Granular Thin Films Research Articles

Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets.

Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared with their ferromagnetic counterparts. Whereas nanoscale thin-film materials are key to the development of future antiferromagnetic spintronic technologies, existing experimental tools tend to suffer from low resolution or expensive and complex equipment requirements. We offer a simple, high-resolution alternative by addressing the ubiquitous surface magnetization of magnetoelectric antiferromagnets in a granular thin-film sample on the nanoscale using single-spin magnetometry in combination with spin-sensitive transport experiments. Specifically, we quantitatively image the evolution of individual nanoscale antiferromagnetic domains in 200 nm thin films of Cr2O3 in real space and across the paramagnet-to-antiferromagnet phase transition, finding an average domain size of 230 nm, several times larger than the average grain size in the film. These experiments allow us to discern key properties of the Cr2O3 thin film, including the boundary magnetic moment density, the variation of critical temperature throughout the film, the mechanism of domain formation, and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into the magnetic ordering mechanism of Cr2O3 and firmly establishes single-spin magnetometry as a versatile and widely applicable tool for addressing antiferromagnetic thin films on the nanoscale.

Piezoresistive granular metal thin films of platinum–boron nitride and platinum–alumina at higher strain levels

Granular metal thin films have a strain sensitivity much larger than continuous metal films. Experiments at high strain can help reveal their piezoresistive mechanisms. We deposit films of platinum nanoparticles in boron nitride (Pt:BN) as well as platinum particles in aluminum oxide (Pt:Al2O3) on polyimide foil as strain gauges. Under low strain of 0.1%, the films exhibit enhanced gauge factors, k=23 for Pt:BN and k=6 for Pt:Al2O3. Toward higher strain of 1.5%, Pt:BN shows reproducible and linear resistance-strain curves. In contrast, Pt:Al2O3 exhibits anomalies: The resistance-strain curves are highly nonlinear with an increasing slope before reaching saturation. The differential gauge factor versus strain increases from 9 to 9500, and the return curve shows large hysteresis. With scanning electron microscopy unstrained and in situ strained films are compared, Pt:BN shows no changes, whereas in Pt:Al2O3, large cracks develop. The relatively soft BN is less prone to cracks than the hard and brittle Al2O3. Hence, the gauge factor in Pt:BN can still be attributed to an electron tunneling mechanism, whereas Pt:Al2O3 becomes dominated by the influence of cracks. A model is presented, and we argue that the reproducible opening and closing of these cracks leads to the gigantic resistance increases at high strain.

Microstructure Analysis on Size Distribution During Film Growth in HAMR Media

Grain size and size distribution reduction in L10 FePt granular thin film is becoming critical to increase the areal storage density and realize the full potential of heat-assisted magnetic recording media. FePt and amorphous carbon or mixture with other segregants are often deposited at elevated temperature in order to promote the L10 ordering in FePt. Due to the materials system and high temperature involved in the fabrication process, the bimodal distribution of the grains is often observed with a significant amount of small grains with diameter <3 nm. The bimodal distribution of the grains could potentially lead to a significant microstructure nonuniformity and thus grain-to-grain properties variation, such as Curie temperature (Tc), texture, ordering, and magnetic properties. In this paper, we performed a detailed analysis on microstructure evolution of FePt-C thin film during the growth process on different initial layer conditions to understand the origin of bimodal grain size distribution in the FePt-C media. Experimental modeling study of depositing FePt on continuous carbon layer at elevated temperature was conducted to understand the nucleation and growth process.

Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

Ultrafast laser generated strain in granular and continuous FePt thin films

We employ ultrafast X-ray diffraction to compare the lattice dynamics of laser-excited continuous and granular FePt films on MgO (100) substrates. Contrary to recent results on free-standing granular films, we observe in both cases a pronounced and long-lasting out-of-plane expansion. We attribute this discrepancy to the in-plane expansion, which is suppressed by symmetry in continuous films. Granular films on substrates are less constrained and already show a reduced out-of-plane contraction. Via the Poisson effect, out-of-plane contractions drive in-plane expansion and vice versa. Consistently, the granular film exhibits a short-lived out-of-plane contraction driven by ultrafast demagnetization which is followed by a reduced and delayed expansion. From the acoustic reflections of the observed strain waves at the film-substrate interface, we extract a 13% reduction of the elastic constants in thin 10 nm FePt films compared to bulk-like samples.

