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Journal of Magnetism and Magnetic Materials 310 (2007) e384–e386

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The magnetic properties of S ¼ 12quasi-one-dimensional-quantum spin

system DMACuCl3 in magnetic fields

Osamu Wadaa,�, Yasuo Yoshidaa, Yuji Inagakia, Takayuki Asanob, Tatsuya Kawaea,Kazuyoshi Takedaa, Yoshitami Ajiroc

aDepartment of Applied Quantum Physics, Kyushu University, Fukuoka 812-8581, JapanbDepartment of Physics, Kyushu University, Fukuoka 812-8581, JapancDepartment of Chemistry, Kyoto University, Kyoto 606-8502, Japan

Available online 7 November 2006

Abstract

Magnetic susceptibility wðTÞ of a quasi-one-dimensional S ¼ 12 quantum spin system DMACuCl3 (DMA ¼ (CH3)2NH2) has been

measured in a magnetic field. This compound exhibits a unique magnetization curve MðHÞ. MðHÞ rises with increasing magnetic field up

to Hc1 ¼ 2T, and shows a 12plateau between Hc1 and Hc2 ¼ 3:5T followed by a gradual increase to the saturation at Hs ¼ 14T. In the

previous studies, the specific heat CðTÞ shows a sharp peak in the low field region for HpHc1 and in the high field region for Hc2pH

[Y. Yoshida, O. Wada, Y. Inagaki, T. Asano, K. Takeo, T. Kawae, K. Takeda, Y. Ajiro, J. Phys. Soc. Japan 74 (2005) 2917. [1]]. The

field dependence of the two peaks, however, is unusual in comparison with that in conventional quantum spin systems investigated so far.

In wðTÞ, the antiferromagnetic ordering in HpHc1 is observed, although its field dependence is inconsistent with that of CðTÞ. The origin

of the discrepancy in the two measurements is discussed.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.30.Cr; 75.40.Cx; 61.66Hq; 75.10.Jm

Keywords: Quantum spin system; Magnetization plateau; Specific heat; Magnetic susceptibilities

Recently, quasi-one-dimensional (1D) quantum spinsystems with a finite spin-gap have attracted considerableinterest. These systems have an energy gap D above asinglet ground state at zero field and are thereforemagnetically inactive at low temperatures. When amagnetic field is applied to these systems, one of the firstexcited states goes down due to Zeeman effect, and the gapdecreases as DðHÞ ¼ D� gmBH, where g, mB and H areg-factor, Bohr magneton and external magnetic field,respectively. At a critical field, Hc ¼ D/gmB, the gapvanishes, and then a magnetically active state can beinduced above Hc. By the presence of weak interactionsbetween 1D chains, a magnetic ordering can be alsorealized at low temperatures.

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.10.356

onding author. Tel./fax: +81 92 201 8898.

ddress: wadatap@mbox.nc.kyushu-u.ac.jp (O. Wada).

The compound we focus on here is an S ¼ 12quasi-1D

quantum spin gap system DMACuCl3, which showscharacteristic features in the magnetization curve MðHÞ.MðHÞ increases steeply below Hc1 ¼ 2T and exhibits a 1

2

magnetization plateau between Hc1 and Hc2 ¼ 3:5Tfollowed by a gradual increase up to the saturation fieldHs ¼ 14T [2]. This peculiar MðHÞ curve has beenexplained qualitatively by an alternating ferromagnetic(F)–antiferromagnetic (AF) dimer model, proposed pre-viously on the basis of the crystal structure, which containsF-dimers and AF-dimers alternatively within a 1D chain.Thus, it is expected that there are two kinds of orderedphases separated by the 1

2magnetization plateau, i.e. a

spontaneous magnetic ordered (SMO) phase and a field-induced magnetic ordered (FIMO) phase, corresponding tothe ordering of F-dimers through AF-dimers in the singletstate below Hc1 and of AF-dimers activated by theapplication of the magnetic field above Hc2, respectively.

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SQUID(0.1T)

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Su

sce

ptib

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Fig. 2. Temperature dependence of wðTÞ and CðTÞ. The result of AC wðTÞat zero field and DC wðTÞ measured with SQUID at 0.1 T are shifted by

0.02 for clarity. An arrow indicates the change in the slope of the

temperature dependence of wðTÞ at 1T.

