R-t-b based sintered magnet

An R-T-B based sintered magnet comprising a first heavy rare earth element, wherein

• R includes Nd,
• T includes Co and Fe,
• the first heavy rare earth element includes Tb or Dy,
• the R-T-B based sintered magnet comprises a region having a concentration of the first heavy rare earth element decreasing from a surface toward an inside,
• a first grain boundary phase which contains the first heavy rare earth element and Nd but does not contain Co is present in one cross section including the region, and
• an area occupied by the first grain boundary phase in the cross section is 1.8% or less.

A sintered magnet, wherein

• a first heavy rare earth element contained in a heavy rare earth compound is diffused from a surface toward an inside of an R-T-B based sintered magnet by attaching the heavy rare earth compound to at least a part of the surface of the R-T-B based sintered magnet and heating the heavy rare earth compound, wherein
• R includes Nd,
• T includes Co and Fe,
• the first heavy rare earth element includes Tb or Dy,
• a first grain boundary phase which contains the first heavy rare earth element and Nd but does not contain Co is present in one cross section including a region having the first heavy rare earth element diffused,
• an area occupied by the first grain boundary phase in the cross section is 1.8% or less,
• a second grain boundary phase which contains Nd and Co but does not contain the first heavy rare earth element is further present in the region, and
• a ratio of an area of the first grain boundary phase to an area of the second grain boundary phase is 2.0 or less.