Journal of Magnetism and Magnetic Materials 323 (2011) 2883–2887

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

Aizawl

E-m

journal homepage: www.elsevier.com/locate/jmmm

Electronic and magnetic properties of SmCrSb3 and GdCrSb3:A first principles study

Sandeep a, M.P. Ghimire b, R.K. Thapa a,c,n

a Department of Physics, Mizoram University, Tanhril, Aizawl 796009, Mizoram, Indiab Faculty of Science, Nepal Academy Of Science and Technology, Khumaltar GPO 3323, Kathmandu, Nepalc Condensed Matter Physics Research Center, Butwal, Rupendehi, Nepal

a r t i c l e i n f o

Article history:

Received 3 December 2010

Received in revised form

9 June 2011Available online 29 June 2011

Keywords:

DFT

FP-LAPW

Density of state

Magnetic moments exchange splitting

53/$ - see front matter & 2011 Elsevier B.V. A

016/j.jmmm.2011.06.045

esponding author at: Department of Physics,

796009, Mizoram, India. Tel.: þ91 389 23280

ail address: r.k.thapa@gmail.com (R.K. Thapa)

a b s t r a c t

The density of states (DOS) and the magnetic moments of SmCrSb3 and GdCrSb3 have been studied by

first principles full-potential linearized augmented plane wave (FP-LAPW) method based on density

functional theory (DFT). For the exchange-correlation potential, the local-spin density approximations

with correlation energy (LSDAþU) method have been used. Total and partial DOS have been computed

using the WIEN2k code. DOS result shows the exchange-splittings of Cr-3d and rare-earth (R) 4f states

electrons, which are responsible for the ground state ferromagnetic (FM) behavior of the systems. The

FM behavior of these systems is strongly influenced by the average number of Cr-3d and Sm (Gd)

4f-electrons. The effective moment of SmCrSb3 is found to be 7.07 mB while for GdCrSb3 it is 8.27 mB. The

Cr atom plays a significant role on the magnetic properties due to the hybridization between Cr-3d and

Sb-5p states.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The co-existing localized and itinerant spins of RCrSb3 com-pounds have attracted interest for their major technologicalapplications in spin electronics, Read head, magnetic RAM,nano-particles, etc. [1]. A series of RCrSb3 compounds crystallizein orthorhombic phase with space group Pbcm. The structure isvisualized as two dimensional infinite layers of the compositionR–Sb and CrSb2 extending perpendicular to the long translationalunit a as shown in Fig. 1. The Cr atom situated in somewhatdistorted Sb6 octahedra. These chains are connected to each othervia edge sharing at the octahedral to form two dimensionalinfinite double layer of CrSb2 [2]. Jackson et al. [3] have reportedthe effect of rare-earth doping to find a single ordering tempera-ture for Cr magnetic sublattice with anti-align Gd3þ sublattice forGdCrSb3. SmCrSb3 has been reported [3] with unique first orderphase transition due to the coupling between 3d and 4f momentsof Cr and rare-earth elements in terms of de-Gennes factor.Numerous studies on the phase transition of the RCrSb3 com-pounds have been reported experimentally [4–8] but the theore-tical investigation is very much limited. Raju et al. [9] studiedLaCrSb3 to suggest the ferromagnetic behavior of the systemby the Huckel’s method. They found the itinerant electron

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Mizoram University, Tanhril,

44; fax: þ91 389 2330522.

.

ferromagnetic behavior due to strong electron correlation in theCr-3d bands. They have also reported the band structures to becomprised of Cr–Sb and Sb–Sb layers interacting weakly alongstacking direction a-axes. Choi et al. [10] studied LaCrSb3, LaVSb3

and YbCrSb3 using linear muffin-tin orbital (LMTO) methods.Their observations showed that LMTO fails to explain the rigidband model of LaCrSb3 and YbCrSb3 as Yb2þ ions are in a divalentstate with fully occupied 4f bands, which differ from the trivalentLa3þ in LaCrSb3.

We have studied the spin-polarized DOS and magneticmoments of SmCrSb3 and GdCrSb3 by FP-LAPW method withinDFT formalism [11]. The exchange-correlation was treated usingLSDAþU method [12].

