Magnetism and Mn Clustering in (In,Mn)Sb Magnetic Semiconductors

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Magnetism and Mn Clustering in (In, Mn)Sb Magnetic Semiconductors Jindong Liu, Micah Hanson, John Peters, and Bruce W. Wessels ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07471 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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ACS Applied Materials & Interfaces

Magnetism and Mn Clustering in (In, Mn)Sb Magnetic Semiconductors Jindong Liu, Micah P. Hanson, John A. Peters, and Bruce W. Wessels*

Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, Illinois 60208, USA

KEYWORDS: magnetic semiconductors, ferromagnetism, InMnSb, XPS, Mn electronic structure, spin-orbit splitting energy, Mn clusters, MOVPE

ABSTRACT: Previously high-temperature ferromagnetism with a Curie temperature in excess of 400K was reported in the magnetic semiconductor (In, Mn)Sb films grown by metal-organic vapor phase epitaxy (MOVPE). To determine the role of Mn distribution on its magnetic properties, the Mn 2p core-level X-ray photoelectron spectroscopy (XPS) of (In, Mn)Sb films was measured. For films grown on an InSb substrate, Mn composition is spatially inhomogeneous and its concentration increases with increasing deposition temperature. Spinorbit splitting energy of the Mn 2p core-level was found to increase with increasing Mn

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concentration. From the dependence of the measured spin-orbit splitting energy on the Mn concentration, evidence of atomic-scale Mn cluster formation was observed. The measured magnetic moment per Mn atom decreases from 3.0µB/Mn to 1.8µB/Mn with increasing Mn concentration, which is attributed to atomic-scale clusters that are ferromagnetic or ferrimagnetic. This detailed investigation gives an insight to the Mn distribution, phase composition and origin of magnetism in MOVPE-grown (In, Mn)Sb magnetic thin films.

1. INTRODUCTION In recent years, significant research efforts have been made on spintronics, which has vast potential in information storage, processing, and spin logic as well as magnetic field sensors.1-3 Magnetic semiconductors prepared by doping host semiconductors with transition metal (TM) elements are considered as an ideal system due to their potential for low power consumption and high spin injection rate for spintronics devices.4,5 Among these materials, (III, Mn)V semiconductors have attracted considerable attention and prototype devices have been demonstrated in both theory and experiment.6-10 Magnetic semiconductors with a Curie temperature (Tc) in excess of 300K are desired for most practical applications. Intrinsic ferromagnetism in transition metal doped III–V compounds has been generally attributed to carrier-mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which relies on the semiconductor layers possessing a high carrier density.11 Thus the narrow-gap semiconductors like (In,Mn)As, (In,Mn)Sb, and (Ga,Mn)Sb, which have shallow Mn acceptor levels and resulting high conductivity, show carrier-mediated ferromagnetism.12-14 However, the origin of ferromagnetism in magnetic semiconductor remains under debate. Previously, carrier-mediated ferromagnetism in (In,Mn)Sb magnetic semiconductors was studied


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via co-doping with Be acceptors, which increases hole concentration independently of Mn concentration.15 In contrast to carrier-mediated RKKY interaction, measurements showed that the Curie temperature decreases rather than increases with Be co-doping.15 Both theory and experiment show growing evidence that correlated substitution of Mn needs to be considered.13,14,16-19 Previous calculations using DFT in magnetic semiconductors indicated that the formation of atomic-scale transition metal clusters is energetically favorable and it can enhance Tc.16 Recently, density-functional theory (DFT) calculations combined with a novel thermodynamic analysis showed that reduced magnetization is not consistent with random distribution of Mn dopants.20 It is proposed that the metal clusters play a role in stabilizing magnetism in transition metal (TM) doped compounds.17 These transition metal clusters, (TM)n where n=2-4, are formed by substitution on adjacent cation sites in the magnetic semiconductors.16-18 In a previous report, we have observed the presence of Mn clusters in (In,Mn)As by X-ray absorption fine structure, which derive from correlated substitution of magnetic atoms.19 These atomic-scale Mn clusters were considered to be responsible for the room-temperature ferromagnetism in the (In,Mn)As films.19 The magnetic behavior of these semiconductors is largely influenced by magnetic atom distribution, electronic structure, and chemical state.14 In previous work, we have synthesized single phase (In,Mn)Sb epitaxial films with a Tc exceeding 400K by metal-organic vapor phase epitaxy (MOVPE).21 Cross-sectional scanning tunneling microscopy (X-STM) measurements reveal that Mn acts as a shallow acceptor in (In,Mn)Sb films at low Mn concentration.22 No further evidence of secondary phase formation is observed by X-STM or high-resolution TEM for In0.98Mn0.02Sb layer.21,22 However, for (In,Mn)Sb with a higher Mn concentration of 0.035, two magnetic phases are observed by X-ray magnetic circular dichroism (XMCD), which was


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attributed to random and correlated substitution of Mn, respectively.23 In contrast, for (In,Mn)Sb films grown on GaAs substrate with a Mn concentration higher than 0.1, hexagonal MnSb and MnAsSb precipitates are observed by TEM.24,25 While MOVPE-grown (In, Mn)Sb epitaxial films show considerable promise as magnetic semiconductor material, little is known about the nature of Mn substitution in the film and its magnetic properties including Tc. As for (In, Mn)Sb films grown by low-temperature molecular beam epitaxy (MBE), Tc of thin films is less than 8K.26 On the other hand, for polycrystalline (In, Mn)Sb which is prepared by direct melting of indium antimonide, manganese, and antimony, a Tc exceeding 600K has been reported.27 The magnetic behavior is attributed to both atomic-scale Mn clusters and MnSb microprecipitates.27 Thus for an understanding of the magnetic properties of MOVPE-grown InMnSb, a detailed investigation of Mn distribution and phase composition is required. X-ray photoelectron spectroscopy (XPS) is a powerful tool for measuring cluster formation in magnetic materials.28-31 From analysis of spin-orbit splitting energy of Mn 2p core levels, detailed information about the local environment and phase composition can be obtained. In this work, XPS analysis of a series of (In, Mn)Sb films deposited on InSb and GaAs substrates was undertaken. We report the dependence of Mn 2p spin-orbit splitting energy on local Mn concentration or phase composition. Spin-orbit splitting energy of the Mn 2p core level increases with increasing Mn concentration. XPS analysis shows that Mn phases composition in the films grown on GaAs strongly depend on deposition conditions. From the XPS analysis, evidence of atomic-scale Mn clustering is obtained. The magnetization measurements show that the magnetic moment per Mn atom decreases with the increasing Mn concentration that is attributed to formation of ferrimagnetic clusters. Correlation of XPS spectra and magnetic behavior in


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MOVPE-grown (In, Mn)Sb films shows that atomic-scale Mn clusters play an important in its magnetic properties. 2. EXPERIMENTAL SECTION MOVPE-grown (In, Mn)Sb films with Mn concentrations ranging from 0.1-13 at. % were deposited on both InSb (100) and GaAs (100) substrates at temperatures ranging from 390420°C. Trimethylindium, trimethylantimony, and methylcyclopentadienyl manganese tricarbonyl (TCMn) were used as the indium, antimony, and manganese source, respectively. Purified hydrogen was the carrier gas for all reactions. Details of the growth have been described previously.21 The typical film thickness was ~200nm. The structure was determined by X-ray diffraction (XRD) using Cu Kα1 radiation. XPS with a monochromatic Al Kα radiation source at a pass energy of 50eV was applied to analyze the chemical composition and electronic structure. The detector of the XPS apparatus has a resolution of 0.1eV. All the XPS spectra were chargecorrected to the carbon 1s peak at binding energy 284.5 eV. Backgrounds in XPS spectra were subtracted using the Shirley method.32 Gaussian line shape analysis was used to determine the exact peak positions. Depth profiling was carried out using a focused ion gun with 3 keV Ar+ with a sputtering rate of ~5.9 Å/s, as determined from measurements using a reference InSb sample calibrated with a profilometer. The XPS 2p energy spectra in the first series of transition metals exhibit characteristic peak separations with a specific value of spin-orbit splitting energy (∆so) for each chemical phase.33 Thus the spin-orbit splitting energy ∆so can be used for chemical phase identification. 33 In addition, for metal clusters, XPS is a method where the cluster size is reflected in the value of spin-orbit splitting energy. 34-38 The effective coordination number can be employed to determine the atom’s local environment.34-38


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Magnetization measurements are performed using a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS-7, Quantum Design). All the magnetization data of (In, Mn)Sb films were corrected by subtracting diamagnetic susceptibility of InSb (GaAs) substrate. 3. RESULTS AND DISCUSSION 3.1 Compositional Analysis using XPS. The Mn distribution in (In, Mn)Sb film was determined using XPS in conjunction with argon ion sputtering. The MOVPE deposition conditions and Mn atomic fractions obtained from XPS are summarized in Table 1. We assign sample labels for films deposited on InSb and GaAs at z °C with a Mn flow rate of y sccm as IFy -Tz and G- Fy -Tz, respectively. For example, we refer to the (In, Mn)Sb film grown on InSb substrate at 400°C using a Mn precursor flow rate of 5sccm as sample I-F5-T400. In order to determine the effect of deposition temperature on Mn distribution in (In, Mn)Sb films, we measured the Mn concentration depth profiles for films grown at various deposition temperatures, as shown in Figure 1. Figure 1(a) shows the measured Mn atomic fraction depth profiles for the films deposited on InSb substrate for three different deposition temperatures (390, 400, and 420 °C). All profiles in Figure 1(a) show that the Mn composition is spatially inhomogeneous. There is a corresponding broader profile for the Mn distribution in the films for a higher deposition temperature. The total amount of Mn incorporated also increases with increasing deposition temperature presumably due to a more efficient pyrolysis of the Mn precursor. The concentration is highest at ~60nm and decreases towards the layer/substrate interface.


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Table 1. Experimental Deposition Conditions, Mn Atomic Fractions, and Saturation Magnetic Moments per Mn Atom of (In, Mn)Sb Filmsa

Sample

FMn (sccm)

Td (°C)

CMn (at. %)

µ (µB/Mn atom)

Substrate

I-F5-T390

5

390

0.1

3.0

InSb

I-F5-T400

5

400

0.5

2.2

InSb

I-F5-T420

5

420

1.7

2.1

InSb

I-F10-T420

10

420

4.2

1.9

InSb

I-F15-T420

15

420

10

2.0

InSb

I-F25-T400

25

400

3.6

-

InSb

I-F25-T420

25

420

13

1.8

InSb

G-F5-T400

5

400

0.8

-

GaAs

G-F5-T410

5

410

-

-

GaAs

G-F5-T420

5

420

-

-

GaAs

G-F10-T420

10

420

-

-

GaAs

G-F25-T420

25

420

-

-

GaAs

a

FMn, Td, CMn, and µ are the Mn flow rate, deposition temperature, spatial average value of Mn

atomic fraction over the entire film depth, and saturation magnetic moment per Mn atom at 5K, respectively. The Mn concentration depth profiles for (In, Mn)Sb films deposited on GaAs substrate are shown in Figure 1(b). The depth profiles in Figure 1(b) clearly differ from those shown in Figure 1(a). For the samples grown at 400 °C, the Mn profile at shallow depth (less than 150nm) is similar to that in films grown on InSb substrate. Near the interface between (In, Mn)Sb layer and


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GaAs substrate, Mn fraction rapidly drops to zero. For the samples grown at temperatures of 410 and 420°C, a large accumulation of Mn near the interface between film and substrate is observed. No Mn can be detected in the film layer far from the interface within the experimental resolution. Upon comparison between the depth profile of two samples (G-F5-T420 and G-F5T410), we find that larger amounts of Mn are detected near the interface for the samples deposited at 410°C. This phenomenon is also observed for the samples grown using a larger Mn precursor flow rate of 25sccm at 410 and 420°C (not shown here).

Figure 1. Mn atomic percentage depth profiles of (In, Mn)Sb films grown at various temperatures on (a) InSb and (b) GaAs substrate. The Mn concentration depth profiles for the samples deposited at 420°C on InSb and GaAs substrates using three flow rate values (5, 10, and 25 sccm) are presented in Figure 2. As shown in Figure 2(a), the spatial average Mn atomic fraction increases with increasing flow rate for the samples grown on InSb. The Mn concentrations are listed in Table 1. As can be seen from Figure 2(b), for the three films grown on GaAs substrate, the Mn profiles all have a similar distribution. The Mn composition turns out be distributed over a broader range near the interface as the Mn


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flow rate increases. From this data, we conclude that Mn flow rate plays an important role on the Mn concentration but doesn’t influence the shape of Mn distribution profile.

Figure 2. Mn atomic percentage depth profiles of (In, Mn)Sb films grown at 420°C using three Mn precursor flow rates (5, 10, and 25 sccm) on (a) InSb and (b) GaAs substrate. 3.2 Phase Composition Analysis using XPS. The local chemical environment surrounding substitutional Mn and phases present in (In, Mn)Sb films were determined. It has been proposed that Mn atomic clusters or Mn-containing nanoprecipitates can exist in (In, Mn)Sb layer, which can be the origin of the high-temperature magnetism.14,19 To determine the local environment and chemical state of Mn in the film, the peak shift of Mn core-level XPS spectra was measured. In XPS analysis, the multiplet splitting can be used in identifying phase composition. Because of spin-orbit coupling, the Mn 2p spectrum shows two peaks (Mn 2p3/2 and Mn 2p1/2), separated by a spin-orbit splitting energy ∆so. The values of ∆so for atomic manganese, metallic phase Mn and compound MnSb are 10.6, 11.25, and 11.6eV, respectively.39-41 Figure 3 shows the Mn 2p corelevel XPS spectra of the samples grown on InSb at 420°C. All the spectra shown are measured at the depth where there is a maximum Mn concentration in the films. The values of ∆so extracted


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from the spectra in Figure 3 show a decrease from 11.1 to 10.6 eV with reducing Mn concentration in (In, Mn)Sb, revealing a change of the component phases. The position of the Mn 2p1/2 component peak remains almost unchanged while the Mn 2p3/2 component peak moves to higher energy as the Mn concentration decreases. The three Mn spectra can’t be associated with any possible Mn-containing bulk compounds since no corresponding values of ∆so are found in the standard XPS spectra of manganese compounds.40 Thus the decrease of spin-orbit splitting energy is not due to different bulk compounds. However, for atomic-scale clusters, the spectral characteristics depend mostly on the average coordination of the cluster atoms and different ∆so values from that of bulk can be obtained.34-38 The effective coordination number is reduced as metal cluster size decreases, resulting in a narrowing of splitting energy. From bulk towards atomic, a decrease of ∆so has been observed in various metal clusters. 34-38 Since the possible presence of Mn atomic clusters in (In, Mn)Sb has been previously proposed, 19 it is reasonable to attribute the observed decrease of ∆so to the reduction of atomic Mn cluster size.


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Figure 3. Mn 2p core-level spectra measured at the depth with maximum Mn concentration for (In, Mn)Sb layers grown on InSb at 420°C. The local Mn concentration values range from 2.3 to 14.1 at. %. The spin-orbit splitting energy ∆so for each spectrum has been marked in the figure.

Since an inhomogeneous Mn distribution has been determined in (In, Mn)Sb by XPS composition depth profile, the phase composition distribution along the depth in an identical sample is of interest. Figure 4(a) and (b) show the Mn 2p core-level spectra obtained at various depths in two samples (I-F25-T420 and I-F10-T420), respectively. According to the Mn depth profiles, local Mn concentration value varies with the depth in the film. The corresponding values of local Mn concentration have been marked near each curve. From the spectra of sample I-F25-T420 in Figure 4(a), it is seen that the spin-orbit splitting energy ∆so decreases from 11.1 to 10.6 eV with reducing local Mn concentration in the range from 14 to 4 at. %. A similar dependence of ∆so has also been observed for sample I-F10-T420 in Figure 4(b): the energy ∆so decreases from the maximum value 10.8 to 10.6 eV with decreasing Mn concentration. For an identical sample, splitting energy and intensity of spectra change successively with local Mn concentration. By comparing the Mn 2p core-level spectra in two samples (I-F25-T420 and I-


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F10-T420), we note that same value of ∆so is obtained in these two samples with approximate local Mn concentrations. Hence, for (In, Mn)Sb films deposited at same temperature (420°C) but different Mn precursor flow rate, spin-orbit splitting energy ∆so just depends on the local Mn concentration.

Figure 4. Mn 2p core-level spectra obtained at different depths within the same film for samples grown with two different Mn flow rates on InSb at 420 °C: (a) I-F25-T420 and (b) I-F10-T420.

Figure 5 shows Mn 2p core-level spectra of a sample (I-F25-T400) obtained at three different depths so as to determine the distribution or phase composition in the sample grown at a lower temperature (400°C). The energy ∆so also decreases with the reduction of local Mn concentration. It should be noted that the value of ∆so with Mn concentration~6.6 at. % is 11.6eV, which equals the splitting energy value for the MnSb compound.41 In contrast, for the samples grown at 420 °C, the value of ∆so with approximate same local Mn concentration (~6.3 at. %) is 10.8eV, as shown in Figure 4(a). Thus even with the approximate same Mn


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concentration, the ∆so values differ for the samples deposited at 400 and 420°C presumably due to differences in Mn coordination.

Figure 5. Mn 2p core-level spectra obtained at different depths in sample I-F25-T400 grown on InSb at 400 °C. Spectrum shown by blue curve is magnified by 5 times to clarify the spin-orbit splitting energy ∆so.

To determine the dependence of coordination environment on deposition temperature, we have measured the spin-orbit splitting energy ∆so of Mn 2p spectra as a function of local Mn concentration for the films grown at two different temperatures (400 and 420°C). The plot of the spin-orbit splitting energy ∆so vs local Mn concentration for two samples (I-F25-T400 and I-F25T420) are shown in Figure 6. As can be observed from the curve of the sample with deposition temperature of 400°C (I-F25-T400), ∆so remains at a constant value of 10.6 eV up to 2.5 at. %, which is the same ∆so value as that of atomic Mn. A similar dependence has been observed in Au clusters when the number of atoms per cluster n is smaller than 20.35 For Mn concentration between 2.5 at. % and 4.5 at. %, the energy ∆so increases from 10.6 to 11.6 eV, indicating the


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formation of secondary phase precipitate MnSb at the higher concentration. At higher local Mn concentration, ∆so remains at 11.6 eV, which is characteristic of MnSb.41 The existence of MnSb precipitates in (In, Mn)Sb films with Mn concentration higher than 4.5 at. % is consistent with our previous work on formation of Mn-rich layers.24 On the other hand, for the sample grown at 420°C, ∆so as a function of local Mn concentration shows different dependence: ∆so remains unchanged at 10.6 eV up to ~4 at. %; for intermediate local Mn concentration, ∆so increases successively with increasing local Mn concentration, much more slowly than the increase rate of the sample grown at 400°C. This increase of splitting energy can be attributed to increase of atomic-scale Mn cluster size. 34-38 Hence, it is concluded that the Mn coordination or phases present in MOVPE-grown (In, Mn)Sb films depends on deposition temperature.

௅ Figure 6. Measured Mn 2p spin-orbit splitting ∆so as a function of local Mn concentration ‫ܥ‬ெ௡

for the samples grown on InSb at two different temperatures. Arrows indicate reference values in a free manganese atom, metal Mn and compound MnSb.


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3.3 Magnetic Properties. To determine the magnetic properties of the (In, Mn)Sb films, first we show in Figure 7 the hysteresis curves of magnetization (M) vs magnetic field (H) for the films grown on both InSb and GaAs substrate using a very low Mn flow rate of 5sccm and two different deposition temperatures (400 and 420 °C). No secondary phase peaks can be detected from the XRD patterns (not shown here) for all the four samples. Nevertheless, as can be observed from Figure 7, all the measured films show clear hysteretic behavior with coercive fields (Hc) ranging from 400 to 1000Oe. On the other hand, for an identical sample deposited on InSb substrate, the hysteresis loops with similar shape are observed at 5 and 300K. The presence of hysteresis indicates that the films are ferromagnetic and not superparamagnetic. The measured saturation magnetizations (Ms) just decreases slightly from 5 to 300K. Larger magnetization values are obtained for the sample deposited at a higher temperature. Furthermore, for the (In, Mn)Sb films grown on GaAs substrate, magnetic behavior differs for the two deposition temperatures. For the one grown at 400°C, the M vs H curves exhibit similar values for Ms and Hc at 5 and 300K, as shown in Figure 7(c). In contrast, a large difference in both Ms and Hc between 5 and 300K is clearly observed for the sample grown at 420°C (G-F5-T420) in Figure 7(d). This difference indicates that different magnetic species exist in the (In, Mn)Sb on GaAs.


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Figure 7. Hysteresis loops (M vs H) of (In, Mn)Sb films grown using a Mn flow rate of 5sccm and deposition temperatures of (a) 400°C on an InSb substrate, (b) 420°C on an InSb substrate, (c) 400°C on a GaAs substrate, and (d) 420°C on a GaAs substrate.

