#ferromagnetism

Levitated nano-ferromagnet confirms a 160-year-old physical prediction

Ferromagnets, such as iron, cobalt, and nickel, are materials with a strong, spontaneous, and permanent magnetic field. Over 150 years ago, the physicist and mathematician James Clerk Maxwell speculated that under specific conditions, non-spinning ferromagnets or electromagnets would behave as gyroscopes, objects that maintain their orientation, typically due to the angular momentum arising from spinning. Maxwell hypothesized that this unique gyroscopic behavior would arise from the relationship between a ferromagnet's magnetism and its angular momentum within a specific set-up. While numerous studies tested this prediction, so far it had never been proven experimentally.

Laser beam flips a ferromagnet's polarity without heating the material

Researchers at the University of Basel and the ETH in Zurich have succeeded in changing the polarity of a special ferromagnet using a laser beam. In the future, this method could be used to create adaptable electronic circuits with light. In a ferromagnet, combined forces are at work. In order for a compass needle to point north or a fridge magnet to stick to the fridge door, countless electrons spin inside them, each of which only creates a tiny magnetic field, all need to line up in the same direction. This happens through interactions between the spins, which have to be stronger than the disordered thermal motion inside the ferromagnet. If the temperature of the material is below a critical value, it becomes ferromagnetic. Conversely, to change the polarity of a ferromagnet, one usually needs to first heat it up above its critical temperature. The electron spins can then reorient themselves, and after cooling down, the magnetic field of the ferromagnet eventually points in a different direction.

Mirror symmetry prompts ultralow magnetic damping in 2D van der Waals ferromagnets

Two-dimensional (2D) van der Waals (vdW) ferromagnets are thin and magnetic materials in which molecules or layers are held together by weak attractive forces known as vdW forces. These materials have proved to be promising for the development of spintronic devices, systems that operate leveraging the spin (i.e., intrinsic angular momentum) of electrons, as opposed to electric charge. A crucial parameter in the context of magnetization is the so-called Gilbert damping coefficient, which indicates how quickly a material's magnetization loses energy and returns to a state of equilibrium after being disturbed. A lower damping coefficient is more favorable for the development of spintronics, as it means that less energy is lost once a material's magnetization is set into motion. Researchers at Beijing Normal University, Shanghai University and Fudan University carried out a study aimed at better understanding the underpinnings of low Gilbert damping in 2D vdW ferromagnets.

"It is anticipated that within just a few decades, the surging volume of digital data will constitute one of the world's largest energy consumers. Now, researchers at Chalmers University of Technology, Sweden, have made a breakthrough that could shift the paradigm: an atomically thin material that enables two opposing magnetic forces to coexist—dramatically reducing energy consumption in memory devices by a factor of 10."

"The article, "Coexisting Non-Trivial Van der Waals Magnetic Orders Enable Field-Free Spin-Orbit Torque Magnetization Dynamics" has been published in Advanced Materials.

Memory units are essential components in virtually all modern technologies that process and store information—AI systems, smartphones, computers, autonomous vehicles, household appliances and medical devices. Magnetism has emerged as a key player in the evolution of digital memory."

"In physics and engineering, two fundamental magnetic states are typically considered: ferromagnetism and antiferromagnetism. Ferromagnetism is the familiar phenomenon (seen in everyday magnets) that attracts materials like iron, nickel or cobalt. In this state, electrons align uniformly (...) creating a unified magnetic field that is externally visible.

In contrast, antiferromagnetism involves electrons with opposing spins, causing their magnetic states to cancel each other out. Combining these two opposing forces offers significant scientific and technical advantages, making them perfect for computer memory and sensors. But until now, this has only been possible by stacking different ferromagnetic and antiferromagnetic materials in multilayer structures.

"Unlike these complex, multilayered systems, we've succeeded in integrating both magnetic forces into a single, two-dimensional crystal structure. It's like a perfectly pre-assembled magnetic system—something that couldn't be replicated using conventional materials."

"For decades, ferromagnetic materials have driven technologies like magnetic hard drives, magnetic random access memories and oscillators. But antiferromagnetic materials, if only they could be harnessed, hold out even greater promise: ultra-fast information transfer and communications at much higher frequencies—a "holy grail" for physicists.

Now, researchers have taken a meaningful step towards utilizing antiferromagnets for new technologies. In "Spin-filter tunneling detection of antiferromagnetic resonance with electrically-tunable damping," published in Science, they describe their innovative approach for both detecting and controlling the motion of spins within antiferromagnets using 2D antiferromagnetic materials and tunnel junctions.

Both types of materials contain atoms that act like tiny individual magnets, each having "spin." In a ferromagnet, all of these atomic spins are aligned, producing an external magnetic field. In an antiferromagnet, atomic spins cancel when they are added up, so no external magnetic field is produced. That's why it's difficult to not only detect the motions of spins within antiferromagnets but also control the motion of their spins."

