🔬 applied physics

Structural, electronic, and magnetic properties of HoCo $_2$ thin films grown by pulsed laser deposition

This study reports the successful growth of smooth, high-quality HoCo $_2$ thin films on Si substrates via pulsed laser deposition, which retain bulk-like ferrimagnetic ordering at 77 K but exhibit a continuous second-order transition and significantly enhanced coercivity compared to the bulk material.

Original authors: Ajay Kumar, Christopher Burgio, Anis Biswas, Yaroslav Mudryk

Published 2026-10-06

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CC BY 4.0

Original authors: Ajay Kumar, Christopher Burgio, Anis Biswas, Yaroslav Mudryk

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where cooling a room or preserving food requires no electricity, no noisy compressors, and no harmful gases. Instead, the cooling happens simply by applying and removing a magnetic field. This is the promise of magnetic refrigeration, a technology that relies on materials that get hot when magnetized and cold when the magnetism is removed. For this process to work efficiently, scientists look for materials that undergo a dramatic change in their magnetic state at a specific temperature, a shift that releases or absorbs a large amount of heat. Among the most promising candidates for this job are compounds made from a rare-earth metal and cobalt. In their natural, bulk form, these materials act like a switch that flips abruptly from one magnetic state to another. However, to use them in real-world devices, scientists often need to shrink them down into very thin layers, like the coating on a microchip. The big question is: when you squeeze these materials into such a thin sheet, do they keep their powerful, abrupt switching ability, or does the thinness change how they behave?

Researchers at the Ames National Laboratory set out to answer this question by creating a specific type of thin film made from holmium and cobalt. They used a technique called pulsed laser deposition, which involves firing rapid bursts of a high-energy laser at a solid block of the material. This process blasts tiny particles off the block, which then travel across a vacuum chamber and land on a silicon surface, building up a layer atom by atom. The team grew films that were incredibly smooth, with a thickness of about 35 nanometers, which is roughly one ten-thousandth the width of a human hair. To ensure the quality of their work, they examined the films with powerful microscopes and X-ray beams, confirming that the surface was as smooth as a polished mirror and that the atoms were arranged in the correct, orderly pattern expected for this material.

When the scientists tested how these thin films reacted to temperature changes, they found that the material still underwent its magnetic transition at the expected temperature of about 77 Kelvin, which is roughly minus 300 degrees Fahrenheit. This was a crucial finding, proving that the essential magnetic character of the material survived the journey from a thick block to a microscopic film. However, the nature of that transition had changed in a significant way. In the thick, bulk version of this material, the magnetic switch flips suddenly and sharply, like a light switch being toggled on or off. This sudden flip creates a small gap in temperature between heating and cooling, known as thermal hysteresis. In the thin films, this gap disappeared. The transition became smooth and continuous, more like a dimmer switch that gradually brightens or dims rather than snapping instantly. This suggests that the physical constraints of being a thin film altered the way the atoms interacted, softening the abruptness of the change.

The researchers also looked at how the films behaved when exposed to a magnetic field at very cold temperatures. While the thick blocks of material showed almost no resistance to changing their magnetic alignment, the thin films held onto their magnetism much more strongly. When the magnetic field was removed, the films required a significant push to reset, showing a level of resistance that was hundreds of times stronger than the bulk material. This indicates that the thinness of the film introduced new internal forces that made the magnetic state more stubborn. Despite these changes in magnetic behavior, the electronic structure of the material remained largely intact. By analyzing the energy levels of the electrons inside the films, the team confirmed that the atoms were still in the same chemical state as they were in the bulk material, preserving the fundamental electronic environment necessary for the material to function.

These results offer a clear picture of what happens when powerful magnetic materials are scaled down. The thin films successfully retained the core magnetic transition temperature, proving they are viable candidates for further study. Yet, the shift from a sharp, sudden change to a smoother, gradual one suggests that the geometry of the material plays a major role in how it responds to heat and magnetism. This discovery is vital for anyone hoping to build magnetic refrigerators, as the smoothness of the transition affects how efficiently the device can cycle between hot and cold. The study confirms that while the material can be made into a thin film, engineers must account for the fact that the thinness itself changes the rules of the game, turning a sharp magnetic switch into a more gradual dimmer. Future work will need to explore whether changing the thickness of the film can restore the sharp switch or if this smooth behavior is an unavoidable feature of the thin-film form.

