Single Atom Magnets on Thermally Stable Adsorption Sites: Dy on NaCl(100)

This study demonstrates that single Dysprosium atoms on NaCl(100) films, particularly those substituting surface sodium ions, exhibit thermally stable magnetic bistability up to 300 K and long spin relaxation times, establishing NaCl as a viable platform for single-atom magnets.

The Gist

Imagine you are trying to build a tiny, ultra-powerful hard drive, but instead of silicon chips, you are using individual atoms as the storage units. This is the dream of Single Atom Magnets (SAMs). If we can make a single atom act like a tiny magnet that holds its direction (North or South) for a long time, we could store massive amounts of data in a space smaller than a speck of dust.

However, there's a big problem: Heat. Just like a spinning top eventually wobbles and falls over when the table shakes, these atomic magnets get jostled by heat and lose their magnetic direction almost instantly. For years, scientists have struggled to find a "parking spot" for these atoms that is stable enough to keep them from wandering off or flipping over when things get warm.

This paper reports a major breakthrough: They found the perfect parking spot.

Here is the story of how they did it, explained simply:

1. The Problem: The Wobbly Table

Previously, scientists put these magnetic atoms (like Holmium or Dysprosium) on top of thin films of materials like Magnesium Oxide (MgO). It worked okay at very cold temperatures, but as soon as the temperature rose even a little (above -200°C), the atoms would start to slide around like marbles on a tilted table. Once they slid, they lost their magnetic power.

2. The Solution: The "Invisible Hole"

The researchers decided to try a different surface: Sodium Chloride (NaCl), which is just common table salt. But they didn't just put the atoms on top of the salt; they made the atoms swap places with the salt atoms.

• The Analogy: Imagine a checkerboard made of red and blue squares (the salt). Usually, you put a special gold piece on top of a square. But here, the researchers took a gold piece and swapped it with a red square. The gold piece is now part of the board, sitting snugly in the hole where the red square used to be.
• Why this matters: Because the gold piece is now part of the structure, it can't slide around. It's locked in place. The paper shows that these "swapped" atoms stay put even at room temperature (300 K), which is a huge leap forward.

3. The Two Types of Magnets

The team discovered that depending on how they placed the atoms, they got two different types of magnetic behavior:

• The "Locked-in" Magnet (Substitutional): These are the atoms that swapped places with the salt. They are incredibly stable. They can hold their magnetic direction for about 10 seconds at very low temperatures. In the world of atoms, 10 seconds is an eternity (like a human living for a million years). Most importantly, they don't move even when it's warm.
• The "Perched" Magnet (Adatom): These are atoms sitting on top of the salt surface. They are even more magnetic, holding their direction for 550 seconds! However, they are like a bird sitting on a fence post; if the wind (heat) blows too hard, they might fly away. They are powerful but less stable.

4. Why Salt?

You might wonder, "Why table salt?"
Think of the metal substrate (the metal underneath the salt) as a noisy, chaotic crowd that distracts the magnetic atoms. The thin layer of salt acts like a soundproof wall or a quiet cushion. It isolates the atom from the noisy crowd below, allowing the atom to focus on its own magnetic spin without getting disturbed by electrons or vibrations.

5. The Big Picture

This paper is a game-changer because it solves the two biggest hurdles at once:

1. Stability: The atoms stay put at room temperature (no more sliding!).
2. Memory: They can hold their magnetic direction for a long time (no more flipping!).

The Takeaway:
By using a simple trick of swapping atoms into a salt crystal, the researchers have created the first "Single Atom Magnet" that is both thermally stable and magnetically robust. This proves that table salt (NaCl) is a perfect platform for building the ultra-dense, atomic-scale hard drives of the future. It's like finding a way to park a car in a storm without it getting blown away, finally paving the way for computers that are millions of times smaller and more powerful than today's technology.

Technical Summary

1. Problem Statement

The development of Single-Atom Magnets (SAMs) aims to create atomic-scale magnetic memory with high thermal stability and long spin relaxation times ($T_1$). While significant progress has been made with rare-earth (RE) atoms like Holmium (Ho) and Dysprosium (Dy) on oxide surfaces (e.g., MgO), a critical bottleneck remains: thermal diffusion.

• Current Limitation: The most stable SAMs (e.g., Ho on MgO) exhibit long $T_1$ but are thermally unstable. They diffuse from their magnetic adsorption sites (e.g., top-O) to bridge sites at relatively low temperatures (58–70 K), where they lose their magnetic bistability and become paramagnetic.
• Goal: To identify a system where the adsorption site is thermally stable up to room temperature (RT) while maintaining the electronic configuration necessary for slow magnetic relaxation.

2. Methodology

The researchers employed a multi-modal approach combining experimental characterization with theoretical modeling:

• Sample Preparation:

  • Grown ultrathin NaCl films (2–10 monolayers) on Ag(111) and Cu(111) substrates.
  • Deposited Dysprosium (Dy) atoms at varying temperatures: Room Temperature (RT, 300 K) and Low Temperature (LT, 4–10 K).
  • Varied film thickness to control the preferred adsorption site (substitutional vs. adatom).

