Magnetic and microstructural properties of melt-spun FeGaSmC permanent magnets

ELSEVIER

Journal of Magnetism and Magnetic Materials

167

( 1997)

43-46

Magnetic and microstructural properties of melt-spun FeGaSmC permanent magnets J. van Lier *, M. Seeger, H. Kronmdler Mtr.u-P Itrnck-/n\tirIrt,fiir Mercd&wKhu~‘ hsriturfiir q. Ph~sik. Heisrhr;q.stK 1.D7056Y

StLrttIyr-t. Genrrtrrl\

Received 22 July 1996

Abstract The influence of an over-stoichiometric Sm content on the magnetic and microstructural properties of melt-spun Fe,,Ga,Sm ?+ ,C, alloys with 6 = 0.13-0.23 has been investigated. A strong increase in the coercivity I*,, H, up to 2.2 T at room temperature has been found with increasing 6 values. Room-temperature values of the maximum energy density are ( HH ),,,:,, = 61.7-64.3 kJ/m”. For elevated temperatures (7’2 450 K) the sample with the highest Sm content (6 = 0.23) exhibits the largest values of the maximum energy density, e.g. (BH),,,,, = 3I.4 kJ/m3 at 500 K. From the reversible part of the recoil curves. the temperature dependence of the intrinsic magnetic properties ~I,. K, and K, has been determined. The microstructural parameters cyK and NC,-,describing the influence of the non-ideal microstructure on the coercive field were determined from the temperature dependence of the coercive field and compared with the real microstructure. Krworrls:

High coercivity: Intermetallic compounds

1. Introduction The discovery of the properties of the Fe,,Sm, 3) [I] with high values of the anisotropy field pLoHA tization

.I,

(e.g.

excellent hard magnetic nitrides Fe,,Sm,N,. (.v I the Curie temperature Tc. and the saturation

TC = 745

K.

magne-

p0 HA = 14 T and

J, = I .47 T for J = 2 [2]), was followed

by many further investigations. However, the potential of these alloys for technical applications has been found to be limited because of their low thermal stability. They decompose at T = 670°C into SmN and c-w-Fe [3]. The same problems were found in the carbides Fe,, Sm ?C (‘, which have similar magnetic properties

Corresponding

author.

Fax:

varllier~~?vaxph.mpi-stuttgart.mpg.de.

0304.8853/97/$17.00 P/I

Copyright 0

SO.304-SXS.~~96~00624-S

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and decompose for high carbon contents at T = 600700°C [4]. Thus sintered magnets of these alloys cannot be produced. Recently Shen et al. [5] found that by partially substituting Fe by Ga. these carbides become stable up to high carbon contents and up to T = 1000°C. We have prepared samples of the composition Fe,,GazSm,+,YC, with 6 = 0.13-0.23 in order to obtain high-coercivity materials with good hard magnetic properties at high temperatures.

2. Sample preparation A FeC pre-alloy was melted in an arc furnace together with elementary Sm and Ga for several times under an argon atmosphere. The purity of the elements was 99.9%. The ingots were quenched with

1997 Elsevier Science B.V. All rights reserved

.I. uan Lier et al. /Journal

44

of Magnetism and Magnetic Materials 167 (1997143-46

the melt-spinning technique into amorphous ribbons. The best magnetic properties could be reached by a subsequent annealing treatment at 775°C for 15 min in high vacuum. The Sm contents of the samples (S = 0.13, 0.15 and 0.23) were determined using WDX measurements.

I

o_

‘..

----.- -.

'.

6 = 0.23 6 = 0.15 6 =,0.13

-

z

y;

3. Magnetic

I

.

:~_._~

measurements I

The room-temperature hysteresis loops of the samples Fe,,Ga,Sm,+,C, measured in a PAR VSM are shown in Fig. 1. In all three samples the hysteresis is rectangular with a narrow field range where the irreversible demagnetization processes take place. With increasing 6 there is a significant increase in the coercivity I”,, H, from 1.7 T for 6 = 0.13 to 2.2 T for S = 0.23. The disadvantage of the high Sm content is a weak decrease in the remanent polarization JR from 0.59 to 0.58 T, followed by a decrease in the maximum energy density (BH),,,from 64.3 to 61.7 kJ/m3. Comparing these values with the theoretical upper limit for the energy density of isotropic samples (BH)maxs J~/~J+, = 69 kJ/m3 (6 = 0.13), we have obtained about 93% of the theoretical value. Fig. 2 shows the temperature dependence of the coercivity, compared with the theoretical upper limit J.L,Hcindeduced from the so-called nucleation model for misoriented grains. This value can be much lower than the nucleation field J_L~ H, of oriented grains. In the whole ferromagnetic temperature range the coercivity was found to increase with increasing 6.

