Magnetocrystalline anisotropies of RTiFe11N x compoundsYingchang Yang, Xiaodong Zhang, Linshu Kong, Qi Pan, and Senlin Ge Citation: Applied Physics Letters 58, 2042 (1991); doi: 10.1063/1.105007 View online: http://dx.doi.org/10.1063/1.105007 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/58/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of the M / Co substitution on magnetocrystalline anisotropy and magnetization in SmCo 5 − x M xcompounds ( M = Ga ; Al ) J. Appl. Phys. 107, 09A732 (2010); 10.1063/1.3339776 Magnetocrystalline Anisotropy of Nd3(Fe1−xCox)27,7Ti1,3Ny Compounds AIP Conf. Proc. 899, 650 (2007); 10.1063/1.2733391 Calculations of the magnetocrystalline anisotropy of R2Fe17N x compounds J. Appl. Phys. 73, 6937 (1993); 10.1063/1.352440 Magnetic and crystallographic properties of novel Ferich rareearth nitrides of the type RTiFe11N1−δ(invited) J. Appl. Phys. 70, 6001 (1991); 10.1063/1.350074 Theoretical explanations of magnetocrystalline anisotropy behaviors in RTiFe11N x compounds J. Appl. Phys. 70, 6574 (1991); 10.1063/1.349861

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Magnetocrystalline anisotropies of RTiFel .#4, compounds Ying-chang Yang, Xiao-dong Zhang, Lin-shu Kong, and Qi Pan Department of Physics, Peking University, Beijing 100 871, People’s Republic of China

Sen-lin Ge Box 123, Beijing University of Posts and Telecommunications, Beijing 100 088, People’s Republic of China

(Received 25 October 1990; accepted for publication 8 February 1991)

We have discovered that the rare-earth-iron intermetallics of the type &TiFe,, can absorb moderate quantities of nitrogen at 500 “C, giving the approximate composition RTiFelIN0.5 at 1 atm. X-ray diffraction showed that the tetragonal structure (14/mmm) is retained but that the unit cell volume is slightly increased, More significantly, profound change of magnetocrystalline anisotropy have occurred upon the absorpl:ion of nitrogen. In this letter, we report the lattice parameters and magnetocrystalline anisotropic properties of RTiFer ,N, Compounds. -

In 1987 worldwide efforts have started to investigate the magnetic properties of R(Ti,V)Fer, compounds, in- tending to provide a new alloy for use in permanent mag- nets.ls2 Among them, the SmTiFert compound seems to be the most potential candidate. However, on the basis of experiments to date the SmTiFeit compound appears to offer less practical use in permanent magnet development than the Nd,Fe,,B compound. The reasons for this are twofold: ( 1) its Curie temperature is much closer to the relatively low Curie temperature of the Nd2Fe,,B com- pound, and (2) its saturation magnetization is substan- tially lower than that of the Nd2Fe,,13 compound, which leads to a conclusion that the theoretical maximum energy product of SmTiFeir is only one half that of Nd2FetdB. The high price of Sm metal, of course, is also a disadvan- tage.

In our present work, we discovered that the RTiFe,, compounds can absorb moderate quantities of nitrogen at 500 “C, giving the approximate composition RTiFe, IN0.5 at 1 atm. The absorption of nitrogen in RTiFe,, compounds not only increases the Curie temperature and saturation magnetization,3 but also gives rise to profound changes of magnetocrystalline anisotropy. One significant result of these changes is that NdTiFe,iN, now emerges out as a novel powerful permanent magnet material free of the shortages of SmTiFe,,. In fact, nitrogen absorption makes the sign of the second-order crystal field parameter A? of the tetragonal structure of RTiFe,, compounds become positive. Therefore, in this letter we stress systematically reporting the crystallographic parameters and the magne- tocrystalline anisotropies of nitrides of RTiFe,, com- pounds.

The samples were prepared by arc melting 99.9% pure materials in a purified argon atmosphere. Nitrides were prepared by passing purified nitrogen gas at atmospheric pressure over finely ground powder samples at 500 “C for 2 h and at sufficiently high rates of Bow to ensure enough absorption of nitrogen, then followed by rapidly cooling to room temperature. X-ray diffraction using MO K, radia- tion was employed to detect the structure. In addition, the weight percentage of nitrogen was obtained by chemical analysis.

The powder samples of cylindrical shape were aligned

in 10 kGe field and fixed in epoxy resin. Magnetization curves along and perpendicular to the orientation direc- tion, respectively, were measured on aligned samples with a field of up to 60 kGe at 1.5 K and 300 K by extracting a sample magnetometer. Besides the magnetic measure- ments, x-ray diffraction experiments performed on powder samples aligned in a field of 10 kQe were made in order to investigate the easy magnetization direction.

As found by x-ray examination, all samples are single phase except for a small amount of a-Fe in the light rare- earth compounds. The nitrides of RTiFe, i compounds are found to maintain their original tetragonal structure, space group 14/mmm, but with slight increases in unit cell vol- umes. The lattice parameters of these nitrides and their counterparts are listed in Table I. It is well known that the atomic radius of nitrogen is smaller than that of iron. Therefore, the lattice expansion suggests that the nitrogen atoms should enter into the lattice interstitially rather than substitutionally.

More significantly, nitrogen absorption gives rise to profound changes of magnetocrystalline anisotropies in RTiFet t N, compounds. The magnetic measurement re- sults are summarized in Table II. As we already know for YTiFeir, SmTiFe,,, and GdTiFe,,, the easy magnetization direction is along the c axis in the temperature range from 0 K to Curie temperature. Nevertheless, the x-ray diffrac- tion experiments performed on aligned powder samples of these nitrides indicate that at room temperature the easy magnetization direction for SmTiFe,tN, is in the basal plane, whereas the easy direction for YTiFe,,N, and GdTiFe, ,N, still remains along the c axis. Figure 1 shows the x-ray diffraction patterns of the aligned powder sam- ples and those of the nonaligned powder samples for SmTiFe, iN, and SmTiFerrN,. Contrary to the SmTiFetiN, compound, the x-ray diffraction patterns of the aligned powder samples and those of the nonaligned powder samples of NdTiFettN, and TbTiFetrN,, shown in Fig. 1, illustrate that the easy direction is along the c axis at room temperature. The magnetization curves measured at room temperature and 1.5 K along the orientation di- rection respectivel,y, for NdTiFei ,N, and HoTiFe, ,Nx, are plotted in Fig. 2 from which we obtain the anisotropy field NA of NdTiFe, t N,:, are about 80 kOe at room temperature

2042 Appl. Phys. Lett. 68 (18), 6 May 1991 0003-6951 I91 I1 82042.03$02.00 @ 1991 American Institute of Physics 2042 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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TABLE I. Lattice parameters of RTiFe, t compounds and their nitrides. The quantity AV/V represents the change in unit cell volume upon ni- trogen absorption.

Compound a (A) cm V(A’) AV/V(%)

YTiFett 8.503 4.800 347.0 . . . YTiFe, ,N, 8.611 4.821 357.5 3 NdTiFe,, 8.574 4.907 360.7 . . . NdTiFet ,N, 8.701 4.844 366.7 1.7 SmTiFet , 8.557 4.800 351.4 . . . SmTiFe,,N, 8.641 4.788 357.5 1.7 GdTiFe, t 8.548 4.800 350.7 . . . GdTiFe, ,N, 8.595 4.782 353.3 0.7 TbTiFe, , 8.537 4.808 350.4 . . . TbTiFe,,N, 8.581 4.798 353.3 0.8 HoTiFe,, 8.506 4.799 347.2 * * . HoTiFe, ,N, 8.561 4.787 350.8 1.0 ErTiFe, t 8.495 4.795 346.0 . . . ErTiFe, ,N, 8.548 4.792 350.1 1.2

and 115 kOe at 1.5 K, respectively. This large anisotropy field HA of NdTiFetiN, is very favorable for permanent magnet applications. ‘I’bTiFettN, has the largest anisot- ropy field HA of the RTiFettN, system seen in Table II. Although NdTiFet , and TbTiFei 1 compounds exhibit spin reorientations, no spin reorientations have been observed in NdTiFet ,N, and TbTiFet ,N, compounds, indicating they have uniaxial anisotropy at all temperatures. Figure 3 gives the temperature-dependent magnetizations of NdTiFet lNx, TbTiFet tN, along two different directions in a field of 1 kOe.

