The electronic structures of YFe12-xMoxNy , where x=1.0, 2.0 and y=0, 0.7, have been studied with photoemission and spin-polarized calculations. The peak near the Fermi level in the energy distribution curves (EDC) becomes successively broader with larger Mo concentration. The features in the calculated density of state at 1.3 and 2.7 eV are not readily seen in the EDC, and this may be due to lifetime effects in these compounds. Finally, changes in Curie temperature (Tc) with the change of N or Mo concentration are compared with prediction of the theory of Mohn and Wohlfarth. Reasonable agreement is obtained in the N case but not in the Mo case, the latter most likely due to hybridization of Fe and Mo d bands

Fernando, A.S.

Jaswal, Sitaram

Patterson, B.M.

Sellmyer, David J.

Welipitiya, D.

Woods, J.P.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy May 1994 Electronic structure and Curie temperature of YFe12-xMoxNy compounds A.S. Fernando University of Nebraska - Lincoln J.P. Woods University of Nebraska - Lincoln Sitaram Jaswal University of Nebraska, sjaswal1@unl.edu D. Welipitiya University of Nebraska - Lincoln B.M. Patterson University of Nebraska - Lincoln See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Fernando, A.S.; Woods, J.P.; Jaswal, Sitaram; Welipitiya, D.; Patterson, B.M.; and Sellmyer, David J., "Electronic structure and Curie temperature of YFe12-xMoxNy compounds" (1994). David Sellmyer Publications. 105. https://digitalcommons.unl.edu/physicssellmyer/105 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors A.S. Fernando, J.P. Woods, Sitaram Jaswal, D. Welipitiya, B.M. Patterson, and David J. Sellmyer This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicssellmyer/105 Electronic structure and Curie temperature of YFe, 2-xMoxN, compounds A. S. Fernando, J. P. Woods, S. S. Jaswal, D. Welipitiya, B. M. Patterson, and D. J. Sellmyer B&h Laboratory of Phyv’cs and Center for Materials Research and Analysis, Univwsiq of Nebraska, Linr:oln, Nebraska 68!i%%0111 The electronic structures of YFe, ,-,Mo,N, , where x = 1.0, 2.0 and y =0, 0.7, have been studied with photoemission and spin-polarized calculations. The peak near the Fermi level in the energy distribution curves (@DC) becomes successively broader with larger MO concentration. The features in the calculated density of state at 1.3 and 2.7 eV are not readily seen in the EDC, and this may be due to lifetime effects in these compounds. Finally, changes in Curie temperature (T,) with the change of N or MO concentration are compared with prediction of the theory of Mohn and Wohlfarth. Reasonable agreement is obtained in the N case but not in the MO case, the latter most likely due to hybridization of Fe and MO d bands. I. INTRODUCTION Superior permanent-magnet materials require large satu- ration magnetization, high Curie temperature (T,), and large uniaxial anisotropy. The best permanent magnetic materials are rare-earth(Rj-transition-metal(T) compounds. The tran- sition metals Fe and Co have large magnetic moments and high Curie temperatures, and the R-T compounds often have large uniaxial magnetic ani&otropies. The Fe-rich ternary compound Nd,Fe,4B has an energy product as high as 45 h*fgQe, I-’ but the relatively low I’,: (x300 “C) of this mate- rial limits its application. Permanent-magnet materials re- search is currently investigating other Fe-rich R-T com- pounds. The R T, Z body-centered-tetragonal compound with the ThMn12 structure does not exist in the binary phase, but the structure is stabilized with the partial substitution of a nomnagnetic element i&f) for Fe. The compounds RFe12...,M’, with III -Ti, V, Cr, MO, W, and Si have be.en fabricated and their magnetk properties examined.” The ad- dition of interstitial nitrogen increases the Curie temperature of the materials and the magnetic anisotropy may change.’ For example, T, of NdFeIITi increases by 30% upon nitrid- ing and the magnetic anisotropy changes from in-plane to uniax?aL7 The magnetic properties of the compound YFe,,-lMo., with s=O.5, 1, 2, 4 have been pub1ished.s T, decreases with increased Mo concentration, and increases with nitriding; the volume increases both with increased MO and N concentm- tion. The MO atoms substitute for Fe atoms on the (ij site with the largest magnetic moment.” The magnetic properties of the compounds are deter- mined by the electronic conllguration, and the increase in T, of Y,Fe17 (Ref. 10) and NdFe,,Ti (Ref. 1 I) upon nitriding has heen modeled using electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth.‘” Briefly, t 9’, jai- lv= cM$,~02 W where hl!, is the magnetic moment per Fe site, ,ycI is the enhanced susceptibility? and C is a constant. Jn earlier work the electronic structure of Sm2FeI,N,, with y = 0 and y = 2.6 has been measured with photoelect;on spectroscopy (PES), and a decrease in the density of state (DOS) at EF is ob- served as nitrogen is added which agrees with the calculated DOS.‘” In this paper the electronic structure of YFeIZmxMo,N,, withx=I, 2, andy-0, 0.7 is examined with PES, and the calculated partial DOS for Fe, Y, and MO in YFellMo are given. Comparison with the expected changes in T, are presented. II. EXPERIMENTAL PROCEDURE Bulk samples of YFe12-,Mq, with x= I, 2 were pre- pared by arc melting the elemental constituents in a water- cooled copper boat in flowing-argon gas atmosphere. All the starting elements used were at least 99.98% purity. The al- loys were melted several times to insure homogeneity. The samples were wrapped separately in Ta foils and heat treated in vacuum below 3X 10Uh Torr at 1100 “C for about 50 h,s and subsequently quenched in water. X-ray diffraction mea- surements on powder samples using Cu K, radiation showed that the samples were composed of primarily the ThMn12 structure with a small amount of O-Fe. The magnetization of the samples was measured at 5 and 300 Ii; with supercon- ducting quantum interference device WXJID) and alte.rnat- ing force gradient magnetometers, respectively, and the satu- ration magnetic moments agree with published resu1ts.s The arc-melted buttons were spark cut into discs --I mm thick to expose an interior surface which was then polished to provide an optically smooth surface for PES measure- ments. The samples were mounted on a tungsten sample ma- nipulator using Odfi-mm-diam tantalum wires in an ultrahigh vacuum chamber with a base pressure of 2 X IO-‘” Torr. Each sample was cleaned in situ with several cycles of 2 keV Ar sputtering and subsequent annealing at 3.50 “C. Auger elec- tron spectroscopy (AFZS) was employed to monitor surface cleanliness and to estimate the nitrogen concentration. After cleaning the sample, nitrogen was introduced by ion implan- tation with a kinetic energy of 2 keV and a current density of 25 PA/cm’ for 10 min.13 The subsequent nitrogen conce.ntra- tion was measured with AELS as ~~=8.3+0.8. The samples were then radiantly heated to 350 “C for several minutes to promote nitrogen diffusion from the nitrogen-rich surface into the subsurface region. The nitrogen-rich surface layer was subsequently removed with Ar sputtering to expose the subsurface nitride and the resulting surface nitrogen concen- tration measured with AES was y=O.7%0.3. Quantitatively, the nitrogen concentration per formula unit of 0.7 at the sur- J. Appi. Phys. 75 (IO), 15 May ‘1994 0021-8979/94/75(10)/6303/3/$6.00 Q 1994 American institute of Physics 6303 Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspI ’ 18 I ’ I * I * I * 1 L 1 r YFe,,,Mo; hv=22.5 eV 8 7 6 5 4 3 2 1 0 -1 Binding Energy (eV) FIG. 1. Calculated partial spin-polarized DOS of Fe, MO, and Yin YFe,,Mo and experimental energy distribution curves t.EDC) for clean YFe,,...ZMo, (x=1.0, 2.0). The EDCs are normalized to the area under the EDC curves between 0 and 3.5 eV and shifted vertically. The zero on the binding energy axis refers to the Fermi energy (EF). face