Some recent developments in magnetism in Czechoslovakia

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Some recent developments in magnetism in Czechoslovakia

Luboš Valenta

To cite this version:

Luboš Valenta. Some recent developments in magnetism in Czechoslovakia. J. Phys. Radium, 1959,

20 (2-3), pp.414-420. �10.1051/jphysrad:01959002002-3041400�. �jpa-00236060�

SOME RECENT DEVELOPMENTS IN MAGNETISM IN CZECHOSLOVAKIA

By ^LUBO0160 VALENTA,

Faculty ^of Technical and Nuclear Physics, Prague, Czechoslovakia.

Résumé. 2014 On présente

^{résumé de} quelques-uns ^des derniers résultats obtenus

magné-

tisme

Tchécoslovaquie. ^Il

^les questions suivantes : 1. Relaxations magnétiques ^dans MnFe2O4. ^2. Variations de la largeur ^{de la raie de} la résonance

fonction de l’aimantation spontanée ^{de ferrites} polycristallins ^de manganèse ^{et de} manganèse ^{et de zinc et} la vérification

expérimentale ^de la théorie de Clogston ^{et al. 3.} Une nouvelle possibilité d’interprétation ^{des dévia-}

tions de la loi pour la perméabilité initiale dans le

du mécanisme de rotation du vecteur d’aiman- tation. 4. L’approche ^à la saturation. 5. Une nouvelle méthode de

de l’effet magnétocalorique

des ferrites. 6. L’introduction conséquente ^de l’idée de L. Néel des sous-réseaux magnétiques ^dans

la théorie de Heisenberg ^et

généralisation pour le

de ferrimagnétisme ^et antiferromagnétisme

et des lames minces de spin arbitraire. 7. L’influence de l’inhomogénéité- ^du champ démagnétisant

sur

la résonance ferromagnétique. ^8. La théorie de la structure des domaines élémentaires

lames minces de MnBi. 9. L’interprétation ^de l’hystérésis ^du coefficient démagnétisant balistique.

10. Sur la définition du paramètre p caractérisant les dimensions des échantillons nonellipsoïdaux.

11. La désaimantation des matériaux ferromagnétiques par

champ alternatif magnétique.

Abstract. 2014 Some

results obtained recently ⁱⁿ magnetism ⁱⁿ Czechoslovakia

briefly

summarized : 1. Magnetic relaxation in MnFe2O4. ^2. Dependence of the line breadth of the

resonance ^line

^the spontaneous magnetization ⁱⁿ polycrystalline ^{manganese and manganese}

zinc ferrites and the question ^{of the} experimental verification of the theory ^of Clogston ^{and al.}

3. A

possibility ^of explaining the deviations from the initial permeability ^{law for the}

of rotation of the vector of the spontaneous magnetization. ^4. Concerning ^the spontaneous magne- tization

saturation. 5. A

method for measuring ^{of the} magnetocaloric effect in ferrites.

6. About the consequent introduction of the Néel’s idea of the magnetic sublattices in Heisenberg’s theory ^{and its} generalization ^for ferrimagnetics, antiferromagnetics and thin films with

arbi- trary spin. ^{7. Influence of the} inhomogenity ^{of the} demagnetizing ^field

^the ferromagnetic

resonance.

8. The theory ^{of the domain} structure in thin films of ^{MnBi. 9.} The explanation

of the hysteresis of the ballistic demagnetizing ^{factor. 10.} Concerning the definition of the _para- meter p characterizing ^the non-ellipsoidal specimens. ^11. Demagnetization ^of ferromagnetic

materials by

alternating magnetic ^field.

JOURNAL DE

PHYSIQUE 20, 1959,

1. Introduction.

In this _paper

brief sum-

mary is given ^{of some new results obtained} recently

in magnetism ⁱⁿ Czechoslovakia. Most of them

are as yet unpublished, ^{some are} preliminary

results or concern

part ^{of à wider} programme. -

In addition ^some papers ^are mentioned which were

published ^{in Czech or Slovak} only.

2. Ferrimagnetism ^and antiferromagnetism.

a) EXPERIMENTAL PART.

1. Krupicka ^and

Vilim [1] ^have recently found well pronounced

relaxations of the Richter type in manganese fer- rites with the general ^formula

MnFe204.xMn3O4-YO (0 x 0,37).

