Exchange interactions and critical temperature of bulk and thin films of MnSi : A density functional theory study

Mahboubeh Hortamani,1,2 Leonid Sandratskii,2 Peter Kratzer,1,3 Ingrid Mertig,2,4 and Matthias Scheffler1 1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany 2Max-Planck Institut für Mikrostrukturphysik, D-06120 Halle, Germany 3Fachbereich Physik, Universität Duisburg-Essen, D-47048 Duisburg, Germany 4Fachbereich Physik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle, Germany Received 4 June 2008; published 4 September 2008


I. INTRODUCTION
One of the main goals of spintronics is to combine semiconductor technology with magnetic materials, aiming at the injection of a spin-polarized current from a ferromagnet into a semiconductor.The thin films of transition metals and their compounds on semiconductor substrates are considered as promising candidates for the fabrication of devices with desired functionality.In order to be employed for spin injection, the film should satisfy a number of conditions: ͑i͒ structural stability as an epitaxial film on the substrate; ͑ii͒ sharp and structurally well-defined interface with the substrate; and ͑iii͒ high temperature of the magnetic ordering and high degree of spin polarization at the Fermi level.
Earlier theoretical studies suggested the magnetic intermetallic compounds grown epitaxially on Si as promising candidates for spintronic applications. 1,2The natural crystal structure of bulk MnSi is B20 type.4][5] We found 2,3 description of finite-temperature thermodynamic quantities, such as the Curie temperature, is not straightforward.7][8][9] For a multisublattice crystalline system, the Heisenberg Hamiltonian can be written in the form where the parameters J ij describe the exchange interaction between two atoms; i and j are unit-cell indices; and are sublattice ͑basis site͒ indices; and e is the unit vector pointing in the direction of the magnetic moment at site ͑ , i͒.Because of the crystal periodicity J ij = J i−j,0 .The on-site exchange parameters in Eq. ͑1͒ are equal to zero: J ii = 0.The vector e is assumed to take arbitrary directions that correspond to the classical treatment of atomic magnetic moment.This is a plausible description of itinerant-electron magnet.
The procedure of the mapping of an itinerant-electron system on the Heisenberg Hamiltonian consists of the determination of the interatomic exchange parameters J ij on the basis of the first-principles calculations.In this paper we determine the exchange parameters by fitting the Heisenberg energies to the total energies of various collinear magnetic configurations.The details of the first-principles calculations are given below.
The Heisenberg Hamiltonian is used to estimate the Curie temperature.In the calculation of the Curie temperature, we employ two approaches: the mean-field approximation ͑MFA͒ and the random-phase approximation ͑RPA͒.For three-dimensional solids the MFA is expected to overestimate the value of the Curie temperature, whereas the RPA is expected to underestimate T c . 10 The RPA gives better weighting of the spin-wave excitations than MFA and is supposed to give better estimation of the Curie temperature.
For the two-dimensional ͑2D͒ systems there is an additional complexity related to the Mermin-Wagner theorem.This theorem states that an isotropic 2D ferromagnet cannot order at nonzero temperature. 11The reason for this is the gapless excitation spectrum of isotropic magnets.In the 2D systems the low-energy long-wavelength magnetic fluctuations destroy the long-range magnetic order at any nonzero temperature.If, however, the magnetic anisotropy is taken into account, the smallest excitation energy becomes nonzero.The value of the gap depends on the strength of the magnetic anisotropy.The gap in the excitation spectrum violates the conditions of the Mermin-Wagner theorem and leads to a nonzero value of the Curie temperature.The dependence of the Curie temperature on the anisotropy parameter has a logarithmic character and is relatively weak outside the range of very small anisotropy values.Since the dependence of the Curie temperature on the exchange parameters is approximately linear, for characteristic values of the magnetic anisotropy of the 2D ferromagnets, the magnitude of the Curie temperature is mostly determined by the strength of the exchange interactions.
