A -Type Magnetic Semiconductor (Sr, Na)(Zn, Mn)2Sb2 Isostructural to 122-Type Iron-Based Superconductors
A new diluted magnetic semiconductor (Sr, Na)(Zn, Mn)2Sb2 has been successfully synthesized by doping Na and Mn into the parent compound , which has a -type crystal structure (space group , No. 164, ) isostructural to the 122-type iron-based superconductor . No magnetic ordering has been observed when only spins are doped by (Zn, Mn) substitution. Only with carriers codoped by (Sr, Na) substitution, a ferromagnetic ordering occurs below the maximum Curie temperature ∼9.5 K. Comparing with other -type diluted magnetic semiconductors, we will show that negative chemical pressure suppresses the Curie temperature.
Possessing advantages of both semiconductors and ferromagnetism, ferromagnetic semiconductors are one of the most prospective materials for the application in spintronics . In the past, concentrated magnetic semiconductors such as EuO once triggered great interests , but their Curie temperature is far lower than room temperature and the heterojunctions are difficult to be prepared. In the 1990s, III–V (Ga, Mn)As films became one of the most extensive studied diluted magnetic semiconductors (DMSs) [3–5]. As of today, the highest Curie temperature observed for (Ga, Mn)As is ∼200 K ; however, 200 K is still far below the room temperature requested for practical applications.
Recently, a series of new bulk-form DMSs isostructural to iron-based superconductors have been synthesised, including 111-type Li(Zn, Mn)As , Li(Zn, Mn)P , 122-type (Ba, K)(Zn, Mn)2As2 , (Ca, Na) (Zn, Mn)2Sb2 [10, 11], 1111-type (La, Ba)(Zn, Mn)AsO , and SrF(Zn, Mn, Cu)Sb . In these bulk-form DMSs, Mn substitution for Zn introduces spins, and atoms with low valence substitution for atoms with high valence introduce hole carriers, respectively. Measurements such as muon spin rotation/resonance/relaxation (SR) and nuclear magnetic resonance (NMR) have proved the intrinsic and homogeneous ferromagnetic ordering [14, 15]. Among the fabricated bulk-form DMSs, a Curie temperature as high as ∼230 K was achieved for a 122-type DMS (Ba, K)(Zn, Mn)2As2 ; meanwhile, a N-type DMS Ba(Zn, Co)2As2 with ∼45 K has been successfully fabricated . Both of (Ba, K)(Zn, Mn)2As2 and Ba(Zn, Co)2As2 have a -type tetragonal crystal structure. Ferromagnetic ordering has also been successfully induced in -type hexagonal crystal structure; they are (Ca, Na)(Zn, Mn)2Sb2 [10, 11], (Sr, Na)(Zn, Mn)2As2 , (Ca, Na)(Zn, Mn)2As2 , (Sr, K)(Zn, Mn)2As2 , (Sr, Na)(Cd, Mn)2As2 , and (Ba, K)(Cd, Mn)2As2 .
In this paper, we report the successful synthesis of a new DMS (Sr, Na)(Zn, Mn)2Sb2, in which carriers and spins are introduced by (Sr, Na) and (Zn, Mn) substitutions into the parent compound . has a band gap of ∼0.26 eV and is classified as a direct-gap semiconductor [23, 24]. It has a -type hexagonal crystal structure (space group , No. 164, ) isostructural to the 122-type iron-based superconductor . Ferromagnetic ordering has been observed below the maximum ∼9.5 K. Compared with other -type 122-type DMSs, we will show that negative chemical pressure suppresses the Curie temperature.