The activation energies ^{obtained from their} experi-

ments ^are in good agreement ^with those obtained from acoustical relaxation effects [2], [3]. ^Measu-

rements of the temperature dependence ^{of the}

electrical conductivity, ^made recently by Zâvéta [4]

also show similar results with one exception ^{in the}

stoichiometric ferrite MnFe204, ^{where the acti-}

vation energy measured ^was about 0,08 ^{eV while}

that obtained from magnetic relaxation is approxi- mately 0,3 ^{eV. It seems to} be reasonable to con-

clude from this that there exist some divalent ions

of Fe in MnFe2O4 ^{and hence some} manganese ions with higher ^{valence than two.}

From the preliminary results it seems probable

that the presence of divalent iron ions causes some new relaxations. Hence the relaxation spectrum consists, ^at least, ^{of two bands} corresponding ^to

the changes ^of valence of iron resp. manganese ions. In the _{range of} frequencies used up ^to now, the corresponding ^{maxima are} superposed, ^and

their existence _may be inferred from the asymetry

of the temperature dependence ^of tg 8, where ⁸

denotes the loss angle.

Because of the rather ^lower activation energy in the relaxation caused by ^iron ions,

superposition

of iron and manganese maxima appears with increas-

ing frequency. ^A lowering ^{of the} effective acti- vation energy follows in consequence. ^In this _way the decrease of the activation energy in manganese ferrite with the decreasing oxygen ^content may also be explained (see ^table I).

It should be mentioned, that relaxations of the

same type ^{have been found} by Krupicka ^{in manga-}

nese zinc ferrites and in manganese magnesium

ferrites also.

2. Reinvestigating ^{his recent results} [5], [6]

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3041400

415 TABLE 1

obtained on polycrystalline manganese and ^man- ganese zinc ferrites, Krupicka ^{bas found}

^well-

defined linear dependence ^of v’M: (Ma ^{is the} spon- taneous magnetization)

^AH (line ^{breadth of the}

resonance line) ^{in the} region ^of temperatures

-180 °C-0 °C (see fit. 1). ^{The examined} speci-

FIG. 1.

Dependence of the line breadth

on

spontaneous magnetization.

niens differ in the content of the _oxygen which

implies

différence in Curie temperature (81 ^OC

resp. 111 OC). Nevertheless the curves for the

spontaneous.magnetization plotted against ^reduced temperature T/Tg ^{were the same.}

These results are in good agreement ^{with the} theory proposed ^for monocrystals by Clogston ^et

al. [7] ^{where a} linear increase of AH with VE,

and for the same materials with the différent Curie

temperatures ^the decrease with increasing T, ^{is to}

be expected. ^{For the} comparison ^{of the} theory

with experiment ^{it is} supposed that, ^{due to the low}

value of the crystal anisotropy, ^{the AH in} poly- crystals ^is mainly determined by ^the size of the individual crystals. Furthermore the slope ^{of the}

curves in fig. 1 should, according ^to [7], ^be pro-

portional ^to the ^mean _square deviation of the effective exchange ^{field which} really ^{seems to be} greater ⁱⁿ specimens ^{with the} higher ^{content of}

oxygen and hence of the vacancies in the lattice.

3. An interesting observation concerning ^the

initial permeability ⁱⁿ sintered -ferrites has been made by Sternberk. ^{It concerns} the deviations from the usually ^{used law}

where _uo is the initial permeability, K ^{the first} anisotropy ^{constant and k}

^{constant more} closely

defined by ^{the nature of the} crystal anisotropy.

As is known, (1) ^has been derived for the _process of the rotation of the vector of magnetization ^and

the deviations are interpreted ^{as caused} by ^the

mechanism of the motion of Bloch walls. Stern-

berk’s idea is the following : ^One may suppose the ferrite in the first approximation ^{to be com-} posed ^of crystal grains situated in the hollows of the surrounding ^medium. The effective internal field is then the sum of the external field He plus

the Lorentz’ field 4 _{M. Sternberk has shown} the ^Lorentz’ field u ^{3 7rM. Sternberk has shown}

(see ^also [8], [9]) ^{that for the relative volume occu-} pied by the ferrite grains lying ⁱⁿ the limits

0,75 ^V 1 and for £ > 10 where uo ^{is the}

initial permeability of the tested material Ma J[K[

should normally ^lie between 1-2. In table II there are compared different values taken from many papers. The agreement ^with the rather crude model is quite satisfactory.