• Experimental Techniques:

  • Scanning Tunneling Microscopy (STM): Used to visualize atomic positions, determine adsorption sites (substitutional vs. adatom), and verify thermal stability (absence of diffusion/clustering) at RT.
  • X-ray Absorption Spectroscopy (XAS) & X-ray Magnetic Circular Dichroism (XMCD): Performed at the M4,5 edges of Dy to determine electronic configuration ($4f$ occupancy), magnetic anisotropy (easy axis direction), and spin relaxation times ($T_1$). Measurements were taken at Normal Incidence (NI) and Grazing Incidence (GI) with magnetic field sweeps.
  • Inelastic Electron Tunneling Spectroscopy (IETS): Used to check for spin polarization of outer $5d6s$ shells.

• Theoretical Modeling:

  • Density Functional Theory (DFT): Used to calculate adsorption energies, defect formation energies, and charge density distributions. Gd and Eu were used as analogues for Dy (due to similar $f-d$ promotion energies) to simplify calculations while retaining valency characteristics.
  • Multiplet Calculations: Combined with DFT-derived crystal field (CF) parameters (modeled as point charges) to simulate XAS, XMCD, and magnetization curves.

3. Key Contributions & Results

A. Discovery of Thermally Stable Substitutional Sites

• Observation: When Dy is deposited at RT on 2–3 ML NaCl films, the atoms substitute Na cations in the topmost NaCl layer rather than sitting on top as adatoms.
• Stability: STM measurements confirmed that these substitutional Dy atoms do not diffuse or cluster even at 300 K. DFT calculations revealed a high energy barrier (>3.5 eV) for diffusion, making them the first SAMs with a thermally stable adsorption site up to room temperature.
• Electronic State: These substitutional atoms exhibit a $4f^9$ electronic configuration (due to high coordination and charge transfer), distinct from the $4f^{10}$ state of Dy adatoms.

B. Magnetic Properties of Substitutional Dy ($4f^9$)

• Anisotropy: XMCD data shows a strong out-of-plane easy magnetization axis.
• Relaxation Time ($T_1$): At 2.5 K, the spin relaxation time is approximately 10 seconds.
• Hysteresis: The system displays magnetic hysteresis with a coercive field of $\approx \pm 0.1$ T and a remanence of $\approx 25\%$ of saturation.
• Mechanism: The $4f^9$ Kramers ion has a ground doublet protected against Quantum Tunneling of Magnetization (QTM) at zero field, enabling magnetic remanence.

C. Magnetic Properties of Dy Adatoms ($4f^{10}$)

• Sites: At low deposition temperatures (4–10 K) on thicker films (8 ML), Dy forms adatoms on top-Cl and bridge sites.
• Electronic State: These adatoms exhibit a $4f^{10}$ configuration.
• Relaxation Time: The top-Cl adatoms show an exceptionally long $T_1$ of 550 s at 0.3 T and 2.5 K. However, they lack structural stability at RT (they diffuse).
• QTM: The ground state of top-Cl Dy ($4f^{10}$) is a non-Kramers doublet with a small zero-field splitting ($\Delta E = 0.65$ neV), leading to QTM and no magnetic remanence at zero field.
• Bridge Sites: Also $4f^{10}$ but with an in-plane easy axis and strong mixing of states.

D. Role of the NaCl Substrate

• NaCl acts as an effective decoupling layer, suppressing electron and phonon scattering from the metallic substrate.
• The crystal field provided by NaCl is sufficient to induce slow spin relaxation without the need for complex molecular ligands.
• The study confirms that the $5d6s$ shells are not spin-polarized for these systems, meaning the magnetism is purely driven by the $4f$ electrons.

4. Significance

• Thermal Stability Breakthrough: This work resolves the primary limitation of previous SAMs (thermal diffusion) by demonstrating that NaCl(100) supports Dy atoms in substitutional sites that are stable up to 300 K.
• Platform for Quantum Technologies: The combination of thermal stability (RT) and long spin lifetimes (seconds) makes NaCl a superior platform for single-atom magnetic memory and quantum information processing compared to MgO.
• Site-Dependent Magnetism: The paper elucidates how the adsorption site (substitutional vs. adatom) and film thickness dictate the electronic configuration ($4f^9$ vs. $4f^{10}$), anisotropy, and relaxation mechanisms (Kramers vs. non-Kramers).
• Methodological Advance: The successful integration of DFT-derived point charge models with multiplet simulations provides a robust framework for predicting magnetic properties of RE atoms on insulating surfaces without arbitrary fitting parameters.

In conclusion, the authors demonstrate that Dy on NaCl(100) represents a paradigm shift in SAM research, offering a system where the atom is structurally locked in place at room temperature while retaining the magnetic bistability required for atomic-scale data storage.