Li ( Zn , Co , Mn ) As : A bulk form diluted magnetic semiconductor with Co and Mn co-doping at Zn sites

We report the synthesis and characterization of a series of bulk forms of diluted magnetic semiconductors Li(Zn1-x-yCoxMny)As with a crystal structure close to that of III-V diluted magnetic semiconductor (Ga,Mn)As. No ferromagnetic order occurs with single (Zn,Co) or (Zn, Mn) substitution in the parent compound LiZnAs. Only with co-doped Co and Mn ferromagnetic ordering can occur at the Curie temperature ∼40 K. The maximum saturation moment of the this system reached to 2.17μB/Mn, which is comparable to that of Li (Zn,Mn)As. It is the first time that a diluted magnetic semiconductor with co-doping Co and Mn into Zn sites is achieved in “111” LiZnAs system, which could be utilized to investigate the basic science of ferromagnetism in diluted magnetic semiconductors. In addition, ferromagnetic Li(Zn,Co,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs share square lattice at As layers, which may enable the development of novel heterojunction devices in the future.


INTRODUCTION
2][3][4][5][6] It is now widely accepted that the ferromagnetism found in (Ga,Mn)As system arises from the hole mediated interaction between the local magnetic moments of the Mn, and is homogeneous. 4,7For practical applications, exploring diluted magnetic semiconductors (DMSs) with Curie temperature (T C ) at room temperature is a necessity.It is predicted that T C above room temperature would be achieved with higher concentration of carriers and spins in materials. 4The highest T C of (Ga,Mn)As up to ∼200 K with ∼12% of Mn doping has been reported. 8,9However, the (Ga,Mn)As system still faces several serious challenges.For example, substitution of divalent Mn 2+ into trivalent Ga 3+ provides both holes carriers and magnetic atoms.The hetero-valent substitution, which leads to low solubility of Mn, and the lack of independent control of charge and spin, inevitably brings several limitations to the synthesis methods, and physical properties. 10,11Therefore, the origin of ferromagnetism in (Ga,Mn)As has not been fully understood.To overcome these difficulties, seeking independent of spin and charge injections on bulk DMSs become one of the major missions for these materials.
LiZnAs is a direct-gap semiconductor which can be synthesized by the solid-state reaction. 12,13he crystal structures and band structures of LiZnAs are very similar to those of GaAs, 13,14 which indicates that LiZnAs can be an excellent candidate of the host material of DMSs.The DMS Li(Zn,Mn)As was successfully synthesized by Deng et al. 11 with a ferromagnetic transition temperature up to 50 K.
In this system, the individual control of the magnetic atoms via (Zn 2+ , Mn 2+ ) substitutions and carriers via off-stoichiometry of Li concentrations are successful achieved.Hence, Li(Mn,Zn)As is not similar to (Ga,Mn)As but rather to (Zn,Mn)Te, as Mn does not introduce holes and, in order to induce ferromagnetism, acceptor co-doping is necessary, by e.g., Zn-substitutional Li, as in the case of (Zn,Mn)Te. 15,16In addition, the same chemical valence (2+) of Mn and the host Zn allows for a rather high chemical solubility of Mn, which makes it possible to obtain bulk forms of specimens.8][19][20][21][22][23][24][25][26][27] The availability of bulk forms of specimens make it possible to investigate the microscopic magnetism of DMSs based on typical bulk probes, 28 such as nuclear magnetic resonance (NMR), 29 muon spin relaxation (µSR), 17,18,20,30 neutron scattering, absorption spectroscopy (XAS), 31 resonance photoemission spectroscopy (RPES), 31 and angle-resolved photoemission spectroscopy (ARPES). 32It has been confirmed by the XAS measurements that the valence of Mn in (Ba,K)(Zn,Mn) 2 As 2 is bivalence.The NMR and µSR measurements confirmed that the ferromagnetism in these bulk DMSs is homogeneous. 11,18,20,29n this article, we report the successful synthesis and characterization of a new DMS with co-doped both Co and Mn into the same Zn sites of LiZnAs.With Co and Mn simultaneously substitution for Zn sites, the system exhibits ferromagnetic order up to 40 K.The influence of Co and Mn concentration on the ferromagnetic ordering state is discussed in detail in this work.The coercive field of this system is less than 100 Oe.Hall effect measurements indicate that the carriers are p-type with the density of ∼10 19 /cm 3 with 10% of Co content.Anomalous Hall Effect (AHE) was observed under T C .Moreover, square lattice of As layers in Li(Zn,Mn)As are commonly included in its variants, i.e., antiferromagnets LiMnAs and superconductors LiFeAs with lattice matching within 10%, 11 which could provide the possibility to make heterojunction devices with these materials through the As layers.The present work could have important consequences in basic science of DMSs and in applications in the future.