of 1:12 compounds here and previously published’* is in agre.ement with published bulk values7 and supports the as- sertion that the bulk atomic structure is present up to the surface which is probed by AES and PES. The electronic structure was measured with PES experiments which we.re performed at the Synchrotron Radiation Center in Wisconsin. The photoelectron energy distribution curves (EDC) pre- sented were obtained with a photon energy of 22.5 eV and a total energy resolution of 0.14 eV. III. ELECTRONIC STRUCTURE The EDCs from the Ar-sputter-cleaned and annealed YFe,,-XMoX samples with bulk values of x= 1.0, 2.0, and the calculated partial DOS for each element in YFe,,Mo are shown in Fig. 1. The EDC curves have been normalized to the same areas between 0 and 3.5 eV binding energy and the baseline for each EDC is shifted vertically. The DOS are determined with self-consistent spin-polarized calculations using linear-muffin-tin orbitals in the scalar relativistic approximation.‘” A supercell consisting of four formula units was used to simulate the ternary compound with correct stoichiometry. The total DOS (not shown) is the sum of the three partial DOS curves, and the total DOS is dominated by the Fe DOS, due to the large atomic percentage (85%) of Fe in the compound. The present Fe DOS is substantially dif- ferent from Fe DOS in NdFe,,Ti compounds” and the dif- ference is due to the strong hybridization with MO. The EDCs are comparable to the calculated DOS with a 1-2 eV wide peak at the Fermi energy. The features in the calculated DOS at 1.3 and 2.7 eV are not readily discernible in the EDC, and this may be due to lifetime effects in these compounds.r5 The change in EDCs at E, after nitriding YFe,,Mo and YFe,eMoZ compounds are shown in Fig. 2. The YFertMo YFe,p-J.b.& hv=22..5 eV I  I  * , , , , , * , I .  I  I  I  I  I  2 1 0 Binding Energy (eV) 1 FIG. 2. Expand view of the experimental EDCs of YFettMoN, for y ~0.0, y =0.7 (upper two curves) and of YFe,eMozNy for y =tl.O, y=O.7 (lower two curves!. The zero on the binding energy axis corresponds to the Fermi energy. sample (upper two curves) shows a significant decrease in EDC at EF upon nitriding; the YFe,,Mo, sample (lower two curves) does not show any significant change in E.DC at E, upon nitriding. The comparison of EDCs for x = 1 and x = 2 without nitrogen (upper two curves) and with nitrogen (‘lower two curves) are shown in Fig. 3. The EDCs at EF for YFe,,Mo and YFer,,Mo, show no significant change; the ni- trided samples YF~,,MoN,,~ and YFe1aMoZNe7 show a slight increase in EDC at E, as the Mo:Fe ratio is increased. In previous experiments on 1:12 and 2:17 compounds”**” the change in the EDCs at E, was compared 52 .‘;i 5 d k? x .e i z ,“~‘,-‘~~t”~~!‘**~ YFe,&.to& hv=22.5 ev 2 1 0 Binding Energy (eV) FIG. 3. Expand view of EDCs of unnitrided t.upper two curves) and nitrided (lower two curves) samples of YFe,, x MO, ix= 1.0, 2.0). The zero on the binding energy axis refers to the Fermi energy. 6304 J. Appi. Phys., Vol. 75, No. IO, 15 May 1094 Fernando et a/. Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspTABLE 1. Calculated values of (J’:ITc)M-W #and (T~./T,),, for YFe,,_,Mo,N, samples. x,y (M;,M,)2a cX&,;fb ‘r’$!!-w (r;,~,),, 1.0 NT(&) (1 /eV> 1,0.7 2,0.7 0.62 0.75 ‘Reference 8. hBased on experimental NT(Ep) values. 0.46 0.72 FIG. 4. CaluIated k,, vs calculated Nr(E,) for the samples listed. IV. CONCLUSIONS ” to the calculated”‘~‘” total DOS, N1’(EP)=N,(EF)+NI(EF), where N-ljll’,c) and Nt(EFj are the up- and down-spin DOS at the Fermi level, respectively. The calculated DOS, Stoner parameter, and magnetic moments used with Mohn and Wohlfarth theory predict a change in T,. upon nitrogenation which agrees very well with measured c.hange in T, . In the present experiment the calculated DOS is available only for the YFqIMo compound, and a comparison of changes in N,(E,) as the nitrogen concentration or the Fe:Mo ratio change.d is not possible. In order to compare changes in the measured electronic structure with changes in the magnetic properties CT, and Me), a dependence of x0 on NT(EF) is required. Calculated VdU~S of Xi1 for listed samples, where ~$“=[l/Ar#Z,)-t- iIN,(BZ/]/4,& are plotted as a func- tion of hr,(E,) in Fig. 4. There is a linear dependence be- tween ,xs and N,(E,) in the small range of N7(EF) for the 1:12 and 29’7 Fe-based compounds considered. With this plot, the electronic structure is related to T, and lllu as M$Tc.