2624 IEEE TRANSACTIONS ON MAGNETICS. VOL. 15. NO. 3 . MAY 1989

A Simple One-Cycle Power Supply for Approach to Saturation Studies in Soft Magnetic Poly crystalline Toroidal Samples

M. LE FLOC” A N D M. P. LE GUEN

Abstract-The ring shape is a good solution to the study of the mag- netization processes of magnetic materials because demagnetizing fields are easily avoided. However, when small samples are investigated, the magnitude of the field applied is necessarily limited by the low number of primary winding turns, and by the cross section of the wire itself.

The use of sequential-type techniques for high current supply allows this problem to be avoided.

The one-cycle power production from the ac-line voltage is a simple example of such techniques. An electronic device designed around a commercial integrated circuit is described. A full period of current of 20-111s duration is applied to the load with a magnitude ranging from 0 to 100 A.

Magnetization parameters of soft polycrystalline ring ceramics ap- proaching the saturation region are studied as an application. The fit- ness to measure such characteristical parameters as saturation mag- netization and magnetic hardness is shown. A comparison of our results with those in the literature is made.

I. INTRODUCTION ANY AUTHORS have studied the behavior of mag- M netic materials near saturation and have attempted

to establish empirical relationships between the macro- scopic magnetization and such characteristic parameters of the magnetic substance as spontaneous magnetization [l], magnetic hardness [2], [ 3 ] , and the anisotropy con- stant [4], [ 5 ] . The most classical procedure for investi- gating the approach to saturation region is to set the sam- ple in the air gap of an electromagnet or in an air- magnetizing coil. These techniques allow the production of high magnetic fields.

However, under these circumstances, the difficulty one encounters is to determine the field strength in the sample because of the appearance of magnetic poles. This diffi- culty is increased even for a further nonsaturated material which is always the case when the objective of a study is the magnetization process.

A better solution is to use ring-shaped materials. How- ever, in this case, the small size of certain samples does not allow the application of permanent high fields pro- duced by high current flow in a primary winding. These limitations are overcome by use of sequential-type tech- niques. This work shows an example of this approach. A simple method for special high-current pulses production

Manuscript received January I . 1988: revised October 17. 1988. The authors are with U . F. R . Sciences. 29287 Bresl Cedex. France IEEE Lop Number 8926640.

is proposed with an application to the measurement of two basic magnetic parameters: the spontaneous magnetiza- tion and the magnetic hardness of some polycrystalline ferrimagnetic ceramics.

11. DESCRIPTION OF A SIMPLE ONE-CYCLE POWER SUPPLY

The RCA-CA 3059 (Fig. 1) is a zero-voltage switch circuit used as a trigger function for the control of thyris- tors and triacs [6]. The output (terminal 4) supplies nar- row pulses of current in phase with the ac-line voltage applied to terminal 5 . Therefore, the triac is triggered as long as pulses are generated by keeping an input voltage at terminal 13 higher than the internal reference level. This last operation is made with a simple manual switch and a series of half-cycles of ac-line current is applied to the load.

In order to form a one-full cycle, two additional ter- minals (1 and 6) are used in connection with four D-flip- flop circuits and two external transistors. The general pro- cedure is simply explained by a timing diagram illustrated in Fig. 2.

111. APPLICATION TO MAGNETIC MEASUREMENTS I N

APPROACH TO SATURATION REGION Various techniques are available for magnetization

measurements in medium and high fields [7], [SI. In these measurements, the sample is put in the air gap of an elec- tromagnet or in an air coil supplied by an ac or dc power source. In both cases, the magnetic induction measure- ments are based on the appearance of a voltage induced in the sensing coil by a flux change. However, it is well known that, under these conditions, the magnetic medium gives rise to demagnetizing fields making the knowledge of the internal field very hazardous. This difficulty is in- creased by the nonuniformity of the magnetization and it is the case when the sample has an arbitrary shape or when it is not saturated. The difficulty is easily avoided by using ring-shaped materials. The simplest principle for magne- tization measurements is then the transformer method which gives at the secondary winding output a voltage proportional to the magnetic flux change.

However, a new difficulty appears for small samples: the limitation of magnetic field magnitude due to the lim-

001 8-9464/89/0500-2624$01 .OO 0 1989 IEEE

L E FLOC” AND LE GUEN: A ONE-CYCLE POWER SUPPLY 2625

I TRIG H

ACQUISITION

SYSTEM B

Fig. 1. General diagram of the one-cycle power supply with the ring-sample in the load circuit.