2. Materials and Methods
Polycrystalline specimens of (x = 0.05, 0.075, 0.10, 0.125, 0.15) were synthesised through a solid-state reaction method. Specifically, Sr, Na, Zn, Mn, and Sb elements of high purity were mixed, followed by heating at 200°C for 10 h and 750°C for 10 h in evacuated silica tubes. Subsequently, the mixture was subjected to grinding. The mixture was then pressed into pellets and placed in alumina crucibles. The crucibles were then sealed in evacuated silica tubes and subjected to heating at 750°C for 30 h. Afterwards, they were allowed to cool to room temperature. All the mixing and grinding procedures were carried out in a high-purity Ar-filled glove box. Both the and contents were below 0.1 ppm. Powder X-ray diffraction (XRD) was applied to determine the crystal structure at room temperature using a powder X-ray diffractometer (Model EMPYREAN) equipped with a monochromatic radiation. A Quantum Design Magnetic Property Measurement System was employed for DC magnetization measurements, and a typical four-probe method was used to measure the resistivity.
3. Results and Discussion
The X-ray diffraction patterns of (x = 0.05, 0.075, 0.10, 0.125, and 0.15) are shown in Figure 1(a). All the patterns match well with the -type crystal structure with the space group. The () has the 1a (0, 0, 0) site, while () and Sb atoms occupy the 2d (1/3, 2/3, z) site in a -type unit cell. The specific structure is displayed in the inset of Figure 1(b). In all samples, trace of ZnSb impurities was also observed and marked by stars. These nonmagnetic impurities have no influence on the ferromagnetism discussed in the following. We carried out the Rietveld refinement for (x = 0.05, 0.075, 0.10, 0.125, and 0.15) by GSAS-II , and the result is shown in Figure 1(b) and Table 1. We obtained the unit cell volume, and they are shown in Figure 1(c). The volume increases from 135.468 Å3 to 135.634 Å3 monotonically, implying that Na and Mn were successfully doped into the ionic sites. Doping Na and Mn more than 15% was not successful.
The temperature dependence of DC magnetization for is shown in Figure 2. The measurement was conducted under field-cooled (FC) conditions with the external field ∼100 Oe. No ferromagnetic transition has been observed down to 2 K, indicating that only spin doping by (Zn, Mn) substitution cannot induce ferromagnetic ordering. The results can be well fitted by the Curie formula with and as shown in the inset of Figure 2. We have also observed similar results in other bulk-form DMS systems such as Sr(Zn, Mn)2As2  and La(Zn, Mn)AsO .
The temperature dependence of DC magnetization for (x = 0.05, 0.075, 0.10, 0.125, and 0.15) is shown in Figure 3(a). The measurement was conducted under field-cooled (FC) as well as zero-field-cooled (ZFC) conditions with the external field ∼100 Oe. The magnetization is found to increase abruptly below ∼15 K. For , the FC and ZFC curve starts to separate, and the separate temperature is the spin freezing temperature . The spin freezing temperature is ∼3.5 K and ∼4.5 K for x = 0.10 and 0.125, respectively, and increases to ∼7.0 K for x = 0.15 finally. As shown in Figure 3(b), the minimum value from is defined as the Curie temperature . The values are ∼3.5 K, ∼5.0 K, ∼8.0 K, and ∼8.0 K for x = 0.05, 0.075, 0.10, and 0.125, respectively. For x = 0.15, the Curie temperature finally increases to ∼9.5 K. We fit the curves above with the Curie–Weiss formula , in which represents a constant indicating the effective moment, is a temperature-independent contribution, and is the Weiss temperature. As shown in Figure 3(c), the reverse of versus temperature is plotted. The intercept with the x-axis of the linear fitting curve of data at high temperature is the Weiss temperature . The values are determined to be ∼5.6 K, ∼6.3 K, ∼9.3 K, and ∼10.3 K for x = 0.05, 0.075, 0.10, and 0.125, respectively. Eventually, increases to ∼12.0 K for x = 0.15. We plotted the fitted results of Curie temperature, Weiss temperature, and spin freezing temperature in Figure 4 and tabulated them in Table 2.