~,yu =ANI.(:EP) -+ B, where A and B are constants and R/A i- - 0.45 eV-‘. Upon alloying with either N or MO, the new value of T,: (Tc!.) can be related through Eq. (1) to the measured changes in magnetization and electronic structure (2) where 8yJ$1 is given by [IIIT(EF) - 0.48]/[N+(EF) - 0.481. The values of NT(EI) are obtained from the experimental EDC curves. The ratio (T,~IT,),,, measured directly, and (T~lTc)M-lft are shown in Table T. As shown in Table I, these two values agree fairly well when nitrogen is added to the two parent compounds. HOW- ever, when the nitrogen concentration is held fizzed and the Fe:Mo ratio is varied, there is a large disagreement between these two values. The disagreement is due to the fact that substitution of MO for Fe in going from YFettMo to YFel&02 lowers the Fe concentration and causes a strong hybridization of the Fe and MO d bands thus changing the proportionality constant in Fq. (I). PES experiments probing the electronic structure agree approximately with the calculated DOS. The samples with fixed MO concentration follow the Curie-temperature theory of Mohn and Wohlfarth upon nitriding. However, the theory does not correctly predict changes in 2”, on alloying with MO. The reason for this appear to be that nitrogen goes into the lattice interstitially and mainly expands the lattice, whereas MO substitution for Fe causes a significant change in the electronic structure of the material. ACKNOWLEDGMENTS We are grateful to the United States Department of En- ergy for support under Grant No. DE-FG@2-86ER45262, Ne- braska Energy Office and to the Cornell National Supercom- puting Facility which is funded by the National Science Foundation. The authors thank the staff of the Synchrotron Radiation Center at the University of Wisconsin. ‘M. Sagawa, S. Fujimura, N. Togawa, and Y. Matsuura, J. Appl. Phys. 55, 2083 (1983). “G. C. Hadjipanayis, R. C. Hazeton, and K. R. Lawless, Appl. Phys. Lett. 43, 797 (lY83). 3J. J. Croat, J. F. Herbst, R. W. Lee, and F. E. Pinkerton, Appl. Phps. L&t. 44, 148 (1984). ‘K. S. V. L. Narasimhan, in Proceedings of the 8th Internutional Workshop on Rare-Earth Magnets and their Applications, edited by K.’ J. Strnat (University of Dayton, Dayton, OH, 198% p. 459. ‘D. B. de Mooij and K. H: J. Buschow, J. Less Common Met. 136, 207 (1988). “J M D. Coey. H. Sun, and D. P. F. Hurley, J. Magn. Magn. Mater. 101, 310 ;1991i. ‘Y.-C. Yang, X,-D. Zhang, L.-S. Kong, and Q. Pan, Solid State Commun. 78, 317 $991). “H. Sun, M. Akayama, K. Tatami, and H. Fujii, Physica B 183, 33 (19Y3). “C. J. M. Denissen, R. Coehoorn, and K. H. J. Buschow, J. Magn. Magn. Mater. 87, 51 (1990). t”S. S. Jaswal, W. B. Yelon, G. C. Hadjipannyis, Y. Z. Wang, and D. J. Sehmyer, Phys. Rev. L.&t. 67, 644 (1991). “A S . . Fernando J P, Woods, S. S. Jaswai, B. M. Patterson, D. Welipitiya, I . A. S. Nazareth, and D. J. Sellmyer, J. Appl. Phys. 73, 6919 (lYY3j. “P. Mohn and E. P. Wohlfarth, J. Phys. F 17, 2421 (lY87j. 13J. P. Woods, A. S. Fernando, S. S. Jaswal, B. M. Patterson, D. Welipitiya, and D. J. Sellmyer, J. Appl. Phys. 73, 6913 (1993). “H. L. Skriver, The I.hfT0 Method, Vol. 41 of the Springer Series in Solid State Sciences (Springer, New York, 1984). “M M. Steiner, R. C. Alhers, L. J. Sham, Phys. Rev. B 45, 13272 (1992). ‘“S’S. Jaswal, Phys. Rev. B 48, 6156 (1993). J. Appt. Phys., Vol. 75, No. 10, 15 May 1994 Fernando et al. 6305 Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

