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Magnetic and electronic transport properties of Mn-doped silicon
Solid State Communications 140 (2006) 192–196 www.elsevier.com/locate/ssc
Magnetic and electronic transport properties of Mn-doped silicon S.B. Ma, Y.P. Sun ∗ , B.C. Zhao, P. Tong, X.B. Zhu, W.H. Song Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Received 26 May 2006; received in revised form 12 July 2006; accepted 28 July 2006 by C. Lacroix Available online 15 August 2006
Abstract Polycrystalline Si1−x Mnx (x = 0.005, 0.01, and 0.015) samples were prepared by the arc-melting method. Powder x-ray diffraction analysis demonstrates that the light Mn doping does not change the crystalline structure of silicon. Magnetic studies reveal that the ferromagnetism can be developed in all Mn-doped samples and the Curie temperature (TC ) increases with increasing Mn doping content x. The effective magnetic moments are 4.15, 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The undoped sample shows semiconducting behavior in the whole studied temperature range, whereas a metal–insulator transition can be observed near TC for all doped samples. The thermally activated conducting mechanism dominates the low temperature transport properties of the doped samples. The activation energy obtained from the fitting decreases monotonously with increasing x. In addition, the anomalous Hall effect below TC was observed from the magnetic field dependence of the Hall resistivity curves. c 2006 Elsevier Ltd. All rights reserved.
PACS: 72.25.Dc; 75.50.Gg; 75.50.Pp; 75.60.Ej Keywords: A. Diluted magnetic semiconductors; A. Mn-doping; D. Ferromagnetism; D. Anomalous Hall effect
1. Introduction In recent years, much attention has been paid to diluted magnetic semiconductors (DMSs) due to the possibility of integrating charge and spin degree of freedom in a single material [1]. GaAs doped with manganese is the most widely studied material of this class in which ferromagnetic (FM) ordering is found with the Curie temperature (TC ) up to 170 K and has been successfully utilized as spin-polarized devices [2– 6]. Room temperature ferromagnetism has been found in Mndoped CdGeP2 , ZnGeP2 [7–9] and ZnGeAs2 [10]. The groupIV-based DMSs materials have been earnestly expected to the favorable category with TC above room temperature due to their compatibility with conventional integrated circuits. Recently, it has been very exciting due to the discovery of ferromagnetism in Ge1−x Mnx (x = 0.05) epitaxial thin films with TC up to 110 K [11]. In addition, several authors have reported the ferromagnetism formed in the manganese doped silicon system. For instance, the epitaxial thin film Si1−x Mnx (x = 0.05) has TC of ∼70 K [12]. The room temperature ferromagnetism ∗ Corresponding author. Tel.: +86 551 559 2757; fax: +86 551 559 1434.
E-mail address: [email protected] (Y.P. Sun). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.039
is observed in the Mn-ion implanted silicon thin films [13]. The Mn-doped Si film with TC ∼ 250 K and above room temperature has been prepared by sputtering technique and vacuum vaporizing deposition [14,15]. These controversial results show that the magnetism of Si-based DMSs is very sensitive to the preparation method. As we know, there exist a lot of dangling bonds because of the two dimensionality of the film sample. It is not clear whether the observed ferromagnetism in Mn-doped Si film is related to these dangling bonds. In order to test whether the observed ferromagnetism in Mn-doped Si sample is intrinsic, it is necessary to perform the investigation for Mn-doped Si bulk samples. In this paper, we report the magnetic and transport results of a series of Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) polycrystalline samples synthesized by the arc melting technique. All doped samples show ferromagnetism with TC at about 250 K. The metal–insulator (M–I) transition was observed near TC from temperature-dependent resistivity. 2. Experiment Mn-doped silicon polycrystalline Si1−x Mnx samples with nominal compositions of x = 0.005, 0.01, and 0.015 were
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fabricated by the arc melting technique. High purity manganese (99.97%) and silicon (99.999%) powder were weighed in appropriate ratios and homogenized in an agate pestle. The homogenized mixture was pressed into pellets and melted in an arc furnace under a high purity argon atmosphere. After melting, the ingots were cut into slices. The slices were annealed for 10 h at 850 ◦ C under flowing H2 and Ar ambient. The structure and phase purity of the samples were examined by powder x-ray diffraction (XRD) using Cu K α radiation at room temperature. The magnetic measurements were carried out on a superconducting quantum interference (SQUID, Quantum Design) magnetometer. The temperature dependence of longitudinal resistivity ρ and the magnetic field dependence of Hall resistivity ρx y using the four-probe method were measured in a commercial Physical Property Measurement System (PPMS). Hall resistivity was given by reversal of the magnetic field direction to cancel the offset voltage due to the geometrical asymmetry of transverse electrodes.
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Fig. 1. Powder x-ray diffraction patterns for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples. The inset shows the shift of the (311) peak position.
3. Results and discussion Fig. 1 shows the powder XRD patterns of polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples at room temperature. The powder XRD reveals that all samples display the characteristic peaks of the silicon cubic structure without detectable second phases, indicating that Mn atoms are incorporated in the lattice of silicon. The conclusion can also be confirmed by the shift of the (311) peak position toward smaller angles as shown in the inset of Fig. 1, implying the increase of lattice parameters. This result is consistent with the fact that the Mn ion has larger ionic radii than that of silicon. Therefore, we can conclude that the mixed phase does not exist in the present samples, though support from a more accurate analysis technique is needed. Fig. 2 plots the temperature dependence of magnetization M(T ) for Si1−x Mnx between 5 and 300 K under zero-field cooling (ZFC) mode at 1000 Oe. An obvious paramagnetic (PM) to ferromagnetic (FM) phase transition is observed in the M(T ) curves of all doped samples. The transition temperature obtained TC (defined as the one corresponding to the peak of dM/dT in the M vs. T curve) is 225, 238, 262 K for the samples with x = 0.005, 0.01, and 0.015, respectively. It is apparent that TC increases monotonously with the increase x. In addition, there exists an anomaly in the M(T ) curves at low temperatures around T ∗ ∼ 40 K. Meanwhile, a considerable increase of magnetization with the decrease of temperature below T ∗ is observed, which can be interpreted as the non-uniform distribution of Mn ions in the polycrystalline samples [16]. The magnetic hysteresis loop M(H ) curves of all studied samples at 5 K are shown in the main panel of Fig. 3. It is clearly shown that the FM ground state exists in all doped samples. Meanwhile, no obvious hysteresis is observed for the undoped sample. The saturation magnetization increases with the increase x, which is consistent with the results observed in the M(T ) curves as shown in Fig. 2. The M(H ) measurement has also been performed at 77 K. As an example, the M(H )
Fig. 2. Magnetization as a function of temperature for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples measured at H = 1000 Oe under the zero-field-cooled (ZFC) mode.
curves at both 5 and 77 K for the sample with x = 0.01 are displayed in the inset of Fig. 3. The coercive field is ∼80 Oe at both 5 and 77 K, which is almost temperature independent. In order to further understand the magnetic properties of the Mndoped Si, the temperature dependence of the inverse magnetic susceptibility (χ −1 = H/M) for the x = 0.015 sample is plotted in Fig. 4 as an example. It can be seen that the experimental curve in the PM phase above TC can be well described by the Curie–Weiss law, i.e. χ = C/(T − Θ), where C is the Curie constant, and Θ is the Weiss temperature. The effective magnetic moment µeff can be obtained from the fitting as 4.15 and 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The results indicate that µeff does not vary considerably with x. Considering the theoretical value of µeff : 4.90 and 3.87µ B /Mn for Mn3+ and Mn4+ , we suggest that both Mn3+ and Mn4+ ions coexist in the present doped samples. The latter will continue to discuss. The temperature dependence of resistivity ρ in logarithmic scale for all the investigated samples is shown in Fig. 5. The experimental data is obtained at zero magnetic field in the temperature range of 5–300 K. As can be seen from Fig. 5(a), for the undoped sample (x = 0), ρ increases monotonously
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Fig. 3. Magnetic hysteresis loops at 5 K for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples. The inset shows the hysteresis loops for Si0.99 Mn0.01 at 5 and 77 K, respectively.
Fig. 4. The temperature dependence of the inverse of the magnetic susceptibility for the sample with x = 0.015. The solid line is the fitting curve according to the Curie–Weiss law.
with decreasing temperature which is a typical feature of semiconductors. For all doped samples, the resistivity has a typical characteristic of a metal–insulator (M–I) transition. It shows a distinct resistive minimum corresponding to the temperature Tρ min (as shown in Fig. 5(b)). That is to say, as T > Tρ min , the sample behaves as a metallic character with a positive resistance temperature coefficient, dρ/dT > 0; as T < Tρ min , the sample shows a semiconducting character with a negative resistance temperature coefficient, dρ/dT < 0. In addition, Fig. 5(b) also displays that Tρ min increases with increasing Mn-doping content x as shown in Table 1. It should be noted that the value of Tρ min is almost in accordance with Tc as observed in the M(T ) measurement. The result suggests that the M–I transition may have a magnetic origin similar to the Ge-based DMSs [17,18], where the M–I transition is assumed to originate from the spin-flip scattering near the magnetic transition. We should note that the M–I transition observed in present samples is “slow” compared with other DMSs, such as GaMnAs, where a well-defined hump is observed at the M–I transition with a spin-scattering
Fig. 5. (a) Temperature dependence of the resistivity for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples at zero field in a logarithmic scale. (b) The partial enlargement plot of temperature-dependent resistivity. A metal–insulator transition near the respective TC for all studied samples is obviously observed.
Table 1 Magnetic and electrical characteristics of Si1−x Mnx polycrystalline bulk samples Sample
TC (K)
θ (K)
Tρ min (K)
E a (meV)
µeff (µ B /Mn)
x = 0.005 x = 0.01 x = 0.015
225 238 262
222 232 258
221 240 256
24 17 15
– 4.15 4.05
From left to right, the columns indicate sample composition; Curie temperature; Weiss temperature; M–I transition temperature; activation energy; effective magnetic moment.
effect in the corresponding temperature range, respectively [19, 20]. This inconspicuous M–I transition near TC for the doped samples may be the scattering of crystal face of polycrystalline sample. In order to obtain more information from the electronic transport properties, we attempt to fit the low temperature ρ(T )
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Fig. 6. The inverse temperature dependence of the resistivity for polycrystalline Si1−x Mnx (x = 0.005, 0.01, and 0.015) samples at zero field in a logarithmic scale. The solid line represents the linear fit to the data for 80–260 K.
data using several models. It is found that the best fitting result is given by the formula: ρ = ρ0 exp(E a /k B T ) for doped samples (as shown in Fig. 6), where ρ0 is a prefactor, E a is the activation energy, and k B is the Boltzmann constant, which has the typical feature of the thermal activated conduction behavior for semiconductor [15]. From the fitting curves, the activation energy E a calculated by the aforementioned formula is 24, 17, and 15 meV for the samples with x = 0.005, 0.01, and 0.015, respectively (as shown in Table 1). The activation energy decreases with increasing Mn content. The activation energies are compatible with a variable-range hopping conduction in the impurity band formed by the heavy Si doping. The activation energy E a obtained from ρ(T ) data below TC varies from 15 meV as x = 0.005 to 24 meV for x = 0.015. The high values of E a in the corresponding temperature range seem to exclude a variable hopping conduction mechanism in an impurity band. Meanwhile, the activation energies are not corresponding to any acceptor level of Mn in Si. For DMSs showing an insulating character, a percolation model has been suggested by Kaminski and Das Sarma to explain both their electrical and magnetic properties [21,22]. According to this model, at temperatures of the order of TC and below, charge carrier (hole) transport in present samples occurs mainly through nearest-neighbor hopping at the localization sites. The value E a discussed above is the hopping activation energy among these localization sites. To further investigate transport properties, the magnetic field dependence of Hall resistivity ρx y for Si1−x Mnx (x = 0, 0.01, and 0.015) samples is measured at 50 K and the results are shown in the main panel of Fig. 7. For the x = 0 sample, the Hall resistivity increases linearly with increasing magnetic field in the whole measured field range, which is a typical characteristic of semiconducting materials. However, the ρx y (H ) curve for doped samples deviates from the linear behavior at relative low fields (less than 1.0 T). This exotic behavior can be taken into account of the anomalous Hall effect (AHE) [23]. The AHE has also been observed in other DMSs system previously, where the authors ascribed the AHE
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Fig. 7. Hall resistivity ρx y as a function of magnetic field for polycrystalline Si1−x Mnx (x = 0, 0.01, and 0.015) samples measured at 50 K. The inset shows magnetic field dependence of Hall resistivity ρx y for Si0.99 Mn0.01 at 50, 100, 200, and 300 K, respectively.
to skew-type scattering of polarized carriers by magnetic impurities [24,25]. In general, the Hall resistivity ρx y is expressed as ρx y = R0 B + Rs M, where B is an external magnetic induced intensity, M is a magnetization, R0 and Rs is the normal and anomalous Hall coefficient, respectively. Although the AHE makes a determination of densities of holes be less accurate, an estimate can be obtained from the Hall data at high magnetic field where the magnetization is saturated. The charge carrier concentration p at T = 50 K deduced from the normal Hall coefficient obtained from the fitting of the high field ρx y (H ) curve are 1.6 × 1019 , 6.6 × 1019 , and 3.5 × 1020 /cm3 for the sample with x = 0.005, 0.01, and 0.015, respectively. All samples have positive Hall coefficient, implying that the present samples belong to the p-type semiconductors. The carrier concentration increases monotonously with increasing x. According to the above results, this conclusion that the doped Mn in silicon serves as both acceptor (hole) and magnetic impurity is reasonable. In this scenario, the ferromagnetism induced by Mn in the present samples can be explained as follows: the magnetically ordered phase arises from the indirect interaction between magnetic ions mediated through carriers (holes), because the direct interaction between Mn will result in an antiferromagnetic (AFM) coupling as in bulk Mn [13,26,27]. The inset of Fig. 7 shows the Hall resistivity ρx y plotted as a function of the applied magnetic field at several temperatures below and above TC for the sample Si0.99 Mn0.01 . It can be seen that the AHE only appears at low temperature FM phase, but it is essentially absent at temperatures higher than TC . The result is not difficult to understand, because ferromagnetism disappears above TC . At last, we note that the ferromagnetism in present samples is not related to the second phase of SiMn compounds. As we know, a pure Si does not have a net magnetic moment (as shown in Figs. 2 and 3) and a metallic Mn is an AFM material [28]. In addition, among all the SiMn compounds, only SiMn compound has ferromagnetism with a Curie temperature less than 30 K [29]. Therefore, the observed magnetic properties in
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present samples with TC of around 250 K appear to exclude the possible contribution from the second phase of SiMn compounds. Our prepared Si1−x Mnx compounds are suggested to belong to a Si-based DMSs type.
[7]
4. Conclusion
[9]
In summary, we have prepared polycrystalline Si1−x Mnx DMSs with x = 0, 0.005, 0.01 and 0.015 using the arc melting method. All doped samples undergo a PM–FM transition near 250 K. The effective magnetic moments are 4.15 and 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The M–I transition around the respective Curie temperature has also been observed in ρ(T ) curves. The low temperature ρ(T ) data for the doped samples can be fitted by the thermally activated model. The activation energy calculated from fitting decreases with increasing x. The distinct anomalous Hall effect is found for Mn-doped Si samples. All these results indicate the possible application of the present system as Si-based diluted magnetic semiconductors.