The isothermal magnetization for (x = 0.05, 0.075, 0.10, 0.125, and 0.15) at 2 K is shown in Figure 3(d). All samples displayed clear hysteresis loops, which indicates the existence of ferromagnetic ordering for all doping levels at 2 K. The coercive field is ∼3.5 Oe, ∼8.8 Oe, ∼40.2 Oe, and ∼47.4 Oe for x = 0.05, 0.075, 0.10, and 0.125, respectively. For x = 0.15, finally increases to ∼88.9 Oe. This value is smaller than ∼300 Oe of (Ca, Na)(Zn, Mn)2Sb2  and ∼110 Oe of (Sr, Na) (Zn, Mn)2As2 . We also plotted and tabulated the coercive field versus doping level for in Figure 4 and Table 2.
The temperature dependence of electrical resistivity for (x = 0, 0.05, 0.075, 0.10, 0.125, and 0.15) is shown in Figure 5. All samples show metallic behaviour. The resistivity decreases monotonically from for x = 0 to for x = 0.15, which suggests that the carrier concentrations are increased via higher doping of Na atoms. According to a previous study, another -type DMS (Sr, Na)(Zn, Mn)2AS2 also displayed the similar trend . We also tabulated the resistivity of at 4 K in Table 2.
The maximum Curie temperature of (Sr, Na)(Zn, Mn)2Sb2 is lower than ∼10.8 K of (Ca, Na)(Zn, Mn)2Sb2 , ∼20 K of (Sr, Na)(Zn, Mn)2As2 , and ∼33 K of (Ca, Na)(Zn, Mn)2As2 , which all have the same structure and smaller unit cell. To further discuss the relationship between chemical pressure and the Curie temperature, we tabulated the Curie temperature and the parent compounds’ lattice parameters for different -type diluted magnetic semiconductors in Table 3. From (Ca, Na)(Zn, Mn)2As2 to (Sr, Na)(Zn, Mn)2Sb2, the lattice parameter kept increasing from a = 0.4162 nm and c = 0.7010 nm to a = 0.4503 nm and c = 0.7721 nm. The volume of unit cell increased from 0.1052 nm3 to 0.1356 nm3. Comparing the Curie temperature of the same Mn doping level for different compounds, we can see that larger unit cell volume leads to lower Curie temperature. For example, with the same 5% Mn doping, the Curie temperature is ∼20 K for (Ca, Na)(Zn, Mn)2As2 , ∼15 K for (Sr, Na)(Zn, Mn)2As2 , and ∼6 K for (Ca, Na)(Zn, Mn)2Sb2  and decreases to ∼3.5 K for (Sr, Na)(Zn, Mn)2Sb2, indicating that negative chemical pressure suppresses the Curie temperature.
In summary, a new bulk-form -type DMS (Sr, Na)(Zn, Mn)2Sb2 isostructural to 122-type iron-based superconductors was successfully synthesized. Spins and carriers are introduced by (Zn, Mn) substitution and (Sr, Na) substitution, respectively. Spin doping alone cannot induce any type of magnetic ordering. The ferromagnetic ordering occurs below the maximum Curie temperature ∼9.5 K only with simultaneous introduction of spins and carriers into the sample. Through comparison of different 122-type DMSs with crystal structure, we find that the chemical pressure and Curie temperature are closely correlated in these DMSs; i.e. negative chemical pressure suppresses the Curie temperature. Our fabrication of the new DMS (Sr, Na)(Zn, Mn)2Sb2 enriches the DMS families and enlightens the searching for DMS with high Curie temperature. The same crystal structure between (Sr, Na)(Zn, Mn)2Sb2 and 122-type iron-based superconductor may enable the development of novel devices in future.
The authors confirm that the data supporting the findings of this study are available within the article.
Conflicts of Interest
The authors declare conflicts of interest regarding the publication of this paper.
This work was supported by the National Basic Research Program of China (No. 2016YFA0300402), NSF of China (No. 12074333), and the Key R&D Program of Zhejiang Province, China (2021C01002).
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