Structual and magnetic properties of RMo1.5Fe10.5N xYingChang Yang, Qi Pan, XiaoDong Zhang, MingHou Zhang, ChangLi Yang, Yan Li, SenLin Ge, and BaoFeng Zhang Citation: Journal of Applied Physics 74, 4066 (1993); doi: 10.1063/1.354451 View online: http://dx.doi.org/10.1063/1.354451 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/74/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic and magnetothermal properties of La 1 − x Nd x Fe 11.5 Al 1.5 compounds J. Appl. Phys. 103, 07B338 (2008); 10.1063/1.2836711 Microstructural evolutions in Nd Fe 10.5 Mo 1.5 N X and Pr 13 Fe 80 B 7 compounds duringhydrogenation-disproportionation desorption recombination process J. Appl. Phys. 99, 08B503 (2006); 10.1063/1.2159208 Investigation of magnetic properties and phase evolution of Nd x Febal.TiCB10.5 (x=9, 9.5, 10, and 11)melt spun ribbons J. Appl. Phys. 91, 8171 (2002); 10.1063/1.1449447 Effect of interstitial nitrogen on the structural and magnetic properties of NdFe 10.5 V 1.5 N x J. Appl. Phys. 83, 2700 (1998); 10.1063/1.366991 Structural and magnetic properties of RFe10.5V1.5N x J. Appl. Phys. 75, 3013 (1994); 10.1063/1.356168

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Structual and magnetic properties of RlMol~5Fe10.5Mx Ying-Chang Yang, Qi Pan, Xiao-Dong Zhang, Ming-Hou Zhang, Chang-Li Yang, and Yan Li Department of Physics, Peking University, Beijing 100871, People’s RepubIic of China

Sen-Lin Ge Box 123, Beijing University of Posts and Telecommunications, Beijing 100088, People’s Republic of China

Bao-Feng Zhang Department of Physics, Tianjing University, People’s Republic of China

(Received 5 October 1992; accepted for publication 20 May 1993)

A systematic study of the structural and magnetic properties of the RMol,sFelo.sN, series, where R is Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er, has been carried out using x-ray diffraction, magnetic measurements, and the Mossbauer effect. The 1-12 phase with Ce and Pr has been successfully stabilized. PrMoi.sFetc,,N, p assesses excellent intrinsic magnetic properties, favorable for permanent magnet applications. The 1-12 phase, RMo1.5Fe10,5Nx, was also stabilized. Compared to RMo,FereN,, this compound shows a higher Curie temperature and a larger saturation magnetization due to the lower MO content.

1. INTRODUCTION

In 1990 we reported that the RTiFe,, compounds can absorb moderate quantities of nitrogen at 500 “C, yielding a compound RTiFellNl-s,l where S ranges from 0 to 0.5. Based on a neutron-diffraction study, the nitrogen atoms are found to occupy the 26 interstitial sites2 It has been shown that a significant improvement of magnetic proper- ties is achieved upon nitrogenation in RTiFe,iNi-a com- pounds. The nitrogen atoms not only have the effect of increasing the Curie temperature and saturation magneti- zation, but also give rise to a profound change in magne- tocrystalline anisotropy. One significant result of these ef- fects is that the NdTiFellNl-g may qualify as a new hard magnetic material.3 In fact, the presence of interstitial ni- trogen atoms forces the sign of the second-order crystal- field parameter A,, of the ThMn&ype tetragonal struc- ture to become positive. Therefore, all of the rare-earth ions that possess a negative second-order Stevens’ factor azjt such as Pr3+, Nd3+, Tb3+, Dy’+, and Ho3+, are ex- pected to have an easy axisav5 This conclusion is valid for the whole series of R(Fe,M) izNx compounds, where M is

I Ti, V, Cr, MO, Mn, W, Si, Al, etc. Recently, several studies have been done on the nitrides of Nd( M,Fe) 12N,.,6-g and the magnetic properties of RMo,Feic and their nitrides are already reported.@” However, since a large amount of MO reduces the saturation magnetization and Curie tem- perature, a smaller amount of MO is desirable. On the other hand, some studies have indicated that it should be easier to create a high-coercivity material when M=Mo.~ Thus, it is interesting to study the formation of R(Mo,Fe)r,N, and to optimize its intrinsic magnetic properties for per- manent magnet applications.

