University of Groningen
Temperature-induced magnetization reversal in a YVO3 single crystal
Ren, Y; Palstra, T.T.M.; Khomskii, D.I; Pellegrin, E.; Nugroho, A.A.; Menovsky, A.A.;
Sawatzky, G.A
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Nature
DOI:
10.1038/24802
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Ren, Y., Palstra, T. T. M., Khomskii, D. I., Pellegrin, E., Nugroho, A. A., Menovsky, A. A., & Sawatzky, G. A.
(1998). Temperature-induced magnetization reversal in a YVO3 single crystal.
Nature
,
396
(6710), 441 -
444. https://doi.org/10.1038/24802
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8
®eld lines are puzzling, however. In principle, the plasma should fall
freely along such lines, reaching speeds of 50 km s
-1
or more, but
that con¯icts with the speeds in fact seen (a few kilometres per
second). A somewhat similar situation prevails in spicules, also
without a satisfactory explanation.
Why has counter-streaming not been detected in previous Dop-
pler observations of ®laments? We made Dopplergrams from pairs
of red-wing and blue-wing images, as have many other authors
4±14
.
We concluded that our line-of-sight velocity maps alone do not
reveal counter-streaming because a simple subtraction of moderate-
resolution images, taken in opposite wings, will usually result in a
null Doppler signal. Second, motions of the chromospheric back-
ground can corrupt the signal in the ®lament, unless spatial
resolution is suf®ciently high. Last, isolated pairs of images do
not reveal the long distances traversed by the moving knots of mass.
Detection of the counter-streaming requires high spatial resolu-
tion, a favourable ®lament orientation, time-series of observations
in both wings simultaneously over several hours, carefully registered
images and a high rate of image projection. The ®lament we
observed is quite typical. We therefore suggest that counter-stream-
ing is a feature common to all quiescent ®laments.
Our observations contradict all contemporary prominence
models, even those of barbs, as these models do not allow any
vertical motions, let alone counter-streaming. Simply allowing
horizontal ®eld lines to drift up and down, while carrying blobs
of mass, will not easily explain counter-streaming, for then alternate
®eld lines would have to pass each other and travel long diagonal
distances. Such complex motions seem unrealistic to us.
We propose instead a conceptual model
18
in which ®eld lines bend
down from the spine to the chromosphere (see Fig. 2 centre). In this
model, the mass ¯ows along the ®eld lines: it is not carried by
moving horizontal ®eld lines, and does not cross the ®eld lines. The
tension of such steeply inclined ®eld lines, which are embedded in
the prominence sheet, might act to restrain the prominence from
erupting. Certainly the existence of such ®eld lines needs to be
incorporated in any acceptable model of prominence eruption.
M
Received 24 April; accepted 10 September 1998.
1. Jackson, B. V. & Howard, R. CME mass distribution derived from SOLWIND coronagraph
observations. Sol. Phys. 148, 359±370 (1993).
2. Priest, E., McKay, D. & Longbottom, A. Dipped magnetic ®eld con®guration associated with ®laments
and barbs. Astron. Astrophys (in the press).
3. Aulanier, G., Demoulin, P., van Driel-Gesztely, L., Mein, P. & DeForest, C. 3-D magnetic con®gura-
tions supporting prominences II. Astron. Astrophys 335, 309±322 (1998).
4. Dunn, R. B. A Photometric Investigation of the Solar Chromosphere. Thesis, Harvard Univ. (1960).
5. Engvold, O. The small-scale velocity ®eld of a quiescent prominence. Sol. Phys. 70, 315 (1981).
6. Engvold, O. in New Perspectives on Solar Prominences (eds Webb, D., Rust, D. m. & Schmeider, B.)
(IAU Colloq. 167, Kluwer Academic Press, in the press).
7. Engvold, O. The ®ne structure of prominences I. Sol. Phys. 49, 283±295 (1976).
8. Liggett, M. & Zirin, H. Rotation in prominences. Sol. Phys. 91, 259±267 (1984).
9. Mein, P. in Solar Coronal Structures (eds Rusin, V., Heinzel, P. & Vial, J. C.) 289±296 (IAU Colloq. 144,
Veda Publishing House, 1993).
10. Martres, M. J., Mein, P., Schmeider, B. & Soru-Escaut, I. Structure and evolution in quiescent
prominences. Sol. Phys. 69, 301±312 (1981).
