Nano Res

1

Growth of skyrmionic MnSi nanowires on Si: critical

importance of the SiO2 layer

Siwei Tang1,2

, Ivan Kravchenko2, Jieyu Yi

1,2, Guixin Cao

2, Jane Howe

3, David Mandrus

1 () and Zheng

Gai2 ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0538-4

http://www.thenanoresearch.com on July 7, 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0538-4

TABLE OF CONTENTS (TOC)

Growth of skyrmionic MnSi nanowires on Si: critical

importance of the SiO2 layer

Siwei Tang1,2, Ivan Kravchenko2, Jieyu Yi1,2, Guixin Cao2,

Jane Howe3, David Mandrus1,* and Zheng Gai2,*

1 Department of Materials Science and Engineering,

University of Tennessee, Knoxville, TN 37996, USA 2 Center for Nanophase Materials Sciences, Oak Ridge

National Laboratory, Oak Ridge, TN 37831, USA 3 Hitachi High Technologies Inc., Rexdale, ON M9W

6A4, Canada

The thickness of the SiO2 layer on the Si substrate plays the key role in

obtaining high yield growth of cubic B20 MnSi skyrmion nanowires.

A growth phase diagram was constructed based on systematic studies

of various growth conditions.

0.0 0.2 0.4 0.6 0.8-0.1

0.0

0.1

0.2

0.3

0.4

0.5 B+

A2

0dM

/dB

, R

e

ac

B (T)

0dM/dB

Re ac

Im ac

T = 28 K

Bc2B

c1

B-

A1

0.0

0.1

0.2

M

M ( f.u

.)

Growth of skyrmionic MnSi nanowires on Si: critical

importance of the SiO2 layer

Siwei Tang1,2

, Ivan Kravchenko2, Jieyu Yi

1,2, Guixin Cao

2, Jane Howe

3, David Mandrus

1 () and Zheng

Gai2 ()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Skyrmion, spintronics,

oxide-assisted growth,

nanowires,

magnetic materials

ABSTRACT

MnSi in B20 structure is a prototypical helimagnet that forms a skyrmion lattice,

a vortex-like spin texture under applied magnetic field. We have systematically

explored the synthesis of single crystal MnSi nanowires via controlled

oxide-assisted chemical vapor deposition and observed a characteristic

signature of skyrmion magnetic ordering in the MnSi nanowires. The thickness

of the SiO2 layer on the Si substrate plays the key role for obtaining a high yield

of B20 MnSi skyrmion nanowires. A growth mechanism was proposed that is

consistent with the existence of an optimum SiO2 thickness. A growth phase

diagram was constructed based on the extensive studies of various growth

conditions for various MnSi nanostructures. The persistence of both the

helicoidal and skyrmion magnetic ordering in the one-dimensional wires was

directly revealed by ac and dc magnetic measurements.

Introduction

A magnetic skyrmion lattice, a vortex-like spin

texture recently observed in chiral magnets, is of

great interest to future spintronic data storage and

other information technology applications[1–14]. The

origin of the magnetic skyrmion phase can be traced

to the anti-symmetric Dzyaloshinski-Moriya (DM)

interaction that is allowed in space groups lacking

inversion symmetry [15–17]. The combined effect of a

large ferromagnetic exchange and a weak DM

interaction is to twist the magnetization into a

long-period spiral that can be tens to hundreds of

nanometers in length. As these spirals are only

weakly bound to the underlying lattice in cubic

systems, they can be readily manipulated with

modest applied fields. Prototypical materials with

skyrmion ordering are those compounds with cubic

B20 structure, like MnSi and FeGe. The skyrmion

lattice in MnSi appears in a small region (known as

the A phase) of the H-T phase diagram in bulk

samples, but in 2D samples like thin films the

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Zheng Gai, gaiz@ornl.gov; David Mandrus, dmandrus@utk.edu.

Research Article

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2 Nano Res.

skyrmion phase is much more robust[4,11,14,18]. It is

of great interest to determine the properties of the

skyrmion phase in quasi-1D nanowires to see the

effect of an extra dimensionality limitation. The

persistence of the skyrmion ordering in

one-dimensional MnSi nanowires may be very

promising for spintronics applications as the

magnetic domains and individual skymions could be

manipulated with small currents[8,10,19]. Recently,

several reports have appeared about the successful

synthesis of high-quality MnSi nanowires via

chemical vapor deposition, using MnCl2 or Mn vapor

as a precursor gas [20–23]. Real-space observations

have confirmed the existence of skyrmions in

quasi-1D MnSi nanowires by using Lorentz TEM [24].

