1 On Curie temperature of B20 -MnSi films Zichao Li12 Ye Yuan13 Viktor Begeza14 Lars Rebohle1 Manfred Helm14 Kornelius

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On Curie temperature of B20-MnSi films

Zichao Li1,2, Ye Yuan1,3*, Viktor Begeza1,4, Lars Rebohle1, Manfred Helm1,4, Kornelius

Nielsch 2,4,5, Slawomir Prucnal1, Shengqiang Zhou1*

1Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials

Research, Bautzner Landstrasse 400, D-01328 Dresden, Germany

2Institute of Materials Science, Technische Universität Dresden, 01069 Dresden, Germany

3Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, People's Republic of

China

4Institute of Applied Physics, Technische Universität Dresden, 01062 Dresden, Germany

5Institute for Metallic Materials, IFW-Dresden, Dresden, 01069, Germany

* Corresponding author: Ye Yuan, Shengqiang Zhou

E-mail address: yuanye@sslab.org.cn, s.zhou@hzdr.de

Abstract: B20-type MnSi is the prototype magnetic skyrmion material. Thin films of MnSi

show a higher Curie temperature than their bulk counterpart. However, it is not yet clear what

mechanism leads to the increase of the Curie temperature. In this work, we grow MnSi films

on Si(100) and Si(111) substrates with a broad variation in their structures. By controlling the

Mn thickness and annealing parameters, the pure MnSi phase of polycrystalline and textured

nature as well as the mixed phase of MnSi and MnSi1.7 are obtained. Surprisingly, all these

MnSi films show an increased Curie temperature of up to around 43 K. The Curie temperature

is likely independent of the structural parameters within our accessibility including the film

thickness above a threshold, strain, cell volume and the mixture with MnSi1.7. However, a

pronounced phonon softening is observed for all samples, which can tentatively be attributed

to slight Mn excess from stoichiometry, leading to the increased Curie temperature.

Keyword: B20 MnSi, Thin films, Curie temperature

Introduction

Bulk manganese monosilicide (MnSi) is a weak itinerant helical magnet with B20 crystal

structure [1]. At ambient pressure, bulk MnSi shows magnetic order with a Curie temperature

(TC) of ~ 29.5 K. It has been under investigation for a few decades regarding its intriguing

physical properties, such as magnetic quantum phase transition [2, 3] and the formation of a

non-Fermi liquid phase [4, 5]. With respect to practical applications, the most attractive

property of MnSi is the formation of a magnetic skyrmion lattice, which is a topologically stable

spin configuration and promising for spintronic application. A magnetic skyrmion lattice was

experimentally observed in bulk MnSi by Mühlbauer et al. by using small angle neutron

scattering [6]. This work has greatly motivated the development of MnSi thin films on Si

substrates. Generally, MnSi thin films have been prepared by molecular beam epitaxy (MBE)

[7-11], solid-state phase epitaxy [12-14] and magnetron sputtering [15]. Interestingly,

independent of the preparation methods, epitaxial-like MnSi films grown on Si(111) show

much enhanced TC to 35-45 K [11-13]. On Si(111) substrates, MnSi(111) is rotated by 30°

with the orientation relationship of Si(111) ‖ MnSi(111) and Si[112] ‖ MnSi[110], leading to a

lattice mismatch of around -3.0% ([aMnSi cos(30º) – aSi]/aSi = -3.0%). This induces an in-plane

lattice expansion in the MnSi films. It has been shown experimentally that for bulk MnSi

hydrostatic pressure decreases its TC [2] while negative chemical pressure can increase its TC

[16, 17]. Therefore, the increased TC in MnSi films was presumably attributed to the tensile

strain from the mismatch with Si substrate [10, 12]. However, the detailed analysis does not

support this assumption, since thinner films show lower TC than thicker films [12, 18].

Karhu et al. have systematically checked the change of TC on MnSi films with different

thicknesses and strain [12]. Indeed, it was found out that the thinner films show lower TC and

all thicker (>10 nm) films exhibit a similar TC at around 43 K. A proportional correlation is

observed between TC and the ratio between the out-of-plane and the in-plane strain. Li et al.

also found that the TC of 50 nm thick MnSi film is almost identical to that of the 10 nm film

[13]. López et al. found that a 30 nm-thick MnSi film does not develop any long-range magnetic

order, while the 150 nm MnSi film has a TC at 34 K [15]. In general, it is known that with

increasing thickness of the thin films, the strain originating from the interface should relax [19,

