Introduction
Combining the features of carrier control in semiconductors and spin control in ferromagnets, diluted magnetic semiconductors (DMS) have significant advantages in spintronics and have thus been extensively studied in recent decades.[1–3] For example, spin-orbit torque, which was first discovered in DMS, is now an important mechanism for the new generation of magnetic memories and sensors.[4] In the 1990s, the typical DMS, (Ga,Mn)As, was successfully conducted spin and carrier doping via the substitution of Mn2+ into Ga3+ sites.[3,5] However, a single-element substitution such as this can lead to a lack of independent tuning for conducting or magnetic properties.[6,7] Furthermore, the dual roles of Mn complicate the underlying theoretical understanding, including whether double exchange is a relevant mechanism in these systems.[8] To overcome the aforementioned difficulties, a series of new type DMS materials with independent spin and carrier doping have been discovered, that is, Li(Zn,Mn)As and (Ba,K)(Zn,Mn)2As2 (BZA).[9–13] Among these new DMS materials, the reliable Curie temperature (TC = 230 K) of BZA was the highest.[3,14,15]
Owing to the application potential of high-TC BZA, it has been proposed to develop isostructural heterojunctions between BZA, superconducting (Ba,K)Fe2As2, and high Néel temperature semiconductor BaMn2As2 to explore their emerging phenomena and functionalities.[3,14,16] Particularly, their matching ThCr2Si2-type crystal structure and the small mismatch (less than 5%) within the a-b plane between them represent the unique features of BZA and analogs and could lead to unprecedented fabrications of potential heterojunctions for DMS materials, as shown in Figure 1. Taking the Andreev reflection junction as a typical example, the first theoretical consideration of the spin-flip Andreev reflection was studied with DMS, and recently it led to spin-triplet superconductivity at the interface of ferromagnet/superconductor.[17,18] Furthermore, the versatile design of materials properties, for example, superconductivity with ferromagnetism, topological magnetism, and enhanced spin–orbit couplings, could be accomplished via proximity effects.[19] These prospects have inspired further trials to improve TC of BZA.[20–23] The intuitive way of improving TC is to increase the doping level of magnetic cations in DMS materials.[16] For example, TC of (Ga,Mn)As has been enhanced to 200 K via heavy Mn doping.[24,25] It is worth noting that further Mn doping is a long-term challenge for (Ga,Mn)As, due to heterovalent (Ga3+, Mn2+) substitution. The latter results in not only limited chemical solubility but also interstitial Mn defects, which act as electron donors to balance hole carriers from substitutional Mn.[9,26] Such a disadvantage is overcome by isovalent (Zn2+, Mn2+) substitution in BZA. The Mn concentration over 15% is theoretically tolerable owing to the existence of isostructural BaMn2As2 with fully occupied Mn.[27]
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It is well known that the magnetic exchange interaction between nearest Mn ions is antiferromagnetic superexchange, and it tends to dominate the interactions among all Mn ions and results in antiferromagnetism in DMS materials with increasing Mn-doping. Taking the advantage of independent spin and carrier doping, it is possible to study the individual doping effect on ferromagnetism. Our calculations indicate that carrier-doping, namely K-doping, can suppress the antiferromagnetic superexchange in BZA. Moreover, the sufficient carrier-doping can overcome the dominating antiferromagnetic interaction and further enhance TC in heavy Mn-doped BZA. However, extra Mn doping has not been successful with conventional synthesis conditions. Generally, the doping levels in most of the DMS materials are limited due to either structural distortion or valence disequilibrium which can be induced by the impurity doping. High-pressure synthesis or annealing has shown the capability to recover structural distortions and maintain unstable valences in many materials.[28] With high-pressure and high-temperature annealing, the Mn concentration of BZA was increased to 28%, ≈2 times that of conventional synthesized samples. The high-pressure stabilized high doping levels in polycrystalline BZA finally led to the highest TC of 260 K.
Results and Discussion
Crystal Structure
K and/or Mn-doped BZA samples crystalize into ThCr2Si2-type structure as shown in Figure 1a. In previous reports, Mn-related impurities formed when Mn doping level was over 15%.[8] Taking advantage of the high-pressure technique, higher concentrations of Mn can be stabilized in bulk BZA. The purity of all the samples ((Ba0.7K0.3)(Zn1-xMnx)2As2 with x = 0.18, 0.2, 0.22, 0.24, 0.26, 0.28) was confirmed with powder X-ray diffraction (XRD) measurements. Figure 2a shows the XRD patterns and corresponding Rietveld refinement of (Ba0.7K0.3)(Zn0.76Mn0.24)2As2 as a typical example. All the Bragg peaks are indexed under the space group of I4/mmm, indicating the high purity of the samples. Rietveld refinements were used to calculate the lattice parameters of samples. The dopant dependences of the lattice parameters, a and c, are shown in Figure 2b. We can see that the lattice parameters change monotonically with increasing Mn concentrations, suggesting successful chemical substitution.
