“111” type DMSs
LiZnAs[23, 24] is a direct-gap semiconductor that has a cubic crystal structure with space group F-43m as shown in Figs. 1(a) & 1(b) and a band gap (1.61 eV) similar to that of GaAs (1.52 eV). Note that LiZnAs could be viewed as a zinc blende structure, in which Ga sites in (Ga,As) are replaced with Li and Zn. Benefitting on the LiFeAs iron superconductor fabrication experiences and facilities. Jin’s group firstly reported the successful synthesis of bulk form polycrystals Li(Zn,Mn)As. In principle, the Zn2+/Mn2+ replacement offers spins, and the off-stoichiometrical Li cation adjust the carrier’s type and concentration. But the long range ferromagnetic ordering was only observed with the excess Li doping and it is a p-type rather than n-type as the theory prediction. The reason is that the excess Li1+ prefers to occupy the Zn2+ sites based on a DFT calculation. The temperature dependence of M at 2 kOe (no difference between field cooling and zero field cooling procedures) was illustrated in Fig. 2(a), and external field H from 0 to 20 kOe dependence of M at 2 K is shown in Fig. 2(b). Clear signatures of ferromagnetic order were seen in these figures with the highest TC of 50 K in Li1.1(Zn0.85Mn0.15) As sample. Semiconducting behavior of resistivity of LiZnAs (green line in Fig. 2(c)) changes to T-independent behavior for Li deficient systems, wheras much smaller resistivity and definite metallic behavior were found for Li excess samples. The resistivity increases monotonically with increasing Mn concentration in Fig. 2(d) which suggests that Mn acts as a scattering center. The onset of magnetic order reduces this scattering rate, as can be seen in the negative magnetoresistance of Li1.1(Zn0.9Mn0.1)As sample in Fig. 2(e) below TC. Fig. 2(f) shows representative anomalous Hall effect of Li1.1(Zn0.95Mn0.05)As at 2 K, which exhibits p-type carriers with concentrations of n ~1020 cm–3.
In order to examine volume fraction and the ordered moment size, μSR measurements were performed on sintered polycrystalline specimens of Li1.1(Zn0.95Mn0.05)As. The time spectra in zero field (ZF), shown in Fig. 3(a), clearly show an increase of the relaxation rate below ~25 K on Fig. 3(b), and measurements in longitudinal fields confirmed that the increase is from static magnetic order. This is in correspond with the earlier result in (Ga,Mn)As. The volume fraction of regions with static magnetism on Fig. 3(c) indicates a sharp transition at TC ~ 30 K, and full volume magnetic order achieved when T < 10 K. Further studies on fabrication methods and heat treatments might help improve spatial homogeneity of the transition. The weak transverse field (WTF) of 30 Oe, shown in Fig. 3(d), also provide direct signal of the magnetic volume fraction. When the internal magnetic field is much larger than the applied external field, the scale of oscillation can reflect the paramagnetic volume. The consistenct of the ordered fraction from the measurements in ZF and WTF, shown in Fig. 3(c), supports the validity of our analysis of ZF-μSR spectra.
The discovery of Li(Zn,Mn)As sparked extensive researches in this 111 type DMSs[28, 32–36]. Fig. 4 shows only three of these new DMSs with independent spin and charge doping, e.g. Li(Zn,Mn)P, Li(Cd,Mn)P and Li(Zn,Co,Mn)As. Isostructural to Li(Zn,Mn)As, Li(Zn,Mn)P is also a p-type DMSs with excess lithium providing charge doping. The highest TC could reaches 34 K. The saturation moment per Mn (Msat) is about 1μB–2μB, which is comparable to that in (Ga,Mn)As, Li(Zn,Mn)As, etc. ρ(T) of Li(Zn,Mn)P decreases with increasing temperature, which shows semiconductor behaviors. Li(Zn,Mn)P is a soft magnetic material with about less than 100 Oe, shown in Fig. 4(a), based on the magnetic hysteresis loop and magnetoresistance curves. Magnetoresistivity ρH(T) of Li1.04(Zn0.9Mn0.1)P at different external fields are shown in Fig. 4(b). ρH(T) increases monotonically with decreasing temperature, showing a rapid rise below TC. As shown in the inset of Fig. 4(b), the negative magnetoresistance is far from saturation in rather high magnetic field, in which spin orientation is fully aligned. In this condition, the negative magnetoresistance presumably results from the weak localization effects. Systematic μSR measurements also confirmed that the magnetic volume fraction on Li1.15(Zn0.9Mn0.1)P could reaches nearly 100% at 2 K. Compared to Li(Zn,Mn)P, DMSs Li(Cd,Mn)P with optimum doping exhibits a higher TC of 45 K as shown in Fig. 4(c). But more than 80% negative magnetoresistance, shown in Fig. 4(d), is a record in this 111 type DMSs. For this new type DMSs, spins and carries are indispensable.
