Journal of Magnetism and Magnetic Materials 226}230 (2001) 1973}1975

Ultrasonic experiments in SrCu (BO ) and NH CuCl
in magnetic "elds up to 50 T
B. Wolf *, S. Zherlitsyn , S. Schmidt , H. Schwenk , B. LuK thi , H. Kageyama,Y. Ueda, H. TanakaPhysikalisches Institut Universita( t Frankfurt, Robert Mayerstrasse 2-4, D-60054, Frankfurt, Germany
Institute for Solid State Physics, University of Tokyo, Tokyo 106-8666, JapanDepartment of Chemistry, Tokyo Institue of Technology, Tokyo 1528551, JapanAbstract
We present sound velocity data in high "elds for the two plateau substances SrCu (BO ) and NH CuCl .
Pronounced sound velocity and attenuation anomalies are observed in the region between the magnetization plateaus.
We interpret these as a resonant interaction between the sound wave mode and the soft magnetic excitations.  2001
Elsevier Science B.V. All rights reserved.
Keywords: Low dimensional spin system; Magnetization plateau

Low dimensional spin systems exhibit many surprisingproperties. For example, dimerised spin chains or twodimensional spin dimers exhibit distinct plateaus in magnetization as a function of magnetic "eld. In SrCu
(BO ) [1] and in NH CuCl [2] these have been mea
sured for various temperatures.
With a quadrature method [3] we are able to measureultrasonic velocity and attenuation in pulsed "elds up to50 T. We applied this technique to various substanceswith interesting magnetic properties: CsCuCl [3],
YbInCu [4], (VO) P O [5], SrCu (BO ) [6] and  NH CuCl [7]. Here we brie#y discuss the latter twosubstances which exhibit plateaus in the magnetization.
SrCu (BO ) has a two dimensional structure whichconsists of alternately stacked CuBO - and Sr-layers.
Within the CuBO plane the Cu> form dimers, whichare orthogonally connected giving rise to an exact orthogonal dimer ground state [8]. In Fig. 1 we show a longitudinal sound velocity (c mode) versus magnetic "eld* Corresponding author.Fax.:#49-69-79828409.
E-mail address: wolf@physik.uni-frankfurt.de (B. Wolf).

for ¹"1.5 K. The magnetization for the same temperature is also given in the same "gure. We observe for therelative elastic constant a broad minimum at 25 T followed by very sharp minima at 27, 36, and 42 T. Thecorresponding magnetization exhibits plateaus for
m/m ",  and  It is seen that the sound velocity  minima are in the region where the magnetization changes from one plateau to the next one. The broad minimum at 25 T can arise from localized triplet excitations(triplet-phonon-bound state). The sharp minima forhigher "elds arise due to a resonant interaction between
the magnetic excitations and the soundwave mode.
The interaction between soundwaves and a triplet
magnetic state was investigated experimentally in thechain substance (VO) P O [5]. The coupling between  a longitudinal sound mode and the lowest branch of thetriplet in a magnetic "eld is due to the exchange striction
mechanism. This leads to a strain dependence of thesinglet}triplet splitting. A mean "eld type description
gives the salient features of the "eld and temperaturedependence of the sound velocity for this substance [5].
From the temperature dependence of the c elastic
mode and from the resonant soundwave-spin triplet excitation we get a magnetoelastic coupling constant of ca0304-8853/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 0 9 3 - 2

1974

B. Wolf et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1973}1975

Fig. 1. Relative change of the elastic constant c in SrCu (BO )for the magnetic "eld sweep up (solid circles) and sweep down
(open circle) measured at ¹"1.5 K. Field dependence of relative magnetization m/m at the same temperature is also shown(solid line). Here m is a saturation value of the magnetization.
The magnetic "eld applied along the a!axis. The elastic constant minima occur between the magnetization plateaus.

800 K for SrCu (BO ) [6] This large coupling constantmeans that the Cu-dimers are coupled very strongly tothe lattice. But it is not large enough to induce a structural transition. Note that the exchange constants areclose to a cirtical value to induce an antiferromagnetic
phase transition [8]. Further studies with other elastic
modes are necessary to gain a complete picture of thestrain-dimer coupling.
NH CuCl belongs to the ABX family of compoundswhich plays a prominent role in low dimensional magnetism. KCuCl , TlCuCl and NH CuCl have a monoclinic structure at room temperature [9]. NH CuCl
has a structural order-disorder type transition at 70 K.
Several elastic modes exhibit pronounced anomalies inthe vicinity of this transition [7].
In Fig. 2 we show elastic constant results forNH CuCl . The shear mode c is plotted as a functionof magnetic "eld at 1.5 K. Pronounced anomalies areobserved at the onset and the end of the plateau states i.e,
for 4.5, 13, 17, 24 and 28 T. One can compare the resultsin Fig. 2 with magnetization and di!erential susceptibility results for this substance [2]. The similarity of therelative soundvelocity results with the negative susceptibility is remarkable. In both cases the e!ect between the and  plateau is the largest. Note that for SrCu (BO ) such a similarity does not hold. For the "eld up and the"eld down curve we notice some hysteresis in the magni-Fig. 2. Magnetic "eld dependence of the transversal modec together with the magnetic susceptibility form Ref. [9]. (solidline) for NH CuCl . Both measured at ¹"1.5 K. For theelastic constant only the magnetic "eld sweep down (opencircles) is shown. The external magnetic "eld is applied parallelto the a!axis.

tude (not shown). We do not know yet whether this is anintrinsic e!ect. As in the case of SrCu (BO ) the sound
wave anomalies arise due to a resonant interaction withthe soft magnetic mode between the di!erent plateaus.
The size of the e!ect for the c mode is similar in sizewith the longitudinal mode for SrCu (BO ) shown in
Fig 1. The longitudinal mode c for NH CuCl exhibits
almost an order of magnitude smaller e!ect in the "elddependence [7].
Various magnetic properties for NH CuCl are not
understood. Contrary to the related compounds KCuCl
and TlCuCl one observes no singlet-triplet gap for B"0
[9]. In addition the absence of a plateau for m/m " is a mystery. This sound wave-magnetic excitation e!ectallows to describe the spin lattice coupling in this substances more completely.
This work was supported in part by the BMBF via thegrant 13N6581A/3 and the Sonderforschungsbereich
252.

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