Soft Magnetic Materials & High Frequency Materials

Soft Magnetic Materials/Soft Magnetic Composites/High Frequency Magnetic Materials

Soft magnetic materials are characterized with high magnetic induction (B), high magnetic permeability (μ’-jμ”=dB/dH), low coercivity (H_c), and low loss tangent (tanδ= μ”/ μ’). Loss tangent determines the energy (power) loss in soft magnetic materials or soft magnetic composites.  The power losses is expressed as:

(1)

where  is the hysteresis loss and is proportional to the operating frequency (f ) and the area enclosed by BH loop at dc,  the second term is the eddy current loss and is proportional to f ², material thickness (t) perpendicular to the B direction, and is inversely proportional to electrical resistivity ρ, P_an is the anomalous loss, and P_FMR­ is the loss due to ferromagnetic resonance that is usually around GHz range.   Soft magnetic materials or soft magnetic composites are widely used in motors and generators, transformers, inductors, magnetic sensors, electromagnetic interference (EMI) shielding materials, and various microwave magnetic devices.   The operating frequency ranges from dc to microwaves. Depending on the operating frequency, one must chose different types of soft magnetic materials or soft magnetic composites that are typically classified as silicon steel/electrical steel/lamination steel, amorphous magnetic materials/nanocrystalline magnetic materials, ferrites, soft magnetic composites, and nanostructured magnetic materials.

Silicon Steel/Electrical Steel/Lamination Steel

Silicon steel, also known as electrical steel or lamination steel, is FeSi alloys with silicon content up to 6.5%.  The addition Si significantly increase the electrical resistivity to about 50 μΩ-cm.  The saturation magnetic induction is above 1.5T and the relative permeability (in reference to that of vacuum) is about 4000.  Two main forms of silicon steels are available: (a) cold-rolled non-grain-oriented steel (CRNGO) has isotropic magnetic properties in any direction and is cheap, and (b) cold-rolled-grain-oriented steel (CRGO) has a higher saturation magnetic induction in along the rolling direction but is more expensive.  Silicon steel is usually coated between laminations to reduce the eddy current loss, to improve corrosion resistance, and to act as lubricant during die cutting.  The magnetic properties of electrical steel are tested according to internationally standard test methods.  Silicon steels are often used in motors, generators, and transformers.  The operation frequency typically is below 20 kHz.  Mechanical strain can significantly deteriorate magnetic properties, which, sometimes, requires the test of materials under stress.

            Amorphous Magnetic Materials/Nanocrystalline Magnetic Materials

Because of glass structure of amorphous magnetic materials, they have higher resistivity above 100 μΩ-cm and low magnetic anisotropy, resulting in both lower eddy current loss and hysteresis loss.  In addition, by adjusting the composition, amorphous magnetic materials also have very low magnetostriction, which reduces the energy loss and noise of transformers.  By heat treatment, amorphous magnetic materials can be partially crystallized to form nanocrystalline magnetic materials which further reduces the magnetic anisotropy and increases the saturation magnetic induction.  Typical amorphous magnetic materials contains Fe-Co-Si-B-P-Cu-Nb where Si, B, and P are glass forming elements, Cu promotes nucleation, Nb impedes crystalline grain growth, Co improves the magnetic induction and magnetic phase transformation temperature (Curie temperature).  Amorphous magnetic materials are commonly made by melt-spinning technique that produces ribbon/sheet forms of materials.   Amorphous magnetic materials are fragile and cannot be easily machined.  There are needs to make bulk amorphous magnetic materials that can be achieved via casting methods which are widely used in aluminum die casting and stainless steel casting industries.   Recent researches have also suggested a steel die in which the molten metal flows at low rate and high temperature can be used to produce complete glassy materials.  This provides a unique approach to produce shaped amorphous magnetic parts.   Due to the metallic nature of amorphous materials, the operating frequency of these magnetic materials is still limited below about 100 kHz.

Ferrites

Ferrites are magnetic oxides with insulating properties.  Therefore, ferrites are commonly used in high frequency up to microwave frequency of GHz range.  There are two main classes of ferrites.  One has cubic spinel structure and the other has hexagonal structures.

Cubic Spinel Ferrites

The cubic spinel structure has the composition of MeFe₂O₄ where Me²⁺ represents metal cations including Fe²⁺.  Since the site occupation is very important to understand their magnetic properties,  the cubic spinel ferrites have also been expressed according to their site occupation referred to as normal spinel ferrites with {Me²⁺}_A[Fe₂³⁺]_BO₄^2- and inverse spinel ferriteswith {Fe³⁺}_A[Me²⁺Fe³⁺]_BO₄^2-_,  where A and B represent the tetrahedral and octahedral sites, respectively.  Many cubic spinel ferrites are inverse or mixed (normal+inverse) spinel ferrites.   The magnetic moments within each sites are typically ferromagnetically coupled (parallel aligned) but between two sites are antiferromagnetically coupled (antiparallel).  Therefore, one can easily compute the total magnetization based on the compositions.   The magnetic properties of ferrites can be modified with substitution of different elements, which have preference of occupying tetrahedral A or octahedral B-sites.   For example, Co²⁺, Ni²⁺, Li¹⁺ cations tend to occupy octahedral B-sites whereas Zn²⁺, Cd²⁺ prefer tetrahedral A-sites.

