Formation and characteristics of hard magnetic materials: Pr(Mo,Fe)12N xYingchang Yang, Qi Pan, Xiaodong Zhang, Jun Yang, Minghou Zhang, and Senlin Ge Citation: Applied Physics Letters 61, 2723 (1992); doi: 10.1063/1.108073 View online: http://dx.doi.org/10.1063/1.108073 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/61/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crystallographically anisotropic Sm2Fe17Nδ and Nd(Fe,Mo)12N x hard magnetic flakes J. Appl. Phys. 115, 17A711 (2014); 10.1063/1.4862849 Preparation of anisotropic Nd ( Fe , Mo ) 12 N x magnetic materials by the strip-casting technique J. Appl. Phys. 99, 08B517 (2006); 10.1063/1.2171954 Synthesis and magnetic properties of PrFe12−x Mo x and PrFe12−x Mo x N y (0.5≤x≤1.0, y1) J. Appl. Phys. 76, 6722 (1994); 10.1063/1.358179 Study of the coercivity of Pr(Mo, Fe)12N x J. Appl. Phys. 75, 5441 (1994); 10.1063/1.355702 Aligned high anisotropy Pr(Fe,Co,Mo)12N film samples J. Appl. Phys. 75, 6006 (1994); 10.1063/1.355490

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Formation and characteristics of hard magnetic materials: Pr(Mo,Fe)12N, Ying-chang Yang, Qi Pan, Xiao-dong Zhang, Jun Yang, and Ming-hou Zhang Department of Physics, Peking University, Beijing 100871, People’s Republic of China

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

(Received 4 May 1992; accepted for publication 5 October 1992)

We succeed in stabilizing the PrMo,.5Felo,S compounds and their nitrides with ThMn,,-type structure. The single phase was identified by using x-ray diffraction technique and thermomagnetic measurements. In PrMo1,5Fe10,5 N the c-axis is the easy magnetization X, direction from 0 K to Curie temperature. The anisotropy field is up to 147 K Oe at 1.5 K and 110 K Oe at room temperature. In combination with a high Curie temperature of 640 K and a large saturation magnetization of 141.70 emu/g, these intrinsic magnetic properties are favorable for permanent magnet applications. As a preliminary attempt, a coercive force of 3.0 and 10.0 K Oe was obtained at 300 and 1.5 R, respectively, by using hydrogenation disproportionation and desorption (HDD) processing.

In our previous work, we discovered that the RTiFet , compounds can absorb moderate quantities of nitrogen at 500 “C!, giving a composition RTiFellNl-s,l where 6 is the range from approximately 0 to 0.5. The nitrogen atoms were found to occupy the 2b interstitial site.’ The intersti- tial nitrogen atoms not only have a sensitive effect of’ in- creasing Curie temperature and saturation magnetization, but also give rise to profound changes in magnetocrystal- line anisotropy. One significant result of these effects is that the NdTiFe, ,N, -$ can be qualified as a new potential hard magnetic material.3’4 In fact, the interstitial nitrogen atoms make the sign of the second order crystal field parameter A,, of the ThMnt,-type tetragonal structure become posi- tive. Therefore, all of the rare-earth ions which possess a negative second Stevens’ factor CZ~, such as Pr, Nd, Tb, Dy, and Ho, are expected to have an easy axis.4*5 Recently, a large number of studies have been done on the permanent performance of Nd (M, Fe) t2Nx, where M=Ti, V, MO, W, Cr, etc.“*7 Among the light rare-earth elements, since aJ of Pr3+ is negative as in the case of Nd3+. Accordingly,

FIG. 1. X-ray diffraction patterns of PrMo,,, Fe,,,, (a) nonaligned sam- FIG. 2. X-ray diffraction patterns of PrMo,,5Fe,0.,N; (a) nonaligned ple, (b) aligned sample. sample, (b) aligned sample.

Pr (M,Fe) t2Nx are assumed to have similar magnetic prop- erties to those of Nd(M,Fe)t2 nitrides, when M=Ti, V, MO, W, Cr, etc. However, it is difficult to stabilize the 1-12 phase with Pr. In this work, we succeed in synthesizing the Pr(Mo,Pe,-,),, ‘single phase and their nitrides Pr(Mo$%-,) 12Nx, where y is ranging approximately from CL.1 to 0.3, and x is about 1 and the magnetic prop- erties of novel hard magnetic materials based on the ni- trides of the type Pr(Mo,,Fe,-,) i2Nx will be reported.

