Room-temperature Ferromagnetism, Perpendicular Magnetic Anisotropy, and Anisotropic Magnetoresistance in Epitaxially Stabilized Air-stable Chromium Tellurides

doi:10.21203/rs.3.rs-4861088/v1

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Room-temperature Ferromagnetism, Perpendicular Magnetic Anisotropy, and Anisotropic Magnetoresistance in Epitaxially Stabilized Air-stable Chromium Tellurides

https://doi.org/10.21203/rs.3.rs-4861088/v1

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Chromium telluride (Cr_(1+δ)Te₂) thin films of various δ were prepared using a hybrid pulsed laser deposition technique by varying the flux rate of a Te-molecular beam source. We found that the thickness and nanocrystalline quality of the films increased with the Te-flux. All three films prepared showed hexagonal layering with the same out-of-plane lattice constant. Increasing the Te-flux rate stabilized the CrTe, Cr₂Te₃, and CrTe₂ phases of the telluride, and they showed different magnetic transition temperatures (T_C) from film to film, corresponding to their composition. The tellurides show anisotropic magnetoresistance (AMR) and a metal-to-insulator transition (MIT) at the vicinity of their respective T_Cs, with the highest T_C of 324 K unveiled by Cr₂Te₃. These stoichiometry-controlled room-temperature ferromagnetic air-stable 2D materials prepared using the hybrid deposition technique can significantly advance further development of 2D materials for magnetic device applications.

Physical sciences/Materials science/Nanoscale materials/Synthesis and processing

Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials

Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials

epitaxy

2D materials

tellurides

ferromagnetism

perpendicular magnetic anisotropy

In the past years, a wide range of interesting properties, such as perpendicular magnetic anisotropy (PMA) and the anomalous and topological hall effect (AHE and THE), have been observed in chromium tellurides, Cr_(1+δ)Te₂. ^1–4 These phenomena are interesting for possible applications in spintronics, like in magnetic random access memory (MRAM) storage devices. Furthermore, the recent discovery of room-temperature skyrmions in Cr_(1+δ)Te₂ could make them candidates for skyrmion-based memories. ^3,5

Like other transition metal dichalcogenides (TMD), CrTe₂ consists of two chalcogenides (Te) layers sandwiching a transition metal (Cr) layer. ⁶ Other compounds of Cr_(1+δ)Te₂ like CrTe, Cr₃Te₄, Cr₂Te₃, or Cr₅Te₈ only differ in the number of Cr intercalants denoted by δ. ^7–11 Most of them exist in a hexagonal or monoclinic crystal structure and exhibit different degrees of magnetic anisotropy, with Curie temperatures (T_C) ranging between 150 K to 340 K for thin films and reaching up to 360 K for bulk Cr₇Te₈. ^12–15 Magnetoresistance (MR) of up to 5% has also been observed in Cr_(1+δ)Te₂ thin films. ^16,17

2D materials are usually prepared via exfoliation from the bulk material. Though exfoliated films support flexible devices, defects and microcracks are prone to be present in mechanically exfoliated films, limiting their functionalities and lowering air stability. ^18–20 Epitaxially stabilized thin films are free from such defects as they are grown on a substrate and endure no friction from mechanical exfoliation. The substrate/film interfacial energy also provides extra stability while handling the films compared to exfoliated flakes. Thin film deposition methods like chemical vapor deposition (CVD) or molecular beam epitaxy (MBE) have been commonly used to deposit Cr_(1+δ)Te₂ thin films. ^15,21 A few works also reported using pulsed laser deposition (PLD) to fabricate Cr_(1+δ)Te₂ thin films. ^2,11 While PLD has advantages like higher growth rates, lower costs, and higher experimental flexibility compared to methods like MBE, there are also some drawbacks. ²² Deviations between the target and film stoichiometry tend to depend on the mass ratios of the species in the target and mass transfer; additionally, it takes a lot of resources to produce PLD targets of different atomic percentages. ²³ In order to circumvent those drawbacks of PLD while keeping its advantages, we built a hybrid PLD-MBE setup for the deposition of Cr_(1+δ)Te₂ thin films with different compositions. PLD was used to ablate a Cr target in combination with a cracker cell molecular beam source for the evaporation of Te. Similar approaches have been previously used to deposit TMD materials like MoS₂ and WSe₂. ^24,25

In this work, we report on three Cr_(1+δ)Te₂ films successfully deposited with different compositions using a hybrid PLD setup. The crucible temperature of the evaporation source and, thereby, the Te-flux was varied, leading to the development of very different ferromagnetic properties in each film. These properties, including PMA and MR, are compared amongst the films, showing significant differences depending on the Cr: Te ratio.

