Where the magnets sit in a joint

Almost every humanoid joint motor is a permanent-magnet synchronous machine. A wound stator sits on the outside; a rotor carrying a ring of alternating north and south magnet poles turns on the inside. Current in the copper builds a rotating field, the magnets chase it, and the reducer or screw behind the rotor turns that spin into joint torque. It’s the same principle in the frameless torque motor of a hip, the motor behind a roller screw in a knee and the tiny motors in a dexterous hand. A single humanoid carries 30 or more of them.

The magnet of choice is sintered NdFeB, roughly 30% rare earths (mainly neodymium and praseodymium), 69% iron and 1% boron. It stores around 400 kJ/m³ of magnetic energy, more than any other commercially viable material, and that energy density is what lets a palm-sized motor produce the torque a biped needs without the limb turning into a club.

Why not cheaper ferrite? Because mass compounds in a walking machine. A ferrite motor has to be about 30% heavier to deliver the same torque, and in a leg that extra weight loads the joint below it, which then needs a bigger motor of its own. Engineers can trade magnet grade for copper, current and cooling, but they can’t trade it for nothing.

Heat is why dysprosium matters

Neodymium magnets have a weak spot. Their coercivity – the resistance to being demagnetised – falls as they warm up, and a joint motor is a warm place. The magnets sit centimetres from windings that dump heat during every torque peak, and the same peak current throws an opposing field straight at them. Push a magnet past its limit and it loses strength permanently. The joint gets weaker for good.

The fix is chemistry. Magnet makers substitute some neodymium with the heavy rare earths dysprosium and, to a lesser extent, terbium, which raise the anisotropy field and with it the coercivity. The more heat a grade must survive, the more heavy rare earth it carries. That’s the hidden bill: analyst estimates put a humanoid at 0.57–1.3 kg of neodymium-praseodymium and 35–79 g of dysprosium. Terbium is roughly 2.5 times as effective as dysprosium per gram, and priced accordingly – more than four times as much.

Those temperatures are nominal, not promises. A magnet’s real limit depends on the magnetic circuit it sits in and how hard the drive pushes the current, which is why suppliers such as Mainrich tell actuator designers to size grades from hot demagnetisation curves rather than the suffix. The practical lever is grain-boundary diffusion: instead of mixing dysprosium through the whole alloy, it’s diffused into the grain boundaries where demagnetisation starts, reaching the same coercivity with less heavy rare earth. Savings are real, but every new route needs qualifying part by part.

The chokepoint map

China doesn’t just mine rare earths. It owns nearly every step between the ore and the finished magnet, and its share rises the closer you get to the part that goes into a joint.

Beijing has used that position twice in eighteen months. On 4 April 2025 it put licence requirements on seven medium and heavy rare earths – samarium, gadolinium, terbium, dysprosium, lutetium, scandium and yttrium – together with their metals, oxides, alloys and the permanent magnets that contain them. Humanoids felt it within weeks: Elon Musk said that month that Optimus production was being affected and that Tesla was working with Chinese authorities on an export licence for the magnets. Those April controls are still in force.

In October 2025 China went further, with extraterritorial rules reaching foreign-made products that contain Chinese-origin rare earths or were made with Chinese technology. After the Trump–Xi meeting that month, those measures were suspended for one year. Unless the pause is extended, they return on 10 November 2026. The price gap already shows where the pressure sits: in late August, dysprosium and terbium oxide in Rotterdam cost roughly four to five times the Chinese export price, and Fastmarkets reported that no dysprosium or terbium oxide had been exported to Japan at all in 2026.

There’s an asymmetry here that matters for the humanoid race. Chinese makers built more than 40,000 humanoids in the first half of 2026, around 97% of reported global shipments, and they buy magnets on the domestic market with no export licence in the loop. A Western OEM designing the same joint pays more, waits longer and carries licensing risk on every shipment.

The volume math: it’s not the tonnage

Humanoids won’t strain the world’s magnet supply by sheer weight any time soon. At 2–4.5 kg each, a million robots a year would need 2,000–4,500 tonnes of NdFeB. Electric cars already used about 30,100 tonnes in 2025 – 21 million vehicles at roughly 1.4 kg each.

Non-Chinese capacity is coming, but late. MP Materials’ 10X campus in Texas is sized at around 10,000 tonnes of magnets a year, with commissioning from 2028. USA Rare Earth’s South Carolina plant targets 6,400 tonnes a year of sintered NdFeB, also from 2028. Even with those, projections leave China with more than 82% of global NdFeB capacity in 2030. For the next two years, a humanoid maker outside China that needs heat-resistant grades is buying from Chinese magnet plants or from the small pool of alternatives in Japan and Europe.

Ways out, none of them quick

Actuator teams have four levers. Each costs something.

• Cut the dysprosium, not the magnet. Grain-boundary diffusion and Dy-lean grades hit the same coercivity with less heavy rare earth, at the price of requalifying every part.
• Run the magnets cooler. Better heat paths from winding to housing let a designer drop a temperature class – from UH to SH, say – and every class down means less Dy or Tb in the alloy.
• Accept ferrite where mass matters least. Torso and waist joints can swallow a 30% heavier motor more easily than an ankle can.
• Wait for new materials. Niron Magnetics’ iron nitride is rare-earth-free by design; its July 2026 agreement with ASPINA covers brushless DC and stepper motors, backed by a Minnesota plant with a conditional $150 million federal loan. No humanoid joint runs on it yet.

The date to watch is 10 November. If the October 2025 rules come back, the licence question stops being about magnets shipped from China and starts covering motors and actuators built elsewhere with Chinese material inside – which today means almost all of them.

FAQ

Sources