Journal of Magnetism and Magnetic Materials 102 (1991) 175-180 North-Holland

The effect of impurities Fe2+, Co2’, Ni2+ on antiferromagnetic resonance in KMnF,

D. Slcrzypek

Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland

Received 20 March 1991

The X-band antiferromagnetic resonance spectrum of KM@ with impurities Fe*+, Co*’ and Ni*+ has been studied at temperatures from 20 K to TN. The admixture of 3d ions dramatically influence the AFMR of KMnFs. The admixtures containing 0.4 at% Fe*+ and 1.0 at% Cos’ cause a decay of resonance mode within an interval of 2 K below TN. In this work, the X-band AFMR of crystals KMnc,,Ni o,olFs and KMnFs : Fe was examined. A broadening of the resonance linewidth and “anomalous” temperature relationship are probably caused by a spin-lattice relaxation through ion relaxation processes.

1. Introduction

The single crystals of potassium manganese flu- oride, doped Fe*+, Co*‘, Ni*+ have been studied by X-band antiferromagnetic resonance for impur- ity concentrations not exceeding 1 at%. KMnF, shows a great number of both magnetic and structural phase transitions. The crystal structure of KMns belongs to the cubic perovskite (Pm3m) at room temperature. At 186 K a structural phase transition to tetragonal symmetry (14/mcm) is observed. Below 91 K other structural phase tran- sition to monoclinic symmetry was observed. The monoclinic distorsion is very slight (see Ratuszna and Glazer in ref. [l] and references cited there).

The magnetic structure of KMnF, has been studied by Hidaka [2]. In the uniaxial antiferro- magnetic region (T, = 81.5 K -c T < TN = 88.3 K), the structure is of the G-type with the sublattice magnetization parallel to the z-axis, while in the weak-ferromagnetic region (below T, = 81.5 K) the structure is of the pseudo-G-type with the sub- lattice magnetization nearly parallel to the x (or y)-axis and slightly canted towards the y (or x)- axis.

The magnetic properties of the crystal KMns have been studied by the electron spin resonance method by Saiki et al. [3]. The analysis shows that

a second-order magnetic transition associated with the spin reorientation at Td = 50 K exists in this crystal.

2. Preparation of the samples and experimental technique

The examined samples were crystallized by the Bridgman method. The details on their prepara- tion can be found in the work of Skrzypek et al. [4]. The single crystals were cloven into the size 0.5 x 1.0 x 2.0 mm3 whose edges were along the (110) directions. The samples of KMnF, doped Fe2+, Co*‘, Ni2+ with admixture concentration not exceeding 1 at% were examined. The composi- tion and homogeneity of the crystals were checked using X-ray microanalysis. The antiferromagnetic resonance (AFMR) was observed at v = 9.3 GHz (X-band) with standard EPR spectrometer, in the temperature range from 20 to 90 K. The demagne- tization effects of the samples were neglected. The relations of the resonance field shift and the reso- nance linewidth broadening versus temperature were examined.

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176 D. Skrzypek / A&ferromagnetic resonance in KMnF, and impun’ties

3. Experimental results and discussion

KMnF, is antiferromagnetic below TN = 88.3 K. The combination of a large effective exchange field (HE = lo6 Oe) and small anisotropy field (HA < 10 Oe) permits AFMR to be observed con- veniently at X-band frequencies. This paper pre- sents the studies on the effect of 3d-ion impurities on antiferromagnetic resonance in KMnF,. The magnetic properties such as the Neel temperature and exchange field are unaffected by low con- centrations of Fe2+, Co2’, Ni2+. The mentioned ions have a non-zero orbital magnetic momentum and characterize in a strong (Fe2+, Co”) spin- orbit coupling. This coupling is a main source of crystalline anisotropy.

Figs. 1 through 4 show the temperature depen- dence of the resonance fields for KMnF,, KMnF, : Fe (iron concentration in the sample is about 0.05 at%), KMn,,s,NiO,,,F, and KMn,,,

BCmTl

KMnF3

x2 .

.

TCKI

Fig. 1. Temperature dependence of the resonance fields for KMnF,.

Co,,.OiF,. The X-band AFMR is visible for three first samples. For KMnO,,,Co,,O,Fe,, a decay of resonance mode occurs within an interval of 2 K below TN. A similar behaviour was observed for

KMn0.9%F%.m 3 F samples. It is a principal result of this work. Minor admixtures of Fe2+ and Co2’ ions in a KMnF, host change the resonance constant [(21i,H,)l12] in the theoretical formula of the AFMR so far, that no X-band antiferro- magnetic resonance is observed in such materials.

