# ACaAl2Si2-Type Magnetic Semiconductor (Sr, Na)(Zn, Mn)2Sb2 Isostructural to 122-Type Iron-Based Superconductors

Authors: Yilun Gu; Rufei Zhang; Haojie Zhang; Licheng Fu; Guoxiang Zhi; Jinou Dong; Xueqin Zhao; Lingfeng Xie; Fanlong Ning
Journal: Advances in Condensed Matter Physics
 (2022)

---

## Abstract

A new diluted magnetic semiconductor (Sr, Na)(Zn, Mn)2Sb2 has been successfully synthesized by doping Na and Mn into the parent compound SrZn2Sb2, which has a CaAl2Si2-type crystal structure (space group P3¯m1, No. 164, hP5) isostructural to the 122-type iron-based superconductor CaFe2As2. 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 TC∼9.5 K. Comparing with other CaAl2Si2-type diluted magnetic semiconductors, we will show that negative chemical pressure suppresses the Curie temperature.


---

## Body

## 1. Introduction

Possessing advantages of both semiconductors and ferromagnetism, ferromagnetic semiconductors are one of the most prospective materials for the application in spintronics [1]. In the past, concentrated magnetic semiconductors such as EuO once triggered great interests [2], 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 [6]; 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 [7], Li(Zn, Mn)P [8], 122-type (Ba, K)(Zn, Mn)2As2 [9], (Ca, Na) (Zn, Mn)2Sb2 [10, 11], 1111-type (La, Ba)(Zn, Mn)AsO [12], and SrF(Zn, Mn, Cu)Sb [13]. 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 (μSR) 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 [16]; meanwhile, a N-type DMS Ba(Zn, Co)2As2 with TC∼45 K has been successfully fabricated [17]. Both of (Ba, K)(Zn, Mn)2As2 and Ba(Zn, Co)2As2 have a ThCr2Si2-type tetragonal crystal structure. Ferromagnetic ordering has also been successfully induced in CaAl2Si2-type hexagonal crystal structure; they are (Ca, Na)(Zn, Mn)2Sb2 [10, 11], (Sr, Na)(Zn, Mn)2As2 [18], (Ca, Na)(Zn, Mn)2As2 [19], (Sr, K)(Zn, Mn)2As2 [20], (Sr, Na)(Cd, Mn)2As2 [21], and (Ba, K)(Cd, Mn)2As2 [22].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 SrZn2Sb2. SrZn2Sb2 has a band gap of ∼0.26 eV and is classified as a direct-gap semiconductor [23, 24]. It has a CaAl2Si2-type hexagonal crystal structure (space group P3¯m1, No. 164, hP5) isostructural to the 122-type iron-based superconductor CaFe2As2 [25]. Ferromagnetic ordering has been observed below the maximum TC∼9.5 K. Compared with other CaAl2Si2-type 122-type DMSs, we will show that negative chemical pressure suppresses the Curie temperature.


