Alloying elements other than cobalt and iron in solid solution in nickel lower the Curie temperature. Sufficient copper is present in Monel to lower the Curie temperature so that although Monel is usually slightly magnetic at room temperature, it will become non-magnetic when warmed mildly. When the alloy contains about 1.25% Si, it is non magnetic at room temperature. A Si content of 4% lowers the Curie temperature to approximately -70F. The magnetic properties are not affected greatly by cold work or thermal treatment.
Source: Metals Handbook, Volume 1, Eight Edition