Nanoparticles: Synthesis and Size-Induced Ferromagnetism

Letter pubs.acs.org/NanoLett

Mn5Si3 Nanoparticles: Synthesis and Size-Induced Ferromagnetism Bhaskar Das,†,‡ Balamurugan Balasubramanian,*,†,‡ Priyanka Manchanda,†,‡ Pinaki Mukherjee,†,‡ Ralph Skomski,†,‡ George C. Hadjipanayis,§ and David J. Sellmyer*,†,‡ †

Nebraska Center for Materials and Nanoscience, ‡Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, United States § Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States ABSTRACT: Mn-based silicides are fascinating due to their exotic spin textures and unique crystal structures, but the low magnetic ordering temperatures and/or small magnetic moments of bulk alloys are major impediments to their use in practical applications. In sharp contrast to bulk Mn5Si3, which is paramagnetic at room temperature and exhibits low-temperature antiferromagnetic ordering, we show ferromagnetic ordering in Mn5Si3 nanoparticles with a high Curie temperature (Tc ≈ 590 K). The Mn5Si3 nanoparticles have an average size of 8.6 nm and also exhibit large saturation magnetic polarizations (Js = 10.1 kG at 300 K and 12.4 kG at 3 K) and appreciable magnetocrystalline anisotropy constants (K1 = 6.2 Mergs/cm3 at 300 K and at 12.8 Mergs/cm3 at 3 K). The drastic change of the magnetic ordering and properties in the nanoparticles are attributed to low-dimensional and quantum-confinement effects, evident from firstprinciple density-functional-theory calculations. KEYWORDS: Nanoparticles, Mn-based alloys, quantum confinement, ferromagnetism

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shown in Figure 1. A composite target made of manganese and silicon is sputtered using a direct current (DC) magnetron

anostructuring of magnetic materials is of considerable interest due to its unique ability to tune their behavior for many advanced applications including extremely high-density magnetic ordering, spintronic devices, and ultrastrong permanent magnets.1−3 Particularly, in the case of nanoparticles, nanoscale effects have been observed to modify the electronic structure and subsequently to result in large spin polarization, entirely different magnetic ordering or transitions, and new spin structures as compared to the corresponding bulk alloys.4−8 Mn-based intermetallic compounds recently have become a central focus of research in magnetism from the viewpoints of developing new rare-earth-free permanent magnets and magnetic materials with interesting spin-electronic properties.9,10 The elemental magnetic moment of Mn is larger than those of Fe and Co, but Mn atoms tend to exhibit antiferromagnetic interactions, which reduce net magnetization and Curie temperature.11,12 In this regard, Mn5Si3 is a promising material due to its hexagonal structure that has potential for creating a high magnetocrystalline anisotropy. However, bulk Mn5Si3 is paramagnetic at room temperature and transforms at around 100 K into an orthorhombic phase that exhibits only antiferromagnetic ordering.11,13−16 In this study, we synthesize novel Mn5Si3 nanoparticles and achieve ferromagnetic ordering with a high Tc ≈ 590 K and an appreciable ⟨m⟩ = 2.2 μB/Mn by exploiting nanoscale effects. Fabrication and Characterization of Nanoparticles. The Mn5Si3 nanoparticles were fabricated using a gas aggregation-type cluster-deposition method, as schematically © XXXX American Chemical Society

Figure 1. Fabrication of Mn5Si3 nanoparticles. A schematic of the cluster-deposition process.

sputtering at high power (225−250 W) using a mixture of Ar [500 standard cubic centimeter per minute (SCCM)] and He (100 SCCM) as sputtering gas. The sputtered atoms are condensed to form Mn−Si nanoparticles, which are extracted as a collimated beam and deposited as dense films on singlecrystalline Si (001) substrate for X-ray diffraction measurement (Rigaku D/Max-B X-ray diffractometer that uses Co K α radiation of about 1.7889 Å wavelength) and magnetometry Received: October 27, 2015 Revised: December 27, 2015

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Figure 2. TEM studies on Mn5Si3 nanoparticles. (a) A low-resolution TEM image and (b) the corresponding particle-size histogram, where d and σ/ d are the average particle size and rms standard deviation, respectively. (c) A high-angle annular dark-field (HAADF) image and the corresponding EDS elemental color mappings showing (d) Mn, (e) Si, and (f) Mn and Si distributions.