4. Let us remark, ^{that Sternberk has been led} [9]

to similar conclusions also from the investigation

of the behaviour near saturation of some

nickel, manganese and manganese zinc ferrites.

5. The low heat conductivity ⁱⁿ ferrites makes the measurement of the magnetocaloric ^{effect in}

these materials very difficult. This has been studied classically (see e.g. the _papers [10, 11, 12, 13]) using thermo-electrical ^éléments. Zâveta _[14]

has therefore proposed ^{the use of} the adiabatic

change ^of the electrical resistance connected with the change ^{of the} magnetic ^field.

His idea is the following : First of all the depen-

dence ^of the electrical resistance

the tempe-

rature R(T)H-O ^and R(T)H=Hm ^{is found. Then the}

adiabatic change ^{of the} resistance (AR)s ^with

H

Hmis ^measured. Nowthe difference between

(dR)s ^and (OR)T ^{for the} isothermic change (see

416

TABLE II

(1) Calculated from the

data

effective field of the crystal anisotropy 2Kl/Ms resp. 0,2 KIM..

(2) ) ^Precise composition ^unknown.

(8) ^Measured

^the powdered ^material (see ^Birks [38]).

(4) Calculated

the basis of Guillaud’s [47] and Pauthenet’s [46] ^data using ^{for the} density p the value p::: ⁵ g’/cn13

FiG. 2.

To the derivation of the Eq. (3).

(fin. 2) ^{enables us to find the} required (LlT)B. ^This

is because

where e is practically ^{the same for} R(T)H-o

and R(T)H"’Hm (k ^{is the} Boltzmann’s constant).

From the geometry ^one may ^see that

If elk is ^found graphically ^from (2) ^{then we} may find (A T)s ^from (3).

The applicability ^of the method has been tested

by ^Zàvéta

the manganese ferrite

MnOFe2O3 + 0,08 Mn3o4 + 0,055 0 (fig.3).

FIG. 3.

Temperature dependence

of the magnetocaloric ^effect.

A typical anomaly ^{is seen at} the Curie point.

Thus the usefulness of the proposed ^{method is}

quite ^evident.

417

b) ^{THEORY. - 6. The} great progress made in the last years ⁱⁿ the theory ^of ferrimagnetism ^and antiferromagnetism ^is intimately connected with the use of the idea of the magnetic sublattices ^as introduced by ^Néel (see ^first of all his fundamental papers [15, 16]). ^The thorough investigation ^of

the applicatibility ^{of this idea in the} theory ^thus

seems désirable.

The author [17] ^{tried to find what the results}

would be if this idea were introduced into Hei-

senberg’s theory ^when using ^the modification given by ^{Van Vleck [18].} The hamiltonian used had the usual form for the exchange interaction. The

crystal lattice is supposed to ^be composed ^{of Néel’s}

lattices. Similarly ^to [19], [20] ^the existence of the sublattices is characterised quantum-mecha- nically by ^the supposition ^{that the} spins ^{of the} particular sublattices have the definite eigenvalues simultaneously. The hamiltonian of this system is formulated in terms of the " rot’ated " spin operators

^used by Oguchi [21]. Using ^the approximations equivalent ^to those of Van Vleck [18] ^{one obtains the} dependence of the spon- taneous magnetization ^{on the} temperature ^{and the} intensity ^{of the} magnetic ^field.

The theory includes for this model the theory ^of

the ferrimagnetism, antiferromagnetism ^{and the} theory ^{of thin films} proposed recently by ^the

author [20]. ^{The results obtained are} analogous

to those obtained from the molecular field theory

as

given by ^Néel [16], ^{Yafet and Kittel} [22] ^etc...

and to those obtained by ^{Van Vleck} [23] using ^the

method of Stoner [24]. ^{For the} special ^case

S =1/2 ^the theory agrees with the theory ^of

Vlasov and lshmukhametov [19] ^and with that of the author [20]. ^The angle ^between the directions of the magnetization ⁱⁿ the sublattices _may be

expressed ^{in terms of the} exchange integrals.

Hence the principal possibility follows for the

experimental determination of the exchange ^inte- grals.

The corresponding situation in the region ^of

low temperatures ^is

being investigated.

3. Concerning ^the influence of the inhomogenity

of the demagnetizing ^{field on} ferromagnetic

résonance.

