Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

Journal of Alloys and Compounds 370 (2004) 31–35

Magnetic phase transitions in Nd1−x Yx Mn2 Ge2 (0 ≤ x ≤ 0.6) A. Elmali∗ , I. Dincer, Y. Elerman Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler-Ankara, Turkey Received 23 July 2003; received in revised form 3 September 2003; accepted 3 September 2003

Abstract The structure and magnetic properties of Nd1−x Yx Mn2 Ge2 (0.0 ≤ x ≤ 0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr2 Si2 -type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn2 Ge2 shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering. © 2003 Elsevier B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Magnetic measurements

1. Introduction The ternary rare-earth RMn2 Ge2 compounds (R: rareearths, Ba, Ca) have been extensively studied because of their interesting magnetic properties [1,2]. The characteristic feature of the RMn2 Ge2 compounds is the presence of long-range ordering of the manganese moments. The magnetic ordering in the Mn sublattice persists up to temperatures higher than 300 K, while the rare-earth sublattice orders magnetically at low temperature only. These compounds crystallize in the body-centered tetragonal ThCr2 Si2 -type structure with the space group I4/mmm, in which R, Mn and Ge atoms occupy 2a (0,0,0), 4d (0,1/2,1/4) and 4e (0,0,z) sites, respectively [3]. The magnetic properties of these compounds are very sensitive to the intralayer Mn–Mn spacing a a dMn –Mn . Roughly, if dMn–Mn > 2.87 Å (a > 4.06 Å), the intralayer in-plane coupling is antiferromagnetic and the intera layer coupling is ferromagnetic. When 2.84 Å < dMn –Mn < 2.87 Å (4.02 Å < a < 4.06 Å), the intralayer in-plane coupling is antiferromagnetic, but the interlayer coupling is a also antiferromagnetic. In the case dMn –Mn < 2.84 Å (a < 4.02 Å), there is effectively no intralayer in-plane spin component, and the interlayer coupling remains antiferromagnetic [4,5].

∗ Corresponding author. Tel.: +90-312-2126720; fax: +90-312-2232395. E-mail address: [email protected] (A. Elmali).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.021

Due to the strong dependence of the interlayer Mn−Mn exchange interaction on the lattice constant a, one can vary a the dMn –Mn -value in the neighbourhood of the critical value by alloying another rare-earth or transition metal. This leads to different changes of the magnetic properties of these systems and one can observe the destruction, stabilization, and variation of different types of ferromagnetic and antiferromagnetic phases [6,7]. Mössbauer spectroscopy [8], neutron diffraction [2,9], magnetization measurements on single crystal [10] and polycrystal [2,11] studies show four magnetic phase transitions in NdMn2 Ge2 . The Mn sublattice of NdMn2 Ge2 orders antiferromagnetically along the a-axis below the Néel temperature TN2 (Mn) ≈ 415 K. Below the Curie temperature TC (Mn) ≈ 330 K, a c-axis component of the magnetization develops leading to a canted ferromagnetic spin structure. Below the spin reorientation temperature TSR (Mn) ≈ 215 K, the spin structure becomes conical with the cone axis (the ferromagnetic alignment) along the a-axis. The Nd sublattice orders ferromagnetically along the a-axis parallel to the Mn moments below TC (Nd) ≈ 100 K. In YMn2 Ge2 , the spins in each of the Mn layers align parallel along the c-axis, and spins in alternating Mn layers align in the up-down sequence of a collinear antiferromagnet with the Néel temperature of 440 K [12,13]. With the aim of gaining a deeper insight into intralayer and interlayer magnetic properties in R1−x Rx Mn2 Ge2 , we have studied magnetic properties of Nd1−x Yx Mn2 Ge2 compounds by means of magnetization measurements.

