Journal of Alloys and Compounds, 210 (1994) 325-330 325 JALCOM 1169

Coercive field and domain wall properties in (DyxYl_x)Fe2 and (ErxYI_.)Fe2 intermetallic compounds

K.M.B. Alves*, L.C. Sampaio, A.P. Guimar f i e s a n d S.F. da C u n h a Centro Brasileiro de Pesquisas Fisicas, R. Xavier Sigaud 150, CEP22290 Rio de Janeiro, RI (Brazil)

(Received January 4, 1994; in final form February 7, 1994)

Abstract

Isothermal d.c. magnetization measurements were performed for (RxYt_~)Fe2 (R-=Dy, Er, 0 <x < 1) at several temperatures. The observed pronounced decay of the coercive field He with temperature was fitted with an exponential curve at low temperatures for concentrations near 30%. The coercive field Hc and the propagation field Hp showed maximum values in the concentration range 30%-40% for Dy or Er. These results can be explained by intrinsic domain wall pinning. Time relaxation effects of the remanent magnetization were detected and attributed to domain wall surface deformations.

1. Introduction

Many intermetallic compounds with Laves' phase and of AB2 type present domain wall pinning. In particular, high anisotropy energies present in some compounds give rise to an intrinsic coercive field, as a result of very narrow domain walls. This behaviour has been observed in many pseudo-binary compounds, particu- larly in those where the magnetic moment of the A elements couples anti-parallel to the Fe moments in B sites. Examples are the R(Fel_~Alx)2 compounds with R=Dy, Tb, Er, Hf [1-5].

In a previous paper [6], we have reported that magnetic measurements in (RxY~-x)Fe2 for R-= Tb, Ho, Dy showed that Dy has a much larger propagation field compared with other rare earths.

In the present work, we study the behaviour of magnetic properties, such as the coercive field H~ and propagation field Hp, with variation of the temperature and time effects of the remanent magnetization for some concentrations of R = Dy, Er in (RxY~_~)Fe2 in the range 0 <x ~< 1.

2. Experimental details

The polyerystalline (R~YI_x)Fe2 samples were pre- pared in an arc melting furnace under a purified argon atmosphere, using stoichiometric proportions of high

*Permanent address: UFES, Departamento de Fisica e Quimica, 29069 Vitoria, ES, Brazil.

purity metals. Heat treatment at 900 °C for 100 h in an argon atmosphere was performed, followed by X- ray analysis of the powdered samples at room tem- perature, which showed a single phase with C15 crystal structure.

The magnetization measurements were made in a PAR vibrating sample magnetometer. The samples were zero-field cooled and the measurements above 4.2 K were made by continuously increasing the temperature.

3. Results

We have performed measurements of hysteresis loops after zero-field cooling (ZFC) at several temperatures, with applied fields of up to 12.5 kOe for the compounds with R = Dy, Er. No differences between the ZFC and field cooling (FC) hysteresis loops were detected. These loops are shown in Fig. 1 for R--Dy and in Fig. 2 for R - E r .

We observe that the coercive fields depend on the temperature for both Dy and Er. The curves of the coercive field Hc obtained in the ZFC measurements v s . temperature for some concentrations of R are shown in Fig. 3 for Dy, and in Fig. 4 for Er. The continuous line is an exponential fit from the low temperature points.

In the demagnetizing region of the hysteresis loop (mainly near Ho), a time dependence of the magnet- ization was detected for all intermediate concentrations. ZFC measurements of the magnetization as a function of time were made for several fields, by stopping the

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326 K.M.B. Alves et al. / Coercive field and domain wall in Dy, Er intermetallics

20 15 10 5 0 -5

15 10

0

-15

5 0

-5 -10

-15

15 10

5 0

-5 -10

-15 -20

2__ 20% Dy

~ ~ 2Ki

_) 20% Dy

, , ,131~

20% Dy 65K

I I I i

Y

~ ] 20% Dy 120K

I I I i

H(kOe)

30% Dy , , , , , 4 . 2 ~

Y ~ 40% Dy ) 30K ~ 40K

I I I | i i I

F , 6 0 K , 8 0 K

| I i

30% Dy J i 40% Dy , , 8 0 K I 120K

i t I i i i

H(kOe) H(kOe)

for 20% Dy, 30% Dy and 40% Dy. Fig. 1. (DyxYI_~)Fe2 hysteresis loops at different temperatures

hysteresis loop at a field near the coercive field and recording the time variation of the magnetization. The results obtained with this procedure for 20% Dy and with the compound at different temperatures are shown in Fig. 5. For each temperature, we have taken the curve with the lowest starting magnetization (different in each case). Since these correspond to different points on the hysteresis loops, the curves do not follow a temperature sequence.

