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From 170Yb 3+ Mössbauer Measurements
J. Hodges, P. Bonville, P. Imbert, A. Pinatel-Phillipot
To cite this version:
J. Hodges, P. Bonville, P. Imbert, A. Pinatel-Phillipot. Magnetic Interactions In YbBa 2Cu 3O x (x
= 7.0, 6.0) From 170Yb 3+ Mössbauer Measurements. Journal de Physique I, EDP Sciences, 1995, 5 (4), pp.501-515. �10.1051/jp1:1995143�. �jpa-00247075�
Classification Physics Abstracts
75.30E 74.70 76.80
Magnetic Interactions In YbBa2Cu30x (x
= T.o, 6.0) àom ~"Yb~+
Môssbauer Measurements
J-A- Hodges, P. Bo~lville, P. Imbert and A. Pi~1ateLPhillipot
CEA/DSM/DRECAM/Service de Physique de l'Etat Condensé, Centre d'Etudes de Saclay,
91191 Gif sur Yvette, France
(Received 19 October 1994, received in final form 19 December 1994, accepted 19 December
1994)
Abstract. Trie magnetic interactions in superco~lducting YbBa2Cu307 a~ld Don.supercon-
ducting YbBa2Cu306 bave bee~lexam1~led using ~"Yb~+ Môssbauer spectroscopy on grain- orie~1ted samples. In YbBa2Cu307 where the Yb~+ sublattice orders at 0.35 K a~ld the saturated mag~letic moment is 1.75 pB, trie saturated molecular field is 0A T a~ld it is directed perpendicu-
lar to the c-axis. In YbBa2Cu306, where the Cu(2) sublattice also orders, the average saturated molecular field on Yb~+ is 0.2 T. It is disordered in direction and contains contributions from both the Yb~+ and the Cu(2)~+ sublattices. The preseI1ce of
a Cu(2)~+ derived molecular field shows the Yb~+ ions
are trot at a centre of magnetic inversion symmetry relative to trie ordered
Cu(2)~+ moments.
1. Introduction
Most of trie rare earths can be substituted into YBa2Cu307 without influeI1ciI1g trie super-
conducting properties [ii, The magI1etic interactions leading to magnetic order within trie
rare earth sublattice are of interest as they link widely spaced sheets stacked along trie c~axis and pass through trie Cu(2) O bilayers wherein lies trie source of superconductivity. Trie
layered arrangement of trie rare earth ions leads to two-dimensional features aI1d trie persis~
tence of magnetic correlations above tue three~dimensional ordering temperature [2,3] wuicu
is generally quite low (< i K),
In this structure, trie rare earth site which is at a site of orthorhombic symmetry, is eight.
fold coordinated by oxygens lying in trie Cu(2) O bilayers. Yb3+ bas an interesting crystal
field scheme. Trie 4f13, ~F~/2 multiplet is split by trie crystal field to leave trie lowest Kramers doublet about 1000 K below trie first excited doublet [4]. Trie ground doublet, derived from a cubic r~ state, shows only a weak anisotropy with tue single ion hard magnetisation direction
lying along tue c-axis and tue easy magnetisation direction along tue b-axis [5]. Tue magnetic ordering direction is along trie easy axis [6].
Previous l~°Yb Môssbauer experiments [7] on YbBa2Cu30~ (on a sample which was later
found to coI1tain a higher amount of the impurity phase Yb2BaCu05 than imtially estimated)
© Les Editions de Physique 1995
and neutron diffraction measurements [6] bave provided trie Yb3+ sublattice ordering temper.
ature (TN " 0.35 K).
We present here a study of the magnetic ordering in superconducting YbBa2Cu30~ and in
non.superconducting YbBa2Cu3 06, carried ont using 1"Yb Môssbauer measurements on grain oriented samples of good quality-
From an analysis of trie Môssbauer lineshapes at 0.05 K, where trie results are influenced by
trie Boltzmann populations of trie exchange split Yb3+ S'
= 1/2 ground doublet, we obtain trie size of tue saturated molecular field and deduce trie size of trie saturated magnetic moment.
The use of grain oriented samples provides information concerning trie directions of the ordered
moments. Of particular interest is trie comparison between YbBa2 Cu30~ and YbBa2Cu3 06 In
YbBa2Cu30~ only trie Yb3+ sublattice orders whereas in YbBa2Cu306 trie Cu(2) sublattice is also expected to magnetically order m trie same way as it does in YBa2Cu306 where TN
~
400 K and trie moments lie perpendicular to trie c.axis [8,9]. Trie present results show that while trie molecular field acting on a Yb3+ in YbBa2Cu30~ comes from within trie Yb3+ sublattice, trie molecular field on Yb3+ in YbBa2Cu306 contains also a contribution coming from trie
ordered Cu(2)~+ sublattice.