As indicated above, among the light rare-earth ele- ments, Pr(M,Fe)12N, is anticipated to have permanent magnet properties similar to those of Nd(M,Fe) iZN, be- cause aJ is negative for both rare-earth ions. We managed to synthesize the RMo,.,Fe,,, single phase and the nitride RMo,.,Fe,,-,,N, for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy,

Ho, and Er. The structual and magnetic properties of the RMo1.5Fe10.5N, series are presented in this article.

II. EXPERIMENT

The samples were prepared by arc melting of 99.5% pure materials in a purified argon atmosphere, for the light rare earth, followed by a heat treatment at 900-1000 “C for 1 week. Nitrides were prepared by passing purified nitro- gen gas at atmospheric pressure over finely ground powder samples (about 20-30 ,um) at 500 “C! for 2 h, then rapidly cooling to room temperature. X-ray diffraction and chem- ical analysis were made to determine the structure and the weight percentage of nitrogen.

Powder samples of cylindrical shape were aligned in a 10 kOe fleld and embedded in epoxy resin. Magnetization curves were taken, respectively, on the aligned samples along and perpendicular to the orientation direction. Mea- surements were made in an applied field up to 70 kOe and in a temperature range of 1.5-300 K by using an extracting sample magnetometer. The Curie temperatures were deter- mined from o-T curves obtained with a vibrating sample magnetometer operating in a temperature range from 300 to 1000 K. Besides the magnetic measurements, x-ray- diffraction experiments on aligned powder samples were made to detect the easy magnetization direction.

The 57Fe Miissbauer spectra of the powder samples were obtained at room temperature using a constant- acceleration spectrometer, which utilized a 57Co (Rh) source, and were calibrated with a-iron. The Miissbauer spectra were fitted by least-squares techniques with Lorent- zian lines.

Ill. RESULT AND DISCUSSION

A. Crystallographic structure

From the x-ray-diffraction patterns, the RMol.sFelo.5 series is identified as single phase and the RMo,.,Fe,,,N, series is found to retain the same structure, but with an

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TABLE I. The lattice parameters a and c, unit-cell volume V, relative change in unit-cell volume upon nitrogenation A V/V, Curie temperature T,, and saturation magnetization o, of RMo,,,Fe,e,,N, in comparison with RMol,sFelasNx.

Compounds fJ (A) c Lb v (A’) AV/V (5%) Tc W T=1.5 K T=300 K

Yh%4%0.5 8.486 4.757 342.6 475 129.86 99.43 YM%P%.~Nx 8.626 4.786 356.1 4.0 650 142.55 125.51 ~Mo&%o.5 8.535 4.766 347.2 386 112.36 76.04 ~M~d+d% 8.619 4.821 358.1 3.2 612 131.56 108.76 PrMo3elo.5 8.590 4.805 354.6 455 134.38 111.81 PrMoSed% 8.675 4.864 366.0 3.2 640 141.70 120.85 NdMod%o.5 8.588 4.787 353.1 470 140.40 118.20 NdMo~&o.~Nx 8.665 4.802 360.5 2.1 635 143.42 122.90 SmM%&o.5 8.577 4.787 352.2 492 122.22 104.29 SmMod%o.d% 8.665 4.802 360.5 2.4 626 130.74 114.24 GdMod%o.5 8.552 4.795 350.7 540 87.07 74.34 GdMo&%9& 8.620 4.804 357.0 1.8 727 109.54 99.72 ~Mo#eto.5 8.530 4.785 348.2 477 54.74 54.08 ~M%P%.& 8.620 4.795 356.3 2.3 660 82.23 77.49 DyMo2eto.S 8.515 4.797 347.8 471 52.2 60.46 DWod%& 8.655 4.802 359.7 3.4 658 73.57 14.25 HoMod%. 8.506 4.780 345.8 450 65.76 73.83 HoMoSed% 8.621 4.781 355.3 2.7 649 81.54 85.16 ErMol.5Felo.5 8.49 1 4.771 344.0 438 66.12 74.31 ~rMd%dC 8.63 4.792 356.9 3.8 645 83.57 91.09