EXPERIMENT
Polycrystalline specimens of Li(Zn,Co,Mn)As were synthesized using solid-state reaction.The starting materials, precursors of Li 3 As and ZnAs, and high purity of Co and Mn powders, were mixed according to the nominal composition of Li(Zn 1-x -y Co x Mn y )As.The mixtures were pressed into pellets and sealed inside evacuated Ta tubes to prevent the evaporation of Li.Then, the tantalum tubes were sealed inside evacuated quartz tubes.The tubes were heated to 700 • C and held for two days before the furnace slowly cooled to room temperature.The crystal structure and phase purity of the resulting powers were examined by power X-ray diffraction (XRD; Philips X'pert diffractometer) using Cu − K α radiation.Lattice parameters were determined via Rietveld analysis using the GSAS software package. 33The DC magnetic susceptibility measurements were performed with a superconducting quantum interference device (SQUID-VSM; Quantum Design).The electric transport was measured by a four-probe technique using silver paste electrodes on a Quantum Design PPMS.

Structural characterization
The crystal structures of co-doped Li(Zn,Co,Mn)As were characterized by XRD.The subsequent XRD measurements indicate that the doping does not change the cubic structure of LiZnAs when the doping concentration is below 15%.For doping above 15%, some impure phases from CoAs and LiMnAs appeared.The X-ray diffraction patterns of Li(Zn 1-2x Co x Mn x )As for x = 0, 0.05, 0.075, 0.1, respectively, are shown in Figure 1c.All the Bragg peaks can be indexed into the same structure of the host material LiZnAs, which has a cubic structure with the space group of F-43m (No.216).This structure is similar to that of zinc-blend GaAs, as shown in Figure 1a and Figure 1b, where Ga 3+ is replaced by (Li + and Zn 2+ ). Figure 1d shows the lattice parameters of all polycrystalline samples, which were calculated from the X-ray diffraction data.The lattice parameters of LiZnAs were calculated to be a = 5.930 Å, which is consistent with the previously reported values of

