CHIN. PHYS. LETT. Vol. 27, No. 8 (2010) 087502
Magnetic Properties of Spin-Ladder Compound Sr14 (Cu1−𝑦 Fe𝑦 )24 O41
*
HU Ni(胡妮)1,2** , LU Zhi-Hong(卢志宏)3 , CHENG Li(程莉)1 , XIONG Rui(熊锐)1 , SHI Jing(石兢)1,4 1
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, and School of Physics and Technology, Wuhan University, Wuhan 430072 2 School of Science, Hubei University of Technology, Wuhan 430068 3 School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430072 4 International Center of Materials Physics, Chinese Academy of Sciences, Shenyang 110016
(Received 16 November 2009) Magnetic properties of spin-ladder compounds Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 (0 ≤ 𝑦 ≤ 0.05) are investigated in the temperature range from 10 to 300 K. The result reveals that all the samples exhibit magnetic crossover behavior in the paramagnetic range, and Fe3+ doping can efficiently increase the susceptibility due to the large moment of Fe3+ . Both the observations are consistent with our previous investigation on transport behaviors, indicating the strong correlation between the magnetism and transport behaviors. The spin gap is evidenced in all the samples, and strengthens as Fe3+ doping level increases, which can be associated with the antiferromagnetic interaction between Fe3+ and Cu cations.
PACS: 75. 30. Cr, 75. 25. +z, 75. 50. Ee
DOI: 10.1088/0256-307X/27/8/087502
Quasi-one-dimensional spin-ladder compound Sr14 Cu24 O41 has attracted considerable interest in experiment[1−8] and theory[9−11] due to the discovery of a spin-gap and superconductivity.[12] The compound possesses a complex layered structure of alternating Cu2 O3 and CuO2 sheets along the 𝑏 axis,[9] which are separated by Sr2+ layers. The CuO2 planes are composed of 𝑆 = 1/2 spin chains of edge-sharing Cu-O-plaquettes. Along the chains the nearest neighboring Cu spins are coupled with weak ferromagnetic (FM) interactions via nearly 90∘ Cu-O-Cu bonds. The Cu2 O3 layers consist of two-leg ladders, in which the nearest neighboring Cu spins are coupled by strong antiferromagnetic (AFM) superexchange interaction via 180∘ Cu-O-Cu bonds, both along the legs and the rungs.[13] One of the unique features of the compound is that the Cu cations have a formal valence of +2.25, which indicates that Sr14 Cu24 O41 is inherently a hole doped compound. Generally, most of the holes locate in the CuO2 chains, and each hole in the chain couples with one Cu2+ to form the so-called Zhang–Rice (ZR) singlet.[9] Along the 𝑐 axis, two Cu2+ spins of nearest-neighboring ZR singlets form a dimer, giving rise to a spin gap ∆gap of about 140 K.[3,14] A much larger spin gap about 400 K is formed in the spin ladders, and contributes little to the low-temperature magnetic property.[15−17] Recently, investigations have revealed that the physical property of Sr14 Cu24 O41 can be significantly modulated by the 𝐵-site doping.[18−20] Chen et al.[21] have systematically investigated the effect of the 𝐵-
site nonmagnetic doping in Sr14 Cu24 O41 , and reported that only the magnetic properties in the chains can be affected, which are ascribed to the dimer formation between the impurity’s spins and the neighboring Cu2+ spins. However, the effect of 𝐵-site magnetic doping is rather complicated. It was reported that 𝐵-site Ni doping does not change the spin gap of the ladder, but largely enhances the number of spin-singlet pairs.[22] 𝐵-site Co doping can cause localized holes in CuO2 chains to transfer to Cu2 O3 ladders, which results in Curie–Weiss (CW) behavior as well as a metal-insulator transition, in a full temperature range (5 K < 𝑇 < 300 K).[23,24] Furthermore, our previous works have shown that a 𝐵-site Co doping induces strong antiferromagnetic interaction, and forms the antiferromagnetic dimerized state.[25] Up to now, the 𝐵-site doping effect in spin-ladder compound Sr14 Cu24 O41 is not fully understood, more investigations are needed. Magnetic cation Fe3+ has a half filled 3𝑑 shell with an electronic configuration 𝑡32𝑔 𝑒2𝑔 (𝑆 = 5/2), and antiferromagnetically coupled with other magnetic cations.[26] Our previous results[27] revealed that the transport behavior strongly depends on the Fe3+ -doping level. In this work, we reveal the effect of 𝐵-site Fe3+ doping on the magnetic properties and the transport behavior of Sr14 Cu24 O41 . The results show that Fe3+ doping can significantly increase the low-𝑇 magnetic susceptibility, and strengthens the spin gap for all the samples. The increase of magnetic susceptibility should be responsible for the decrease of resistivity reported in Ref. [27].