In this paper, we have systematically explored the

synthesis of free-standing single crystal MnSi

nanowires via controlled oxide-assisted chemical

vapor deposition. We have also observed the

characteristic signature of skyrmion magnetic

ordering in MnSi nanowires with magnetization

measurements. In contrast to the other MnSi wire

growth techniques and the conventional

oxide-assisted growth method employed in the

growth of Si nanowires [25–29], in this work the SiO2

layer on a Si substrate was used to assist and

precisely control the growth process. The thickness of

the SiO2 layer plays a critical role in order to obtain a

high yield of stoichiometric, well-crystallized B20

MnSi nanowires; a growth phase diagram was

constructed based on the investigated growth

parameters. The ac and dc magnetic properties of

MnSi nanowires reveal the persistence of both the

helicoidal and skyrmion magnetic ordering in the

one-dimensional wires.

1 Experimental

1.1 Sample Preparation.

All the samples were synthesized in a

custom-designed chemical vapor deposition (CVD)

system. The system has precise control of growth

temperature, gas flow rate, gas pressure, and

distance from substrate to precursor, as schematically

shown in Fig. 1(a). In order to get reproducible

growth, it was necessary to first pump the system

down to a fixed pressure (3×10-2 Torr for all the

samples) before filling with argon gas. Mn powder

was spread flat on the bottom of a porcelain crucible,

and then a Si(001) wafer with different SiO2 thickness

(0, 1, 50, 100, 200, 300, 500 and 1000 nm) was placed

above (avoid direct contact) the Mn powder, with the

surface of the wafer face down. The distance between

the Mn powder and Si wafer was adjusted using

layers of supporting wafers of Si to avoid

contamination from other elements. The Mn powder

was evaporated and deposited onto the Si wafer

under an Ar gas environment. The Ar gas is used to

adjust the environmental pressure in order to control

the Mn vapor; the flow rate was varied from 140 to

300 sccm (standard cubic centimeters per minute),

and the pressure was maintained between 50 and 625

Torr. The syntheses were carried out at temperatures

between 750°C and 950°C, lasting from 40 minutes to

160 minutes. Hydrogen terminated Si wafers were

produced by HF acid etching which removes the

native oxide [30]; they are contamination-free and

chemically stable for subsequent processing as the

surface silicon atoms are covalently bonded to

hydrogen. An oxidation free Si(001) surface can be

achieved after annealing the hydrogen terminated Si

wafer to 600C in an Ar gas enviroment.

Controllable SiO2 layers were thermally grown on Si

substrates at 1200°C. There are 6 parameters involved

in the MnSi nanowire growth (growth temperature,

duration of growth, Ar gas flow rate, Ar pressure,

distance between the wafer and the precursor, and

SiO2 layer thickness). After extensive initial screening

tests, the optimized growth conditions for

stoichiometric and crystalline MnSi wires was

determined to be 850°C, 80 minutes, 140 sccm, 625

Torr, 500 m from sample to precursor, and 100 nm

of SiO2 on the substrate.

1.2 Analytical Techniques.

A Zeiss Merlin Scanning electron microscope (SEM)

equipped with a Bruker SDD energy dispersive X-ray

spectrometer (EDX) were used to characterize the

microstructure and composition of the nanowires in

this study. Transmission electron microscopy (TEM)

was also carried out on selected specimens, using a

Hitachi HF-3300 TEM/STEM at 300 kV. The EDX

analysis was carried out in both the SEM and TEM.

For the SEM analysis, the nanowires were analyzed

either on the original substrates or after dry transfer

to SEM carbon tapes. For the TEM study, the

nanowires were first removed from the substrate and

transferred onto a holey carbon film supported by a

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3 Nano Res.

200-mesh copper grid. The results were consistent.

The data shown in this manuscript are from

SEM/EDX. The optimized growth was repeated

many times. For each growth, SEM/EDX mapping

was performed on at least 4 nanowires. All the

nanowires were confirmed to be single crystal

although some of the wires were covered by

amorphous silicon dioxide layer. X-ray diffraction

was used to confirm the primary phase of the

samples (peaks of MnSi, Mn5Si3 and Mn4Si7). As the

signal from the wires was very small compared to the

substrate, the omega offset was set to 2-3° to

minimize the substrate signal. For magnetization

measurements, dc and ac magnetic properties of the

synthesized material were characterized with a

Quantum Design magnetic property measurement

system (MPMS)[31], [32] with and without substrates.