2. Computational details

We have performed our calculations using the experimentallydetermined lattice parameters and the atomic positions [13–15]for SmCrSb3 and GdCrSb3. Since Sm and Gd 4f-orbitals are ratherlocalized, the 4f-electron correlations are expected to be strong.Consequently, the LSDAþU calculations have been chosen toinclude the on-site Coulomb interaction. The onsite Coulombenergy (U) and the exchange parameter (J) applied are 0.52 and0.00 Rydberg (Ry), respectively. We have used 47 k points in theirreducible Brillouin zone, and the muffin-tin radii for Sm, Gd, Cr,Sb1, Sb2 and Sb3 are 2.5, 2.5, 2.5, 2.35, 2.35 and 2.35 in atomicunit, respectively. The density plane cut-off RMTnKMAX is 7.0,

Fig. 1. Crystal structure of RCrSb3 using Xcrysden.

Sandeep et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2883–28872884

where KMAX is the plane wave cut-off and RMT is the muffin-tinradii. The self-consistency is better than 0.001 e/au3 for chargedensity and spin density and the stability is better than 0.01 mRyfor total energy per cell. For computations of DOS and magneticmoments, WIEN2k code [16] is used.

Fig. 2. Total and partial DOS plots for (a) SmCrSb3 (b) Sb-5s, p (c) Cr-3d (d) Cr-deg

and Cr-t2g and (e) Sm-4f states.

3. Results and discussions

3.1. Density of states

The total and partial DOS plots of SmCrSb3 are shown inFig. 2(a–e). From the total DOS plots [Fig. 2(a)] of spin-up anddown configurations, we find a peak at �9.0 eV below the Fermilevel (EF), which arises due to Sb-5s states [Fig. 2(b)]. In thevalence region of the spin-up case, peaks in the energy range�5.7 upto �0.2 eV are contributed due to Cr-3d states and Sb-5p

state electrons [Fig. 2(b, c)]. In spin-down case, the DOS contribu-tions in these regions were mostly due to Sb-5p state electrons.A sharp peak is observed at EF due to Sm-4f state in spin-up case[Fig. 2(e)]. In the conduction region, a uniform distribution of DOSis found in spin-up, which is due to Sb-5p and Cr-3d stateelectrons [9–10]. In spin-down channel, a sharp peak isobserved at �4.0 eV, which is due to Sm-4f state electrons.

Exchange-splitting of Cr-3d state orbitals were found to occur interms of decomposed eg and t2g orbitals [Fig. 2(d)]. The exchangesplitting between eg up and eg down states for Cr-3d is 10.5 eV,and that between t2g up and t2g down states is 9.0 eV. Also,exchange splitting for Sm-4f between up and down spins is4.0 eV. These energy differences between the exchange-splittingof the states give rise to the magnetic behavior of Cr and Sm inSmCrSb3. Also, Cr-3d shows the itinerant while Sm-4f shows thelocalized behavior [9–10,17] supporting the ferromagneticground state for SmCrSb3. The ground state energy of SmCrSb3

was calculated as �247490.18 Ry.The total and partial DOS plots of GdCrSb3 are shown in

Fig. 3(a–e). The ferromagnetic behavior is reflected in the total

Fig. 3. Total and partial DOS plots for (a) GdCrSb3 (b) Sb-5s, p (c) Cr-3d (d) Cr-deg

and d-t2g and (e) Gd-4f states.

Sandeep et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2883–2887 2885

DOS (Fig. 3a) [18,19]. From the DOS plots, peak is observed at�9.0 eV in the core region, which is due to Sb-5s states in bothspin channels [Fig. 3(b)]. A sharp peak at �4.8 eV is due to Gd-4f

state electrons in spin-up channel [Fig. 3(e)]. Also, the contribu-tions due to Cr-3d and Sb-5p are found in the valence region in anenergy range �4.6 eV up to the Fermi level [Fig. 3(b, c)]. In thevalence region of spin-down channel, only Sb-5p state is con-tributing to the total DOS. A sharp peak at EF of the spin-downchannel is due to Gd-4f state electrons. In the conduction region,Sb-5p states with few Cr-3d states contribute in spin-up channel,while in spin-down channel, Cr-3d state dominates over Sb-5s

and 5p states. The crystal field produced by the Sb-octahedraresults into a splitting between t2g and eg states of Cr and Gd-4f

states in GdCrSb3 [Fig. 3(d, e)]. The exchange splitting between t2g

up and t2g down for Cr is 10.5 eV, and that between eg up and eg

down is 10.0 eV. The exchange splitting is also observed for Gd-4f

states in spin-up and spin-down. This exchange splitting wasfound to be 5.0 eV due to which the Gd moment is higher thanthat of Cr moment in GdCrSb3. The ground state energy ofGdCrSb3 was calculated as �254256.46 Ry.