increase in the lattice parameters, as seen in Table I. As an example, x-ray-diffraction patterns of CeMol.sFe,o,s and CeMoi ,Fe,c.,N, are illustrated in Figs. 1 (a) and 2(a). The unit-cell volume expansion varies from 1.8% to 4.0%. As the radius of nitrogen atoms is smaller than that of iron, MO, and the rare-earth atoms, the lattice expansion sug- gests that the nitrogen atoms enter into the lattice intersti- tially rather than substitutionally. The ability to absorb nitrogen seems to vary from one rare earth to another, and this is reflected by the variation of quantity A V/ V with the rare-earth species. Compared to the RTiFeii compounds,

RMol.,Fele,s seems to possess a stronger ability to absorb nitrogen atoms since the x in RMol.5Felo.sN, is about 1.

B. Curie temperature and saturation magnetizatlon

The Curie temperature T, and saturation magnetiza- tion o, of RMo~~~F~,,~,N, are summarized in Table I, and compared with their original counterparts.

FIG. 1. X-ray-diffraction patterns of (a) nonaligned and (b) aligned CeMo,,,Felo,5 powder samples.

5 Cef%$m.5Nx

FIG. 2. X-ray-diffraction patterns of (a) nonaligned and (b) aligned CeMo,,,Feio,sN, powder samples.

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CeMho.5

H=lkOe

T(K)

FIG. 3. The thermomagnetic curves of CeMo,,,Fe,,,N, and CeMod%o.~

The Curie temperatures of R-Fe intermetallic com- pounds are determined by the Fe-Fe, R-Fe, and R-R in- teractions. In general, the Fe-Fe interaction is dominant, and the R-R interaction negligible. The interstitial nitrogen atoms have the effect of increasing the Curie temperature. Figure 3 shows the thermomagnetic curves of CeMo,.,Fe,,, and CeMol.sFelo.sN,. Since Y and Ce are nonmagnetic, the Curie temperature of Y and Ce com- pounds are determined only by the Fe-Fe interactions. The large increase of T, of CeMo1,5Fe,,,,NX compared to CeMo1.5Fe10.5, and of YMol.sFe,o.s compared to YMol.5Fe10,5NX, demonstrates that the Fe-Fe interactions are enhanced by the nitrogenation. Taking into account the R-Fe interaction, we can write the following formula based on the molecular-field theory:

T,=T;eFe+a(gJ-1)2J(J+1), (1)

where TFvFe is the contribution of the Fe-Fe interaction to the Curie temperature. The second term is derived from the R-Fe interaction, g, and J are the Land6 factor and

: 300 Y Ce R Nd P m S m Eu Gd Tb Dy tt~ Er T m Yb

FIG. 4. The Curie temperature of RMo,,Fe,,, and R,,,Fqo,,NX with their theoretical curves as a function of R.

4068 J. Appl. Phys., Vol. 74, No. 6, 15 September 1993

TABLE II. Exchange interaction coefficients of RMo1.5Fe10,S and ~od+lo.5Nx.

