Ba(Zn1−2xMnxCux)2As2: A Bulk Form Diluted Ferromagnetic Semiconductor with Mn and Cu Codoping at Zn Sites

We report the synthesis and characterization of a bulk form diluted magnetic semiconductor Ba(Zn1−2xMnxCux)2As2 with the crystal structure identical to that of “122” family iron based superconductors and the antiferromagnet BaMn2As2. No ferromagnetic order occurs with (Zn, Mn) or (Zn, Cu) substitution in the parent compound BaZn2As2. Only when Zn is substituted by both Mn and Cu simultaneously, can the system undergo a ferromagnetic transition below TC ~ 70 K, followed by a magnetic glassy transition at Tf  ~ 35 K. AC susceptibility measurements for Ba(Zn0.75Mn0.125Cu0.125)2As2 reveal that Tf strongly depends on the applied frequency with and a DC magnetic field dependence of , demonstrating that a spin glass transition takes place at Tf. As large as −53% negative magnetoresistance has been observed in Ba(Zn1−2xMnxCux)2As2, enabling its possible application in memory devices.

Scientific RepoRts | 5:15507 | DOi: 10.1038/srep15507 variants, i.e., antiferromagnets and superconductors with lattice matching within 5%, which provides the the possibility to make junctions with these materials thorough the As layer 19 . In addition, the bulk form specimens would enable the magnetic techniques to provide complementary information at a microscopic level, such as nuclear magnetic resonance (NMR) and neutron scattering 12 . Among them, the T C of (Ba, K)(Zn, Mn) 2 As 2 single crystal has been reported to reach 230 K 24 . (Ba, K)(Zn, Mn) 2 As 2 was synthesized by doping Mn and K into the parent compound β-BaZn 2 As 2 which is a direct gap (0.2 eV) semiconductor 25 , where the substitution of Mn for Zn and K for Ba introduces spins and hole carriers, respectively.
In this paper, we report the successful fabrication of a new DMS material with a rather new synthesize route, which is different to the previously reported ~10 DMSs [8][9][10][11][12][13][14][15][16][17][18][19] . Instead of doping at different sites, we co-doped both Mn and Cu into the same Zn sites of BaZn 2 As 2 to introduce local moments and carriers, respectively. A new series of DMS compounds Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (0.025 ≤ x ≤ 0.2) have been successfully fabricated. While the system remains semiconducting, 20% Mn and Cu doping results in a ferromagnetic transition below T C ~ 70 K, followed by a magnetic glassy transition below T f ~ 35 K. AC susceptibility measurements on an x = 0.125 sample indicate that T f strongly depends on the applied frequencies and magnetic fields, which confirms the spin glass nature at T f . In addition, as large as ~− 53% negative magnetoresistance (MR) at a magnetic field H = 50 KOe has been achieved in Ba(Zn 0.75 Mn 0.125 Cu 0.125 ) 2 As 2 , which is attributed to the suppression of spin fluctuations by magnetic field. Future work is needed to gain deeper understanding of the magnetic behavior of this system and achieve higher T C values.

Results and Discussion
Synthesis and structural characterization. The polycrystalline specimens of Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (x = 0.025, 0.075, 0.125, 0.200) were synthesized by the solid state reaction method. Details of the synthesis and facilities used for characterization are described in the Methods section. In Fig. 1, we show the X-ray diffraction patterns for polycrystalline Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (0.025 ≤ x ≤ 0.200). The Rietveld refinement for Ba(Zn 0.85 Mn 0.075 Cu 0.075 ) 2 As 2 with parameters R WP = 10.52 %, R P = 7.58 %, χ 2 = 1.348 shows that the Bragg peaks can be well indexed into the tetragonal structure with space group I4/mmm. The lattice parameter a increases and c decreases monotonically with the doping concentration x, indicating the successful doping of Mn and Cu into the lattice. We show the crystal structure in Fig. 1(c), which is isostructural to the parent compound of 122-type Fe-based superconductor Ba(Fe 1−x Co x ) 2 As 2 26 with T c = 22 K and antiferromagnet BaMn 2 As 2 with Néel temperature T N = 625 K 27 . This feature provides the possibility to make junctions with these systems though As layer. No peaks of impurities are detected for the doping levels of x = 0.025 and x = 0.075. α-BaZn 2 As 2 with space group of Pnma appears for x = 0.125 and becomes markable for x = 0.20, as marked by * in Fig. 1(a). Small traces of non-magnetic Ba 3 As 14 impurity are marked as #. Both α-BaZn 2 As 2 and Ba 3 As 14 are Pauli paramagnetic, which will not affect the magnetic behavior of Ba(Zn 1−2x Mn x Cu x ) 2 As 2 discussed in the following.