4. Discussion

In the ZFC hysteresis loops, there appears a prop- agation field Hp in the initial curve, defined as the field beyond which the domain walls can move. Its value decreases with increasing temperature; Hp was deter- mined from the initial magnetization curves. The systems (R, Y)Fe2 (R--Dy, Er) present maxima in the curves of the propagation field Hp as a function of the con- centration. For a concentration of 30% R, Hp values of about 8 kOe for R----Dy and 3 kOe for R - E r are observed at 4.2 K (see Fig. 6). In a recent paper [6], we have associated the observed reduction in the spin echo intensities in nuclear magnetic resonance (NMR)

measurements with an increase in Ho, i.e. to a blocking of domain walls for fields of less than Hp.

The same behaviour was observed for the coercive field Hc as a function of the concentration (Fig. 6). However, in the demagnetization region of the hysteresis loop, a time effect was observed which is ascribed to the appearance of deformations in the domain wall. Therefore, the field value given for the coercive field is that value necessary to cancel the magnetization in the hysteresis loop, measured with a rate of 54 Oe s-1 under a maximum field of 12.5 kOe.

The presence of a maximum in the curves of Hp and Hc vs. the concentration near the isoconcentration range shown in Fig. 6 was expected, since, in this range, one finds a maximum dispersion of exchange and magne- tocrystalline anisotropy energies. However, another phe- nomenon can contribute to an explanation of these maxima and justify why they do not occur for a con- centration of precisely 50%.

It is known [7] that systems of (RxYI _x)Fe2 ( R - Dy, Gd, Tb, Er) compounds present compensation tem- peratures in a certain range of concentration; these are values of T for which the sum of the magnetization of the two sublattices is near zero. As shown by Piercy and Taylor [8], for R = Dy, equilibrium points, i.e. values of the concentration where the moment cancellations occur, appear near 30%-40% Dy at 4.2 IC Also, the

K.M.B. Alves et al. / Coercive field and domain wall in Dy, Er intermetallics 327

m

E v

Fig. 2.

2O 15

'Oo 5 r

I0 / 4.2 K 15

I I I I

~ 0 % Er 8K

5

0

5

I0 / 16K 15

I I I I

I I I I

I0 5 o

5 20 % Er I0 62K 15

I I I I

/ o

I I I I

15 I0 5 0 5

I0 15

-15

40 K ~ r ~ ~ ~ r

I I I I

~ _ ~ 20% Er 82K

t i I I

-I0 -5 0 5 I0 H (kOe)

15 -15

58 K ~ r

I I I I -I0 -5 0 5 I0

(ELYI-x)Fe2 hysteresis loops at different temperatures

H (kOe) for 20% Er and 30% Er.

15

maximum of Hc occurs at about 35% Dy at the same temperature. The field required to reverse the mag- netization at these points of cancellation is larger, so Hc approaches a maximum. Our results for the de- pendence of H~ on the Dy concentration agree with those of Piercy and Taylor. The same phenomena are valid for Er compounds.

The most interesting feature of our results is the strong temperature dependence of the coercive field, which is interpreted in the following in terms of wall pinning. In Figs. 3 and 4, the temperature dependence

of Hc is fitted with an exponential function. This be- haviour is related to the decrease in the domain wall pinning and is important for the magnetization process at low temperatures.

