The ferromagnetic properties of a substance appear only at temperatures below the Curie point.

The vast majority of atoms have their own magnetic field. Almost any atom can be thought of as a tiny magnet with north and south poles. This magnetic effect is explained by the fact that electrons, when moving in orbits around the atomic nucleus, create microscopic electric currents, which generate magnetic fields (cm. Oersted’s discovery). Adding the magnetic fields induced by all the electrons of the atom, we get the total magnetic field of the atom.

In most substances, the magnetic fields of atoms are oriented randomly, as a result of which they are mutually quenched. However, in some substances and materials (primarily in alloys containing iron, nickel or cobalt), the atoms are ordered so that their magnetic fields are directed in one direction and reinforce each other. As a result, a piece of such a substance is surrounded by a magnetic field. Of these substances called ferromagnets, since they usually contain iron, and receive permanent magnets

To understand how ferromagnets are formed, imagine a piece of hot iron. Due to the high temperature, the atoms in it move very quickly and chaotically, leaving no room for ordering atomic magnetic fields in one direction. However, as the temperature decreases, the thermal motion weakens and other effects begin to prevail. In iron (and some other metals), a force acts at the atomic level that tends to unite the magnetic dipoles of neighboring atoms with each other.

This force of interatomic interaction, called exchange force, was first described by Werner Heisenberg (cm. Heisenberg Uncertainty Principle). It is due to the fact that two neighboring atoms can exchange external electrons, and these electrons begin to belong simultaneously to both atoms. The exchange force firmly binds the atoms in the crystal lattice of the metal and makes their magnetic fields parallel and directed in one direction. As a result, the ordered magnetic fields of neighboring atoms are mutually reinforced rather than quenched. And this effect can be observed in the volume of a substance of the order of 1 mm.3containing up to 10sixteen atoms. Atoms of such magnetic domain (cm. below) are arranged in such a way that we have a pure magnetic field.

At high temperatures, the action of this force is hampered by the thermal motion of atoms, while at low temperatures, atomic magnetic fields can reinforce each other. The temperature at which this transition occurs is called Curie point metal – in honor of the French physicist Pierre Curie who discovered it.

In reality, the structure of ferromagnets is much more complex than described above. Typically, individual domains include only a few thousand atoms, the magnetic fields of which are unidirectional; however, the fields of different domains are directed randomly and in the aggregate, the material is not magnetized. Therefore, an ordinary piece of iron does not exhibit magnetic properties. However, under certain conditions, the magnetic fields of the domains that make up the ferromagnet are also ordered (for example, when hot iron is cooled in a strong magnetic field). And then we get a permanent magnet. The presence of the Curie point also explains why, when a permanent magnet is strongly heated, at some point, its complete demagnetization.

Marie Sklodowska Curie, 1867-1934

Polish, then French chemist. She was born in Warsaw in an intellectual family during the difficult period of the Russian occupation, which fell to the lot of Poland. While studying at school, she helped her mother maintain a boarding house, serving in it as a maid. After leaving school, she worked for some time as a governess in wealthy families in order to earn money to receive a medical education for her sister. During this period, the engagement of Sklodowska to a young man from the family where she served (the parents considered such a marriage of their son unworthy of their social status and missed a brilliant opportunity to improve their family gene pool) fell on the upset by the groom’s parents. After her sister received a medical education in Paris, Skłodowska herself went to study there.

The brilliant results of the entrance examinations in physics and mathematics attracted the close attention of leading French scientists to the young Pole. The result was her engagement in 1894 to Pierre Curie and a marriage to him the following year. In those years, studies of the phenomenon of radioactivity were just beginning, and there was a lot of work in this area. Pierre and Marie Curie set about extracting radioactive samples from ores mined in Bohemia and studying them. As a result, the spouses managed to discover several new radioactive elements at once (cm. Radioactive decay), one of which was named curium in their honor, and another – polonium in honor of Mary’s homeland. For these studies, the Curies, together with Henri Becquerel (1852-1908), who discovered X-rays, were awarded the Nobel Prize in Physics for 1903. It was Marie Curie who first coined the term “radioactivity” – after the name of the first discovered by Curie radioactive element radium.

After the tragic death of Pierre in 1906, Marie Curie refused the pension offered by the Sorbonne University and continued her research. She managed to prove that as a result of radioactive decay, the transmutation of chemical elements occurs, and, thereby, laid the foundation for a new branch of the natural sciences – radiochemistry. For this work, Marie Curie was awarded the Nobel Prize in Chemistry in 1911 and became the first scientist to win twice the most prestigious prize for achievements in the natural sciences. (In the same year, the Paris Academy of Sciences rejected her candidacy and did not accept Marie Curie into its ranks. Apparently, two Nobel Prizes seemed to the gentlemen of the academicians insufficient to overcome their tendency to discrimination on the basis of nationality and gender.)

During the First World War, Marie Curie was engaged in active applied medical research, working at the front with a portable X-ray unit. In 1921, a subscription was opened in America to raise funds for the purchase of 1 gram of pure radium for Marie Curie, which she needed for further research. During her triumphant trip to America with public lectures, the key to the casket with precious radioactive metal was presented to Curie by the President of the United States, Warren Harding.

The last years of Marie Curie’s life were filled with important international initiatives in the field of science and medicine. At the beginning 1930s Marie Curie’s health deteriorated sharply – the huge doses of radioactive exposure she received in the course of many years of experiments affected – and in 1934 she died in a sanatorium in the French Alps.

Pierre Curie
Pierre Curie
Pierre Curie, 1859-1906

French physicist. Born in Paris into the family of a prominent physician. Received education at home. Initially, he studied pharmacology at the Sorbonne, but very soon became interested in natural science experiments with crystals, which were conducted by his brother Jacques, and eventually became director of the School of Physics and Chemistry (École de Physique et Chimie). In 1895 he married Maria Sklodowska and in the same year he defended his doctoral dissertation on the magnetic properties of paramagnets (cm. Curie’s law). Together with his wife, in the most difficult working conditions, he conducted experiments at the School to study the properties of radioactive substances. In 1904 he was appointed to the post of professor of physics and director of the laboratory (soon transformed into the Radium Institute) of the Sorbonne. In April 1906, Pierre Curie died in a ridiculous accident, falling under the wheels of a cab. He didn’t even have time to complete the equipment for his new laboratory.

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