NUCLEAR RELAXATION, SPIN ECHO AND PARALLEL PUMPING IN Y3Fe5O12

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NUCLEAR RELAXATION, SPIN ECHO AND PARALLEL PUMPING IN Y3Fe5O12

M. Petrov, A. Paugurt, V. Pashin

To cite this version:

M. Petrov, A. Paugurt, V. Pashin. NUCLEAR RELAXATION, SPIN ECHO AND PARALLEL PUMPING IN Y3Fe5O12. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1144-C1-1145.

�10.1051/jphyscol:19711409�. �jpa-00214450�

RESONANCE RELAXA T/ON DANS LES ISOLANTS

NUCLEAR RELAXATION, SPIN ECHO AND PARALLEL PUMPING IN Y3Fe,012

M. P. PETROV, A. P. PAUGURT and V. F. PASHIN Institute of Semiconductors, Academy of Science of U. S. S. R.,

Leningrad, U. S. S. R.

Rbume.

- L'influence d'un pompage parallele sur le signal

NMR

dans

Y

3Fe50

¹²

a etk 6tudi6e. On a considkre les mkcanismes possibles de la relaxation nuclkaire parallkle causke par l'amortissement des ondes de spin paramktriques.

La relaxation du rkseau de spins dans un echantillon polycristallin a kt6 etudike dans l'intervalle de tempkratures

4,2 a 400

OK. Le minimum de

T1

qu'on a attribuk

a

l'kchange Fezf-Fe3+ a kt6 trouvk

a 300

OK.

Abstract. - The influence of parallel pumping on

NMR

echo signal in

Y

3Fe~O12 have been studied. Possible mecha- nism of transverse nuclear relaxation caused by the damping and scattering of parametric spin waves is considered.

The spin lattice relaxation in polycrystalline sample have been investigated in the

4.2-400

OK temperature interval. The minimum of

T I

which it was supposed due to Fez+-Fe3+ valence exchange was found at

300 OK.

I. Interaction of nuclear moment with parametric spin waves. - We have investigated the influence of parallel pumping on nuclear relaxation and intensity of the spin echo. The usual double electron nuclear magnetic resonance equipment was used. The sample and the coil for the observation of the spin echo were placed in the resonator. The 9 460 mc/s parallel pumping pulse with one microsecond lenght was applied. The decreasing of the intensity of the nuclear echo signal (hl) was observed, when microwave pulse with the power more than spin-wave threshold was applied a t any time between the first radio pulse and the signal echo (Fig. 1). If the pair of 900 and 1800

RG. 1.

- Signal echo intensity versus the delay time,

T = 150 OK, H = 800

Oe. Solid curve

-

the microwave field is absent. Dotted curve

-

microwave pulse is on the distance of

400

mcsec from the first radio

-

frequency pulse.

radio pulses have been established, the decreasing of echo intensity can be absent, if the microwave pulse coincides with the center of 180° radio-pulse (Fig. 2).

The influence of parallel pumping is absent if one observes stimulated echo and microwave pulse is applied between the second and the third radio-pulse.

Such behaviour of the echo signal intensity can be explaned supposing existence of additional fluctuations of longitudinal local field component on nuclei, which

FIG. 2. -

Dependence

of the

intensity

of

effect

(AZlI)

for usual echo from the position of the microwave pulse.

causes the decreasing of the transverse relaxation time T2, due to microwave pumping action.

Then the changing of the echo signal is equal exp(TmW/T2,,) where T,, is the length of microwave pulse, T2,,

-

nuclear transverse relaxation time caused by parametric spin waves.

The observed change of the echo intensity (with the power microwave pulse 5-10 db more, than the spin wave threshold) shows, that TZmw changes from the original value (0.5 x s) till lo-'

-

s. It is possible to calculate T,,,. For the simplest ferroma- gnet taking into account the effect of spin wave damping and the two magnon scattering on nuclei moments we have

where

w,

is the NMR frequency, S

⁼

3 is the electron spin, nK is the number of the parametric spin waves, N is the number of magnetic ions in a sample,

y e

is the electron gyromagnetic ratio and AHK is spin wave damping factor.

Considering nK/N

21

and AHK

=

0.2 Oe [I], we have 1/T2,, - ^{lo5 -} lo6 s- I, that is in agreement with experimental data. The value 1/T2,, increases with increasing of n,. This behaviour agrees with the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711409

NUCLEAR RELAXATION, SPIN ECHO AND PARALLEL PUMPING

experimental data for dependence of AI/Im, on the

applied power level (Fig. 3).

FIG. 3. - The decreasing of signal echo intensity from supplied

Psupplied

,

T p = 80 mcs, Ho = 800 Oe.

power level, P =

-

Ptreshold

FIG.

4.

-Temperature dependence of the longitudinal rela-

The influence of parallel am ping on ~ u c l e a r longi-

xation time ( T ~ ) on nuclei 57Fe for cc d >> positions. External

tudinal relaxation is a rather small one in our case.

magnetic field H = 2 000 Oe, polycrystalline sample.

For isotropic hyperfine interaction we have

:

11. The longitudinal relaxation.

-

The experimental data on TI (with no parallel pamping) for nuclei in

cc

d

D

positions of Y3Fe5012 are given in figure 4. A minimum of

TI

has been observed near

300 O K .

The position of this minimum in temperature scale does not depend on the external magnetic field magnitude.

Practically, the same value of TI have been measured in temperature interval 77-400

^OK

for nuclei in << a

^)>

sublattice of Y,Fe,012.

Two origins of nuclear spin lattice relaxation in Y3Fe50,, are discussed in literature. The first one is the many magnons (two and three magnons) process of nuclear relaxation 12, 31 and the second one is the absorption and emisson of one spin wave by nuclear spins if the damping of spin waves takes place [4, 51.

Many magnons process affords long time TI (of order lo3 s) at liquid helium temperatures and do not give minimum of TI and so disagrees with our experimental data. One magnon process calculated by de Gennes and Hartmann-Boutron [5] gives the relationship

l/TTl -- AH, where AH is the line width of ferroma- gnetic resonance. At high temperatures there is no maximum of AH in Y,Fe,OI2 with temperature position not depending on external magnetic field. So, we cannot explain the experimental data by this mechanism.

In order to explain the minimum of TI at 300

^{O K}

we suppose the valence exchange of electron between Fe2+ and Fe3+ in Y,Fe,O,,, due to admixture of

~ e ions. Then minimum of TI must exist if ~ + w,

z, =

I.

Here

z,

is the correlation time for electron jumps between ~ e and Fe3+. It is known ~ + [6, 7,

81

where

^{2 ,} =

5.4.10-l5 s, W

=

0.27 eV. Then

and position of minimum does not depend from exter- nal magnetic field. Better agreement with experiment one can reach if changes a little values of

z,

and W.

The authors wish to thank professor G. A. Smo- lensky for interest to this work and helpful discussions.

References

[I] LE CRAW (R. C.), SPENCER

(E.

G.),

J. Phys. Soc. Jap.

[5] DE GENNES (P. G.), HARTMAN-BOUTRON (G.),

Compt.

S,

1962,

17,

401.

^Rend.,

1961,

253,

2922.

[2] BEEMAN (D.), PINCUS

(P.), Phys. Rev.,

1968,

166,

[6] YAKOVLEV (Y.), LEBED

(B.), SOV. Physics Solid State,

359. 1965,

6,

2354.

[3] MYERS (S. H.), MEYER (H.), REMEIKA (I. P.),

J. Phys.

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(D.

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11.

55.

[4] ROBERT

(C.),

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(T.

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