The main physical quantity in the MFA is the effective exchange field experienced by a given magnetic moment from the side of all other magnetic atoms.The statistical mechanics in the MFA is the statistical mechanics of individual atomic spins in an effective exchange field originating from the interaction with the environment.The longwavelength fluctuations are not present in the MFA and the conditions of the Mermin-Wagner theorem are violated; the MFA gives ͑for 2D isotropic ferromagnets͒ a nonzero value of the Curie temperature.On the other hand, the RPA deals with the wave-vector dependent spin-wave excitations and, in agreement with the Mermin-Wagner theorem, gives ͑for a 2D isotropic ferromagnet͒ a zero value of the Curie temperature.Thus, for the 2D systems the use of the RPA is strongly preferable.The MFA estimation of the Curie temperature can still be considered as a useful characteristic of the exchange interactions in the system.Therefore, in this paper we present both MFA and RPA estimations of the Curie temperature.
The MFA estimation of the Curie temperature of a multisublattice system can be obtained by solving the system of coupled equations, 12,13 where J 0 ϵ͚ j J 0j .Equation ͑2͒ can be rewritten in the form of an eigenvalue matrix problem, where ⌰ = ͑2 / 3k B ͒J 0 , I is a unit matrix, and S is the vector set of ͗e z ͘.The largest eigenvalue of the matrix gives the value of T c MFA . 12,13he RPA approach to the calculation of the Curie temperature of multiple-sublattice systems was recently discussed in Ref. 14.The derivation of the RPA method starts with the consideration of the Heisenberg Hamiltonian of quantum spins.Rusz et al. 14 arrived at the following formula: where s ˜i z = S i z / S .Here S i z is the z component of the spin of the site ͑ , i͒ and S is the value of the spin of the atoms of the type.The average value of s ˜i z does not depend on i. ͓N −1 ͑q͒ ͔ in Eq. ͑4͒ is the diagonal element of the matrix inverse to matrix N defined by Here k B is the Boltzmann constant and ⌬ gives the magnetic anisotropy energy.The value of ⌬ can be estimated on the basis of first-principles calculations with the spin-orbit coupling taken into account.It is given by the energy difference of the ferromagnetic ͑FM͒ states with different directions of the magnetic moments.Equation ͑4͒ is written separately for each atomic type .Therefore, the number of equations is equal to the number of sublattices.These equations are interconnected through the matrix N ͓Eq.͑5͔͒.Rusz et al. 14 suggested an iterational procedure for solving the system of Eq. ͑4͒.Since in this paper we consider the case of only two inequivalent sublattices, the system of two equations is solved graphically.
As stated above for ⌬ = 0, the Curie temperature of a 2D system vanishes in agreement with the Mermin-Wagner theorem. 11he classical limit of Eq. ͑4͒ is obtained as a limit S → ϱ.Correspondingly, ͑S +1͒ / S tends to be equal to one.
The DFT calculations were performed using the allelectron full-potential augmented plane wave plus localorbital method 15 implemented in the WIEN code. 16The exchange-correlation potentials are treated in the generalized gradient approximation ͑GGA-PBE96͒.The muffin-tin radii in the calculations for bulk MnSi are chosen to be R MT = 1.06 Å for both Mn and Si.The convergency of the bulk calculations was achieved at the cutoff energy of E cut wf = 14.5 Ry for the plane-wave expansion of the wave functions in the interstitial region.We used a ͑15ϫ 15ϫ 15͒ k mesh in the first Brillouin zone.
The surface calculations are performed by means of a three-dimensional periodic supercell consisting of ten Si͑001͒ layers followed by a maximum of five alternating atomic layers of Mn and Si and a vacuum spacer of about 10-11 Å ͑Fig.1͒.The number of the substrate Si layers was chosen sufficiently large to reproduce the bulklike behavior of the central layers.The MnSi layers were assumed to have B2-type crystal structure.To make the system invariant with respect to space inversion, the MnSi films were put symmetrically at both the top and the bottom of the Si slab.The calculated equilibrium lattice constant of bulk Si ͑5.48 Å͒ is used for the Si͑001͒ substrate.8,19 The energy cutoff convergency for the film was achieved using E cut wf = 13.8Ry for the plane-wave expansion in the interstitial region.A set of ͑10ϫ 10ϫ 1͒ special k points is used for integrations over the Brillouin zone of the ͑1 ϫ 1͒ surface cell of Si͑001͒.In the film calculation, all Mn and Si layers except the two central Si layers in the slab are relaxed until the calculated atomic forces are smaller than 0.03 eV/ Å.

