Effect of Mn doping on magnetic and transport properties of Nd0.5Sr0.5Co(1−y)MnyO3

Journal of Magnetism and Magnetic Materials 325 (2013) 1–6

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Effect of Mn doping on magnetic and transport properties of Nd0.5Sr0.5Co(1  y)MnyO3 S. Kundu, T.K. Nath n Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

a r t i c l e i n f o

abstract

Article history: Received 11 April 2012 Received in revised form 18 July 2012 Available online 23 August 2012

Investigation of magnetic, electronic- and magneto-transport properties of Nd0.5Sr0.5Co(1  y)MnyO3 has been carried out with y¼ 0–1. These studies, in effect, simultaneously reveal the effect of Mn doping on a ferromagnetic-metallic system Nd 0.5Sr0.5CoO3 as well as the effect of Co doping on the antiferromagnetic-insulating system Nd0.5Sr0.5MnO3. Our linear/nonlinear ac susceptibility and dc magnetization measurements indicate that in both the cases ferromagnetic–paramagnetic transition temperature decreases and resistivity increases. The magnetoresistance of the samples in the lowest temperature which is nearly zero for Nd0.5Sr0.5CoO3, has been found to increase monotonically with the increase of Mn substitution. This effect is attributed to the phase separation scenario. Upon increasing the concentration of Co in Nd0.5Sr0.5MnO3 no indication of ferromagnetic–metallic phase is observed. & 2012 Elsevier B.V. All rights reserved.

Keywords: Cobaltite Electronic-transport Disorder Phase separation

1. Introduction Cobaltites and manganites are some of the most interesting materials amongst the strongly correlated electron systems having various fascinating electronic and magnetic properties. Similar to manganites, cobaltites posses close interplay between charge, orbital, spin and lattice degrees of freedom [1–5]. However, cobaltites have one extra degree of freedom, the spin state of Co ion. Multiple spin state found in Co ion is a consequence of comparable value of the crystal field splitting energy and Hund’s coupling energy [1–3]. In a material like La1  xSrxCoO3 the Co3 þ and Co4 þ ions may have three different spin states each namely, the low spin (LS: t 62g e0g for Co3 þ and t 52g e0g for Co4 þ ), the intermediate spin (IS: t 52g e1g for Co3 þ and t 42g e1g for Co4 þ ) and the high spin (HSt 42g e2g for Co3 þ and t 32g e2g for Co4 þ ) state [1–3]. Spin state transition may occur under influences like change of temperature [2]. The observed ferromagnetism in cobaltites originates most likely from double exchange interaction between Co3 þ and Co4 þ species similar to the case of doped manganites, but dependent on Co spin state. Another most interesting feature of cobaltites is the phase separation effect [6–8] like in maganites. The low temperature phase of cobaltites materials like La1  xSrxCoO3 have been found to be inhomogenous where ferromagnetic (FM) regions coexist with various non-FM regions or spin glass regions [7,8]. Similar to the La-based cobaltites, the system

n

Corresponding author. Tel.: þ91 3222 283862. E-mail addresses: [email protected] (S. Kundu), [email protected] (T.K. Nath). 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.015

Nd1  xSrxCoO3 with a slightly lower bandwidth is also known to have a variety of magnetic and electronic properties depending on the hole concentration. For instance, in the low doped regime this material displays a phase separated state and glassy magnetic behavior [9,10]. This system also exhibits ferrimagnetic ordering below certain temperature in a particular doping regime [11]. As far as electronic-transport property is concerned Nd1  xSrxCoO3 display insulating behavior in the low doped regime and metallic behavior in the heavily doped regime [9]. However, in any case, the physics of cobaltites remained less understood compared to that of manganites till date. We, here present our results of investigations on different magnetic and transport properties of Mn substituted (for Co) Nd0.5Sr0.5CoO3. The two extreme compositions of Nd0.5Sr0.5Co(1  y)MnyO3 are Nd0.5Sr0.5CoO3 and Nd0.5Sr0.5MnO3, having completely different physical properties. For instance, Nd0.5Sr0.5CoO3 has a ferromagnetic-metallic ground state while Nd0.5Sr0.5MnO3 is known to have a charge/orbital order antiferromagnetic (AFM)-insulating state below certain temperature ( 150 K) [12]. So, our investigation, effectively, involves two issues simultaneously—the effect of Mn substitution in place of Co in a half doped cobaltites as well as the effect of Co substitution in place of Mn in a half doped manganites.