No qualitative differences were observed. The data

shown in this paper were obtained without

substrates. The nanowires were transferred by gently

applying Kapton tape to the substrate to pick up

many nanowires and then firmly affixing the tape to

a clean silicon substrate. The tape and clean silicon

substrate were checked and confirmed to be

diamagnetic. The presence of the wires on the tape

was verified using SEM and EDX.

Figure 1 Controlled growth of the MnSi nanowires. (a)

Schematic drawing of the bespoke chemical vapor deposition

system which has temperature, pressure, gas flow rate, distance

between sample and precursor, as well as O2 controls. SEM

images of nanowires prepared on Si substrate with different SiO2

thicknesses are shown in (b) 100nm, (c) 300 nm, (d) 500 nm and

(e) 1000 nm.

2. Results and discussion

The growth of MnSi nanowires was first tried on

both a Si wafer with a natural oxidation layer of a

few nanometers and hydrogen terminated Si wafers

without oxidation layer. Following the procedure

described in Refs. [20,21] and fine-tuned in our study

(experiment section), MnSi nanowires were only

occasionally found to grow around the edges of

wafers. We found that the number of nanowires

that will grow on an oxidation-free Si wafer is very

small. Furthermore, while somewhat more wires

were observed to grow on wafer with a native oxide

layer, the yield was still small. Interestingly, we

noticed that if the growth was conducted under a

poor vacuum (the base pressure of the growth

system worse than 3×10-2 Torr), the number of wires

on the silicon wafer increased dramatically. This

observation clearly indicates that the growth is not a

simple physical vapor deposition of Mn on Si, but a

chemical reaction with O2 is involved.

To verify and quantitatively control the effect of

oxygen on the growth of MnSi wires, the thickness of

the SiO2 layer on the Si substrates was varied in

series of comparison experiments. For precise

comparison, the growth was repeatedly carried out

with all the other experimental conditions remaining

exactly the same while changing the SiO2 thickness

from 0 nm up to 1000 nm. The number of wires on

the substrate increased dramatically when the SiO2

layer was introduced. Fig. 1 and Fig. 2 show the

morphological, crystalline and compositional results

from wafers with different SiO2 layer thicknesses.

The diameter of the nanowires is in the range of 100

to 600 nm, and the length is of the order of tens of

micrometers. Fig. 1(b) to (e) are the SEM images of

the wires from Si wafers with 100 nm, 300 nm, 500

nm and 1000 nm SiO2 layer thicknesses, respectively.

Wires from the 100 nm SiO2 wafer have the best

morphology in terms of quantity, size uniformity and

even distribution on the surface. The crystalline

structures of the wires on various SiO2 thicknesses

are very different, too (Fig. S1).

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4 Nano Res.

Figure 2 Growth phase diagram of the well-controlled growth of

MnSi nanowires. (a) Morphology of nanowires prepared under

optimal growth conditions (100 nm SiO2 thin film, 600 Torr,

850 °C, 140 sccm and 500µm distance). The inset shows the

electron diffraction pattern and TEM image of one wire. (b)

Comparison of EDX spectra from individual nanowires grown on

three different SiO2 thicknesses. The inset shows Mn and Si

chemical element mapping on two wires. Growth phase diagram

obtained from samples prepared under different experiment

parameters (c) temperature and (d) distance vs growth time. The

final batch of samples are marked using diamonds (see text).

The Mn/Si composition ratio of the wires can be

found using EDX analysis. Shown in Fig 2 (a) is an

SEM image of nanowires grown on 100 nm SiO2 at

850°C, for 80 minutes. The TEM image and electron

diffraction pattern confirm the single crystal nature

of the wires. Fig. 2 (b) shows the comparison of the

EDX spectrum (normalized to the Si peak) from

individual wires of samples with different oxide

layer thicknesses. The atomic ratio of Mn/Si from

EDX depends on the SiO2 layer thickness. The Mn/Si

ratio varies from 0.6 (low oxide thickness) to around

5:3 (thickness of 200 nm and above) (Fig. S2).

These observations point to the existence of an

optimum SiO2 layer thickness for the growth of MnSi

nanowires with high yield and correct stoichiometry

and reveal the significant role of the SiO2 layer in the

growth, which can be understood as an

oxide-assisted growth. The elemental Si required for

the MnSi wires is unlikely to come from the surface

of SiO2 layer for two reasons: the growth behavior is

strongly dependent on the thickness of SiO2 layer,

and the reaction temperature (850 C) is much lower

than the dissociation temperature of SiO2 (1400 C).

