Xueqin Zhao, Jinou Dong, Licheng Fu, Yilun Gu, Rufei Zhang, Qiaolin Yang, Lingfeng Xie,Yinsong Tang, and Fanlong Ning,2,3,?

1Zhejiang Province Key Laboratory of Quantum Technology and Device and Department of Physics, Zhejiang University,Hangzhou 310027, China

2Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

3State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China

Abstract: We report the successful synthesis and characterization of a novel 1111-type magnetic semiconductor (Ba1-xNax)-F(Zn1-xMnx)Sb (0.05 ≤ x ≤ 0.175) with tetragonal ZrSiCuAs-type structure, which is isostructural to the layered iron-based superconductor La(O,F)FeAs. Na substitutions for Ba and Mn substitutions for Zn introduce carriers and local magnetic moments, respectively. Ferromagnetic interaction is formed when Na and Mn are codoped, demonstrating that local magnetic moments are mediated by carriers. Iso-thermal magnetization shows that the coercive field is as large as ～ 12 000 Oe, which is also reflected in the large split between the temperature-dependent magnetization in zero-field-cooling and field-cooling condition. AC susceptibility under zero field demonstrates that samples evolve into spin-glass state below spin freezing temperature Tf. The measurements of temperature-dependent resistivity indicate that (Ba1-xNax)F(Zn1-xMnx)Sb exhibits semiconducting behaviour.

Key words: magnetic semiconductors; ferromagnetic interaction; carriers; spin-glass

1. Introduction

Due to the potential applications in spintronics devices,diluted magnetic semiconductors (DMSs) that simultaneously combine the spin and charge degrees of electrons have received extensive attentions[1-6]. In 1990s, the III-V DMS (Ga,Mn)As was successfully prepared by low temperature molecular beam epitaxy (LT-MBE) method. The Curie temperatureTChas reached as high as ～200 K[7-9], which is much higher than those of some II-VI DMSs, such as (Zn,Mn)Te[10]and (Cd,Mn)Te[11]. To explain the magnetic mechanism of III-V DMSs, Dietlet al. proposed a mean-field Zener's model that described the ferromagnetic interactions between the local magnetic moments and carriers, and predicted thatTCof some DMSs may reach room temperature if more Mn atoms are doped[12,13]. Subsequently, Reedet al. reported thatTCof(Ga,Mn)N was up to ～370 K[14]. The application of DMSs in spintronic devices will be practical once the Curie temperature of some DMSs are above room temperature. However, the development of III-V DMSs has also encountered some limitations.For example, in (Ga,Mn)As, magnetic atoms Mn easily enter the interstitial positions of the lattice, which makes it difficult to accurately determine the actual amount of doped Mn atoms. Meanwhile, the substitutions of Mn for Ga introduce magnetic moments and carriers simultaneously, which constrains us to study their individual contribution to the formation of the ferromagnetic ordering. In addition, films cannot be used for microscopic probes that are based on bulk materials, such as nuclear magnetic resonance (NMR), neutron scattering and muon spin relaxation (μSR)[5,15-17]. On that account, the study of bulk DMSs with decoupled charge and spin doping are worthy of attentions.

Fig. 1. (Color online) (a) The polycrystal powder X-ray diffraction patterns of (Ba1-xNax)F(Zn1-xMnx)Sb (x = 0.00, 0.05, 0.075, 0.10, 0.125, 0.15, and 0.175). Traces of impurities ZnSb are marked as stars (*). (b) The Rietveld refinement result of (Ba0.875Na0.125)F(Zn0.875Mn0.125)Sb. Inset shows the tetragonal ZrCuSiAs-type crystal structure of parent compound BaFZnSb. (c) Lattice parameters a and c versus doping level x of (Ba1-xNax)F(Zn1-x-Mnx)Sb (x = 0.00, 0.05, 0.075, 0.10, 0.125, 0.15, and 0.175). (d) The unit cell volume of (Ba1-xNax)F(Zn1-xMnx)Sb (x = 0.00, 0.05, 0.075, 0.10, 0.125,0.15, and 0.175).