Resistivity. In Fig. 2, we show the temperature dependent resistivity of the parent compound BaZn 2 As 2 , Ba(Zn 0.9 Cu 0.1 ) 2 As 2 and Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (x = 0.025, 0.075, 0.125, 0.20). The resistivity of the parent semiconductor BaZn 2 As 2 displays a typical semiconducting behavior. With 10% Cu doping, the resistivity of Ba(Zn 0.9 Cu 0.1 ) 2 As 2 is heavily suppressed by an order of 4, indicating that carriers are doped. The semiconducting behavior for Mn and Cu codoped case has been conserved for x up to 20%, i.e., resistivity continuously increases with temperature decreasing from room temperature down to 4 K. The absolute value of resistivity at 4 K, however, drops from 10 3 Ω cm for x = 0.025 to 10 Ω cm for x = 0.20. We roughly fit the resistivity of Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (x = 0.025, 0.075, 0.125, 0.20) near room temperature in terms of a thermal activation function 13 . Similar approach has also been employed to (La, Sr)(Zn, Mn)AsO 13 . The fitting result for Ba(Zn 0.6 Mn 0.2 Cu 0.2 ) 2 As 2 is shown in Fig. 2(b) as an example. The values of energy gap E g are between 0.031 and 0.048 eV, which are about an order of magnitude smaller than that of the parent compound BaZn 2 As 2 25 . We have conducted Hall effect measurement on Ba(Zn 0.75 Mn 0.125 Cu 0.125 ) 2 As 2 , but the large resistivity prevents us to accurately determine the carrier density. A preliminary result shows that the carriers are p-type with the concentration in the order of p ~ 10 19 cm −3 . And the corresponding mobility is estimated to be in the order of 10 −1 cm 2 V −1 s −1 . This value of carrier density is not unusual in bulk form DMSs, which is comparable to that of (Ba 0.9 K 0.1 ) (Cd 2−x Mn x )As 2 16 and two orders of magnitude larger than that of Li(Zn, Mn)P 11 , but an order of magnitude smaller than that of (Ba, K)(Zn, Mn) 2 As 2 9 and Li(Zn, Mn)As 10 . Fig. 3(a,b), we show the temperature dependence of magnetization for Ba(Zn 0.9 Mn 0.1 ) 2 As 2 and Ba(Zn 0.9 Cu 0.1 ) 2 As 2 , respectively. No anomaly or transition is observed in the measured temperature range, and the moment at 2 K is only ~0.001 μ B /(Mn or Cu atom) for both Ba(Zn 0.9 Mn 0.1 ) 2 As 2 and Ba(Zn 0.9 Cu 0.1 ) 2 As 2 . We fit the magnetization data to a Curie-Weiss law M = M 0 + C/(T − θ) and obtained C = 0.00456 μ B K/Mn, θ = − 2.74 K for Ba(Zn 0.9 Mn 0.1 ) 2 As 2 , and C = 0.00028 μ B K/Cu, θ = − 1.45 K for Ba(Zn 0.9 Cu 0.1 ) 2 As 2 , indicating the paramagnetic ground state. These results indicate that doping either Mn or Cu alone into BaZn 2 As 2 can not form any type of magnetic ordering. This feature has also been observed in LaZnAsO, where doping Mn or Fe only does not result in ferromagnetic ordering 8,12 . The magnetic character of Cu in 122-type arsenides has been investigated by density functional calculations 28 and intensive transport properties measurements 29 . Cu 3d bands are ~3 eV below Fermi energy (E F ), and contribute little to the density of states at E F

28
. The 3d shell of Cu is completely filled with 3d 10 electronic configurations 28,29 . Therefore, the valence of Cu in 122-type arsenides is + 1 with nonmagnetic state S = 0 28,29 . The paramagnetic state of Ba(Zn 0.9 Cu 0.1 ) 2 As 2 are consistent with the previous reports 28, 29 .
