Physica B 326 (2003) 436–439

Spin dynamics in the quantum spin system KCu5 V3 O13
H.-H. Klaussa,*, P. Lemmensa, M. Birkea, D. Baabea, D. Mienerta, F.J. Littersta,cA. Amatob, J. Pommerc, A. Ionescuc, K.-Y. Choic, G. Guntherodt.
,
H. Kageyamad, Z. Hiroid, M. Takigawad
aInstitut f. Metallphysik, TU Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen, SwitzerlandcII. Physikalisches Institut, RWTH Aachen, D-52056 Aachen, Germany
dISSP Tokyo, Kashiwa, Chiba 277-8581, JapanbAbstract
The complex Oxo-Cu-vanadate KCu5 V3 O13 with 5 independent Cu-sites ðs ¼ 12Þ per unit cell has an exchange
topology described as a ladder of spin tetrahedra or triangles. Magnetic susceptibility measurements on single crystals
show a steplike anomaly at 213 K and an antiferromagnetic transition at 7:5 K: ZF mSR reveals only nuclear relaxation
above 10 K and two spontaneous precession signals in the long range ordered regime. Below 3:7 K; a spin reorientation
leads to a broad frequency distribution typical for an incommensurate spin structure.
r 2002 Elsevier Science B.V. All rights reserved.
PACS: 71.70.Gm; 75.30.Et; 76.75.+i
Keywords: Low-dimensional quantum spin system; Spin ladder

1. Introduction
The magnetic properties of low dimensional
cuprates and vanadates have been studied intensively in recent years as they represent quantum
spin systems with interesting properties. For someexchange topologies that are realized in thesesystems, peculiar phase diagrams are postulated
that include short-range and long-range orderedphases as functions of exchange coupling parameters or doping. These phases are divided byquantum critical points. Of particular importance*Corresponding author. Tel.: +49-531-391-5128; fax: +49531-391-5129.
E-mail address: h.klauss@tu-bs.de (H.-H. Klauss).

are systems that realize competing interactions,e.g. spin frustration due to antiferromagnetic
exchange on triangular or tetrahedral lattices.
Systems with ladder topologies are also interesting,as they may have spin liquid ground states that arepredisposed to unconventional electronic andmagnetic instabilities [1,2].
In the course of investigating the complex OxoCu-vanadates Cux V2 O5þx with a linkage ofdifferently coordinated chains of CuO5 pyramids
and VO4 tetrahedra, we studied the magnetic
properties of the related compound KCu5 V3 O13 :While the former systems form effectively onedimensional chain or more complicated dimerizedplane structures, the latter compound consists ofladders of Cu2þ s ¼ 12 triangles or tetrahedra [3]. It

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 5 7 - 5

H.-H. Klauss et al. / Physica B 326 (2003) 436–439

was the purpose of this study to investigate themagnetic properties of KCu5 V3 O13 using nuclear
probes and other experimental techniques to gaina better understanding of the effect and interplayof frustration with anisotropies in a quantum spinsystem.

2. Magnetic susceptibility and speciﬁc heat
Single crystals of KCu5 V3 O13 show a Curie–
Weiss-like magnetic susceptibility for T > 213 K
with a yCW ¼ 145 K pointing to antiferromagnetic correlations. At T n ¼ 213 K a jump in thesusceptibility is observed (Fig. 1). The magnitude
and sign of this jump depend on the ﬁeld
orientation with respect to the crystallographic
axes. In measurements on polycrystalline samples
this effect is totally nulliﬁed. Therefore it must berelated to anisotropies or orbital degrees of freedom. The respective anomaly in the speciﬁc heat isshown in the inset of Fig. 1.
Low temperature magnetic susceptibility andspeciﬁc heat measurements give evidence formagnetic instabilities at 7:5 and 3:7 K: The relatedentropies at these temperatures are evident, but
smaller than expected. Susceptibility and high ﬁeld
magnetization of polycrystalline KCu5 V3 O13 havebeen reported and are in rough agreement with our437

data [4]. The transitions at 3:7 and 213 K have notbeen found.

3. Nuclear probes
We performed zero ﬁeld mSR experiments on apolycrystalline sample of KCu5 V3 O13 in thetemperature range from 1:7 to 250 K: Between
room temperature and 10 K only a static gaussian
relaxation due to nuclear moments and noelectronic relaxation is observed (Fig. 2). Therefore no slow spin ﬂuctuations occur in thistemperature range.
Below 7:5 K a static internal ﬁeld distribution ofelectronic origin is found (Fig. 3). Between 7:5 and
3:8 K an isotropic internal ﬁeld distribution with
two well deﬁned muon precession frequencies withequal amplitude are observed. Both signals can be
described as exponentially-relaxing precession,
Pm ðtÞ ¼ 23ð12 cosð2pn1 t þ f1 Þ elT1 t
þ 12 cosð2pn2 t þ f2 Þ elT2 t Þ þ 13 elL t :ð1Þ