2. Experimental Details
2.1. SAMPLE PREPARATION. YbBa2Cu307 was prepared by sintering appropriate amounts
of Yb203 (isotopically enriched in 1"Yb), BaC03 and CUO in air. Four intermediate grindings
were used to optimise trie sample quality. In order to limit trie amount of impurity phases (these form much more readily in YbBa2Cu30~ than in YBa2Cu30~), we limited trie maximum
annealing temperature to 910°C- A further heat treatment in oxygen maximised trie oxygen
content. A room temperature X.ray analysis provided trie lattice parameters (a
= 3.808,
b
= 3.859, c = 11.675 À) and identified trie impurity phases Yb2BaCu05 and BaCu02 each of which 15 present near trie 5% level. Trie YbBa2Cu306 sample was obtained by annealing in
a dynamic vacuum at 600°C- The X.ray analysis provided trie lattice parameters (a
= 3.846,
c = Il.849 À) and showed an unchanged impurity content. Trie weight difference produced by
trie vacuum anneal corresponded to an estimated lowering of oxygen content of about 0.9 per
formula unit. We recall that for trie two oxygen levels, trie rare earth is generally thought to be at a centre of crystallographic inversion symmetry-
2.2. GRAIN ORIENTATION. Trie grain oriented samples of YbBa2Cu307 and YbBa2Cu306
were obtained by setting grains, sifted to 20p, in a resin (stycast) in presence of a 2T magnetic field applied parallel to trie plane of trie disc shaped sample. During trie treatment, trie sarnple
was rotated about an axis perpendicular to trie disc plane. Tuis configuration is appropriate
in tue present case wuere tue grains uave an easy plane of magnetisation (arising from tue weak single ion amsotropy of tue Yb~+ ions), and it orients tue diilicult magnetisation axis
(the c-axis) perpendicular to the disc plane. Trie quality of trie orientation obtained with this method depends on trie percentage of trie grains which are in single crystal form. To
maximise this percentage, it is advantageous to mcrease trie average grain size by a suitable
ueat treatment before crusuing and sifting tue sample. Tuis works well for YBa2Cu30~ but
not for YbBa2Cu30~ because tue annealing temperature required to swell tue grain size in trie latter case is higher than trie temperature at which impurity phases (mainly Yb2BaCu05 and BaCu02) begm to develop strongly. Trie grain orientations were thus carried out on trie samples as imtially prepared.
Trie X-ray diffraction patterns with trie X-rays diffracted from trie plane of trie sample
disc are shown m Figure 1- In both cases a strong enhancement of trie (001) lines is visible
x10~
1 80
62
1.44
ce
~ i
Z
~ o go O Q
o
ce
O =
m 18
~ lO
~~~4
1
1 35
1 20
1 05
~'l 0
E~
§
U
o.45
o
io o
Ù(deg)
Fig.
bBa2Cu307 (top)
bBa2Cu30~ (bottom)- The lhanceme~lt of the (001) peaks evidences well
developed c-axis rientation but the
co~lt1~lued prese~lce of the (103) a~ld (110) l1~les
unoriented fraction is also
present- The c-aXis riented
fraction amounts to about a third of
trie
sample volume at both oxygen levels- indicating that grains bave their
curve analysis on trie (006) line provided FWHM
values
of 5° for and
YbBa2Cu~06 showing tuat in both
sarnples, trie c-axis alignment bas a slight dispersion. This
dispersion is ignored in trie following as it was found to bave negligible influence of trie analysis. igure 1 shows that the powder diffraction fines (trie strongest of
which are
the (103) and (i10) fines) although much weaker thon tue (001) lines, are still visible. This indicates that a fraction of the sample remains unoriented. Estimates of the relative amounts of the two fractions can be obtained by combining the X-ray data and the Môssbauer data.
The X.ray estimate, obtained for each of the two oxygen levels, is based on the comparison of trie ratio of trie intensity of a line coming from the unoriented fraction with trie intensity of trie same line coming from a separate completely unoriented sample which was also set in
a resin [loi. The Môssbauer estimate, obtained for the O~ sample, is provided by the relative intensities of particular absorption lines as described below. For each of the two oxygen levels,
we estimate that the c.axis oriented fraction accounts for about one third of the sample, the two thirds remaining randomly oriented. These values emphasize again that a grain-oriented sarnple may contain a substantial amount of unoriented crystallites even if its X~ray pattern is
dominated by the (001) lines. The fact that trie two samples have essentially the same degree
of grain orientation will be of use in the analysis presented below.
2.3. MôSSBAUER LINESHAPE ANALYSIS. The ~~°Yb Môssbauer measurements (Ig = 0,
I~x = 2, E~ = 84 kev, lmm/s
= 68MHz) were made down to 0.05 K using a neutron activated TmB12 source displaced with a triangular velocity sweep.