Fig. 2 . Sequential timing for a one-period of the ac-line voltage

itation of the number of turns and the cross section of the wire wound around the sample. Most of our specimens have an external diameter of approximately 15 mm, an internal diameter of 5 mm, and a thickness of about 5 mm. However, in particular cases, much smaller sizes have been encountered.

From the Ampere theorem, an approximate relationship between magnetic field and primary current is derived as follows:

NI H = - TD

where N is the number of turns, I the current intensity, and

For a copper wire of 0.5-mm diameter, a simple com- putation shows that the number of turns cannot exceed 30. Consequently, the magnetic field is limited to about 10 Oe/A. Under these conditions, the approach to saturation

the mean diameter of the ring (2D = Di + 0,).

of magnetically soft materials needing a field of several hundred oersteds, involves the application of several tens of amperes to the coil. This is impossible in the case of a continuous current wave. A sequential method consisting of applying a current of high magnitude during a short time must be used. High current pulse generators might be used; however, a one-period of ac-line voltage is a simple and particularly cheap solution.

Experiments related to the influence of the overheated wire on the magnetic properties of our samples have shown that, for 100 A the one-cycle waves of 20-ms du- ration have to be spaced in 10-s intervals. This result has been established empirically from a study of the thermal behavior of the initial susceptibility up to the Curie tem- perature, the normal shape of which is shown in Fig. 9. The overheating of the coil is then well controlled by avoiding the appearance of anomalous changes in the thermal record of this parameter.

A . System Capabilities The system magnetizes the ring-core by supplying, in

the primary winding, a one-full cycle of 50-Hz ac-line. The output signal of the secondary coil of the transformer is amplified and integrated by operational amplifiers (Fig. 1).

A digital storage voltmeter is used for data acquisition of field ( H ) and induction ( B ) voltages during the 20-ms duration of the current period. The procedure is entirely computer monitored and results are automatically con- verted and plotted in physical quantities.

The system can be used as a loop tracer, but it has been adopted to study the magnetization processes during its approach to the saturation region for which only the first magnetization curve is useful.

In order to test the system capabilities in its most gen- eral aspect, the magnetic ring sample is simply substituted by an air core. So, in the whole current range (0- 100 A)

2626 IEEE TRANSACTIONS ON MAGNETICS. VOL 15. NO. 3. MAY 19x9

the output signal V, becomes directly proportional to the input current I .

The signal voltage v, comes from the amplification- integration of the secondary voltage “e” according to the following equation:

with

where G and RC are, respectively, the amplifier gain and the time constant of the integrator. N ‘ is the number of turns of the secondary coil, and S is its cross section.

By substituting “e” in (2), V, becomes

G RC

V, = - N 'Spa H .

And then, eliminating H from ( 1 )

v, = - G NN’ 2) - z. ( RC aD

(4 )

(5)

The theoretical result is thus a straight line the slope of which is fixed by the experimental conditions.

= 1 cm, S = 0.225 cm2, the slope equals 0.0092 V . A-’, which agrees well with the experimental result of Fig. 3.

Another test of calibration consists, of course, of actual measurements on reference samples for which physical (grain structure) and magnetic (magnetization laws, in- trinsic parameters) properties are well known.

For this purpose, the polycrystalline samples we have studied with our system obey these last conditions [9].

Two parameters have been investigated between room temperature and 540 K for YIG and Ni-Zn ferrimagnetic materials: the saturation magnetization 4aM, and the magnetic hardness “a.”

B. Some Theoretical Considerations

development of 1 / H terms as follows [lo], [ I I]:

For G = 100, RC = 470 ps, N = 4, N ’ = 12,

The approach to saturation law is commonly given as a

where is the saturation magnetization, “a” the mag- netic hardness [2], and “b” a term of magnetocrystalline anisotropy origin [5]. This last parameter is classically interpreted by spins rotations in high fields [ 101 and found to be equal to

(7)

K is the magnetocrystalline anisotropy constant, but may be extended to the magnetoelastic anisotropy when the internal stresses are large enough [ 1 13.