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Magnetic Properties
Temperature-dependent magnetization (M(T)) measurements were performed to determine TC of the aforementioned samples. For the sake of simplification, M(T) under the field cooling (FC) process for three selected samples are shown in Figure 3a. Similar to the previous report of BZA with lower doping levels, the upturns on the M(T) curves, which indicate the forming of ferromagnetic ordering, are defined as TC.[18] Figure 3b shows a clearly dome-shaped x dependence on TC and coercive force (HC). TC increases with increasing Mn concentrations in the first place, and the highest TC = 260 K is obtained in the sample with (Ba0.7K0.3)(Zn0.76Mn0.24)2As2. The overdoped Mn damages ferromagnetism due to the increased antiferromagnetic interaction between the nearest neighboring Mn2+.
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Field-dependent magnetizations (M(H)) at 5 K and in the vicinity of TC are shown in Figure 3c,d, respectively. At low temperatures, one can find robust magnetic loops with HC of ≈0.5 T. The doping-level dependence of HC also shows a dome-shaped relationship similar to the x dependence of TC. The sample (Ba0.7K0.3)(Zn0.76Mn0.24)2As2 with the highest TC exhibits the largest HC of 0.8 T. The saturate magnetizations (MS) are ≈1 μB/Mn consistent with lower Mn-doped samples.[9,22] In the vicinity of TC, magnetic loops are measured to confirm the ferromagnetic ordering. At 257 K, (Ba0.7K0.3)(Zn0.76Mn0.24)2As2 still shows an open loop indicating the forming of ferromagnetic ordering, while the rest of the samples exhibit nearly linear loops indicating paramagnetic states.
Magnetoresistance and Hall Effect
Negative magnetoresistance (MR) is also strong evidence of ferromagnetism in DMS materials, due to the reduction in spin scattering with increasing external magnetic fields.[5,29–31] The temperature dependence of resistivity shows semiconducting behavior within the measuring temperature range (5 to 300 K). Below TC, MR can be found for all the samples. At 5 K, butterfly-like MR(H) curves are observed, as shown in Figure 4a. The peaks at around 0.5 T are consistent with HC from M(H) curves. It is worth noting that the MR is far from reaching saturation at 7 T, where the magnetic moment is nearly saturated. In such conditions, the MR is presumably from the weak localization effects.[32,33] At high temperatures, negative MR is also clearly present in the vicinity of TC, as shown in Figure 4b, evidencing the forming of ferromagnetic ordering. For the sample (Ba0.7K0.3)(Zn0.76Mn0.24)2As2 with TC of 260 K, the maximum MR are −7.5% and −1.7% at 5 and 300 K, respectively. The MR with similar magnitudes have been reported in BZA with lower doping levels.[27]
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To determine the carrier concentrations, Hall effect measurements were conducted. Figure 4c shows the anomalous Hall effect of (Ba0.7K0.3)(Zn0.76Mn0.24)2As2 at 5 K. For a ferromagnetic DMS material, the carrier scattering by the magnetic ions causes the carriers to accumulate asymmetrically in the transverse direction of the current. This scattering results in an additional contribution to the normal Hall effect, namely, the anomalous Hall effect. This is an important feature of long-range ferromagnetic ordering in a DMS material. The Hall resistance (RHall) can be phenomenologically described by Equation (1)
First-Principles Simulations
To study the magnetic interactions between Mn impurities in BZA, a Heisenberg Hamiltonian with the nearest coupling J1, the next-nearest coupling J2, the third-nearest neighbor couplings J3, and the interlayer couplings J⊥ of the Mn─Mn pairs is employed, as described by Equation (2),
Using the density functional theory calculations, the exchange couplings J1, J2, J3 and J⊥ for the different supercells of Ba(Zn0.75Mn0.25)2As2, (Ba0.875K0.125)(Zn0.75Mn0.25)2As2, (Ba0.75K0.25)(Zn0.75Mn0.25)2As2 and (Ba0.5K0.5)(Zn0.75Mn0.25)2As2 are calculated. Further details can be found in the Supporting Information. The results are listed in Table 1, with ferromagnetic for J > 0, and antiferromagnetic for J < 0.