III. BULK MnSi
The intermetallic compound MnSi is known to form a cubic crystal structure B20 with four Mn and four Si atoms per unit cell.Other transition-metal silicides such as FeSi, CoSi, and CrSi also crystallize in this type of structure.The point group is P2 1 3.It includes four threefold rotation axes pointing in the ͑111͒, ͑11 ¯1 ¯͒, ͑1 ¯11 ¯͒, and ͑1 ¯1 ¯1͒ directions.The calculated lattice parameter in the present work is 0.9% smaller than the experimental value.The Mn atom has one Si neighbor at 2.28 Å along the ͓111͔ direction, three Si neighbors at a distance of 2.37 Å, and three Si neighbors at 2.52 Å ͓see Fig. 1͑a͔͒.
Below the magnetic critical temperature of 29 K, MnSi has a helical magnetic structure with a long spiral period of 180 Å.The formation of the helical structure in MnSi is a consequence of relativistic effect and the lack of inversion symmetry. 17,20he B20 crystal lattice cannot be matched with the Si͑001͒ substrate.We consider the CsCl-type ͑B2͒ lattice ͓Fig.1͑b͔͒ as an epitaxially grown structure of MnSi on Si͑001͒.The B2 lattice matches well with the Si surface.The B2 films of FeSi and CoSi have been successfully grown by von Känel et al. 21on Si͑111͒.A recent work 1 predicted the B2 crystal structure for MnSi films on Si͑001͒.
In this section we report the results of the calculations for bulk MnSi with both B20 and B2 structures.The comparison of the calculated properties of the bulk, the surface, and the film of MnSi allows one to get a deeper insight into the physics of the systems.For the B2 lattice, we will study the role of the tetragonal distortions ͑B2-t͒.We will refer to the undistorted B2 lattice as cubic B2 lattice ͑B2-c͒.In Fig. 2 we present the total energy versus volume plot for the B20 and the B2-c lattices.The curves were fitted using the Mur-  naghan equation.The equilibrium energy of the B2-c structure exceeds the equilibrium energy of the B20 structure by 0.25 eV/formula unit.This energy difference is less than the corresponding differences for FeSi ͑0.04 eV͒ and CoSi ͑0.42 eV͒. 4 The calculations show that equilibrium MnSi in B20 structure is ferromagnetic while in B2-c structure it is paramagnetic ͑PM͒.In the B20 phase, the antiferromagnetic ͑AFM͒ state is energetically higher by 52 meV/formula unit than the FM state.The atomic Mn moment in the AFM structure of the B20-MnSi is 0.48 B .The induced moment on Si is in this case zero.In the FM state the corresponding atomic moments are 1.03 ͑0.04͒ B , which are larger than the experimental moment of 0.4 B per formula unit.The calculated atomic moments are comparable to the results of recent calculations by Jeong and Pickett 22 performed with the fullpotential local-orbital method.We calculate the atomic spin moments for the muffin-tin spheres with a radius of 1.06 Å.This radius is 5% smaller than the radius of the touching spheres for the B20 lattice.The consideration of the touching atomic spheres does not change the atomic moments substantially.
The equilibrium lattice constant for the metastable B2-c structure is 2.79 Å ͑Fig.2͒ that is close to half of the calculated lattice constant of Si 5.48 Å.Therefore, the mismatch between lattices is less than 2% and Si can be considered as a good substrate to grow B2 MnSi layers.
The B2 structure can be stabilized by high pressure.The GGA calculations show a phase transition from the B20 to the B2 structure at a pressure of about 40 GPa ͑Fig.2͒.The value of the critical pressure is given by the slope of the common tangent to the energy-volume curves for both B2 and B20 structures: p =−͑‫ץ‬E / ‫ץ‬V͒ T .This pressure is larger than the critical pressure that suppresses the magnetism in the B20 structure. 23 the corresponding equilibrium volumes, the B20 MnSi is ferromagnetic while the B2-c MnSi is nonmagnetic.The difference in the magnetic state of these two phases can be explained on the basis of the Stoner criterion, 24 which states that a nonmagnetic state of a system is unstable with respect to the formation of the ferromagnetic state if IN nm ͑⑀ F ͒ Ͼ 1, and it is stable if the value of the product is smaller than one.
