| Stan Zurek, Magnetic materials, Encyclopedia Magnetica
Magnetic materials - a term commonly used for materials which exhibit strong magnetic properties, such as ferromagnetic or ferrimagnetic, further broadly classified as magnetically soft, hard, or semi-hard.1) The “soft-hard” naming convention is related to the ease of magnetisation rather than to the mechanical properties.
However, in general all materials are “magnetic” but they respond in various ways to magnetic field, depending on their atomic structure and ambient conditions, or at least do not significantly obstruct magnetic field. In that sense, even vacuum is “magnetic” because magnetic field can propagate through it and by definition2) its magnetic permeability $μ_0$ is a universal physical constant in the SI system of units.3)
On the other hand, antiferromagnetic materials have internally ordered magnetic structure, but such that does not produce large values of susceptibility (i.e. appears to be “non-magnetic”). Yet, these materials find use in special magnetic applications such as magnetoresistive sensors in magnetic hard drives.4)
Magnetic field can be completely expelled from a superconducting body (Meissner effect) but this happens due to the induced surface electric currents, rather than magnetic response as such. However, from the outside, type I superconductors behave as if they were perfect diamagnets, with permeability of zero (comparing to vacuum which has permeability of unity).5)
Magnetic properties of all pure chemical elements of practical importance were measured, with at least an order-of-magnitude accuracy (see the large illustration with the periodic table below). The elements can be broadly classified into diamagnetic and paramagnetic (weak magnetic properties) and ferromagnetic (strong magnetic properties). At room temperature only three elements are ferromagnetic: iron, cobalt, and nickel.
However, pure elements are rarely used because of their magnetic properties (but they can be used for other reasons, like for instance copper for making electric wires or noble gases for providing protective chemical atmosphere).
From engineering viewpoint, metal alloys and chemical compounds, even made from non-ferromagnetic elements, can exhibit very strong magnetic effects, which need to be optimised or tailored, by many means: chemical composition, mechanical forming, thermal processing (annealing), with or without magnetic field, etc.
There are several ways in which materials can respond, and these different types of response are described by various types of magnetism:
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A wide range of materials is widely used in engineering applications, both “magnetic” and “non-magnetic”:
These are described below.
Soft magnetic materials or rather “magnetically soft materials” are used in energy generation and transformation, mechanical force generation, and signal processing such as sensing and transmission.
The “softness” in the name refers to their magnetic response (ferromagnetism or ferrimagnetism), as they can be easily magnetised and demagnetised (especially when compared to “hard” materials). The B-H loop is narrow, so that the coercivity values are low. For example the standard IEC 60404-1 defines magnetically soft materials as such that have coercivity Hc below 1000 A/m.8)
Soft magnetic materials are used extensively in conversion of electric energy, both: to a different level (transformer) as well as to a different form (motor, generator). The largest power transformers can have magnetic cores weighting several hundreds tons.
Under AC conditions, maximum power converted by a magnetic core is a function of the level of flux density (the higher magnetic saturation the better) and magnetic losses (the lower the better). In low-frequency applications the saturation is the limiting factor. But in high frequency applications it is the loss, so magnetic cores may need to operate significantly below the saturation limit to avoid thermal damage.
There are several groups of materials, and their use is dictated typically by the cost of a given solution.
|Type of material||Main constituents||Comments|
|Pure iron||Fe||Saturation polarisation up to 2.15 T9), high cost, used in DC applications such as electromagnet or flux guides for permanent magnet circuits|
|Mild steel||Fe||Saturation up to 2.15 T, lowest cost, produced in very high volume, used in DC applications such as electromagnet, non-critical relay, or flux guides for permanent magnet circuits|
|Non-oriented electrical steel||Fe + Si||Saturation up to 2.15 T, higher cost, produced in thin sheets (<1 mm) in very high volume, used predominantly in motors and smaller generators and transfomers, mostly operating at mains frequency|
|Grain-oriented electrical steel||Fe + Si||Saturation up to 2.1 T, higher cost, produced in very high volume, used in power transformers, large generators and motors operating at mains frequency|
|Thin-gauge electrical steel||Fe + Si||Produced in thinner sheets (<0.25 mm) to suppress eddy currents, used in motors and generators operating at higher operating frequencies, e.g. in motors of electric vehicles|
|Fe-based amorphous tape||Fe + B||Saturation up to 1.75 T, higher cost, produced in high volume, used in power transformers and sensing applications. Higher permeability than electrical steels but very brittle (very thin ribbon (<0.05 mm) with amorphous structure through controlled annealing|
|Nanocrystalline ribbon||Fe + B||Saturation up to 1.2 T, high cost, used in medium frequency power transformers and inductors. Very brittle (very thin ribbon (<0.05 mm), crystallised from amorphous structure|
|Co-Fe alloy||Co + Fe||Saturation up to 2.43 T (the highest known for any material). Used for high-performance applications, such as electric motors and generators in aerospace applications.|
|Ni-Fe alloy||Ni + Fe||Saturation between 0.45-1.5 T (depending on the ratio of Ni/Fe). Used typically for low power applications, pulse transformers, sensors, shielding.|
|Soft ferrite||FeO and MnZn or NiZn||Saturation up to 0.5 T, medium cost, used typically for high-frequency applications, mechanically brittle and difficult to machine, produced through sintering|
|Iron and iron-alloy powder||Fe, Fe+Al||Saturation up to 1.5 T, medium cost, used typically for chokes in power supplies up to 100 kHz|
|Co-based amorphous tape||Co||Very high permeability, very expensive, used in pulse applications|
|Garnet, spinel and hexagonal ferrites||Various||Saturation up to 0.5 T, used in GHz applications|
|Heusler alloy||Various||Niche applications|
For soft magnetic materials, the image below shows the summary of four types of characteristics: saturation, permeability, frequency range, and relative cost.
Once magnetised, they retain a significant amount of magnetic energy and become strong sources of magnetic field, without any need for power supply, so they are also referred to as permanent magnets or simply “magnets”.
The energy density of a magnet is proportional to the values of coercivity and remanence. The highest energy magnets such as NdFeB have mostly linear part of the B-H curve in the second quadrant, and the optimum operating point is such that the product B·H reaches the maximum (BHmax), as illustrated in the graph.
Because of the high coercivity of permanent magnets, there is a considerable difference in the B-H and J-H hysteresis loops. For the same reason, there are two values of coercivity BHc and JHc.
Magnets are used as sources of magnetic field, with three main areas of application:
|Type of material||Main constituents||Comments|
|Alnico||Fe+Al+Ni+Co||Lower coercivity, high remanence, low energy density, highest temperature range|
|Hard Ferrite||Fe oxides||Inexpensive, low energy density, low temperature range|
|SmCo||Sm+Co||Expensive, high energy density, high temperature range|
|NdFeB||Nd+Fe+B||Highest energy density, moderate temperature range|
|PtCo||Pt+Co||Most expensive, high temperature range|
Materials are classified as “semi-hard” mostly on the basis of their coercivity, with a range intermediate between the soft (the lower the better) and hard (the higher the better).
However, from a wider perspective, it is the specific mode of use of a given material that defines the classification.14) There are three main types of applications for semi-hard magnetic materials: