OPTICAL STUDIES OF ELECTRON RELAXATION IN MAGNETIC MATERIALS

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OPTICAL STUDIES OF ELECTRON RELAXATION IN MAGNETIC MATERIALS

K. Aoyagi, K. Tsushima, M. Uesugi, S. Sugano

To cite this version:

K. Aoyagi, K. Tsushima, M. Uesugi, S. Sugano. OPTICAL STUDIES OF ELECTRON RELAX-

ATION IN MAGNETIC MATERIALS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-801-C1-

802. �10.1051/jphyscol:19711282�. �jpa-00214311�

JOURNAL DE PHYSIQUE

Colloque C

supplbment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - ⁸⁰¹

OPTICAL STUDIES OF ELECTRON RELAXATION IN MAGNETIC MATERIALS

K. AOYAGI, K. TSUSHIMA, M. UESUGI Broadcasting Science Research Laboratories of NHK,

(Japan Broadcasting Corporation), Kinuta, Setagaya-ku, Tokyo, Japan and S. SUGANO

The Institute for Solid State Physics, Tokyo University Roppongi, Minato-ku, Tokyo, Japan

Rhum6.

- Par des Btudes d'effet Zeeman sur

(Dy3AlsOl~) A I'aide de champs magnttiques pulsbs, nous avons trouvB que certains des ions Dy3+ etaient A

excites au premier niveau de champ cristallin, qui se situe

-

^cm-1

au-dessus du niveau fondamental. En observant de nombreux matk~iaux magnttiques, nous avons trouvt que la relaxa- tion de spin dans 1'6tat du plus bas doublet de Kramers Btait responsable de l'excitation. En dessous de la temptrature de NBel, ce phBnom6ne n'est pas observ6. Ceci montre que les magnons jouent un r61e important dans le transport de l'knergie Zeeman de I'ttat fondamental au bain thermique.

Abstract.

- In the Zeeman studies of

(Dy 3A150

with pulsed magnetic fields, we have found that some of the Dy3+ ions were excited to the first excited crystalline-field level at

which is located at -

cm-1 above the ground level. By performing the observation about many magnetic materials, it was found that the spin relaxation in the lowest Kramers doublet state are responsible for the excitation. Below Nee1 temperature, this phenomenon

not observed.

This indicate that magnons play an important role in carryng the Zeeman energy of the ground state to the heat bath.

The pulsed magnetic field technique has often been used to study the Zeeman effects in strong magnetic fields [I]. With this technique, in some cases, we may observe time dependent phenomena such as electron- spin relaxation process [2] [3]. On the study of the Zeeman effect of DAO [4], it was found that there are absorption lines which appear additionally only in the pulsed magnetic fields. We shall report here some details of this effect and discuss its mechanism.

DAG is one of the antiferromagnetic materials well studied so far (TN

2.49 OK). From the optical studies of DAG, it is known that the ground state of Dy3+ is the Kramers doublet 2, of the 6H15/2 term split by the crystalline-field and is separated by 70.3 cm-I from the higher one 2,. Figure 1 shows the schematic energy-level diagram of the 6H,5,2 and

FIG. 1.

- Schematic

diagram

the

6Fsp

states

6 ~ 5 / 2

states. The absorption spectrum at 4.2

consists of the lines corresponding to the electronic transitions only from the Z, to the various excited states.

Figure 2 shows the shape of our pulsed magnetic

FIG. 2. -

Time dependence

pulsed magnetic field.

field, which is deformed from a sinusoidal shape because of technical reasons. The field strength comes to its maximum value at the time of 0.7 ms measured from the onset of the pulse. The exposure time for the spectro-photograph, i. e., the pulse width of the Xe flash light, was about 30 ps.

Figure 3 shows the Zeeman pattern of the absorption spectrum corresponding to the 6H15/2

6F5/2 transi- tion at 4.20K. The pulsed field (H) were applied parallel to the < 111 > direction of the cubic axes.

Beside the conventional Zeeman spectrum, additional absorption lines (d,,) appear in the fields greater than about 30 kOe. These lines become stronger as the intensity of the field is increased. The intensity of the lines does not depend upon the delaying time but does upon only the field strength at the time of observation.