Here I is the Stoner parameter and N nm ͑⑀ F ͒ is the density of states ͑DOS͒ at the Fermi level in the nonmagnetic phase.
In Figs.3͑a͒ and 3͑c͒ we show the DOS of the nonmagnetic states of both B20 and B2-c structures.The B20 DOS is broader than the B2-c DOS.This can be attributed to the shorter Mn-Si bond length in the B20 structure.However, the Fermi level of the B20 DOS lies on one of the local peaks while for the B2-c structure the Fermi level is at a shoulder of a peak with smaller value of the density of states.
The Stoner parameter can be estimated according to the formula: I = ⌬ ex / m, where ⌬ ex is the exchange splitting between spin-up and spin-down states and m is the corresponding magnetic moment giving the origin to the exchange splitting.The value of ⌬ ex is estimated from the DOS of the B20 structure as the energy distance between corresponding peaks in the spin-up and spin-down DOSs.The calculated ⌬ ex = 0.55 eV and m = 1.0 B yield a Stoner parameter of I = 0.5 eV/ B .This value is comparable with the Stoner parameter of fcc Mn, 0.41 eV/ B , reported by Janak. 25The Stoner parameter can be treated as an atomic characteristic that does not depend substantially on the crystal environment. 26Therefore, we use the same Stoner parameter for both B2 and B20 structures.
The values of the Stoner product are given in Table I.For the B20 structure the Stoner product exceeds unity and predicts the ferromagnetic instability, whereas for the B2-c structure, it is substantially smaller than unity and the ground state of the system is nonmagnetic.TABLE I. Density of states per Mn atom at the Fermi level for both spin channels N ↑ ͑⑀ F ͒ and N ↓ ͑⑀ F ͒, total density of states at the Fermi level for the nonmagnetic case N nm ͑⑀ F ͒ ͑state/eV͒, the exchange splitting ⌬ ex ͑eV͒, the product of Stoner parameter I, and N nm ͑⑀ F ͒. FIG. 3. Density of states for MnSi in the B20 structure using a mesh of ͑18ϫ 18ϫ 18͒ k points in the Brillouin zone.͑a͒ The solid line is the DOS of the B20 structure in the nonmagnetic phase for both spin channels.͑b͒ The solid ͑dash͒ line is the DOS of the majority ͑minority͒ spin channel.͑c͒ and ͑d͒ The solid line is the DOS of the nonmagnetic phase for cubic and tetragonal structures.͑e͒ The solid ͑dash͒ line is the DOS for the B2 in the majority ͑minority͒ spin channel.The Fermi level is set to be the energy zero.
Note that for the B2-c structure, an antiferromagnetic state exists.The Mn moments assume a small value of 0.17 B .The energy of the antiferromagnetic state is 6 meV per Mn atom higher than the energy of the nonmagnetic state.
Since the epitaxial growth of the MnSi films on Si͑001͒ is accompanied by tetragonal distortions, we performed calculations for the bulk MnSi with a distorted B2 lattice.The calculations were performed as follows.The a and b parameters of the base plane of the structure were fixed either to a = b = 2.79 Å ͑the equilibrium lattice constant of the B2-c MnSi͒ or to a = b = 2.74 Å that corresponds to the lattice constant of Si.The c parameter of the tetragonally distorted structure was varied.The magnetic structure appears in the first case at c = 1.05 a, while using the Si lattice constant, the magnetic structure is stabilized at c = 1.10 a.In both cases the transition from the equilibrium nonmagnetic structure to the equilibrium ferromagnetic structure is discontinuous; the magnetic moment per atom jumps from 0 to 0.8 B ͑Fig. 4͒.The tetragonal distortion leads to an increase in the DOS at the Fermi level of the nonmagnetic state ͑Fig.3͒ that is the origin of the ferromagnetic instability in agreement with the Stoner picture.