Figure 3. Structure of Mn5Si3. (a) Hexagonal unit cell (prototype: Mn5Si3 and space group: P63/mcm). (b) X-ray diffraction patterns of bulk alloy and nanoparticles (NP). The vertical lines represent the standard peak positions and corresponding relative intensities of the X-ray diffraction peaks of Mn5Si3. (c) A HRTEM image of a single nanoparticle. (d) The fast Fourier transform (FFT) of the HRTEM image indexed with hexagonal structure along the (1̅20) zone axis.

studies using SQUID (superconducting quantum interference device) and PPMS (physical property measurement system). The nanoparticle films are coated immediately with a carbon cap layer of about 5 nm, using a radio frequency sputtering gun employed in the deposition chamber to prevent oxidation upon exposure to atmosphere. The composition of the target is adjusted to obtain the Mn5Si3 stoichiometry in Mn−Si

nanoparticles. The Mn5Si3 nanoparticles were deposited with a low coverage density on a carbon film supported by copper grids for transmission electron microscopy measurements using FEI Technai Osiris STEM. Note that the substrate is kept at room temperature during the deposition of nanoparticle films and carbon cap layer, and thus the diffusion of carbon into the nanoparticle film is unlikely. We also used SiO2 as a capping B

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Figure 4. Magnetic properties of Mn5Si3. (a) Magnetization (M) measured as a function of temperature (T) in a magnetic field of 1 kOe. The M−T curve for the bulk alloy is magnified in the inset to show clearly the antiferromagnetic transitions AFM1 and AFM2 at 99 and 66 K, respectively. (b) M−T curve for the nanoparticles measured in the temperature range of 300−850 K, where the Curie temperature Tc is indicated by an arrow. (c) The magnetic hysteresis loops of the isotropic (unaligned) nanoparticles measured at 300 and 3 K. (d) The experimental magnetization curves (spheres) of the nanoparticles at high-field region (35−70 kOe) were fitted (lines) using the law-of-approach to saturation method to estimate the magnetocrystalline anisotropy constant K1.

purity Mn and Si were melted in a conventional arc melting process to obtain a homogeneous ingot of Mn5Si3 composition. Then, the ingot was used to prepare melt-spun ribbons, which were mechanically ground into bulk Mn5Si3 powder.19 Bulk Mn5Si3 is paramagnetic at room temperature and shows antiferromagnetic transitions at 99 K (AFM 1) and 66 K (AFM 2), as clearly shown in the inset of Figure 4a. The lowtemperature antiferromagnetic transitions in bulk Mn5Si3 have been attributed to a structural transition from the hexagonal to orthorhombic structure at 100 K.11,13 In comparison, the nanoparticles exhibit an enhanced magnetization by about 3 orders of magnitude, which indicates a possible ferromagnetic ordering with a Curie temperature Tc higher than 300 K. The measured M−T curve at elevated temperatures, Figure 4b, suggests a Tc close to 590 K for the nanoparticles. The magnetization curves of the Mn5Si3 nanoparticles measured as a function of applied field H from −70 kOe to +70 kOe reveal a large saturation magnetization (Ms), and appreciable coercivities (Hc). For example, the expanded M−H loops of the nanoparticles, Figure 4c, show Hc = 0.50 and 0.90 kOe at 300 and 3 K, respectively. Most importantly, M does not saturate even for a high field (70 kOe) as shown in Figure 4d, suggesting a significant magnetocrystalline anisotropy constant K1. In Figure 4d, the experimental magnetization data at high field were fitted using the law-of-approach to saturation method with the expression M = Ms (1 − A/H2) + χH for uniaxial isotropic particles.20−22 χ is the high-field susceptibility, and A is a parameter related to K1 and saturation magnetization Ms through A = (4/15)(K21/M2s ). This analysis yields K1 = 6.2 Mergs/cm3 and Ms = 802 emu/cm3 (Js = 10.1 kG) at 300 K and K1 = 12.8 Mergs/cm3 and Ms = 983 emu/cm3 (Js = 12.4 kG) at

layer and found that the structural and magnetic properties of Mn5Si3 nanoparticle films are not altered by the capping material. Structure. Figure 2a shows a transmission electron microscope (TEM) image of Mn−Si nanoparticles. The corresponding particle-size histogram, Figure 2b, reveals an average size of 8.6 nm with an rms standard deviation of σ/d ≈ 0.2. Figure 2c shows a high-angle annular dark-field (HAADF) image of a Mn5Si3 particle. The corresponding energy dispersive X-ray spectroscopy (EDS) color maps having Mn, Si, and the combined Mn and Si elemental distributions are shown in Figure 2d−f, respectively. The result reveals a uniform distribution of Mn and Si and a composition close to the Mn5Si3 stoichiometry in the nanoparticle, which is also supported by X-ray diffraction (XRD) and high-resolution TEM (HRTEM) measurements. Figure 3a shows the unit cell of the Mn5Si3-type hexagonal structure, which belongs to the space group P63/mcm. The XRD pattern of Mn−Si nanoparticles is compared with that of the bulk Mn5Si3 alloy in Figure 3b. The standard diffraction data of the hexagonal Mn5Si3-type structure is also given as vertical lines in Figure 3b.17,18 It is clear that both Mn5Si3 bulk and nanoparticles form the hexagonal Mn5Si3-type crystal structure. In addition, Mn5Si3 nanoparticles are single crystalline with a high degree of atomic ordering, as shown in the HRTEM image of Figure 3c. The fast Fourier transform (FFT) of the HRTEM image is also indexed to the hexagonal structure along the (1̅20) zone axis as shown in Figure 3d. Magnetic Properties. Figure 4a compares the temperature-dependent magnetization curves of Mn5Si3 bulk and nanoparticles. For the preparation of bulk Mn5Si3 alloy, high C

