MAGNETIC CHARACTERISATION OF AMORPHOUS ANDNANOCRYSTALLINE MATERIALS

A. Mitra , A. K. Panda and S. K. PalNational Metallurgical Laboratory, Jamshedpur, India

ABSTRACTAmorphous and nanocrystalline ferromagnetic materials have animportant place among the metallic materials due to their unique andfavourable association of mechanical, electrical and magnetic properties.The present paper deals with the magnetic properties like hysteresis,magnetostriction, domain wall movement and magnetoimpedance ofthese materials.

INTRODUCTIONThe nanocrystalline and amorphous ferromagnetic alloys reveal much superior

soft magnetic properties compared to their crystalline counterparts. Thenanocrystalline ferromagnetic alloys introduced by Yoshizawa et.al [1] are the newgeneration of magnetic materials with excellent soft magnetic properties. Their highpermeability and moderate saturation induction make them potential candidates forsensor application. They also exhibit ultra low coercivity (-mOe). However, evaluationof magnetic (coercivity, magnetostriction, anisotropy field and Barkhausen noise)needs special attention to establish their superiority to other materials.

PREPARATION OF MATERIALSThe material under study was Fe-Nb-Cu-Si-B ribbon prepared at the authors

laboratory by melt spinning technique. Fig-la shows the photograph of the melt-spinning unit. Melt spinning process parameters like speed of rotation, argon pressure,crucible dimensions were optimized to produce long uniform amorphous ribbons asshown in Fig.1b..

MAGNETIC CHARACTERISATION

Different techniques were used to characterise the various magnetic propertiesof amorphous and nanocrystalline soft magnetic materials . The characterisationtechniques are discussed in the subsequent sections.

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^iatf-Spysstik oir Fxrarr t: ar"<illratrra1t Tv€aa:f ¢ r al th^usfcuy

t ).^Wt. w.'Qptlp' rigla

Fig. 1 : (a) The melt spinning apparatus for the preparation of amorphous ribbon.(b) The prepared ribbon at the laboratory

Magnetic hysteresis loop measurementThe conventional technique of hysteresis loop measurement involves magnetic

testing in closed flux configuration using toroidal specimens. The ribbon shapedsamples were therefore, tested by winding in the form of toroids. The primary andsecondary windings were done for applying magnetic field and sensing the magneticflux respectively. The tedious and long duration required for winding prolongs thecharacterisation time when the specimens are in large numbers. To get quickerresults on hysteresis loop measurements, magnetic evaluation by fluxmetric methodusing Helmholtz Coils in open flux configuration was followed. Although this methodof measurement is very convenient, error comes from the demagnetisation effect ofthe ribbon. The demagnetization field (Hd) caused by the free poles generated atthe ends of the ribbon sample changes the applied magnetic field in open fluxconfiguration of measurement of magnetic properties. Hence the properties dependon the dimension of the sample. Demagnetisation field effectively weakens theapplied field (Hopp) [2] and the effective field (Hoff) experienced by the samplebecomes;

H ,ff = Happ -Hd

= Happ-Nd M (1)where Nd is the demagnetisation factor which depends on the sample dimensionand M is magnetisation value measured at the applied field. The demagnetisationfactor for ellipsoid has been extended for rod [3]. The demagnetisation factor ofrods is given by

Nd = 1K2 [log 2K -11 (2)

where K is the aspect ratio, that is the ratio of length to diameter.

To calculate the demagnetisation factor for ribbon shaped sample, it can beconsidered to the first approximation as a rod with cross sectional area as that ofribbon and ribbon length equal to that of rod. With this assumption the aspect ratiobecomes

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(3)

and,

Nd = [_4Alog L -1

Jn0 L A-/n(4)

The hysteresis loop of Fig . 2a of 6 cm long ribbon was corrected with theabove Nd value (eq.4) and replotted in Fig. 2b.

Ribbon Lengthban a'

ai

m -o m m n m

nz

43 , M rocfdd(A"

E

(a) (b)

Fig. 2: Hysteresis loop of 6 cm long ribbon (a) with demagnetisation correctionand (b) after demagnet/sation correction using eqn. 4

It was found that the calculated Nd using eqn.4 was too high for the hysteresisloop correction for the type of the samples under study. Hence, an empirical pre-factor (4)) which is less than one , has been multiplied with the calculated Nd of eq.4to get modified demagnetisation factor (Ndm).

With modified demagnetization factor, Ndm , corrected hysteresis loop will beindependent of ribbon dimension . Thus, eqn.1 becomes;

Hall = Happ -Ndm M

= Happ -4)Nd M (5)

To find the suitable empirical pre-factor, 4), hysteresis loop of a 20 cm longribbon specimen was taken as a reference in the present study as its aspect ratio, L/D= 675 . 25 which was very high . The pre - factor ¢ was chosen in such a way thatafter demagnetisation correction the hysteresis loops for shorter ribbon of 6cm length(Fig-3b ) would be similar to that of 20cm long ribbons ( Fig-3a ) and hence dimensionindependent . It was found that when 0 = 0.65 , the demagnetisation correctedhysteresis loop of 6cm long ribbon is similar to that of 20cm long ribbon. Similarcorrections were valid for ribbons of different lengths. Thus, the modifieddemagnetisation factor becomes

NM = 0.65x 4A L 1d nL2 (log A/n - )] (6)

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Ribbon length20 cm

o.

40 40 70 40 10

A03 Mepnic Fid4 (Aft)

0.^+ =0.65

Ribbon length6cm

•60 -40 40 00 40 !0

0]

a.^ Magnetic Field (Wm)

Cni

Fig. 3: (a) Hysteresis loop of 20 cm long ribbonand (b) after demagnetisation correction using eqn. factor 0

The variation of modified demagnetisation factor and the correspondingmagnetic parameters with the ribbon length are shown in Table -1

Table 1: Variation of modified demagnetisation factor and the correspondingmagnetic parameters with the ribbon length

RibbonlengthL (cm)

AspectRatio(L/ D)

Remanence ,Br (without

demag .Corr.), T

Ini. Suscept(without

Demag . Corr.)(x103)

Modifieddemag .Factor

(0.65 "Nd )

Remanenc,Br (afterdemag .Corr.) T

Ini.Suscept(after demag.Corr.) (x103)

6 202.58 0.03 4.29 7. 9265 .0.22 6.508 270.10 0.05 4.95 4.7149 0.21 6.4510 337.63 0.07 5.33 3.1448 0.22 6.4012 405.15 0.14 5.71 2.2561 0.22 6.5516 540.20 0.17 5.94 1.3331 0.22 6.4520 675.25 0.22 6.16 0.8850 0.22 6.52

The demagnetisation correction at an optimised empirical pre-factor of 0.65gave consistent remanence and initial susceptibility values even for ribbons of differentlengths. Using this prefactor magnetic coercivity of as-cast and heat-treatedFeNbCuSiB alloy was determined from the demagnetisation corrected hysteresisloop. It was observed that optimum heat-treatment resulted in ultra low coercivityof SmOe (0.32A/m) and very high susceptibility -105

Magnetostriction coefficient

Another important magnetic property is the saturation magnetostrictionconstant (Xs) which is the change in the length due to the presence of magneticfield. The conventional technique like strain gage, LVDT, three terminal capacitanceprobe are not suitable for the measurement of ? for the measured material, as itcan not give sufficient force to activate sensors. The SAMR is determined bysimultaneously applying a small a.c. transverse magnetic field (Hai), saturating d.c.axial magnetic field (HdC) and tensile stress to the samples (4). The method is basedon the detection of induced voltage V2f (the second harmonic of the applied a.c.magnetic field) in a receiving coil set around the amorphous wire. When tensile

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stress s is applied on the wire (at fixed Hac and Hdc), V2r value increases or decreasesas a consequence of the increment of magnetoelastic anisotropy having a transverseor an axial easy axis, respectively. The value of Hdc for which SAMR method issuitable, must be high enough so that (VOOHd&)2 constant and the magnetizationsaturation of the amorphous wire Is achieved. The transverse magnetic field isobtained by passing an electric current through the wire. The current density wassmall enough to neglect the increase in temperature of the sample. The saturationmagnetostriction constant was calculated from the following expression:

7k= -(µo Ms/3)(AHK / AG) for V2r and Hac = const.

Where Ms is the saturation magnetization of the sample.

Magnetic Barkhausen noiseWhen a magnetic material is subjected to a cyclic magnetic field, a burst

type of electromagnetic signal (known as Barkhausen emissions) results from thedomain wall movement. Barkhausen emissions of heat-treated Fe-Nb-Cu-Si-Balloy was carried out and shown in Fig. 4.

^^" -- - reeved

1, ZJ A

Armco a at --- -

i

--- - nneate 23 - --

04 4im! oil.

_-- nnea a at 48

Mc6netisin - Pield nneale ag

In.

m cc

Time (mS)

Fig. 4 : Barkhausen signal of as-received and heat treated material

It is interesting to note that the material showed minimum Barkhausenemissions at the nanocrystalline state where the soft magnetic properties of thematerials are superior. The minimum barkhausen noise preferred for good magneticsensor characteristics was obtained at suitable nanoparticle size obtained at optimumannealing temperature in the amorphous alloy.

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Magneto - impedance

Now-a-days magnetoimpedance technique is also used to characteriseamorphous and nanocrystalline magnetic materials which have very low anisotropyenergy. It is a large variation of impedance of the material in presence of small DCmagnetic field when high frequency alternating current passes through it (5). Themagnetoimpedance behaviour of the materials are used to evaluate the intrinsicmagnetic properties like anisotropy field, permeability, susceptibility, etc. and arediscussed in the paper. Fig-5 shows the estimation of circular anisotropy fieldfrom magnetoimpedance measurement for the (Co94Fe6)72.5Si12.5B15• The DC axialfield corresponding to the maximum MI ratio is attributed to the static circularanisotropy field Hk

350

300

250

200

150

100

500-30 -20

DC Magnetic Field (Oe)

-10 0 10 20 30

Fig. S : Magnetoimpedance measurement of (Co Fe6)72.5Si12.5S15. The dashed lineindicate corresponding anisotrophy field Hk

CONCLUSIONDifferent characterisation techniques for amorphous and nanocrystalline soft

magnetic materials have been discussed. Direct measurement of soft magneticribbons have been done with proper demagnetisation corrections. Properties likemagnetostriction can be evaluated by technique like Small Angle MagnetisationRotation (SAMR), while the magnetic noise can be monitored by Barkhausen signals.The intrinsic properties like anisotropy field, permeability etc can be inferred fromthe magneto-impedance behaviour of the materials.

REFERENCES

1. Y.Yoshizawa, S.Oguma, K.Yamauchl. J. Appl. Phys 64 (1988) 6044

2. B. D. Cullity in "Introduction to magnetic materials ', (Addison-Wesley Publn.company),1972, p.49

3. S. Chikazumi, Physics of Magnetism (J. Wiley Pub., New York, 1964) p.19

4. H.Chiriac, et al, Moldavian Jr. of Physical Sciences, N2, 2002

S. L. V. Panina, K. Mohri, Appl.Phys. Lett 65 (1994) 1189

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