Ferrimagnetism

Ferrimagnetism is a type of permanent magnetism, which means that materials retain their magnetism after being removed from an outside magnetic field. Ferrimagnetism is observed in compounds because these are more structurally complex than pure elements such as iron or cobalt. Ferrimagnets generally have lower saturation levels for magnetization and require less exposure to a magnetizing force.

Ferrimagnetism is one of five types of magnetism, along with paramagnetism, diamagnetism, ferromagnetism, and antiferromagnetism. Although ferromagnetism is the most commonly encountered and the strongest form, it does share some characteristics with ferrimagnetism in regard to temperature. The major difference between ferromagnetism and ferrimagnetism is found on the subatomic level, in the molecular structure of a material.

Background

Magnetism is a natural phenomenon that affects certain materials, inciting an attractive or repulsive force on other materials. It is caused by electrically charged particles called electrons at the subatomic level of the material. The particles' movement creates magnetic fields and encourages alignment with other magnetized materials.

Recognized since ancient times, magnetism was not well understood but was utilized by humankind. Certain stones, known as lodestones, generally existed in bands and contained the mineral magnetite. The name originates from the region of Magnesia in Asia Minor, where lodestones were found in large quantities. Lodestones were used for navigational purposes when it was realized that they always point in a north-south direction and could magnetize objects made of specific materials, such as iron and steel. Originally, magnetite was classified as a ferromagnet, until Louis Néel discovered ferrimagnetism and antiferromagnetism in 1948.

Néel began studying magnetism between 1928 and 1939 in Strasbourg, Germany, under the guidance of Pierre Weiss, who developed the theory of ferromagnetism in 1907. Néel further expanded upon Weiss's work, calculating the effect of temperature fluctuations on magnetism. He was interested in the presence of paramagnetism, which is when certain materials with an attraction to an external magnetic field create their own fields. His first discovery was antiferromagnetism, a property of materials with low positive magnetic susceptibility. This led him to the discovery of ferrimagnetism.

Overview

The molecular structure of a magnetic substance is described as sublattices within a crystal structure. This refers to the placement of magnetic particles and their electrons. The specific ordering or the magnetic behavior that stems from that order is what creates ferrimagnetism. Ferrimagnetism commonly appears in ferrites, which are a magnetic oxide. It is from ferrite that ferrimagnetism gets its name. Magnetite is a "hard" ferrite in its natural state, meaning that its magnetism is permanent and it displays resistant behavior in regard to moisture, salt, and some acids. Ferrite is composed of iron oxide and other metals, and its subatomic structure is made up of many small crystals. Common metals that iron oxide reacts with to create a ferrite include barium, copper, cobalt, aluminum, and magnesium. In the twenty-first century, man-made ferrites have been developed that have uses in various technologies, such as telephones, televisions, and digital electronics.

In the 1930s, two German scientists, E.J.W. Verwey and J.L. Snoek, began studying magnetism, ferrites, and other materials as they believed that ferrite cores could be made with low losses. By 1936, Snoek produced a material that he believed would be useful in industrial electric technology. Several patents were produced by the scientists' research lab; however, much of their work was kept secret due to the political atmosphere before and during World War II. In Japan during the same time, scientists Yogoro Kato and Takeshi Takei studied ferrites and their atomic structure. Their work also increased understanding of ferrimagnetism.

Within the crystal-structure of ferrimagnetic materials, the atoms align both parallel and antiparallel. This is unique because in ferromagnetism, the atoms align in a parallel direction, while in antiferromagnetism they align antiparallel. The atoms align spontaneously, and the strength of the magnetic field is attributed to this structure. The parallel atoms form a stronger magnetic attraction than those that align antiparallel. Unlike ferromagnets, ferrimagnets have weak electric conductivity.

Magnetism is influenced by temperature. In the mid-nineteenth century, French physicist Pierre Curie, for whom the Curie temperature (or Curie point) is named, demonstrated that magnetism was interrupted by high heat. Materials have individual Curie points where the subatomic structure becomes misaligned and magnetism disappears. Once the material drops below its Curie temperature, it can be remagnetized. In ferrimagnetics, there is also a point below the Curie point that can create a magnetic field of zero. This is called the magnetization compensation point and is necessary for high-speed magnetization reversal technology.

Louis Néel used his theory of ferrimagnetism to predict the existence of antiferromagnetism. His research led to the establishment of the Néel temperature, which is similar to the Curie point. At the Néel temperature point, the magnetic order within a material is destroyed, rendering it paramagnetic. The Néel temperature is recognized as the Curie point of an antiferromagnet.

Ferrimagnets and the principles of magnetism that create them are important not only in technological advances but also in the general study of Earth and the universe. Ferrimagnetic minerals in a variety of rocks are used in the field of paleomagnetism to explore the geomagnetic properties of not only Earth but also of other planets. Ferrites are likewise widely utilized. Before random access memory (RAM) was developed, computers required ferrite cores, which relied on a network of wires to store information. In the twenty-first century, ferrites and ferrimagnets are used in computer memory cores, antennas, transformers, microwave frequency devices, and household appliances.

Bibliography

Coey, Michael. "Obituary: Louis Néel (1904–2000)." Nature, vol. 409, no. 302, 18 Jan. 2001, www.nature.com/nature/journal/v409/n6818/full/409302a0.html. Accessed 13 Dec. 2016.

Cullerne, John, editor. Penguin Dictionary of Physics. 4th edition. Penguin, 2009.

"'Development of Ferrite Materials and Their Applications' Recognized as IEEE Milestone." TDK Corporation, 13 Oct. 2009, www.global.tdk.com/news‗center/press/aah30900.htm. Accessed 13 Dec. 2016.

De Vries, Marc J. 80 Years of Research at the Philips Natuurkundig Laboratorium (1914–1994). Amsterdam UP, 2005, pp. 50, 91–7, 269–74.

Friedel, Jacques. "Louis Néel." Physics Today, vol. 54, no. 10, 2001, p. 88.

Hoddeson, Lillian, et al., editors. Out of the Crystal Maze: Chapters from the History of Solid State Physics. Oxford UP, 1992, pp. 394–402.

"Louis Néel – Biographical." Nobelprize.org, www.nobelprize.org/nobel‗prizes/physics/laureates/1970/neel-bio.html. Accessed 13 Dec. 2016.

Mi, Mengjuan, et al. "Variation Between Antiferromagnetism and Ferrimagnetism in NiPS3 by Electron Doping." Advanced Functional Materials, vol. 32, no. 29, 18 July 2022, DOI: 10.1002/adfm.202112750. Accessed 18 Jan. 2023.

Morris, Christopher G., editor. Academic Press Dictionary of Science and Technology. Elsevier Science & Technology, 1992.

Nolting, Wolfgang, and Anupuru Ramakanth. Quantum Theory of Magnetism. Springer-Verlag Berlin Heidelberg, 2009, pp. 317–22.

Spaldin, Nicola A. Magnetic Materials: Fundamentals and Applications. 3rd ed. Cambridge UP, 2013, pp. 89–106, 113–127.

"Type of Magnetism." Gandhi Institute of Technology and Management (GITAM) University, www.gitam.edu/eresource/Engg‗Phys/semester‗2/magnetic/types.htm. Accessed 13 Dec. 2016.

Walker, Laurence R. "Ferrimagnetism." Access Science, Jan. 2020, accessscience.com/content/ferrimagnetism/253900. DOI: 10.1036/1097-8542.253900. Accessed 18 Jan. 2023.