Figure 8 presents field-cooled (FC) (500Oe) temperature-dependent magnetization M vs T curves for these four samples. As can be seen from M vs T curves of all the samples except the one deposited on GaAs at 420°C (G-F5-T420), the magnetization is only weakly temperature dependent over the whole measured temperature range with a Tc higher than 400K. In contrast, for film on GaAs (G-F5-T420), a convex M vs T curve with Tc ~350K is observed, indicating the


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presence of a different magnetic phase. Combined with the XPS analysis of Mn composition depth profiles, the distinct magnetic behavior in this film is attributed to Mn accumulation near the film/substrate interface. Since a similar Tc value is reported for the MnAs, the ferromagnetic component with Tc~350K in this sample is attributed to interfacial MnAs nanoprecipitates.42 For (In, Mn)Sb layers grown on InSb substrate, M vs T curves indicate that Curie temperature is higher than 400K. The high-temperature magnetism is attributed to the atomic-scale Mn clusters in the (In, Mn)Sb films.

Figure 8. Field cooled (500Oe) magnetization curves (M vs T) of (In, Mn)Sb films grown using a Mn flow rate of 5sccm and two deposition temperatures (400 and 420°C) on InSb and GaAs substrate. Films grown on InSb have a Tc greater than 400K.

The detailed dependence of magnetic properties on TM concentration can be obtained by considering small cluster formation. Figure 9(a) shows possible configurations for small substitutional Mn clusters Mnn where n=2-4. Here we assume only cation interactions. For Mn clusters, the magnetic properties largely depend on the cluster size and local Mn environment.43-


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Even for the simplest case of dimer molecule Mn2, both ferromagnetic and antiferromagnetic

spin states have been reported.48,49 In Figure 9(b), we show saturation magnetic moment per Mn atom values (µ) at 5K of all the investigated (In, Mn)Sb layers grown on InSb substrates. One sample (I-F25-T400) was not included since MnSb precipitates were observed by XPS. As shown in Figure 9(b), the value of µ decreases with increasing Mn concentration. A value of µ ~3.0µB/Mn is obtained at ~0.12 at. %, which is consistent with previous calculations for metallic Mn nanostructures.44 The magnetic moment decreases to less than 2µB/Mn for Mn concentration higher than 4 at. %, which is attributed to increase of atomic-scale Mn cluster size.45,46 The structures, magnetic moments per Mn atom µ, and binding energies Eb (the energy required to decompose the isomers) of small metallic Mn clusters Mnn (n=2-7) have been studied by Longo et al. using DFT calculations.46 The most stable isomers of Mnn (n=2-4) reported by Longo et al. are shown in Figure 9(a). For Mn dimers (Mn2), the most stable Mn2 spin state is ferromagnetic (FM) with µ =5µB/Mn. The antiferromagnetic (AFM) spin state, however, is also considered to be stable, with a binding energy only 0.11eV lower that of the FM state.46 Thus for samples consisting of Mn dimers Mn2, the magnetic moment per Mn atom µ is higher than 2.5µB/Mn. In the case of Mn trimers (Mn3), there are two stable spin arrangements for an equilateral triangle: one FM with µ =5µB/Mn and one ferrimagnetic (FIM) with µ =5/3 µB/Mn. The binding energy difference between the FM and FIM spin state is just 0.03eV. 46 For Mn tetramers (Mn4) the AFM spin state with µ =0µB/Mn and FIM spin state with µ =2.5 µB/Mn are nearly equienergetic.46 As for samples consisting of Mn trimers Mn3, we assume that the average magnetic moments per Mn atom µ equal the average value of two stable spin state (3.3 µB/Mn) due to the negligible energy difference. Similarly, we assume that the average value of µ is 1.25µB/Mn for samples consisting of Mn tetramers Mn4. For the cluster Mnn with n>4, the DFT


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calculations indicate the energy preference for an AFM spin state.

45,46

Thus the average

magnetic moment value decreases with increasing cluster size.45,46 All the atomic Mnn clusters for n=2, 3, and 4 can be easily formed in the zinc-blende structure of (In, Mn)Sb by substituting Mn on cation sites.14,16-19 The (In, Mn)Sb films with low Mn concentration exhibit ferromagnetism since single substitutional Mn is stable.14 At higher Mn concentration, dimers with both FM and AFM spin state are formed, resulting in a decrease in magnetic moments per Mn atom µ. We attribute the higher magnetic moment value of 3µB/Mn obtained in (In, Mn)Sb films with Mn concentration4), which decreases spin state.