11. Mahlerbe,J.,Schmeider, B. & Mein, P. Dynamicsin the ®laments.Astron. Astrophys. 102, 124±128 (1981).
12. Mahlerbe, J. M., Schmeider, B., Ribes, E. & Mein, P. Mass motions in ®laments. Astron. Astrophys. 119,
197±206 (1983).
13. Engvold, O. & Keil, S. L. in Coronal and Prominence Plasmas (ed. Poland, A. I.) (NASA Conf. Publ.
2442, NASA, Washington, DC, 1986).
14. Kubota, J. & Uesegi, A. Vertical motion of a prominence. Publ. Astron. Soc. Jpn 38, 903±909 (1986).
15. Leroy, J.-L., Bommier, V. & Sahal-Brechot, S. New data on the magnetic structure of quiescent
prominences. Astron. Astrophys. 131, 33±44 (1984).
16. Hundhausen, J. R. & Low, B. C. Magnetostatic structure of the solar corona 2, the magnetic topology
of quiescent prominences. Astrophys. J. 443, 818±832 (1995).
17. Bothmer, V. & Schwenn, R. Eruptive prominences as sources of magnetic clouds in the solar wind.
Space Sci. Rev. 70, 215±220 (1994).
18. Martin, S. F. & Echols, C. R. in Solar Surface Magnetism (eds Rutten, R. J. & Schrijver, C. J.) 339±346
(Kluwer Academic, Dordrecht, 1994).
Acknowledgements. We thank H. Zirin for the observing run at BBSO, D. Martin for reproducing the
original photographic data in digital movie format, and R. Ewald for her assistance in preparing the
illustrations.
Correspondence and requests for materials should be addressed to J.B.Z. (e-mail: jzirker@noao.edu).
letters to nature
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Figure 2 Images on 16 April, showing the inclined threads. In the blue wing, the
streaming is only westward along the spine and downward into the barbs on the
near side of the ®lament (blue arrows in the middle panel). In the red wing, the
streaming is only eastward along the spine and up in the same barbs on the near
side (red arrows in middle panel). This systematic pattern appears most clearly on
16 April, when the ®lament lay just inside the east limb. Finally, on April 20, when
the ®lament had crossed the central meridian, the pattern reverses, with a
predominant eastward streaming in the blue wing although both eastward and
westward streaming are seen in different ®lament threads.
Temperature-induced
magnetization reversal
inaYVO
3
single crystal
Y. Ren*, T. T. M. Palstra
²
, D. I. Khomskii*, E. Pellegrin*,
A. A. Nugroho
³
§, A. A. Menovsky
³
& G. A. Sawatzky*
* Solid State Physics Laboratory,
²
Inorganic Solid State Chemistry Laboratory,
Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands
³
Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstaart 65,
1018 XE Amsterdam, The Netherlands
.........................................................................................................................
The total energy of a magnet in a magnetic ®eld is lowest when the
magnetic moment is aligned parallel to the magnetic ®eld. Once
aligned, the magnetic moment can be reversed by applying a
suf®ciently large ®eld in the opposite direction. These properties
form the basis of most magnetic recording and storage devices.
But the phenomenon of magnetization reversal in response to a
change in temperature (in a small magnetic ®eld) is rarer. This
effect occurs in some ferrimagnetic materials consisting of two or
§ Jurusan Fisika, Fakultas Matematika dan Ilmu Pengetahuan Alam, Institut Teknologi Bandung,
Indonesia.
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more types of antiferromagnetically ordered magnetic ions
1
, and
forms the operational basis of ferrimagnetic insulators. Here we
report the observation of multiple temperature-induced magne-
tization reversals in YVO
3
. The net magnetic moment is caused by
a tilting of the antiferromagnetically aligned moments of (crystal-
lographically identical) V
3+
ions, due to orthorhombic distortion
in the crystal structure. We observe an abrupt switching at 77 K
associated with a ®rst-order structural phase transition, and a
gradual reversal at ,95 K without an accompanying structural
change. The magnetization always reverses if the crystal is cooled or
warmed through these two temperatures in modest ®elds. We
propose a possible mechanism involving a change in orbital order-
ing which may be generic to a broad class of transition metal oxides.
Transition metal oxides with the perovskite structure display a
large variety of properties such as high-temperature superconduc-
tivity, colossal magnetoresistance
2,3
and very diverse magnetic
properties. We now report on another novel and peculiar phenom-
enon in YVO
3
: multiple and reversible sign changes in the magne-
tization with temperature. Such magnetic-moment reversals have
been observed in those ferrimagnets
1
with strong magnetic aniso-
tropy that exhibit a compensation temperature. The net magnetiza-
tion, initially oriented parallel to the ®eld, changes sign at this
temperature and the metastable, energetically unfavourable state is
®xed by the strong anisotropy. For such an effect to occur,
inequivalent sites must exist. In YVO
3
, however, all magnetic V
sites are equivalent, as shown by the crystallographic data.