Deposition

Cr_(1+δ)Te₂ thin films were deposited on hexagonal Al₂O₃ (0001) substrates using a hybrid PLD-MBE setup, as shown in Fig. S1. A KrF-excimer laser was used with a frequency of 4 Hz and a fluence of 7 J/cm² for the PLD deposition of Cr. During the 8000 laser pulses used for the ablation of the Cr target, Te was evaporated from a valved cracker cell directly into the plasma plume produced by the ablation of Cr. The crucible of the evaporation cell containing the Te was heated to T_EC = 325, 350, and 375°C for the different films of the series. The cracker, which cracks up large molecular clusters formed by the Te during the evaporation and adds kinetic energy to the particles, was heated to a temperature of 650°C for all the films. ²⁶ The substrates were heated to 300°C. Only the Te flux was varied by changing the crucible temperature T_EC, which will be used in this work to distinguish the films. Additionally, a reference film containing only Cr (sample named ‘only Cr’) was deposited with the same growth parameters of PLD.

Crystal structure and morphology

The X-ray diffraction (XRD) measurements of the Cr_(1+δ)Te₂ thin films show a (00l)-oriented crystal structure, which can be seen in Fig. 1a. The grey-colored Miller indices are the planes of the hexagonal substrate, and the black indices denote that of the hexagonal film. For simplicity, we used Miller indices (hkl) instead of (hkil). The only Cr film is amorphous and exhibits no peaks in the XRD measurement. We observed epitaxial growth for all three Cr_(1+δ)Te₂ thin films. The out-of-plane pseudocubic lattice constant (c) is found to be 12.35 ± 0.06 Å for the 325°C-sample and 12.39 ± 0.03 Å for both the 350°C and 375°C samples. The CrTe, Cr₃Te₄, Cr₂Te₃, and CrTe₂ (all of them are ferromagnetic) grown by CVD have a c of 12.46, 12.34, 12.1, and 12.02 Å, respectively, as summarised in a recent review article. ¹⁵ Since we observed the same c for all three tellurides, we tend to assume that they have the exact stoichiometry on the first look. However, we supplied different ratios of Cr: Te during their growth, aiming for a stoichiometry control. Since the films in our work are epitaxially locked to a substrate, an in-plane stress or strain could also lead to biaxial strain or stress, leading to differences in the c than the bulk or reported values. ²⁷ So, it is possible that they differ in stoichiometry but exhibit the same c due to epitaxy.

X-ray reflectivity (XRR) measurements were conducted on all the samples, and by fitting the curves with the GenX software ²⁸ (Fig. 1c.), the film thickness was estimated. The sample thicknesses correlate to the crucible temperature and, thereby, the Te-flux rate during the growth. We observed the film thickness from the XRR with 17.7, 19, 38, and 46 nm values for the only Cr, 325, 350, and 375°C samples, respectively. With the increase in T_EC, more Te atoms are supplied, contributing to its thickness. The Kiessing thickness fringes in the XRR patterns indicate high-quality interfaces in all the samples. Since the 325 and 350°C films lack a homogeneous smooth surface (as will be shown later with microscopy results), GenX could not provide a reliable fit, which is reflected in the deviation between the data and fitted curves. However, the TEM data in the following session show that the XRR values provide an estimate. To explore the crystal quality of the films, a rocking curve around the (004) peak of the Cr_(1+δ)Te₂ films was measured as depicted in Fig. 1b. The rocking curves display that the crystallinity increases with the crucible temperature. Further, we performed the Phi-Scan as shown in Fig. 1d. The 350 and 375°C films have a six-fold symmetry axis in the (222) crystal lattice. ²⁹ The 375°C sample shows smaller peaks between the major peaks, suggesting a domain structure with 30° in-plane rotation between grains. The poor detection of the peaks for 50°≤ φ ≤ 220° for the 350°C sample was due to defects in this sample compared to the 375°C sample. The experiment on the 325°C sample led to too low intensities for a Phi measurement using our experimental set-up. This is due to the lower thickness of the sample compared to 350 and 375°C samples, contributing less to the total diffraction intensity.

