TM61 is an MHz-band low-loss MnZn power ferrite with an initial permeability (μi) of 1000, a saturation flux density (Bs) of 540 mT at 25 °C (440 mT at 100 °C) and a Curie temperature above 260 °C. Pcv at 1 MHz / 50 mT is 190 → 155 kW/m³ and at 3 MHz / 10 mT 100 → 90 kW/m³, both with a negative temperature coefficient; at 100 °C the remanence (Br) is 50 mT and the coercivity (Hc) 15 A/m. A μi of 1000 raises the permeability cut-off frequency, so core loss does not limit efficiency when GaN/SiC devices switch at 1–3 MHz. Before the introduction of TM65, TM61 was the common grade for MHz-class power inductors and transformers, used in storage and filter inductors of MHz DC-DC converters and in GaN/SiC resonant and isolation transformers. Among the high-frequency grades TM61 is the earlier generation: TM65 is recommended for new MHz-class inductor designs (μi 1300, 100 °C I@90 % = 47.3 A against 42.1 A for TM61); TM61 remains in supply for existing designs and remains applicable to 1–3 MHz transformers (155 kW/m³ at 1 MHz / 50 mT, 100 °C). Small-size MHz transformers should use TM63; at 500 kHz / 100 mT it reads 550 → 700 kW/m³, so frequencies below 700 kHz are outside its intended range.