Magnetic properties
The influence of Co and Mn doping on the ferromagnetism was investigated individually.Figure 2a shows the temperature dependence of the magnetization M(T ) in zero-field-cooling (ZFC) and field-cooling (FC) procedures under H = 1000 Oe for Li(Zn 0.9 Mn 0.1 )As.No ferromagnetic order is observed down to 2 K with only Mn doping.We fit the magnetization data to a Curie-Weiss law by the formula: where χ 0 is a temperature-independent paramagnetic term, 34 C is the Curie constant, and θ is the Weiss temperature.C = 0.1198 K emu/mol, θ = 3.63 K are obtained from formula (1), which indicates that Li(Zn 0.9 Mn 0.1 )As is paramagnetic from room temperature down to 2 K. Inset of Figure 2a shows the M(T ) in zero-field-cooling (ZFC) and field-cooling (FC) procedures under H = 500 Oe for Li(Zn 0.95 Co 0.05 )As, which still remains diamagnetism.It suggests that Co doping acts only as charge injection, consistent with resistivity measurements which will be discussed in next section.These results indicate that there is no ferromagnetic ordering when only doping either Co or Mn into Zn sites of LiZnAs.This feature is also observed in other DMSs systems. 25,27,35Only co-doping with Co and Mn into the Zn sites, can the system obtain the ferromagnetic ordering, as discussing below.
The effect of Co and Mn doping on the ferromagnetism was studied by changing concentration of one dopant while fixing another one.Inset the top left of Figure 2b shows the DC magnetic measurements of Li(Zn 1-0.05-yCo 0.05 Mn y )As under field H = 500 Oe with y =0.05 and 0.15, respectively.No obvious difference has been observed between field cooling (FC) and zero-field cooling (ZFC) procedures, where the clear signatures of ferromagnetic order are observed.These results indicate that ferromagnetic order in this system is introduced by the carriers mediated interaction between the local magnetic moments of the Mn.Above T C , the sample is paramagnetic.The magnetic susceptibility data χ(T ) which can be fit by the Curie-Weiss formula of equation ( 1), as shown inset the bottom right of Figure 2b.The field dependence of the magnetization of Li(Zn 1-0.05-yCo 0.05 Mn y )As (y = 0.05, 0.15) at T = 2 K is shown in Figure 2b.Small coercive fields (less than 100 Oe) are observed in this system, which could be promised for spin manipulation in the future.The saturation moment (M sat ) is 2.17 µ B /Mn for y = 0.05, which is comparable to that of Li 1.1 (Zn 0.95 Mn 0.05 )As. 14With the increase of Mn doping, T C increases from 20 K to 30 K, while M sat decreases from 2.17 µ B to 0.68 µ B , which probably reflect the competition between the short-range antiferromagnetic superexchange of nearest-neighbor (NN) Mn moments and a longer-range ferromagnetic interaction of distant Mn moments regulated by carriers, like the RKKY-like interaction. 4,36The direct antiferromagnetic coupling between the Mn-Mn pairs causes antiferromagnetic order in LiMnAs with T N = 373.8K. 37 Similar results were found in other previously reported DMSs. 11,17,18,20,25,35e fixed Mn content at the level of 15%, and enhanced the doping levels of Co from 5% to 15%. Figure 2c shows the measurements of magnetic susceptibility for Li(Zn 1-x -0.15 Co x Mn 0.15 )As (x = 0.05, 0.075, 0.1, 0.15) under field H = 500 Oe.The optical T C = 40 K is achieved with the Co concentrate of 5%.The hysteresis measurements of the corresponding specimens are shown in Figure 2d.Both the Curie temperature T C and M sat are monotonically suppressed by the increasing Co concentration x.T C decreases from 40 K to 18 K.Similarly, The M sat decreases from 0.68 µ B /Mn for x = 0.05 to 0.19 µ B /Mn for x = 0.15.

Resistivity
Results of resistivity studies are discussed below.Figure 3a shows the temperature dependence of electrical resistivity ρ(T ) for Li(Zn 1-x Co x )As with x = 0, 0.05, 0.1, respectively.The resistivity of host material LiZnAs with x = 0 displays a typical semiconducting transport behavior, which is consistent with the previous reported. 11With doping Co into Zn locations, the resistivity decreases in a monotonous way.When Co concentration increase to 10%, the activation-type of ρ(T ) changes to a slow increase with a decrease in temperature, in a manner characteristic of "dirty metals". 38 of spin-dependent scattering by aligning the spins in an applied field, 3,6 negative magnetoresistance, defined as [ρ(H) − ρ(0)]/ρ(0), is clearly observed under T C .The negative magnetoresistance of Li(Zn 0.75 Co 0.1 Mn 0.15 )As reaches to 16.2 % at T = 2 K and H = 7 T, which is consistent with the isothermal measurement as shown in Figure 4a.Temperature dependence of resistivity under various external field for Li(Zn 0.75 Co 0.1 Mn 0.15 )As is shown in Figure 4c.A field induced insulator-to-metal like transition around T C can be observed under the external field of 1 T, due to the suppression of magnetic fluctuations below T C .This feature was also observed in DMS Li(Zn 0.9 Mn 0.1 )As system. 11

