Figure 1(c) shows a HRTEM image with a crystal lattice spacing of 0.65 nm along the c axis, which corresponds to the distance between neighboring (001) planes of the hexagonal CrTe phase. In addition, the corresponding fast Fourier transform (FFT) pattern reveals a single set of spots [inset of Fig. 1(c)], corroborating the high crystalline quality of the CrTe film. The sharp streaky RHEED pattern [Fig. 1(d)] implies high crystalline quality and flat surface of the film. This is supported by the AFM image taken on the CrTe film that has a root-mean-square surface roughness (Rq) of ~ 0.32 nm [Fig. 1(e)]. Elemental mapping and EDS spectrum show that the as-prepared thin film contains only Cr and Te elements and uniformly distributes in the film. A quantitative analysis demonstrates that the atomic ratio of Cr/Te is 49.5:50.5, very close to 1:1 [Fig. 1(f)].
The film thickness is measured by the AFM line scan near the film-substrate boundary. The AFM line scan image and obtained height difference between the surface of the film and that of the STO substrate are shown in Fig. 2(a), 2(b), and 2(c), respectively. There are obvious boundary and height difference between the CrTe film and the STO substrate, as can be seen from the Amplitude Retrace (AmR) mode of AFM image [Fig. 2(a) and 2(b)]. The obtained data show that the thicknesses of the thin-film sample is about 25 nm [Fig. 2(c)]. The fitting of the small angle X-ray reflectivity (XRR) oscillation pattern of the same thin-film sample yields a film thickness of ~ 25 nm, as shown in Fig. 2(d), well consistent with that obtained by AFM line scans.
Figure 2(e) shows XRD θ-2θ scan pattern of the 25-nm CrTe thin-film sample. The diffraction peaks at 2θ ∼ 40.0o and 86.2o result from the STO (111) and (222) lattice planes, respectively. Refer to the standard PDF card of the CrTe (ICSD Card no. 626889), it can be concluded that the other two diffraction peaks located at 2θ = 28.8o and 59.7o are CrTe (002) and (004) diffraction peaks, respectively, indicating the film is (001) oriented. The XRD ϕ-scan patterns show six-fold symmetry for the CrTe film and three-fold symmetry for the STO substrate [Fig. 2(f)], respectively, which implies epitaxial growth of the CrTe film on the STO substrate.
Figure 3(a) shows the ZFC and FC magnetization versus temperature (M-T) curves of the 25-nm CrTe film, as measured by applying 100, 500, and 1000 Oe magnetic fields parallel to the film plane, respectively. The thin-film sample has obvious ferromagnetic characteristics, and the Curie temperature (TC) is above room temperature (TC ~ 330 K). According to the M-T curve, the phases can be roughly divided into three regions, corresponding to the low temperature (< 100 K) antiferromagnetic phase (AFM phase), intermediate temperature ferromagnetic phase (FM phase), and high temperature (> 330 K) paramagnetic phase (PM phase). These M-T curves indicate that the CrTe film has complex magnetic structures. As shown in Fig. 3(a), the in-plane ZFC and FC curves are separated at low temperatures in the magnetic field of 500 Oe, and the ZFC magnetization decreases rapidly near 110 K, implying the transition from the ferromagnetic state to the antiferromagnetic state. However, at the lowest measurement temperature of 2 K, the remanent magnetization doesn’t reach zero. The film has a weak net moment of ~ 0.23 µB/f.u., implying possible spin canting within each Cr sublattice although the moments between Cr1 layer and those of Cr2 layer couples antiferromagnetically [17], as schematically shown in the left panel of Fig. 3(b). As shown in Fig. 3(a), for 500 Oe, the FC moment per Cr ions (3.35 µB/f.u.) is 12.4 time that of the ZFC one (0.27 µB/f.u.). For 1000 Oe, the FC moments per unit cell reaches 7.86 µB/u.c., corresponding to 3.93 µB/f.u., approximately 98.3% of the theoretical moment of Cr3+ ions. This means that the antiferromagnetic state is not so stable. 500–1000 Oe magnetic fields is sufficient to force the moments of both Cr1 and Cr2 layers to parallel to the field direction [right panel of Fig. 3(b)], resulting in almost saturated magnetization for the FC process.
Magnetization versus magnetic field (M-H) hysteresis loops are measured to further understand the magnetic properties of CrTe films. The representative M-H curves, as measured in the AFM, FM, and PM temperature region, are shown in Fig. 3(c). Even in the low temperature antiferromagnetic state (5 and 50 K), the film exhibits macroscopic ferromagnetic behaviors, reaching a saturation magnetization of 2070 emu/cc (3.93 µB/f.u.) at a magnetic field of 1.2 T. This indicates that the in-plane antiferrimagnetic spin configuration could be modified and tends to be aligned parallel to the direction of external magnetic field. The M-H curves at 100 and 200 K show typical characteristics of hard ferromagnetic materials, which is consistent with the M-T curves. At 300 K, the M-H loop still show ferromagnetic behaviors, however, the saturation magnetization decreases to 1000 emu/cc. Upon heating to 350 K, the hysteresis loop basically disappears [Fig. 3(c)], exhibiting PM phase characteristics.
Figure 3(d) shows the out-of-plane ZFC and FC M-T curves, as measured in magnetic fields of 100, 500, and 1000 Oe, respectively, showing different behaviors from those of the in-plane ZFC and FC M-T curves. The M-T curves basically overlap in a low magnetic field of 100 Oe, while the ZFC and FC curves separate at approximately 288 and 300 K in magnetic fields of 500 and 1000 Oe, respectively. As the external out-of-plane magnetic field increases from 100 to 1000 Oe, the in-plane magnetic moments flip toward out-of-plane direction to some degree, resulting in an increase in the out-of-plane magnetization. For example, at 200 K, the FC magnetization increases from 84.8 to 186.7 and finally to 239.5 emuc/cc upon increasing magnetic field from 100 to 500 and finally to 1000 Oe. Whenever in the antiferromagnetic or ferromagnetic state, the FC in-plane magnetization always much larger than that of the FC out-of-plane magnetization, implying strong magnetic anisotropy. This phenomenon is in sharp contrast to that observed in CrTe bulk crystals whose c-axis magnetization is almost the same as the a-axis magnetization [12]. Since the CrTe film is only 25-nm, the observed magnetic anisotropy could be due to the geometry shape effect.
Figures 4 (a-c) shows the temperature (T)-dependent resistivity (ρ) under magnetic fields of 0, 5, and 14 T. The schematic measurement diagrams are shown in the upper inset of each panel. These ρ-T curves show metal-to-insulator-like transitions which couple to the PM-to-FM phase transitions near the Curie transition point (TC~330 K). The lower inset in each panel shows the variation of MR with temperature under 5 and 14 T magnetic fields. Here, MR is defined as \(\:MR\:\left(\%\right)=\left[\rho\:\:\right(H)-\rho\:\:(0\left)\right]/\:\rho\:\left(0\right)\times\:100\%\), where ρ (0) and ρ (H) are the resistivity of the thin films at zero magnetic field and a magnetic field H, respectively. The CrTe thin-film sample shows considerable negative MR at higher temperatures (above ~ 90K). The largest negative MR appears at TC ∼330 K and decreases with increasing or decreasing temperature. The negative MR near the Curie temperature could be qualitatively understood within the framework of the electronic phase separation model within which the ferromagnetic metallic phase coexists with paramagnetic insulating phase and the volume fraction of the former increases at the expanse of the latter with increasing magnetic field, resulting in the MR effect, similar to those observed in perovskite manganites [18, 19].