In YbBa2Cu30~, the crystal field acts on trie Yb3+ ion to leave a S'
= 1/2 ground state
separated by about 1000 K from the first excited state [4]. Trie electro-nuclear levels for l~°Yb
are described by a two term Harniltoman comprising a hyperfine term and a molecular field term.
H = S' A I flH~°' g S' (1)
where the A- and g~tensors relate to the Yb3+ ground doublet. As J is a good quantum number,
the two tensors are proportional. Using the Môssbauer~derived A values and the EPR~derived
g values trie constant of proportionality is A (MHz)
= 259 g. H~°' is the molecular field. For the I~x
= 2 level, both terms of equation (1) are present, whereas for the Ig = 0 level, trie
hyperfine field term vanishes. Trie field dependence of trie two sets of electro-nuclear levels are shown on Figure 2.
Trie Boltzmann populations of the two sublevels associated with the S'
= 1/2, 1
= 0 ground
doublet are governed by the temperature and by the size of the molecular field which splits the sublevels by an amount gflH~'°'. At the lowest measurement temperature used here (0.05 K)
and for trie sizes of trie molecular field which are found below to act on trie trie Yb3+ ion (0.2
to QI T), only trie lower sublevel of this ground doublet is populated. (A molecular field of
m~
0.3T splits trie doublet by an energy equivalent to 0.45 K). Because of Boltzmann population
effects at 0.05 K, each Yb3+ ion is thus blocked in trie lower substate of its S'
= 1/2 doublet.
This means that even if an intrinsic inter-ionic relaxation mechanism is present (for example
due to trie spm-spin interaction between neighbouring Yb3+ ions), it bas no influence on trie Môssbauer lineshapes. Trie absence of fluctuations within trie electronic substates gives rise to trie so-called slow paramagnetic relaxation Môssbauer hneshapes where trie different absorption lines are directly linked to energy differences between trie electro-nuclear levels shown on Figure
2.
As an introduction to trie somewhat novel lineshapes which are encountered experimentally,
we first present (Fig. 3) some simulated slow paramagnetic relaxation lineshapes obtained at 0.05 K from equation (1). Using trie A~ and g~tensor values for Yb~+ diluted in YBa2Cu30~
(A~ = 795, Ai " 935 MHz, g~ = 3.06, gi " 3.60 [iii) and for different values of H~°' The measurement technique is not sensitive to any small orthorhombicity in trie electro-nuclear parameters, so that equation (1) was taken to bave tetragonal symmetry for both YbBa~Cu306 (which is exact) and for YbBa2Cu30~ (which is an approximation).
50
~ w
1, 25
ru 1,
É
Î
OE -25
1
-50
50
~ w
1, 25
c1
11
~
É
01 c
°' -25
1
-50
soc iooo isoo 2000
HIG)
Fig. 2. Calculated field dependence of the electro-nuclear levels for the S'
= 1/2, Iex
= 2 and
S'
= 1/2, Ig
= 0 subsystems of ~~°Yb~+
using the hyperfine pararneters and g-values appropriate to
Yb~+ substituted into YBa2Cu307. The e~lergies are give~l in Môssbauer u~lits (1 mm/s = 68 MHz)
On trie left sidê of Figure 3 we show trie lineshapes corresponding to a randomly oriented sample with trie molecular field perpendicular to trie c.axis. (Because of trie relatively low
anisotropy of trie A- and g~tensors, these lineshapes for a randomly oriented sample depend only weakly on trie direction of trie molecular field). On trie right side we show sets of trie line-
shapes corresponding to a single crystal sample (or fully grain oriented sample) when trie c.axis is aligned along trie ~/-ray propagation direction, and when trie molecular field lies respectively
perpendicular to, at 45° to and parallel to trie c.axis.
For both trie randomly oriented and trie single crystal cases, trie lineshapes depend on H~'°' for values up to roughly o-à T. Beyond this value, trie molecular field term dominates trie
hyperfine interaction term in equation (1), and each electro-nuclear level associated with trie 1 = 2 Môssbauer level comprises just one of the pure nudear substates (mi>. In this case, the energies of trie electro-nuclear levels (Fig. 2) depend linearly on field so that trie energies of trie Môssbauer transitions no longer depend on trie strength of trie field. Trie overall splitting of
trie pattern is then governed by trie value of trie component of trie hyperfine tensor along trie direction of trie quantizing field. In trie other limiting case when there is no molecular field, trie electro-nuclear levels are governed by trie hyperfine interaction which leaves states expressed in
10URNAL OEPHYSIQUE T3.N° 4. ALiRCH 1995 19
à = 90° 45° 0°
10.0T
0.4
U.2
O.1 ~
o.05
0.03
0.