[8]
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[12] [13] [14] [15] [16]
Acknowledgments [17]
This work was supported by the National Key Research under Contract No. 001CB610604, and the National Nature Science Foundation of China under Contract No. 10474100, 10374033 and Director’s Fund of Hefei Institutes of Physical Science, Chinese Academy of Sciences. References [1] H. Ohno, Science 281 (1998) 951; G.A. Prinz, Science 282 (1998) 1660. [2] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69 (1996) 363. [3] H. Akinaga, S. Miyanishi, K. Tanaka, W. van Roy, K. Onodera, Appl. Phys. Lett. 76 (2000) 97. [4] K.W. Edmonds, K.Y. Wang, R.P. Campion, A.C. Neumann, N.R.S. Farley, B.L. Gallagher, C.T. Foxon, Appl. Phys. Lett. 81 (2002) 4991. [5] Y.D. Park, B.T. Jonker, B.R. Bennett, G. Itskos, M. Furis, G. Kioseoglou, A. Petrou, Appl. Phys. Lett. 77 (2000) 3989. [6] X. Chen, M. Na, M. Cheon, S. Wang, H. Luo, B.D. McCombe, X. Liu,
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Magnetic and electronic transport properties of Mn-doped silicon S.B. Ma, Y.P. Sun ∗ , B.C. Zhao, P. Tong, X.B. Zhu, W.H. Song Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Received 26 May 2006; received in revised form 12 July 2006; accepted 28 July 2006 by C. Lacroix Available online 15 August 2006
Abstract Polycrystalline Si1−x Mnx (x = 0.005, 0.01, and 0.015) samples were prepared by the arc-melting method. Powder x-ray diffraction analysis demonstrates that the light Mn doping does not change the crystalline structure of silicon. Magnetic studies reveal that the ferromagnetism can be developed in all Mn-doped samples and the Curie temperature (TC ) increases with increasing Mn doping content x. The effective magnetic moments are 4.15, 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The undoped sample shows semiconducting behavior in the whole studied temperature range, whereas a metal–insulator transition can be observed near TC for all doped samples. The thermally activated conducting mechanism dominates the low temperature transport properties of the doped samples. The activation energy obtained from the fitting decreases monotonously with increasing x. In addition, the anomalous Hall effect below TC was observed from the magnetic field dependence of the Hall resistivity curves. c 2006 Elsevier Ltd. All rights reserved.
PACS: 72.25.Dc; 75.50.Gg; 75.50.Pp; 75.60.Ej Keywords: A. Diluted magnetic semiconductors; A. Mn-doping; D. Ferromagnetism; D. Anomalous Hall effect
1. Introduction In recent years, much attention has been paid to diluted magnetic semiconductors (DMSs) due to the possibility of integrating charge and spin degree of freedom in a single material [1]. GaAs doped with manganese is the most widely studied material of this class in which ferromagnetic (FM) ordering is found with the Curie temperature (TC ) up to 170 K and has been successfully utilized as spin-polarized devices [2– 6]. Room temperature ferromagnetism has been found in Mndoped CdGeP2 , ZnGeP2 [7–9] and ZnGeAs2 [10]. The groupIV-based DMSs materials have been earnestly expected to the favorable category with TC above room temperature due to their compatibility with conventional integrated circuits. Recently, it has been very exciting due to the discovery of ferromagnetism in Ge1−x Mnx (x = 0.05) epitaxial thin films with TC up to 110 K [11]. In addition, several authors have reported the ferromagnetism formed in the manganese doped silicon system. For instance, the epitaxial thin film Si1−x Mnx (x = 0.05) has TC of ∼70 K [12]. The room temperature ferromagnetism ∗ Corresponding author. Tel.: +86 551 559 2757; fax: +86 551 559 1434.
E-mail address: [email protected] (Y.P. Sun). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.039
is observed in the Mn-ion implanted silicon thin films [13]. The Mn-doped Si film with TC ∼ 250 K and above room temperature has been prepared by sputtering technique and vacuum vaporizing deposition [14,15]. These controversial results show that the magnetism of Si-based DMSs is very sensitive to the preparation method. As we know, there exist a lot of dangling bonds because of the two dimensionality of the film sample. It is not clear whether the observed ferromagnetism in Mn-doped Si film is related to these dangling bonds. In order to test whether the observed ferromagnetism in Mn-doped Si sample is intrinsic, it is necessary to perform the investigation for Mn-doped Si bulk samples. In this paper, we report the magnetic and transport results of a series of Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) polycrystalline samples synthesized by the arc melting technique. All doped samples show ferromagnetism with TC at about 250 K. The metal–insulator (M–I) transition was observed near TC from temperature-dependent resistivity. 2. Experiment Mn-doped silicon polycrystalline Si1−x Mnx samples with nominal compositions of x = 0.005, 0.01, and 0.015 were
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fabricated by the arc melting technique. High purity manganese (99.97%) and silicon (99.999%) powder were weighed in appropriate ratios and homogenized in an agate pestle. The homogenized mixture was pressed into pellets and melted in an arc furnace under a high purity argon atmosphere. After melting, the ingots were cut into slices. The slices were annealed for 10 h at 850 ◦ C under flowing H2 and Ar ambient. The structure and phase purity of the samples were examined by powder x-ray diffraction (XRD) using Cu K α radiation at room temperature. The magnetic measurements were carried out on a superconducting quantum interference (SQUID, Quantum Design) magnetometer. The temperature dependence of longitudinal resistivity ρ and the magnetic field dependence of Hall resistivity ρx y using the four-probe method were measured in a commercial Physical Property Measurement System (PPMS). Hall resistivity was given by reversal of the magnetic field direction to cancel the offset voltage due to the geometrical asymmetry of transverse electrodes.