From the spectroscopic analysis, it was revealed that these lines correspond to the transitions from the Z;

or Z ; Zeeman components to the excited states. There- fore, this phenomenon means that the

and Z';. levels became to be populated in the pulsed magnetic field a t such a low temperature as 4.2OK. A phenomeno-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711282

C 1 - 802 K. AOYAGI, K. TSUSHIMA, M. UESUGI AND S. SUGANO

FIG. 3. — Zeeman effect in the axial-polarisation of the absorp- tion spectrum corresponding to the 6His/2 -> 6F5/2 transition at 4.2 °K. HI I < 111 > . The delay time T is 0.7 ms. The tran-

sitions indicated by dsj are given in figure 1.

logical analysis shows that the population of the Z", Z'2

a n d Z2 levels can be" explained by introducing a n appropriate effective temperature (Tef() which is much higher t h a n t h e temperature 4.2 ° K of t h e heat bath.

re f £ seems to be a function of t h e field strength at t h e time of the observation. F r o m figure 4, it is k n o w n that

FIG. 4. — Field dependence of the effective temperature Ten concerning to Z'{, Xi and Z2 levels obtained from the absorp- tion intensity of the Zeeman spectra of the 6HJS/2 -*• *Fs/2 transition at 4.2 °K. H / / < 111 > . Axial-polarisation.

-Hmax = 95,76 or 57 kOe. x = 0.2 ~ 2.5 ms. H is the strength of magnetic field at the time of observation.

Tiff's for the Z2 a n d EZ2 levels coincide with each other in all the time. This fact means t h a t the two levels are in the thermal equilibrium. This p h e n o m e n o n is also observed at H // < 001 > . In this case, there are two kinds of inequivalent sites with respect to the

relative orientation of magnetic field t o the local symmetry axes of D y3 + ion. Because of a large aniso- tropy of the g-value of t h e g r o u n d state, the Zeeman splitting of the ground Kramers doublet at one site is large, while at another site it is nearly equal t o zero.

However, t h e excitation to t h e Z2 levels takes place at b o t h sites.

Our experimental results suggest that there will be a group of p h o n o n s of relatively high energy which couples strongly with t h e D y3 + i o n system but n o t with the heat b a t h , a n d also t h a t the heat is supplied to the system of these p h o n o n s by t h e spin relaxation in the Zt Zeeman levels. I n our high magnetic fields, the Zx Zeeman energies are relatively high, conse- quently t h e p h o n o n s having these Zeeman energies are also called here t h e high-energy phonons.

Figure 5 shows a magnetic ordering effect on the

FIG. 5. — Temperature variation of the Zeeman spectrum corresponding to the 6Hi5/2 -* 6Fs/2 transition. Hjl< 001 > .

T = 0.7 ms.

additional lines ( d2 1 a n d d2 2) . Below TN these lines disappear, which means t h a t magnons play an impor- t a n t role in carrying t h e heat of the electron plus high- energy p h o n o n systems to t h e heat bath. This effect is also observed in t h e case of H // < 111 > .

This p h e n o m e n o n is rather general a n d can be observed in other magnetic materials such as D y A 1 03, E r G G , H o A G a n d H 0 A I O 3 . In the case of H o3 +, t h e intensity of the additional lines are weak in comparison with those of E r3 + and D y3 +. In T m A G , n o additional lines were observed. These results for H o3 + a n d T m3 +

can be understood if one considers t h e fact that t h e ground states of H o3 + a n d T m3 + a r e not K r a m e r s doublet although t h e former is nearly degenerate.

In D y A 1 03, the additional lines appear at H // a ± b but n o t observed at H // c u p to 100 k O e . Here a, b, a n d c refer to the crystal axes. This can be understood if one considers the fact t h a t t h e g-value of the g r o u n d state is highly anisotropic and gc is nearly equal to zero [5].

References [1] AOYAGI (K.), Misu (A.) and SUGANO (S.), / . Phys.

Soc. Japan, 1963, 18, 1448.

[2] AOYAGI (K.), Misu (A.), KUWABARA (G.) and SUGANO

(S.), / . Phys. Soc. Japan, 1964, 19, 412.

[3] Misu (A.) et al., Japan. J. appl. Phys., 1969, 8, 57.

[4] AOYAGI (K.), TSUSHIMA (K.) and UESUGI (M.), / .

Phys. Soc. Japan, 1969, 27, 49.

[5] SCHUBERT (H.), HUFNER (S.) and FAULHABER (R.),

Z. Physik, 1969, 222, 105.