Kondo-like transport and magnetic field effect of charge carrier fluctuations in granular aluminum oxide thin films

Granular aluminum oxide is an attractive material for superconducting quantum electronics. However, its low-temperature normal state transport properties are still not fully understood, while they could be related to the unconventional phenomenon of the superconductivity in this material. In order to obtain useful information on this aspect, a detailed study of charge carrier fluctuations has been performed in granular aluminum oxide films. The results of electric noise measurements indicate the presence of a Kondo-type spin-flip scattering mechanism for the conducting electrons in the normal state, at low temperatures. Moreover, the magnetic field dependence of the noise amplitude suggests that interface magnetic moments are the main source of fluctuations. The identification of the nature of fluctuation processes is a mandatory requirement for the improvement of quality and performance of quantum devices.

Laser molecular beam epitaxy growth of porous GaN nanocolumn and nanowall network on sapphire (0001) for high responsivity ultraviolet photodetectors

We report on high-responsivity metal-semiconductor-metal (MSM) structure based ultraviolet (UV) GaN photodetectors fabricated on various GaN nanostructures such as porous nanocolumn network (PNCN), nanowall networks (NWNs), and granular and compact thin films. Different GaN nanostructures were hetero-epitaxially grown on c-sapphire using laser molecular beam epitaxy by tuning the AlN buffer layer growth parameters. High resolution x-ray rocking curve measurements indicate that the crystalline quality of GaN critically depends on the selection of AlN buffer layer growth conditions such as type of ablation target and growth temperature. The porous GaN nanostructures revealed a nearly stress-free wurtzite structure as deduced by Raman spectroscopy measurements. Room temperature photoluminescence spectroscopy showed that the GaN films possess a near band emission (NBE) peak at ∼ 3.39 eV along with a broad yellow luminescence (YL) with maxima at 2.25 eV. For GaN PNCN and NWN structures, the NBE-to-YL emission ratio is more than an order higher compared to the GaN films. The fabricated MSM based UV-detector on GaN PNCN and NWN exhibited a high photo-responsivity of ∼27.72 and 24.8 A/W, respectively, under 2 V applied bias at room temperature. The GaN PNCN and NWN nanostructures with excellent photo-responsivity and optical quality prove to be promising candidates for the fabrication of efficient UV photodetectors due to their continuity in lateral direction, high surface area-to-volume ratio and tailored surfaces.

Effect of Pd Buffer and Interlayer on the Magnetic Properties of CoCrPt-Oxide Granular Thin Films

Morphostructural and magnetic properties of CoCrPt-oxide (TiO2, CoO) granular thin films deposited on Pd( $y$ )/Ru/Pd( $x$ )/NiTa intermediate layer have been investigated as a function of the Pd thickness ( $x = 5$ and 10 nm; $y = 1, 3$ , and 5 nm). The Pd( $x$ ) buffer layer shows an fcc (111) texture that leads to a gradual improvement of both the Ru and CoCrPt hcp (0002) crystallographic orientations while increasing the Pd thickness. This results on a significant enhancement of both the effective magnetic anisotropy ( ${K} _{\mathrm {eff}} =4.1 \times 10^{6}$ erg/cm3) and out-of-plane coercivity ( ${H} _{c\bot } = 5.7$ kOe) with respect to the reference CoCrPt-oxides/Ru/NiTa sample ( $K_{\mathrm {eff}} = 3.4 \times 10^{6}$ erg/cm3, ${H} _{c\bot }= 4.7$ kOe) when a 10 nm-thick Pd buffer is used. The inclusion of a Pd( $y$ ) intermediate layer between Ru and Co leads to an increase of the effective anisotropy (up to $4.1 \times 10^{6}$ erg/cm3 for $y = 5$ nm) with respect to the reference sample, which can be ascribed to a combined effect of an interface contribution and a term arising from a CoCrPt lattice distortion.

