Measurement of Dielectric and Magnetic 



Properties of Ferromagnetic Materials 



at Microwave Frequencies 



By WILHELM VON AULOCK and JOHN H. ROWEN 



(Manuscript received August 15, 1956) 



Some experimental techniques are discussed which permit measurement 

 of the magnetic and dielectric properties of ferrite materials in the micro- 

 wave region by observing the perturbation in a cylindrical cavity due to 

 insertion of a stnall ferrite sample. A comparison of the properties of thin 

 disc samples with those of small spheres shows that discs yield more accurate 

 results in the region below ferromagnetic resonance whereas spheres are pre- 

 ferable for the study of ferrite properties near resonance. A short description 

 of instrumentation for cavity measurements at 9,200 mc is given and experi- 

 mental results of disc measurements are reported for a low-loss BTL ferrite 

 and several disc diameters. A cornparison of experimental results with 

 Polder's theory indicates that the loss of poly crystalline ferrites below reson- 

 ance is considerably lower than that predicted from an evaluation of the 

 width of the resonance absorption line. 



1. INTRODUCTION 



The dielectric and magnetic properties of semi-conducting ferromag- 

 netic materials such as ferrites have been the subject of intense study 

 in recent years. Analytical expressions for the components /z and k of 

 the permeability tensor of a loss-free single-crystal ferrite were derived 

 by Polder. 1 These expressions were later modified to include a loss factor 

 a.~- 2 Yager and others'* measured the resonance absorption of single 

 crystals of nickel ferrite and found very good agreement with theoiy 

 provided the loss factor a was determined from the width of the meas- 

 ured resonance absorption line. However, when Artman and Tannen- 

 wald^ measured the real and imaginary parts of m and k for polycrj'stal- 

 line ferrites they found that agreement with theory was somewhat less 

 than perfect if a was also determined from the measured line width. 

 Discrepancies were observed for both real and imaginary parts of /i + 



427 



