Half-Metallic Ferromagnetism in Chalcopyrite (AlGaMn)P2 Alloys

We studied the electronic and magnetic properties of (Al1−yMny)GaP2 (Ga-rich) and Al(Ga1−yMny)P2 (Al-rich) with y = 0.03125, 0.0625, 0.09375, and 0.125 by using the first-principles calculations. The ferromagnetic Mn-doped AlGaP2 chalcopyrite is the most energetically favorable one. The spin polarized Al(GaMn)P2 state (Al-rich system) is more stable than spin polarized (AlMn)GaP2 state (Ga-rich) with the magnetic moment of 3.8 /Mn. The Mn-doped AlGaP2 yields strong half-metallic ground states. The states of host Al, Ga, or P atoms at the Fermi level are mainly a P-3p character, which mediates a strong interaction between the Mn-3d and P-3p states.


Introduction
Diluted magnetic semiconductor (DMS) materials have become a great interest because the charge from the  and  electrons of the nonmagnetic semiconductor and the spin from the magnetic dopant can be used in spintronics devices [1][2][3][4][5][6].Ferromagnetism has been reported in various semiconductor groups including II-VI [3,7], III-V [3,8], and II-IV-V 2 [9].However, to date, the opportunities of DMS's applications have been limited due to the low solubility of magnetic ions in nonmagnetic semiconductor hosts.Thus it is one of the primary challenges to create the ferromagnetic (FM) semiconductors due to the difficulty in the spininjection into the semiconductor to form DMS at room temperature or above room temperature.
When FM metals are used as spin injectors, the polarization in the semiconductor tends to be quickly lost via spin-flip scattering.It has been reported that the Mn-doped chalcopyrite such as ZnSnAs 2 [9] and ZnGeP 2 [10] shows FM ordering at 320 and 312 K, respectively.A newly synthesized MnGeP 2 ternary compound has been reported as a semiconductor whose crystal structure is chalcopyrite.It has been reported that MnGeP 2 exhibits ferromagnetism with   = 320 K and a magnetic moment per Mn at 5 K of 2.58  B , and an indirect energy gap of 0.24 eV.
Overberg and coworkers have investigated ion implantation of Cr or Mn at concentrations of 1-5 at.% in Al  Ga 1− P ( = 0.24, 0.38) epilayers grown by gas source molecular beam epitaxy [11,12].The FM-like ordering has been observed above 100 K for Cr and 300 K for Mn in AlGaP.The Mn dopant appears to be a more promising choice than Cr for high temperature ferromagnetism in AlGaP.AlP is unstable in air and oxidizes rapidly.However, ternary AlGaP material is used in the device structures.We see it can enhance the Curie temperature while maintaining material stability.Other recent experiments for (Ga,Mn)P show ferromagnetism above 300 K [13][14][15].The Curie temperature is strongly influenced by the carrier density and type in the material with highly p-type samples showing much higher values than ntype or undoped samples.Finally, it can be found that the Curie temperature increases with Mn concentration.
In the paper, the electronic and magnetic properties of (Al,Mn)-codoped to fabricate GaP-based DMS have been studied by using first-principle calculations.In the system of AlGaP, Mn, we observed the FM ordering with high magnetic moment (∼3.8  B /Mn).We predicted that AlGaP 2 Advances in Condensed Matter Physics ternary compound can have a character of chalcopyrite semiconductor with a direct band gap.Chalcopyrites, which are genealogically related to the more familiar tetrahedrally coordinated zinc-blende materials, are a class of semiconductors recognized as promising materials for nonlinear optical applications.

