Coherent control of spin waves in terahertz frequencies promises fruitful applications in ultrafast spintronic devices and has been receiving increasing attention. Although materials with low spin-wave damping are especially desirable for such purposes, quality factor values reported in previous studies were limited to the order of several tens. Here, we prepared a high quality orthoferrite ErFeO3 single crystal and investigated the precession motion (quasi-ferromagnetic resonance mode), by using terahertz pulse pumping and near infrared probe pulse to observe the transient Faraday rotation. As a result, we found a quality factor of spin precession exceeding 1000 in a slab sample. The results from rod shaped samples of various widths fabricated from the same single crystal suggest that the spatial coherence length is larger than 100 μm. The results indicate that the high quality ErFeO3 crystals can be an ideal platform for ultrafast spintronics and quantum information technology.

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