The scanning transmission electron microscopy (STEM) results shown in Fig. 2 confirm an increased crystalline quality with increasing T_EC as suggested by the XRD. Firstly, the 325°C and 350°C samples exhibit frequent precipitates (Fig. 2a and 2b), while the 375°C film is smooth and no precipitates were found (Fig. 2c). Voids are found at the substrate/film interface underneath some of the precipitates suggesting that the films detach from the substrate upon precipitate formation. However, since the focussed ion beam (FIB) thinning during STEM sample preparation can lead to the refilling of voids and forming similar artifacts due to differential milling, no particular emphasis is put on this finding. Secondly, while the 325°C sample shows a very high defect concentration and nanocrystals of the size of a few nanometers (Fig. 2d), increasingly large columnar grains evolve at higher T_EC (Fig. 2e and 2f), in which the typical hexagonal ABA stacking can be found as indicated by the yellow zig-zag lines in the insets. The thickness of the films observed during STEM imaging is 25, 35, and 46 nm for 325, 350, and 375°C samples, respectively. For the reasons discussed with XRR results, the values obtained from TEM were deemed more reliable and used later in calculations. The same thickness observed from both experiments for the 375°C sample demonstrates a smoother surface and confirms the higher homogeneity of this film. We used the thickness obtained from XRR for the only Cr sample since it is considered sufficiently accurate.

To analyze the origin of the precipitates and the chemical composition of the films more, energy-dispersive X-ray spectroscopy (EDX) measurements are presented in Fig. 3a-c. The RGB overlay of counts of the Cr K- and Te L-lines clearly demonstrate that the precipitates are Cr-rich, and the cyan halo at the surface of the films additionally indicates overall Cr-enrichment at the surface. Since oxygen, as a light element, can be better detected with electron energy-loss spectroscopy (EELS), the O K-edge EELS signal of the same precipitate presented in Fig. 3a is shown in Fig. 3d. The result proves that the Cr-rich surface layer contains oxygen and is thus likely chromium oxide. In contrast, the Cr-rich precipitate reveals no oxygen signal and is thus likely pure chromium. An additional chromium oxide layer is formed at the interface between the film and the void. The composition of the films is found via EDX; the area in the yellow rectangular areas marked in Fig. 3a, 3b, and 3c are measured (for 325°C and 350°C samples, only the area outside the precipitate was considered)). We found 51(5), 61(7), and 70(3) at-% of Te in the 325, 350, and 375°C samples, respectively, and thus, with the increase in T_EC, the amount of Te in the samples increased, as expected. These values correspond to δ values of 0.9(2), 0.3(3), and − 0.1(2) for 325, 350, and 375°C, respectively, with closest previously reported stable phases CrTe, Cr₂Te₃, and CrTe₂. The average stoichiometry can also change in the presence of defects due to segregation, phase separation between Cr and Cr_(1+δ)Te₂, and nanosized grains, which could also lead to average concentrations matching those of CrTe or Cr₂Te₃. However, further magnetic and transport characterizations tend to bear the stoichiometry assumptions from these results.

The atomic force microscope (AFM) measurements shown in Fig. 4 confirm the presence of precipitates independent of the FIB thinning necessary for STEM sample preparation. The precipitates are spread evenly across the surfaces of the 325 (Fig. 4b) and the 350°C (Fig. 4c) samples, while the 375°C sample (Fig. 4d) has a remarkably smooth surface with a root mean square (RMS) roughness of only 0.62 nm. The former samples have RMS values of 14.79 and 18.11 nm, respectively. This confirms that the precipitates of the 350°C sample in Fig. 3d are more protruding than the precipitates of the 325°C sample. The pure Cr sample, which is assumed to be amorphous, has an RMS value of 0.29 nm (Fig. 4a). Island/precipitate formation is typical in TMD, where excess elements accumulate on the surface. ³¹ Cr₂Te₃ and Bi₂Te₃ were reported to exhibit Te- and Bi-rich islands, respectively. ³¹

