Muon spin rotation experiments in α-Fe2O3 and Cr2O3

Muon spin rotation experiments in α-Fe2O3 and Cr2O3

MUON SPIN ROTATION EXPERIMENTS IN ot-Fe203 AND Cr203 K. RfJEGG, C. BOEKEMA, A. DENISON, W. H O F M A N N and W. KLrNDIG Physik-lnstitut, Universitat Z...

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MUON SPIN ROTATION EXPERIMENTS IN ot-Fe203 AND Cr203 K. RfJEGG, C. BOEKEMA, A. DENISON, W. H O F M A N N and W. KLrNDIG Physik-lnstitut, Universitat ZiJrich, 8001 Zi4rich, Switzerland

The behaviour of the stopped muons in the isostructural antfferromagnetic insulators a-Fe203 and Cr203 indicate a localized state below 500 K. Local fields and relaxation rates are measured and interpreted.

By means of the/zSR technique [1] the antiferromagnetic insulators ot-Fe203 (Ttq = 963 K) and Cr203 (T N -- 308 K) have been investigated. Our main interests in the behaviour of the muon in these oxides are the stopping sites, the local fields B~ and the electronic state of the muon [2, 3]. In a #SR experiment the muon polarization is measured at the time of decay. In the/~SR spectra the Larmor precession signal of the muon spin, due to an effective magnetic field, is superimposed on the muon decay pattern. The amplitude of this signal can be described by the following formula: A(t)

=

A o e x p ( - ~ t ) cos(0~t + {~),

with o~ the precession frequency, ~, the relaxation or depolarization rate and A o the initial asymmetry of the #SR signal. The isostructural sesquioxides a-Fe203 and Cr~.O3 have the rhombohedral corundum structure (R3c). Magnetically, they differ by the arrangement of the magnetic moments of the metal ions along the trigonal c-axis. In Cr203 the spins are parallel ( + ) or antiparallel ( - ) to the c-axis alternatively as ( - + - +). In Fe203, below the Morin temperature (T M = 263 K) the spin arrangement is (+ +--). Above TM the Fe 3+ spins are perpendicular to the c-axis and lie within the basal plane. The temperature dependence of the observed frequencies and relaxation rates in a-Fe203 in zero external field are shown in figs. 1 and 2. There is a jump in the frequency at the Morin temperature TM by a factor of 2.15. This indicates that the local field is mainly due to dipolar fields. Applying an external magnetic field along the e-axis at T = 203 K and T - 80 K a splitting of the signals is observed, corresponding to exactly twice the external field strength. This shows that for T < TM the local fields B~ are directed along the c-axis. At high temperatures, the relaxation rate starts to increase around 400 K and the signal is no

longer seen above 500 K. The interpretation of the behaviour between 400 K and 500 K in terms of an Arrhenius law gives Ea ~ 0.5 eV. The increase in is interpreted as being due to the onset of thermally activated translational diffusion of the muons, which average out the oppositely oriented local fields of the two sublattices. For T < TM one observes an increase of ,~ with decreasing temperature to a split temperature Ts = 120 K. Below this temperature three distinct frequencies are seen. Although the origin of these signals is not clear, the onset of an interaction with the chemical environment or bonding to the neighbouring oxygens as the temperature is lowered may be responsible for the more complicated #SR spectra. A possible explanation in terms of formation of muoxyl bridges (analog to hydroxyl bridge or hydrogen bond) is presently being studied. Supporting evidence for such weak bonding is found in the activation energy associated 2<.0,

220

t

210

,

, 17

- Fe 2 0 3 P6 _ :_.__,i:::

"

>.. 2o¢

.J pu.

IOC

" 9C 80 TM 70

I~)O

2(~)0 ' 3~0 ' 400 TEMPERATURE [K]

5 5 0' 0

Fig. I. /~SR frequencies of a-F¢~O s (singe crystal) in zero applied field as a function of temperature. The errors are-unless indicated-smaller than the points plotted. The jump in the/~SR frequency is due to the spin rotation at the Morin temperature TM. The/~SR signal disappears at about 500 K due to the onset of diffusion.

Journal of Magnetism and Magnetic Materials 15-18 (1980) 669-670 ~North Holland

669

K. Ri~egg et al./ Muon spin rotation experiments

670 50

IO0

, SINGLE CRYSTAL • POWDER

-+°k

(I - F e 2 0 3

4O

~-.,,t~..

1%1

uJ 3 0 z

_o ~ 2o N_

L 209MHz~212

MHz-

I

t

I

l!

I

~40

98MHz--

Cr2 0 3

0

~ ~ 't I I00

I I I 1 200 300 400 TEMPERATURE [K]

I

500

Fig. 2. Depolarization rate ~ for a-Fe203 as a function of temperature. The frequency of the signals from which }~ is estimated is indicated. with the depolarization rate k over this temperature region. A n interpretation of 2~ in terms of an Arrhenius behaviour gives an effective activation energy E a ~ 0.1 eV. Such energies are characteristic of hydrogen bonding. The temperature dependent behaviour of the frequencies observed in Cr203 in zero applied field is shown in fig. 3. The situation is similar to that in Fe203. Below T s = 150 K two distinct frequencies are observed. The intensities of the signals in Cr203 are much weaker than in Fe203. Within the experimental error no discontinuity is measured at T~ for the lower frequency signal in single crystals in contrast to the situation in a-Fe203. The curve approximately follows the sublattice magnetization. Single crystal measurements with an applied field parallel to the c-axis revealed the occurrence of a non-linear splitting. This is consistent with the assumption that the local field direction makes an angle 0 with respect to the c-axis. The explanation implies 0 -- 76 ° and 104 ° for the lower frequency

i

I00--

.L~ [ 200 TEMPERATURE [KI

L

l i 300

1

0

Fig, 3. #SR frequencies of Cr203 (single crystal and powder) in zero applied field as a function of temperature.

signal, and 0 = 80.5 ° and 99.5 ° for the higher one. In other words, Ba is at an angle of about 10 ° with respect to the basal plane. In conclusion, the observation of the /~SR signals in ct-Fe203 and Cr203 in zero applied field shows the localization of the muons. Above a specific temperature T s the behaviour of the muon is free-muon-like. N o muonium is observed. The interpretation of the observed signals below Ts may be sought in terms of a muoxyl bridge. The observed change of the local field at T M in a-Fe203 and the temperature dependence of the frequencies indicate a large dipolar contribution to the local field.

References

[1] Muon Spin Rotation, Proc. First Int. Topical Meeting on Muon Spin Rotation, eds. F. N. Gygax, W. Kfindig and P. F. Meier (North-Holland, Amsterdam, 1978). [2] H. Graf, W. Hofmann, W. Kiindig, P. F. Meier and B. D. Patterson, Solid State Commun. 25 (1978) 1079. [3] K. Rfiegg, C. Boekema, W. Hofmann, W. Kfindig and P. F.

Meier, Hyperfine Interactions 6 (1979) 99.