(Invited) Non-Aqueous Atomic Layer Deposition of SnO2 for Gas Sensing Application

Tin dioxide thin films are grown at low-moderate temperature using a non-hydrolytic atomic layer deposition process. Granular SnO2 thin films are obtained from tin(IV) tetra-butoxide reacting with acetic acid, at temperatures as low as 75 °C. Influence of pulse length of both reactants and of the deposition temperature on the saturation is studied. A narrow ALD window is established between 150 and 175 °C with a growth per cycle of 0.07 nm and with a linear growth as a function of the number of cycles. Above 200 °C decomposition of the tin precursor is not negligible. Microstructure and morphology of the as-prepared films as well as the influence of the deposition parameters are investigated using electron and atomic force microscopies. The band gap of the obtained films is determined using UV-visible spectroscopy. Finally the use of the SnO2 coated carbon nanotubes as gas sensing layer is discussed.

Long-lived photoinduced absorption in granular molybdenum disulfide thin films

We present results of the differential photoinduced absorption spectra investigation of the molybdenum disulfide nanogranular thin films. The films were produced by means of a sulfurization of pre-deposited by magnetron sputtering thin molybdenum films. It has been shown that photoexcitation with the light quanta with the energy higher than the bandgap leads to a modification of the absorption spectra in the visible range. Possible nature of the photoinduced absorption spectra will be discussed.

Highly selective PbS thin film based ammonia sensor for inert ambient: In-situ Hall and photoelectron studies

Granular thin films of p-type semiconducting PbS (Cubic) with predominantly a-axis oriented grains having an average grain size of about 300 nm were deposited on polycrystalline alumina by pulsed laser deposition technique. Optical measurements show a band gap of 2.25 eV. The thin films were highly selective to ammonia at 473 K even in the presence of hydrogen and they operate in inert ambient with a typical response and recovery times of about 25 s and 120 s respectively. In-situ Hall measurements show an exponential decrease in hole carrier concentration with increasing NH3 level. XPS analysis of Pb 4f and S 2p levels reveals that the valence state of Pb and Sulphur remain in +2 and −2 respectively after exposure to the reducing ammonia and the appearance of N 1s peak at 398.1 eV confirms dissociative chemisorption of ammonia on PbS surface.

Weak Antilocalization in Granular Sb2Te3 Thin Films Deposited by MOCVD

Topological insulator (TI) properties of granular Sb2Te3 thin films, grown by MOCVD at room temperature on 4″ SiO2/Si substrates, are investigated. The negative magneto‐conductance (MC), analyzed in terms of the Hikami–Larkin–Nagaoka theory, reveals the presence of a clear weak antilocalization (WAL) effect, thus demonstrating the TI character of our system. The extracted WAL prefactor (α) values are explained in terms of the film conductance, in agreement with the existing models for WAL in transport disordered systems. The dephasing length () dependence on temperature is affected by the grain size of the film and might in turn explain the observed temperature dependence of α and of the conductance. These results demonstrate the feasibility of technologically‐relevant processes to synthesize TIs films, at the same time providing a tool to investigate the effects of the film microstructure on the topological character of their transport properties.

Enhanced magnetoresistance in pulsed laser deposited stable chromium oxide thin films

Granular thin films of primarily rutile type tetragonal chromium dioxide (t-CrO2), along with a minor phase of hexagonal Cr2O3 were deposited on rutile type TiO2 layers by KrF excimer laser based pulsed laser deposition (PLD) technique using Cr2O3 targets. The TiO2 under-layer was also deposited by PLD process on oxidized Si substrates followed by annealing at 1100 °C for 10–15 min under O2 ambient. The lattice-matched interfacial TiO2 layer stabilizes the metastable Cr (IV) phase of CrO2, otherwise it readily converts into its stable phase of Cr (III) oxide, Cr2O3, under ambient conditions. Studies with X-ray diffraction (XRD) and Raman spectroscopy confirmed the stable t-CrO2 phase in the films. The microstructure in the films was studied with field emission scanning electron microscope (FESEM) and magnetic force microscope (MFM). As revealed by electrical analysis, the ferromagnetic (FM) t-CrO2 grains in conjunction with the antiferromagnetic (AFM) Cr2O3 phase in the tailored thin films effectively support an electrical transport process based on inter-granular spin-dependent tunneling (SDT) mechanism, giving rise to significantly high magnetoresistance (MR). A typical sample exhibits a markedly improved MR-value, viz., −30% at an applied field (H) of 438 kA/m at 278 K, than reported values in CrO2 thin films. Such a large MR value in the Coulomb blockade regime arises primarily not only because of the considerably suppressed spin flipping but also as a result of the highly effective SDT mechanism through an interlinked FM-AFM structure of chromium oxides in this specially engineered microstructure of the thin films. Such type of stable half-metallic thin films with low-H switching behavior is attractive for pertinent use in spintronics and magnetic sensing based applications.