Electronic structure and Curie temperature of YFe\u3csub\u3e12-x\u3c/sub\u3eMo\u3csub\u3ex\u3c/sub\u3eN\u3csub\u3e\u3ci\u3ey\u3c/i\u3e\u3c/sub\u3e compounds

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
May 1994 
Electronic structure and Curie temperature of YFe12-xMoxNy 
compounds 
A.S. Fernando 
University of Nebraska - Lincoln 
J.P. Woods 
University of Nebraska - Lincoln 
Sitaram Jaswal 
University of Nebraska, sjaswal1@unl.edu 
D. Welipitiya 
University of Nebraska - Lincoln 
B.M. Patterson 
University of Nebraska - Lincoln 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Fernando, A.S.; Woods, J.P.; Jaswal, Sitaram; Welipitiya, D.; Patterson, B.M.; and Sellmyer, David J., 
"Electronic structure and Curie temperature of YFe12-xMoxNy compounds" (1994). David Sellmyer 
Publications. 105. 
https://digitalcommons.unl.edu/physicssellmyer/105 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Authors 
A.S. Fernando, J.P. Woods, Sitaram Jaswal, D. Welipitiya, B.M. Patterson, and David J. Sellmyer 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicssellmyer/105 
Electronic structure and Curie temperature of YFe, 2-xMoxN, compounds 
A. S. Fernando, J. P. Woods, S. S. Jaswal, D. Welipitiya, B. M. Patterson, 
and D. J. Sellmyer 
B&h Laboratory of Phyv’cs and Center for Materials Research and Analysis, Univwsiq of Nebraska, 
Linr:oln, Nebraska 68!i%%0111 
The electronic structures of YFe, ,-,Mo,N, , where x = 1.0, 2.0 and y =0, 0.7, have been studied 
with photoemission and spin-polarized calculations. The peak near the Fermi level in the energy 
distribution curves (@DC) becomes successively broader with larger MO concentration. The features 
in the calculated density of state at 1.3 and 2.7 eV are not readily seen in the EDC, and this may be 
due to lifetime effects in these compounds. Finally, changes in Curie temperature (T,) with the 
change of N or MO concentration are compared with prediction of the theory of Mohn and 
Wohlfarth. Reasonable agreement is obtained in the N case but not in the MO case, the latter most 
likely due to hybridization of Fe and MO d bands. 
I. INTRODUCTION 
Superior permanent-magnet materials require large satu- 
ration magnetization, high Curie temperature (T,), and large 
uniaxial anisotropy. The best permanent magnetic materials 
are rare-earth(Rj-transition-metal(T) compounds. The tran- 
sition metals Fe and Co have large magnetic moments and 
high Curie temperatures, and the R-T compounds often have 
large uniaxial magnetic ani&otropies. The Fe-rich ternary 
compound Nd,Fe,4B has an energy product as high as 45 
h*fgQe, I-’ but the relatively low I’,: (x300 “C) of this mate- 
rial limits its application. Permanent-magnet materials re- 
search is currently investigating other Fe-rich R-T com- 
pounds. The R T, Z body-centered-tetragonal compound with 
the ThMn12 structure does not exist in the binary phase, but 
the structure is stabilized with the partial substitution of a 
nomnagnetic element i&f) for Fe. The compounds 
RFe12...,M’, with III -Ti, V, Cr, MO, W, and Si have be.en 
fabricated and their magnetk properties examined.” The ad- 
dition of interstitial nitrogen increases the Curie temperature 
of the materials and the magnetic anisotropy may change.’ 
For example, T, of NdFeIITi increases by 30% upon nitrid- 
ing and the magnetic anisotropy changes from in-plane to 
uniax?aL7 
The magnetic properties of the compound YFe,,-lMo., 
with s=O.5, 1, 2, 4 have been pub1ished.s T, decreases with 
increased Mo concentration, and increases with nitriding; the 
volume increases both with increased MO and N concentm- 
tion. The MO atoms substitute for Fe atoms on the (ij site 
with the largest magnetic moment.” 
The magnetic properties of the compounds are deter- 
mined by the electronic conllguration, and the increase in T, 
of Y,Fe17 (Ref. 10) and NdFe,,Ti (Ref. 1 I) upon nitriding 
has heen modeled using electronic structure calculations and 