3.1. Temperature dependence of the resonance field for T, < T< TN

Fig. 5 shows the temperature dependence of the resonance field for the examined samples within a temperature interval of T, -c T-z TN. Introduction of trace Fe’+ admixtures and 1% Ni2+ admixture to KMnF, does not change the nature of the phase transition to uniaxial antiferromagnetic phase. The values of the resonance field and its dependence on temperature are the same as for KMnF,. How- ever, the width of the resonance line and its inten- sity undergo a considerable change. They will be discussed in section 3.3. The resonance constant was calculated from the AFMR condition for axial anisotropy [3]:

(w/v)4- (o/y)2[H2(a2COS2J,+1)

+2&H*{ 3 cos’($ - +) - l}]

- H2(2H,H*){ a cos’q cos 2( J, - cp)

+fxcos\c, sin+sin(lC,-+)

+sin#coscpsin($-+)+(cwcos2#-sin2J,)}

+ (2H,H*)2 cos 2( l/J - I$) cos”( JI - +)

+ ar2H4 cos2J, = 0, (1)

where H is the applied field forming an angle + with easy axis; m = (Mi - M,) is the vector defi- ning the equilibrium direction of sublattice mag- netizations; the vector m forms an angle J/ with the applied field; HE and HA are the exchange and anisotropy fields, respectively;

(2)

D. Skrzypek / Antiferromagnetic resonance in KMnF’, and impurities 177

where x,, and x I represent the parallel and the perpendicular susceptibilities, respectively.

When the applied field is perpendicular to the easy axis (+ = n/2), then $ = 7r/2 and eq. (1) reduces to:

(w/y)*=&-2H&. (3)

The value of the resonance constant (ZH,H,)‘/’ calculated from eq. (3) is presented in fig. 6. For the examined concentrations of Fe2+ and NiZf admixture, this value does differ from that of the resonance constant for KMnF,.

3.2. Temperature dependence of the resonance field for T< TN

Two kinds of resonance lines coexist at temper- atures around T,. New lines appear in higher fields and characterize a strong temperature de- pendence (see figs. l-4). In %&i’s works [3],

50

40

30

21

10

BCmTl *. . -.

*-. KMnF,:Fe

-. Htl<llO> -. . .

. m . . x

resonance conditions for KMnF, in the canted phase were recorded. The field Hi, due to the Dzyaloshinsky-Moriya interaction appears in the resonance condition. Hence, the phase transition T, is discontinuous (first order magnetic transi- tion) and resonance lines are in higher fields.

For admixtured samples, a qualitative nature of the temperature relation for resonance field is the same as for KMnF,. However, a shift towards lower fields is visible. It can be interpreted as a change of resonance condition. In fig. 2 of ref. [3c], the temperature dependence of the factor HEHA for KMnF, at a temperature interval of 4.2 K to TN is presented. In the region Td < T < T, a strong increase of the anisotropy field and almost constant value of the HD field are observed. It seems that a change in the theoretical formula of the AFMR for samples containing Fe’+ and Ni2+ will change the value of the HA field.

For KMnF,, the analysis of antiferromagnetic resonance shows that a second-order magnetic

0

4 KMnF,:Fe 0

H 11<001> 0 4

0

4

M 1 TCKI (b) 20 40 60 80 20

TCKI

40 60 80

Fig. 2. Temperature dependence of the resonance fields for KMnFs : Fe.

178 D. Skrzypek / Antiferromagnetic resonanq in KMnF, and impurities

transition associated with the spin reorientation exists at Td = 50 K. Such behaviour is also ob- served in admixtured samples.

However, it should be stated that the best sep- arated spectrum and the most explicit results are obtained for KMnF,. The 3d ion admixture to KMnF, causes a broadening of linewidths and a reduction of their intensity resulting in a reduced splitting of the spectrum and broadening of phase transition regions.

3.3. AFMR linewidths

For small values of the applied field, the mag- netization in our samples is not flopped. At tem- peratures below the critical temperature TN, the spectra observed in the AFMR of KMnF, are composed of the resonance lines corresponding with different types of antiferromagnetic domains. These lines are narrow, far distant from each other

and considerably different in intensity. In the ad- mixtured materials, the broadening of the reso- nance line is observed also in a paramagnetic phase (see table 1). Hence, the spectrum is not well resolved in these materials. Fig. 7 presents a relationship of ABpp versus temperature for the samples under examination. The discontinuity ob- served in a course of the curves corresponds with the phase transition to the canted phase.

The AFMR linewidth of KMnF, showed a very little temperature dependence. A greater increase of A Bpp is related to fluctuation occurring within a phase transition region. For compounds such as KMnF, involving S-state ions, Rezende and White [5] proposed different mechanisms based on multi- magnon-relaxation processes.

For admixtured samples, a strong increase in linewidth is observed with temperature decrease. When a relaxation problem was considered in magnetically ordered materials it was found that

t

O-

0.