These phenomena are not at all surprising. In RTiFeit compounds, point charge calculations indicate that the sign of At is negative in this tetragonal structure,4 and those rare-earth ions which possess positive second-order Steven’s factors are expected to prefer the magnetization to lie along the c axis, such as Sm3 + , Er3 + . However, after a nitrogenation of these compounds, if the nitrogen atoms occupy the positions where they can considerably affect the crystal field at the rare-earth ions and make the sign of A; become positive, the Nd3 +, Tb3+, Dy3 +, and Ho3 + ions, which possess a negative second-order Steven’s factor, can exhibit uniaxial anisotropy, whereas Sm3 + and Er3 + ions can exhibit the basal-plane anisotropy. The measure- ments of the anisotropy field in YTiFettN, and GdTiFe, lN, suggest that the Fe sublattice exhibit uniaxial anisotropy at all temperatures. Therefore, NdTiFe, iNx,

TABLE II. Magnetocrystalline anisotropic properties of RTiFe,,N, com- pounds.

HA (kOe) Compound YTiFettN, NdTiFe, ,N, SmTiFe,,N, GdTiFe, ,N, TbTiFe, ,N, HoTiFettN, ErTiFei ,N,

“Easy direction” c axis (0 K-T,) c axis (0 K-T,)

basal plane (0 K-T,) c axis (0 K-T,) c axis (0 K-T,) c axis (0 K-T,) c axis (45 K-T,) cone (o-45 K)

1.5 K 3OOK 45 30

115 80 . . . . . . 50 35

140 110 110 90 . . . 35

NdTiF%Nx

I I 12 16 20 24’ (a) 28

Cb)

(d)

FIG. 1. X-ray diffraction patterns of the powder samples of SmTiFe, ,N, GdTiFe, ,N, NdTiFe, ,N, and TbTiFe, ,N,. (a) non- aligned, (b) aligned, in which the aligned field direction is perpendicular to the diffraction surface.

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-a-l.5K * ‘ .30OK

/’ NdTiFqrNx

I OO

(a)

I I 20 40 60

HWle)

-*- l.5K l l 9300K

100 - zi /t 2

‘. ’ l

. . . . . .

--*-, -;-.-‘-’

L

/‘/----- ’ .

/ .

sl -.

50 -.I ’

. .d

i

I

:

H-‘-y

. J . ./.--

/ .A*

. / . /

. /*-

- . /*-

, 1 H0TiFe,,Nx

01' 1 I J 0 20 40 60 (b) twoa)

FIG. 2. Magnetization curves of NdTiFettN, and HoTiFettN, at 1.5 and 300 K in the direction parallel and perpendicular to the aligned direction, respectively.

TbTiFet tNx, and HoTiFet t N, compounds have uniaxial anisotropy at all temperatures, while ErTiFettN, com- pounds are likely to exhibit spin reorientation due to the competing anisotropies of the rare-earth and iron sublat- tices. Figure 3 illustrates the spin reorientation occurring in the ErTiFe,tN, compound; the spin reorientation tem- perature r,, for ErTiFettN, is about 45 K.

In conclusion, we have discovered that by introducing interstitial nitrogen atoms, the magnetocrystalline aniso- tropic properties of RTiFelt compounds can be changed completely. The authors hold the view that nitrogen atoms change the sign of the second-order crystal field parameter Ai to positive. Considering the large electronegativity dif- ference between nitrogen and the rare earth, it is reason- able to assume that nitrogen invariably occupies a position adjacent to a rare earth. This work also opens up a field for

--TbTiFe,,N* 1

100 200 300

T(K)

I””

ErTIFe,,Nx

II

1

0 t I 0 100 200 300

FIG, 3. Temperature-dependent magnetizations of NdTiPettN, TbTiFe, tN, and ErTiFet ,N, along two different directions in a field of 1 kOe.

finding iron-rich rare earth alloys for permanent magnet applications through RTiFei iN, compounds. Among them, the NdTiFettN, compound is the most potential candidate for praadcal use because it combines a large an- isotropy field with a high Curie temperature (740 K) and a strong spontaneous magnetization, which means the es- timated maximum energy product is 56 MGOe.3

This work was supported by the National Science Foundations of China and the Opening Magnetism Labo- ratory of Chinese Academy of Sciences.

‘K. H. J. Buschow, J. Appl. Phys. 63, 3130 (1988). *Ying-chang Yang, Lin-shu Kong, Shu-he Sun, Dong-mei Gu, and Ben-

pei Cheng, J. Appl. Phys. 68, 3702 (1988). ‘Ying-chang Yang, Xiao-dong Zhang, Lin-shu Kong, Qi Pan, and Sen-

lin Ge, Solid State Commun. (to be published). 4Li Hong-shuo, Hu %-ping, and J. M. D. Coey, Solid State Commun,

66, 133 (1988).

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