X-ray photoelectron spectroscopy (XPS) measurements were performed on the samples for the survey scan spectrum, the valence band in combination with Te 5s and O 2s, the C 1s region, and a combined Cr 2p and Te 3d regions. The survey scan, as shown in Fig. S2, shows minor traces of impurities of Si and N, which are attributed to contamination during the measurements conducted on the sample. The energy of the only Cr sample was calibrated using the valence band, as shown in Fig. S3 of the supporting information. The resulting peak position of C 1s (B.E. = 284.2 eV) was assumed to apply to the C 1s peaks of the Cr_(1+δ)Te₂ samples as well since all the samples were handled similarly. Thus, the Cr_(1+δ)Te₂ samples were calibrated using this C 1s position, as shown in Fig. 5b. The shoulder peak of the only Cr sample at ~ 282 eV is likely due to Cr-C bonding. ³³ The shoulder peak at ~ 287 eV is due to the C-O bond, which is prominent in the only Cr sample and less in the telluride samples. ³⁴ The Cr 2p and Te 3d doublets are displayed after calibration and normalization in Fig. 5a for all samples. Additionally, they were shifted along the y-axis to make them visible. The amorphous film of only Cr sample exhibits the Cr 2p doublets for Cr(III) from Cr₂O₃ and metallic Cr with no traces of Te. ³⁵ The wide Cr(III) peak is typical of Cr compounds, which occupy multiplet spectra due to changes in the initial oxidation state of cation due to crystal field geometry and effects of charge transfer from back bonding. ^35,36

The XPS valence band spectra of the samples together with O 2s and Te 5s, normalized to the O 2s peak, are shown in Fig. 5c. Interestingly, the 325 and 375°C samples exhibit the same surface valence band spectra and 350°C sample hints the presence of more surface oxygen than the other two tellurides. The valence band is divided into three regions viz. i, ii, and iii. The region i originates from the Cr 3d electrons. The region ii + iii mainly comes from a broad O 2p emission together with Cr 3d - O 2p and/or Cr 3d - Te 2p hybridization. ^37,38 Peak ii for the Cr sample is at a higher binding energy than that for tellurides due to the presence of Cr-O hybridization in the former, whereas the Cr-Te hybridization exists in the latter. This is due to the electronegativity (Pauling scale) difference of the two anions, O (3.44) and Te (2.1). Region iii is the same for all the samples when normalized with respect to O 2s since this region is coming as a result of the O 2p emission. ³⁸ The satellite in the only Cr sample at ~ 13 eV, at the region of the Te 5s is due to the d² final states of Cr cation. ³⁸ The XPS valence spectra display the presence of (/absence of) Cr-Te (/Cr-O) hybridization in the tellurides. Thus, we believe that the O-anions exist only at the surface of these UHV-grown films and do not participate in the lattice, which is in accordance with the EELS results presented earlier in Fig. 3d.

Magnetic properties

The magnetic properties of the films were measured using a superconducting quantum interference device (SQUID), MPMS® XL from Quantum Design (QD). We used standard QD sample holder straws (AGC2) and gelatin capsules to load the thin film samples. First, the magnetization (M vs. T) was measured under field-cooled (FC) and zero field-cooled (ZFC) conditions between 5 and 350 K under magnetic fields of 0.01, 1, and 5 T. This was done for both perpendicular (⟂) and parallel (∥) alignment of the samples with respect to the direction of the magnetic field. The magnetic hysteresis (M vs. H) loop was measured at 5 K under ZFC conditions. M vs. H was performed additionally at 300 K for the 350°C sample since we found a T_C above room temperature for this film. The M vs. T for the only Cr sample was measured under FC mode at 0.01 T in ⟂ and ∥ orientation to check the presence of any ordered magnetic contributions. Diamagnetic contributions of the sample holder and substrate were subtracted from the linear part of the high-field region of the M vs. H plot and for comparison, the values were then divided by the thin film volume (area of the film multiplied by its thickness obtained from TEM). The T_Cs for the Cr_(1+δ)Te₂ samples were deducted from the minimum of the temperature coefficient of magnetism (TCM), which is calculated by $\:\frac{dlog\left(M\right)}{dT}*100$, and given in %/K for the FC measurements at 0.01 T.

In Fig. 6. the TCM curves and temperature-dependent SQUID measurements (M vs. T) under 0.01 T, as well as the hysteresis (M vs. H) at 5 K can be seen. The ZFC and FC M vs. T for the samples under other fields are shown in Fig. S4. and Fig. S5. respectively. An enlarged M vs. H for H = ± 1.5 T is shown in Fig. S6. The hysteresis loop shows that all films have PMA, although to varying degrees. While the 375°C sample has the most pronounced anisotropy, the 325°C sample reaches nearly the same saturation magnetization (M_S) for both sample orientations with respect to the magnetic field. Ideally, the M_S of a bulk material should reach the same value when saturated, overcoming the magnetocrystalline anisotropy under a high external magnetic field. However, the PMA of thin films exhibits effects similar to those in our samples, with different M_S values in both directions. This is due to varying contributions from magnetocrystalline anisotropy together with interface and shape anisotropies. ^40,41 The M_S, remnant magnetization (M_r), and coercivity (H_C) obtained from ZFC M vs. H at 5 K and the T_C obtained from FC M vs. T under 0.01 T of the telluride samples grown at different T_ECs together with the composition found from the STEM/EELS are tabulated in Table 1.

