Thermal expansion anomaly and magnetic properties of

Nd2AlFe11Mn5 compound

ZHAO Miao(赵 淼), ZHOU Yan(周 严), FU Bin(傅 斌), HAO Yan-ming (郝延明)

Department of Physics, Tianjin Normal University, Tianjin 300074, China

Received 11 April 2005; accepted 8 August 2005

Abstract: Materials with negative thermal expansion have many important applications such as constituents of composite materials designed to reduce their overall thermal expansion. The structural and magnetic properties of Nd₂AlFe₁₁Mn₅ compound were investigated by means of X-ray diffraction and magnetization measurements. The result shows that the Nd₂AlFe₁₁Mn₅ compound crystallizes in a rhomhedral Th₂Zn₁₇-type structure. The Curie temperature T_c is about 150 K. The negative thermal expansion coefficient of Nd₂AlFe₁₁Mn₅ compound is found by X-ray diffraction in temperature range of 122－203 K. There exists an anisotropic and strong positive spontaneous magnetostriction in Nd₂AlFe₁₁Mn₅ compound. The magnetostriction deformations were discussed.

Key words: Nd₂AlFe₁₁Mn₅ compound; spontaneous magnetostriction; negative thermal expansion

1 Introduction

Materials with negative thermal expansion have many important applications as constituents of composite materials designed to reduce their overall thermal expansion. In 1991, Andreev and his co-workers investigated the properties of rare earth(RE)-iron intermetallic compounds based on the type RE₂Fe₁₇ on the thermal expansion and spontaneous magnetostriction. The result showed that RE₂Fe₁₇ compounds undergo strong spontaneous magnetostriction(SM) deformations below their Curie temperatures (T_c), which leads to a large negative volume change anomaly near their Curie temperatures[1－4]. Recent attention has focused on the magnetic substitution of Mn and Cr on the magnetic and structural properties of RE₂Fe₁₇[5－10]. It has been reported that both the T_c and saturation magnetization(M_s) decrease rapidly with increasing x in Pr₂AlFe_16-xMn_x and Y₂Al₃Fe_14-xMn_x compounds[3, 11]. However, they show a large negative volume change anomaly near T_c. For example, in Y₂Al₃Fe₁₁Mn₃ compound[11], the average thermal expansion coefficient is =?v/(v?T)≈－7.5?10^－5/K in the temperature range 185－200 K.

In this paper, the thermal expansion behavior of the unit-cell volume of Nd₂AlFe₁₁Mn₅ compound and its magnetic properties are investigated by means of X-ray dilatometry and magnetization measurements. Perhaps the studies on the negative thermal expansion and further insights into the mechanism play an important role in theory and applications.

It has been known that the arc-melted Mn-rich RE₂(Fe, Mn)₁₇ compound is unstable in the air, and is easy to oxidize. For instance, arc-melted Dy₂Fe₁₁Mn₆ ingot of 12 g is oxidized entirely and becomes gray powder in the air for one day and night[12]. In this work, we substitute one Al atom for one Fe atom in Nd₂(Fe,Mn)₁₇ compound so that it becomes much steadier than before.

2 Experimental

The Nd₂AlFe₁₁Mn₅ compound was prepared by arc melting starting materials of at least 99.95% purity in argon atmosphere. The ingot was remelted three times to ensure their homogeneity. Afterwards the arc-melted ingot was sealed in an evacuated silica tube. After vacuum annealing for seven days at 950 ℃, the sample was subsequently quenched in cold water. X-ray diffraction(XRD) with Cu K_α radiation was used to check whether the sample was single phase and to determine the lattice parameters. The experimental error in the determination of a and c was 10^－4 nm. It has been reported that the normal thermal expansion curves extrapolated from the paramagnetic to the ferromagnetic range can be obtained using the Debye and Grüneisen relation[1, 8]. The value of the Debye temperature T_D, which is necessary for the extra- polation, was estimated from acoustic measurements to be 450 K for Y₂Fe₁₇ and 400 K for other RE₂Fe₁₇ compounds[13]. We used the same value 400 K for the extrapolation of the temperature dependences of the lattice parameters of the sample.

3 Results and discussion

From the powder X-ray diffraction patterns shown in Fig.1, it can be seen that the annealed sample of Nd₂AlFe₁₁Mn₅ compound crystallizes in a rhombo- hedral structure with Th₂Zn₁₇-type (space group,), which is as same as Nd₂Co₉Mn₈ compound[14]. The indices of crystallographic plane (hkl) of reflections are marked on it. The unit-cell parameters a, c, v of the Nd₂AlFe₁₁Mn₅ derived from the whole X-ray pattern are 0.869 2 nm, 1.254 4 nm, and 0.820 9 nm³, respec- tively.