The contribution of Si from the silicon spacers cannot

be ruled out, but the fact that the stoichiometric and

B20 MnSi wires grow well only on 100 nm SiO2 layer

wafer proves the key role of the thickness of the SiO2

layer, because the contribution of the Si spacers is the

same for all the samples. These results also suggest

the role of SiO2 in assisting the growth, as the direct

supply of Si from the spacers is constant and

unlimited for all samples. The similar sensitivity to

SiO2 thickness was also observed in the growth of

other B20 silicide nanowires although the SiO2

thickness was much thinner (1-2nm), and the authors

pointed out that “a new NW growth mechanism

might be at play” [33–35].

There is an optimum SiO2 thickness for the growth

(100 nm in our specific setup), in which the supplies

of SiO, Si and Mn reach an equilibrium condition.

This fact eliminates the most straightforward growth

mechanism in which Mn directly chemically reacts

with SiO2, because it does not utilize the “thickness”

of the SiO2 layers. At elevated temperature, the

intermediate product SiO can be formed by chemical

reaction of at the interface of the Si

wafer and the adjacent SiO2 layer [36,37]. The initial

formation of the SiO at the interface should have

happened during the growth of the SiO2 layer

(1200C). The two key roles that SiO plays during

the growth are: first, after diffusing from the interface

to the surface of wafer, SiO acts as a nucleation site

which adsorbs surrounding Si and Mn atoms for the

one-dimensional wire growth. It was observed

previously in Si nanowire growth by laser ablation

that much higher nanowire yield could be achieved

by introducing SiO2 powder into the Si target to form

SiO [38]. Secondly, during the diffusion process from

the Si/SiO2 interface to the surface, the reverse

reaction also takes place, creating Si interstitials, and

the reaction repeats at the newly created interfaces.

This diffusion process not only diffuses SiO from the

interface to surface, but also largely enhances the

self-diffusivity of Si atoms through the SiO2 layer to

the surface [37] to achieve correct Mn and Si

stoichiometry. The significant diffusion of SiO and

reverse reactions could happen around 900C or even

lower [39–41]. The mechanism by which the

decomposition of SiO2 to SiO occurs in the

temperature range of 900-1050C is not very clear,

although it was proposed based on TEM and SEM

observations that after the formation and lateral

development of holes in the oxide, elemental Si

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5 Nano Res.

inside the holes supplies Si for reaction with SiO2 at

the periphery, so the volatile product of SiO can be

formed [39][40]. At the surface of the SiO2, chemical

reactions such as ,

or are needed

to form the MnSi nanowires. It is worth emphasizing

that this growth process involves an

oxidation-reduction reaction, or an oxide assisted

synthesis method, to differentiate it from other

growth mechanisms [42][43].

The MnSi nanowires with correct stoichiometry and

B20 structure can be grown only under narrow

growth conditions; small changes of any parameter

lead to a dramatic change of the product structure

and composition because of the complexity of the

growth process. As there are multiple parameters

involved in the growth, we designed a final batch

of samples as follows to construct a growth phase

diagram: 5 samples were grown under optimum

conditions by changing only growth temperature, 4

samples by only changing growth time while

keeping all other optimum conditions, and 4 more

samples by only changing distance between the

wafer and the precursor while other parameters

remained constant using the optimum values.

Additionally, 8 more samples (0, 1, 50, 100, 200, 300,

500 and 1000 nm of SiO2 layer) were grown under

optimum conditions to finalize the best SiO2

thickness. The growth phase diagrams summarizing

results on those samples are plotted in Fig. 2 (c) and

(d), in which only the final batch of samples are

marked using diamonds. Other than the oxidation

layer thickness which was discussed above, the

growth temperature, reaction time, and the distance

between substrate and precursor, the argon pressure

and gas flow rate were also systematically

investigated. From the phase diagram, we can see

that the B20 MnSi nanowires can only be grown in

the red area. The width of the wires can be tuned

from 100 nm to 600 nm by changing the growth

temperature within this area.