The M_S value per total volume decreases with T_EC due to the low Cr: Te ratio, where the Cr spins contribute to the total magnetic moment. Cr₂Te₃ shows the highest M_r. The 375°C sample has a large H_C of 0.95 T for the ⟂ orientation, while the coercivity for the ∥ alignment is 0.24 T. The H_Cs of the other two samples provide comparable values under ⟂ and ∥ orientations. The T_C varies for the 325°C and 350°C samples between the ⟂ and ∥ measurements. For the 325°C sample, the ∥ T_c of 287 K is larger than the ⟂ T_c with 280 K. The 350°C sample shows the opposite behavior with 297 K (∥) and 324 K (⟂). The 350°C sample reaches a T_C well above the room temperature for a perpendicular magnetic field. The T_C of the 375°C film is the lowest, with a value of 155 K, where they are the same in ⟂ and ∥ orientations. The difference in T_C in ∥ and ⟂ orientations is due to the low applied magnetic field (0.01 T) of the M vs. T used for calculating the T_C. The difference in M_r values has been reflected in the T_C measured at low magnetic fields (as noted in Table 1); high M_r caused an increase in the observed T_C. Under high-applied magnetic fields of 1 T and 5 T, the FC M vs. T shown in Fig S5 indicates similar values of T_C in ∥ and ⟂ orientations. Due to low data points at high-temperature regions, taking derivatives of these plots was not plausible. Another remarkable feature visible only in the hysteresis for the 375°C sample is a large kink at 0 T. Similar kinks were obtained for Cr₂Te₃ by several other groups. ^21,42,43 This zero-field kink is associated with the high magnetocrystalline anisotropy of this material due to the near degeneracy of magnetic orderings upon reversing the magnetic field. ⁴⁴

The samples have different magnetic properties that vary with the composition. All films show magnetic anisotropy, with the 375°C sample having the strongest PMA and lowest magnetization. Room temperature and near-room temperature ferromagnetism was observed for the 350 (Cr₂Te₃) and 325°C (CrTe) samples. According to previous works, the films with more Cr intercalants have higher T_Cs, while the 375°C sample has a T_C of only about 155 K, similar to other results reported for CrTe₂. ¹⁷ Heisenberg model with finite-range exchange interaction predicts that a 2D material cannot show magnetic ordering due to fewer neighboring spins, ⁵² and Ising model ferromagnetism with scalar spins was suggested in 2D magnetic materials. ^44,45,53 The Presence of longitudinal crystal field anisotropy along the c-axis stabilizes the ferromagnetic ordering in 2D Cr-based honeycomb-kagome lattices. ^44,45 We noticed strong PMA in our samples, which proclaims them to be Ising ferromagnets at finite temperatures, even at room temperature. Usually, the Ising scalar spin interaction modifies from 2D to 3D upon increasing the layer thickness of these materials, which causes an increase in the T_C. ⁵⁴ In our samples, the thickness also increases with the Te-flux rate; however, the thickest film shows the lowest T_C, asserting that the effect is solely related to the composition of these systems.

Table 1

The saturation magnetization (M_S), remnant magnetization (M_r), coercivity (H_C) and transition temperature (T_C) of the telluride samples grown at different T_ECs together with the composition suggested by STEM-EELS.
T_EC (⁰ C)	Material	M_S (emu/cc)		M_r (emu/cc)		H_C (T)		T_C (K)
T_EC (⁰ C)	Material	∥	⟂	∥	⟂	∥	⟂	∥	⟂
325	CrTe	453	472	175	148	0.16	0.11	287	280
350	Cr₂Te₃	389	450	179	216	0.14	0.14	297	324
375	CrTe₂	187	256	27	150	0.24	0.95	154	155

