Magnetic after-effect in the bulk amorphous alloy Tb52Ag48

HAL Id: jpa-00219009

https://hal.archives-ouvertes.fr/jpa-00219009

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Magnetic after-effect in the bulk amorphous alloy Tb52Ag48

B. Boucher, B. Barbara

To cite this version:

B. Boucher, B. Barbara. Magnetic after-effect in the bulk amorphous alloy Tb52Ag48. Journal de

Physique Colloques, 1979, 40 (C5), pp.C5-250-C5-252. �10.1051/jphyscol:1979592�. �jpa-00219009�

JOURNAL DE PHYSIQUE Colloque CS, supplgment au no 5, Tome 40, Mai 1979, page C5-250

Magnetic after-effect in the bulk amorphous alloy Tb52Ag4e

B. Boucher

Service de Physique du Solide et de RCsonance MagnCtique C.E.A. Saclay, BP no 2, 91 190 Gif-sur-Yvette, France

B. Barbara

Laboratoire L. Neel. Grenoble 166X, 38042 Grenoble, France

RCsumC. - On a fait des mesures de trainage magnCtique A diffkrentes tempkratures et sous differents champs.

L'aimantation dkcroit comme In t . On donne la loi de variation du champ coercitif avec la tempbrature et le temps. L'expression reliant le temps a la tempkrature et au champ a la forme d'une loi d'activation. L'Cnergie correspondante varie comme H-' et T-'3'. On donne une valeur estimCe du volume dans lequel l'aimantation se retourne sous l'influence du champ et de I'activation.

Abstract. - Magnetic after-effect measurements have been carried out at various temperatures and under dif- ferent values of applied field. The magnetization decreases as In t . The temperature and time dependence of the measured coercive field have been determined. The relation between the time, the temperature and the field is similar to an activation law. The corresponding energy varies as H-' and T-2.1. The volume in which the magne- tization is inverted under the simultaneous influence of applied field and activation is estimated.

The magnetic properties of Tb,,Ag,, have been measurements have been done with the apparatus found to exhibit, at low temperature, a high coercive and sample described in reference [l]. The variation field and an after-effect (Fig. 1) [l]. The after-effect of the magnetization as a function of time t has been measured for given values of the temperature T and the field H.

M e.m.u/g At a given temperature, the hysteresis loop was

1-125 saturated ; then the applied field was reduced to zero and then inverted. The reverse field was increased up to the value for which the measurement of M(t) was done. Between each measurement at different temperatures, the sample was heated above the magnetic ordering temperature.

For given values of T and H, the measurements were carried out from time ti, which is about the time used by the field for increasing from zero to H.

The after-effect was measured during 30 or 40 min.

Beyond this time, the variation of the magnetization was very small in every case. Thus, with this time

40 scale, we have no information for t small (t < ti) and t large ; with these conditions, an after-effect has been observed only for H, < H < H,. H, is dependent on T (table I) and H , is about 18 kOe at 5.6 K (Fig. l). The after-effect has been measured for 5.6 ,< T ,< 14 K.

=l00 1 . Experimental results.

1 . 1 For given values of T and H, we find :

Fig. 1. - Hysteresis loop at 5.6 K after ref. [l] where M,,,(t) is the measured magnetization. M;,,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979592

MAGNETIC AFTER-EFFECT IN THE BULK AMORPHOUS ALLOY Tb,,Ag,,

Table I 8 3 10-~

Exp.

H, W e ) ref. [l] Calc. Eq. (3)

At - ⁶⁰

3.5 3.7 Fig. 3. -Variation of 6 as a function of M'.

2.75 2.8

the magnetization at ti, that is to say the magnetization connected to the hysteresis loop and a,,, a negative coefficient.

1.2 As in references (2, 31, a time tf is defined as the time for which M(t) = M', M f being a chosen final magnetization state (e.g. M' = 0). One can write : At = tf - ti (At # t, if M' is small or negative (Fig. 1)). At is dependent on H and Mf. These expe- riments show that (Fig. 2) :

Fig. 2. - In At = f ( H - l ) .

- that, whatever the value M f chosen, the line converges at the point l/Ho, to.

1.3 For given values of T and H. the final magne- tization state M' = 0 is reached after time At. At this time the sign of the magnetization changes and H is equal to the coercive field H,. From (2) :

Using (3), the values of H, given in reference [l], are obtained, if one takes At 60 s (table I). This value of At is in good agreement with the experimental 'conditions described in reference [l].

2. Interprhtation. - 2.1 Relation (l) shows that the characteristic time z for reversal of the moments has a large distribution [4].

2 . 2 From relation (3), H, is the value of the coercive field at T = 0 K. But it is also the limiting value of the coercive field for all T, when At tends to to. It is not possible to measure this value directly, Because one is not able to obtain a variation in the field from zero to H in a time as short as to. When H > H,, no after-effect is possible.

2 . 3 From (2), to is the shortest time necessary for magnetization reversal. The value of to is large in comparison with the values observed in crystal- lized alloys (10- " ^{s) [2].}

2 . 4 From (l), we have :

1 1 E

- = -exp - -

t t,. kT

with

E = kTlnAt/t,=

where t, and H,, are constants ; (H, = 22.7 kOe, t, = 6 l o p s S)-and 6 is a paramete; independent

The relation (4) is formally an activation law.

of T and H, but dependent on M* (Fig. 3). We point

According to relation (5), the energy E : out :

- that no after-effect exists, for the time scale ^Q) varies as / H as has been observed for crystal- used, if M > M, = 40 emuig regardless of T. The ^lized I51 ;

values H; of the field, for which M, is obtained from b) is dependent on the initial (Mi) and final (Mf) the hysteresis loop at different T values, are in agree- magnetization states, but in a first approximation is ment with the values of H, (table I). dependent only on the final state. That is to say

- that the slope of the straight line representative that E is dependent on the time at which the measure-

of In At = f ( l / H ) varies linearly with M'. ment is done ;

C5-252 B. BOUCHER AND B. BARBARA

c ) varies like T - ^'.l ; the magnetization has been inverted under the simulta- d ) has an average value of about 10- ^l4 erg (with neous Tnfluence of the field and the activation. u is the time scale chosen). about 103 W3 or 20 to 25 magnetic atoms. u would be larger if the measurements were extended for longer times. When T increases, the energy connected 2.5 If the E value is compared to the energy spent to the hysteresis loop varies as T - 2 [l], E being for magnetization reversal (- 6 X 10-6 erglat [l]), it proportional to T - l . ' , u varies approximation linearly is possible to estimate the size of the volume u in which with T.

References

[l] BOUCHER, B., Phys. Status Solidi (a) 40 (1977) 197.

[2] BARBARA, B. and UEHARA, M., ZEEE Tram. on Magn. 12 (1976) 997.

[3] BARBARA, B. and UEHARA, M., Physica 86-8813 (1977) 1481.

[4] HFXPIN, A., Thiorie du Magnetisme, Bibl. des Sciences et Tech- niques Nuclkaires, chap. XXIV (1968) 848.

[S] BARBARA, B., FILLOW, G., GIGNOUX, D. and LEMAIRE, R.,

Solid State Commun. 10 (1972) 1149.