What are the structural, electronic, and magnetic properties of pulsed-laser-deposited HoCo2 thin films?

Kiessig fringes fix the film thickness at 35 nm

The paper's structural answer comes first from x-ray reflectivity. Well-defined Kiessig fringes appear over a broad angular range, which the authors read as evidence of a continuous film with smooth surface and interface profiles. Two independent analyses of those oscillations agree: a fast Fourier transform gives 37 ± 2 nm, while a linear fit of the modified Bragg equation over oscillation orders m = 5–25 gives 35 ± 1 nm. The reported thickness is therefore 35 ± 1 nm, with the FFT value as a cross-check rather than a competing number.

Atomic force microscopy supports the same picture at the surface. Over a 1 × 1 µm² area, the films show subnanometer roughness of about 170 pm. That matters because the introduction notes that the high fluence needed to ablate rare-earth intermetallic targets in pulsed laser deposition can promote droplet formation and complicate smooth growth. Here the morphology is smooth, so the magnetic and spectroscopic results are not being read through a heavily roughened film.

One Bragg reflection confirms crystallinity but not phase purity

Room-temperature grazing-incidence x-ray diffraction at 0.1° incidence shows a distinct reflection at 2θ ≈ 50.75°, which corresponds to the (400) Bragg reflection of the cubic HoCo2 Laves phase. Increasing the incident angle weakens this film reflection while Si-related reflections become more prominent, indicating a growing substrate contribution. The authors state plainly that because only one film-related reflection is resolved, the data do not allow a full structural refinement or an unambiguous assessment of texture and phase purity.

Composition comes from SEM-EDS. The elemental maps show homogeneous lateral distribution of Ho and Co, and the spectrum yields approximately 35.3 at.% Ho and 64.7 at.% Co. That is slightly Ho-rich relative to the nominal 1:2 HoCo2 stoichiometry, but the authors caution that the deviation may come from EDS quantification uncertainty in a film this thin, where the signal can be influenced by the underlying Si substrate. The bulk target used for deposition, by contrast, measured 33.9% Ho and 66.1% Co and was confirmed by Rietveld refinement as single-phase cubic MgCu2-type C15 with a = 7.1813 Å.

Two films reproduce the 77 K ferrimagnetic transition

Magnetic susceptibility measured under a 1 kOe applied field on two films deposited under identical conditions shows an ordering transition near TC ≈ 77 K, identified more clearly from dχ/dT curves. Both samples give TC = 77 ± 3 K, and the paper treats this reproducibility across independently prepared films as evidence that the characteristic magnetic ordering of bulk HoCo2 is retained even at ~35 nm thickness.

The comparison the authors draw is with prior pulsed-laser-deposited Co-rich films. In Tb31Co69 and Dy31Co69 films on Si, only broad anomalies near the Laves-phase transition temperature appeared in dM/dT, not a well-defined bulk-like transition. The HoCo2 films here instead show a sharp, reproducible transition. Two features in the data are flagged as not intrinsic to the film magnetism: a weak reproducible feature between 50 and 60 K that may be residual oxygen freezing in the SQUID chamber, and a sharp upturn below 10 K from paramagnetic impurities in the Si substrate. The authors present the data without subtracting the substrate contribution to avoid background-correction uncertainty.

Missing thermal hysteresis separates films from bulk HoCo2

The most consequential result is about transition order, not transition temperature. Heating and cooling susceptibility curves measured near TC show no resolvable thermal hysteresis. The authors interpret this as a predominantly continuous, second-order-like magnetic transition in the films. Bulk HoCo2, in the same comparison, shows a sharp transition with clear thermal hysteresis of approximately 1 K.

This is where the film departs from the bulk behavior that motivates interest in HoCo2. Bulk HoCo2 undergoes a first-order ferrimagnetic transition near TC ≈ 76 K accompanied by a cubic-to-tetragonal structural transformation, and the introduction notes that this first-order character can be driven toward second order by rare-earth substitution or hydrostatic pressure. The paper's own framing is that reduced dimensionality likely modifies the character of the transition relative to bulk. That is the authors' interpretation of the hysteresis data, not a direct structural measurement of the transition order in the film.

Coercivity at 5 K rises far above bulk HoCo2

Field-dependent magnetization loops at 5 K for both films show clear magnetic hysteresis with a coercive field of ~600 Oe. The paper contrasts this with the negligible coercivity of bulk HoCo2, so the enhancement is large relative to the bulk reference. It is not, however, unusual among related films: the value is close to the ~1000 Oe and ~900 Oe coercivities reported for Tb31Co69 and Dy31Co69 films at 150 K and 80 K respectively.