the spin-fluctuation theory of Mohn and Wohlfarth.‘” Briefly, 
t 9’, jai- lv= cM$,~02 W 
where hl!, is the magnetic moment per Fe site, ,ycI is the 
enhanced susceptibility? and C is a constant. Jn earlier work 
the electronic structure of Sm2FeI,N,, with y = 0 and y = 2.6 
has been measured with photoelect;on spectroscopy (PES), 
and a decrease in the density of state (DOS) at EF is ob- 
served as nitrogen is added which agrees with the calculated 
DOS.‘” In this paper the electronic structure of 
YFeIZmxMo,N,, withx=I, 2, andy-0, 0.7 is examined with 
PES, and the calculated partial DOS for Fe, Y, and MO in 
YFellMo are given. Comparison with the expected changes 
in T, are presented. 
II. EXPERIMENTAL PROCEDURE 
Bulk samples of YFe12-,Mq, with x= I, 2 were pre- 
pared by arc melting the elemental constituents in a water- 
cooled copper boat in flowing-argon gas atmosphere. All the 
starting elements used were at least 99.98% purity. The al- 
loys were melted several times to insure homogeneity. The 
samples were wrapped separately in Ta foils and heat treated 
in vacuum below 3X 10Uh Torr at 1100 “C for about 50 h,s 
and subsequently quenched in water. X-ray diffraction mea- 
surements on powder samples using Cu K, radiation showed 
that the samples were composed of primarily the ThMn12 
structure with a small amount of O-Fe. The magnetization of 
the samples was measured at 5 and 300 Ii; with supercon- 
ducting quantum interference device WXJID) and alte.rnat- 
ing force gradient magnetometers, respectively, and the satu- 
ration magnetic moments agree with published resu1ts.s 
The arc-melted buttons were spark cut into discs --I mm 
thick to expose an interior surface which was then polished 
to provide an optically smooth surface for PES measure- 
ments. The samples were mounted on a tungsten sample ma- 
nipulator using Odfi-mm-diam tantalum wires in an ultrahigh 
vacuum chamber with a base pressure of 2 X IO-‘” Torr. Each 
sample was cleaned in situ with several cycles of 2 keV Ar 
sputtering and subsequent annealing at 3.50 “C. Auger elec- 
tron spectroscopy (AFZS) was employed to monitor surface 
cleanliness and to estimate the nitrogen concentration. After 
cleaning the sample, nitrogen was introduced by ion implan- 
tation with a kinetic energy of 2 keV and a current density of 
25 PA/cm’ for 10 min.13 The subsequent nitrogen conce.ntra- 
tion was measured with AELS as ~~=8.3+0.8. The samples 
were then radiantly heated to 350 “C for several minutes to 
promote nitrogen diffusion from the nitrogen-rich surface 
into the subsurface region. The nitrogen-rich surface layer 
was subsequently removed with Ar sputtering to expose the 
subsurface nitride and the resulting surface nitrogen concen- 
tration measured with AES was y=O.7%0.3. Quantitatively, 
the nitrogen concentration per formula unit of 0.7 at the sur- 
J. Appi. Phys. 75 (IO), 15 May ‘1994 0021-8979/94/75(10)/6303/3/$6.00 Q 1994 American institute of Physics 6303 
Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
I ’ 18 I ’ I * I * I * 1 L 1 r 
YFe,,,Mo; hv=22.5 eV 
8 7 6 5 4 3 2 1 0 -1 
Binding Energy (eV) 
FIG. 1. Calculated partial spin-polarized DOS of Fe, MO, and Yin YFe,,Mo 
and experimental energy distribution curves t.EDC) for clean YFe,,...ZMo, 
(x=1.0, 2.0). The EDCs are normalized to the area under the EDC curves 
between 0 and 3.5 eV and shifted vertically. The zero on the binding energy 
axis refers to the Fermi energy (EF). 
face of 1:12 compounds here and previously published’* is in 
agre.ement with published bulk values7 and supports the as- 
sertion that the bulk atomic structure is present up to the 
surface which is probed by AES and PES. The electronic 
structure was measured with PES experiments which we.re 
performed at the Synchrotron Radiation Center in Wisconsin. 
The photoelectron energy distribution curves (EDC) pre- 
sented were obtained with a photon energy of 22.5 eV and a 
total energy resolution of 0.14 eV. 