3.2. Band structures plots

The spin-up and spin-down band structures for both systemsare shown in Figs. 4 and 5. The DOS results were supplemented bythe calculated bands. The peaks in the DOS were observed in theform of flat energy bands at corresponding energy regions. In caseof SmCrSb3, these peaks were observed at EF in the spin-upchannel and at 4 eV in the spin-down channel [Fig. 4]. Thecontributions due to Cr-3d states in the DOS were also observedin the valence region of spin-up and conduction regions of spin-down channel, respectively. For GdCrSb3, flat bands wereobserved mainly in valence regions due to both Cr-3d and Gd-4f

states as observed in the DOS plots for the spin-up channel[Fig. 5]. In the spin-down channel, we observe flat bands at andabove EF due to Gd-4f state electrons. Also, in the conductionregion, we observe higher number of bands compared to spin-upchannel due to the contributions from Cr-3d state electrons asobserved in the DOS plots of GdCrSb3.

3.3. Magnetic moments

We have calculated the effective moments of SmCrSb3 andGdCrSb3 to be 7.07 and 8.27 mB, respectively. For both thesystems, the moments were mostly contributed by Cr-3d andrare-earth (Sm-4f and Gd-4f) ions. In Table 1, we have summar-ized the total and individual spin moments of SmCrSb3 andGdCrSb3. It is observed that the magnetic moments of Sm, Gdand Cr are somewhat smaller than the corresponding experi-mental values [2,20]. These differences were partly due to theadditional magnetic moments distributed at the Sb sites and theinterstitial region. The magnetic moment datas have also showedstrong evidences for Cr moments in these anitimonides. We haveobserved that the sum of the magnetic moment of the constituentatoms is not equal to the total magnetic moment of the materials.This is because, while calculating the electronic and magneticproperties of these compounds, we use the muffin-tin sphereswhere some of the interstitial areas are excluded [21]. We havecompared our results with the experimental results provided byothers [2,20]. Compared to GdCrSb3, the experimental value ofmoment of SmCrSb3 is quite low. The discrepancy in the momentof SmCrSb3 can be well attributed to the temperature factor [2].The present calculation is a ground state study, which gives fairlydifferent results [9]. With the increasing magnetic moments forthe lanthanide ions in RCrSb3, starting from LaCrSb3, the valueobtained by us for SmCrSb3 is obvious.

4. Conclusions

We have studied the electronic and magnetic properties ofSmCrSb3 and GdCrSb3 by the FP-LAPW method. The ground stateof SmCrSb3 and GdCrSb3 shows the ferromagnetic behavior due tothe exchange splitting between different states of the Cr-3d andthe rare-earth 4f ions as well as the high moments. The exchangesplitting of eg and t2g of the Cr-3d state as well as the rare-earth 4f

ions in spin-up and spin-down were explained on the basing onthe DOS plots. The energy difference between these states showsthe variation of magnetic moments in the system. The rare-earthmoments of these systems were found to be greater than the Cr

Fig. 4. Spin polarized band structures for SmCrSb3: (a) spin-up and (b) spin-down.

Fig. 5. Spin polarized band structures for GdCrSb3: (a) spin-up and (b) spin-down.

Sandeep et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2883–28872886

moment, which also shows the presence of strong 4f states of Smand Gd atoms. The Cr atom plays a significant role on themagnetic properties of these compounds due to the hybridizationbetween 3d states of Cr and 5p states of Sb atoms. We found that

the moments due to individual Cr-3d and rare-earth (Sm-4f andGd-4f) ions are mainly contributing to the total moment. Themoment for SmCrSb3 is 7.07 mB and for GdCrSb3 it is 8.27 mB,respectively. The magnetic moments obtained are compared with

Table 1Total and individual magnetic moment for SmCrSb3 and GdCrSb3.

Components SmCrSb3

(mB/f.u)

Previous results

(mB/f.u)

GdCrSb3

(mB/f.u)

Previous results

(mB/f.u)

Rare-earth 5.59 (Sm) 1.69 [2] 6.60 (Gd) 5.6 [20]

Cr 3.68 3.01[2] 3.97

Sb1 �0.17 �0.17

Sb2 �0.18 �0.11

Sb3 �0.06 �0.02

Total 7.07 2.04 [2] 8.27 8.16 [20]

Sandeep et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2883–2887 2887

the experimental results [2,20]. We find a close agreement forGdCrSb3 while for SmCrSb3 it is different, which may be to thetemperature factor in the experiment.

Acknowledgements

RKT acknowledges a research project and SD a JRF fromDAE(BRNS), Mumbai, India.

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