Lattice-Mismatch-Induced Oscillatory Feature Size and Its Impact on the Physical Limitation of Grain Size

Size control of nanocrystals is becoming more crucial in growing, for example, granular ferromagnetic thin films for heat-assisted magnetic recording, or self-assembled quantum dots for quantum computing. This study identifies granular patterns with surprisingly ``quantized'' feature sizes in Fe-Pt thin film grown on lattice-mismatched MgO. This arises from the periodic nature of the misfit-strain energy in such a system; island lengths that are integer multiples of the misfit dislocation period are energetically favored. These results point to synthesizing nanostructured materials with well-controlled grain size by tuning the interlayer lattice mismatch.

Novel torque magnetometry for uniaxial anisotropy constants of thin films and its application to FePt granular thin films

A technique based on torque magnetometry is proposed for precise determination of the uniaxial magnetic anisotropy constant, Ku, of perpendicularly magnetized thin films, in particular for Ku larger than 106 J/m3, by electrical detection of the anomalous Hall effect and exact determination of the magnetic field direction. This technique can measure the Ku value of several-nanometer-thick polycrystalline films. Further, the Ku value of FePt–C granular films, which are expected to be useful for high-density magnetic recording media, is evaluated. The intrinsic Ku value of 4-nm-thick FePt–C thin films fabricated at 450 °C is approximately 3 MJ/m3 for carbon contents up to 34 vol % C.

On the depolarization in granular thin films: a Mueller-matrix approach.

We describe a general method to disclose the information hidden in Mueller matrices experimentally obtained from depolarizing samples. Although spectroscopic Mueller-matrix ellipsometry allows for a model-free characterization of inhomogeneous samples, i.e., independently from any assumption on the sample structure, the interpretation of the obtained results is often challenging. The proposed method combines three different decomposition techniques applied to the measured Mueller matrices in transmission and reflection of granular thin films with different thicknesses and densities. We demonstrate that the comparative analysis of the respective differential-, product-, and sum-decomposition of the Mueller matrices, together with correlation effects and the visualization as a Poincaré sphere, reveals the particular underlying physical processes of depolarization. As an example, we apply this method on granular BaSO4 thin films. This method is general and can be applied to a wide variety of intrinsically inhomogeneous materials with applications in physics, industry, biology, or medicine.

Structural and Galvanomagnetic properties in Mn-Bi2Te3 thin films

Bismuth-based binary chalcogenide compounds such as Bi2Te3 and Bi2Se3 are well known materials for their excellent thermoelectric properties due to their near-gap electronic structure. In the last few years these materials have received attention for exhibiting new physics of 3D topological insulators (TI). Possible applications of TI based devices range from quantum computing, spin based logic and memory to electrodynamics. The 3D TIs present spin-momentum-locked surface states by time reversal symmetry (TRS). Introducing magnetic doping in a TI, brakes the TRS and is predicted to open the gap at Dirac point, resulting in exotic quantum phenomena. This interaction between magnetism and topologically protected states is of potential attention for applications in modern spintronics. Quantum phenomena such as weak antilocalization observed in these nanostructures are described. In this work, granular Mn-Bi2Te3 thin films were grown by DC magnetron sputtering on Si(111) substrates and were submitted to ex situ annealing. We present results for the crystal structure of sputtered and annealed films characterized with X-ray diffraction and high-resolution scanning electron microscopy (HRSEM). The surface analysis was studied with atomic force microscopy (AFM). Magnetotransport measurements were performed using standard four probe technique with Hall and MR configurations, with perpendicular magnetic fields up to 9T and temperatures from 300 to 3K.