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A. Elmali et al. / Journal of Alloys and Compounds 370 (2004) 31–35

Understanding the details of the magnetic properties of such compounds certainly relies very much on neutron diffraction studies. However, one can also take advantage of the intrinsic pinning effect in these systems to identify the various magnetic transitions [6,7,14]. Depending on whether the sample is cooled with or without the application of a small external field from the high-temperature antiferromagnetic phase to below the Curie temperature, the ferromagnetic components are “pinned” into different configurations. It is then possible to identify the transitions from the field-cooled (FC) or zero-field-cooled (ZFC) data, or both, depending on the magnitude of the features they present. This method is especially useful in studying quasi-binaries of these compounds such as R1−x Rx T2 X2 , RT2−x Tx X2 (T: transitional metal and X: Si or Ge), etc. when the properties of the parent compounds are known from neutron diffraction experiments.

Nd1-xYxMn2Ge2 180

3

V (Å )

185

175

2.88 2.84

a

dMn-Mn (Å)

2.92

c (Å)

10.92 10.89 10.86

a (Å)

4.10

2. Experimental details Polycrystalline samples Nd1−x Yx Mn2 Ge2 with x = 0, 0.2, 0.4, 0.45, 0.50, 0.55 and 0.60 were synthesised by arc melting the elements in a water-cooled Cu hearth under purified argon gas. The purity of the elements was 99.9% for Nd and 99.9% for Y, 99.98% for Mn and 99.9999% for Ge. The mass loss of Mn during melting was compensated by adding 2% excess Mn. The ingots were melted five times to attain homogeneity. The samples were characterized by powder X-ray diffraction on a Rigaku D-Max 2200 diffractometer using Cu K␣ radiation and a secondary monochromator. X-ray analysis of samples showed that any impurity was less than 3%. The magnetization was measured using a physical properties measurement system (PPMS) magnetometer in the temperature range 5–350 K. The samples were first heated above TC (Mn) and then measured in a ZFC-FC sequence. Magnetization above TC (Mn) was measured with a vibrating sample magnetometer in an external magnetic field of 5 kOe.

4.05 4.00 0.0

0.2

0.4

0.6

0.8

1.0

x Fig. 1. The concentration dependence of the lattice constants a and c, a the intralayer Mn–Mn spacing dMn –Mn and the unit cell volume V. For YMn2 Ge2 , these values are taken from reference [12,13]. Table 1 a The lattice constants a and c, the intralayer Mn–Mn spacing dMn –Mn and the unit cell volume V for Nd1−x Yx Mn2 Ge2 x

a (K)

c (K)

a dMn –Mn (K)

V (K)

0.0 0.2 0.4 0.45 0.5 0.55 0.6 1.0

4.105 4.084 4.066 4.057 4.053 4.047 4.035 3.989

10.906 10.898 10.889 10.886 10.884 10.881 10.877 10.855

2.903 2.888 2.875 2.869 2.866 2.862 2.854 2.821

183.777 181.815 180.051 179.190 178.830 178.206 177.122 172.726

For YMn2 Ge2 , these values are taken from reference [12,13].

3. X-ray diffraction The X-ray diffraction patterns obtained at room temperature for all investigated samples exhibit lines characteristic for the body-centered tetragonal ThCr2 Si2 -type structure with space group I4/mmm. The composition dependence of the lattice constants a and c, the intralayer Mn–Mn spacing a dMn –Mn and the unit cell volume V are shown in Fig. 1 and listed in Table 1. The substitution of Y for Nd causes a linear decrease of the lattice constants a and c, and the unit cell volume V, indicating the validity of Vegard’s law for these pseudo-ternary compounds. Decreasing of these parameters is associated with the smaller atomic radius of Y compared with that of Nd. As a consequence, the decrease of the lattice

parameters results in the decrease of the in-plane Mn–Mn a spacing dMn –Mn and interlayer nearest Mn–Mn distance. 4. Results and discussions 4.1. High-temperature susceptibility The temperature dependence of the susceptibility in the range 300–610 K in an applied field of 5 kOe was measured to determine the Néel temperature of intralayer Mn alignment and the temperature dependence of the inverse susceptibility χ−1 is given in Fig. 2 for all samples. The

