Microwave Circulators Using Ceramic & NdFeB Magnets

by Rodger Billings
M2 Global Technology, Ltd.

The purpose of this discussion is to compare briefly magnetic and mechanical properties of Ceramic and Neodymium Iron Boron (‘Neo’) Biasing Magnets when used in a Drop-in Circulator operating above Gyromagnetic Resonance mounted on a 90°C base-plate with 40 watts of through-power continuously applied.

The Ferrites being biased well above saturation are magnetically equivalent to an air gap, 2-2.5 mm long, sustaining a Magnetic Flux Density in the order of 2300 Gauss (.23 T).  For each Magnet type the Magnetic Circuit is individually configured to apply the same Bias Field to the Ferrite stack.  A single Magnet is disposed above the Ferrite stack in the Ceramic Magnetic Circuit, while in the Neo Magnetic Circuit three Magnets are disposed along flats cut into the Ferrite edges with 120° symmetry.  The Ceramic Magnetic Circuit produces magnetic field lines which are more nearly uniform and orthogonal to the plane of the Ferrite than does the Neo Circuit.  This results in a lower Insertion Loss Circulator.

Temperature Compensation is achieved when each Magnet’s temperature coefficient compensates the de-tuning effect of the thermal gradient of the Saturation Magnetization (4piMs) of the Ferrite material.  These naturally balance in the PCN/PCS bands with Ceramic Magnets and an Aluminum doped Ferrite material.  Other combinations use Temperature Compensation Steels (30% Nickel) to balance these off.  Thus the higher Temperature Coefficient of Ceramic Magnets (-.20°C) as against Neos (-.13°C) does not lead to degraded electrical performance at temperature.

Historically, Ceramic Magnets appeared first, in the late ‘40’s.  They were originally Barium (later Strontium) Ferrites having similar manufacturing methods and mechanical properties to Microwave Ferrites.  Neo Magnets appeared later, in the early ‘80’s and became very popular as their price was comparable to Ceramics and they had a Maximum Energy Product an order of magnitude higher.

Salient magnetic properties are:

  • Type BHmax (MGOe) Tc (°C) Tmax (°C)
  • Ceramic 3.45 460 300
  • NdFeB 20-40 310 150

(Data courtesy of Dexter Corp.)

The Energy Product, BHMax in MegaGauss-Oersteds (1 MGOe = 7956 J/m^3), is a measure of the Maximum Energy Density (proportional to Magnetic Field Strength squared) that can be produced in an given air gap.  The Curie Temperature, Tc in °C, is the temperature at which the material loses its magnetic properties.  Maximum Service Temperature, Tmax in °C, is the maximum long term operating temperature.

It can be seen from the above data that while the Energy Product of Neo magnets is much higher than Ceramics (allowing the design of thin side-magnet devices ), the Curie Temperature is much lower and severely limits the ambient temperature at which stable long term operation may be expected.  It should be noted that Tmax is a maximum temperature.  It is attainable with only certain grades of Neo, ‘pre-aged’ (this involves demagnetizing the fully charged Magnet by a predetermined amount), and enclosed in a Magnetic Circuit biased for optimal temperature stability.  Under ‘real world’ conditions, Tmax often is 115° or even lower.

Mechanically, both materials are hard and brittle, the Neos slightly less so.  They should not be and are not used as structural components within the Circulator.  Ceramics are very strong in compression, having similar mechanical properties to the ferrites which they bias.  The packaging technique used with Ceramic units is very robust and has evolved over 35 years.  Many millions of units have been produced, some capable of withstanding even pyrotechnic shock.  Neos reside to the side of the ferrite stack.  Their major vulnerability is to lateral shock which can be minimized with the use of appropriate adhesives.

A more insidious difference is a susceptibility to oxidation and corrosion of Neo Magnets.  Over time a rust-like coating grows on an unprotected surface and penetrates into the material.  The oxidized material has a lower Coercive Force which ‘shunts’ the remaining Magnet, further reducing the Flux Density.  This metallurgical deterioration is irreversible and exacerbates the low Curie Temperature problem in providing sufficient magnetic bias to the Ferrites at elevated temperature.  This leads to a runaway condition, eventually demagnetizing the Circulator.  Certain coatings or Nickel plating can eliminate most of the difficulty, but the Neo surface is notoriously difficult to clean and process.  Any imperfection in the protective layer will initiate the deterioration cycle.  Ceramics do not oxidize or corrode.

In conclusion then, a Circulator optimally designed with Neo or Ceramic Magnets will exhibit no major electrical difference, other than loss, under low power conditions over the specified temperature range.  The Neo unit will be thinner and lighter.  However, under the power and temperature conditions met in a High Power Amplifier application, the long term effects described above have in the past led to demagnetization of the Neo Magnet units.  Ensuing high reflected power causes catastrophic failure of the Amplifier and other stages.