Researchers at Shenzhen University, the Japan Atomic Energy Agency, the National Institutes for Quantum Science and Technology (QST), the University of Tokyo, the University of Science and Technology of China, Xi'an Jiaotong University, Shenzhen Technology University (SZTU), and The Hong Kong Polytechnic University, have grown a wafer-scale hyper-porous graphene network (HGN) that shows intrinsic ferromagnetism at room temperature.
Unlike a conventional graphene film, the HGN's layers stand roughly perpendicular to the substrate and curve, split, and crosslink into a three-dimensional mesh riddled with pores smaller than about 2 nm.
Atomic-resolution transmission electron microscopy revealed that these pores expose abundant zigzag graphene edges, many of them separated by more than roughly 1.4 nm.
To confirm the structural origin of the effect, the researchers annealed the porous network, collapsing it into a denser carbon structure; the carbon-specific magnetic signal disappeared and the magnetization dropped to roughly the amorphous-carbon control's level.
The pore geometry, not simply the number of edge or defect sites, is therefore what determines whether the material's magnetic moments add up or cancel.
Researchers at Shenzhen University, the Japan Atomic Energy Agency, the National Institutes for Quantum Science and Technology (QST), the University of Tokyo, the University of Science and Technology of China, Xi'an Jiaotong University, Shenzhen Technology University (SZTU), and The Hong Kong Polytechnic University, have grown a wafer-scale hyper-porous graphene network (HGN) that shows intrinsic ferromagnetism at room temperature.
Optical image of HGN on a 2-inch Si wafer. Image from: Nanowerk
The film was produced across a full 2-inch oxide-coated silicon wafer using a plasma-assisted vapor process combined with low-energy electron irradiation, and reached a magnetic moment density roughly 1,000 times higher than defected highly oriented pyrolytic graphite (HOPG).
Unlike a conventional graphene film, the HGN's layers stand roughly perpendicular to the substrate and curve, split, and crosslink into a three-dimensional mesh riddled with pores smaller than about 2 nm. Atomic-resolution transmission electron microscopy revealed that these pores expose abundant zigzag graphene edges, many of them separated by more than roughly 1.4 nm. The same deposition setup, run under different conditions, also produced an amorphous carbon film with a much more disordered atomic arrangement and no comparable network of exposed edges, giving the researchers a directly comparable control material made in the same growth environment.
At room temperature, the porous network reached a saturation magnetization of 4.7 emu/g, more than three orders of magnitude above the graphite reference, corresponding to a magnetic moment of about 0.06 μB per carbon atom from bulk magnetometry. To rule out the usual culprit behind claims of magnetic carbon, trace magnetic contamination, the team used X-ray magnetic circular dichroism (XMCD) tuned to the carbon absorption edge, which reversed sign as expected when the magnetic field or X-ray polarization was reversed, while graphite and the amorphous carbon control showed no comparable signal. XMCD scans at the characteristic absorption energies of iron, cobalt, and nickel detected none of their signatures, and mass spectrometry showed that trace magnetic impurities in the sample could account for at most about 0.23% of the measured magnetization.
To confirm the structural origin of the effect, the researchers annealed the porous network, collapsing it into a denser carbon structure; the carbon-specific magnetic signal disappeared and the magnetization dropped to roughly the amorphous-carbon control's level. Density functional theory calculations showed why edge spacing matters: when opposing zigzag edges sit roughly 1.4 nm or more apart, their unpaired spins favor parallel alignment and reinforce each other, while closer spacing flips the preferred alignment and causes the moments to cancel out. The pore geometry, not simply the number of edge or defect sites, is therefore what determines whether the material's magnetic moments add up or cancel.
The authors describe the result as the first element-resolved confirmation of intrinsic carbon magnetism at room temperature in a bulk (wafer-scale) carbon system, extending mechanisms previously demonstrated only in individual graphene nanoribbons and nanographene molecules to a scalable film. Because the pore geometry appears to be the controlling variable, the authors suggest that if that geometry can be reliably tuned, the resulting magnetic state could become tunable as well, opening a route to explore spintronic and magnetic applications of carbon allotropes.