In this paper, we report the successful fabrication of a new 1111-type magnetic semiconductor, (Ba1-xNax)F(Zn1-x-Mnx)Sb (x= 0.05, 0.075, 0.1, 0.125, 0.15, and 0.175), via doping Na into Ba sites and Mn into Zn sites in the parent compound BaFZnSb to introduce hole carriers and local magnetic moments, respectively. BaFZnSb shares the same crystal structure with that of LaOZnAs and LaOCuS, which have been reported to be the parent semiconductors of 1111-type DMSs (La,Ba)O(Zn,Mn)As (TC～ 40 K)[17]and (La,Sr)O(Cu,Mn)S(TC～ 200K)[23],respectively. Comparingwith oxides, the ionicradiusoffluoride issmallerand itselectronegativity is stronger, which results in constituting stronger ionic bonds[24]. The Weiss temperatureθof (Ba1-xNax)F(Zn1-xMnx)Sb is up to ～ 16 K forx= 0.175, which is a sign of ferromagnetic interaction, and followed by a magnetic glassy transition belowTf～ 14 K.

2. Experiments

We synthesized the polycrystalline specimens of (Ba1-x-Nax)F(Zn1-xMnx)Sb through the solid-state reaction method.High-purity starting materials Ba (99.2%, Alfa Aesar), BaF2(99.99%, Aladdin), Na (99.7%, Aladdin), Zn (99.9%, Alfa Aesar),Mn (99.95%, Alfa Aesar) and Sb (99.999%, Prmat) were mixed according to stoichiometric proportions and were placed in alumina crucibles before sealing in evacuated silica tubes.The mixture was heated slowly to 200 °C and held for 10 h,then heated to 750 °C and held for another 10 h, followed by furnace cooling to room temperature. After that, the intermediate products were grounded, pressed into pellets, placed in alumina crucibles, sealed in evacuated silica tubes again and then reheated to 750 °C for 30 h. To protect the contamination from air and H2O, all operations except the sealing of silica tubes were executed in a glove box filled with high-purity Ar (O2and H2O ＜ 0.1 ppm).

The crystal structures of the samples were characterized at room temperature by a powder X-ray diffractometer (Model EMPYREAN) using monochromatic Cu-Kα1radiation withλ(Kα1) = 1.540598 ?. The detailed information of lattice constants and unit cell volume was calculated by Rietveld refinement method using the GSAS-II software package[25]. The DC magnetization measurements were performed on a quantum design magnetic property measurement system (MPMS). The AC susceptibility measurements were measured on a quantum design physical property measurement system(PPMS). The electrical resistivity measurements were conducted on sintered pellets by the four-probe method.

3. Results and discussion

In Fig. 1(a), we show the powder X-ray diffraction patterns of polycrystals (Ba1-xNax)F(Zn1-xMnx)Sb withx= 0.00,0.05, 0.075, 0.1, 0.125, 0.15, and 0.175, respectively. The Bragg peaks can be well indexed by a ZrSiCuAs-type tetragonal structure with space group P4/nmm[26], which is isostructural to the 1111-type superconductor La(F,O)FeAs[27], indicating that Na substitution for Ba and Mn substitution for Zn have no influence on the tetragonal crystal structure. In Fig. 1(b),we show the Rietveld refinement result of (Ba0.875Na0.125)F-(Zn0.875Mn0.125)Sb. The resultant weighted reliable factorRwpis 7.962%, indicating the samples are in good quality. With doping levelxincreasing, some small peaks of ZnSb impurities marked as * in Fig. 1(a) were observed. ZnSb impurities are non-magnetic, thus they will not affect the discussion of magnetism in the following.

Fig. 2. (Color online) (a) The temperature dependence of DC magnetization for parent phase BaFZnSb and (Ba0.95Na0.05)FZnSb under field-cooling mode in an external magnetic field of 100 Oe. The data (open circles) are the data dots, and the solid lines are the Curie-Weiss fitting results.(b) The temperature dependent magnetization (M) for (Ba1-xNax)F(Zn1-xMnx)Sb (x = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.175) in both zero-field-cooling (ZFC) and field-cooling (FC) procedures under an external magnetic field of 100 Oe. Inset shows the enlarged M(T) curves for all specimens at low temperature. Arrow marks Tirr for x = 0.10. (c) The plot of 1/( X-X0) versus T for (Ba1-xNax)F(Zn1-xMnx)Sb (x = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.175) under FC condition. Arrows mark the Weiss temperatures. Inset shows the enlarged plot of 1/( X-X0) versus T for all of specimens below 30 K. (d) Iso-thermal magnetization for (Ba1-xNax)F(Zn1-xMnx)Sb (x = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.175) at 2 K. Inset shows the enlarged M(H)curves for all of specimens under an external magnetic field Be xt from -20 000 to 20 000 Oe.

