2
EFFECT OF HYDROSTATIC PRESSURE ON THE INITIAL AND REVERSIBLE PARALLEL SUSCEPTIBILITIES OF POLYCRYSTALLINE GARNETS M. Le FLOC'H, J. LOAEC Faculte des Sciences et Techniques, 29283 Brest, France and A. GLOBUS Equipe de Recherche Mat~riaux Magn~tiques, CNRS, 92190 Meudon-Bellevue, France We show the pressure effect on the initial susceptibility of garnets up to To, a typical dispersion of the parallel reversible susceptibility curves caused by the pressure, and how, using a domain wall size model, a reduction of these curves to a single curve is obtained by plotting (# - l)lel(P)/(/x - I)¢(P) versus (/~ - 1)e(P)H. I. Introduction This study is a further development of results about the pressure influence on the magnetic prop- erties of ceramics [1] executed for garnets and in a wide temperature range. We investigated the do- main wall behaviour during reversible magneti- zation processes in two magnetic states namely the demagnetized state (initial susceptibility 41rX) and a magnetized state (reversible parallel susceptibility 4~-Xtt). 2. Results 4~rx~(P ) = (/~ - 1)~(P), 4~rxc(e ) = (la -1)c(P), and 4~rxc(0)= (#- 1)c(0) refer to the corrected susceptibilities at the pressures P and zero. The corrective factor is dx/d (d x = X-ray density, d = sample density). Fig. 1 shows the susceptibility thermal spectra of YIG at different values of the hydrostatic pressure. Fig. 2 shows the reduced curve obtained, between 420 and 550 K, by plot- ting the ratio (/~- 1)c(P)/(/t- 1)¢(0) and its in- verse, versus the product (/~ - 1)¢(0)P. This brings out the general law previously obtained [1]. Fig. 3a indicates that the pressure effect is more important near H = 0 than for the upper values of the dc applied field H where it becomes negligible. Never- theless, the successful reduction of the curves ob- tained in fig. 3b by plotting (/~- I)~(P)/(~- 1)¢(P) versus (/~ - I)~(P)H seems to indicate that the same mechanism is responsible for the pressure effect in the initial state and in any magnetized state. I ::L 400 1 bar YIG A --400 bar ~900 bar 20(] -1200 bar o , '~ 370 460 550 T ( K Fig. I. Pressure effect on the thermal spectrum of YIG. 1 ~-- o i .5 o549K =495K I :L 0 v (J I ::L 2 e531K .477K .441K n513 K •459K .423K m i 0 0 10 20 30 (l~.l)c(O).P(lO4bar) Fig. 2. Left: reduction of the initial susceptibility curves; fight: determination of the empirical linear law: (/~- l)c(O)/(/~- I)=(P) = 1 + (~(F - 1)e(O)P. Journal of Magnetism and Magnetic Materials 15-18 (1980) 1437-1438 ©North Holland 1437

Effect of hydrostatic pressure on the initial and reversible parallel susceptibilities of polycrystalline garnets

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Page 1: Effect of hydrostatic pressure on the initial and reversible parallel susceptibilities of polycrystalline garnets

EFFECT OF HYDROSTATIC PRESSURE ON T H E INITIAL AND REVERSIBLE PARALLEL SUSCEPTIBILITIES OF POLYCRYSTALLINE GARNETS

M. Le FLOC'H, J. LOAEC Faculte des Sciences et Techniques, 29283 Brest, France

and A. GLOBUS Equipe de Recherche Mat~riaux Magn~tiques, CNRS, 92190 Meudon-Bellevue, France

We show the pressure effect on the initial susceptibility of garnets up to To, a typical dispersion of the parallel reversible susceptibility curves caused by the pressure, and how, using a domain wall size model, a reduction of these curves to a single

curve is obtained by plotting (# - l)lel(P)/(/x - I)¢(P) versus (/~ - 1)e(P)H.

I. Introduction

This study is a further development of results about the pressure influence on the magnetic prop- erties of ceramics [1] executed for garnets and in a wide temperature range. We investigated the do- main wall behaviour during reversible magneti- zation processes in two magnetic states namely the demagnetized state (initial susceptibility 41rX) and a magnetized state (reversible parallel susceptibility 4~-Xtt).