-z o z -z o z -z o z -z o z
v (cm/s) v (cm/s)
Fig. 3. Simulated o-05 K slow paramag~letic relaxation ~~°Yb~+ Môssbauer1i~leshapes obtained with the electro-nuclear Hamilto~lia~l give~l in the text. The depe~ldence of lineshapes are show~l as a fu~lctio~l of the molecular field act1~lg on the Yb~+ electro~lic spin. The lineshapes relate to a randomly
orie~lted sarnple (left side) a~ld to a s1~lgle crystal (or c-axis grain-orie~lted) sample (right strie) for three diIfere~1t values of 0 the angle between the field a~ld the i~ray propagation direction (trie c-axis
direction).
terms of trie quantum number F = I +S' [1Ii. For fields of intermediate strength, trie hyperfine
interaction term and trie molecular field term in equatiôn (1) are of comparable importance, and each electro-nuclear level linked to trie 1
= 2 state, may now comprise more than one nuclear substate- In this situation (25'+1)~(21+1)
= 20 separate Môssbauer transitions are
possible.
In trie intermediate field range, where trie overall form of trie pattern depends markedly on
trie field strength, trie o-os K lineshapes (Fig. 3) also show trie novel feature that the centre of gravity of trie pattern depends on trie field strength. At fields above roughly o.5 T, trie centre of gravity of trie absorption pattern corresponds to trie isomer shift value which is essentially
zero. As trie size of trie field decreases, trie centre of gravity initially moves to negative Doppler
velocity values. Trie shift is most clearly visible on Figure 3 for fields of o.l T where negligible absorption occurs outside trie shown Doppler velocity span. As trie field continues to decrease to zero trie centre of gravity of trie slow paramagnetic relaxation pattern moves back to trie
isomer shift value. Trie shift of trie centre of gravity m trie intermediate field range (for example
for H~°~
= o-1 T) is due to trie influence of population dioEerences within trie two substates of trie S'
= 1/2, (mi
" o) ground state shown on Figure 2. Trie absorption associated with trie
substate having trie lower energy gives rise to a pattern centred at negative Doppler velocities whilst trie absorption associated with trie upper substate is centred at positive Doppler veloc~
ities. At 0.05 K where kT < gflH trie upper substate is essentially not thermally populated
and does not contribute to trie absorption pattern which, thus, bas its centre of gravity shifted to negative Doppler velocities. We emphasize trie particular feature of these slow pararnagnetic relaxation lineshapes : for the range of molecular fields encountered below (0.2 0A T), the
Sl
o.5 °
-3 -Z -1 0 3
v (cm/s)
Fig. 4. ~~°Yb~+ Môssbauer absorption at o.05 K in YbBa2Cu307. The satisfactory total data fit (upper part) was obta1~led in terms of trie two show~l subspectra, o~le coming from trie randomly
oriented fraction (subspeclrum with 5 individual fines) and one from trie c~axis oriented fraction
(subspectrum with 4 individual fines)- The data fit provides the size of the saturated molecular field acting on the Yb~+ ion (oA T). The poor data fit (lower part) was obtained by imposmg the molecular field to have a size of 0-2 T. (see trie text).
overall splitting of trie Môssbauer pattern does not depend on trie size of trie molecular field, however, because trie field controls trie centre of gravity of trie absorption pattern, the size of the field cari be determined.
The simulations for a single crystal (or fully grain-oriented) sample with its c-axis oriented parallel to the ~/-ray propagation direction, which are shown on trie right hand side of Figure 3, indicate that trie lineshapes depend both on trie size of trie field (which again influences trie centre of gravity of trie absorption pattern) and on trie angle between trie field and trie c~axis
(~/.ray propagation direction). Trie strong dependence on angle shows that in principal, the direction of trie molecular field can be obtained with good accuracy.
3. Experimental Results
3-1. YBBA2CU30~ AT 0.05 K, The absorption in trie magnetically saturated state with trie ~/.rays propagating along trie c~axis of trie oriented fraction, is shown on Figure 4. Trie
absorption takes trie form of a s4ine pattern with four equally intense outer lmes and with
a weaker central line. Trie absorption is weil fitted (upper part of Fig. 4) in terms of two subspectra corresponding respectively to the randomly oriented (two thirds of trie sample) and c~axis oriented (one third of trie sample) fractions with a common value for trie electro-nuclear
parameters and for trie molecular field (0.4 + 0.04 T). Trie two subspectra which are included in trie data fit shown on trie upper part of Figure 4, correspond to trie two simulated 0A T
spectra illustrated on Figure 3 for respectively, random orientation (spectrum with 5 lines of
equal intensities), and = 90° c.axis orientation (trie central component of the 5.line pattern