Compound

2’:” (K) a (K)

~od%o.5 ~%Fe~o.5Nx

416 619 1.35 5.92

total angular momentum of the rare-earth atoms, and a is a factor which has no relation to the magnetic moment. If we neglect the variation of a as a function of the rare-earth elements, the variations of T, are obtained through the rare-earth series by fitting the experimental Curie temper- atures, as shown in Fig. 4. Furthermore, we can estimate the exchange interaction coefficients. The estimated values of the Fe-Fe interaction and R-Fe interaction in RMol.SFe,o.5 and RMol,5Felo.5N, are listed in Table II. It is clear the interstitial nitrogen atoms have an effect of enhancing the Fe-Fe interaction and reducing the R-Fe interaction. For the heavy rare-earth compounds, the cal- culated curves coincide with experimental data perfectly; but, for the light rare-earth species, the calculated values are relatively low, as also had been found in the series of R2Fe14B, R2Fel, and RTiFell structures. This may be due to the fact that we neglect the variation of a with the different rare-earth partners. The T, of YMol.,Felo,, and YMo,,,Fe,,,N, have some difference with the TryFe, which may stem from the fact that Y is not a true lan- thanide and its electronic structure is different.

Table III offers the molecular magnetic moment p,, average magnetic moment pcLFe of iron atoms, magnetic mo- ment PR of the rare earths, and theoretical values of gJ for RMo,,,Fe,,.,N, compounds at 1.5 K.

It is evident that large increases in the spontaneous magnetizations are achieved upon nitrogenation. Since the 4f electrons of rare-earth atoms are well localized, in prin- ciple, the magnetic moments of rare-earth ions are close to ggpB. Accordingly, the effect of increasing the spontane- ous magnetizations is related to a modification of the 3d electron band. In RMo,.,Fe,o.,NX, the iron moment de- duced from the experimental values of yttrium compounds is 2.01,~~ at 1.5 K and 1.78,~~ at room temperalure. The increase of the atomic magnetic moment of Fe is approxi-

TABLE III. The saturation magnetization Do, molecular magnetic mo- ment p, (pLg), average magnetic moment pFe (pa) of iron atoms, magnetic moment pLR (Pi) of the rare earths, and their g,J of RM%J%o.&, .

R cs (emu/g) Pm (ILB) PFe bd YR (PB) g3 J (PA Y 142.55 21.09 2.01 0 . . . Ce 131.56 20.67 ... . . . . . . Pr 141.70 22.28 2.01 1.19 3.2 Nd 143.42 22.64 2.01 1.55 3.3 S m 130.74 20.78 2.01 -0.31 0.7 Gd 109.54 17.55 2.01 -3.55 7.0 Th 82.23 13.20 2.01 - 7.9 9.0 DY 73.58 11.85 2.01 -9.24 10.0 Ho 81.54 13.17 2.01 -- 7.92 10.0 Er 83.57 13.54 2.01 -- 7.55 9.0

Yang et al. 4068

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TABLE V. The magnetocrystalline anisotropy of RMo,,sFelo,sNN,.

- --lx - 0 -300K

Yf’hFelo.&

0’ ’ ’ sl0m~5060 1

HIKOel

Temperature region HA We)

R Easy direction W T= 1.5 K T=300 K

Y Ce Pr Nd SIIl Gd Th DY Ho Er

c axis c axis c axis c axis basal plane c axis c axis c axis c axis c axis cone

O-T, 22 10 @-TC 10 5 0-i-C 163 110 O-T, 171 110 0-l-C CT, 10 8 O-T, 181 152 O-T, 156 126 O-T, 132 70 50-T, 10 O-50

by an amount higher than that predicted by the volume expansion. The isomer shift in a Miissbauer experiment can be expressed as

HMOel IS=const Ap(O)AR(O)/R, (2)

where FIG. 5. The magnetization curves of YMol,sFelo,s and YMo,,,Fe,,,N, at 1.5 and 300 K along and perpendicular to the orientation direction, re- spectively.

Ap(O)=~(O)absorber-~(O)radiater

and

(3)

mately 12% at 1.5 K and 28% at 300 K compared to the value before nitrogenation.