7. It is interesting ^{to know how the} departures ^{from the} homogeneity ^{of the} magnetics

field within the specimens change the results obtain- ned in the case of the homogenous magnetization.

The first qualitative study ^{has been} given by Pilscikov _[25] and Frait [26]. Moreover Frait has

now

shown [27] ^{how to treat the} problem quanti- tatively ^{in the case when we are able to calculate}

the internal magnetic ^{field in the} ferromagnetic body. ^{It is tben} possible ^to calculate the _permea-

bility ^as

^{function of the} coordinates. The mean

value of some function of u, say f(u) >, e,g, of

the components ^{of the} complex permeability, ^may

be then obtained from

where ha(x, y, z) ^{is the} intensity ^{of the} high ^fre-

quency magnetic ^{field and the} integration ^is

extended ^over the whole volume of the specimen.

Calculating f({J..) > in dependence ^{on the inten-} sity ^{of the} static external field, ^{one obtains the}

resonance curves, which can be compared ^with experiment.

FIG. 4.

Résonance

for the absorption ⁱⁿ plane parallel rectangular ^FeNi plate (78 % Ni).

theoretical curve,-.-.-... experi-

mental

fi- external magnetic field, frequency

9 000 MHz.

In fig. ⁴

comparison is made of the theoretical

curve with the experimental ^{one obtained from the}

measurement ^on

Fe-Ni alloy ^{with 78} % ^{Ni for a} specimen ^{in the form of a} plane parallel rectangular plate. ^The agreement ^is quite satisfactory.

4. Thin films.

8. As shown by ^{Màlek and} Kambersky [28] ^an explanation may be given ^for

the domain structure found recently by ^Williams

et al. [29] ^{on thin films of MnBi. An} explanation

may also be given, ^based

Kittel’arefined theory ([30], § 3,lb). The différence lies in the f act, that ⁱⁿ

Màlek’s and Kambersky’s Itreatment ^the magnetics

interaction of both surfaces is incittded. They

obtained the density ^{of the} demagnetizing energy per square centimeter

where T is the thickness of the film and D is thé

domain size. The surface density y of thé Blocli

wall has been estimated at 15 erg cm-2. The energy of the Bloch walls is therefore

From the minimum condition for the ^sum of both

énergies they obtained the dependence ^{of tbe size}

FIG. 5.

Dependence ^{of the} domain size

on

the thickness in MnBi films.

of the domains on the thickness T of the thin film (see fig. 5). ForT--10-5emitisD2-3.10-5em.

For T 10-g cm the domain structure practically disappears. ^{For T} > 10- 5

D increases

^Tl/2

similarly ^{to the} theory ^of Kittel. Williams et al.

have found D ~ 3-6.10-5

for T ss 10- 5 cm.

It has been shown that neither

more precise

calculation of the Bloch wall energy, ^nor

change

in the form of domain patterns (e.q. chess-board

pattern) essentially change ^the results. As

con-

sequence of the great anisotropy ^{there is}

possi- bility for domains magnetized parallel ^{to the sur-}

face.

At the end it follows that for thin films of MnBi used

the storage elements, ^{their resolution}

power is limited ^to 109 informations/cm2.

5. Magnetostatics. ^{- 9. In} 1950 Janus et al. [31]

have discovered the hysteresis ^{of the ballistic} demagnetizing ^factor NB (see Fig. 6). Hajko ^and

Daniel-Szabô [32] ^{have found}

simple expla-

nation : The infiniteness of NB is inferred from the fact that in the centre of

rod where M == 0 we

fin d that there is

demagnetizing ^field Hdem ⁰

which is formally expressed

^Hdem

^{NB M.}

Thus the origin ^{of the} demagnetizing ^{field must be}

in the other parts ^{of the} specimen. ^{Hence the}

traditional suppositions usually ^{made about the} magnetization ^{in rods must} be abandoned and

replaced by some new one which respects ^{the real}

situation. These conclusions have been confirmed

experimentally by Hajko ^and Daniel-Szabô as

shown in fig. ^{7. There the ratio of the} magne- tization at the place ^{studied to the absolute value}

of the simultaneous ^one in the centre of the rod is plotted against ^{the distance from the centre.}

The sequence of the curves from 1 to 6 corresponds

Fie. 6.

Ilysteresis ^{of tbe ballistic} deinagnetization factor (measured by V. Ilajko ^{and J.} Daniel-Szabô).

FIG. 7.

