Antiferromagnetic Materials
Antiferromagnetic materials are a type of magnetic material where neighboring atomic magnetic moments align in opposite directions, resulting in a net magnetization of zero. This causes antiferromagnetic materials to exhibit no macroscopic magnetic field. They are used in various technological applications, such as in magnetic storage devices and spintronics, due to their unique magnetic properties.
4 Key excerpts on "Antiferromagnetic Materials"
eBook - ePubNanoparticles - Nanocomposites – Nanomaterials
An Introduction for Beginners
- Dieter Vollath(Author)
- 2013(Publication Date)
- Wiley-VCH(Publisher)
...This situation is depicted in Figure 8.2 b. Figure 8.2 Antiferromagnetic crystals show antiparallel ordering of the elementary dipoles (a), which are arranged in two different sublattices. As the elementary dipoles compensate each other, such an object has no resulting magnetic moment. This is different for ferrimagnets (b), where the compensation of the spins is not complete; therefore, these materials have a resulting magnetic moment. In view of nanoparticles, Antiferromagnetic Materials, especially the ferromagnetic variety, are of special importance. In antiferromagnetic compounds like MnO, FeO, α-Fe 2 O 3, etc. an equal number of spins with antiparallel orientation is arranged in two different sublattices (Figure 8.2 a). The magnetic moment of these two sublattices cancels out. This is different in the case of ferrimagnetic compounds, where the magnetic moment of these two sublattices is different. All ferrimagnetic oxides contain iron. They consist either of iron ions with different valency or besides the iron ions a second metal oxide. Typical examples are for an iron compound or MgFe 2 O 4 = MgO⋅Fe 2 O 3 as ternary iron compound. A special case in this context is maghemite, γ-Fe 2 O 3. This compound is ferrimagnetic, because the second sublattice is partly occupied by vacancies. Therefore, the exact formula would be, where stands for vacancies replacing the second type of ions. The temperature range where ferromagnetism and ferrimagnetism exist is limited. The temperature where ferromagnetic or ferromagnetic materials transform into paramagnetic material is called the Curie temperature. In the case of Antiferromagnetic Materials this temperature is the Néel temperature...
eBook - ePubNanoparticles for Biomedical Applications
Fundamental Concepts, Biological Interactions and Clinical Applications
- Eun Ji Chung, Lorraine Leon, Carlos Rinaldi, Eun Ji Chung, Lorraine Leon, Carlos Rinaldi(Authors)
- 2019(Publication Date)
- Elsevier(Publisher)
...When uncompensated spins are coupled antiparallel to one other, materials are antiferromagnetic. Due to spin canting or lattice defects, they might possess a net magnetization. Ferromagnets in which the spins of neighboring lattices are antiparallel and of unequal magnitude are called ferrimagnets. The net magnetization of ferrimagnets is greater than for antiferromagnets. Magnetite, commonly found in biological organisms, tends to exhibit ferrimagnetism. Superparamagnetism: The magnetic response of a material to an external magnetic field depends mainly on the prevalence and interaction between uncompensated electron spins and the system's temperature. Beyond a certain crystallite size, the uncompensated spins in ferro- and ferrimagnetic materials interact and arrange into domains separated by a domain wall to maintain the lowest energy state. Conversely, there is a critical size (80–100 nm for mixed ferrites 44) below which it is energetically unfavorable for domain walls to form, resulting in single domain nanoparticles. 53 As predicted by Louis Nèel, 54 at a high enough temperature, nanoparticles in the single domain regime do not exhibit hysteresis behavior in an applied magnetic field, while their magnetization (volume density of magnetic dipoles) quickly increases and then saturates as the strength of the applied magnetic field increases. This condition is referred to as superparamagnetism. Small ferromagnetic nanoparticles tend to behave as superparamagnets. The size below which superparamagnetism is predominant depends on the balance between magnetocrystalline anisotropy energy, thermal energy, and the magnetic energy of the applied magnetic field. 55 Analogous to paramagnets, superparamagnets tend to lose their magnetism in the absence of a field and do not exhibit any hysteresis. As illustrated in Fig. 13.1, high initial susceptibility, saturation of magnetization, and negligible coercivity and remanence are hallmarks of superparamagnets. 13.3.2...