* Supported the National Natural Science Foundation of China under Grant No 10974148, the National Science Fund for Talent Training in Basic Science (No J0830310), and the National Basic Research Program of China under Grant No 2009CB939705, the Fund of Wuhan University (5082003), and Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials (Hubei University). ** To whom correspondence should be addressed. Email:
[email protected] c 2010 Chinese Physical Society and IOP Publishing Ltd ○
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CHIN. PHYS. LETT. Vol. 27, No. 8 (2010) 087502
(10
-3
=0.01
(e)
=0
=0
4
1T
1T
0.4
2
=0.01
(b)
3
(f)
4
=0.01 1T
1T
3T
3T
2
0
0
(c)
3
(g)
=0.03
=0.03 1T
1T
2
3T
3T
1
=0.03
1.5
=0.05
0 2
0
1.0
(d)
3
=0.05
(h)
1T
=0.05 1T 3T
3T
0.5
(103 emu/mol(Cu+Fe))-1
=0
(a)
0.8
1/
(a) 2.0
with 𝑦 = 0, while this deviation becomes ambiguous and gradually shifts towards low temperature range with the increase of 𝑦. The 𝜒−1 − 𝑇 curve becomes much more linear in the sample with 𝑦 = 0.05. This feature strongly suggests that the Fe3+ doping causes a large paramagnetic (PM) contribution. In order to study the effects of magnetic field on susceptibility, we measured the magnetic susceptibility of the Fe3+ doped samples under two selected fields, as shown in Figs. 2(b)–2(d). The susceptibilities measured in both fields are very similar, and the differences between the 𝜒−𝑇 curves measured are very small. This feature indicates that in all the samples, the robust AFM interaction dominates, and can not be suppressed even in a magnetic field of 𝐻 = 3 T.
(10-3 emu/mol(Cu+Fe))
emu/mol(Cu+Fe))
A series of samples Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 (0 ≤ 𝑦 ≤ 0.05) were prepared using the standard solid-state reaction. Firstly, highly purified starting reagents, SrCO3 , CuCO3 , and Fe2 O3 , were mixed in stoichiometric ratios, ground, and calcined at 850∘ C for 24 h in air. The obtained black powders were reground and cold pressed into disks with a diameter of 15 mm, and a thickness of about 1.4 mm. Then, these pellets were sintered at 950∘ C in air for 24 h. The crystal structures of the samples were investigated using a Bruker D8 advanced x-ray diffractometer with Cu 𝐾𝛼 radiation at room temperature, and the results reveal that all the samples possess a single phase.[27] The 𝑇 -dependences of magnetic susceptibilities were measured using a Quantum Design physical properties measurement system in fields of 𝐻 = 1 T and 𝐻 = 3 T. Figure 1(a) presents the 𝑇 -dependence of magnetic susceptibility 𝜒 of Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 (0 ≤ 𝑦 ≤ 0.05) in a field of 𝐻 = 1 T. Upon cooling, the susceptibility of the sample with 𝑦 = 0 increases continuously, and reaches a maximum at 𝑇 = 𝑇𝑁 ∼ 65 K due to the AFM interaction between Cu cations, and finally a CW tail appears below 𝑇 ∼ 28 K. This is consistent with the earlier report.[19]
1
4
0
0
(b)
0
100
200
(K)
3
300 0
100
200
300
(K)
Fig. 2. (a)–(d) 𝑇 -dependence of the susceptibility 𝜒 and (e)-(h) the inverse susceptibility 1/𝜒 for Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 with 𝑦 = 0, 0.01, 0.03 and 0.05, respectively, for measurement fields 𝐻 = 1 T and 𝐻 = 3 T.
2
1
(10
3
emu/mol(Cu+Fe))
-1
0.0
0 0
100
200
300
(K)
Fig. 1. Temperature-dependence of (a) the susceptibility 𝜒, (b) the inverse susceptibility 1/𝜒 measured under 𝐻 = 1 T for all the samples.
