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This paper presents a reference design of a solid-nitrogen cooled 300-MHz/25 mm Nb3Sn NMR magnet based on a novel design/operation concept. The magnet operates in the temperature range 8-10 K with its cryocooler thermally decoupled from the magnet and idled. A mass of solid nitrogen in thermal contact with the magnet prolongs the magnet warm-up time to 17 h, during which NMR measurement may be performed under a quiet environment free of the cryocooler's microphonics. At 10 K, the magnet is re-coupled to the cryocooler for recooling to 8 K and ready for another 17-h measurement.