Abstract
1. Introduction
Ferromagnetic semiconductors possess both properties of semiconductors and ferromagnetism, which have attracted many attentions due to their potential applications for spin-sensitive electronic devices[
In 1990s, the research on DMSs has been focusing on III–V DMSs. Among them, (Ga,Mn)As has been the most thoroughly investigated system[
Recently, a series of novel bulk form DMSs isostructural to iron-based superconductors have been reported. They are 111-type Li(Zn,Mn)P[
2. Microscopic methods
NMR and μSR are powerful to measure spin dynamics and magnetic excitations in magnetically ordered systems. However, each experimental probe has its own requirements for samples. For NMR, the testing sample is usually needed to be put into a cylindrical coil, and signal to noise (S/N) ratio is largely relying on the filling factor of samples into the coil. S/N ratio is very small for film specimens, which makes it very hard to measure (Ga,Mn)As and other film specimens. While for μSR, bulk samples are also preferred. Thanks to the development of technology at Paul Scherrer Institute, it is now possible to measure films with thickness of tens of nanometers by low energy muons.
2.1. NMR
NMR is a local, site-selective probe. It is powerful to measure both static and dynamic susceptibilities. Due to the combinations of nucleons, some nuclei have certain nuclear spin
The spin contribution to
where
Using the Gaussian approximation for the spin-spin correlation function, we can express
where
and
2.2. μSR
μSR is another powerful method to investigate DMSs. Due to the parity breaking, muons are nearly 100% polarized even without the application of an external field. After stopping in the sample muons begin to precess in the local field. By measuring the anisotropic distribution of the positrons emitted by muons, we can infer the internal magnetic environment around muons. μSR is a high field-sensitive method and we can obtain volume fraction of magnetic ordered state using zero field (ZF-) μSR and weak transverse field (wTF-) μSR methods. For the analysis of the ZF μSR time spectra in DMS system, we usually use a two-component function. We write the function as
The first term on the right hand side represents the magnetic component, and the second term represents the paramagnetic component, where
where
as observed in diluted-alloy and spin glasses. The series expansion for Eq. (6) in terms of
The series expansion for an exponential decay function
which was employed for the analysis of ZF-μSR spectra in (Ga,Mn)As[
3. NMR and μSR results of DMSs
3.1. 111-system
Masek et al. firstly predicted that I–II–V Li(Zn,Mn)As could become a new diluted magnetic semiconductor[
3.1.1. NMR
As explained above, we need to choose a proper nucleus for conducting successful NMR experiment. Our initial attempt was to measure NMR signal from phosphorus in Li(Zn,Mn)P since P has a nuclear spin 1/2 and the measurement should have been easier. Unfortunately, the p-orbital of P atoms strongly hybridize with d-orbital of Mn atoms, which induces a very broad NMR lineshape especially below the Curie temperature. We then switched to measure Li NMR. Li has a nuclear spin 3/2, which should have given rise to three NMR lines if the charge distribution of nuclei is deformed from a spherical shape. This is because the nuclear quadrupole moment will interact with electric field gradient of the charge environment, which shifts the Zeeman levels. However, in LiCdP, as can be seen from the first picture in Fig. 1, six Cd atoms sit at nearest neighbor (N.N) sites of Li atoms. This means that no quadrupole interaction and only one NMR line exist, as shown in Fig. 2(a). This Li line is named as Li(0) site since no Mn is doped yet. But once Mn atoms are doped, the situation changes. For each Li atom, it can have zero, 1 to 6 Mn atoms at its nearest neighbor sites. We show five different possibilities in Fig. 1. Doping Mn directly changes the line shape of Li. As can be seen from Fig. 2(b), a broad hump appears at the left hand side of Li(0) site. This hump is from Li atoms with 1–6 Mn atoms at its N.N. sites, and is defined as Li(Mn) sites. Li(Mn) sites include Li(0), Li(1), Li(2), Li(3), Li(4), Li(5) and Li(6) sites as depicted in Fig. 1. Focusing on the shifts of Li(0) and Li(Mn) sites, we can readily obtain the static susceptibilities for each of them.
Figure 1.(Color online) The probability to find Li(0), Li(1), Li(2), Li(3), Li(4) for 10% Mn doped into Cd sites in LiCdP. The number in bracket means the number of Mn atoms at N.N. Cd sites.
Figure 2.(Color online) The representative 7Li line shapes of (a) Li1.1CdP and (b) Li1.1(Cd,Mn)P.
In a similar way, Ding et al. conducted the NMR measurements on Li(Zn,Mn)P that has the maximum Curie temperature ~ 34 K[
Figure 3.(Color online) (a) The temperature dependence of the 7Li NMR Knight shifts, – 7K, at the Li(Mn)sites. The HHFW of Li(0) in Li(Zn
3.1.2. μSR
For 111-type DMS, we show μSR results of Li
Figure 4.(Color online) (a) The zero field
3.2. 1111-system
Following the research trend in Fe-based superconductors, we have also discovered a series of new materials named 1111-type DMSs. The first 1111-type DMS reported is (La,Ba)(Zn,Mn)AsO with
Ding et al. conducted μSR measurements on (La,Ba)(Zn,Mn)AsO. The result is shown in Fig. 5 (adopted from Ref. [18]). Below
Figure 5.(Color online) (a) The time spectra of LF-
Figure 6.(Color online) Correlation between the static internal field parameter
3.3. 122-system
Different from 111-system and 1111-system, the crystal structures of 122-type DMSs are not identical. The crystal structure of (Ba,K)(Zn,Mn)
Man et al. observed ferromagnetism in a new DMS Ba(Zn,Mn,Co)2As2 with n-type carriers[
For 122-type DMSs, we use (Ba,K)(Zn,Mn)
Figure 7.(Color online) (a) ZF-
4. Summary
Many other microscopic methods focusing on bulk form DMSs have also been performed. Suzuki et al. studied (Ba,K)(Zn,Mn)
In conclusion, a series of new bulk form diluted magnetic semiconductors isostructural to iron-based superconductors have been synthesized. The new DMSs have the advantage of decoupled carrier and spin doping, and bulk form is beneficial to microscopic measurements. In addition, appropriate carrier doping is beneficial to promote exchange interactions between Mn atoms and form a long range ferromagnetic ordering state, thereby improving
Acknowledgments
The work was supported by MOST (No. 2016YFA0300402), NSF of China (No. 11574265) and the Fundamental Research Funds for the Central Universities. Authors acknowledge helpfuldiscussions with J. H. Zhao, C. Q. Jin, Y. J. Uemura and T. Imaiand the help from G. D. Morris, B. S. Hitti, and other staff in theprocess of μSR measurements at TRIUMF.
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