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Fig. 1. Powder x-ray diffraction patterns for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples. The inset shows the shift of the (311) peak position.
3. Results and discussion Fig. 1 shows the powder XRD patterns of polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples at room temperature. The powder XRD reveals that all samples display the characteristic peaks of the silicon cubic structure without detectable second phases, indicating that Mn atoms are incorporated in the lattice of silicon. The conclusion can also be confirmed by the shift of the (311) peak position toward smaller angles as shown in the inset of Fig. 1, implying the increase of lattice parameters. This result is consistent with the fact that the Mn ion has larger ionic radii than that of silicon. Therefore, we can conclude that the mixed phase does not exist in the present samples, though support from a more accurate analysis technique is needed. Fig. 2 plots the temperature dependence of magnetization M(T ) for Si1−x Mnx between 5 and 300 K under zero-field cooling (ZFC) mode at 1000 Oe. An obvious paramagnetic (PM) to ferromagnetic (FM) phase transition is observed in the M(T ) curves of all doped samples. The transition temperature obtained TC (defined as the one corresponding to the peak of dM/dT in the M vs. T curve) is 225, 238, 262 K for the samples with x = 0.005, 0.01, and 0.015, respectively. It is apparent that TC increases monotonously with the increase x. In addition, there exists an anomaly in the M(T ) curves at low temperatures around T ∗ ∼ 40 K. Meanwhile, a considerable increase of magnetization with the decrease of temperature below T ∗ is observed, which can be interpreted as the non-uniform distribution of Mn ions in the polycrystalline samples [16]. The magnetic hysteresis loop M(H ) curves of all studied samples at 5 K are shown in the main panel of Fig. 3. It is clearly shown that the FM ground state exists in all doped samples. Meanwhile, no obvious hysteresis is observed for the undoped sample. The saturation magnetization increases with the increase x, which is consistent with the results observed in the M(T ) curves as shown in Fig. 2. The M(H ) measurement has also been performed at 77 K. As an example, the M(H )
Fig. 2. Magnetization as a function of temperature for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples measured at H = 1000 Oe under the zero-field-cooled (ZFC) mode.
curves at both 5 and 77 K for the sample with x = 0.01 are displayed in the inset of Fig. 3. The coercive field is ∼80 Oe at both 5 and 77 K, which is almost temperature independent. In order to further understand the magnetic properties of the Mndoped Si, the temperature dependence of the inverse magnetic susceptibility (χ −1 = H/M) for the x = 0.015 sample is plotted in Fig. 4 as an example. It can be seen that the experimental curve in the PM phase above TC can be well described by the Curie–Weiss law, i.e. χ = C/(T − Θ), where C is the Curie constant, and Θ is the Weiss temperature. The effective magnetic moment µeff can be obtained from the fitting as 4.15 and 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The results indicate that µeff does not vary considerably with x. Considering the theoretical value of µeff : 4.90 and 3.87µ B /Mn for Mn3+ and Mn4+ , we suggest that both Mn3+ and Mn4+ ions coexist in the present doped samples. The latter will continue to discuss. The temperature dependence of resistivity ρ in logarithmic scale for all the investigated samples is shown in Fig. 5. The experimental data is obtained at zero magnetic field in the temperature range of 5–300 K. As can be seen from Fig. 5(a), for the undoped sample (x = 0), ρ increases monotonously
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Fig. 3. Magnetic hysteresis loops at 5 K for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples. The inset shows the hysteresis loops for Si0.99 Mn0.01 at 5 and 77 K, respectively.