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Figure 9. (a) Geometries of atomic-scale Mnn (n=2, 3, and 4) clusters reported in Ref. 46, with their magnetic moments per Mn µ and binding energy Eb. The Mn atoms shown in black have “spin-up” (majority) magnetic moments, whereas the atoms shown in yellow have “spin-down” (minority) magnetic moments. (b) Saturation magnetic moment per Mn atom of InMnSb films at 5K, plotted against Mn concentration. (c) Field cooled (500Oe) M vs T curve of sample I-F10T420.


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As aforementioned, XPS analysis of the spin-orbit splitting also indicates an increase of Mn cluster size for the (In, Mn)Sb films with local Mn concentration higher than 4 at. %. For a sample (I-F10-T420) with maximum local Mn concentration ~6 at. %, Mn clusters Mnn with large size (n>4) presumably exist in this film due to the increase of spin-orbit splitting energy value, which results in a decrease of the magnetic moment. 45,46 Figure 9(c) shows the fieldcooled (500Oe) M vs T curve of sample I-F10-T420. The magnetization increases with increasing temperature from 300 to 400K, indicating the presence of an AFM interaction. This increase agrees well with the previous calculations reported by Bobadova-Parvanova et al., which indicate that the optimal spin state for Mnn clusters with n>4 changes from FM to AFM and the magnetic moment value decreases with increasing cluster size.45 For (In, Mn)Sb films with Mn concentration higher than 4 at. %, the magnetism stems from a combination of Mn clusters in FM, FIM and AFM spin states. Thus the magnetization measurements in conjunction with XPS analysis indicate that the high-temperature magnetism in (In, Mn)Sb films originates from atomic-scale Mn clusters in the p-doped InSb matrix. As to stabilization of hightemperature ferromagnetism in the (In, Mn)Sb, the M vs H curves showed well-resolved hysteresis. No supermagnetism is observed, indicating that the ferromagnetic state is stable. This work is instructive for spintronics device applications as well as for fundamental understanding of high-temperature magnetism. Magnetic semiconductor (In, Mn)Sb is a promising material for future spintronics devices due to its high carrier mobility. 50 Mn clusters, acting as spin-dependent scattering center, decrease hole mobility in (In, Mn)Sb films. 50 Even so, the hole mobility in (In, Mn)Sb films is two orders of magnitude higher than (Ga,Mn)As.50,51 Moreover, large effective g-factors have been previously observed in (In, Mn)Sb films, indicating that (In, Mn)Sb should be suitable for magnetic bipolar transistors.52 The effective g-


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factors and therefore the magnetoresistance can be tuned by varying Mn concentration in Mndoped magnetic semiconductors.53 This work demonstrates the potential of controlling the magnetic properties by the variation of Mn concentration and phase composition, indicating that (In, Mn)Sb is a suitable material for the realization of novel spin logic devices. 4. CONCLUSIONS The Mn distribution and phase composition in MOVPE-grown (In, Mn)Sb films were determined using XPS. XPS composition depth profiles demonstrate that Mn distribution depends on deposition temperature. The spin-orbit splitting energy ∆so of Mn 2p core-level spectra as a function of local Mn concentration shows that the phase composition in MOVPEgrown (In, Mn)Sb films depends on deposition temperature. The spin-orbit splitting energy ∆so of Mn 2p core level increases with increasing Mn concentration and it is attributed to the increase of atomic-scale Mn cluster size. The measured magnetic moment per Mn atom decreases with increasing Mn concentration which is attributed to a model whereby the atomic-scale clusters are either ferromagnetic or ferrimagnetic depending on cluster size. The magnetization properties in conjunction with XPS analysis indicate that atomic-scale Mn clusters are responsible for the high-temperature magnetism in (In, Mn)Sb films. These results demonstrate the potential of controlling the magnetic properties of (In, Mn)Sb films by the variation of Mn concentration or phase composition. Furthermore, XPS and magnetization measurements enable correlation between magnetization, Mn distribution, and phase composition in the MOVPE-grown (In, Mn)Sb films.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was supported by the NSF under grant DMR-1305666. Use of the Center for Nanoscale Materials at Argonne National Lab, supported by Department of Energy, Office of Science, Office of Basic Energy Sciences (Grant No. DE-AC02-06CH11357), is acknowledged. Extensive use of the microfabrication facilities of the Materials Research Center at Northwestern University supported by the NSF (No. DMR-1121262) is acknowledged. Jindong Liu acknowledges the support from the China scholarship council (File No. 201306160050).

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Magnetism and Mn Clustering in (In,Mn)Sb Magnetic Semiconductors

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