Close to our situation is the observation of a `diamagnetic'
response in LaVO
3
(refs 4±6). Upon weak-®eld cooling, this
system exhibits a magnetization opposite to the applied magnetic
®eld below a structural phase transition temperature
T
t
138 K , T
N
142 K, where T
N
is the magnetic ordering
(Ne
Â
el) temperature. It has been suggested that this diamagnetic
response is due to a reversal of a canted-spin moment on traversing
the ®rst-order Jahn±Teller phase transition at T
t
, below which the
orbital angular momentum is maximized
6±8
, and that the response
of the orbital moment to the forces generated at the ®rst-order
phase transition can reverse the Dzyaloshinsky vector so as to create
a canted spin in a direction opposite to the applied ®eld, given that
T
t
is close to T
N
. Other polycrystalline samples of V
3+
perovskites
show weak indications of anomalous behaviour
7±9
. We demonstrate
the dramatic behaviour of single crystals of YVO
3
in which multiple
magnetization switching is clearly observed. In contrast with
LaVO
3
,YVO
3
provides a wide temperature window between T
N
and the ®rst-order phase transition which we refer to as T
s
, enabling
us to study the effect in detail. Furthermore, an additional magne-
tization reversal is observed, irrespective of the cooling conditions.
Figure 1 Temperature dependence of the magnetization in an applied magnetic
®eld of 1 kOe along the a-, b- and c-axes, respectively. Upon cooling the sample in
a ®xed magnetic ®eld H , 4 kOe below T
N
116 K, the magnetization after ®rst
increasing starts decreasing and crosses zero at T
p
< 95 K to a large negative
value. With further cooling, it jumps at T
s
77 K to a large positive value. This
second transition exhibits hysteresis that is consistent with a ®rst-order transition,
as inferred from the structural data. In contrast with LaVO
3
, we observe not only
an abrupt magnetization reversal at the ®rst-order transition T
t
, but also a gradual
reversal near 95 K.
Figure 2 Magnetization versus temperature in a ®eld of 100 Oe, demonstrating
the memory effect upon the application of large ®elds. In the curve marked by
®lled circles in a we follow M with decreasing temperature down to a temperature
below T
p
; a high ®eld is then applied to ¯ip the magnetization positive, after which
the ®eld is lowered again to 100 Oe and the temperature decreases. The curve
marked by diamonds is measured without intermediate application of the high
®eld. In b, we show the curves with increasing temperature starting from below T
s
:
the curve marked by diamonds without having `trained' the sample, and the curve
marked by ®lled circles after `training' as described for a. This demonstrates the
reversibility: upon warming, M now switches from negative below T
s
, to positive
for T
s
, T , T
p
, and becomes negative for T . T
p
. it is thus nearly a `mirror image'
of the behaviour shown in Fig.1 and the diamond marked curve in b, except that it
becomes positive again close to T
N
.
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YVO
3
has a distorted perovskite structure of the type GdFeO
3
(crystal symmetry Pbnm) at all temperatures. It has two magnetic
phases: one at T
s
< 77 K , T , T
N
116 K and another at T , T
s
.
The detailed magnetic structure of these phases is controversial
10,11
but is not crucial for the experimental observations reported here.
The distorted crystal structure naturally leads to canted spin
structures. As the oxygen ions mediating the superexchange inter-
action between the two nearest neighbour V ions are not at an
inversion centre, the antisymmetric Dzyaloshinsky±Moriya (DM)
interaction of the form D×S
1
3 S
2
will be present, where D is the
so-called Dzyaloshinsky vector and S
1
(S
2
) are the V spins corre-
sponding to the two magnetic sublattices. The DM interaction,
which prefers canted spin arrangements, is in competition with the
quite different ordinary Heisenberg exchange JS
1
×S
2
, which prefers
collinear spin arrangements. Also, because the oxygen octahedra
coordinating the V ions are twisted to form a staggered V±O bond
direction along the c-axis, the single-ion anisotropy easy axis is
staggered. Both of these mechanisms can lead to weak
ferromagnetism
12
. The fact that YVO
3
is a weak ferromagnet with
spin canting has been established from measurements on powder
samples
13
. Several batches of crystallographically pure large single
crystals (5 3 5 3 5mm
3
) were grown by the ¯oating-zone method.