The high yield one-dimensional growth can be

qualitatively explained by the theory of

supersaturation, with the SiO2 layers as acting to

control the supersaturation ratio. In order to enhance

one-dimensional growth, the two-dimensional

growth probability PN should be suppressed. This

probability is given by: ,

where α is the supersaturation ratio which is equal to

p/p0, where p is actual vapor pressure and p0 is

equilibrium vapor pressure at growth temperature T,

σ is surface energy of a solid whisker, B is a constant

and k is the Boltzmann constant [44]. Reducing

temperature and supersaturation ratio are two

options to enhance one-dimensional growth. While

the growth temperature cannot be changed too much

because of the activation energy of the phase

formation, the supersautration ratio can be tuned by

the thickness of the SiO2 layer. In this case the thicker

the SiO2 layer becomes the lower the Si or SiO

concentration becomes as these species need to

diffuse from the bottom to the surface of SiO2 layer.

This analysis suggests that the diameter of the

nanowires can be further reduced by further

increasing the SiO2 layer thickness and growth time,

and meanwhile decreasing the growth temperature.

The magnetic measurements of the samples support

the structural and compositional results. The

nanowires were transferred by gently applying a

Kapton tape to the growth substrate and then firmly

affixing the tape to a clean silicon substrate. Shown in

Fig. 3 (a) are the magnetization curves near the 29 K

helimagnetic transition plotted on an expanded scale.

The transition is clearly apparent for the MnSi wires

grown on 100 nm SiO2 layer measured at 10 Oe.

The cusp close to Tc is a clear signature of the

formation of the helical state, which only exists at

small field (the cusp disappears when the field

exceeds 1000 Oe) [3,45]. The magnetic

measurements were done with magnetic field

parallel and perpendicular to the surface of the wafer

(in-plane and out-of-plane). Although there are a few

nanowires pointing away from the growth substrate,

the majority of wires are aligned on the substrate,

while the orientation in plane of the substrate is

random. In this configuration, the out-of-plane

magnetic data shows the sum of the magnetization

perpendicular to the wires’ long axis; while the

in-plane data shows the azimuthal average of all the

wires on the surface plane. From both temperature

dependent and field dependent data in Fig. 3 (a), the

magnetic behavior in-plane looks a little bit stronger

than out-of-plane. The growth direction of the wires

is [110] [20,21], and the helical domains point out of

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6 Nano Res.

the (111) face, which is 35 from (110). So the

projection of the helical contribution out-of-plane is

around 71% of the in-plane contribution. This is

likely the reason for the small difference in the two

directions although as addressed above the magnetic

data is the average of the wires. Further studies using

local techniques such as micro SQUID are necessary

to fully quantify the magnetic anisotropy.

The isothermal ac susceptibility and DC

magnetization as a function of magnetic field confirm

the existence of the skyrmion phase (A phase) in the

nanowires. Fig. 3 (b) shows the comparison of the ac

susceptibility with the susceptibility 0dM/dB

calculated from the dc magnetization M at 28 K, just

below the transition temperature. The isothermal ac

susceptibility as a function of field was measured

under an excitation amplitude of 1 Oe and 8 Hz ac

magnetic field applied in-plane. Measurements

performed as the magnetic field is increasing and

decreasing show characteristic features which have

been shown to be signatures of the A-phase [6,46–49].

Other than the reduced value of ac susceptibility, we

also observed the extra peaks in the 0dM/dB which

is a characteristic feature of A-phase formation as

explained in detail in Ref [46]. The 0dM/dB curve

shows two lower kink regions (700-1150 Oe and 1150

-2300 Oe) and three transition maxima (700, 1150,

2300 Oe). By increasing the field, the magnetic

structure goes from helical to conical ordering (from

Bc1 to ), then from conical to A-phase (in between

and ). At 26.5 K, the first maximum in

0dM/dB (which represents the helical-to-conical

transition) is still visible, but the two peaks related to

the A-phase transition almost disappear. Please

note that as the mass of the nanowires is much

smaller (0.06 mg for the sample shown) than the bulk

single crystals, the signal-to-noise ratio of the data is

not as good as that of the literature[46]. Other than the

extra noise, our data are consistent with the

above-mentioned reference.

The field dependence of the ac susceptibility Re(ac)

and dc magnetization changes strongly in the narrow

temperature window from 26 K to 28.5 K and for B ≤

4000 Oe. Fig. 3(c) shows the evolution of the Re(ac)

approaching from low T to Tc. The blue and red

dashed lines are guides to the eye for Bc1, Bc2 and ,

. At 26 K, there is a shallow dip in the middle; a

well-pronounced local minimum has developed near

27.8 K. The characteristic feature is present up to 28.5

K and by 29 K is completely smeared. The quasi-1D

effect, in the range we studied (100 nm to 600 nm), is

very weak.

Figure 3 dc and ac magnetization obtained on MnSi nanowires.