Hall effect measurements
Figure 4d shows the Hall resistivity of Li(Zn 0.8 Co 0.1 Mn 0.1 )As at T = 2 K.The Hall effect measurements indicate that p-type carriers is obtained for the present Co and Mn co-doped system, with Co concentration of 15%.The anomalous Hall effect resulting from spin-orbit coupling is clearly observed around zero field, which confirm the ferromagnetic order in this system.We calculate the hole concentration following the formula: where R H is Hall coefficient and n p is carrier density, and obtain n p ∼7.74×10 19 cm -3 , which is close to that of Li(Zn,Mn)As system. 11The p-type carrier of this DMS system probably due to the following two facts: (1) Li occupies substitutional Zn sites, and (2) the substituted Zn atoms either escape from the system or remain neutral without ionization, as mentioned in Ref. 11.

CONCLUSIONS
In summary, we presented the successful synthesis and characterization of a new ferromagnetic DMS Li(Zn,Co,Mn)As with the same structure of LiZnAs.With Co and Mn co-doping, the system undergoes a ferromagnetic order with T C up to 40 K, where Mn substitution for Zn introduces spin and Co substitution for Zn introduces carriers, respectively.The maximum M sat of the present system reached to 2.17 µ B /Mn, which is comparable to that of Li (Zn,Mn)As.With increasing applied field, the interaction of the local spins and conduction electrons gives rise to an insulator-metal transition like behavior.This new DMS is suitable for spin manipulation with a relatively small coercive field (less than 100 Oe) in the future.In addition, it is the first time that ferromagnetic ordering has been observed in co-doped Co and Mn into Zn sites, which could challenge our understanding of magnetism in DMSs and explore new DMSs in other co-doped systems.Hence, more theoretical and experimental work is expected for further investigate the properties and physics of this co-doped system.

115014- 3
FIG. 1.(a) Crystal structure of (Ga,Mn)As.(b) Crystal structure of Li(Zn,Co,Mn)As (c) Powder XRD patterns of Li(Zn 1-2x Co x Mn x )As with several Co and Mn concentrations x taken at room temperature.(c) Lattice constants of a axis of Li(Zn,Co,Mn)As with different Co and Mn doping obtained from X-ray diffraction patterns.

FIG. 2 .
FIG. 2. (a) Temperature dependent magnetic susceptibility for LiZn 0.9 Mn 0.1 As with H = 1000 Oe, inset shows the temperature dependent magnetization M for LiZn 0.95 Co 0.05 As with H = 500 Oe.(b) The isothermal magnetization of Li(Zn 10.05y C0 0.05 Mn y )As (y = 0.05, 0.15) measured at 2 K. Inset the top left shows the T -dependent magnetization M of Li(Zn 10.05y Co 0.05 Mn y )As (y = 0.05,0.15)under H = 500 Oe.Inset the bottom right shows the temperature dependence of the inverse susceptibility of Li(Zn 0.9 C0 0.05 Mn 0.05 )As.(c) T -dependent magnetization M for Li(Zn 1x 0.15 Co x Mn 0.15 ) with several Co doping in the zero field cooling (ZFC) and field cooling (FC) modes with an external field of H = 100 Oe.(d) The hysteresis curves of Li(Zn 1x 0.15 Co x Mn 0.15 ) (x = 0.05, 0.075, 0.1, 0.15) measured at 2 K.

6 Chen
FIG. 4. (a) T -dependent resistivity for Li(Zn 0.75 Co 0.1 Mn 0.15 )As under H = 0, 0.2, 1, 2 and 7 T. (b) Field dependence of magnetoresistance at T = 2, 10 and 50 K from 7 T to 7 T. (c) T -dependent resistivity of Li(Zn 0.7 Co 0.15 Mn 0.15 )As under various fields.(d) Hall resistivity of Li(Zn 0.8 Co 0.1 Mn 0.1 )As at T = 2 K, which exhibits p-type carriers with concentrations of n p ∼7.74×10 19 cm -3 and the anomalous Hall effect due to spontaneous magnetization at H = 0 Oe.