The samples were prepared by arc melting of 99.9% pure materials in a purified argon atmosphere, followed by a treatment at 1000 “C for a week. Nitrides were- prepared by passing purified nitrogen gas at atmospheric pressure over finely ground samples at 500 “C for 2 h. X-ray diffrac- tion and chemical analysis were made to determine the structure and the weight percentage of nitrogen.

The powder samples of cylindrical shape were aligned in a 10 KOe field and fixed in epoxy resin. Magnetization curves along and perpendicular to the orientation direction were measured, respectively, on aligned samples with a

c c 2 f! d z f

5 5

E

2723 Appl. Phys. Lett. 61 (22), 30 November 1992 0003-6951/92/472723-03$03.00 @ 1992 American institute of Physics 2723 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 = and c unit cell volume V, and relative change in unit cell volume upon nitrogenation A V/V of PrMo, 5Fe,0.5Nn in comparison with PrMo,.,Fe,,,,.

a(A) c(A) UA3) AV/V(%) PrMod%o.5 8.590 4.805 354.6 f..

PrM%Fed% 8.675 4.864 366.0 3.2

field of up to 70 KOe in a temperature range from 1.5 K to 300 K by using an extracting sample magnetometer. Be- sides the magnetic measurements, x-ray diffraction exper- iments on aligned powder samples were made to detect the easy magnetization direction. The magnets were made by using HDD processing. The ingots were ground into fine powders. A high-temperature treatment in 1 bar hydrogen gas for 2-5 h was carried out for absorbing hydrogen and hydrogen decrepitation, then followed by a nitrogen ab- sorption treatment at 500 “C! for 2-5 h.

All samples are of single phase. The x-ray diffraction patterns of PrMo,.,Fe,,, and PrMot.,Fe,,,N, are illus- trated in Figs. 1 and 2. The nitrides of PrMot.sFelo.sN, are found to maintain their original tetragonal structure, space group 14/mmm, but with a slight increase in lattice pa- rameters, shown in Table I. It is well known that the ni- trogen atom radius is smaller than that of Pr, MO, and Fe. Thus, the lattice dilating suggests that the nitrogen atoms should enter into the lattice interstitially rather than sub- stitutionally. The nitrogen atom location has been investi- gated by a neutron diffraction study, and was found to occupy the 2b interstitial site. Detailed discussions of the neutron data will be reported elsewhere.

The intrinsic magnetic properties of PrMol.SFelO.s and PrMol~sFetO,sNX, such as Curie temperature T,, saturation magnetization up easy magnetization direction EMD and anisotropy field HA, are summarized in Table II. As shown in Table II, the nitrogen atoms have an effect of increasing Curie temperature and saturation magnetization. More- over, the interstitial nitrogen atoms give rise to a significant change in magnetocrystalline anisotropy.

Figure 1 (b) shows the x-ray diffraction patterns of an aligned sample of PrMo,,Fe,,,. A drastic increase of the (h&,0) reflection and diminuation of (O,O,Z> are charac- teristic of an easy plane. Figure 3 presents thermomagnetic curve of PrMo,.,Fete.,. No distinct spin reorientation is observed. Thus, it is concluded that the easy magnetization direction of PrMot,, Fe,,, lies in the basal plane from 0 K to Curie temperature. A dramatic change in magnetocrys- talline anisotropy occurs upon nitrogenation. For

r 20 t PrMwA0.5

00’ too 200 300 TI Kl

FIG. 3. Thermomagnetic curve of PrMo,,Fe,,, (H= 1 KOe).

PrMo,.,Fe,,,,N,, the c-axis becomes the easy magnetiza- tion direction. As evidence, the x-ray diffraction patterns of magnetically aligned powder sample of PrMol,sFeto,5NX are illustrated in Fig. 2(b), in which. the situation is re- versed to that of PrMol,sFelo,s. Figure 4 shows the mag- netization curves measured along and perpendicular to the orientation directions at 1.5 and 300 K, respectively, from which the anisotropy field HA is estimated. HdA of PrMo,.,Fe,,.,N, is up to 147 KOe at 1.5 K and 1 ib KOe at room temperature.