(a) In and out of plane anisotropic magnetization (see text for

definition) under dc field of 10 Oe. The kink around 29 K is the

signature of helical magnetic order. The inset shows the field

dependence of the magnetization at 10 K. (b) The comparison of

the ac susceptibility Re(ac) (blue), Im(ac) (red) with the

susceptibility 0dM/dB (black) calculated from the field

dependence of the dc magnetization M (purple, right Y axis) at

28 K. The isothermal ac susceptibility as a function of field

measured under excitation field of 1 Oe and 8 Hz ac magnetic

field applied in-plane at 28 K. (c) The evolution of the Re(ac)

approaching from 26 K to Tc 29 K. The blue and red dash-lines

are guides to the eye for Bc1, Bc2 and , .

3. Conclusions

The growth conditions of MnSi nanowires have been

studied. Different Mn-Si compositions can be

selected by tuning growth parameters. The key

parameter for high yield, stoichiometric, and

well-crystallized growth of cubic B20 MnSi wires is

the thickness of the SiO2 layer. A growth phase

diagram was constructed that provides a systematic

guide for selective growth of nanowires with

different compositions. The ac and dc magnetic

properties of MnSi nanowires reveal the presence of

helimagnetic and skyrmion magnetic phases in the

one-dimensional wires. Theoretical modeling will be

required to fully understand growth phase diagram

and validate the conjectured growth mechanism.

Acknowledgements

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7 Nano Res.

We thank N. J. Ghimire for stimulating discussions

and a critical reading of the manuscript. This

research was conducted at the Center for Nanophase

Materials Sciences, which is sponsored at Oak Ridge

National Laboratory by the Scientific User Facilities

Division, Office of Basic Energy Sciences, U.S.

Department of Energy.

Electronic Supplementary Material: Supplementary

material (further details of the XRD, EDX and

magnetization on different structures) is available in

the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

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www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.

Electronic Supplementary Material

Growth of skyrmionic MnSi nanowires on Si: critical

importance of the SiO2 layer

Siwei Tang1,2

, Ivan Kravchenko2, Jieyu Yi

1,2, Guixin Cao

2, Jane Howe

3, David Mandrus

1 () and Zheng

Gai2 ()

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Fig. S1 XRD spectroscopy of nanowires samples prepared on Si substrate with different SiO2 thickness, 1

nm, 100 nm and 1000 nm. Different crystalline structures were obtained for different samples. On the

sample with only native oxide layer (1 nm), peaks of Mn4Si7 are dominant (black). However, on sample with

100 nm SiO2 layer, the dominant structure is the MnSi with exact ratio of 1:1 composition (red). While for

samples with 1000 nm oxide layer, as showing blue, the main structure become Mn5Si3.

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Nano Res.

Fig. S2 Mn/Si atomic ratio obtained using EDX spectroscopy from nanowires samples prepared on Si

substrate with different SiO2 thickness. SiO2 layer thickness dependence of the quantified atomic ratio of

Mn/Si from EDX. The Mn/Si ratio increases when the oxide layer thickness increases. At low oxide

thickness, the Mn/Si ratio is as low as 0.6; when the oxide layer thickness becomes 200 nm and above, the

composition start to approaching 5:3. The Mn/Si atomic ratio plot is consistent with the crystalline structure

development in the XRD (from Mn4Si7 to MnSi and Mn5Si3).

Fig. S3 Temperature dependences of the dc magnetizations from different samples measured at 1000 oe. The

magnetic measurements of the samples support the structural and compositional results. The comparison of

the temperature dependent magnetization measured using SQUID magnetometer, which shows three

different magnetic transitions between 2 K up to 300 K (Fig. S3). The 29 K transition is from skyrmion

helimagnetic ordering in MnSi, and the transition around 65 K corresponds to a antiferromagnetic transition

between a non-collinear AF1 and a collinear AF2 magnetic structure of Mn5Si3 (100 K). The ferromagnetic

transition near 220 K is quite intriguing as it has received only a little attention in the literature. The origin of

this transition is likely to be the formation of a dilute magnetic semiconductor with composition Si1-xMnx

(x<<1) in which a very small percentage of Mn atoms are incorporated into Si during the sample processing.

Mn4Si7 is a very weak ferromagnetic phase with Tc of 40 K and a saturation moment of 0.012 µ B/Mn, so the

contribution to the phase to the magnetization curve is very small.

Address correspondence to Zheng Gai, gaiz@ornl.gov; David Mandrus, dmandrus@utk.edu.