## 2. Materials and Methods

Polycrystalline specimens ofSr1−xNaxZn1−xMnx2Sb2 (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 H2O and O2 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 CuKα1 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 ofSr1−xNaxZn1−xMnx2Sb2 (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 CaAl2Si2-type crystal structure with the P3¯m1 space group. The (Sr1−xNax) has the 1a (0, 0, 0) site, while (Zn1−xMnx) and Sb atoms occupy the 2d (1/3, 2/3, z) site in a CaAl2Si2-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 Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15) by GSAS-II [26], 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.Figure 1
(a) The X-ray diffraction patterns forSr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15). Trace of impurities ZnSb(∗) are marked. (b) The rietveld refinement of Sr0.9Na0.1Zn0.9Mn0.12Sb2. Inset is the crystal structure of SrZn2Sb2. (c) The lattice parameters a and c for Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15). Inset is the unit cell volume V for Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15).
(a)(b)(c)Table 1
Lattice parametersa, c, and V for Sr1−xNaxZn1−xMnx2Sb2.
Doping levela (nm)c (nm)V (Å3)Rwp (%)0.050.450090.77215135.4610.90.0750.450200.77215135.539.10.100.450240.77216135.569.60.1250.450240.77249135.6210.70.150.450310.77306135.6310.1The temperature dependence of DC magnetization forSrZn0.9Mn0.12Sb2 is shown in Figure 2. The measurement was conducted under field-cooled (FC) conditions with the external field Bext∼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 M=C/T+M0 with C=2.35×10−2μBK/Mn and M0=2.46×10−4μB/Mn 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 [18] and La(Zn, Mn)AsO [12].Figure 2
The temperature dependence of magnetization forSrZn0.9Mn0.12Sb2 measured under field-cooled conditions with an external field Bext∼100 Oe. Inset: the plot of 1/M−M0 versus temperature.The temperature dependence of DC magnetization forSr1−xNaxZn1−xMnx2Sb2 (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 Bext∼100 Oe. The magnetization is found to increase abruptly below T∼15 K. For x≥0.10, the FC and ZFC curve starts to separate, and the separate temperature is the spin freezing temperature Tf. The spin freezing temperature Tf 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 dM/dT is defined as the Curie temperature TC. 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 TC with the Curie–Weiss formula M=C/T−θ+M0, in which C represents a constant indicating the effective moment, M0 is a temperature-independent contribution, and θ is the Weiss temperature. As shown in Figure 3(c), the reverse of M−M0 versus temperature is plotted. The intercept with the x-axis of the linear fitting curve of data at high temperature is the Weiss temperature θ. 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, θ 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.Figure 3
(a) The temperature dependence of magnetization forSr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15) measured under field-cooled and zero-field-cooled conditions with an external field Bext∼100 Oe. (b) The plot of dM/dT versus temperature for Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15). (c) The plot of 1/M−M0 versus temperature for Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15). (d) Isothermal magnetization for Sr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15) at 2 K.
(a)(b)(c)(d)Figure 4
The Curie temperature, Weiss temperature, spin freezing temperature, and coercive field versus doping levelx of Sr1−xNaxZn1−xMnx2Sb2.Table 2
Curie temperatureTC, Weiss temperature θ, effective magnetic moment Meff, saturation moment Msat, coercive field HC, and resistivity at 4 K for Sr1−xNaxZn1−xMnx2Sb2.
Doping levelTC (K)θ (K)Meff (μB/Mn)Msat (μB/Mn)HC (Oe)Resistivity at 4 K (Ω⋅cm)0—————5.7 × 10−30.053.55.63.821.193.51.3 × 10−30.0755.06.33.541.058.81.0 × 10−30.108.09.33.551.2240.21.0 × 10−30.1258.010.33.490.8847.45.4 × 10−40.159.512.04.290.7588.95.2 × 10−4The isothermal magnetization forSr1−xNaxZn1−xMnx2Sb2 (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, HC finally increases to ∼88.9 Oe. This value is smaller than HC∼300 Oe of (Ca, Na)(Zn, Mn)2Sb2 [10] and HC∼110 Oe of (Sr, Na) (Zn, Mn)2As2 [18]. We also plotted and tabulated the coercive field versus doping level for Sr1−xNaxZn1−xMnx2Sb2 in Figure 4 and Table 2.The temperature dependence of electrical resistivity forSr1−xNaxZn1−xMnx2Sb2 (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 5.7×10−3Ω⋅cm for x = 0 to 5.2×10−4Ω⋅cm for x = 0.15, which suggests that the carrier concentrations are increased via higher doping of Na atoms. According to a previous study, another CaAl2Si2-type DMS (Sr, Na)(Zn, Mn)2AS2 also displayed the similar trend [18]. We also tabulated the resistivity of Sr1−xNaxZn1−xMnx2Sb2 at 4 K in Table 2.Figure 5
The temperature dependence of resistivity forSr1−xNaxZn1−xMnx2Sb2 (x = 0.05, 0.075, 0.10, 0.125, and 0.15).The maximum Curie temperature of (Sr, Na)(Zn, Mn)2Sb2 is lower than TC∼10.8 K of (Ca, Na)(Zn, Mn)2Sb2 [10], TC∼20 K of (Sr, Na)(Zn, Mn)2As2 [18], and TC∼33 K of (Ca, Na)(Zn, Mn)2As2 [19], which all have the same CaAl2Si2 structure and smaller unit cell. To further discuss the relationship between chemical pressure and the Curie temperature, we tabulated the Curie temperature TC and the parent compounds’ lattice parameters for different CaAl2Si2-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 TC is ∼20 K for (Ca, Na)(Zn, Mn)2As2 [19], ∼15 K for (Sr, Na)(Zn, Mn)2As2 [18], and ∼6 K for (Ca, Na)(Zn, Mn)2Sb2 [11] and decreases to ∼3.5 K for (Sr, Na)(Zn, Mn)2Sb2, indicating that negative chemical pressure suppresses the Curie temperature.Table 3
The Curie temperatureTC and parent compounds’ lattice parameters for different CaAl2Si2-type diluted magnetic semiconductors.
MaterialTC of 5% Mn doping (K)TC of 10% Mn doping (K)a (nm)c (nm)Volume (nm3)(Ca, Na)(Zn, Mn)2As2 [19]20280.41620.70100.1052(Sr, Na)(Zn, Mn)2As2 [18]15170.42230.72680.1123(Ca, Na)(Zn,Mn)2Sb2 [11]6100.44340.74490.1269(Sr, Na)(Cd, Mn)2As2 [21]690.44610.74240.1278(Ba, K)(Cd, Mn)2As2 [22]<280.45130.76740.1354(Sr, Na)(Zn, Mn)2Sb2∗3.580.45030.77210.1356∗Current work.


## 4. Conclusions

In summary, a new bulk-formCaAl2Si2-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 TC∼9.5 K only with simultaneous introduction of spins and carriers into the sample. Through comparison of different 122-type DMSs with CaAl2Si2 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 CaFe2As2 may enable the development of novel devices in future.



---

*Source: 4291923-2022-01-10.xml*