Formation kinetics of a weak ferromagnetic domain from an AFM domain wall in DyFeO3

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Formation kinetics of a weak ferromagnetic domain from an AFM domain wall in DyFeO3

N.F. Kharchenko, S.L. Gnatchenko, A.B. Chizhik

To cite this version:

N.F. Kharchenko, S.L. Gnatchenko, A.B. Chizhik. Formation kinetics of a weak ferromagnetic do- main from an AFM domain wall in DyFeO3. Journal de Physique, 1989, 50 (10), pp.1153-1156.

�10.1051/jphys:0198900500100115300�. �jpa-00210985�

Short Communication

Formation kinetics of a weak

ferromagnetic

DyFeO3

N.F Kharchenko, ^S.L Gnatchenko and A.B. Chizhik

Institute for Low Temperature Physics ^and Engineering, ^Ukr.SSR Academy ^of Sciences, 310164 Kharkov, ^U.S.S.R

(Reçu ^le 16 janvier 1989, accepté ^{le 17 mars} 1989)

Resume. 2014 On utilise la photographie à grande ^{vitesse et la} magnéto-optique pour étudier la for- mation d’un domaine à ferromagnétisme ^{faible à} partir ^d’une paroi antiferromagnétique à ^{180° £ la}

transition de premier ^{ordre induite} par ^le champ ^{entre un état} antiferromagnétique ^{et un} état faible- ment ferromagnétique ^dans l’orthoferrite de dysprosium

Abstract. ²⁰¹⁴ In this work high-speed photography ^and magnetooptical studies have been performed

of the formation process ^{of a weak} ferromagnetic domain from a 180° antiferromagnetic ^domain

wall at the pulsed field-induced first-order phase transition from an antiferromagnetic ^{state to a weak} ferromagnetic ^{one in} dysprosium orthoferrite.

Classification

Physics ^Abstracts

75.60C - 75.30K - 75.50E

Domain walls of an initial phase ^can be the nuclei of a new magnetic ^{state at the} magnetic

first-order phase transitions [1]. The formation of paramagnetic [2, 3] ^{and weak} ferromagnetic (WFM) [4] domains from an antiferromagnetic (AFM) domain wall was visualized earlier in a

quasi-stationary magnetic field. We have performed visual and magneto-optical,studies ^{of forma-}

tion kinetics of a WFM domain from a 180° domain wall between AFM domains during ^the pulsed

field-induced phase transition from an AFM state to a WFM one in dysprosium orthoferrite.

The studies were carried out in a DyFeo3 plate of about 40 pm thick ^cut normal to the crystal optical ^{axis. The} sample ^was placed ^{in a} magnetic field with the constant H, ^and pulse Hp compo-

nents. An applied field is oriented along ^the crystal optical axis that makes an angle ^{of about 600}

with the c-axis in the (bc)-plane. Below the Morin point (TM ^{= 49} K) this field leads to the AFM state symmetry lowering ^from ri (Gy ) ^{to r 14} (Fz ^Gx Goy) ^and induces the first order phase ^transi-

tion r14 (Fz ^G. Gaz) ^{- r4} (Fz Gx) from the AFM to WFM state. The sum over the constant and pulsed fields exceeded the phase transition field Ht ^{in the} experiments ^{done. The} pulsed ^field

duration was 45 Jls and the pulse ^{rise was} 1 ps.

Preliminary ^visual investigations ^{in the} polarized light ^under stationary fields made it possible

to identify the domain wall between AFM domains Fz Gx Gt ^{and Fz Gx} Gy -. ^{As known, in} DyFe03

there occurs the AFM G-vector rotation in the AFM domain wall between F, G x G00FF ^{and F, Gz} G-

states in the crystal (ab)-plane. ^The ferromagnetism ^{vector F} changes ^{in a wall} only in absolute

value, its direction (F il c) being unchanged. It takes the maximum value in the center of the domain wall at G 11 a. Thus, ^{at H} Ht, ^{the AFM domain} wall contains the nucleus of the WFM

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500100115300

1154

state r 4 (Fz Ga,). ^A magnetic field which induces the r14 ^---+ r4 phase transition leads to the AFM wall broadening ^and formation of a WFM domain at H ⁼ Ht [4].

Formation and evolution of a WFM domain were studied by high-speed photography. Figure

1 presents photographs illustrating ^the broadening process ^{of a WFM domain. The} photographs

were taken for various T delay ^{times of a short} light ^laser pulse (about ⁶ ns) ^with respect ^{to the}

front edge ^{of a field} pulse. ^{A WFM} domain formed from the AFM wall broadening is visualized in 1-2 ps ^{after the} pulsed ^{field is switched on. As the} delay ^time increases, ^{the domain} broadening

is observed in photographs. Polarizers are aligned ^so that there is no contrast between AFM and WFM phases ; ^{dark walls are} only ^seen between them. In photographs 3 and 4 WFM domains can

be seen as well, ^formed by fluctuations. These domains were visualized for the given relation of