Cubic spinel ferrites have very high permeability but low operating frequency, typically less than 100 MHz.  The most commonly used cubic spinel ferrites are MnZn ferrites of Mn_xZn_1-xFe₂O₄, and NiZn ferrites of Ni_xZn_1-x­Fe₂O₄.  MnZn ferrites has the highest permeability over 2,000, but the operating frequency is limited below 1MHz.  NiZn ferrites has slightly lower permeability but higher operating frequency up to about 20 MHz.

            Z- and Y-type Hexagonal Ferrites

Hexagonal structure promotes magnetic anisotropy and increases ferromagnetic resonant frequency and thus operating frequency.  The hexagonal structure is much more complicated than the cubic spinel structure.  Hexagonal ferrite structure is made of stacked S-, R-, and T-blocks.  S-block with Me₂Fe₄O₈ (Me: metal cations) consists of two hexagonal close-packed oxygen layers.  R-block [BaFe₆O₁₁]^2- consists of three layers with one oxygen ion in the middle layer replaced by Ba²⁺ ion substitutionally.   T-block (Ba₂Fe₈O₁₄) consists of four layers with one Ba²⁺ ion substituting an oxygen ion in each of the two middle layers.  Two commonly used soft hexagonal ferrites are Co₂Z or Z-type and Co₂Y or Y-type ferrites, referring to Ba₃Co₂Fe₂₄O₄₁ and Ba₂Co₂Fe₁₂O₂₂, respectively.   Co₂Z is made of RSTSR*S*T*S* (* refers to the rotation around z-axis by 180°) and Co₂Y is made of TSTSTS.   The operating frequency of Co₂Z is typical up to 1 GHz whereas Co2Y can be used for above 1 GHz.

Our researches in this area focus on the development of high frequency, low loss ferrites derived from NiZn, Co₂Z, and Co₂Y.

Soft magnetic composites

The magnetic permeability and ferromagnetic resonance follows the Snoek’s Law:

(2)

where μ_r is the relative magnetic permeability, f_FMR is the ferromagnetic resonant frequency, γ is the gyromagnetic ratio of 28 GHz/T, and 4πM_s is the saturation magnetization.  Therefore, the product of μ_r f_FMR is proportional to the saturation magnetization of the material.  Consequently, it is desirable to use materials with high saturation magnetization to increase the permeability and ferromagnetic resonant frequency. Metallic ferromagnetic materials have much higher saturation magnetization compared with that of ferrites.  The highest saturation magnetization is found in CoFe alloys of about 2.4T.  Fe has the saturation magnetization of 2.2T.    In order to use metallic ferromagnets at high frequency, one must make them insulating so as to prevent eddy current losses.  Soft magnetic composites or SMC were developed for this purpose.  Soft magnetic composites or SMC typically consists metallic magnetic inclusions, particles or other shaped objects, each of them is coated with insulating materials.  The most common coatings are polymeric materials.   Other oxides such as SiO₂, MgO are also used.  The coated particles are compacted to form various core materials.

Our researches in this area focus on the development of soft magentic composites or SMC based on magnetic flakes.

Nanostructured magnetic materials

One important factor to consider in developing high frequency composite materials is the demagnetization effect.  As a magnetic material is placed inside an applied magnetic field (H_a), magnetic poles are induced at the two ends of the material.  The induced poles generates an internal magnetic field, which is opposite to the direction of the applied field.  This field is called demagnetization field (H_d).   Consequently, the total field inside the material is H_tot=H_a-H_d < H_a, resulting in the apparent permeability (μ_app) that is smaller than the true permeability of the material (μ_r):

(3)

where N  is the demagnetization factor of the inclusion in the direction of the applied field.   Demagnetization factors of several common shapes are shown in the table.  It immediately becomes clear that for spherical-like particles, N is about 4π/3, which leads to μ_app » 3 even if μ_r >> 1.  The best choice is N=0, which are true for film (flake) and rod geometries.  Alternatively, it is to have multidomain particles where closure domain will minimize the magnetic poles and consequently minimize the demagnetization factor.  Furthermore, by bringing particles closer to introduce exchange coupling among particles will significantly reduce the effect due to the demagnetization factor.

Our researches in this area focus on the development of various nanostructured materials based on nanowires, nanofibers, nanoflakes, etc.