The effect of nitrogenation on magnetocrystalline an- isotropy is easy to understand. Usually, the second field parameter A, is dominant in determining the magneto- crystalline anisotropy for rare-earth ions. It has been shown previously that in the R (Mo,Fe) i2 compounds, the sign of A, is negative, and Pr3+, which possess a positive second order Stevens’ factor, is assumed to prefer the mag- netization direction to lie in the basal plane. Although, the Fe sublattice has an easy axis in the R (Mo,Fe) i2 com- pounds, the planner anisotropy of Pr sublattice overcomes the axial anisotropy of Fe sublattice. As a result, PrMo,.,Fe,,, has an easy plane over the investigated tem- perature range. Upon nitrogenation, the nitrogen atoms occupy the 2b sites which possess the same 14/mmm point

TABLE II. The Curie temperature ?-, saturation magnetization a, easy magnetization direction EMD and anisotropy field H,4 of PrMo,.SFelO,sNn in comparison with PrMo,,,Fe,,,.

T,(K)

455

640

aJemu/d

Z-=1.5 K Z-=300 K

134.38 111.81

141.70 120.85

H4(KOe)

Z-=1.5 K T=3OO K

. . . .._

147 110

EMD

T~1.5 K Z-=300 K

plane plane

c-axis c-axis

2724 Appt. Phys. Lett.,Vol. 61, No.22,30 November1992 Yang etal. 2724 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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e 3 5 r -e-- i.5K

l -• 300K

PrMol.gFe1o.5 NX

Q

H(KOe1

FIG. 4. Magnetization curves of PrMo,,sFe,e,sN,.

symmetry as the rare earth on 2a site. However, the elec- tronic charge of nitrogen ions is opposite to that of rare- earth ions. The interstitial nitrogen atoms affect consider- ably the crystal field interactions on the rare-earth sites and make the signs of A,, become positive. Accordingly, in the case of Pr3+, since the second Stevens’ factor is negative, an easy axis is expected.

The interstitial nitrides of Pr( MoFe, -v) r2Nx; which combine a high Curie temperature, large saturation mag- netization, and strong antisotropy field, are favorable for permanent magnet applications. The estimated theoretical maximum energy product is 5 1 MGOe at 1.5 K, or 37 MGOe at 300 K. For a preliminary attempt, a coercive force of 10.0 and 3.0 KOe was obtained at 1.5 K and room temperature, respectively. Figure 5 shows hysteresis loops for Pr ( MoyFq -,J 12Nx.

Finally, it is important to notice that the processing is available also for preparing other Pr(M,Fe) r,N, com-

I I I -10 0 IO ;

HlKOel

FIG. 5. Hysteresis loops of Pr(Mo,,Pe, -$ iIN,.

pounds, where M=Ti, V, W, Cr, etc. A systematic study on these Pr nitrides is in progress.

This work was supported by National Natural Science Foundation of China and the Open Magnetism Laboratory of Chinese Academy of Sciences.

‘Y.-C. Yang, X.-D. Zhang, S.-L. Ge, L.-S. Kong, and Q. Pan, Proc of Sixth International Symposium on Magnetic Anisotropy and Coercivity in Rare Earth-Trunsion Metal Alloys, edited by S. G. Sankar (Carnegie- Mellon University, Pittsburgh, 1990). p. 190.

‘Y.-C Yang, S.-L. Ge, X.-D. Zhang, Q. Pan, J.-L. Yang, B.-S. Zhang, and Y.-F. Ding, Solid State Commun. 78, 313 (1991).

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4Y.-C Yang, X.-D. Zhang, L.-S. Kong, Q. Pan, and S.-L. Ge, Appl. Phys. Lett. 58, 2042 (1991).

‘Y.-C. Yang, X.-D. Pei, H.-L. Li, X.-D. Zhang,~ L.-S. Kong, Q. Pan, and M.-H. Zhang, J. Appl. Phys. 70, 6574 (1991).

“L. Schultz, K. Schnitzke, J. Wecker, and C. Kuhrt, J. Appl. Phys. 70, 6339 (1991).

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2725 Appl. Phys. Lett., Vol. 61, No. 22, 30 November 1992 Yang et al. 2725 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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