This paper reports a magnetically tunable U-band metallic waveguide isolator based on the ferromagnetic resonance (FMR) absorption effect. The device features a BaFe₁₂O₁₉ (BaM) single-crystal array integrated into a rectangular waveguide. By leveraging the high intrinsic magnetocrystalline anisotropy and narrow FMR linewidth of the single-crystal material, the isolator achieves high-frequency operation with a significantly reduced external bias field. Experimental results demonstrate a broad continuous tuning range from 50 GHz to 66 GHz. The device exhibits exceptional efficiency, with a typical insertion loss of less than 0.5 dB (minimum 0.24 dB) and an isolation exceeding 15 dB across the operating band. The cascaded array configuration ensures uniform magnetization and stable performance. This combination of ultra-low insertion loss and frequency agility makes the proposed isolator an ideal candidate for next-generation adaptive millimeter-wave communication and radar systems.