Table 1 First-principles magnetic exchange couplings of Mn─Mn pairs in Ba(Zn0.75Mn0.25)2As2, Ba(Zn0.75Mn0.25)2As2, (Ba0.875K0.125)(Zn0.75Mn0.25)2As2, (Ba0.75K0.25)(Zn0.75Mn0.25)2As2 and (Ga0.91Mn0.09)As. J1, J2, J3, and J⊥ are intralayer couplings of the nearest, the next-nearest, the third-nearest neighbors and interlayer couplings respectively. Two Mn impurities are coupled ferromagnetically for conditions with J > 0 and antiferromagnetically for conditions with J < 0.
| First-principles calculations | ||||
| J1 [meV] |
J2 [meV] |
J3 [meV] |
J⊥[meV] | |
|
Ba(Zn0.75Mn0.25)2As2 (Ba0.875K0.125)(Zn0.75Mn0.25)2As2 |
−8.76 −5.46 |
−2.47 −0.71 |
0.17 0.58 |
−1.47 1.15 |
| (Ba0.75K0.25)(Zn0.75Mn0.25)2As2 | −1.49 | 1.73 | 1.11 | 2.62 |
|
(Ba0.5K0.5)(Zn0.75Mn0.25)2As2 (Ga0.91Mn0.09)As |
0.31 8.31 |
2.95 −0.63 |
2.95 0.81 |
2.32 ∖ |
To estimate the Néel temperature (TN) for the antiferromagnetic cases and the TC for the ferromagnetic cases, the following mean-field formula is used, as described by Equation (3),
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For comparison, the magnetic exchange couplings J1, J2, and J3 and TC of (Ga1-xMnx)As are also calculated in a similar way. The rescaled method is also adopted to counteract the overestimation of the mean-field result, where the calculated mean-field TC of (Ga0.figur84Mn0.16)As is approximated to correspond to the TC of 200 K of (Ga0.84Mn0.16)As in experiments.[24,36] As shown in Figure 5a, the TC increases with increased Mn concentration for (Ga1-xMnx)As and (Ba0.75K0.25)(Zn1-xMnx)2As2 with x in the range of 5% to 30%. This behavior is consistent with the experimental result of (Ba0.7K0.3)(Zn1-xMnx)2As2. As shown in Figure 5b, (Ba1-yKy)(Zn0.75Mn0.15)2As2 shows antiferromagnetic behavior without hole doping (namely, K doping). With the increased carrier concentration y, the antiferromagnetic coupling is suppressed and becomes ferromagnetic for y reaching 25%. For K-underdoped samples (Ba0.875K0.125)(Zn1-xMnx)2As2, it is antiferromagnetic coupling, and the TN increases with increased x as depicted in Figure 5c.
Conclusion
The new type of diluted magnetic semiconductors (Ba0.7K0.3)(Zn1-xMnx)2As2 with a record high TC are synthesized via the proper high-pressure annealing. With Mn content of 24%, we obtain the highest reliable TC of 260 K among diluted magnetic semiconductors with independent spin and carrier doping, showing promising application potential. Moreover, first-principles calculations of magnetic interactions between nearest-neighbor Mn ions show that the antiferromagnetism can be suppressed by increasing K doping in the DMS (Ba,K)(Zn,Mn)2As2. Thus our results suggest a promising method to achieve room temperature or even higher Tc ferromagnetism for (Ba,K)(Zn,Mn)2As2 and analogs. It is noted that further K doping over 30% could not be accomplished at current synthesis conditions. Although Ba2+ and K+ have similar ionic sizes, Ba(Zn,Mn)2As2 and K(Zn,Mn)As crystalize into a tetragonal and a hexagonal structure, respectively.[37] Much higher synthesis pressures would be required to break the limitation of the (Ba,K) solution in (Ba,K)(Zn,Mn)2As2.
On the other hand, the DMS (Ba,K)(Zn,Mn)2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2 have the identical crystal structure. In addition, there is quite good lattice matching within the a-b plane (mismatch <5%). This unique feature provides distinct advantages in fabricating new functional devices based on the heterojunctions of various combinations of aforementioned diluted magnetic semiconductors, superconductors, and magnetic states.[3,17,19] The near room-temperature ferromagnetism means that (Ba,K)(Zn,Mn)2As2 shows promise for the development of new spintronic devices.