III. ELECTRONIC STRUCTURE 
The EDCs from the Ar-sputter-cleaned and annealed 
YFe,,-XMoX samples with bulk values of x= 1.0, 2.0, and 
the calculated partial DOS for each element in YFe,,Mo are 
shown in Fig. 1. The EDC curves have been normalized to 
the same areas between 0 and 3.5 eV binding energy and the 
baseline for each EDC is shifted vertically. The DOS are 
determined with self-consistent spin-polarized calculations 
using linear-muffin-tin orbitals in the scalar relativistic 
approximation.‘” A supercell consisting of four formula units 
was used to simulate the ternary compound with correct 
stoichiometry. The total DOS (not shown) is the sum of the 
three partial DOS curves, and the total DOS is dominated by 
the Fe DOS, due to the large atomic percentage (85%) of Fe 
in the compound. The present Fe DOS is substantially dif- 
ferent from Fe DOS in NdFe,,Ti compounds” and the dif- 
ference is due to the strong hybridization with MO. 
The EDCs are comparable to the calculated DOS with a 
1-2 eV wide peak at the Fermi energy. The features in the 
calculated DOS at 1.3 and 2.7 eV are not readily discernible 
in the EDC, and this may be due to lifetime effects in these 
compounds.r5 
The change in EDCs at E, after nitriding YFe,,Mo and 
YFe,eMoZ compounds are shown in Fig. 2. The YFertMo 
YFe,p-J.b.& hv=22..5 eV 
I  I  * , , , , , * , I .  I  I  I  I  I  
2 1 0 
Binding Energy (eV) 
1 
FIG. 2. Expand view of the experimental EDCs of YFettMoN, for y ~0.0, 
y =0.7 (upper two curves) and of YFe,eMozNy for y =tl.O, y=O.7 (lower two 
curves!. The zero on the binding energy axis corresponds to the Fermi 
energy. 
sample (upper two curves) shows a significant decrease in 
EDC at EF upon nitriding; the YFe,,Mo, sample (lower two 
curves) does not show any significant change in E.DC at E, 
upon nitriding. The comparison of EDCs for x = 1 and x = 2 
without nitrogen (upper two curves) and with nitrogen 
(‘lower two curves) are shown in Fig. 3. The EDCs at EF for 
YFe,,Mo and YFer,,Mo, show no significant change; the ni- 
trided samples YF~,,MoN,,~ and YFe1aMoZNe7 show a slight 
increase in EDC at E, as the Mo:Fe ratio is increased. 
In previous experiments on 1:12 and 2:17 
compounds”**” the change in the EDCs at E, was compared 
52 .‘;i 
5 
d 
k? 
x .e 
i 
z 
,“~‘,-‘~~t”~~!‘**~ 
YFe,&.to& hv=22.5 ev 
2 1 0 
Binding Energy (eV) 
FIG. 3. Expand view of EDCs of unnitrided t.upper two curves) and nitrided 
(lower two curves) samples of YFe,, x MO, ix= 1.0, 2.0). The zero on the 
binding energy axis refers to the Fermi energy. 
6304 J. Appi. Phys., Vol. 75, No. IO, 15 May 1094 Fernando et a/. 
Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
TABLE 1. Calculated values of (J’:ITc)M-W #and (T~./T,),, for 
YFe,,_,Mo,N, samples. 
x,y (M;,M,)2a cX&,;fb ‘r’$!!-w (r;,~,),, 
1.0 
NT(&) (1 /eV> 
1,0.7 
2,0.7 
0.62 0.75 
‘Reference 8. 
hBased on experimental NT(Ep) values. 
0.46 0.72 
FIG. 4. CaluIated k,, vs calculated Nr(E,) for the samples listed. IV. CONCLUSIONS ” 
to the calculated”‘~‘” total DOS, N1’(EP)=N,(EF)+NI(EF), 
where N-ljll’,c) and Nt(EFj are the up- and down-spin DOS 
at the Fermi level, respectively. The calculated DOS, Stoner 
parameter, and magnetic moments used with Mohn and 
Wohlfarth theory predict a change in T,. upon nitrogenation 
which agrees very well with measured c.hange in T, . In the 
present experiment the calculated DOS is available only for 
the YFqIMo compound, and a comparison of changes in 
N,(E,) as the nitrogen concentration or the Fe:Mo ratio 
change.d is not possible. 
In order to compare changes in the measured electronic 
structure with changes in the magnetic properties CT, and 
Me), a dependence of x0 on NT(EF) is required. Calculated 
VdU~S of Xi1 for listed samples, where 
~$“=[l/Ar#Z,)-t- iIN,(BZ/]/4,& are plotted as a func- 
tion of hr,(E,) in Fig. 4. There is a linear dependence be- 
tween ,xs and N,(E,) in the small range of N7(EF) for the 