Constant “ a , ” called magnetic hardness, has been in- terpreted by Nee1 (21 as due to cavities or nonmagnetic

0 10 30 1 ( A ) - Fig. 3 . Air-core transformer test. In the power supply of Fig. 2, the sam-

ple is simply substituted by a nonmagnetic ring sample. Voltage output of the amplification-integration circuit is measured as a function of the one-cycle current magnitude. The theoretical response is, of course, lin- ear. Experimental deviations from this law (circles) brings out the global error of the whole apparatus.

inclusions always present in materials. In a convenient range of the applied magnetic field, the magnetization law simply becomes

The validity of this law is limited on the high field side by the fact that is does not lead to a finite energy in infinite values of the field. According to NCel [2], this law must be progressively changed in a ( 1 / H )* law as the mag- netization is reaching the limit M,.

C. Experimental Results 1) Measurement Method: On the YIG and Ni-Zn fer-

rite samples we have studied, the field range, where the magnetic hardness is visible, is 10-300 Oe. This range depends basically on the granular structure and on tem- perature.

Saturation magnetization and magnetic hardness are de- termined together by plotting the product 4aM . H versus H. The two parameters investigated appear, then, as the two typical constants of a linear law as follows:

4aM * H = 4 a M , . ( H - a ) . ( 9 ) The slope of the straight line gives the saturation mag-

netization and its intersection with the abscissa axis leads to the magnetic hardness value.

Fig. 4 is an illustration of such a law where the coeffi- cient “a,” in the example of this particular porous YIG sample ( p = 0.22) is found equal to 11 Oe at room tem- perature.

The slope gives 1367 G for the measured saturation magnetization 4aMm which corresponds to the corrected value 4aM,,

4aMtt1 1 - P

4aM, = - 1753 G

withp = 0.22

L E FLOC" A N D LE G U E N : A O N E - C Y C L E POWER SUPPLY

t - B - m

10

5 -

t 5 P

?

i - I

U

2

1

-

Y I G

Porosity : 2 2 %

M.H = M,(H - a )

4 ' 1 M , = 1 3 6 7 G

a = l l C k

L

Fig. 4. Approach to saturation law for a porous YIG. The product M . H leads to a linear function of H with the magnetic hardness a measured by the offset.

z Y IG

0 Present results

0 Vibrating sample magnelomeler (Foner s y s t e m )

U IUU LUU I ( L J - C

Fig. 5 . Fitness of the apparatus to measure the saturation magnetization of polycrystalline YIG ring samples. The results agree well with the Foner method.

2) Saturation Magnetization Measurement: Figs. 5 and 6 show the saturation magnetization of polycrystalline ferrimagnetic samples, YIG and Ni-Zn ferrite, measured at various temperatures by the electronic system described above.

The comparison with other results obtained from well- tried techniques, such as the vibrating sample magnetom- eter, demonstrates good agreement between the two meth- ods.

3) Magnetic Hardness Measurement: In Fig. 7 we present a comparative study of the magnetic hardness and the saturation magnetization between room temperature and the Curie point.

In the case of this relatively porous material ( p =

0.22), the magnetic hardness is found to be directly pro- portional to 4nM, as shown in Fig. 8. It can be then ex- pressed as

a = 0 (4aM,) (11) with /3 = 0.0066.

This result agrees well with NCel theory [2] according to which the parameter 0 is also proportional to the sam- ple porosity.

2627

v

I k 'I 5

0 Present r e s u l t s

0 V i b r a t i n g sample m a g n e t o v e t e r

0 100 200 T("C)+

Fig. 6 . Saturation magnetization of a spinel ferrite as a function of tem- perature. Comparison with the Foner method.

4 7 M ,

t - 2 1

porous Y I G

0 100

2 4 7 M , 'I 1

t l

1 0 100 T ( " C ) --C

Fig. 7. Temperature dependence of apparent spontaneous magnetization 47rM," and magnetic hardness deduced from the analysis of the function M . H (see Fig. 5 ) .

(p,o 22)

Y IG /// I,,"

0 1 4 7 M s ( k G ) +

Fig. 8. Linear correlation between the magnetic parameters 47rM," and "a" (see Fig. 8). This result agrees well with the Nee1 theory.

Therefore, (1 1) becomes

a = CY . p * (47rM,) (12) where p is the porosity, and CY a new constant the numer- ical value of which is 0.03.