As shown in Fig. 1(b), the parent compound BaFZnSb has a layered crystal structure, which consists of two layers in the ab plane: one is [BaF]+layers (BaF4tetrahedra) and the other is [ZnSb]-layers (ZnSb4tetrahedra)[28]. These layers are conventionally regarded as the tetragonal fluorite and anti-fluorite structure types separately[28], and they are stacked alternately along thec-axis. The lattice parameters obtained from Rietveld refinement are shown in Fig. 1(c). The lattice parameters of parent compound BaFZnSb area= 4.43605 ? andc=9.81786 ?, which is close to previous reported values ofa=4.4384 ? andc= 9.7789 ?[28]. Meanwhile, with doping levelxincreasing,amonotonically increases from 4.43720 to 4.44271 ?, andccontinuously decreases from 9.80767 to 9.78259 ?, repectively. This should be ascribed to the fact that the ionic radius of Na+(1.02 ?) is smaller than that of Ba2+(1.35 ?), but the ionic radius of Mn2+(0.83 ?) is larger than that of Zn2+(0.74 ?). The monotonical behaviours of lattice parametersaandcwith doping levelxdemonstrate the successful doping of Na into Ba sites and Mn into the Zn sites, respectively. In Fig. 1(d), we find that the unit cell volume doesn't vary monotonically with increasing doping levelx. This is due to the fact that lattice parametersaandcbehave in opposite direction.

Table 1. The Weiss temperature θ, the base temperature magnetic moment Mb ase, the effective magnetic moment μeff and the coercive fieldHC for (Ba1-xNax)F(Zn1-xMnx)Sb for x = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.175.

Fig. 3. (Color online) The (a) real part X' and (b) imaginary part X'' of AC susceptibility with varying frequencies f under zero field for(Ba0.9Na0.1)F(Zn0.9Mn0.1)Sb. (c) A frequency dependence of spin freezing temperature Tf for (Ba0.9Na0.1)F(Zn0.9Mn0.1)Sb.

Fig. 4. (Color online) Temperature-dependent resistivity measurements for (Ba1-xNax)F(Zn1-yMny)Sb (x = 0.00, y = 0.00; x = 0.05, y =0.00; x = 0.05, y = 0.05; x = 0.075, y = 0.075; x = 0.10, y = 0.10; x = 0.125,y = 0.125; x = 0.15, y = 0.15; x = 0.175, y = 0.175).

Electrical transport measurement is another important characterization method of magnetic semiconductors. In Fig. 4, we perform temperature-dependent resistivityρfor(Ba1-xNax)F(Zn1-yMny)Sb (x= 0.00,y= 0.00;x= 0.05,y= 0.00;x= 0.05,y= 0.05;x= 0.075,y= 0.075;x= 0.10,y= 0.10;x=0.125,y= 0.125;x= 0.15,y= 0.15;x= 0.175,y= 0.175). Both of the parent compound BaFZnSb and (Ba0.95Na0.05)FZnSb where only carriers are introduced show metallic behaviour.The semiconducting behaviour is displayed for all samples with substitutions of (Ba,Na) and (Zn,Mn) simultaneously. Remarkably, the resistivity increases monotonically with the doping levelxincreasing (except forx= 0.175), similar to that in(La,Ca)(Zn,Mn)SbO[41]. The main cause for that is that carriers are scattered by magnetic fluctuations caused by doped Mn atoms. Similar phenomenon has also been reported in(Ga,Mn)As[42].

4. Summary

Acknowledgements

The work was supported by the Key R&D Program of Zhejiang Province, China (2021C01002) and NSF of China (No.12074333).