Fig.1 XRD pattern of Nd₂AlFe₁₁Mn₅ compound at room temperature

Fig.2 gives the temperature(T) dependence of the magnetization (M) of the Nd₂AlFe₁₁Mn₅ compound from 79 to 205 K in a low field of 40 kA/m. It shows that M decreases slowly at the beginning with temperature rising and then drops sharply during the temperature range 130－165 K, and eventually disappears gradually. We confirm that the rapid change in 130－165 K results from the disappearance of magnetic ordering, and it can be also seen in Fig.4. The Curie temperature T_c is about 150 K which is derived by extrapolating square of spontaneous magnetization (M²) to zero in the plot of M² versus T. It is obvious that the T_c value is much lower than that of the Nd₂AlFe₁₆ compound (T_c≈470 K)[15]. As a rule, we confirm that T_c is determined by the intensity of the exchange interaction and by 3d-sublattice magnetiza- tion[2]. Therefore manganese substitution for Fe is likely to reduce the 3d-sublattice magnetization and cause the low T_c value. Otherwise it is possible that the Mn atom substitution for Fe atom reduces the T-T interaction. This may be due to the antiferromagnetic coupling between Mn and Fe in 3d sublattice.

Fig.2 Temperature dependence of magnetization of Nd₂AlFe₁₁Mn₅ compound at low field of 40 kA/m

Fig.3 shows the temperature dependence of the lattice parameters a and c of Nd₂AlFe₁₁Mn₅ compound which are derived from X-ray diffraction patterns of the (220) and (303) reflections of the rhombohedral lattice with step-scanning (at 0.01° intervals). Along the c axis, the value decreases monotonously with increasing temperature from 103 to 162 K, and then increases linearly above T_c. In the basal plane the variety of parameter a is more complicated. Unlike the transformation of c, it increases at first, and then decreases in the temperature range of 142－203 K. This indicates that the negative thermal expansion is anisotropic. In the magnetic state of Nd₂AlFe₁₁Mn₅ compound, there are two reasons working on unit-cell size. One is magnetic ordering which decreases with temperature increasing and brings on the contraction of the unit-cell volume. The other reason comes from the normal thermal expansion effect (in other words, the phonon contribution to the thermal expansion) which results in the expansion of the unit-cell with temperature increasing. Being at high temperature, there is only normal thermal expansion effect. We suppose that the parameters a and c changed variously is due to the anisotropy of magnetic ordering disappear. Compared with being along c axis, the magnetic ordering in basal plane disappears at higher temperature.

Fig.3 Temperature dependences of lattice parameters a and c of Nd₂AlFe₁₁Mn₅ compound(dash lines represent phonon contribution to thermal expansion)

There is a negative thermal expansion behavior in temperature range of 122－203 K as shown in Fig.4 and the average thermal expansion coefficient is =?v/(v?T)=－1.7×10^－5/K. Furthermore, the negative thermal expansion of the sample is anisotropic (see Fig.3). Previous studies indicated that in the case of Mn substitution, the Mn atoms preferentially occupy 6c/4f sites of the crystallographic structure. The unusual behavior of the unit-cell volume can be understood by considering the magnetovolume effect in compound even though the influence of the 6c/4f site preferential occupation by Mn cannot be completely neglected. From 203 to 654 K, the sample is in paramagnetic state and its average thermal expansion coefficient is =?v/(v?T)=2.4×10^－5/K.

Fig.4 Temperature dependence of unit-cell volume of Nd₂AlFe₁₁Mn₅ compound(dash line represents phonon contribution to thermal expansion)

The temperature dependences of the extrapolated values v_p, a_p and c_p are given in Fig.3 and Fig.4, respectively. Here the signs m and p represent the unit cell parameters in the magnetic state and paramagnetic state, respectively. In terms of the deviation from the extrapolated curves from the paramagnetic to ferromagnetic (see Figs.3 and 4), we can easily obtain the uniaxial deformation λ_c, basal-plane linear deformation λ_a and volume effect ω_s (see Fig.5). From the figure, it can be seen that the linear deformation differs considerably from 1.6×10^－3 at 103 K to around zero at 203 K. In addition, λ_c is larger than λ_a at the same temperature below 122 K. This indicates that there is much stronger spontaneous magnetostriction along the c axis than in basal plane at low temperature. The volume effect ω_s is close to zero at 203 K which is about 50 K higher than T_c. It may be due to the short-range ordering.

Fig.5 Temperature dependences of spontaneous magneto- striction deformations ω_S, λ_c and λ_a of Nd₂AlFe₁₁Mn₅ compound

4 Conclusions

There exists an anisotropic strong spontaneous magnetostriction in Nd₂AlFe₁₁Mn₅ compound and there is a negative thermal expansion coefficient of Nd₂AlFe₁₁Mn₅ compound in the temperature range from 122 to 203 K.

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Foundation item: Project(50271022) supported by the National Natural Science Foundation of China; Project(1999) supported by the Excellent Young Teachers Program of MOE, China; Project(04362011) supported by the Natural Science Foundation of Tianjin, China

Corresponding Author: HAO Yan-ming; Tel: +86-22-83913802; E-mail: yanminghao@eyou.com

(Edited by YUAN Sai-qian)