The magnetic measurements made on a large number of our single
crystals all reveal the same unusual behaviour, with several magne-
tization reversals upon cooling in a weak magnetic ®eld (Fig. 1).
Even more striking, is the observation of a memory effect shown
in Fig. 2a. This indicates that the sign of the magnetization can be
reversed by the application of a large enough ®eld, but that upon
lowering the ®eld, the temperature-dependent net magnetic
moment M(T) always changes sign, irrespective of what its sign
was, when crossing T
s
both on cooling and warming. The same is
also true for the second crossing at T
*
.
Although we do not as yet have a detailed microscopic theory for the
magnetization reversal with temperature, we propose the model
shown in Fig. 3. As mentioned above, there are two mechanisms for
producing a canted spin structure in these materials: single-ion
magnetic anisotropy and DM coupling. These two canting mechan-
isms produce the net moment in the a±c plane, and in our model we
assume that they are oriented in opposite directions. We checked that
the observed phenomena are connected to spin canting: the differential
susceptibility dM/dH is always positive (even in the `diamagnetic'
state) and the net moment is ,0.01 Bohr magneton per V ion, which
for an S 1 system corresponds to a canting angle of ,0.28.
Using this model, we present in Fig. 3 a pictorial view of what
could happen as a function of temperature. We start at temperatures
just below T
N
(Fig. 3a). Here, the two sublattice spins prefer to lie
close to a local easy axis if the local magnetic anisotropy is large,
resulting in a net magnetic moment, as shown. As T decreases, this
net moment will ®rst grow because of the development of a
sublattice magnetization due to superexchange. However, as the
sublattice magnetization develops, so does the DM coupling, which
tends to cant the spins in the opposite direction. Consequently, the
net moment will reach a maximum and then decrease. It crosses
zero at the temperature below which the DM interaction dominates.
This will result in a moment opposite to the small applied ®eld. It
could only reverse to its lowest energy state in the ®eld by reversing
the two sublattices on a macroscopic scale, resulting in a frozen in
metastable state (Fig. 3b). A large external magnetic ®eld can
overcome the barrier for rotation of the sublattice spins, resulting
in a reversal of the sublattice spin orientation (Fig. 3c). The net
moment is now oriented parallel to the ®eld and will remain so
upon lowering the ®eld. If we now increase the temperature, we
return to the situation where magnetic anisotropy dominates. Thus,
the net moment will change sign again, now turning negative for
T . 95 K (the curve marked by ®lled circles in Figs 2b, 3d). Once we
get close enough to T
N
, the energy barrier for the reversal of the
sublattice magnetization will eventually become very small. Any
®nite ®eld will ¯ip the net moment again to a positive value and
reach a maximum just below T
N
, dropping to zero at T
N
(Fig. 2).
This model explains all the data from high temperatures down to
the ®rst-order phase transition at 77 K.
The magnetization reversal at T
s
remains more of a puzzle. It
follows from our results that the ferromagnetic moment is oriented
along the a-axis both above and below T
s
. From neutron scattering,
we know that the magnetic structure changes at T
s
from C-to
G-type, as proposed by Kawano et al.
11
This is de®nitely connected
with a change of orbital ordering occurring at T
s
. Low-temperature
single-crystal X-ray diffraction (Y.R. et al., unpublished results) of
YVO
3
shows that, whereas above T
s
the V±O bond lengths are
almost equal, the ®rst-order phase transition is accompanied by a
strong distortion of these bond lengths, resulting in pairs of long
(2.052 A
Ê
), intermediate (1.992 A
Ê
) and short (1.975 A
Ê
) V±O bonds.
The long and short bonds are oriented alternately along the [110]
and [11
Å
0] in the basal a±b plane, whereas the intermediate one of
1.992 A
Ê
is along the c-axis, similar to the structure of LaMnO
3
;
such a distortion corresponds to orbital ordering, with the d
xy
Figure 3 Pictorial view of the temperature dependence of the magnetization. a,Just
below T
N
, the two sublattice spins prefer to lie close to a local easy axis if the local
magnetic anisotropy is large, resulting in a net magnetic moment parallel to the
applied ®eld. b,AsT decreases below T
p
95 K, the DM interaction dominates
which tends to cant the spins in opposite direction. A large external magnetic ®eld
can overcome the barrier for rotation of the sublattice spins, resulting in a reversal of
the sublattice spin orientation. c, The net moment is now orientedparallel to the ®eld
and will remain so upon lowering the ®eld. If we now increase the temperature, we
come back to the situation where the magnetic anisotropy dominates. d, Thus, the
net moment will change sign again, turning negative for T . 95 K.