Andrei, E. Y. et al. The marvels of moiré materials. Nat. Rev. Mater. 6, 201–206 (2021). ADS  CAS  Google Scholar  Kennes, D. M. et al. Moiré heterostructures as a condensed-matter quantum simulator. Nat. Phys. 17, 155–163 (2021). CAS  Google Scholar  Angeli, M. & MacDonald, A. H. Γ valley transition metal dichalcogenide moiré bands. Proc. Natl Acad. Sci. 118, e2021826118…

Quantum Computer Shows Zero-Temperature Phase Transition, Challenging Physics Predictions

For the first time, a worldwide team replicated spontaneous symmetry breaking (SSB) at zero temperature, a major advance in condensed matter physics and  quantum computing. A superconducting quantum processor with over 80% fidelity was employed for this landmark work on short-range phase transitions. Brazilian UFSCar, Danish Aarhus University, and Chinese Southern University of Science and Technology researchers published in Nature Communications.

The experiment focused on spontaneous symmetry breaking, crucial to condensed matter physics and the standard model. Symmetry creates conservation laws in physical systems, yet breaking it can create complex structures. The Mermin-Wagner theorem prohibits the development of ordered phases like FM and AFM in short-range interacting systems at any finite temperature in one or two dimensions. One-dimensional systems were widely believed to restrict Spontaneous Symmetry Breaking even at zero temperature.

Symmetry Breaking Spontaneously

In condensed matter physics and the standard model, spontaneous symmetry breakdown is essential. This idea is linked to physical systems and complex structures.

Spontaneous Symmetry Breaking is described in detail from the references:

Central Idea:

Symmetry causes conservation laws in most physical systems. Noether's theorem shows this conservation-symmetry relationship.

However, breaking symmetry can create complicated structures. Spontaneous Symmetry Breaking occurs when a system's ground state or lowest-energy state spontaneously transitions to a state without symmetry in its governing principles.

Traditional Understanding and Challenges (Mermin-Wagner Theorem):

Unplanned Symmetry Breaking in one- and two-dimensional systems is a hot topic for quantum phase transitions at limiting temperatures. In physical systems with long-distance interactions, long-range order like ferromagnetism (FM) or antiferromagnetism (AFM) generally emerges.

According to the Mermin-Wagner theorem, short-range interacting systems in one or two dimensions cannot create correlated antiferromagnetic (AFM) and ferromagnetic (FM) states at any finite temperature. This theorem covers Hubbard and Kondo lattices, Heisenberg chain spin systems, and metal electron interactions.

Spontaneous Symmetry Breaking is assumed to be forbidden for one-dimensional systems even at zero temperature. Compared to its limited temperature equivalent, this field has attracted less attention.

The Experimental Breakthrough in SSB Observation:

The new work simulates Spontaneous Symmetry Breaking at zero temperature for the first time. More than 80% fidelity was achieved using a superconducting quantum processor.

This work focused on replicating dynamics at zero temperature, which is unachievable in reality due to the unattainability principle, the third law of thermodynamics. Quantum computers mimicked absolute zero.

The experiment proved that symmetry breaking can occur in local particle interactions, such as between first neighbours, when the temperature is zero. This violates traditional physics expectations that certain phases are banned for short-range interacting systems at any finite temperature.

SSB in Experiment:

Digital quantum annealing powered the device, which had seven qubits in a superconducting lattice resembling a three-generation Cayley tree (allowing only nearest-neighbor interactions).

The starting state was a classical antiferromagnetic (AFM) 'flip-flop configuration' in which particle spins alternated. Initial state was called “Classical Néel state”.

After that, the system spontaneously transformed into a ferromagnetic (FM) quantum state with quantum correlations and particle spins aligned.

This phase transition from classical AFM to quantum FM-like is clearly attributed. From the ground state of the Néel field Hamiltonian, the AFM phase developed, whereas from the excited state, an ordered quantum FM-like state emerged. Depending on the initial Néel state, FM-like or AFM-like phases produced the system's energy split.

Symmetry in System Dynamics:

The initial Hamiltonian for the Néel state is symmetric in (\hat{M}{z}) (total magnetisation), ([\hat{H}{\text{Néel}}, \hat{M}_{z}]=0)21.

Thus, each adiabatic development from ground or excited Néel states will occur across a separate magnetisation plane. The excited state is positively magnetised, while the ground state is negatively magnetised.

The system also maintained the parity defined by the operator (\hat{\Pi}_z = \prod \hat{\sigma}_z) ([\hat{\Pi}_z, \hat{H}(s)]=0)22. This parity conservation law prevented degraded states with differing parities from mixing during evolution, preventing the annihilation of associated phases22.

Witnessing and Quantifying SSB/Entanglement:

The phase transition and ordered patterns (({C}_{x}^{(i,j)}) were identified and quantified using two-point correlation functions. These functions showed the system's dynamical symmetry breakdown.

The quantum character of FM-like and AFM-like phases was revealed by studying entanglement entropy, particularly the second-order Rényi entropy.

The Rényi entropy was invented by Hungarian mathematician Alfréd Rényi to assess entanglement and its distribution in quantum systems. It monitors subsystem entanglement even in mixed quantum states.The Rényi entropy of a subsystem increases when it is entangled (assuming the system is pure).