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[1]. In past, scientists mainly focused on two research streams of ferromagnetic semiconductors: one is the concentrated magnetic semiconductors where magnetic atoms locate in each unit cell, for example, EuO[2] and CdCr2S4[3]; the other one is diluted magnetic semiconductors (DMSs) in which small amount of magnetic atoms have been doped into non-magnetic semiconductors and magnetic ordering has developed. In 1980s, Mn doped II–VI semiconductors, such as (Zn,Mn)Se[4], (Zn,Mn)Te[5], (Cd,Mn)Se[6] and (Cd,Mn)Te[7], etc. have been widely investigated. The difficulty encountered in II–VI DMSs is the low carrier concentrations, which prevents the formation of ferromagnetic long range ordering.
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[8–11]. As of today, the highest Curie temperature in (Ga,Mn)As has been reported as ~200 K[12–14]. In (Ga,Mn)As, Mn substitution for Ga introduces both spins and carriers simultaneously, which makes it difficult to control individual concentrations of spins and carriers, respectively. On the other hand, the mismatch valences of Mn2+ and Ga3+ limits the solid solution and only metastable (Ga,Mn)As films can be synthesized by MBE (molecular beam epitaxy) method. More progresses of III–V DMSs can be found in a recent review article by Tanaka et al.[15].
Recently, a series of novel bulk form DMSs isostructural to iron-based superconductors have been reported. They are 111-type Li(Zn,Mn)P[16] and Li(Zn,Mn)As[17], 1111-type (La,Ba)(Zn,Mn)AsO[18], and 122-type (Ba,K)(Zn,Mn)2As2[19] and Ba(Zn,Co)2As2[20], etc. In (Ba,K)(Zn,Mn)2As2, the highest Curie temperature
m C} $
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
m B})chi_{
m spin} $
m B} $
m B} $
The spin contribution to
$ begin{array}{l} displaystylefrac{1}{T_1} propto Tdisplaystylesum_q|A({q})|^2displaystylefrac{chi''({{q}},{ f_0)}}{ f_0} end{array}, $ | (1) |
where
Using the Gaussian approximation for the spin-spin correlation function, we can express
$ begin{array}{l} displaystylefrac{1}{T_1} = sqrt{2pi}displaystylefrac{S(S+1)}{3omega_{ m e}}left(frac{A_0}{hbar} ight)^2 end{array} ,$ | (2) |
where
$ begin{array}{l} omega^2_{ m e} = displaystylefrac{2}{3}zS(S+1)left(displaystylefrac{J}{hbar} ight)^2 end{array} ,$ | (3) |
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
$ begin{array}{l} A(t) = A_{ m mag}G_Z^L(t) + A_{ m para}{ m exp}left[-({lambda}t)^{beta} ight] end{array}. $ | (4) |
The first term on the right hand side represents the magnetic component, and the second term represents the paramagnetic component, where
$ begin{array}{l} f(H_i) = displaystylefrac{{gamma}_{mu}}{pi}displaystylefrac{a}{a^2+{gamma}^{2}_{mu}H^2_i} end{array} ,$ | (5) |
where
m s}^{-1}{
m G}^{-1} $
$ begin{array}{l} G^L_Z(T) = displaystylefrac{1}{3}+displaystylefrac{2}{3}(1-at){ m exp}(-at) end{array} ,$ | (6) |
as observed in diluted-alloy and spin glasses. The series expansion for Eq. (6) in terms of
$ begin{array}{l} G^L_Z(T) = 1-displaystylefrac{4}{3}at+a^2t^2+cdots .end{array} $ | (7) |
The series expansion for an exponential decay function
m exp}(-{varLambda}t) $
$ begin{array}{l} G^L_Z(T) = 1-{varLambda}t+displaystylefrac{1}{2}{varLambda}^2t^2+cdots ,end{array} $ | (8) |
which was employed for the analysis of ZF-μSR spectra in (Ga,Mn)As[23]. Comparing the Eqs. and (8), we notice that
m s} $
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[24]. In 2011, Deng et al. successfully synthesized the samples and observed ferromagnetic ordering below the Curie temperature ~ 50 K[17]. LiZnAs has a similar cubic crystal structure to that of GaAs. Doping Mn into Zn sites could be as high as 10%. In Fe-based superconductor family, LiFeAs is named as 111-type according to the chemical formula. For convenience, we also name Li(Zn,Mn)As as 111-type DMS. This family includes four other members, Li(Zn,Mn)P[16], Li(Cd,Mn)P[25], Li(Zn,Cr)As[26] and Li(Zn,Mn,Cu)As[27]. They all have the cubic crystal structure [16, 17, 25–27].
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.
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Figure1.
(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.
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Figure2.
(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[16]. In Fig. 3, we show the results of Li(Zn
m s}^{-1} $
m C} $
m C} $
m C} $
m C} $
m C} $
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Figure3.
(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
m C} $
3.1.2
μSR
For 111-type DMS, we show μSR results of Li
m C} sim $
m C} $
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Figure4.
(Color online) (a) The zero field μSR time spectra of Li
m C} $
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
m C} $
m C} $
Ding et al. conducted μSR measurements on (La,Ba)(Zn,Mn)AsO. The result is shown in Fig. 5 (adopted from Ref. [18]). Below
m C} $
m s} $
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Figure5.
(Color online) (a) The time spectra of LF-μSR in (La
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Figure6.
(Color online) Correlation between the static internal field parameter
m s} $
m C} $
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)
m C} $
m C} $
m C} $
Man et al. observed ferromagnetism in a new DMS Ba(Zn,Mn,Co)2As2 with n-type carriers[39], and they managed to obtain high quality polycrystal. Lately, Ba(Zn,Co)
For 122-type DMSs, we use (Ba,K)(Zn,Mn)
m C} $
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Figure7.
(Color online) (a) ZF-μSR time spectra obtained in polycrystalline specimen of (Ba
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
m C} $
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.