In  levels. With 20% doping, T C increases to 70 K and T f increases to 35 K. In Fig. 3(d), we present the results of isothermal magnetization measurements. For x ≥ 0.075, clear hysteresis loops have been observed at 2 K. The coercive field becomes lager for higher x, and reaches 1600 Oe for x = 0.20. We should note that this value is much smaller than ~10 4 Oe of (Ba, K)(Zn, Mn) 2 As 2 9 . The contrasting ground states shown in Fig. 3(a,b,c) unequivocally demonstrate that only when Zn is substituted by both Mn and Cu simultaneously, can the ferromagnetic ordering develop, which also indicates that the ferromagnetic signals result from the doping of Mn and Cu rather than impurities.
We fit the T-dependent magnetization above T C to the Curie-Weiss formula χ = χ 0 + C/(T − θ) in order to obtain the Weiss temperature (θ) and effective paramagnetic moment of Mn (μ eff ). The best fittings show that the effective moment μ eff is 4.8 ~ 5.7 μ B /Mn for 0.025 ≤ x ≤ 0.20, indicating the high spin state of Mn with the valence of + 2 in the system of Ba(Zn 1−2x Mn x Cu x ) 2 As 2 . We tabulate the Curie temperature T C , the spin freezing temperature T f (the temperature where ZFC and FC curves split), the base temperature moment μ BT (the values at 2 K measured from FC curves with H = 100 Oe), the coercive field H c and the energy gap E g (fitted from the resistivity data) in Table 1. T C , T f , θ and H c show a trend of increasing with higher doping level x, indicating the strengthening of ferromagnetic exchange interaction with higher concentration of Mn and Cu. Meanwhile, the systematic changes of these magnetic parameters suggest that the magnetic signals in this system are not caused by impurities. On the other hand, we notice that μ BT first increases from 0.027 μ B /Mn for x = 0.075 to 0.110 μ B /Mn for x = 0.125, but decreases to 0.079 μ B /Mn for x = 0.20. This may be due to the competition of ferromagnetic and antiferromagnetic exchange interactions between Mn atoms.
To further investigate the valence of Cu and Mn, we conducted the X-ray photoelectron spectroscopy (XPS) measurements for Ba(ZnMn 0.2 Cu 0.2 ) 2 As 2 . Ba and Zn have been observed from the peaks of binding energy. But unfortunately, after very careful comparison, we haven't detected effective peaks of Cu or Mn from the binding energy. No conclusion about the valence of Cu or Mn has been achieved from the XPS measurements. We can't obtain evidence from XPS that whether Cu contribute magnetic moments . We tentatively attribute the negative magnetoresistance to the suppression of spin fluctuations by applied field. AC susceptibility. We measured the AC susceptibility, χ′, for the x = 0.125 sample at various frequencies ν under zero external field, and show the results in Fig. 5. We found that the maxima of the real part, χ′, drop obviously, and T f shifts slightly to higher temperature with the increasing AC frequencies. This feature is typically taken as signs for spin glass systems [30][31][32][33][34][35][36][37] . This kind of behavior has also been observed in CaNi 1−x Mn x Ge 30 , CeCu 4 Mn 31 , La(Fe 1−x Mn x ) 1.4 Si 1. 6 32 and II-VI family DMS 23 . The Vogel-Fulcher law [38][39][40][41] is usually used to describe the dependence between T f and ν, where E a is the activation energy, T 0 is the Vogel-Fulcher temperature, and ν 0 is the fitted frequency. We tried different values of ν 0 from 10 10 Hz to 10 13 Hz, which showed that the best linear fitting can be obtained when ν~10 0 13 Hz, in good agreement with expectation for a spin-glass ν (~1 0 0 13 Hz) rather than a cluster-glass ν ( − 10 10 0 7 9 Hz) 42 . So ν 0 is considered as a constant value of 10 13 Hz for this system in the following discussion. In Fig. 5(b), T f is plotted as a function of 1/ln(ν 0 /ν). The well fitted is usually used to distinguish the frequency sensitivity of T f in a spin glass 40,43 . K is the order of 0.01 for spin glass systems, while K > 0.1 for superparamagnets 36 . For the x = 0.125 sample, K is estimated to be ~0.008 ± 0.002, in good agreement with the typical values reported for spin glasses 30,31,37,40 .