Thermal stability and magnetization reversal mechanism in granular L1 FePt thin films

The relationship between coercivity HC and magneto-crystalline anisotropy field HK of L10 FePt granular alloy thin films is investigated as a function of film thickness in the range of 3.5–12.5 nm. While HK exhibits a decrease from 82 kOe to 71 kOe with increasing film thickness, HC displays a pronounced peak at a critical film thickness of tCR ≅ 7 nm. In order to explain the non-monotonic behavior of HC as a function of film thickness, the time dependence of HC at ambient temperature (TRT = 300 K) and the temperature dependence of the AC susceptibility in the range TRT – 800 K are measured as a function of film thickness and interpreted in the frame of the Stoner–Wohlfarth model of coherent rotations. It is demonstrated that the HC decrease with increasing film thickness above tCR is a consequence of a transition from coherent to an incoherent magnetization reversal mechanism in isolated grains. For a 7 nm thick film (tCR), the average grain size of ∼7.4 nm is comparable with the film thickness, suggesting that the domain-wall (DW) width δ ≅ tCR. Previous theoretical work has demonstrated a strong dependence of δ on the orientation of the DW with respect to the (001) planes of an L10 FePt lattice. By using the values of the micromagnetic exchange coupling A theoretically evaluated for parallel and vertical DW orientation with respect to the (001) planes, one obtains δ = 5.2 nm for parallel and δ = 6.7 nm for vertical DWs. The latter is closer to the experimental value of δ, suggesting that the nucleation of vertical DWs inside the grains (probably at grain boundaries) is the dominant mechanism responsible for the incoherent magnetization reversal evidenced in the investigated films.

Skew scattering dominated anomalous Hall effect in Cox(MgO)100−x granular thin films

We investigated the mechanism(s) of the anomalous Hall effect (AHE) in magnetic granular materials by fabricating 100 nm-thick thin films of Cox(MgO)100−x with a Co volume fraction of 34 ⩽ x ⩽ 100 using co-sputtering at room temperature. We measured the temperature dependence of longitudinal resistivity () and anomalous Hall resistivity () from 5 K to 300 K in all samples. We found that when x decreases from 100 to 34, the values of and respectively increased by about four and three orders in magnitude. By linearly fitting the data, obtained at 5 K, of anomalous Hall coefficient () and of to , we found that our results perfectly fell on a straight line with a slope of 0.97 ± 0.02. This fitting value of in clearly suggests that skew scattering dominated the AHE in this granular system. To explore the effect of the scattering on the AHE, we performed the same measurements on annealed samples. We found that although both and significantly reduced after annealing, the correlation between them was almost the same, which was confirmed by the fitted value, = 0.99 ± 0.03. These data strongly suggest that the AHE originates from the skew scattering in Co–MgO granular thin films no matter how strong the scattering of electrons by the interfaces and defects is. This observation may be of importance to the development of spintronic devices based on MgO.

Tuning magneto-transport properties of Fe–Au granular thin films by cluster organization

A comparative study of morpho-structural, magnetic and magneto-transport properties of two Fe–Au granular films with different concentrations of Fe nanoclusters of almost similar size is reported. Different organizations of the Fe clusters, i.e. in lamellar-like or random-like configuration, were obtained by varying the amount of Fe in the Fe–Au films. The specific magnetic behaviour was investigated with respect to local structure and morpho-structural aspects by combining magneto-optic Kerr effect and superconducting quantum interference device magnetometry, 57Fe conversion electron Mössbauer spectroscopy and a wide range of electron microscopy techniques. A strong in-plane magnetic texture with uniaxial anisotropy was observed in the case of the lamellar-like organization of the clusters (specific to the Fe–Au film with higher Fe concentration) whereas a superparamagnetic behaviour was evidenced in the case of random distribution of the clusters (specific to the Fe–Au film with lower Fe concentration), despite the similar average size of the clusters in the two samples. Specific magnetoresistance effects were investigated with respect to both the involved magnetic configurations and magnetic interactions of the Fe clusters.