Ferromagnetic materials come in many forms and can serve RF/microwave applications in many ways. These materials are often recruited for high-frequency circuits for their resonant qualities as building blocks for such components as filters and oscillators. Ferromagnetic materials are so named because they have magnetic properties and can be made into magnets; they are materials that will exhibit spontaneous magnetism and remain magnetized when exposed to an external magnetic field and the field is then removed. These materials are typically used with printed-circuit-board (PCB) materials to add inductance and resonance and enable the fabrication of resonant circuits at specific frequencies.

As an example, most engineers working with higher-frequency instruments and military systems will be familiar with yttrium-indium-garnet (YIG) substrates, a ferromagnetic material which has long served as a building block for tunable RF/microwave filters and oscillators. Components formed of these materials can be varied in frequency according to applied current, often over a considerably wide bandwidth.

Ferromagnetic materials are based on magnetic elements, such as cobalt, iron, and nickel, and have been formulated as ceramic-based materials with the high magnetic permeability needed to store magnetic fields. These materials possess many unpaired electrons, which will align under the effects of an applied electromagnetic (EM) field to form a magnetic field. Commercial ferromagnetic materials are available as soft, formable ferrite materials and harder, machinable materials, such as ceramic-based materials commonly found in magnetically based microwave components, such as circulators and isolators. Typically, a small disk of ferromagnetic material is machined as a main component in a circulator or isolator circuit, with that material contributing a great deal to the electrical performance of the circulator or isolation, including isolation and insertion loss.

These materials have grown more popular in recent years not only for fabricating some of the high-frequency components noted but for such additions to PCBs as planar electromagnetic bandgap (EBG) structures that can add electromagnetic-interference (EMI) shielding to critical segments of a PCB. When used in this way, ferromagnetic materials are extremely useful in mixed-signal (analog and digital) circuits to isolate RF/microwave transmission lines from the noise that can be produced from the digital portions of the circuit.

Ferromagnetic materials will remain magnetized to some extent after being subjected to an external magnetic field. In fact, different types of magnetic materials will maintain more or less of an applied magnetic field. Hysteresis refers to the capabilities of a material to “remember” the applied magnetic field, and the material’s remanence is the amount of magnetism that is retained when the applied magnetic field has been removed, which is a critical parameter for fabricating permanent magnets.

Ferromagnetic materials are one of several types of magnetic materials used in electronic circuits, with other materials, such as diamagnetic and paramagnetic materials, offering somewhat different magnetic properties. These three types of materials, for example, are categorized by their bulk magnetic susceptibility, which is a measure of how much magnetism a material will retain when exposed to a magnetic field. Ferromagnetic materials feature positive susceptibility, reproducing a healthy portion of an applied magnetic field even when that field has been removed. Paramagnetic materials, such as aluminum and manganese, also have a positive susceptibility, but retaining a much smaller amount of magnetism from the applied field, and failing to keep the magnetism once the field has been removed. Diamagnetic materials, such as copper and silver, have a small and negative susceptibility, resisting magnetism from an applied magnetic field. Yet another form of magnetic material, ferrimagnetic materials such as garnet, are somewhat less magnetic than ferromagnetic materials

Ferromagnetic materials intended for use for their resonant properties are often doped with different materials and different doping concentrations to achieve a target resonant frequency or ferromagnetic resonance (FMR) frequency. YIG films, for example, are doped with different materials, including aluminum, to achieve different magnetization responses. Ferromagnetic films can be doped in different ways to respond with different resonant frequencies when exposed to a magnetic field.

Evaluating and comparing different ferromagnetic materials is a matter of understanding some of the essential properties of these materials and what those properties mean for different applications. For example, every ferromagnetic material has a specific temperature, known as the Curie temperature, above which they no longer exhibit magnetic behavior. Data sheets for ferromagnetic materials usually list a maximum temperature along with a recommended operating temperature range. The operating temperature range is usually considerably lower than the Curie temperature for any magnetic material, but the Curie temperature can be an important parameter to consult when reviewing any material-processing steps for ferromagnetic materials. Bonding ferromagnetic materials to PCBs and other materials can require high temperature and pressure, and the electrical and mechanical characteristics of these materials can be impacted by extremely high temperatures. As with many other electronic materials, for example, ferromagnetic materials exhibit a coefficient of thermal expansion (CTE) with mechanical/dimensional changes that occur as a result of temperature and which can play a role in performance and reliability.

Some of the other properties of ferromagnetic materials that can be compared and contrasted include permeability, resistivity, quality factor (Q), magnetic loss, and magnetic anisotropy. For example, the inductance of an inductor formed with a particular ferromagnetic film depends not only on the configuration of the inductor but on the permeability of the material, with higher permeability to be preferred. Since the effective permeability of a ferromagnetic material will decrease with increasing frequency, the inductance of a given inductor formed on that material will decrease with frequency as well. Using a ferromagnetic film or material with a higher value of effective permeability can help reduce the size of inductors and transformers for any applicable frequency.

Ferromagnetic materials with high electrical resistance or resistivity (measured in Ohms-cm) will exhibit low eddy current loss. This characteristic makes these materials suitable for inductors, transformers, and electromagnetic, but also for applications such as radar absorption and for in-circuit control of EMI. Saturation (measured in Gauss or Tesla) is a point in a ferromagnetic material in which an increase in current or in magnetic field strength no longer results in an increase in magnetic flux in the material or inductor formed from the material. A ferromagnetic material that reaches its saturation point will also exhibit a decrease in the inductance of the material or inductors formed from the material, as the material can no longer increase its level of magnetism with any increase in magnetic field or current. Magnetic materials can usually be compared in terms of their saturation behavior by a parameter known as saturation flux density, with higher numbers indicating greater potential to achieve larger magnetic fields for a given material. If a ferromagnetic material is approaching saturation for a particular application, it may be beneficial to specify a magnetic material with lower permeability or to run the selected material/application at a lower current level to avoid saturation.  

Ferromagnetic materials are a large part of electronic designs across a wide range of frequencies, from audio through higher microwave frequencies. Research continues to advance these materials, including in US government laboratories, on the development of multiferroic composite materials that are formed from a blend of ferrite and other materials. These multiferroic materials bring the benefits of voltage-tunable frequency response to magnetic circuits and components, allowing adjustments to the FMR frequency of a multiferroic magnetic material. For such components as tunable antennas, circulators/isolators, and filters, these materials offer tremendous potential for improving the performance and reliability of such systems as RF/microwave radios and radars.

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