Hard ferrite magnetic insulators revealing giant coercivity and sub-terahertz natural ferromagnetic resonance at 5–300 K

Abstract

The temperature behavior of the magnetic properties is crucial for the application of magnetic materials. Recently, giant room temperature coercivities (20-36 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) frequencies (160-250 GHz) were observed for single-domain M-type hexaferrites with high aluminum substitution. Herein, the temperature dependences of the magnetic properties and natural ferromagnetic resonance are studied at 5-300 K for single-domain Sr_{1-<i>x</i>/12}Ca_<i>x</i>/12Fe_12-<i>x</i>Al_<i>x</i>O₁₉ (<i>x</i> = 1.5-5.5) particles. It is shown that the samples maintain their magnetic hardness over the whole temperature range. The coercivity and NFMR frequencies have a maximum shifting to the low-temperature region with a rise in aluminum concentration. The highest coercivity of 42 kOe and the maximum NFMR frequency of 297 GHz are observed for <i>x</i> = 5.5 at 180 K.