We further confirmed the room temperature magnetization and PMA of the 350°C sample. An M vs. H loop was measured for this sample at 300 K, which can be seen in Fig. 7a. They show PMA with respect to the magnetic moment, and M_S. A small H_C of ~ 63 Oe was observed in both orientations, as can be seen in the inset of Fig. 7a. We measured the FC M vs. T measurement of the only Cr sample (Fig. 7b) as a reference. Though bulk Cr is an antiferromagnetic material ³⁹, we found that the only Cr sample is non-magnetic and only initiates a small bump at ~ 270 K in ⟂ orientation, originating from the T_N of Cr₂O₃ formed at the surface of amorphous Cr. ^45,46 Since this sample is not (anti)ferromagnetic, any contribution from Cr precipitates to the ordered magnetic properties of the telluride films are primarily ruled out.

The air stability of the magnetic films was confirmed on the 350 and 375°C samples by repeating the FC measurements for 0.01 T after several weeks. As can be seen in Fig. 8, the curves barely deviate from one another despite the films’ exposure to air and humidity during all of this time while also undergoing different measurement methods. Coughlin et al. reported extreme air sensitivity in Cr₂Te₃ nanoplates grown by the CVD technique. ⁴⁷ They discussed the air stability reported in other works like in Cr₅Te₈ grown by CVD, ⁴⁸ Cr₃Te₄ grown by atmospheric pressure CVD, ⁴⁹ CrTe₂ grown by CVD, ¹⁷ CrTe exfoliated from CVD-grown material,⁵⁰ etc. as a possible misinterpretation of exerted surface sensitive experiments like Raman or XPS and showed that the magnetism was drastically changing after keeping Cr₂Te₃ a few days in the air. They demonstrated that the oxidation of Cr_1+δTe₂ is thermodynamically favored and causes a reduction in magnetization after a mere 12 days. We reproduced the M vs. T measurements of the 350 (Cr₂Te₃) and 375°C (CrTe₂) samples after weeks and found their stability. This could be attributed to the difference in the method of sample preparation. We believe that the micro-cracks present in the CVD-grown and exfoliated samples, as reported by Wu et al.,⁵¹ could be a reason for fast oxidation leading to such an instability. Our UHV-grown thin films are epitaxially stabilized on the hexagonal Al₂O₃ (001) substrate and are stable compared to materials prepared with other techniques. Moreover, compared to exfoliated flakes, the epitaxial thin films are ‘bulk,’ and the air stability is further supported by the surface protective layer formed, thus keeping the rest of the film intact. On the other hand, exfoliated films offer a large surface area exposed to air.

Electrical transport properties

The resistance of the samples was measured between 5 and 350 K in a physical property measurement system (PPMS) with and without applying a magnetic field of 5 T. The magnetic field was switched on after the sample was cooled down to 5 K, as in the case of ZFC SQUID measurements. Then, the resistance was measured during heating, and another measurement was taken while the sample was cooled down to 5 K while keeping the magnetic field on, as in FC SQUID magnetometry. The measurement without a magnetic field was undertaken similarly, both during the heating and cooling cycles. The resistance was also measured while varying the magnetic field and keeping the sample at constant temperatures of 5, 100, 150, 200, 300, and 350 K. This measurement was done as in ZFC M vs. H experiments, from 0 T to 5 T to -5 T and then up again to 0 T. For the FC measurement, the cycle started at 5 T. The resistance R measured was converted into the specific resistance using $\:\rho\:=R\cdot\:d\cdot\:3.5749$ where d is the sample thickness and a correction factor of 3.5749 was applied, to consider the contact distance and sample width. ⁵⁵

The resistivity (ρ) vs. T plots of all the samples with and without magnetic field, under heating and cooling cycles are depicted in Fig. 9. The only Cr sample has a ρ value of ~ 148 x 10^− 6 Ωcm at room temperature, as reported for thin Cr films. ⁵⁷ It shows a semiconducting behavior with temperature and no change in the conductivity with the application of a magnetic field or heating/cooling cycles. Ultra-thin Cr films are reported to show similar semiconducting behavior, contrary to the antiferromagnetic metallic bulk Cr.⁵⁶ The thermomagnetic data of the only Cr sample shown in Fig. 7b is an indication of the suppression of antiferromagnetism in this sample and explains the semiconducting behavior as reported for ultrathin Cr films. ^56,57 All the tellurides are metallic, considering that their ρ values lie in the range of 590 − 490 x10^− 6 Ωcm at room temperature. However, the telluride films show a metal-to-insulator type transition (MIT) near their respective T_Cs, as can be seen in Fig. 9b-d. The red line marks the T_c for each Cr_(1+δ)Te₂ sample. The ferromagnetically aligned spins below the T_C offer less scattering and better conductivity in these systems. Under the application of an external magnetic field of 5 T, the alignment of the spins has been further improved, resulting in lower resistivity, thus leading to magnetoresistance (MR). The 325 and 350°C samples show another inflection in the ρ vs. T curve at lower temperatures with an increasing resistivity below ~ 100 K and ~ 25 K for 325 and 350°C samples, respectively. This is attributed to the presence of Cr precipitates in these samples. The ρ is dominated by the characteristics of the precipitate at low temperatures where the resistivity of the telluride is less, and that of thin Cr film is higher.

