Researchers at the Lawrence Berkeley National Laboratory have successfully demonstrated that two-dimensional (2D) layered crystals held together by van der Waal forces—these include graphene and molybdenum disulfide—can exhibit intrinsic ferromagnetism. Not only did the team demonstrate that it exists in these materials, but the researchers also demonstrated a high degree of control over that ferromagnetism. The discovery could have a profound impact for applications including magnetic sensors and the developing use of spintronics for encoding information.
In research described in the journal Nature, the Berkeley scientists worked with a 2D chalcogenide layered material called chromium germanium telluride (CGT), a layered ferromagnetic insulator that has garnered interest because of its potential in spintronic devices. While the material has been around in bulk form for decades, only recently has it been made into 2D flakes, joining the list of other van der Waals crystals.
The researchers used an optical technique known as the magneto-optic Kerr effect that involves the use of a scanning Kerr optical microscope to observe the material. This technique detects how the rotation of linearly polarized light is changed when it interacts with electron spins in the material. This made it possible to detect unambiguously that the magnetism was originating from the atomically thin materials.
“Our discovery of intrinsic ferromagnetism in 2D van der Waals crystals has opened a scientific research field,” said Xiang Zhang, a senior faculty scientist at Berkeley Lab's Materials Sciences Division and UC Berkeley professor, in an e-mail interview with IEEE Spectrum.
While ferromagnetism in 2D van der Waals crystals was not thought to be theoretically impossible, it was extraordinarily difficult to detect and even to create the circumstances in which it exists, according to Zhang. This is because at non-zero temperatures, thermal energy inevitably enters into a ferromagnetic material and excites the aligned electron spins. This thermal excitation is much stronger in 2D materials than in their 3D counterparts. As a result, when a given material shrinks from 3D to 2D regime, the critical temperature at which the ferromagnetism survives drops substantially. Due to this strong thermal effect, 2D ferromagnetism is fundamentally fragile.
Although thermal effect plays a critical role in suppressing the magnetic order, the 2D van der Waals crystal studied by the Berkeley Lab researchers has an inherent magnetic anisotropy—that is, the magnetization orientation has a preferred direction. The energy difference between the preferred and non-preferred directions stabilizes the 2D magnetic order against the thermal excitations, making it possible for the Berkeley scientists to observe the 2D ferromagnetism.
“Thin films of metals like iron, cobalt, and nickel, unlike 2D van der Waals materials, are structurally imperfect and susceptible to various disturbances, which contribute to a huge and unpredictable spurious anisotropy,” said Cheng Gong, a postdoctoral researcher in Zhang's lab and co-author of the study, in a press release. “In contrast, the highly crystalline and uniformly flat 2D CGT, together with its small intrinsic anisotropy, allows small external magnetic fields to effectively engineer the anisotropy, enabling an unprecedented magnetic field control of ferromagnetic transition temperatures.”
This discovery could be the answer to the demand for increasing information storage density as device feature sizes decrease, says Zhang. “Sooner or later, people have to address the ferromagnetism issues in 2D materials, when 3D materials shrink down to 2D regime,” said Zhang. “In other words, 3D materials have to be thinned down to 2D in many fields as a result of the constantly-increasing device density.”
In general, electronics and optics are turning to 2D materials because they are more suitable for tiny devices. In addition, the properties of 2D materials can be easily controlled and modulated, making them particularly well suited for sensors and modulators. And because the 2D materials are normally optically transparent, with limited light absorbance, they make sense for a range of optical applications.
“We envision that 2D ferromagnetic van der Waals materials would also have a broad range of potential applications such as nanoscale memories, magnetic sensors, transparent magnets, magneto-optic modulators,” said Zhang.
In continuing research, Zhang and his colleagues intend to direct their focus two directions: fundamental physics and application-oriented approaches. The 2D van der Waal crystals the researchers demonstrated provide an ideal experimental platform to address the intriguing and rich physics of electron spins, which are strictly confined in a perfect and real 2D material system.
Zhang adds: “We hope to engineer and manipulate the magnetic properties of such 2D materials to make them suitable for various application purposes.”