The narrow line shape (Fig. 4) proves a commensurate long range ordered spin system in thistemperature range. For temperatures below 3:7 K;
the internal ﬁeld distribution is changed qualitatively. A strong asymmetric broadening of the twosignals is observed. The signals are described byﬁrst-order Bessel functions j0 which have been
derived for an incommensurate modulation vector

0.24

asymmetry

0.18

0.12

10 K
140 K
220 K

0.06

Fig. 1. Magnetic susceptibility of KCu5 V3 O13 in B ¼ 2500 Oe
perpendicular to the crystallographic a-axis. Right inset: hightemperature anomaly at 213 K: Left inset: high temperature
speciﬁc heat of KCu5 V3 O13 ðB ¼ 0Þ:024time (µs)

68Fig. 2. Zero ﬁeld muon spin relaxation spectra of KCu5 V3 O13
at 10, 140 and 220 K:

H.-H. Klauss et al. / Physica B 326 (2003) 436–439

43814

KCu5V3O13

Frequency (MHz)12asymmetry

10 K

0.21

5.3 K

0.14

ν11086ν242001

0.07

0.2

0.4
0.6time (µs)

1.5Fourier Amplitude (a.u.)567890.8Fig. 3. Low temperature zero ﬁeld muon spin relaxation
spectra of KCu5 V3 O13 : Spectra at 5:3 and 10 K are shiftedvertically for clarity. Lines are ﬁts using the different models
described in the text.

5.3 K
1.7 K

ω11
ω204Fig. 5. Spontaneous muon spin precession frequencies inKCu5 V3 O13 :1.7 K
003Temperature (K)00.52

ω21020Frequency (MHz)30Fig. 4. Fourier spectra of KCu5 V3 O13 below and above 3:7 K
showing a signiﬁcant line broadening at 1:7 K with respect tothe data at 5:3 K:from susceptibility and speciﬁc heat (Fig. 5). This
proves a homogeneous sample with static magnetic order observed at two magnetically inequivalent muon sites. The change of the line shapeand the step-like increase of the precession
frequency n2 are interpreted with a spin reorientation to an incommensurate ordered phase. To
draw more speciﬁc conclusions on the magnetic
structures Knight shift experiments on a single
crystal are needed to identify the magneticallyinequivalent muon sites in the lattice.
51
V-NMR measurements in low (E1 T) andhigh (E8 T) external ﬁeld reveal a temperature
independent hyperﬁne coupling in the paramagnetic temperature regime above 10 K: This result
supports the dynamic nature of the switching
phenomenon observed in the susceptibility. The
magnitude of the hyperﬁne coupling constantpoints to a spin singlet pairing of four Cu spins
and one additional unpaired Cu spin per unit cell.

4. Conclusionsof the local ﬁeld at the muon site [5]:Pm ðtÞ ¼ 23ð12 j0 ð2pn1 t þ f1 Þ elT1 t
þ 12 j0 ð2pn2 t þ f2 Þ elT2 t Þ þ 13 elL t :ð2ÞThe onset temperatures of both precession signals
coincide with the ordering temperature derived

The Oxo-Cu-vanadate KCu5 V3 O13 shows acoexistence of unpaired and dimerized Cu spin-12
spin species and a complex and low symmetry
exchange topology. This is proposed to lead to theanisotropic switching phenomena in the magnetic
susceptibility between 20 and 220 K: The zero ﬁeldmSR experiments reveal no electronic relaxation

H.-H. Klauss et al. / Physica B 326 (2003) 436–439

above 7:5 K; conﬁrming the dynamic nature of themagnetization switching. Below 7:5 K; two welldeﬁned spontaneous muon spin precession signals
conﬁrm long range magnetic order. At 3:7 K astrong broadening of the spectrum is interpreted asa spin reorientation. The discrepancy between thelarge Curie–Weiss temperature and the smallordering temperatures are taken as evidence for aconsiderable competition of magnetic exchange
processes. These are compatible with a local singlet
formation of a part of the spins.

Acknowledgements
We acknowledge support by DFG SPP1073.

439

References[1] E. Dagotto, Rep. Prog. Phys. 62 (1999) 1525.
[2] P. Lemmens, G. Guntherodt,.
C. Gros, Phys. Rep. (2002), inprint.
[3] F.D. Martin, Hk. Muller-Buschbaum,.
Z. Naturforsch. B:
Chem. Sci. 49 (1994) 1137.
[4] H. Sakurai, K. Yoshimura, K. Kosuge, A. Mitsuda, T.
Goto, J. Phys. Soc. Japan 70 (2001) 3209.
[5] L.P. Le, A. Keren, G.M. Luke, B.J. Sternlieb, W.D. Wu,Y.J. Uemura, J.H. Brewer, T.M. Riseman, R.V. Upasani,L.Y. Chiang, W. Kang, P.M. Chaikin, T. Csiba, G. Gruner,.
Phys. Rev. B 48 (1993) 7284.