The energies of the structures as a function of volume are presented in Fig. 4͑a͒.The corresponding dependencies of the atomic spin moments are given in Fig. 4͑b͒.The characteristic feature of the tetragonally distorted B2-t is the pres-ence of two discontinuous phase transformations.As already mentioned, in the low-volume region the system is nonmagnetic.With increasing volume, the system first transforms into a low-spin ͑LS͒ ferromagnetic state.Further increase in the volume leads to the discontinuous transformation to a high-spin ͑HS͒ state.These transformations lead to discontinuities of the first derivative of the energy as a function of volume.In contrast to the tetragonally distorted systems, the volume variation for the B2-c does not lead to a discontinuous phase transformation.With an expansion of the system, it reaches a critical point of the appearance of ferromagnetism.With further increase in the volume, the magnetic moment increases, continuously staying, however significantly, lower than the high-spin moment of the distorted phases.These results show that the tetragonal distortions of the B2 phase inherent for the epitaxially grown films enhance the trend to the formation of a magnetic state.
Next we estimate the magnetic transition temperature for the B2 structure with the Si lattice parameters a = b = 2.74 Å and c = 3.1 Å.For these lattice parameters the ferromagnetic state has the LS type.Our calculation shows that the antiferromagnetic state, which has a checkerboard structure in the xz plane, has its energy lower than the ferromagnetic state by 16 meV/Mn.Therefore, we estimate the Néel temperature of this antiferromagnetic state.To estimate the exchange field experienced by the magnetic moments, we compare the energy of the ground antiferromagnetic state and the energy of the configuration in which one of the spins is reversed.This energy difference is 40 meV.Under a simplified assumption about the character of the interatomic exchange interactions, we get on the basis of the mean-field approximation the value of the Néel temperature of 60 K.

IV. THIN FILMS
The epitaxial thin films of some materials grow in a crystal structure that is different from their bulk structure ͑pseudomorphical growth͒.This property is explained by the presence of the strain and/or interface and surface energy contributions.The studies of the structural stability and magnetic properties of pseudomorphically grown MnSi in B2 structure on Si͑001͒ are reported earlier. 1,2,5The unit cell of the B2 structure is rotated by 45°with respect to the Si͑001͒͑1 ϫ 1͒ surface unit cell.The surface unit cell of a one monolayer ͑ML͒ B2 film contains two inequivalent Mn at- oms that sit at the bridge sites of the Si͑001͒͑1 ϫ 1͒ surface.We have shown that the Mn moments in this film are ferromagnetically coupled.We found previously that the films are ferromagnetic up to two MLs.With further increase in the film thickness, an interlayer antiferrimagnetic order becomes favorable. 1,2An important question is if the ferromagnetic order of the one and two ML films of MnSi is retained at room temperature.Answering this question requires the knowledge of the interatomic exchange interactions.
In this section we estimate the Curie temperature on the basis of first-principles calculations of the Mn-Mn exchange parameters.The exchange parameters are evaluated by fitting the DFT energies of a number of collinear magnetic configurations with a Heisenberg Hamiltonian.A larger number of magnetic configurations leads to a larger number of the exchange parameters that can be obtained by the fitting.The number of the considered collinear magnetic configurations is restricted by the size of the magnetic supercell used in the calculations.Since the calculation for large super cells with the WIEN package is very expensive, we evaluated parameters for near neighbors only.Previous calculation of exchange interaction for various systems show that, in compounds, the exchange parameters are expected to decay rapidly with an increasing distance between the magnetic atoms, and the restriction to nearest neighbors is reasonable. 27In the following the Curie temperature is calculated for the films consisting of one or two MLs.
We begin with the consideration of one ML.The in-plane exchange couplings for the first-and second-nearest neighbors are obtained for the case of one ML of MnSi using a ͑2 ϫ 1͒ surface unit cell containing four MnSi formula units ͓Fig.5͑a͔͒.
The first-neighbor interaction is between Mn atoms in the corner ͑Mn 1 ͒ and in the middle ͑Mn 3 ͒ of Fig. 5͑a͒.In the second Mn neighbor interactions, Mn atoms are both located either in the corner ͑Mn 1 and Mn 2 ͒ or in the middle ͑Mn 3 and Mn 4 ͒ ͓Fig.5͑a͔͒.For the selected supercell there are five inequivalent collinear magnetic configurations ͑MCs͒.MC1 and MC2 are antiferrimagnetic while MC3 and MC4 are antiferromagnetic.The energies and magnetic moments of these configurations are collected in Table II.The inspection of the energies of the magnetic configurations shows that the energy of MC3 is much higher than the other configurations.Since a characteristic feature of MC3 is an antiparallel orientation of the moments of all nearest neighbors, this high energy reveals a strong ferromagnetic interaction between nearest neighbors.