Technical Summary: Structural, Electronic, and Magnetic Properties of HoCo₂ Thin Films Grown by Pulsed Laser Deposition

Problem Statement
Rare-earth transition metal intermetallics, specifically the cubic Laves phases $RCo_2$ (where $R$ is a rare earth), serve as critical platforms for studying the interplay between localized rare-earth magnetism, itinerant Co 3d electrons, and magnetoelastic coupling. Heavy rare-earth members like HoCo₂ exhibit a first-order ferrimagnetic transition near 77 K, accompanied by a large magnetocaloric response and a structural transformation from cubic to tetragonal. While these properties make them promising for cryogenic magnetic refrigeration, extending these materials to thin-film geometries presents significant challenges. Achieving targeted stoichiometry in sputtered films is difficult due to deposition sensitivity, and pulsed laser deposition (PLD) of rare-earth intermetallics often suffers from droplet formation and surface roughness due to the high fluence required. Furthermore, it remains unclear how reduced dimensionality and film-specific structural constraints affect the thermodynamic nature of the magnetic transition (e.g., whether the first-order character is preserved or modified) and the intrinsic electronic structure.

Methodology
The authors synthesized bulk HoCo₂ targets via arc melting and prepared them for PLD. Thin films were grown on Si(100) substrates using a 248 nm KrF excimer laser under ultrahigh-vacuum conditions at a substrate temperature of 800°C. Two films were prepared under identical conditions to ensure reproducibility.

The study employed a comprehensive suite of characterization techniques:

Structural Analysis: X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GI-XRD) were used to determine film thickness, interface quality, and crystallinity. Atomic force microscopy (AFM) assessed surface morphology and roughness.
Compositional Analysis: Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) mapped elemental distributions and quantified stoichiometry.
Magnetic Characterization: Temperature-dependent magnetic susceptibility ( $\chi(T)$ ) and field-dependent magnetization ( $M(H)$ ) loops were measured using a SQUID magnetometer.
Electronic Structure: X-ray photoelectron spectroscopy (XPS) was utilized to examine core-level spectra (Ho 4d, Co 2p) and valence-band structures to compare the electronic environment of the films against the bulk material.

Key Results

Film Quality and Morphology: XRR data revealed well-defined Kiessig fringes, indicating smooth surface and interface profiles. Fast Fourier transform and modified Bragg equation analyses yielded a film thickness of $35 \pm 1$ nm. AFM confirmed high morphological quality with a root-mean-square (RMS) surface roughness of approximately 170 pm.
Composition: SEM-EDS analysis showed a homogeneous lateral distribution of Ho and Co. The films exhibited a composition of 35.3 at.% Ho and 64.7 at.% Co, indicating a slight Ho-rich deviation from the nominal 1:2 stoichiometry, likely influenced by substrate signal contributions in thin films.
Magnetic Behavior:
- Transition Temperature: Both films exhibited a reproducible ferrimagnetic transition at $T_C = 77 \pm 3$ K, retaining the characteristic ordering temperature of bulk HoCo₂.
- Transition Order: Unlike the bulk material, which displays a sharp first-order transition with a thermal hysteresis of ~1 K, the thin films showed no resolvable thermal hysteresis. This suggests a predominantly continuous, second-order-like transition in the film geometry.
- Coercivity: At 5 K, the films displayed clear magnetic hysteresis with a coercive field of ~600 Oe. This is substantially larger than the negligible coercivity observed in bulk HoCo₂, though comparable to values reported for other PLD-grown $RCo_2$ -related films.
Electronic Structure: XPS analysis revealed that the Ho 4d core-level line shapes and Co 2p spectra in the films were broadly similar to those in the bulk. The preservation of multiplet structures and asymmetric metallic Co features indicates that the essential local electronic environment and occupied electronic structure are retained despite the thin-film growth.

Significance and Claims
The paper establishes the feasibility of growing high-quality, smooth HoCo₂ thin films on Si substrates using PLD while preserving the characteristic magnetic transition temperature. The primary finding is that reducing the dimensionality to ~35 nm modifies the thermodynamic nature of the magnetic transition from first-order (bulk) to second-order-like (film), evidenced by the absence of thermal hysteresis. Additionally, the films exhibit enhanced magnetic coercivity compared to the bulk.

The authors conclude that while the essential electronic structure is retained, the modification of the transition order suggests that film-specific structural constraints significantly alter spin-lattice coupling. The study highlights the need for future investigations into film thickness dependence to identify critical thicknesses for this transition-order change and temperature-dependent structural measurements to determine if the cubic-to-tetragonal transformation persists in the thin-film geometry. These factors are deemed essential for understanding how the giant magnetocaloric response of HoCo₂ evolves with reduced dimensionality.

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