20]. In thicker MnSi films, the TC is expected to be lower than in thinner films. To understand

the relationship between strain, atomic bonds and TC in MnSi films, Figueroa et al. have

investigated thick MnSi films by polarization-dependent extended X-ray absorption fine

structure and found that the Mn positions are unchanged. They concluded that for thick MnSi

films the unit cell volume should be essentially the same as for bulk MnSi. They attributed the

enhanced TC to the interface, whose particular unidentified characteristics strongly affect the

magnetic properties of the entire MnSi film, even far from the interface. However, in other

literature the very thin (below 5 nm) MnSi film shows lower TC [12, 18] or the absence of

magnetic order [15]. Engelke et al. proposed that the film morphology may play a critical role

[18]. Just recently, Sukhanov et al. reported an improved TC of bulk MnSi lamellae with μm

dimensions embedded in MnSix ( x~1.7) matrix [21]. The lattice mismatch between MnSi

lamellae and the MnSi1.7 matrix produces a tensile strain in MnSi. To understand the increased

TC, it has been assumed that the interface influences the μm thick lamellae. Therefore, the origin

of the increased TC in MnSi films is still illusive.

Here, we report a systematic investigation on the Curie temperature of MnSi thin films

with a large variation in their structural properties (see Table 1). These thin films were prepared

by the solid-state reaction of metallic Mn layers with Si during ms-range flash lamp annealing.

By controlling the Mn thicknesses and annealing parameters (energy density deposited to the

sample surface by flash lamps), pure phase B20-MnSi and its mixture with MnSi1.7 are prepared

both on Si(100) and Si(111) substrates. All obtained thin films have a high Curie temperature

around 43 K and the characteristic signature of magnetic skyrmions. We attempt to find a

correlation between the Curie temperature and the structural properties, and therefore shed light

on the understanding of the increased Curie temperature in thin films.

Results

Figure 1 (a) shows the XRD pattern of sample G with a 60 nm MnSi film grown on a

Si(100) substrate. The (200) and (400) diffraction peaks of the Si substrate are at 33.05° and

69.2°, respectively. The MnSi (210) and (211) Bragg peaks are observed at 44.6° and 49.2°,

respectively. According to the powder PDF card (n. 01-081-0484) [25], the MnSi (210) peak is

the strongest. Taking into account the intensity ratio between different peaks, the MnSi film

grown on Si(100) exhibits a polycrystalline nature. Furthermore, the MnSi1.7 (104) peak appears

at 26.03°. These two phases (MnSi and MnSi1.7) often coexist [7, 26]. The XRD pattern of

sample L with a MnSi film grown on a Si(111) substrate is shown in Fig. 1 (b). The diffraction

peaks at around 28.4° and 58.9° are from the (111) and (222) of the Si substrate, respectively.

The MnSi(111) and (222) peaks are observed at 34.2° and 72.4°, respectively. The MnSi(210)

peak is also observed at 44.6°, but with much weaker intensity. Considering the intensity ratio

between different peaks, the MnSi phase in this sample is highly (111) textured. Within the

detection limit, there is no visible peak, which can be assigned to the MnSi1.7. From our

measurements for other samples with different thickness (not shown), we have found the

following: (1) MnSi films on Si(100) are always polycrystalline with the co-existence of the

MnSi1.7 second phase; (2) MnSi films on Si(111) are highly (111) textured and we can obtain

either pure MnSi phase or the mixture of two phases with different concentration ratios.

Table 1. The parameters of the samples and their Curie temperature (TC).

Sample

Substrate

Thickness of the

regrown layer (nm)

Flash energy

density (J/cm2)

Anneal

surface

Content of

MnSi1.7 (%)

TC (K)

Si(100)

115

Mn surface

37±2

Si(100)

115

Mn surface

39±2

Si(100)

140

Si surface

42±2

Si(100)

115

Mn surface

45±1

Si(100)

135

Si surface

44±1

Si(100)

140

Si surface

43±2

Si(100)

140

Mn surface

44±2

Si(111)

140

Si surface

39±2

Si(111)

110

Mn surface

44±2

Si(111)

115

Mn surface

46±1

Si(111)

140

Mn surface

42±1

Si(111)

140

Si surface

45±2

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1OnCurietemperatureofB20-MnSifilmsZichaoLi1,2,YeYuan1,3*,ViktorBegeza1,4,LarsRebohle1,ManfredHelm1,4,KorneliusNielsch2,4,5,SlawomirPrucnal1,ShengqiangZhou1*1Helmholtz-ZentrumDresden-Rossendorf,InstituteofIonBeamPhysicsandMaterialsResearch,BautznerLandstrasse400,D-01328Dresden,Germany2InstituteofMate...

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