1:12 and 29’7 Fe-based compounds considered. With this 
plot, the electronic structure is related to T, and lllu as 
M$Tc.~,yu =ANI.(:EP) -+ B, where A and B are constants and 
R/A i- - 0.45 eV-‘. Upon alloying with either N or MO, the 
new value of T,: (Tc!.) can be related through Eq. (1) to the 
measured changes in magnetization and electronic structure 
(2) 
where 8yJ$1 is given by [IIIT(EF) - 0.48]/[N+(EF) - 0.481. 
The values of NT(EI) are obtained from the experimental 
EDC curves. The ratio (T,~IT,),,, measured directly, and 
(T~lTc)M-lft are shown in Table T. 
As shown in Table I, these two values agree fairly well 
when nitrogen is added to the two parent compounds. HOW- 
ever, when the nitrogen concentration is held fizzed and the 
Fe:Mo ratio is varied, there is a large disagreement between 
these two values. The disagreement is due to the fact that 
substitution of MO for Fe in going from YFettMo to 
YFel&02 lowers the Fe concentration and causes a strong 
hybridization of the Fe and MO d bands thus changing the 
proportionality constant in Fq. (I). 
PES experiments probing the electronic structure agree 
approximately with the calculated DOS. The samples with 
fixed MO concentration follow the Curie-temperature theory 
of Mohn and Wohlfarth upon nitriding. However, the theory 
does not correctly predict changes in 2”, on alloying with 
MO. The reason for this appear to be that nitrogen goes into 
the lattice interstitially and mainly expands the lattice, 
whereas MO substitution for Fe causes a significant change in 
the electronic structure of the material. 
ACKNOWLEDGMENTS 
We are grateful to the United States Department of En- 
ergy for support under Grant No. DE-FG@2-86ER45262, Ne- 
braska Energy Office and to the Cornell National Supercom- 
puting Facility which is funded by the National Science 
Foundation. The authors thank the staff of the Synchrotron 
Radiation Center at the University of Wisconsin. 
‘M. Sagawa, S. Fujimura, N. Togawa, and Y. Matsuura, J. Appl. Phys. 55, 
2083 (1983). 
“G. C. Hadjipanayis, R. C. Hazeton, and K. R. Lawless, Appl. Phys. Lett. 
43, 797 (lY83). 
3J. J. Croat, J. F. Herbst, R. W. Lee, and F. E. Pinkerton, Appl. Phps. L&t. 
44, 148 (1984). 
‘K. S. V. L. Narasimhan, in Proceedings of the 8th Internutional Workshop 
on Rare-Earth Magnets and their Applications, edited by K.’ J. Strnat 
(University of Dayton, Dayton, OH, 198% p. 459. 
‘D. B. de Mooij and K. H: J. Buschow, J. Less Common Met. 136, 207 
(1988). 
“J M D. Coey. H. Sun, and D. P. F. Hurley, J. Magn. Magn. Mater. 101, 
310 ;1991i. 
‘Y.-C. Yang, X,-D. Zhang, L.-S. Kong, and Q. Pan, Solid State Commun. 
78, 317 $991). 
“H. Sun, M. Akayama, K. Tatami, and H. Fujii, Physica B 183, 33 (19Y3). 
“C. J. M. Denissen, R. Coehoorn, and K. H. J. Buschow, J. Magn. Magn. 
Mater. 87, 51 (1990). 
t”S. S. Jaswal, W. B. Yelon, G. C. Hadjipannyis, Y. Z. Wang, and D. J. 
Sehmyer, Phys. Rev. L.&t. 67, 644 (1991). 
“A S . . Fernando J P, Woods, S. S. Jaswai, B. M. Patterson, D. Welipitiya, I . 
A. S. Nazareth, and D. J. Sellmyer, J. Appl. Phys. 73, 6919 (lYY3j. 
“P. Mohn and E. P. Wohlfarth, J. Phys. F 17, 2421 (lY87j. 
13J. P. Woods, A. S. Fernando, S. S. Jaswal, B. M. Patterson, D. Welipitiya, 
and D. J. Sellmyer, J. Appl. Phys. 73, 6913 (1993). 
“H. L. Skriver, The I.hfT0 Method, Vol. 41 of the Springer Series in Solid 
State Sciences (Springer, New York, 1984). 
“M M. Steiner, R. C. Alhers, L. J. Sham, Phys. Rev. B 45, 13272 (1992). 
‘“S’S. Jaswal, Phys. Rev. B 48, 6156 (1993). 
J. Appt. Phys., Vol. 75, No. 10, 15 May 1994 Fernando et al. 6305 
Downloaded 21 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1104&amp;context=physicssellmyer

Electronic structure and Curie temperature of YFe\u3csub\u3e12-x\u3c/sub\u3eMo\u3csub\u3ex\u3c/sub\u3eN\u3csub\u3e\u3ci\u3ey\u3c/i\u3e\u3c/sub\u3e compounds

Abstract

Similar works

Full text

Available Versions

DigitalCommons@University of Nebraska

UNL | Libraries