2. Results

4~rx~(P ) = (/~ - 1)~(P), 4~rxc(e ) = (la -1)c (P) , and 4~rxc(0)= ( # - 1)c(0) refer to the corrected susceptibilities at the pressures P and zero. The corrective factor is dx/d (d x = X-ray density, d = sample density). Fig. 1 shows the susceptibility thermal spectra of YIG at different values of the hydrostatic pressure. Fig. 2 shows the reduced curve obtained, between 420 and 550 K, by plot- ting the ratio ( / ~ - 1 ) c ( P ) / ( / t - 1)¢(0) and its in- verse, versus the product (/~ - 1)¢(0)P. This brings out the general law previously obtained [1]. Fig. 3a indicates that the pressure effect is more important near H = 0 than for the upper values of the dc applied field H where it becomes negligible. Never- theless, the successful reduction of the curves ob- tained in fig. 3b by plotting ( / ~ - I ) ~ ( P ) / ( ~ - 1)¢(P) versus (/~ - I)~(P)H seems to indicate that the same mechanism is responsible for the pressure effect in the initial state and in any magnetized state.

I

::L

400

1 bar

YIG

A - - 4 0 0 bar

~ 9 0 0 bar

20(] -1200 bar

o , '~ 370 460 550 T ( K

Fig. I. Pressure effect on the thermal spectrum of YIG.

1

~-- o

i

.5

o 5 4 9 K = 4 9 5 K

I

:L

0 v

( J

I

::L 2

e 5 3 1 K . 4 7 7 K . 4 4 1 K n 5 1 3 K • 4 5 9 K . 4 2 3 K

m i 0 0 10 20 30 ( l ~ . l ) c (O ) .P ( lO4ba r )

Fig. 2. Left: reduction of the initial susceptibility curves; fight: determinat ion of the empirical linear law: ( / ~ - l )c(O)/( /~-

I)=(P) = 1 + (~(F - 1)e(O)P.

Journal of Magnetism and Magnetic Materials 15-18 (1980) 1437-1438 ©North Holland 1437

Page 2: Effect of hydrostatic pressure on the initial and reversible parallel susceptibilities of polycrystalline garnets

1438 M. Le Floc'H et al . / Hydrostatic pressure effect on the susceptibility

P1 - - YIG N 526 K P1; 20 bar 300 ' k

~ P 2 ~ (a) 1:)2 =110 ,, ~ P3 =260 ,,

P 4 ~ 1 0 1 0 ,,

~oo P5 " ~ . . - ' ~

~" I ~" (b) ~ ' ~ u ~ • 20bar

~" ~t r~ o 110 ,, . 5 ~ • 260 ,,

"~• n 470 ,,

" ~ ~ 1010 ,,

reduced Curve -

I I

0 1;0 H. (IJ. _ 1)c(P) (Oe)

Fig. 3. (a): pressure effect on the reversible parallel susceptibil- ity; (b): reduction of the curves of (a).

3. Discussion and interpretation

Guyot and Globus [2] investigating the parallel reversible susceptibility law versus the mean grain diameter D m of YIG, obtained a similar reduction of their curves by plotting (fig. 4b) ( / ~ - l)lwl/Dm versus HD m. The striking similarity of the two results added to the fact that, at a given tempera- ture, the ratio M2~/K~ may be regarded as a con- stant because of the weak variations of M s and K n under pressure [3], permits us to extend the Globus formula (/t - 1)c ~ M2Dm/Kn [4], to the case of ceramics under hydrostatic pressure. In this case Dm must be regarded as a fictitious mean grain diameter sensitive to the pressure effect This "ap- parent reduction of Din", inducing a proportional decrease of susceptibilities, has previously found a plausible explanation [1] in terms of a modification of the 180 ° wall topography due to inhomogeneous local straining.

i

300

200

100

YIG 1 4 6 6 K

(a) o Din1 = 4.7 I.tm

• Din2 = 2 ,, . Din3= 1.2 ,, • Dm4 < 0.2 .,

Dm4 (I-t;1)rot i I i

500 1000 H (rnOe)

E a

i

:L

60

40

20

YIG ~ ~ . 466 K

(b) * ~ o

reduced curve e ~ e o

,// , .# ( l~- l )w = t l~-lJc - ( l~ - l l ro t

i I i

0 5 0 0 1 0 0 0 H. Din(toOl . tAm)

Fig. 4. (a): effect of the mean grain diameter on the reversible parallel susceptibility; (b): reduction of the curves of (a).

4. Conclusion

It appears that changes of the initial susceptibil- ity by pressure are due, not to the weak compressi- bility of the cubic materials, but to modifications in the domain wall topography, and the same laws which control the effect of pressure on the initial susceptibility are valid for the reversible parallel susceptibility.

References

[1] J. Loa~c, A. Globus, M. Le Floc'H and P. Johannin, IEEE Trans. Magnetism 11 (1975) 1320.

[2] M. Guyot and A. Globus, AlP Conf. Pro(:. 5 (1971) 902. [3] J. P. Kaminov and R. V. Jones, Phys. Rev. 123 (1961) 1122. [4] A. Globus, Soft Magnetic Materials 2, Cardiff Conf. (1975).