Fig. 4. The temperature dependence of the inverse of the magnetic susceptibility for the sample with x = 0.015. The solid line is the fitting curve according to the Curie–Weiss law.
with decreasing temperature which is a typical feature of semiconductors. For all doped samples, the resistivity has a typical characteristic of a metal–insulator (M–I) transition. It shows a distinct resistive minimum corresponding to the temperature Tρ min (as shown in Fig. 5(b)). That is to say, as T > Tρ min , the sample behaves as a metallic character with a positive resistance temperature coefficient, dρ/dT > 0; as T < Tρ min , the sample shows a semiconducting character with a negative resistance temperature coefficient, dρ/dT < 0. In addition, Fig. 5(b) also displays that Tρ min increases with increasing Mn-doping content x as shown in Table 1. It should be noted that the value of Tρ min is almost in accordance with Tc as observed in the M(T ) measurement. The result suggests that the M–I transition may have a magnetic origin similar to the Ge-based DMSs [17,18], where the M–I transition is assumed to originate from the spin-flip scattering near the magnetic transition. We should note that the M–I transition observed in present samples is “slow” compared with other DMSs, such as GaMnAs, where a well-defined hump is observed at the M–I transition with a spin-scattering
Fig. 5. (a) Temperature dependence of the resistivity for polycrystalline Si1−x Mnx (x = 0, 0.005, 0.01, and 0.015) samples at zero field in a logarithmic scale. (b) The partial enlargement plot of temperature-dependent resistivity. A metal–insulator transition near the respective TC for all studied samples is obviously observed.
Table 1 Magnetic and electrical characteristics of Si1−x Mnx polycrystalline bulk samples Sample
TC (K)
θ (K)
Tρ min (K)
E a (meV)
µeff (µ B /Mn)
x = 0.005 x = 0.01 x = 0.015
225 238 262
222 232 258
221 240 256
24 17 15
– 4.15 4.05
From left to right, the columns indicate sample composition; Curie temperature; Weiss temperature; M–I transition temperature; activation energy; effective magnetic moment.
effect in the corresponding temperature range, respectively [19, 20]. This inconspicuous M–I transition near TC for the doped samples may be the scattering of crystal face of polycrystalline sample. In order to obtain more information from the electronic transport properties, we attempt to fit the low temperature ρ(T )
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Fig. 6. The inverse temperature dependence of the resistivity for polycrystalline Si1−x Mnx (x = 0.005, 0.01, and 0.015) samples at zero field in a logarithmic scale. The solid line represents the linear fit to the data for 80–260 K.
data using several models. It is found that the best fitting result is given by the formula: ρ = ρ0 exp(E a /k B T ) for doped samples (as shown in Fig. 6), where ρ0 is a prefactor, E a is the activation energy, and k B is the Boltzmann constant, which has the typical feature of the thermal activated conduction behavior for semiconductor [15]. From the fitting curves, the activation energy E a calculated by the aforementioned formula is 24, 17, and 15 meV for the samples with x = 0.005, 0.01, and 0.015, respectively (as shown in Table 1). The activation energy decreases with increasing Mn content. The activation energies are compatible with a variable-range hopping conduction in the impurity band formed by the heavy Si doping. The activation energy E a obtained from ρ(T ) data below TC varies from 15 meV as x = 0.005 to 24 meV for x = 0.015. The high values of E a in the corresponding temperature range seem to exclude a variable hopping conduction mechanism in an impurity band. Meanwhile, the activation energies are not corresponding to any acceptor level of Mn in Si. For DMSs showing an insulating character, a percolation model has been suggested by Kaminski and Das Sarma to explain both their electrical and magnetic properties [21,22]. According to this model, at temperatures of the order of TC and below, charge carrier (hole) transport in present samples occurs mainly through nearest-neighbor hopping at the localization sites. The value E a discussed above is the hopping activation energy among these localization sites. To further investigate transport properties, the magnetic field dependence of Hall resistivity ρx y for Si1−x Mnx (x = 0, 0.01, and 0.015) samples is measured at 50 K and the results are shown in the main panel of Fig. 7. For the x = 0 sample, the Hall resistivity increases linearly with increasing magnetic field in the whole measured field range, which is a typical characteristic of semiconducting materials. However, the ρx y (H ) curve for doped samples deviates from the linear behavior at relative low fields (less than 1.0 T). This exotic behavior can be taken into account of the anomalous Hall effect (AHE) [23]. The AHE has also been observed in other DMSs system previously, where the authors ascribed the AHE
195
Fig. 7. Hall resistivity ρx y as a function of magnetic field for polycrystalline Si1−x Mnx (x = 0, 0.01, and 0.015) samples measured at 50 K. The inset shows magnetic field dependence of Hall resistivity ρx y for Si0.99 Mn0.01 at 50, 100, 200, and 300 K, respectively.