Experimental Section
Synthesis
High-purity raw materials, including Ba, K, Zn, Mn, and As, were mixed and ground in a glove box according to the nominal molar ratio. The mixture loaded in a crucible was sealed in a Nb tube and then a quartz tube, and calcined at 950 K for 12 h. The products were then reground, and annealed under 800 K and 5.5 GPa for 1 h with a high-pressure press. Such high-pressure annealing treatment can increase the chemical solution of Zn-Mn and retain volatile elements simultaneously. It is noted that the high-pressure annealing in this work is different from the high-pressure used in the previous report.[15] For the former, the samples are fabricated via high-pressure annealing at 800 K to overcome the limitation of chemical solution in the lattice of BZA. For the latter, the samples were synthesized under ambient pressure, and then loaded into high-pressure generators, diamond anvil cells, to study the pressure effect on ferromagnetic interaction and TC of BZA within temperature range of 2–300 K.
General Properties Characterization
Powder X-ray diffraction (XRD) was carried out for the component analysis of resulting products, using Cu-Kα radiation with a Philips X'pert diffractometer at room temperature. Energy dispersive X-ray analysis (EDX) of a commercial scanning electron microscope (SEM) was used for the confirmation of chemical compositions of polycrystal specimens. The real doping ratio is consistent with nominal composition. For the sake of simplification, the nominal formulas are used in the following text. Characterization of dc magnetic susceptibility for all the samples was accomplished by superconducting quantum interference device (SQUID). Electricity transport measurements were conducted via a physical property measurement system (PPMS).
First-Principles Calculations
First-principles simulations were performed with the projector augmented wave (PAW) method using the Vienna ab Initio simulation package (VASP).[38,39] The choice of the electron exchange-correlation functional was generalized gradient approximation (GGA) with the form of Perdew-Burke-Ernzerhof (PBE) realization.[40] The GGA plus on-site Coulomb repulsion U (GGA + U) approach by Dudarev et al. was used to describe strongly localized Mn 3d orbitals.[41] The effective on-site Coulomb interaction parameter (U = 4 eV) was applied to Mn 3d orbitals. Lattice constants and atomic positions were fully relaxed until the maximum force acting on all atoms was less than 1 × 10−3 eV and the total energy was converged to 1 × 10−7 eV with the Gaussian smearing method. The kinetic energy cutoff of 450 eV was employed. The Brillouin zone was sampled with a 4 × 4 × 8 Monkhorst–Pack grid for calculations of magnetic exchange couplings with 2 × 2 × 1 supercells of (Ba1-yKy)(Zn1-xMnx)2As2.[42] All doping configurations with corresponding doping probabilities are generated by disorder code.[43,44]
Acknowledgements
The work was supported by the National Key R&D Program of China (No. 2022YFA1403900), and the CAS Project for Young Scientists in Basic Research (No. YSBR-030). Z.D. acknowledges support of the Youth Innovation Promotion Association of CAS (No. 2020007). B.G. is supported by the National Natural Science Foundation of China (No. 12074378), Chinese Academy of Sciences (No. JZHKYPT-2021-08 and No. XDB33000000). Y.P. and X.L. contributed equally to this work.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Abstract
Achieving room‐temperature ferromagnetism is one of the major challenges for diluted magnetic semiconductors (DMS). (Ba,K)(Zn,Mn)2As2 (BZA) belongs to a new type of DMS materials that feature independent spin and carrier doping. In previous studies, BZA shows a reliable Curie temperature (TC) of 230 K, a record among these types of materials. In this work, progress in further experimentally enhancing TC of BZA to 260 K is reported by increasing Mn concentration with parallel K doping, as supported by complementary first‐principles calculations. A sufficient carrier concentration can suppress the short‐range antiferromagnetic interaction of the nearest Mn─Mn pair, which suppresses ferromagnetism in DMS materials. Consequently, a higher TC has been obtained in BZA with improved Mn‐ and K‐doping levels by using high‐pressure synthesis that effectively eliminates structural distortion and overcomes the limitation of chemical solution in BZA. The work demonstrates an effective strategy to enhance TC in DMS systems.
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Details
; Uemura, Yasutomo J. 3 ; Jin, Changqing 1
1 Institute of Physics, Chinese Academy of Sciences, Beijing, China, School of Physics, University of Chinese Academy of Sciences, Beijing, China
2 Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, Beijing, China
3 Department of Physics, Columbia University, New York, NY, USA