We found that the following exchange parameters reproduce well the energies of magnetic configurations collected in Table II.The parameter of the nearest-neighbor interaction is 13.8 meV.The second-nearest-neighbor interactions between the Mn 1 and the Mn 2 ͑the Mn 3 and the Mn 4 ͒ are 3.8 ͑4.9͒ meV.As expected, the exchange interaction with second neighbors is significantly smaller than with the nearest neighbors.The nearest-neighbor interaction determines substantially the value of the Curie temperature.
In the case of the two ML films, we use a ͑1 ϫ 1͒ surface unit cell to evaluate the interlayer exchange parameters, i.e., two sublattices per layer.On the basis of this unit cell, we can calculate the first-neighbor in-plane exchange parameter for both surface and interface layers.Since the surface and interface Mn layers have different Si environments, the exchange interactions within layers can be different.The firstneighbor interlayer exchange interaction is between Mn 2 -Mn 4 and Mn 1 -Mn 3 ͓Fig.5͑b͔͒.The second-neighbor interlayer exchange interaction is between Mn 1 -Mn 4 and Mn 2 -Mn 3 .
To estimate the exchange parameters, we consider five magnetic configurations.Their energy and magnetic moments are given in Table III.The comparison of the energies of magnetic configurations shows that MC1 and MC2 have very similar high energies, whereas the energies of the MC1 and MC4 are much smaller.The following set of parameters  describe these energies: The in-plane parameter for the Mn atoms in the surface layer is J 12 = 7.4 meV, and J 34 = 19.8meV for the interface layer.This large difference reveals a strong coordination influence of the Si states on the Mn-Mn exchange interaction.Note that the number of neighboring Si atoms for a Mn atom in the interface layer is less than for the Mn atom in the surface Mn layer.The firstneighbor interlayer exchange parameters J 24 and J 13 are estimated as 19.6 meV and the second-neighbor interlayer exchange parameters J 14 and J 23 are estimated as −2.9 meV.A negative sign of the exchange parameter shows that an antiparallel spin coupling is more favorable.As a conclusion, the exchange coupling between Mn atoms depends not only on interatomic distances but also strongly on the number of neighboring Si atoms.

A. Free-standing one monolayer of MnSi
The strength of the interatomic exchange interaction is determined by the electronic structure of the system.The relation between the electronic structure and the exchange parameters is, however, very complex.The reason for this complexity can be explained as follows.The exchange parameters describe the change of the total energy of the system with the change of the magnetic configuration.The total energy is an integrated characteristic that depends on the electronic states of the system.The variation of the magnetic structure leads to a complex pattern of the changes in the electronic states.This pattern depends crucially on the energy position of the states, their spin polarization, and hybridization.The contributions to the total-energy change coming from different electronic states largely compensate each other and the final value of the total-energy variation is determined by a fine balance of interactions in the system.
Although we cannot directly predict the values of the effective exchange parameters by the analysis of the electronic DOS of the ground state, to get an insight into the role of the Si atoms in the formation of the magnetic interactions, it is instructive to compare the variation in the electron structure for a series of systems.We performed calculations for five Mn-Si films ͑Fig.6͒.One ML of Mn atoms is the same for all films and corresponds to two Mn atoms in a ͑1 ϫ 1͒ surface unit cell.The Si environment is changed systematically.From the viewpoint of the Si environment, the five systems can be characterized as follows: ͑i͒ pure Mn monolayer, ͑ii͒ a sparse Si layer on one side of the Mn ML, ͑iii͒ a dense Si layer on one side of the Mn ML, ͑iv͒ one dense and one sparse Si ML on both sides of the Mn ML, and ͑v͒ dense Si ML on both sides of the Mn ML ͑Fig.6͒.The dense layer has two Si atoms per ͑1 ϫ 1͒ surface unit cell while the unit cell of the sparse layer contains one Si atom per surface cell.