eBook - ePubMaterials
Engineering, Science, Processing and Design
- Michael F. Ashby, Hugh Shercliff, David Cebon(Authors)
- 2009(Publication Date)
- Butterworth-Heinemann(Publisher)
...Most materials achieve near-perfect cancelation either within the atomic orbits or—if not—by stacking the atomic moments head to tail or randomising them so that, when added, they cancel. A very few, most based on just three elements—Fe, Ni and Co—have atoms with residual moments and an inter-atomic interaction that causes them to line up to give a net magnetic moment or magnetisation. Even these materials can find a way to screen their magnetisation by segmenting themselves into domains: a ghetto-like arrangement in which atomic moments segregate into colonies or domains, each with a magnetisation that is oriented such that it tends to cancel that of its neighbors. A strong magnetic field can override the segregation, creating a single unified domain in which all the atomic moments are parallel, and if the coercive field is large enough, they remain parallel even when the driving field is removed, giving a ‘permanent’ magnetisation. There are two sorts of characters in the world of magnetic materials. There are those that magnetise readily, requiring only slight urging from an applied field to do so. They transmit magnetic flux and require only a small reversal of the applied field to realign themselves with it. And there are those that, once magnetised, resist realignment; they give us permanent magnets. The charts of this chapter introduced the two, displaying the properties that most directly determine their choice for a given application. 15.7 Further reading Braithwaite N., Weaver G. Electronic Materials 1990 The Open University and Butterworth-Heinemann Oxford, UK ISBN 0-408-02840-8. (One of the excellent Open University texts that form part of their materials program.) Campbell P.. Permanent Magnetic Materials and Their Applications. Cambridge, UK: Cambridge University Press; 1994. Douglas W.D. Magnetically soft materials, in ASM Metals Handbook Properties and Selection of Non-ferrous Alloys and Special Purpose Materials 9th ed...
eBook - ePubMagnetic Resonance Imaging
Physical Principles and Sequence Design
- Robert W. Brown, Y.-C. Norman Cheng, E. Mark Haacke, Michael R. Thompson, Ramesh Venkatesan(Authors)
- 2014(Publication Date)
- Wiley-Blackwell(Publisher)
...There is also ‘antiferromagnetic’ and ‘ferrimagnetic’ material where neighboring spins completely and incompletely cancel, respectively. The importance of contrast agents brings up a special topic. The act of continually subdividing ferromagnetic material eventually leads to particles that are each just one domain. Mixed into a background substance, they behave like a set of very large magnetic moments, producing superparamagnetism. With no external field, their thermal motion leads to vanishing magnetization. With an external field, the alignment of these domain particles can produce a strong self-field, and they can saturate in a manner analogous to homogeneous ferromagnetic materials. Iron oxide particulates are an example of such superparamagnetic materials. Thanks to the large local fields created, ferric oxide or iron particulates are used as a contrast agent to produce signal loss in regions where they are deposited or to which they have migrated. These particulates vary in size from hundreds of nanometers to several microns, and they behave like a single magnetic domain. The large magnetic moment associated with a particulate has far-reaching effects. A single particle (say, a particulate sphere with a 1-micron diameter) can affect the signal in a volume millions of times the volume of the original particle volume (see Fig. 25.1 and Prob. 25.2). Fig. 25.1 : A voxel of volume 1000 R 3 containing a single superparamagnetic spherical particle. The shaded spherical region with radius R can completely dephase in a conventional gradient echo experiment. If hundreds of these spheres were present, the entire voxel signal could be lost. Problem 25.2 a) For a particulate of radius a = 1 micron (see Fig. 25.1), find the dipole magnetic field (look ahead at the dipole term in (25.15)) at a point P a distance 100 a away along the direction of the static magnetic field...