Increasing the Fe3+ -doping level causes remarkable changes of the magnetic behavior. Firstly, the magnetic susceptibility increases as 𝑦 increases from 0 to 0.05, indicating a large magnetic contribution of Fe3+ . Secondly, the broad peak around 65 K in the sample with 𝑦 = 0 is greatly suppressed with the increase of 𝑦. The Fe3+ -doping effects can also be clearly observed in the 𝑇 -dependence of the inverse susceptibilities 𝜒−1 , as shown in Fig. 1(b). A clear deviation from the CW behavior can be observed around 60 K for the sample
The fittings of the susceptibility with the CW law 𝜒 = 𝐶/(𝑇 − Θ), where 𝐶 is the Curie constant, and Θ is the Weiss temperature, are shown in Figs. 2(f)–2(h). It is worth noting that the susceptibility deviates from the CW law even in the PM region, indicating the existence of strong magnetic correlation above 𝑇𝑁 ∼ 65 K. For the sample with 𝑦 = 0.01, the PM susceptibility can be well described as two separated linear behaviors with a crossover at 𝑇 ∼ 𝑇𝐷 = 204 K. A similar crossover was also observed in the transport behavior, which indicates the formation of the singlet-state in the spin ladder.[27] Similar features can be observed in other two Fe3+ doping samples. The decrease of 𝑇𝐷 with 𝑦 indicates the suppression of formation of a singlet state due to the Fe3+ doping.
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CHIN. PHYS. LETT. Vol. 27, No. 8 (2010) 087502 Table 1. Values of Curie constant 𝐶, the Weiss temperature Θ, dimmers number 𝑁𝐷 , and exchange coupling constant 𝐽𝐷 of Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 (0 ≤ 𝑦 ≤ 0.05) after using a four-term fitting. Fe3+ 𝑦=0 𝑦 = 0.01 𝑦 = 0.03 𝑦 = 0.05
(10-3 emu/mol(Cu+Fe))
0.8
𝐶 (emuK/(Cu+Fe)mol) 0.0094 ± 0.0002 0.0428 ± 0.0001 0.0511 ± 0.0001 0.0676 ± 0.0006
=0
Experimental data Fitting data Curie-weiss part Dimer Part
0.6
0.4
0.2
0.0 0
50
100
150
200
250
300
(K)
Fig. 3. Two-term fitting of the 𝑇 -dependence of the magnetic susceptibility 𝜒 for the sample Sr14 Cu24 O41 in a magnetic field of 𝐻 = 1 T.
Θ (K) −1.74 ± 0.4 −3.11 ± 0.3 −5.11 ± 0.7 −9.97 ± 1
𝜒total = 𝜒CW + 𝜒dimer ,
(2)
𝜒CW = 𝐶/(𝑇 − Θ),
(3)
𝜒dimer =
2𝑁𝐷 (𝑁𝐴 /24)𝑔 2 𝜇2𝐵 , 𝑘𝐵 𝑇 [3 + exp(𝐽𝐷 /𝑘𝐵 𝑇 )]
(4)
where 𝑁𝐴 is the Avogadro number, 𝑘𝐵 is the Boltzmann constant, 𝑁𝐷 is the number of dimers in the chain, 𝐽𝐷 is the intra-dimer exchange integral, and 𝑔 is the g factor set to 2.2.[28,29] The fitting results are shown in Fig. 3 for the sample with 𝑦 = 0. Obviously, the susceptibility exactly obeys the CW law below 20 K. Therefore, the 𝑇 -dependence of the dimmer contribution 𝜒dimer can be obtained after subtracting 𝜒0 and the CW term 𝜒cw at low temperatures. Upon
(10-4 emu /mol(Cu+Fe)) dimer
(1)
The 𝑇 -independent term 𝜒0 represents the sum of the core diamagnetic (∼ −4.1 × 10−5 emu/mol-Cu) and Van-Vleck PM (∼ 4.47 × 10−5 emu/mole-Cu) susceptibility. The algebraic sum of the above two terms is about 4 × 10−6 emu/mole-Cu,[23] and is almost negligible compared with the experimental data. The term 𝜒ladder is assumed to be the same as that of SrCu2 O3 , which is very small below 300 K (∼ 10−5 emu/mol-Cu) and can also be neglected in the investigated temperature range.[11] Therefore, we use the following equations to fit the experimental data,
𝐽𝐷 (K) 134 ± 1 148 ± 1 144 ± 2 135 ± 2
cooling, the 𝑇 -dependence of 𝜒dimer exhibits a maximum around 𝑇𝑁 = 65 K, and then decreases rapidly towards zero due to the formation of the dimerized state. For comparison, Fig. 4 presents the 𝑇 -dependence of 𝜒dimer for all the samples, measured in a magnetic field of 𝐻 = 1 T. Here the 𝜒dimer data of all the samples change with 𝑇 in the same way. Interestingly, the broad peak does appear in all the 𝜒dimer −𝑇 curves, although this feature can not be seen in the 𝜒−𝑇 curves shown in Fig. 1. Moreover, 𝜒dimer shows clear increase with 𝑦, and the maximum point 𝑇𝑁 in 𝜒dimer (𝑇 ) does not change with 𝑦. From this fitting, we estimate the values of 𝐶, Θ, and 𝑁𝐷 , 𝐽𝐷 , as listed in Table 1.