Nature © Macmillan Publishers Ltd 1998
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444 NATURE
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orbital occupied at each V
3+
site and the second electron occupying,
respectively, the d
xz
and d
yz
orbitals in two sublattices as shown in
Fig. 4. According to the Goodenough±Kanamori rules
14
, this orbital
occupation naturally leads to the G-type antiferromagnetism
11
.A
similar result was also obtained by band-structure calculation
15
.
From Fig. 1, we see that the ferromagnetic component is both above
and below T
s
oriented parallel to the a-axis of the crystal; the
magnetic structure above T
s
must therefore be of C-type. Then,
according to Bertaut's symmetry considerations
12
, the easy axis
below T
s
should be close to the c direction in order for the weak
moment to remain along a. From this, we conclude that the easy axis
must indeed change on going through the phase transition. The C-
type magnetic ordering observed between T
s
and T
N
(ref. 11) should
then correspond to an orbital structure, with the alternation of the
sublattices also being along the c direction and, as discussed above,
with the easy axis almost parallel to b. Such a drastic change of the
magnetic properties at T
s
can lead to a change in the relative role of
the DM interaction and the anisotropy. Careful neutron scattering
experiments are needed to clarify the behaviour of YVO
3
across T
s
.
Our results make it dif®cult to compare LaVO
3
and YVO
3
in
detail: single crystals of LaVO
3
are needed, although the close
proximity of T
N
and T
t
in LaVO
3
will be a complicating factor. M
Received 19 June; accepted 19 August 1998.
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. Jpn Appl. Phys. 30,
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. Phys. Rev. B
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. C.R. Acad. Sci., Paris 319,
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334 (1995).
8. Nguyen, H. & Goodenough, J. B. Magnetic and transport properties of CeVO
3
. J. Solid State Chem.
119, 24±35 (1995).
9. Corti, M., Cintolesi, A., Lascialfari, A., Rigamonti, A. & Rossetti, G. Magnetic properties of YVO
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from
susceptibility and
89
Y NMR measurements. J. Appl. Phys. 81, 5286±5288 (1997).
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investigations of rare-earth orthovanadites. Sov. Phys. Solid State 18, 1165±1166 (1976).
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63, 2857±2861 (1995).
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Acknowledgements. We thank J. B. Goodenough for discussion of the results and for explaining his ideas
on the origin of the magnetization reversal in LaVO
3
; we also thank J. Rodriquez-Carvajal and E. F. Bertaut
for discussion. This work is supported by the Netherlands Foundation for the Fundamental Research of
Matter (FOM) and the Dutch Organization for the Advancement of Pure Research (NWO), and in part by
EU (OXSEN).
Correspondence and requests for materials should be addressed to G.A.S.
Figure 4 Suggested orbital ordering below and above T
s
77 K. a,AC-type
orbital ordering with a G-type spin ordering; and b,aG-type orbital ordering with a
C-type spin ordering. The dotted lines are along the b-axis. The hatched squares
indicate the planes in which the occupied d orbitals lie. The d
xy
orbitals that are in
each case occupied by one electron are omitted for clarity.
Spontaneous ordering of
bimodal ensembles of
nanoscopic gold clusters
C. J. Kiely*, J. Fink*
²
, M. Brust
²
, D. Bethell
²
& D. J. Schiffrin
²
* Materials Science and Engineering, Department of Engineering,
The University of Liverpool, Liverpool L69 3BX, UK
²
Department of Chemistry, The University of Liverpool, Liverpool L69 7ZD, UK
.........................................................................................................................
The controlled fabrication of very small structures at scales
beyond the current limits of lithographic techniques is a techno-
logical goal of great practical and fundamental interest. Impor-
tant progress has been made over the past few years in the
preparation of ordered ensembles of metal and semiconductor
nanocrystals
1±7
. For example, monodisperse fractions of thiol-
stabilized gold nanoparticles
8
have been crystallized into two- and
three-dimensional superlattices
5
. Metal particles stabilized by
Figure 1 An ordered raft comprising Au nanoparticles of two distinct sizes with
R
B
=R
A
< 0:58. Shown are electron micrographs at low (a) and higher (b) mag-
ni®cation. c, The low-angle superlattice electron diffraction pattern obtained from
this bimodal raft structure.