The MR is calculated using the formula $\:MR\left(\text{%}\right)=\frac{R\left(H\right)-R\left(0\right)}{R\left(H\right)}*100$, where R(H) and R(0) are the resistance with a magnetic field of H and zero, respectively.

The MR as a function of temperature for the cooling and heating cycles (calculated using the data shown in Fig. 9) is shown in Fig. S7. The only Cr is showing no MR. The 325 and 350°C samples show magneto resistance at 350 K and minima below their respective T_C. The MR of the 375°C sample is almost zero at room temperatures up to 200 K, and the negative MR increases upon further cooling and shows an inflection in the vicinity of its T_C. Small bumps/local minima are likely due to artifacts from the PPMS measurements. A maximum negative MR of 1.7% and 3.75% are shown by the 325 and 350°C samples, respectively, at ~ 275 K. The 375°C sample has a negative MR of 4.5% at 5 K. It shows that these tellurides exhibit magnetoresistance when in their ferromagnetic state. Near above T_C, weak ferromagnetic correlations exist, causing the small MR values observed in that region. Anisotropic magnetoresistance (AMR) was already reported in Cr-tellurides. ¹⁶ A spin-orbit coupling is key to possessing such AMR effects. In this work, all three tellurides show strong PMA; hence, AMR is expected in them.

To check MR’s field dependence, we have measured the ρ as a function of the magnetic field at various temperatures, as shown in Fig. S8. We found that the ρ shows AMR field dependence with a volcano-shaped MR vs. field at higher temperatures and a butterfly MR vs. field curve at lower temperatures. The calculated MR values are plotted in Fig. 10. Except for the Cr sample, all films show magnetoresistance, which reaches up to -4.5% for the 375°C sample. Small MR at 350 K for those samples is also visible in the MR curves in Fig. 10, where the field-dependent results can be seen. Butterfly magnetoresistance (BMR) ⁵⁸ for the lower temperature samples becomes only slightly visible at 5 K, while the 375°C sample shows the first signs of BMR already at 100 K and a really strong effect at 5 K.

In summary, we fabricated nanocrystalline (00l) oriented Cr-telluride thin films on the sapphire substrates. Structural and morphological properties seem strongly dependent on the Cr: Te ratio. We controlled the Te-flux during the film deposition by using different crucible temperatures (T_EC) to obtain CrTe (T_EC = 325°C sample), Cr₂Te₃ (T_EC = 350°C sample), and CrTe₂ (T_EC = 375°C sample) films. The excess Cr supplied during the deposition of CrTe and Cr₂Te₃ created Cr-precipitates. All the tellurides show perpendicular magnetic anisotropy (PMA), with CrTe₂ exhibiting the strongest PMA. The T_C varies depending on the composition, with a value of 280 K (287 K) for CrTe, 324 K (297 K) for Cr₂Te₃, and 155 K (154 K) for CrTe₂. The room-temperature ferromagnet Cr₂Te₃ also shows room-temperature PMA and anisotropic magnetoresistance (AMR). All the tellurides show AMR with a maximum value of -4.5% for the CrTe₂ at 5 K and a metal-to-insulator transition below their respective T_Cs. We believe this work is a great accomplishment concerning the stoichiometry modification in epitaxially stabilized films, which also affords air-stable ferromagnetism even when kept in air for weeks. This project could serve as a stepping stone for fabricating novel tellurides to investigate many advanced magnetism-related phenomena for future spintronics devices.