0

200

400

600

T [Kl Fig. 2. Temperature dependence of the coercive field pO H, and the minimum nucleation field p,-,HF” of Fe,,Ga,Sm,+,Cz.

Nevertheless, all samples showed significant discrepancies compared with the theoretical limit due to the influence of the non-ideal microstructure. Fig. 3 shows the temperature dependence of the remanent polarization (a) and the maximum energy

-I-Y!

2:(a) ‘4 0

I

I

‘-‘.____

‘_y.

_ - _-..._

-...

-...

zz

6 =

". 0

TF

6 = 0.15

_.- -

6 = 0.13

t 0

y

0.23

-_-.

'\ \

I

i

200

400

600

T [Kl

-,

s

i

2 _

-

6 = 0.23

_---

6 = 0.15

8-

.._.-.-. 6 = 0.13 '\ \.

-5

0

POH

Fig. 1. Room-temperature Fe,,Ga,Sm 2+sC2 samples at 775°C for 15 min.

5

OL 0

ITI

hysteresis with different

I

8

400

200

T loops of melt-spun Sm contents annealed

[Kl

Fig. 3. Temperature dependence of the remanent polarization (a) and the maximum energy density (BH ),,, (b) Fe,SGa?Sm7+SCZ. _ _

I 600

J, of

4s

J. ran Lirr et al. / Journal of Magnetism and Magnetic Materials 167 f 1 Y97) 43-46

density (b). With increasing Sm content a decrease in J, is observed. In the high-temperature range this dependence vanishes because of the low H, values of the samples with low Sm contents. Below 400 K the value of (BH),,, is limited by J, and decreases with increasing S. In the samples with low Sm content the high-temperature values of are not limited by the remanent polariza(BH),,, tion but by the irreversible demagnetization processes, which even take place at small reversed field values, leading to a dependence of the maximum energy density opposite to that due to the Sm content. This effect is of great interest in view of the technical relevance of these magnets, because of the thermal stability of the magnetization at elevated temperatures. For the sample with 6 = 0.23, ( BHjma, reaches a value of 31.4 kJ/m3 at 500 K, which corresponds to 84% of the theoretical value of 37 kJ/m’. 4. Anisotropy constants and the nucleation model At present the temperature dependence of the anisotropy constants K, and K2 of 2: 17 carbides is not well known. Single crystals of these materials are not available, but the anisotropy constants can be determined from fits of the recoil curves of isotropic polycrystalline samples [6,7]. This procedure can be carried out because of the rectangularity of the hysteresis loops, which excludes irreversible processes in the field region between saturation and the remanent state. The determined values for the anisotropy constants as derived from the demagnetization curves are independent of the Sm content 6, as shown in Fig. 4. These quantities K, and K, are intrinsic parameters. This fact leads to the assumption that the excess of Sm mainly forms a Sm-rich intergranular phase and does not influence the ferromagnetic 2~17 phase significantly. The remanent polarization is reduced due to the decrease in the volume fraction of the ferromagnetic phase. The non-magnetic Sm-rich layer also decouples the grains and therefore leads to the above described increase in the coercivity. The fact that the Curie temperature does not vary with varying 6 (Tc = 625 K) is a further hint that the Sm excess does not affect the hard magnetic 2:17 phase. The full line in Fig. 4 is fitted to the experimen-

B

0

200

400

T

600

[Kl

Fig. 4. Temperature dependence of the anisotropy constants K, C2 determined from the experimental and K2 of Fe,,Ga,Sm,+, demagnetization c&es.-

tally determined data for all three samples in order to calculate the so-called minimum nucleation field Hmin according to the nucleation theory. Within the N nucleation model [8,9], the discrepancy between the nucleation field Hr” and the coercive field H, is described by PO&

= ak /-#nmi”

-&J,,

(1)

the influence of inhomowhere (ok describes geneities at the grain boundaries on the crystal anisotropy, and H, m’” is the minimum nucleation field for misoriented grains. The second term considers internal stray fields with the effective demagnetization factor N,,,. For small K, values, Hr’” can be derived approximately from the determined values of the anisotropy constants K, and K? and the saturation magnetization Js, according to &‘(“” = (K, + K,)/Js.

