Elon Musk, the chief executive of Tesla, said on Tuesday that China’s halt on exports of certain magnets was affecting his plans to build humanoid robots, in the clearest signal yet that China’s action is beginning to affect big American businesses.

China this month suspended exports to any country of so-called heavy rare earth metals and magnets made from them, as part of its retaliation for President Trump’s increases in tariffs on U.S. imports of Chinese goods. The Chinese government has stopped allowing shipments until it can devise an export license system.

China produces the entire world’s supply of heavy rare earth metals, from ore mined in China and Myanmar, and 90 percent of magnets made with these metals. Japan produces the rest of the magnets, but uses raw materials from China.

Rare earth magnets are used inside electric motors that need to fit in compact spaces. Robots have many small electric motors, typically one or more for each joint, that each require a magnet. The use of the heavy rare earths in most of these magnets keep them from overheating and malfunctioning when motors are confined in small spaces.

Tesla’s humanoid robot Optimus at an exhibition booth during the World Artificial Intelligence Conference in Shanghai last year.Credit…Agence France-Presse — Getty Images

Rare earth magnets are up to 15 times as powerful as conventional iron magnets of the same size. If traces of heavy rare earth metals are included in a magnet’s material, it can maintain its magnetic force even at temperatures that would boil water.

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