It is known that systems with high anisotropy have narrow domain walls (since 8 a (J/K) a/2, where 8 is the wall width, and J and K are the exchange and anisotropy constants respectively). Van den Brock and Zijlstra [9] have shown that the domain wall energy is different if its centre coincides with an atomic plane or if it is located between two planes; this energy difference AV

328 K.M.B. Alves et al. / Coercive field and domain wall in Dy, Er intermetallics

1.2

1.0

0.8

0.6

0.4

0.2

8

"~ 6 0

2

4

0 0

H c 1550 exp (-0.0:5816 T)

10% Dy

I I I I I

H c = 9529 exp (- 1.03417 T )

I I ! I I

H c = 5806 exp (- 0.02718 T )

Dy

i i i l I

20 40 60 80 I00 Temperature (K)

I0

8

6

4

2

3

2

I

1.5

1.0

0.5

0.5

0 0

0.4

= 9553 exp (- 0.05061 T)

40% Dy

I I I I I

=f065 exp (-0.04349 T)

60% Dy

I I I I I

., Hc= 2025 exp ( - 0.0475 T)

Dy =

H c 1217 exp ( - 0.0521 T ]

Fe 2

I I I I !

20 40 60 80 I00 Temperature (K)

Fig. 3. Temperature dependence of the coercive field He for different concentrations of Dy in (DyxYl-x)Feq.

gives rise to an intrinsic coercive field Ho et AV/D, where D is the interplanar distance. Hihinger and Kronmiiller [10] showed that the intrinsic cocrcivity resulting from intrinsic domain wall pinning is larger for narrow walls, through the relationship HcaAVo~exp [-1r(8/D)].

However, impurities and defects are responsible for another kind of domain wall pinning, i.e. the extrinsic pinning, giving rise to the extrinsic coercive field. As these intrinsic and extrinsic wall pinnings are inde- pendent, the coercive field can be written as a sum of both contributions [11]:

/L(D =/-/o, ,.,.(D + Ho, .=r(D

In Figs. 3 and 4, the two contributions can be detected. As we approach the intermediate concentrations (where the Hc values are larger), we observe that an exponential decay of Ho indicates (according to ref. 10) that the intrinsic domain wall pinning dominates at low tem- peratures.

The time dependence curves of the magnetization (Fig. 5) can be fitted with M(t)=MTo-ctlnt, where M-r0 is a different constant for each temperature [12];

K.M.B. Alves et al. / Coercive field and domain wall in Dy, Er intermetallics 329

5

O v

o "1-

, ~ e = 3.17 exp (- 0.0275 T)

2 20 % Er

,

I I I !

5 " He= 5.53 exp (-0.031 T)

4 "~ r o -= 5 v

co

:z: 2

I

0 ~ I f i 0 2 0 4 0 6 0 8 0

Temperature (K)

Fig. 4. Temperature dependence of the coercive field He in (ErxYi-x)Fez for 20% Er and 30% Er.

10

8

6

";4

~ 2

0

-2

9 K

23 K

13K 4.2 k

- 4 I i I I I L I I I I t t ~ I l l l l I t I I I l l

10 100 1000 Time ( s )

Fig. 5. Time dependence of the remanent magnetization for (Dy, Y1-~)Fez at different temperatures.

i0

8

~.. 6

© ~ 4

10

O 6

2

T=4.2 - Dy

• - Er

T=4.2 - Dy

• - Er

0 10 20 30 40 50 60 70 80 90 100 Composition (% R)

Fig. 6. Concentrat ion dependence of the propagation field Hp and of the coercive field He for R -=- Dy (A), Er (11) in (RxYi _x)Fe2 at 4.2 K.

5. C o n c l u s i o n s

In conclusion, the strong temperature dependence of the coercive fields in the (RxYI-x)Fe2 system, (R = Dy, Er) shows that, at low temperature, intrinsic domain wall pinning is the dominant pinning mechanism, and is caused by high anisotropy and narrow domain walls. The concentration dependence of the Ho and Hp fields shows maxima which are related to the cancellation of the sum of the magnetizations of the two sublattices. Time after-effects were also observed; although more work and measurements specifically on this time de- pendence were need, dM/dt was found to be roughly proportional to the reciprocal magnetic field and to be related to wall surface deformations.

A c k n o w l e d g m e n t s

the plot of dM/dt vs. 1/H for these data gives straight lines that roughly converge to Ho (the coercive field at 0 K) only at low temperatures. Therefore, although we associate the time relaxation with deformations of the wall surface (kink creation) by a thermal activation process [13], we could not draw any quantitative con- clusions on the thermal activation energies.

The authors acknowledge the collaboration of HJ. dos Santos for carrying out some magnetic measure- ments.

R e f e r e n c e s

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330 K.M.B. Alves et aL / Coercive field and domain wall in Dy, Er interrnetallics

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