Usually, the carriers are induced in Li site while spins in Zn site in all these above systems. Different from that idea, a new DMSs Li(Zn,Co,Mn)As was reported in which carriers and spins are both induced in Zn site simultaneously. Fig. 4(e) displays the tempertature dependence of M in H = 100 Oe for Li(Zn1–x–0.15CoxMn0.15)P. There is no difference in ZFC and FC procedures for small coercive fields with the highest TC of 40 K in LiZn0.80Co0.05Mn0.15P. The resistivity of Li(Zn1–x–0.15Mn0.15)P decreases with increase of Co doping. This means much more carries are induced successfully. A field induced insulator-to-metal like transition around TC can be observed with the external field of 1 T, which is due to the suppression of magnetic fluctuations below TC. This feature is also observed in Li(Zn,Mn)As system. Fig. 4(d) demonstrates the Hall resistivity of Li(Zn0.8Co0.1Mn0.1)As at 2 K, which exhibits p-type carriers with concentrations of n ~ 7.74 × 1019 cm–3 together with the anomalous Hall effect due to spontaneous magnetization at H = 0.
“122” type DMSs
BaZn2As2 is a semiconductor synthetized at high temperature (>900 °C) with the tetragonal ThCr2Si2 crystal structure. A new type “122” DMSs (Ba,K)(Zn,Mn)2As2 has been synthesized with the Ba2+/K1+ substitution (hole carries) and Zn2+/Mn2+ (spins) doping. Fig. 5(a) shows the temperature dependence of M in H = 500 Oe for (Ba0.7K0.3)(Zn0.85Mn0.15)2As2 at ZFC and FC procedures with TC 230 K. The hysteresis curves M(H) are shown in the inset of Fig. 5(a). Fig. 5(b) exhibits the spontaneous magnetization curve under 5 Oe of (Ba0.7K0.3)- (Zn0.85Mn0.15)2As2, showing T 3/2 dependence in low temperature, as expected for a homogeneous ferromagnet. Volume fraction of regions with static magnetic order, estimated by μSR measurements in ZF and WTF of 50 Oe are shown in Fig. 5(c). The μSR results indicate that static magnetic order develops in the entire volume with a sharp onset around TC. Resistivity of (Ba1–xKx)(Zn1–yMny)2As2 for with several different charge doping levels are shown in Fig. 5(d). For BaZn2As2 semiconductor, doping K atoms into Ba sites introduces hole carriers, leading to metallic behavior in (Ba,K)Zn2As2. The resistivity curves of (Ba1–xKx)(Zn1–yMny)2As2 for selected values of x up to 0.3, exhibit a small increase at low temperatures due to spin scattering effect caused by Mn dopants. This variation of resistivity is often observed in heavily doped semiconductors. Strictly metallic behavior (with monotonic decrease of resistivity with decreasing temperatures) is not a precondition of having a ferromagnetic coupling between Mn moments mediated by RKKY interaction[12, 30]. Fig. 5(e) shows the magnetoresistance curve measured in the external field up to 7 T at different temperatures, showing obvious negative magnetoresistance below TC. Fig. 5(f) shows the Hall effect results of (Ba0.85K0.15)(Zn0.90Mn0.10)2As2 at several temperatures with hole concentration about 4.3 × 1020 cm–3.