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hard directions of the aligned nanoparticle film meet. This value is somewhat smaller than K1 = 12.8 Mergs/cm3 estimated from the law-of-approach to saturation, but they are of the same order of magnitude. The strong magnetocrystalline anisotropy of Mn5Si3 nanoparticles is also revealed from the easy-axis alignment experiment, generally used for uniaxial nanoparticles.23 Due to this anisotropic nature, the nanoparticles were successfully aligned using an external magnetic field of about 5 kOe, prior to deposition on substrates, as schematically shown in Figure 5a. For example, the magnetic hysteresis loops of the aligned (anisotropic) nanoparticles measured at 3 K, Figure 5b, show an appreciable coercivity, Hc = 1.7 kOe, and remanence ratio Mr/Ms = 0.82 along the easy axis as compared to those measured along the hard axis (Hc = 0.50 kOe and Mr/Ms = 0.3), where Mr is the remanent magnetization. Note that Hc and Mr/Ms along the easy axis for the aligned nanoparticles at 300 K are about 0.9 kOe and 0.84, respectively (not shown here). The coercivity, 1.7 kOe, is much smaller than the anisotropy field, about 14 kOe at 3 K. This effect is known as Brown’s paradox and caused by real-structure effects such as interparticle exchange interactions, defects, and inhomogeneity such as the “core−shell” magnetic structures.24−26 Density Functional Theory. In brief, the experimental results show that Mn5Si3 nanoparticles are ferromagnetic with a high Tc ≈ 590 K, and a large saturation magnetic polarization, Js = 12.4 kG at 3 K, which corresponds to an average magnetic moment ⟨m⟩ = 2.2 μB/Mn. In order to understand the magnetic properties of the nanoparticles, we have performed first-principle density-functional theory (DFT) calculations using the projected augmented wave method (PAW), as implemented in the Vienna ab initio simulation package (VASP).27,28 The exchange-correlation effects were imple-

3 K. Js = 4πMs is the saturation magnetic polarization. Since the law-of-approach to saturation is ideally applicable for noninteracting randomly oriented particles and some spin canting in nanoparticles is also possible, this method gives a semiquantitative estimate of magnetic anisotropy for densepacked nanoparticle ensembles. We also estimated K1 = 7 Mergs/cm3 at 3 K from the magnetic anisotropy field HK (Figure 5b), where the magnetization curves for the easy and

Figure 5. Easy-axis aligned Mn5Si3 nanoparticles. (a) A schematic of the alignment process using an external magnetic field Hal. (b) In-plane hysteresis loops measured along the easy and hard directions. HK represents the anisotropy field, which is roughly equal to the field where easy- and hard-axes loops meet.

Figure 6. DFT results of Mn5Si3 nanoparticles: (a) Schematic of a nanoparticle having 128 atoms. Density of states for (b) surface Mn and (c) core Mn atoms. (d) Radial distribution of magnetic moments m(r) from the center to the surface estimated from DFT calculations. The error bar estimated from the statistical analysis of the DFT magnetic moments is marked at r ≈ 1.1 nm. D

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high Tc and ⟨m⟩ observed in the case of Mn5Si3 nanoparticles is presumably a direct consequence of the modification in the electronic structure due to nanoscale effects, evident from Figure 6b and c. Note that the finite-size effect often leads to a decrease of Tc with a reduction of particle size.39−41 This effect is normally observed in ferromagnetic nanoparticles as the size is reduced, but there are exceptions, such as Gd.42 Unlike Gd, where the effect is moderate (ΔTc = 40 K) and comes from the exchange interactions between Gd surface atoms of fixed moment, the present system exhibits a much stronger effect that primarily originates from the substantially enhanced magnetic moment of the Mn surface atoms. In summary, we have determined saturation magnetic polarization values of 12.4 kG (10.1kG) and average magnetic moments of 2.2 μB/Mn (1.7 μB/Mn) at 3 K (300 K) in Mn5Si3 nanoparticles. The nanoparticles also exhibit appreciable coercivities (Hc = 1.7 kOe at 3 K and 0.9 kOe at 300 K), mainly due to the underlying magnetocrystalline anisotropies at the corresponding temperatures. The first-principle calculations demonstrate that the surface-induced spin polarization is responsible for the high magnetic moment in the nanoparticles. This study explains how the nanoscale effects can be used to improve the magnetic properties of Mn-based silicides, which may be emerging materials for significant magnetic and spintronics applications.