A. Elmali et al. / Journal of Alloys and Compounds 370 (2004) 31–35

33

Nd1-xYxMn2Ge2 B=5 kOe 20000

20000

TN2(Mn) 10000

10000

TN2(Mn)

x=0.2

x=0.0

0

20000

TN2(Mn)

TN2(Mn)

20000

-1

-1

χ (emu g Oe)

χ (emu g Oe)

0

10000

10000

-1

-1

x=0.45

x=0.4 0

0 20000

TN2(Mn)

TN2(Mn)

20000

10000

10000

x=0.5

TN2(Mn)

0 400

500

600

Temperature (K)

400

500

x = 0.55 x = 0.6 0

600

Temperature (K)

Fig. 2. The temperature dependence of inverse susceptibility of Nd1−x Yx Mn2 Ge2 above 300 K in an applied field of 5 kOe.

Néel temperature for x = 0 which is found from a change in slope of the inverse susceptibility of the compound is at about 425 K. This temperature is comparable with the Néel temperature TN2 (Mn) = 415 K which has been determined by magnetic measurements and in a Mössbauer study [2,8,10]. Therefore, TN2 (Mn) for all other concentrations is similarly found from a change in slope of inverse susceptibility. The TN2 (Mn) temperatures are designated by arrows in high temperature susceptibility curves (see Fig. 2). TN2 (Mn) is nearly concentration independent throughout the whole range which these similar behavior was observed in Pr1−x Tbx Mn2 Ge2 [6], and the spin alignment is expected to be within the ab plane as in NdMn2 Ge2 and various other RMn2 Ge2 compounds [15]. On the other hand, from the neutron diffraction study on the La1−x Yx Mn2 Ge2 [13] it was found a decreasing of TN2 (Mn) with decreasing cell parameter. A more detailed neutron diffraction study on Nd1−x Yx Mn2 Ge2 with 0.2 ≤ x ≤ 0.6 is required to clarify this point.

4.2. Low-field magnetization measurements The temperature dependence of the FC and ZFC magnetizations taken in an applied field 50 Oe in the interval 5 K ≤ T ≤ 350 K are shown in Fig. 3. Differences in the FC and ZFC modes are found, which is indication of different configurational pinning in each mode of measurement. The ferromagnetic component is pinned by the anisotropy of the antiferromagnetic component of the Mn sublattice when the system is cooled through TC (Mn) with or without an applied field. By exploring this effect, TC (Mn) can be determined with good precision from the 50 Oe data. The splitting of the FC and ZFC curves for x = 0 occurs at 336 K, which corresponds to TC (Mn) found by neutron diffraction studies [2,9]. In the interval 0 ≤ x ≤ 0.55, the Mn sublattice orders ferromagnetically at TC (Mn) ≈ 336 K almost independent of the rare-earth concentration. The values of the transition temperatures of all samples are listed in Table 2.

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A. Elmali et al. / Journal of Alloys and Compounds 370 (2004) 31–35

Nd1-xYxMn2Ge2 B=50 Oe 2

TSR

x=0.0

1

TC(Nd)

TSR

TC(Nd)

x=0.2

1

TC(Mn)

TC(Mn) x=0.45

x=0.4

TSR TC(Nd)

TC(Nd)

TC(Mn)

TIM

TC(Mn)

0 2

1

1

0 2

0

x=0.55 x=0.6

x=0.5

TC(Nd)

TC(Nd) 1

TIM T (Mn) C

TN1(Mn)

0.5

TN1(Mn) 100

200

300

0

100

1.0

TIM TC(Mn)

0 0

Magnetization (emu/g)

Magnetization (emu/g)

0 2

2

200

0.0

300

Temperature (K)

Temperature (K)

Fig. 3. The temperature dependence of the magnetization of Nd1−x Yx Mn2 Ge2 in an applied field of 50 Oe.