Figure 6 illustrates the influence of the Si atoms on the partial Mn 3d DOS.This influence is very strong and clearly reveals the important role of the Mn-Si hybridization.This hybridization leads to a broadening of the DOS in each of the spin channels that is also reflected in the decrease in the Mn moments with increasing number of the Si atoms in the near environment.
The Mn magnetic moment for an isolated Mn layer is 3.9 B , which reduces to 3.4 and 2.9 B for the structures ͑ii͒ and ͑iii͒ ͓Figs.6͑a͒-6͑c͔͒.
The magnetic moment of the Mn layer sandwiched between two asymmetric dense and sparse Si layers ͓Fig.6͑d͔͒ is 2.3 B , which is strongly reduced to 0.8 B for two symmetric dense Si layers.
The comparison of the total energies of the ferromagnetic and antiferromagnetic configurations shows that for the cases ͑i͒ and ͑ii͒, the exchange interaction between nearest Mn atoms is antiferromagnetic, whereas it becomes ferromagnetic for structures ͑iii͒-͑v͒.This property emphasizes again the importance of the hybridization of the Mn 3d states with the states of the nominally nonmagnetic Si atoms.As stated above, there is no simple way for deriving the values of the exchange interactions on the basis of the electronic structure.A possible qualitative reason for the transition from ferromagnetic exchange to antiferromagnetic exchange might be, for instance, the mechanism discussed by Kanamori and Terakura. 28

B. Magnetic transition temperature
Now we turn to the estimation of the Curie temperature for MnSi ultrathin films.As discussed in Sec.II we apply 1layer Si-Mn-Si(spare) FIG. 6. Density of states of the Mn atom in the unsupported free-standing layer.͑a͒ A single Mn layer, ͑b͒ Mn-Si ͑sparse͒ layer, ͑c͒ Mn-Si ͑dense͒ layer, ͑d͒ Mn layer sandwiched between two asymmetric Si dense and sparse layers, and ͑e͒ Mn layer sandwiched between two symmetric Si layers.The Fermi level is set to the energy zero.The schematic of each structure is shown above the plots, small circles are Si and large circles are Mn atoms.two approaches: mean-field approximation ͑MFA͒ and random-phase approximation ͑RPA͒.
Within MFA and using the exchange parameters derived above, we obtain a Curie temperature of 565 and 757 K for one and two MLs, respectively.These large values of the T c -MFA reflect strong interatomic exchange interaction.This is mainly due to the enhancement of the value of the magnetic moment in a layered structure.
To determine the Curie temperature within RPA, we plot the Curie temperature as a function of the ratio of the sublattice magnetizations for each of the two sublattices ͓Eq.͑4͔͒.The intersection of the curves gives the Curie temperature of the system.
For one ML, J 12 and J 34 are very close to each other.The two sublattices can be considered as almost equivalent.That makes the curves T͓log͑m 2 / m 1 ͔͒ for the two sublattices almost mirror symmetric and results in the intersection of the curves close to the point m 1 =m 2 ͓Fig.7͑a͔͒.
For two ML coverage, the surfacial magnetization drops faster than the interfacial magnetization.The curves intersect at the point where the ratio of magnetizations is 1.8 ͓see Fig. 7͑b͔͒.The inequivalence of the Mn layers results from the different Si environment ͑see Fig. 5͒.
In Fig. 8 we show the dependence of the Curie temperature on the anisotropy parameter for both one and two ML films.The logarithmic dependence of T c on ⌬ is in agreement with the rule proposed by Bruno. 29A calculation that includes spin-orbit interaction gives an anisotropy parameter of 0.4 meV for this system.This is comparable with the value calculated by Pajda et al. 30 for Fe and Co films.We obtain values of the RPA Curie temperature of 241 and 328 K for the one and two ML MnSi films, respectively.Since RPA treats properly the long-wavelength fluctuation, it gives better estimation of T c . 14ue to strong ferromagnetic interlayer coupling, the Curie temperature is enhanced with increasing film thickness from one to two MLs.Additionally we calculate the Curie temperature for a free-standing two ML MnSi films without a Si substrate ͓this correspond to the four top atomic layer of Fig. 5͑b͔͒.In this case the MFA approach gives a value of 211 K while the RPA value is 135 K. Therefore, the Si substrate enhances the Curie temperature of the MnSi film significantly.