The observed Mtissbauer spectra at room temperature of YMol.5Felo s and YMol,Fe,o,,NX and the fitting results are shown in Gig. 5. The fitting parameters are summarized in Table IV. Compared to the parent compound, the aver- age hyperfine field increases by about 56 kOe for YMo1.5Feio.5NX. For the Y-Fe compounds, the average hy- perfine fields can be generally converted to iron moments by means of a conversion factor 147 kOe/p, deduced by Gubbens et a1.13 The increase of iron moment upon nitro- genation is about 29%, which is close to the value (28%) obtained from the magnetic measurements.

The average isomer shifts (IS) increase by 0.0692 mm/s ror YMol~,Feio,,NX. It is known that the volume effect on the iron isomer shift AIWA In V is typically 1.0 mm/s for a close-packed structure; so, the increased value caused by the volume expansion is 0.04 mm/s. This means that the average isomer shifts in YMo,.5Fe,o.,NX increase

TABLE IV. “Fe Mijssbauer effect spectra data of YMol,sFelo,, and YMod%dL.

Compounds Site IS (mm/s) Hhf (kOe) H,,, (kOe) &e

~~ol.Pe~o.5 8i -0.0591 236.156 8j -0.1516 195.075 187.8 1.28 8f -0.1383 150.308

YMd%o.& 8i 0.0493 319.670 8j -0.0358 252.669 244.3 1.66 8f -0.1402 188.742

AR/R = -0.0014 for 57Fe. (4)

IS is proportional to the total s-electron density at the nucleus, and increases with increasing 3d occupation. From the 57Fe Miissbauer data it follows that the 3d elec- tron density in YMo1,5Fe10,5N, increases. This is consistent with the magnetic measurements.

The increases in Curie temperature and saturation magnetization upon nitrogenation are believed to be re- lated to the cell volume expansion. Usually the relatively low Curie temperature of R( M,Fe) ,2 is explained in terms of distance-dependent exchange interactions between the Fe atoms. Because of the short interatomic distance be- tween the Fe neighbors on some sites, such as two Sj sites, an antiferromagnetic interaction is assumed to exist in R( M,Fe) i2 compounds. Obviously the lattice expansion obtained by introducing the nitrogen atoms reduces this effect. Moreover, a band-structure calculation for Y,Fei,N, has indicated that the lattice expansion causes a narrowing the Fe 3d band, with a resulting increases in saturation moment and Curie temperature.14

C. Magnetocrystalline anisotropy

The data related to the magnetocrystalline anisotropy in the RMo1.5Fe10,5N, series are summarized in Table V. The x-ray-diffraction patterns of magnetically aligned pow- der samples of CeMo&e10.5 and CeMol,sFe,o.5N, are shown in Figs. 1 (b) and 2(b). Compared with the non- aligned patterns on CeMo,~,Feio,,,there is a drastic increase in the (002) intensity and a corresponding lowering of the (WO) lines. The preferential orientation reveals that the magnetic moment is directed axially. Figure 6 shows mag- netization curves along and perpendicular to the aligned

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-8 -6 -4 -2 #I

0 2 4 6 8 VELaITYlmrdsl

-8 -6 -4 -2 0 2 4 6 8 VELDCITYlmm/sl

FIG. 6. The Mijssbauer spectra of YMo,,sFe,c,,N, and YMot,,Fe,,,, at room temperature.

direction at 1.5 K and room temperature for YMol.sFelo.5 and YMo,.,Fete,sN, . It can be seen that after nitrogenation the anisotropy of YMot.,Fet,,,N, is decreased to the point that the magnetic alignment cannot be distinguished. Nor- mally, in the R-Fe intermetallic compounds, the magneto- crystalline anisotropy stems from the contribution of the rare-earth sublattice and iron sublattice. In the YMoi.,Fet,.,N, compounds iron is the only one which is magnetic, so the anisotropy behavior of YMot.,Fe,,,,N, represents the decreased anisotropy on the iron sublattice.