Deposition

A hybrid deposition system utilizing PLD ablation for Cr and an MBE evaporation source for Te was used for thin film deposition. For the ablation of the Cr target, a COMPex 205F KrF excimer laser from Coherent (λ = 248 nm) was used with a pulse rate of 4 Hz and a fluence of 7 J/cm². For each film, 8000 pulses were deposited on a sapphire substrate (Al₂O₃ (0001)) heated to 300°C. A valved cracker cell (V-CRC from Createc) evaporated Te into the plasma plume generated by the Cr ablation. The cracker cell is equipped with two independent heating circuits for the crucible containing the Te element and the cracking zone. While the crucible temperature T_EC was varied, the cracking (zone) temperature remained constant at 650°C. The vacuum chamber had a base pressure of ~ 4×10^− 8 mbar and a pressure of ~ 5×10^− 7 mbar during the deposition.

Structure and Composition

XRD, XRR- and Phi-Scans, as well as rocking curves, were measured with a Bruker D8 Discover diffractometer. A monochromator to restrict the beam to Cu Kα₁-radiation (λ = 1.5405 Å) was used for the XRD measurement. The rocking curves were measured for the (004) peaks. The Phi measurement was conducted by setting 2θ and ω to the Bragg angle of the (008) peak and tilting Psi to the interplanar angle of about 75° between the (001) and (222) lattice planes of the Cr_(1+δ)Te₂ films. ³⁰ AFM measurements were performed using an Asylum Research MFP-3D Origin AFM from Oxford Instruments in contact mode. An AXIS Supra + XPS from Kratos was used with a monochromatic Al Kα source for the XPS characterization. The samples were mounted on the holder using copper clamps. Charge neutralization was achieved by attaching the samples with copper masks to the sample holder. The valence band of the only Cr sample was used for energy calibration by differentiating the spectrum and setting the resulting maximum at the Fermi edge to 0 eV. Under the assumption that Carbon contamination is similar for all samples, the spectra of the Cr_(1+δ)Te₂ samples were calibrated using the C 1s position of the Cr-sample.

TEM Analysis

Electron transparent samples were extracted with an FEI Helios G4 dual beam FIB using a standard cross-section geometry and a Ga beam acceleration voltage of 2 kV during the final thinning step. The subsequent STEM investigations were carried out in an FEI Titan G2 E-TEM at an acceleration voltage of 300 kV using a convergence semi-angle of 10 mrad and probe currents of 42 pA for ADF imaging and 100 pA for EELS and EDX mapping. EELS signal intensities were extracted after pre-edge power-law background subtraction, and the EDX quantification was performed using the Cliff-Lorimer method.

Magnetic and Electrical Transport Measurements

To characterize the magnetic and electrical properties of the films, an MPMS® XL SQUID and a PPMS from Quantum Design, respectively, were used. To achieve good reliability of the PPMS measurements, gold contacts were sputtered on the films in a four-point probe configuration. On top of those contacts, the copper wires of the DC-Puck were carefully attached with silver epoxy.

DATA AVAILABILITY

All data related to this work and the files described in the supplementary information are publicly available at the universal research data repository for the Göttingen Campus (GRO.data) using the link https://doi.org/10.25625/LBPMCL.

ACKNOWLEDGEMENTS

Deutsche Forschungsgemeinschaft (DFG) supported this project under project 429646908 and Collaborative Research Centre (CRC) 1073. AT, PH, and JP acknowledge the financial support received from Ministerium für Wissenschaft, Forschung und Kunst (MWK), Lower Saxony under ‘Zukunft.niedersachsen’.

AUTHOR CONTRIBUTIONS

AT conducted the thin film deposition and performed the acquisition, analysis, and interpretation of data with respect to XRD, XPS, AFM, SQUID, and Magnetoresistance. PH supervised the acquisition of data with respect to XRD, XPS, AFM, and SQUID. TM performed acquisition, analysis, and interpretation of TEM results. JP conceived the idea, designed, supervised, and interpreted the work. AT wrote the initial draft of the manuscript. PH and TM edited the draft manuscript. JP edited and finalized the manuscript.

COMPETING INTERESTS

The authors declare no competing interests.

ADDITIONAL INFORMATION

Supplementary information The following file is available: CrTe_SI.pdf

Correspondence and requests for materials should be addressed to Jasnamol Palakkal.

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Room-temperature Ferromagnetism, Perpendicular Magnetic Anisotropy, and Anisotropic Magnetoresistance in Epitaxially Stabilized Air-stable Chromium Tellurides

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