(21

In Fig. 2 the calculated Hti” values are shown as the theoretical limit of an ideal microstructure. These values were used to determine the microstructural is parameters (Yk and N,,. In Fig. 5 woH,/Js plotted against pLoH,“‘“/J, according to Eq. (1 I. As expected from the nucleation theory, this plot yields a linear behavior within a large temperature range. In the high-temperature region there is a deviation from linearity that may be due to incorrectly determined anisotropy constants. The reasons are saturation effects and/or irreversible rotation processes before reaching the remanent state. Both effects influence the determined intrinsic parameters and Hc for temperatures near the Curie temperature. (ok and N,,,

46

J. c,an Lier et al. / Journul qf‘Mqnrtim

and Mugnetic Materials

167 (I9971 43-46

5. Conclusions

I

1

I

0

2

4

I

6

6

Fig. 5. Plot to determine the microstructural parameters N,,, for melt-spun Fe,,GazSm2+8 C,_ samples.

ok

and

are taken from this figure as slope and intersection of the balance straight line. All three samples have nearly the same ax value of 0.77, but N,, decreases from 2.2 (6 = 0.13) to 1.8 ( 6 = 0.23) with increasing Sm content. The improved magnetic decoupling between the grains by adding a non-magnetic intergranular phase decreases the stray fields described by a lower but still rather high effective demagnetization factor. No influence of the Sm content on the microstructure was found in the TEM investigations. Fig. 6 shows a TEM micrograph of a sample with 6 = 0.15. The grains have very sharp edges and bent surfaces, leading to large stray fields and consequently to high values of N,,,. The average grain size is around 50 nm with a broad grain size distribution.

Over-stoichiometric Sm in nanocrystalline meltspun Fe,,Ga,Sm,C, permanent magnets forms an intergranular non-magnetic phase. This has a favorable influence on the decoupling of the grains, leading to enhanced coercivities. The intrinsic material parameters of the hard magnetic 2: 17 phase are not influenced by high Sm contents. TEM investigations show no influence of an Sm excess on the microstructure. The microstructural parameter LYE is independent of the Sm content. The decline of stray fields due to the improved magnetic decoupling is the reason for the decrease in the microstructural parameter N&. with increasing 6. A weak loss of remanent polarization, followed by a loss of the maximum energy density, has been found for high Sm contents at temperatures below 400 K. As predicted, an enhancement of the maximum energy density caused by the excess of Sm was found in the technically interesting temperature region about 450 K.

Acknowledgements The authors wish to thank F. Mehner, R. Henes and W. Maisch for preparing the samples, and B. Heiland for conducting the WDX investigations.

References [I] J.M.D. Coey and H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [2] M. Katter, J. Wecker and L. Schulz, J. Magn. Magn. Mater. 92 (1990) L14. [3] X.C. Kou. R. Grijssinger, M. Katter, J. Wecker, L. Schulz. T.H. Jacobs and K.H.J. Buschow. J. Appl. Phys. 70 (1991 J 2272. [4] X.-P. Zhong, R.J. Radwanski, F.R. de Boer, T.H. Jacobs and K.H.J. Buschow, J. Magn. Magn. Mater. 86 (1990) 333. [5] B.C. Shen, L.S. Kong, F.W. Wang and L. Cao, Appl. Phys. Lett. 63 (1993) 2288. [6] K.-D. Durst and H. Kronmiiller, J. Magn. Magn. Mater. 59 ( 1986) 86. [7] D. Kiihler, Thesis, University of Stuttgart (1992). [S] H. Kronmiiller, Phys. Stat. Solidi (b) 144 (1987) 385. [9] G. Martinek and H. Kronmiiller, J. Magn. Magn. Mater. 86 (1990) 177. Fig. 6. TEM micrograph

of Fe,,Ga,Sm?

,TCz.