The spin reorientation transition, observed in neutron diffraction studies, is observed as a sharp peak at TSR = 213 K. Neutron diffraction studies give a value of about 215 K for the onset of the transition, which corresponds to more rather the inflection point the M(T) data above TSR . Similarly, a spin reorientation transition is also observed for the x = 0.2 compound and the temperature corresponding to the position of the peak at about 160 K is taken TSR . Below TSR , the splitting in M(T) of the FC and ZFC states Table 2 The magnetic transition temperatures for Nd1−x Yx Mn2 Ge2 x

TC (Nd) (K)

TC (Mn) (K)

0.0 0.2 0.4 0.45 0.5 0.55 0.6

100 85 80 75 70 40 30

336 340 350 345 340 338

TN1 (Mn) (K)

TN2 (Mn) (K)

TSR (K) 213 160 90

250 333

425 440 447 450 448 445 440

TIM (K)

280 295 310

indicates the presence of pinning. The occurrence of pinning implies that the rotation of the ferromagnetic easy magnetization direction out of the c-axis to within the basal plane has to be accompanied by the establishment of antiferromagnetic ordering along another direction. The beginning of the Nd sublattice ordering at TC (Nd) ≈ 100 K reported in neutron diffraction studies for x = 0 [2] is not clearly featured in M(T) data. Any feature related to this transition is most probably masked by the ferromagnetic exchange in the Mn layers already existing above TC (Nd). The temperature dependence of the magnetization of the x = 0.4 sample shows a similar behaviour to that of x = 0.0 and 0.2. For the compounds x > 0.4 TSR can not be determined accurately from M(T) curves. The lattice constants of samples in the range 0.45 ≤ x ≤ 0.55 are located close to critical values for ferromagnetic or antiferromagnetic coupling of the Mn sublattice. Below TC (Mn), the magnetization increases with decreasing temperature as expected for a system with ferromagnetic coupling of Mn planes for samples with 0.45 ≤ x ≤ 0.55. These samples show an intermediate (IM) phase, as clearly seen

A. Elmali et al. / Journal of Alloys and Compounds 370 (2004) 31–35

for x = 0.55. In the IM phase bounded by TIM , the canted ferromagnetism progress towards antiferromagnetism as the lattice contracts on decreasing temperature. This is a continuous transition that occurs in a composition range where the critical lattice spacing appears within the temperature range investigated, as found in the Pr1−x Yx Mn2 Ge2 [16], Pr1−x Dyx Mn2 Ge2 [17] and Ce1−x Dyx Mn2 Ge2 [17]. Owing to inhomogeneity of the polycrystal samples, it is difficult to determine the values of TIM and TN1 (Mn) accurately. The compound with x = 0.55 exhibits SmMn2 Ge2 -like magnetic behaviour and phase transitions. For the compound with x = 0.55 and 0.6 the origin of the fact that as the temperature decreases an increase in the magnetization at about 100 K is still not clear. Increasing substitution of Y for Nd shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering. The compound with x = 0.6 exhibits a transition from ab-plane antiferromagnetism to c-axis antiferromagnetism below TN1 (Mn) persisting down to TC (Nd). The lattice parameter a of this compound is 4.035 Å and this is consistent with the spacing condition for this antiferromagnetic phase transition. At lower temperatures, the Nd sublattice begins to order as seen by the rapid increase with decreasing temperature in the FC magnetization. The temperature TC (Nd) is obtained from the inflection point of the derivative of the magnetization.

Acknowledgements The authors would like to thank to Mr. Eyüp Duman for the high temperature magnetization measurements. This work was further supported by the University of Ankara Research Funds (Grant Numbers 20010705044 and BAB

35

2002.07.45.008) and TÜBI˙ TAK-BMBF Bilaterel Program (Grant Number MISAG-JÜLICH-1).

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