IO.

IO-

BCmTI

00 0

0 KM",, %o, F3 II 0 Hll<OOl>

(b) 20

TCKI

10 60 80

Fig. 3. Temperature dependence of the resonance fields for KMn,,Ni,,,F,.

D. Skrzypek / Antiferromagnetic resonance in KMnF, and impurities 179

KMno.99c%.ol F3 3LmTl H II <IO>

ECmTl

,,s ..*

0

. .

TCKI

84 86 88 90 92

I-

L

20 40 60 80 TCKI

Fig. 4. Temperature dependence of the resonance fields for

KM~o.&%.o,F,.

the observed high values of ABpp and “anoma- lous” temperature relationships are usually condi- tioned by the occurrence of the additional relaxa- tion channels [6]. In our case, it is an indirect spin-lattice relaxation that proceeds with par- ticipation of the ion subsystem having a strong spin-orbit interaction. The 3d ions having orbital and spin magnetic momentums can be a channel transferring the energy from the magnetic subsys- tem to the lattice or other types of magnetic modes, by spin-phonon and spin-magnon relaxa- tion processes, respectively. The studies on a so called ion relaxation result in a relation:

AB-r, (4

where r is the relaxation time of the admixture ions. A temperature-dependence of this parameter plays a decisive role in describing the behaviour of the linewidths.

a KM+

l KMnF3:Fe

x KMnl199Ni0.01F3

t

BCmTJ ' Knnw9C00.0,F3

400 H I<llO>

300 -

zoo-

loo-

TCKI

75 80 85 90

Fig. 5. Temperature dependence of the resonance fields for various samples in the temperature range T, < T-C TN.

4. Conclusions

The performed experiment allowed to draw the following conclusions:

a) X-band AFMR for KMnF, has been ob-

0 KMnF3 (fromhik,)

H ' (2HC Hn' CmTl

x KMnF3:Fe

. KMnF0.99NhOl~3

. KMnwr) coo.ol F3

Hil<001>

5 I

x . *. I .

zoo- * x l x 0

** : x .

100 - l . x .X

1 2 .., TCKI

80 02 0L 86 88 90

Fig. 6. Temperature dependence of the resonance constant in the temperature range T, < T < TN.

180 D. Skrzypek / Antiferromagnetic resonance in KMnF, and impurities

Table 1 Resonance linewidths in the pammagnetic phase at room tem- perature

Compounds

KMnFs KMnF, : Fe

KMno.s,sF%.,Fs

KMna.@ia.ctFs

KMno.ssWo~Fs

;f

6.0 10.0

65.0

13.1

46.1

1 e

I’

.

0

BppCmTl KM)

x KHnF3:Fe

D KMn0.99Ni0.0, F3

' KMn0.99cOao1F3 .

Ii II <llO>

.

TCKI

60 70 80 90

Fig. 7. Temperature dependence of the resonance linewidths for various samples.

served up to 4.2 K. The situation dramatically changes, when KMnF, is admixtured with Fe*+ and Coz’ ions. For KM~I~.&o~~~~F~, a decay of the resonance mode occurs within a temperature interval of 2 K below TN. A similar behaviour was observed for KMn 0.9%F%.,F3 samples.

b) X-band AFMR for KMnF, : Fe and KMn0,,9Ni,,,,,F3 was examined with a temperature interval of from 20 K to TN_ AU magnetic phase transitions, characteristic for KMnF, were ob- served. The small shift of the resonance field indicates a change in values of the parameter in the equation for AFMR.

c) The considerably “anomalous” increase in linewidth with decreasing temperature in the mag- netically ordered phase is related to the spin- lattice relaxation through so-called ion-relaxation process.

d) All the effects observed in the samples with admixtures are caused by a pressure of the ions with non-zero orbital momentum.

References

[l] A. Ratuszna and M. Glazer, Phase Transitions 12 (1988) 347.

[2] M. Hidaka, J. Phys. Sot. Japan 39 (1975) 103.

A.J. Heeger, 0. Beckman and A.M. Portis, Phys. Rev. 123 (1961) 1652.

[3] (a) K. Saiki, J. Phys. Sot. Japan 33 (1972) 1284, (b) 38 (1975) 373. (c) K. Saiki and H. Yoshioka, Solid State Commun. 15

(1974) 1067.

[4] D. Sktzypek, P. Jakubowski, A. Ratusna and A. Chelkow- ski, J. Crystal Growth 48 (1980) 497.

[5] S.M. Rezende and R.M. White, Physica B 86-88 (1977)

1277; Phys. Rev. B 14 (1976) 2939. [6] A.G. Gurewicz, Magnitnyj rezonans w ferritach i antiferro-

magnetikach (Nat&a, Moscow, 1973).