The effect of the interstitial nitrogen atoms on the rare- earth sublattice is obvious. PrMol.SFelo,5 has the easy di- rection in the basal plane. After nitrogenation it takes axial anisotropy. Magnetization curves of PrMo,,,Fe10.5NX mea- sured at 1.5 K and room temperature along and perpen- dicular to the preferential orientation are plotted in Fig. 7. From there we find that the anisotropy field HA of PrMo,.,Fe,,.,N, is 163 kOe at 1.5 K and 110 kOe at room temperature. The intensity of anisotropy field of

1801 , 180, I

HlKOel HlKOel 120 150

~Mq5bXNX --l.5K **.XXlK

HlKOd tKKOel

‘Y.-C. Yang, X.-D. Zhang, S.-L. Ge, L.-S. Kong, Q. Pan, Y.-T. Hou, S. Huang, and L. Yang, in Proceedings of the Sixth International Sympo- sium on Magnetic Anisotropy and Coercivity in Rare-Earth-Transion- Metal Alloys, edited by S. G. Sankar (Carnegie Mellon University Press, Pittsburgh, PA, 1990). p. 190.

FIG. 7. Some magnetization curves of KMo,,,Fe,c,,N, at 1.5 and 300 K ‘Y.-C. Yang, S.-L. Ge, X.-D. Zhang, Q. Pan, J.-L. Yang, B.-S. Zhang, along and perpendicular to the orientation direction, respectively. and Y.-F. Ding, Solid State Commun. 78, 313 (1991).

PrMoi.,Fe,,.,N, surpasses that of NdZFe14B; accordingly, a high coercive force should be expected in PrMo,sFelo,sNX. The PrMo,.,Felo,sNX phase, which com- bines a high Curie temperature, a large saturation magne- tization, and a strong anisotropy field, is favorable for per- manent magnet applications. The estimated theoretical maximum energy product is 51 MGOe at 1.5 K or 37 MGOe at room temperature.

The magnetization curves of NdMol.sFelo,sNX, TbMo1,5Fe10.5Nx, and DyMo,,,Fe,,,N, are also plotted in Fig. 7. The anisotropy field of RMoi.sFei,-,~N, is estimated on the basis of corresponding magnetization curves. In contrast with the NdMol.sFelO.s, TbMol.sFele,s, and DyMo1,5Fe10.5, no spin reorientations are observed in their nitrides. The anisotropy competition between Fe and Er sublattice results in a spin reorientation with ErMol,,Felo,,NX at about 50 K.

The changes of the magnetocrystalline anisotropy orig- inate from the effect of the interstitial nitrogen on the crystal-field interaction at rare-earth sites. The nitrogen atoms occupy the 2b sites which possess the same I4/mmm point symmetry as the rare earth on 2a sites; however, the charge of nitrogen ions is opposite to that of rare-earth ions. Using a single-ion model and considering not only the rare-earth ions, but also the nitrogen ions as ligands, we can calculate the crystal field. The calculation shows that the contribution from neighboring nitrogen ions to the crystal field is positive and large while neighboring rare- earth ions contribute negative but small interactions and thus make the sign of A, become positive.

IV. CONCLUSION

It is well known that R-Fe intermetallic compounds with ThMn,,-type structure do not exist. In order to form the 1-12 phase with R and Fe, a third element M is re- quired; however, its addition may reduce the magnetic properties because M is nonmagnetic. In our work we man- aged to cut back the content of MO and increase the in- trinsic magnetic properties. We are the iirst to synthesize the 1-12 phase with Ce, Pr, and their nitrides, and we tind that PrMol,5Felo.5NX and NdMol.,Felo.sNX, which com- bine both a large easy axial anisotropy with a high Curie temperature and strong spontaneous magnetization, are most promising candidates for permanent magnet applica- tions.

ACKNOWLEDGMENTS

This work was supported by National Science Foun- dation of China and the Open Magnetism Laboratory of Chinese Academy of Science.

4070 J. Appl. Phys., Vol. 74, No. 6, 15 September 1993 Yang et al. 4070

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