It is well known that in these quasi-one-dimension spin-ladder systems, the magnetic susceptibility 𝜒 comprises four terms: the 𝑇 -independent term 𝜒0 , the CW term 𝜒CW , the dimer term 𝜒chain contributed from the CuO2 chains, and the 𝜒ladder term related to ladder site Cu cations,[21] i.e. 𝜒total = 𝜒0 + 𝜒CW + 𝜒chain + 𝜒ladder .
𝑁𝐷 1.64 ± 0.02 1.96 ± 0.02 2.07 ± 0.04 2.19 ± 0.05
5 4 3 2
=0 =0.01 =0.03
1
=0.05 =1 T
0 0
50
100
150
200
250
300
(K)
Fig. 4. 𝑇 -dependence of 𝜒dimer for all the samples at 𝐻 = 1 T. Symbols: experimental data, lines: fitting results using a dimer model.
For the sample with 𝑦 = 0, the CW constant 𝐶 is small due to the small amount of remnant free Cu2+ in the CuO2 chains and in the Cu2 O3 planes. After doping, the value of 𝐶 continuously increases with 𝑦, indicating the significant PM contribution of Fe3+ since the moment of Fe3+ is larger than Cu2+ . The negative Weiss temperature Θ indicates the AFM ground state of the system, and the increase of Θ with 𝑦 should be ascribed to the strong AFM interaction between Fe3+ and Cu cations. In the sample with 𝑦 = 0, 𝑁𝐹 = 0.59 per molecular unit, which is estimated from the equation 𝐶 = 𝑁𝐹 (𝑁𝐴 /24)𝑔 2 𝜇2𝐵 /4𝑘𝐵 , represents the free spins of Cu2+ in the chains. While 𝑁𝐷 = 1.64 per molecular unit represents the dimerized state between Cu cations with an intra-dimer integral 𝐽𝐷 = 134 K. Considering the fact that the number of holes will decrease due to Fe3+ doping, the formation of Zhang–Rice singlets may be suppressed with the increase of Fe3+ doping. However, our results show that the number of dimers 𝑁𝐷 increases with 𝑦,
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CHIN. PHYS. LETT. Vol. 27, No. 8 (2010) 087502
indicating the increase of dimers. Therefore, in the Fe3+ doped samples, the dimerized states not only may be formed between Cu cations, but also can be formed between Fe3+ ions, which leads to an increase of 𝑁𝐷 . The sum of 𝑁𝐹 + 2𝑁𝐷 ≈ 3.84 is very close to 4, indicating that there are 6 nonmagnetic Cu3+ existing within each chain.[7,8] Here we note that the exchange integral 𝐽𝐷 is almost independent of Fe3+ doping. This feature indicates that the spin-gap energy is insensitive to the Fe3+ doping. A comparison of the 𝑇 -dependence of 𝜒dimer in different magnetic fields of 𝐻 = 1 T and 3 T is shown in Fig. 5. It is clear that only the value of 𝜒dimer shows an increase with 𝐻, but both the dimer exchange constant 𝐽𝐷 and the maximum point 𝑇𝑁 in 𝜒dimer − 𝑇 do not change with the increase of the magnetic field, indicating the stable AFM ground state in all the samples. 4
(a)
2 =0.01
(10-4 emu /mol(Cu+Fe))
1T 3T
0
(b)
4 2 =0.03 1T
0
3T
6 (c)
4 2
=0.05 1T 3T
0 0
100
200
300
(K)
Fig. 5. 𝑇 -dependence of 𝜒dimer for the Fe3+ doped samples in measuring fields 𝐻 = 1 T and 𝐻 = 3 T.
In conclusion, a series of samples of Sr14 (Cu1−𝑦 Fe𝑦 )24 O41 (0 ≤ 𝑦 ≤ 0.05) have been prepared by conventional solid-state reaction. Detailed magnetic susceptibility measurements are performed under two selected fields in a temperature range from 10 to 300 K. The results reveal that all the samples exhibit a crossover behavior in the PM range, and the Fe3+ doping can efficiently increase the susceptibility due to the PM contribution of Fe3+ . Both the observations are in agreement with our previous transport investigation, indicating the strong correlation between the magnetism and transport behaviors. A four-term fitting shows that the spin gap does exist in
all the samples, and the value of the spin gap increases with 𝑦, which can be associated with the strong AFM interaction between Fe3+ and Cu cations.
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