The dependence of the magnetic transition temperature on the dimensionality of the system is a nontrivial issue since the change of the dimensionality leads to changes in the system that have opposite effect on the magnetic transition temperature.The reduced coordination number of the magnetic atoms leads to a decreasing number of exchange interactions with neighboring atoms.This factor is expected to result in a decreased transition temperature.On the other hand, the decreased coordination number leads to less intense hybridization of the states of the magnetic atom with the states of the environment.Narrower energy bands are the consequence of the decreased hybridization, which can cause a strong increase in the atomic magnetic moments and interatomic exchange interactions.Whichever one of these two trends wins can be different for different systems.In our calculations we found that the strong increase in the magnetic moments and exchange interactions in the films compared to the bulk overcompensates the decreased number of the interatomic interactions and leads to an enhanced Curie temperature of the film.͑See the related experimental and theoretical discussions of the enhanced Curie temperature of the Gd surface. 31,32͒

V. SUMMARY
In the present work we focused our study on the calculation of the ground-state and thermodynamic magnetic properties of the bulk, free-standing, and supported films of MnSi with the B2 crystal structure.We show that decreasing the coordination number of the Mn atoms in the MnSi film leads to an increase in the magnetic moments, the exchange interactions, and the magnetic transition temperature compared to the bulk system.Also, the Si substrate increases the Curie temperature of MnSi film.To get a deeper insight into the role of Si, we perform calculation of a Mn monolayer with different Si environments, and demonstrate that increasing Si coordination of Mn atoms stabilizes ferromagnetism.We study the dependence of the Curie temperature of the films on the magnetic anisotropy energy that has a weak logarith- mic character.For the calculated magnetic anisotropy energy of 0.4 meV per Mn atom, the Curie temperature assumes the value of 241 and 328 K for one and two ML MnSi films, respectively.

FIG. 2 .
FIG.2.͑Color online͒ Energy-volume dependence of B20 ͑black curve͒, the B2-c structure ͑blue curve͒.The red dashed line is the common tangent of both curves.The pressure for the transition phase between the B20 and the B2-c structure is the slope of the red dashed line and is around 40 GPa.

4 FIG. 5 .
FIG. 5. ͑a͒ Top view of one ML ͑with a ͑2 ϫ 1͒ in-plane surface unit cell͒ and ͑b͒ side view of two MLs ͓with a ͑1 ϫ 1͒ in-plane surface unit cell͔ MnSi/Si͑001͒.Black circles are Mn and gray circles indicate Si atoms.

FIG. 7 .FIG. 8 .
FIG.7.͑Color online͒ Dependence of T c on the logarithmic scale of magnetization ratio of two sublattices for ͑a͒ a one ML and ͑b͒ a two ML MnSi films.Red and green curves belong to the sublattices in the surface and interface layers, respectively.The curves intersect at the point where log͑m 2 / m 1 ͒ is 0 for one ML and 0.255 for two ML films.This points correspond to the T c of 241 and 328 K for one ML and 0.255 for two MLs, respectively.The anisotropy energy obtained by spin-orbit calculation is 0.4 meV.

TABLE II .
Total energies and magnetic moments of various magnetic configurations of one ML MnSi.The surface supercell contains four Mn atoms numbered according to Fig.5͑a͒.The energy of the ferromagnetic state is taken as the energy zero.+/− present up/down directions of spins of Mn atoms.FIG.4.͑Color online͒ ͑a͒ B2 in tetragonal structure with inplane lattice constant of 2.74 Å ͑green curve͒ and 2.79 Å ͑red curve͒.Dashed lines are for the nonmagnetic state and solid lines are for the FM state.͑b͒ The magnetic moment per formula unit in the tetragonal B2 structure.The magnetic moment of PM, LS, and HS states are clearly resolved in this plot.

TABLE III .
Total energies and magnetic moments of various magnetic configurations of two ML SiMn.The surface supercell contains four Mn atoms in two layers, numbered according to Fig.5͑b͒.The energy of the ferromagnetic state is taken as energy zero.