The dynamical slowing down of spin fluctuations can also be expressed by the standard power dependence, where τ = 1/ν is the relaxation time, τ 0 = 1/ν 0 is set as 10 −13 s, T G is the spin freezing temperature, η is the dynamic exponent. When T f approaches T G which is the zero frequency limit, the order of τ gets much larger than τ 0 , indicating that spin fluctuations significantly slow down. A linear fit of ln(ν 0 /ν) versus ln(T f /T G − 1) according to Eq. (2) is shown in Fig. 5(c), yielding T G ~ 18.96 K and η . 8 3. The value of η falls into the range of 4-12 for spin glasses [31][32][33]36,40,44,45 , which is not cluster-glass like character 46 . η . 8 3 is close to 7.9, the calculated value for the three-dimensional Ising spin-glass 47,48 . In Fig. 6(a), we show the measurements of T-dependent AC susceptibility at a fixed frequency of 500 Hz with various DC fields for Ba(Zn 0.75 Mn 0.125 Cu 0.125 ) 2 As 2 . The AC susceptibility is strongly affected by the external DC fields, i.e., the cusps smear out, the peak value of χ′ decreases remarkably, and T f shifts to lower temperature with increasing DC fields. These are all characteristic features of spin glasses 30,32,49 . The DC field dependence of the spin freezing temperature T f can be described by the equation,  Fig. 6(b). δ is ~2/3 for Ising spin glass systems, and δ = 2 for Heisenberg systems 50,51 . In the current case, δ is close to 2/3, indicating that the glassy state for Ba(Zn 1−2x Mn x Cu x ) 2 As 2 may be explained by mean-field theory with Ising model. In Fig. 6(c), we show the imaginary component of AC susceptibility at 500 Hz with DC fields up to 3000 Oe. Similar to the case of χ′, T f decreases noticeably with increasing fields. The

Conclusion
A bulk form diluted magnetic semiconductor Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (0.025 ≤ x ≤ 0.2) with maximum T C ~ 70 K has been successfully synthesized. It is the first time that ferromagnetic ordering is observed when Mn and Cu are codoped into the Zn sites, where Mn substitution for Zn introduces spin and Cu substitution for Zn introduces carriers, respectively. The new system displays large negative magnetoresistance while conserving the semiconducting behavior with the doping level up to 20%. The AC susceptibility measurements show that the spin freezing temperature T f is dependent on frequency and external field, confirming the glassy nature below 35 K. Finally, the new DMS system has a tetragonal crystal structure identical to that of "122" family of Fe-based superconductors and the antiferromagnetic system BaMn 2 As 2 , which makes it possible to make various junctions of these systems through As layer. More theoretical and experimental work are expected to further understand the properties and physics of this system.

Methods
The polycrystalline specimens of Ba(Zn 1−2x Mn x Cu x ) 2 As 2 (x = 0.025, 0.075, 0.125, 0.200) were synthesized by the solid state reaction method. High purity elements of Zn (99.9%), Mn (99.99%), Cu (99.9%) and As (99%) were mixed, ground and pressed into pellets. The pellets were sealed in evacuated silica tubes and sintered at 800 °C for 10 hours to make the precursors (Zn 1−2x Mn x Cu x )As. The mixture of Ba (99.2%) and (Zn 1−2x Mn x Cu x )As were then slowly heated to 900 °C and held for 10 hours, then 1150 °C for 15 hours before cooling down to room temperature with the furnace turned off. The handling of materials were performed in a high-purity argon filled glove box (the percentage of O 2 and H 2 O ≤ 0.1 ppm) to protect it from exposure to air. Powder x-ray diffraction was performed at room temperature using a PANalytical x-ray diffractometer (Model EMPYREAN) with monochromatic CuK α1 radiation. The electrical resistance was measured on sintered pellets with the typical four-probe method. The DC magnetization measurements were conducted on a Quantum Design Magnetic Property Measurement System (MPMS-5). The AC susceptibility and magnetoresistance were measured on a Quantum Design Physical Property Measurement System (PPMS-9).