He, Hp ^{and Ht} fields in 14-18 ps only ^after Hp ^{was switched on.}

Fig. 1.- Formation of a WFM domain from an AFM domain wall in a pulsed ^field Hp = ^{1400 Oe at}

Hc ^{= 600} Oe, Ht ^{= 1100} Oe, ^{T = 43} K.1 ) rl ^{= 3 ILs,} 2) ^{r2 = 10 ILs,} 3) ^{r3 = 14 ps,} 4) ^{r4 = 25 ps.}

The time dependence ^{of the} interphase ^{wall shift,} x(t), (Fig. 2, ^line 1) ^was determined by dynamical photographs ^{of a WFM} domain formed from an AFM wall, ^{taken at} various time mo- ments. The x(t) dependence obtained is nonlinear by contrast to that (Fig. ^{2, line} 2) measured in

a similar driving ^{field Hd =} (H, ⁺ Hp) - ^{Ht for the} interphase wall between WFM and AFM domains in a magnetic intermediate state prepared previously ^{in a constant field He = Ht. In the}

initial stage of broadening of the WFM domain formed, ^the interphase wall motion is accelerated.

This motion becomes stationary ^as interphase ^{walls achieve a} velocity equal ^to that of motion in the magnetic intermediate state in a similar driving field. The linear section of the x(t) dependence corresponds ^{to the} stationary ^{motion of} interphase ^walls.

Fig. ² Fig. ³

Fig. Z- The time dependence ^of interphase ^wall displacement. ^{He = 600} Oe, Hp = ¹⁴⁰⁰ Oe, ^{Ht -}

1100 Oe, ^{T = 43 K.}

Fig. 3.- The time dependence of light intensity. ^{Hc =} 600 Oe, Hp ⁼ 1400 Oe, Ht ^{= 1100} Oe, T ^{= 43} K, ,

t N 12 u s.

’ ’

In order to define reasons for the nonstationary interphase wall motion of a WFM domain formed due to thé AFM wall broadening, magnetooptical studies of a WFM phase ^{have been car-}

ried out. The light intensity ^{I of a} helium-neon laser (À ^{= 6328} Â) ^was measured transmitted

through ^a section of the sample placed between crossed polarizers. ^A crystal ^{section studied of}

about 30 nm wide contained an AFM wall transformed into a WFM domain under a pulsed ^field.

The transmitted light intensity depends ^on both the concentration of coexisting phases ^{in the sec-}

tion studied and the Faraday rotation in each phase : ^{I = 7o}

[p Sin2oW

Sin2 OAI

p - 2x/d ^{is the WFM} phase concentration, ^{2x the WFM domain} width, ^{d the width of the sec-}

tion studied, Ow ^and 9A ^the Faraday rotation in WFM and AFM phases, respectively, proportional

to magnetization. ^The I(t) dependence obtained is given ⁱⁿ figure 3. Time t* corresponds ^{to a}

moment when the WFM domain width becomes equal ^to that of the section studied. _p ⁼ 1 and I ⁼ Io sin28w ^{for t} > t* . The light intensity ^{rise at t} > t* implies ^an increase of Ow ^{which is defined} by ^the magnetic ^{moment of a WFM} phase. t* N ^{12 ps for the} I(i) ^{curve from} figure ^3.

The observed intensity rise of transmitted light ^{at t} > t* , ^{which seems to be due to an increase}

of magnetization ^{of a WFM} phase, ^{makes it} possible ^to conclude that a WFM phase formed from the broadening ^{of an AFM domain wall is not} equilibrium thermodynamically ^{because of inten-}

sive excitation of a spin system. ^As follows from the I(t) dependence given ⁱⁿ figure ^{3, the time of}

established equilibrium ^is 25-30 lis for ^the experimental conditions studied. In this period ^inter- phase ^{walls are} displaced by 35-40 jÀm. The comparison between the x(t) ^and I(t) dependences

shown in figures ² (line 1) ^and 3 indicates that the time of establishing equilibrium ^{in the WFM}

state coincides with that of establishing ^the stationary interphase ^{wall motion. Thus, one can make}

a conclusion that at the initial stage ^{of the WFM domain} broadening ^the nonstationary interphase

wall motion is induced by ^a change ^{of the} magnetic pressure ^on interphase ^{walls as the WFM} phase approaches equilibrium.

1156

References

[1] ^{MITSEK A.I.,} GAIDANSKII P.F., ^PUSHKAR V.N., Phys. ^{Status Solidi 38} (1970) ^69.

[2] DILLON J.F. Jr., ^CHEN E.Yi., ^GIORDANO N., ^WOLF W.P., Phys. Rev. Lett. 33 (1974) ^98.

[3] ^{CHEN E.Yi., DILLON J.F. Jr.,} GUGGENHEIM H.J., ^J. AppL Phys. ⁴⁸ (1977) ^804.

[4] GNATCHENKO S.L., ^EREMENKO V.V., KHARCHENKO N.F, Sov. J Low Temp. Phys. ⁷ (1981) ^1536.