Two measurement conditions qualify the number. The magnetic field was applied parallel to the film surface, and at higher fields the diamagnetic contribution from the Si substrate becomes increasingly significant, visible as the high-field slope of the loops. The two films differ in absolute magnetic signal magnitude, which the authors attribute to possible differences in effective magnetic volume from thickness, composition, microstructure, interfacial or surface oxidation, and substrate contribution. Their transition temperatures and overall magnetic behavior remain consistent, which the paper takes as evidence that the observed magnetic response is intrinsic.

XPS shows the local electronic environment survives growth

X-ray photoelectron spectroscopy compares bulk HoCo2 and the Si/HoCo2 film directly. The Ho 4d core-level spectra in both show an intense broad feature centered near 162 eV that deconvolves into two relatively sharp components separated by ~2.1 eV, plus lower-intensity multiplet-derived features near 165.0 and 168.5 eV and a broader feature around 174.0 eV. The paper reads this complex line shape as characteristic of strong multiplet interactions between the Ho 4d core hole and the partially filled Ho 4f shell, and the close similarity between bulk and film as evidence that Ho keeps a comparable local electronic environment and trivalent state.

The Co 2p spectra show principal 2p3/2 and 2p1/2 features near 778 and 793 eV with a spin-orbit separation of ~15 eV, consistent with metallic Co-derived states. Both components have an asymmetric tail toward higher binding energy, modeled with Doniach–Šunjić line shapes, plus weak higher-binding-energy loss features modeled with Voigt profiles. One difference is noted: the loss feature associated with the bulk 2p3/2 component is more pronounced than in the film. Because the corresponding features are weak and partially overlap in the 2p1/2 region, the authors state that a unique microscopic assignment of this intensity difference is not possible from these spectra alone. Valence-band spectra of bulk and film both show a broad envelope with appreciable weight near the Fermi level and two broad features around 6 and 9 eV, which the paper says may contain hybridized Ho 4f and Co 3d contributions together with more delocalized states.

The structural origin of the changed transition remains untested

The paper is explicit about what its data cannot decide. Only one film-related Bragg reflection was resolved, so texture and phase purity are not unambiguously assessed. The XPS comparison is core-level and valence-band line shape only; the authors note that higher-resolution ultraviolet photoemission spectroscopy would be needed to resolve the near-Fermi-level electronic structure more reliably. The coercivity and susceptibility measurements include an unsubtracted Si substrate contribution, and the ~600 Oe value is measured at 5 K with the field parallel to the film surface.

The open question the authors pose is whether the cubic-to-tetragonal structural transformation that accompanies the 77 K transition in bulk HoCo2 persists, is modified, or is suppressed in the thin-film geometry. Answering it requires temperature-dependent structural measurements across TC, which this study does not provide. The paper also identifies a thickness study as the route to finding a possible critical thickness for the first- to second-order-like change. Until those measurements exist, the transition-order conclusion rests on the absence of resolvable thermal hysteresis in susceptibility, not on direct structural evidence.

Transition order, not transition temperature, is the variable to watch

For anyone tracking HoCo2 for cryogenic magnetic refrigeration, this paper redirects attention. The 77 ± 3 K transition temperature is retained in ~35 nm films, so the characteristic magnetic ordering survives reduced dimensionality. What changes is the thermodynamic character of the transition: no resolvable thermal hysteresis, read as predominantly continuous and second-order-like, against the first-order magnetostructural transition of bulk HoCo2. Since the large magnetocaloric response of heavy rare-earth RCo2 compounds is tied to that first-order behavior, the order of the transition is the property that most directly governs whether the film inherits the bulk magnetocaloric advantage.

The practical threshold the paper sets up is therefore a thickness one. The authors propose that films of different thicknesses are needed to identify a possible critical thickness for the first- to second-order-like change, and that temperature-dependent structural measurements across TC are needed to determine the fate of the cubic-to-tetragonal transformation. The ~600 Oe coercivity at 5 K and the preserved Ho 4d and Co 2p line shapes tell a consistent story of retained local electronic environment with altered collective behavior, but the decisive next comparison is between film thicknesses and across the transition temperature, not between this film and bulk alone.

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