Dependence ^of thé rémanent magne fixation ^li

rods

the distance froin the centre of thé rod measured

on

the descendent part ^{of thé} hysteresis loop.

to the motion along ^the decreasing part ^{of the} hysteresis loop ^{from the} upper top ^{to the lower one.}

10. Hajko [33] ^has proved ^{that for} uniformly

magnetized rectangular ^{rods the ballistic dema-}

gnetizing ^{factor is} formally ^{the same}

^{in the case}

419 of the ellipsoidal ^and cylindrical specimens ^when

the parameter p is defined after Schneider [34J

where 1 is the length ^{of the rod} and a, ^{b its trans-}

versal dimensions. _{It may} be easily ^{shown that} Hajko’s arguments may be generalized ^{for rods}

with any ^cross section which have a centre of _sym-

metry. ^Then A’B

^{Q where} Q is the angle ⁱⁿ

which both basic planes perpendicular ^{to the axis}

of the rod, ^{are seen from the centre. For} long ^rods

we have again approximately.

if we de fine

where Fis the surface of the cross-section.

6. On the demagnetization ^of ferromagnetic

materials by alternating magnetic ^{field. -11. Star-} ceva and Shur [35] ^{observed that in} some cases thé

application ^{of a} decreasing alternating magnetic

field results in

increase of the remanent magne- tization and may be accompanied by ^the change ^of

its sign. Hajko ^and Daniel-Szab6 investigated

this effect

toroidal specimens made from magne-

tically soft metallic and ferritic materials. The initial magnetic ^{state has been} obtained in the _way

seen in fig. ^{8. The} results of their experiments

Fie. 8.

To the préparation ^{of the} initiai magne tic ^states.

(fig. 9), show thé variation of the i-eiiiaiienu

magnetization ^{Jr after}

application ^{of an alter-} nating magnetic ^{field with}

^maximum ampli-

tude Il.. ^{In their} experiments ^the amplitude

decreased continuously ^{to zero.} Similarly ^to [35]

all these results _may again ^be qualitatively explained by ^the Kondorsky’s hypothesis ^of

domains in the crystal ^{with different critical fields.}

Similar curves have been found by Hajko ^and

Daniel-Szabô for the dependence ^{of the} apparent

remanent induction in ferromagnetic ^{rods and} ellipsoids.

.Fie. 9.

Dependence of the remanent magnetization Jr

on

the maximum amplitude ^{of the} alternating magnetic

field.

7. Acknowledgements.

^The author would like

to express his hearty ^{tbanks to his friends and} colleagues ^{Dr. S.} Krupicka, ^{Dr. J.} Sternberk,

K. Zâvéta, Dipl. phys., ^Z. Frait, CSc., ^Z. 8/làlek, ^CSc.

V. Kambersky, Dipl. phys., Doc. Dr. V. Hajko

and Doc. J. Daniel-Szabô for the aid in preparation

of this report.

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DISCUSSION

Mr. Schlômann (Comments).

^{I would like to} point ^{out that} the upper ^{one of} the curves shown in fig. 1, ^if extrapolated ^{back to Ms ===} 0, goes ïnto the region ^of negative line breadth. This

indicates, ¹ believe, ^{that other line} broadening

mechanisms are also important ^{in the} present ^case.

Mr. Kurti.

What was the value of Hm ^{used in} the measurements of the magnetocaloric ^{effect ?}

How do the values of Ts compare ^with those calcu- lated from magnetization ^{curves ?}

Mr. Valenta.

The value of the field

5 500 Oe. The comparison ^{was to} my knowledge

not carried out.

Mr. Wohlfarth.

^{The curves} giving ^{the rema-}

nence after A. C. demagnetization ^are analogous

to those reported ⁱⁿ my communication for magne-

tically ^hard powder materials, ^{where the} underly- ing theory ^is perhaps ^{a bit more realistic than for}

the magnetically ^{soft materials} described here.

Mr. Valenta.

Yes, ^I agree.

Mr. Dates.

1) ^{How was} Hdem measured ? 2) ^{How did t,he} measurements agree with those of Düssler ?

Mr. Valenta. - 1) The ^{induction Bis determined}

in the well known fashion by the ballistic method and the field HB

also found ballistically ^with

flat coil which

placed ^{close to the surface of}

the sample.

2) ^{1 am not well} enough acquainted ^with

Düssler’s paper ^{to answer} satisfactorily.