to skew-type scattering of polarized carriers by magnetic impurities [24,25]. In general, the Hall resistivity ρx y is expressed as ρx y = R0 B + Rs M, where B is an external magnetic induced intensity, M is a magnetization, R0 and Rs is the normal and anomalous Hall coefficient, respectively. Although the AHE makes a determination of densities of holes be less accurate, an estimate can be obtained from the Hall data at high magnetic field where the magnetization is saturated. The charge carrier concentration p at T = 50 K deduced from the normal Hall coefficient obtained from the fitting of the high field ρx y (H ) curve are 1.6 × 1019 , 6.6 × 1019 , and 3.5 × 1020 /cm3 for the sample with x = 0.005, 0.01, and 0.015, respectively. All samples have positive Hall coefficient, implying that the present samples belong to the p-type semiconductors. The carrier concentration increases monotonously with increasing x. According to the above results, this conclusion that the doped Mn in silicon serves as both acceptor (hole) and magnetic impurity is reasonable. In this scenario, the ferromagnetism induced by Mn in the present samples can be explained as follows: the magnetically ordered phase arises from the indirect interaction between magnetic ions mediated through carriers (holes), because the direct interaction between Mn will result in an antiferromagnetic (AFM) coupling as in bulk Mn [13,26,27]. The inset of Fig. 7 shows the Hall resistivity ρx y plotted as a function of the applied magnetic field at several temperatures below and above TC for the sample Si0.99 Mn0.01 . It can be seen that the AHE only appears at low temperature FM phase, but it is essentially absent at temperatures higher than TC . The result is not difficult to understand, because ferromagnetism disappears above TC . At last, we note that the ferromagnetism in present samples is not related to the second phase of SiMn compounds. As we know, a pure Si does not have a net magnetic moment (as shown in Figs. 2 and 3) and a metallic Mn is an AFM material [28]. In addition, among all the SiMn compounds, only SiMn compound has ferromagnetism with a Curie temperature less than 30 K [29]. Therefore, the observed magnetic properties in
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present samples with TC of around 250 K appear to exclude the possible contribution from the second phase of SiMn compounds. Our prepared Si1−x Mnx compounds are suggested to belong to a Si-based DMSs type.
[7]
4. Conclusion
[9]
In summary, we have prepared polycrystalline Si1−x Mnx DMSs with x = 0, 0.005, 0.01 and 0.015 using the arc melting method. All doped samples undergo a PM–FM transition near 250 K. The effective magnetic moments are 4.15 and 4.05µ B /Mn for the samples with x = 0.01 and 0.015, respectively. The M–I transition around the respective Curie temperature has also been observed in ρ(T ) curves. The low temperature ρ(T ) data for the doped samples can be fitted by the thermally activated model. The activation energy calculated from fitting decreases with increasing x. The distinct anomalous Hall effect is found for Mn-doped Si samples. All these results indicate the possible application of the present system as Si-based diluted magnetic semiconductors.
[8]
[10] [11]
[12] [13] [14] [15] [16]
Acknowledgments [17]
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