Different from (Ba,K)(Zn,Mn)2As2, another “122” type DMSs with hexagonal CaAl2Si2 was reported subsequently, such as (Ca,Na)(Zn,Mn)2As2, (Sr,Na)(Zn,Mn)2As2 and (Sr,Na)(Cd,Mn)2As2, etc. Fig. 6(a) shows the temperature dependence of M in H = 500 Oe for (Ca0.9Na0.1)(Zn,Mn)2As2 with several different charge doping levels at ZFC and FC procedures. The highest TC is 33 K. In Fig. 6(b), the temperature dependence of the volume fraction of regions with static magnetic order, derived from μSR measurements in ZF, is consistent with that of spontaneous magnetization under 5 Oe. The latter shows T 3/2 dependence in low temperature expected for a homogeneous ferromagnet[22, 30, 38]. Due to the competition between nearest-neighbor antiferromagnetic interactions and ferromagnetic interactions from remote Mn moments, Msat per Mn decreases with increasing Mn concentration. μSR results, shown in Fig. 6(d) are also consistent with the spontaneous magnetization under 10 Oe. This means (Sr0.8Na0.2)(Zn0.85Mn0.15)2As2 is also a homogeneous ferromagnet with almost 100% ordered volume fraction at low temperatures. Fig. 6(e) shows the temperature dependence of M in H = 500 Oe for (Sr1–xNax)(Cd1–xMnx)2As2 with several different charge doping levels x at ZFC and FC procedures with the highest TC 13 K. Magnetotrasport measurements performed on (Sr0.8Na0.2)(Zn0.8Mn0.2)2As2 at 2 K under the field of up to 7 T are shown in Fig. 6(f). The negative magnetoresistance reached –23% at 2 K and 7 T Taking the orbital effect into consideration, the negative magnetoresistance data are fit by kB1/2 rule, indicating weak localization magnetoresistance at low temperature. The similar phenomenon is found in (Ga,Mn)As system[37, 43].
“1111” type DMSs
Isostructural to the well-studied iron-based superconductor LaFeAs(O1–xFx), a new kind of “1111” DMSs were reported after the “111” and “122” DMSs, e.g. (La,Ca)(Zn,Mn)SbO and (Ba,K)F(Zn,Mn)As[46, 47], etc. Fig. 7(a) shows the M(H) for (La0.95Ca0.05)(Zn0.9Mn0.1)SbO measured at 25 and 100 K, respectively. An abrupt increase of magnetization at 25 K suggests that a ferromagnetic state and paramagnetic state transition (TC) occurs. The highest TC reaches 40 K with proper doping in (La,Ca)(Zn,Mn)SbO material. The resistivity increases monotonically with increasing Mn concentration, suggesting that scattering center of Mn. The resistivity of (La0.95Ca0.05)(Zn0.925Mn0.075)SbO is shown in Fig. 7(b), with magnetic field up to 5 T. A negative magnetic resistance is clearly observed in a wide temperature region. This behavior can be well explained by the field suppression of the spin fluctuation. Fig. 7(c) shows the M(T) in ZFC and FC procedures under 500 Oe for the (Ba0.8K0.2)F(Zn1–yMny)As samples with y = 0.025, 0.05, 0.075, 0.1 and 0.15, respectively. The highest TC reaches 30 K for optimal Mn doping (y = 0.1). Above TC, the susceptibility is fit to Curie-Weiss law as shown in the inset of Fig. 7(c), which indicates a ferromagnetic interaction between Mn2+. The temperature dependence of the fast relaxation rate Λ is plotted in Fig. 7(d), exhibiting a monotonic increase with decreasing temperature and reaching a maximum value at the lowest measurement temperature (2 K). The relationships between hole concentration and TC of “111”, “122” and “1111” DMSs and other diluted ferromagnetic semiconductor systems are plotted in Fig. 7(e)[19, 20, 22, 28, 36, 45, 49, 50]. From Li(Zn,Mn)P to (Ba, K)(Zn,Mn)2As2, TC is considerably improved. As the Zener model predicted[51, 52], the ferromagnetic ordering in DMSs is mediated by hole carriers, and the Curie temperature is positive correlated with hole concentration.