O. Wada et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e384–e386 e385

In the previous studies, CðTÞ shows two kinds of peaks forHpHc1 and Hc2pH, where the peak for HpHc1 is calledas the first peak and that for Hc2pH as the second peak inthis paper. On the other hand, no magnetic ordering hasbeen detected between Hc1 and Hc2, which is consistent withthe magnetization plateau in MðHÞ. This is quite naturalbecause there are no magnetic degree of freedom at this fieldrange. We presented a field-temperature (H � T) phasediagram based on the specific heat results as shown in Fig. 1,which shows unusual behaviors. As indicated by opensymbols in Fig. 1, the critical temperature T c discontinuouslychanges at Hc1 and Hc2 as a function of field. Such unusualfield dependence of Tc is inconsistent with conventionalSMO or FIMO phases observed so far [3]. In order to clarifythe origin of peaks in CðTÞ, we measured the magneticsusceptibility wðTÞ at H ¼ 0 and 1T.

The powder sample used in this study was prepared by theslow evaporation method [4]. wðTÞ above 1.8K wasmeasured in DC method with using a SQUID magnetometer(Quantum Design MPMS7), while at low temperatures downto 0.1K, AC (f ¼ 16Hz) measurements were performedwith employing a 3He–4He dilution refrigerator where amagnetic field is generated by superconducting magnet.

The temperature dependence of wðTÞ is shown in Fig. 2.The results of wðTÞ at H ¼ 0:1 and 1T show a goodagreement, indicating that wðTÞ above T ¼ 2K shows nofield dependence below H ¼ 1T. Thus, we plot wðTÞ at H ¼

0T superposing on that at H ¼ 0:1T in Fig. 2. On thecontrary, wðTÞ below T ¼ 2K shows a large field depen-dence. At H ¼ 0, wðTÞ shows a sharp peak due to SMO ataround T ¼ 1:0K and a Curie-like upturn below T ¼ 0:4K.At H ¼ 1T, the sharp peak becomes broad, which can beunderstood as the suppression of the spin fluctuation by theexternal field. As indicated by the arrow in Fig. 2, the slopeof wðTÞ changes at around T ¼ 0:6K. This is considered tobe due to a phase transition. At H ¼ 1T, the Curie-like

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H (

T)

T (K)

HC2

HC1

HSsecond peak

first peak

AF peak (magnetic susceptibility)

FIMO Phase

(Specific heat)

SMO Phase

Fig. 1. Magnetic field versus temperature phase diagram of DMACuCl3.

Peak temperatures of CðTÞ and wðTÞ are plotted by open symbols and

filled circles, respectively.

upturn disappears, which suggests that the origin of theupturn comes from the paramagnetic impurities.The transition temperatures detected at H ¼ 0 and 1T are

plotted by filled circles in Fig. 1. The temperaturedependence of CðTÞ is described also in Fig. 2. It is obvious,from Figs. 1 and 2, that the transition temperatures in CðTÞ

is inconsistent with that in wðTÞ. For example, at H ¼ 0T,the peak in CðTÞ is at T ¼ 0:8K, while that in wðTÞ is atT ¼ 1:0K. A small discrepancy of the peak temperaturebetween CðTÞ and wðTÞ is not unusual. However thediscrepancy in the present result is quite a large. Moreover,at H ¼ 1T, the anomaly temperature of wðTÞ is lower thanthe peak temperature of CðTÞ, which reverses to the result atH ¼ 0T. From these results, the discrepancy is not causedby an experimental error, but reflects a nature of thetransitions. The field dependence of the peak observed in thewðTÞ would be valid for the phase boundary of the SMOphase in accordance with our naive institution that the Tc

should change continuously from finite value at H ¼ 0T tozero at H ¼ Hc1. In this course, the peaks detected by CðTÞ

may reflect another phase transition. Anyhow, this is one ofthe possibilities and we have no clear evidence at themoment. To clarify this point, further experimental studiesare needed. Now we are planning to extend the present wðTÞmeasurements to FIMO phase and measure CðTÞ and wðTÞin magnetic fields using single crystal samples of thismaterial.

This work was partially supported by a Grant-in-Aid forScientific Research on Young Scientists (B) (No. 17740233)from the Ministry of Education, Culture, Sports, Scienceand Technology of Japan.

ARTICLE IN PRESSO. Wada et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e384–e386e386

References

[1] Y. Yoshida, O. Wada, Y. Inagaki, T. Asano, K. Takeo, T. Kawae, K.

Takeda, Y. Ajiro, J. Phys. Soc. Japan 74 (2005) 2917.

[2] Y. Inagaki, A. Kobayashi, T. Asano, T. Sakon, H. Kitagawa,

M. Motokawa, Y. Ajioro, J. Phys. Soc. Japan 74 (2005) 2683.

[3] Y. Yoshida, N. Tateiwa, M. Mito, T. Kawae, K. Takeda,

Y. Hosokoshi, K. Inoue, Phys. Rev. Lett. 94 (2005) 037203.

[4] R.D. Willett, J. Chem. Phys. 44 (1966) 39.