Computational Methodology
The electronic structure and magnetic properties on the (Al,Mn)-codoped GaP semiconductor which consists of a supercell of 64 atoms have been investigated.We checked three doping levels of 6.25%, 9.375%, and 12.5% of Mn atoms, respectively.The first-principles simulations were performed using the full-potential linear muffin-tin orbital (FPLMTO) method [16] based on density functional theory (DFT).All calculations were performed within the generalized gradient approximation (GGA) with the exchange-correlation functional proposed by Perdew-Burke-Ernzerhof scheme [17].The muffin-tin radii of Mn (or Al, Ga) and P were chosen to be 2.4 and 1.9 a.u., respectively, with the plane-wave energy cutoff by 596.6 eV.The convergence tests of the total energy with respect to the plane-wave energy cutoff and -point sampling have been carefully examined.
The LMTO basis set and charge density were expanded in terms of the spherical harmonics up to  = 6 inside each muffin-tin sphere.The LMTO basis functions in the valence energy region were chosen as 4s and 3d for Mn, and 4s, 4p, and 3d for Ga.The basis function of Mn (or Al) for the 4s (or 3s), 4p (or 3p), and 3d is generated with cutoff energy of 159.12 eV, 232.56 eV, and 340.0 eV, respectively.The valence electrons were not assumed to have the spinorbital coupling but generated the self-consistent supercell potential by considering the scalar relativistic effects.The atomic potentials were approximated by spherically symmetric potential; however the full charge density including all nonspherical terms was evaluated in Fourier series in the interstitial region on the FPLMTO method.The charge density is determined self-consistently by using a gammacentered 4 × 4 × 4 grid in the Brillouin zone.Using 64 k points of 4 × 4 × 4 grid insured that the total energies and the magnetic moments were converged on a better 10 meV/cell and 0.01  B /atom scale, respectively.

Results and Discussion
For the AlP, GaP, and pure AlGaP 2 systems, we considered the atomic relaxations for the positions of the structures.The atomic geometry and positions of the structures were fully relaxed until the force between atoms was less than 1.0 mRy/Bohr.However, the distortions of near host atoms by substituting Mn dopant in the AlGaP 2 bulk were neglected.We found the equilibrium lattice parameters for the chalcopyrite structure from first-principles calculations.The equilibrium lattice parameters are  = 5.726 Å and  = 11.452Å for GaP;  = 5.648 Å and  = 11.296Å for AlP.For AlGaP 2 , they are  = 5.685 Å,  = 11.222Å, and / = 1.974.In the case of GaP and AlP systems, we performed a minimization of total energy for the supercell volume keeping the constant / ratio (=2.0).For the AlGaP 2 system, we considered the structural relaxation for each of  and -axis.The calculated parameters for GaP and AlP can be compared with that of the zinc-blende structure.The experimental values are  = 5.451 > and 5.450 > for GaP [18,19] and AlP [18], respectively.The chalcopyrite GaP, AlGaP 2 , and AlP structures exhibit the semiconducting character with energy gaps of 0.276 eV, 1.241 eV, and 1.679 eV, respectively.We can see a downward bowing of the lattice parameter with respect to the Al concentration increases in the Al  Ga 1− P 2 system ( = 0.0, 0.25, 0.75, and 1.0), while the band gap increases from 0.276 eV to 1.679 eV.The band gap is smaller than the experimental value [20].In general, the magnitude of band gap calculated using the GGA or LDA (local density approximation) is less than half of the value obtained in the experiment.The chalcopyrite AlGaP 2 indicates a semiconducting character with direct band gap (not to be seen in the figure).The tetrahedral chalcopyrite structure was displayed in Figure 1.The lattice parameters for each crystalline and the energy gap were listed in Table 1.
We found that the Al(Ga,Mn)P 2 is more energetically favorable than the (Al,Mn)GaP 2 (Ga-rich).However, the difference in the substitution energies between both systems is very small.The substitution energy is −346.

Figure 2 : 4 AdvancesFigure 3 :
Figure2: DOS for Al, Ga, P, and Mn sites of 3.125% Mn-doped AlGaP 2 in the FM state.The Fermi level is set to zero.

Figure 4 :
Figure 4: Band structures for Ga-rich (Al 1− Mn  )GaP 2 of (a) and (b) and Al-rich Al(Ga 1− Mn  )P 2 of (c) and (d) with  = 0.0625 and 0.125 in the FM state.The Fermi level is set to zero.