2628 IEEE TRANSACTIONS ON MAGNETICS. VOL. 25. NO. 3. MAY 1989

4.6‘C-

YIG

100 200

50 100

(b)

50 T (“C)-

3.5

0

Fig. 9 . Thermal variations of the initial susceptibility (thermal spectrum) and the magnetic hardness (a) of a polycrystalline YIG sample and (b) of a polycrystalline Ni-Zn ferrite. Anomalous behavior of this last pa- rameter is caused by progressive demagnetization.

This last value agrees strikingly well with those given, on the one hand, by NCel [2] in an experimental study on polycrystalline iron, and, on the other, by Schlomann [3] in a theoretical development.

Fig. 9 is a further example of the sensitivity of the mag- netic hardness to the demagnetizing effects. The phenom- enon is observed only on substances showing a certain degree of inhomogeneity in the ionic structure. The Ni- Zn spinels are an example (Fig. 9(b)) showing anomalous behavior of magnetic hardness near the Curie temperature due to a progressive process of thermal demagnetization.

Similar observations on nickel alloys have been reported by NCel [2].

V. CONCLUSION When magnetization mechanisms are studied, it is very

important that the magnetic medium has a “looped flux” shape, the better configuration being the ring. Unfortu- nately, this structure does not allow very high field appli- cation because the current magnitude is necessarily lim- ited by the cross section of the winding. Sequential techniques must be used, applying high current levels during a very short time. Then power dissipation remains low.

Such a method is shown in this paper using a one-cycle of the ac line (50 Hz). Hence, the current flows in the primary winding of the sample only for 20 ms. Current magnitudes of up to 100 A can be easily applied without self-heating of the magnetic substance (controlled by the temperature dependence of the susceptibility).

The current supply, computer monitored, is tested by a study of the approach to saturation mechanism of poly- crystalline soft ferrites. The results are compared with those in the literature and found to be in good agreement with them.

REFERENCES [I ] P. Weiss, “Absolute value of intensity of magnetization at satura-

tion,” J . Phys., vol. (4) 9 , p. 373, 1910. [2] L. NCel, “Law of approach to saturation and a new theory of mag-

netic hardness,” J . Phys. Radium, vol. (8) 9 , p. 184. 1948. 131 E. Schlomann, “Properties of magnetic materials with a non-uniform

saturation magnetization. I . General theory and calculation of the static magnetization,” J . Appl. Phys., vol. 38, p. 5027, 1967.

[4] H. Polley, “Approach of magnetization to saturation in nickel,” Ann. Physik. vol. ( 5 ) 36, p. 625, 1939.

151 G. F . Dionne, “Determination of magnetic anisotropy and porosity from the approach to saturation of polycristalline ferrites,” J . Appl . Phys . , vol. 40, p. 1839, 1969.

[6] G. J . Cranieri, Linear Integrated Circuirs App[ication Notes (ICAN 6158). RCA Solid Stade Databook Series, 1972, p. 447.

[7] S. Foner, “Review of magnetometry,” IEEE Trans. Magn . , vol.

[8] V. Cagan, “Basic magnetic measurements: new progress for old prin- ciples,” in 1984 Proc. 4th Inr. Conf. on Ferrites, vol. 15, pt. 1, p. 11.

[9] A. Globus, “Some physical considerations about the domain wall size theory of magnetization mechanisms,” J . d e Phys., vol. 38, suppl. C1, p. 1 , 1977.

[ IO] R. M. Bozorth, Ferromagnetism, 7th ed. Princeton, NJ: Van Nos- trand, 1951, p. 484.

[111 A. Herpin, Theory of Magnetism. Paris, France: Bibliotheque des Sci. et Tech. Nucl., Presses Univ. de France, 1968, p. 831.

[I21 A. Globus and R. Valenzuela-Monjaras. “Influence of the deviation from stoichiometry on the magnetic properties of Zn-rich NiZn fer- rites,” IEEE Trans. Magn. , vol. MAG-1 1, p. 1300. 1975.

MAG-17, p. 3358, 1981.

M. Le Floc’h was born in Pont-I’Abb6, France, on January 28, 1945. He received the Doctorat 3e cycle degree from the University of Rennes, France, in 197 1 , and the Doctorat d’Etat degree from the University of Brest, France, in 1983.

He is Maitre de Confkrences at the University of Bretagne Occidentale. Brest, France. He has nearly 12 years of experience in studies of ferrimag- netic ceramics under stress. He has also worked at the Laboratory of “Etat Polycrystallin Ceramique” (CNRS-Bellevue, France).

M. P. Le Guen, biography not available at the time of publication