic quality over long periods of time. Temporary magnets lose their magnetism quickly. Electromagnets use electric current to generate a magnetic field.
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Permanent magnets hold their magnetic properties for long periods of time. Changes in permanent magnets depend on the strength of the magnet and the magnet's composition. Changes generally happen due to changes in temperature (usually increasing temperature). Magnets heated to their Curie temperature permanently lose their magnetic property because the atoms shift out of the configuration that causes the magnetic effect. The Curie temperature, named for discoverer Pierre Curie, varies depending on the magnetic material.
Magnetite, a naturally occurring permanent magnet, is a weak magnet. Stronger permanent magnets are Alnico, neodymium iron boron, samarium-cobalt, and ceramic or ferrite magnets. These magnets all meet the requirements of the permanent magnet definition.
Magnetite, also called lodestone, provided compass needles from explorers ranging from Chinese jade hunters to world travelers. The mineral magnetite forms when iron is heated in a low-oxygen atmosphere, resulting in the iron oxide compound Fe3O4. Slivers of magnetite serve as compasses. Compasses date back to about 250 B.C. in China, where they were called south pointers.
Alnico Alloy Magnets
Alnico magnets are commonly used magnets made from a compound of 35 percent aluminum (Al), 35 percent nickel (Ni) and 15 percent cobalt (Co) with 7 percent aluminum (Al), 4 percent copper (Cu) and 4 percent titanium (Ti). These magnets were developed in the 1930s and became popular in the 1940s. Temperature has less effect on Alnico magnets than other artificially created magnets. Alnico magnets can be demagnetized more easily, however, so Alnico bar and horseshoe magnets must be stored properly so they don't become demagnetized.
Alnico magnets are used in many ways, especially in audio systems like speakers and microphones. Advantages of Alnico magnets include high corrosion resistance, high physical strength (do not chip, crack or break easily) and high temperature resistance (up to 540 degrees Celsius). Disadvantages include weaker magnetic pull than other artificial magnets.
Ceramic (Ferrite) Magnets
In the 1950s a new group of magnets was developed. Hard hexagonal ferrites, also called ceramic magnets, can be cut into thinner slices and be exposed to low level demagnetizing fields without losing their magnetic properties. They also are cheap to make. The molecular hexagonal ferrite structure occurs in both barium oxide alloyed with iron oxide (BaO∙6Fe2O3) and strontium oxide alloyed with iron oxide (SrO∙6Fe2O3). The strontium (Sr) ferrite has slightly better magnetic properties. The most commonly used permanent magnets are ferrite (ceramic) magnets. Besides cost, advantages of ceramic magnets include having good demagnetization resistance and high corrosion resistance. They are, however, brittle and break easily.
Samarium-cobalt magnets were developed in 1967. These magnets, with a molecular composition of SmCo5, became the first commercial rare-earth and transition-metal permanent magnets. In 1976 an alloy of samarium cobalt with trace elements (iron, copper and zircon) was developed, with a molecular structure of Sm2(Co, Fe, Cu, Zr)17. These magnets have great potential for use in higher temperature applications, up to about 500 C, but the high cost of the materials limits the use of this type of magnet. Samarium is rare even among the rare-earth elements, and cobalt is classed as a strategic metal, so supplies are controlled.
Samarium-cobalt magnets work well in moist conditions. Other advantages include high heat resistance, resistance to low temperatures (-273 C) and high corrosion resistance. Like ceramic magnets, however, samarium-cobalt magnets are brittle. They are, as stated, more expensive.
Neodymium Iron Boron Magnets
Neodymium iron boron (NdFeB or NIB) magnets were invented in 1983. These magnets contain 70 percent iron, 5 percent boron and 25 percent neodymium, a rare-earth element. NIB magnets corrode quickly, so they receive a protective coating, usually nickel, during the production process. Coatings of aluminum, zinc or epoxy resin may be used instead of nickel.
Although NIB magnets are the strongest known permanent magnets, they also have the lowest Curie temperature, about 350 C (some sources say as low as 80 C), of other permanent magnets. This low Curie temperature limits their industrial use. Neodymium iron boron magnets have become an essential part of household electronics including cell phones and computers. Neodymium iron boron magnets are also used in magnetic resonance imaging (MRI) machines.
Advantages of NIB magnets include power-to-weight ratio (up to 1,300 times), high resistance to demagnetization at human-comfortable temperatures and cost-effectiveness. Disadvantages include loss of magnetism at lower Curie temperatures, low corrosion resistance (if the plating is damaged) and brittleness (may break, crack or chip upon sudden collisions with other magnets or metals. (See Resources for Magnetic Fruit, an activity using NIB magnets.)
Temporary magnets consist of what are called soft iron materials. Soft iron means that the atoms and electrons are able to become aligned within the iron, behaving as a magnet for a time. The magnetic metals list includes nails, paper clips and other materials containing iron. Temporary magnets become magnets when exposed to or placed within a magnetic field. For example, a needle rubbed by a magnet becomes a temporary magnet because the magnet causes the electrons to align within the needle. If the magnetic field or the exposure to the magnet is strong enough, soft irons may become permanent magnets, at least until heat, shock or time causes the atoms to lose their alignment.
The third type of magnet occurs when electricity passes through a wire. Wrapping the wire around a soft iron core amplifies the strength of the magnetic field. Increasing the electricity increases the strength of the magnetic field. When electricity flows through the wire, the magnet works. Stop the flow of electrons and the magnetic field collapses. (See Resources for a PhET simulation of electromagnetism.)
The World's Biggest Magnet
The world's biggest magnet is, in fact, the Earth. The Earth's solid iron-nickel inner core spinning in the liquid iron-nickel outer core behaves like a dynamo, generating a magnetic field. The weak magnetic field acts like a bar magnet tilted at about 11 degrees from the Earth's axis. The north end of this magnetic field is the south pole of the bar magnet. Since opposite magnetic fields attract each other, the north end of a magnetic compass points to the south end of the Earth's magnetic field located near the north pole (to put it another way, the Earth's south magnetic pole is actually located near the geographic north pole, though you'll often see that south magnetic pole labeled as the north magnetic pole).
The Earth's magnetic field generates the magnetosphere that surrounds the Earth. Interaction of the solar wind with the magnetosphere causes the northern and southern lights known as the Aurora Borealis and Aurora Australis.
The Earth's magnetic field also impacts the iron minerals in lava flows. The iron minerals in the lava align with the Earth's magnetic field. These aligned minerals "freeze" into place as the lava cools. Studies of magnetic alignments in basalt flows on either side of the mid-Atlantic ridge provide evidence not only for reversals of the Earth's magnetic field but also for the theory of plate tectonics.