mented by generalized-gradient approximation (GGA-PBE)29 and a supercell with 15 Å vacuum spaces in the x, y, and z directions is used to exclude interactions between the neighboring nanoparticles. For the nanoparticle, the Γ-point was used for k-point sampling due to the large supercell. The atomic positions for the nanoparticles were relaxed until the force acting on each atom was less than 0.1 eV/Å, and a convergence criteria of 1 × 10−5 eV has been used for electronic structure calculations. The simulations were performed for a hexagonal nanoparticle having 128 atoms (schematically shown in Figure 6a) and also for the bulk hexagonal structure. The DFT analysis yields an average magnetic moment ⟨m⟩ = 2.7 μB/Mn for the hexagonal nanoparticle and also reveals a large magnetic moment of about 3.4 μB/Mn for surface atoms of nanoparticles as compared to 0.85 μB/Mn for core atoms. This difference is reflected by the calculated densities of states (DOS) for the surface atoms (Figure 6b) and that of core atoms (Figure 6c). Figure 6d shows the inhomogeneous distribution of the calculated magnetic moment across the nanoparticle m(r), where r is the distance from the center of the nanoparticle. In brief, Figure 6b−d demonstrate a modified electronic structure at the nanoparticle surface as compared to the core, leading to a large surface spin polarization. Interestingly, DFT calculations also show that bulk hexagonal structure is ferromagnetic with a moment of 2.2 μB/Mn. In real experiments, the bulk alloys transform to an orthorhombic structure at around 100 K, which is an antiferromagnetic phase.11,13−16 Our experiments and DFT calculations indicate that the Mn5Si3 nanoparticles are ferromagnetic in the temperature range of 3−590 K and do not undergo a structural transition at low temperatures below 100 K. The experimental moment ⟨m⟩ = 2.2 μB/Mn for the 8.6 nm nanoparticle is smaller than that of the nanoparticles having 128 atoms (particle size ≈ 2.4 nm), and this difference can be attributed to the size-dependent increase of surface contribution to the magnetic moment. The radial distribution of magnetic moment, shown in Figure 6d, suggests a decay length or shell thickness of 0.6 nm, where there is a large spin polarization or high surface magnetic moments. If we consider a shell thickness of about 0.6 nm with an average moment of 3.4 μB/Mn, and rest of the “core” atoms have magnetic moments of 0.85 μB/Mn, the average magnetic moment of the 8.6 nm nanoparticle is about 1.8 μB/Mn, which is in reasonable agreement with the corresponding experimental value 2.2 μB/ Mn. Note that 2.2 μB/Mn is also significantly lower than the atomic moment of Mn (5 μB). This is very common in itinerant Mn-based intermetallic compounds, such as MnBi (3.12−3.95 μB/Mn), IrMn3 (2.6 μB/Mn), MnAl (2.3−2.5 μB/Mn), and MnGa (2.6−2.7 μB/Mn), due to the hybridization of the Mn 3d electrons with neighboring atoms.30−33 Bulk magnetic measurements cannot determine detailed atomic spin structure and spin-canting also cannot be ruled out, but the ferromagnetic-like experimental magnetic properties along with DFT results suggest that Mn5Si3 nanoparticles are ferromagnetic. The DFT calculations are based on the assumption of a collinear spin structure. In reality, the surface spins may well exhibit some noncollinearity, but the good agreement between theory and experiment suggests that these noncollinearities are not very pronounced. It is also known that the magnetism of Mn5Si3 and other Mn-based nanostructure are sensitive to small perturbation of alloying and lattice parameters.34−38 The unusual ferromagnetic ordering with a

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Synthesis, experimental characterization, and theoretical analysis were supported by the U.S. Department of Energy, Office of Basic Energy Sciences under the awards DE-FG0204ER46152 (B.D., B.B., R.S., D.J.S.) and DE-FG02-04ER4612 (G.C.H.). Electron microscopy analysis and electronic structure calculations were supported by the Army Research Office under the award WF911NF-10-2-0099 (P.M., P.M.). Research at Nebraska was performed in part in the Nebraska Nanoscale Facility, Nebraska Center for Materials and Nanoscience, which is supported by the National Science Foundation under Award NNCI: 1542182, and the Nebraska Research Initiative. This research also benefited from the Holland Computing Center (simulations).

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REFERENCES

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