2. Experimental details We known Nd2O3, metric

have synthesized our samples through chemical route as pyrophoric reaction [13]. We have taken high purity Sr(NO3)2, Co(NO3)2.6 H2O and Mn(CH3COO)2 in stoichioproportion and dissolved in distilled water with proper

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amount of HNO3. Triethnolamine (TEA) was then added to the solution with a molar ratio to the metal ions ¼4:1:1 (Nd/Sr: Co/Mn:TEA¼1:1:4). This solution was continuously stirred and heated at 180 1C until combustion took place. The obtained fluffy powder was ground thoroughly and calcinated in air (5 h) at 1150 1C to obtain bulk polycrystalline samples of Nd0.5Sr0.5Co(1  y) MnyO3 with y¼0, 0.1, 0.3, 0.5, 0.9, 0.95 and 1. The powder samples were pressed into pellets and again sintered in air at the same temperature for 1 hour. The phase of the samples was checked by high resolution x-ray diffraction (HRXRD) technique. The electronic- and magneto-transport measurements were carried out employing a closed cycle helium refrigeration cryostat fitted inside a superconducting magnet (8 T) in the temperature range of 2–300 K. The magnetic properties of the samples have been investigated employing homemade vibrating sample magnetometer (77–300 K) [14] and homemade ac susceptometer (77– 300 K). Both these instruments employ a high precision Lock-inamplifier (SR830) for signal detection and a high precision PID temperature controller (Lakeshore 331 S) to control and monitor the sample temperature.

3. Experimental results The room temperature HRXRD pattern of Nd0.5Sr0.5Co(1 y)MnyO3 samples are shown in the Fig. 1(a). We find no impurity peaks in the patterns indicating single phase of the samples. The data were refined employing Rietveld method assuming Imma space group. The sample Nd0.5Sr0.5MnO3 is known to crystallize in Imma space

5x104

Nd0.5Sr0.5(Co1-yMny)O3

Intensity (counts)

y=0

4x104 y = 0.1

3x104

y = 0.3

2x104

y = 0.9 y = 0.95

1x104

y=1

0 20

30

40

70

80

exp comp exp - comp

y = 0.95

1.5x104 Intensity (counts)

50 60 2θ (degree)

1.0x104

5.0x103

0.0 20

30

40

50 60 2θ (degree)

70

80

Fig. 1. (a) The room temperature HRXRD pattern of the samples (b) The experimental and computed data on Rietveld refinement of the sample with y¼ 0.95. Inset of (b) shows the variation of unit cell volume with Mn concentration (y).

group of orthorhombic structure at room temperature [15]. On the other hand, the other end member Nd0.5Sr0.5CoO3 has also been found to be crystallize in Imma space group [16]. For these reasons, all the other members in between have also been refined assuming the Imma space group. One representative refinement is shown in the Fig. 1(b) for y¼ 0.95 sample. The quality of fitting looks fair. We observe some mismatch between the computed and the experimental data around the peaks. This possibly arises due to our preparation method which is different from standard solid state reaction method. In our case, a significant size distribution of the grains may be present in the samples. The refined lattice parameters are listed in the Table 1. We find from Table 1 that though the values have some irregularity, there is an overall pattern in the change in the lattice parameters. For example, the lattice parameters ‘a’ and ‘b’ display an overall increase with y, whereas, ‘c’ shows an overall decrease with y. Most importantly, an overall increase of the unit cell volume (except for y¼0.95) with increasing Mn concentration is found from the inset of Fig. 1(a). This increase in unit cell volume is very intriguing. This variation is understood from the fact that the atomic radius of Mn is higher than the atomic radius of Co. So, increasing Mn substitution expectedly increases the cell volume. This shows that the overall increase of unit cell volume with ‘y’ is physically meaningful. The magnetic properties of the sample have been studied in terms of measuring ac susceptibility at low field. We know that the magnetization of a sample can be, in general, expressed in a power series in H as, M ¼ M0 þ w1 H þ w2 H2 þ w3 H2 þ :::. Here, w1 is the linear susceptibility and the higher order ones (w2 ,w3 y) are the nonlinear susceptibilities. Fig. 2(a) shows the variation of real part of linear ac magnetic susceptibility (wR1 ) as a function of temperature for the samples. We find that wR1 for Nd0.5Sr0.5CoO3 sample displays a sharp drop at around 235 K indicating a FMparamagnetic (PM) transition. This temperature (Curie temperature, TC) is in good agreement with previous reports [9,17]. The other end compound Nd0.5Sr0.5MnO3 exhibits a behavior as already reported in literature. This sample (Nd0.5Sr0.5MnO3) displays a drop in the value of wR1 below  150 K (TN) due to AFM ordering in it. The FM–PM transition is also visible at around 260 K. These two temperatures are in accord with already reported values [12]. The close match of the transition temperatures of these samples with those reported in literature also indicates the proper oxygen stoichiometry of these samples. However, unlike standard bulk Nd0.5Sr0.5MnO3, in case of our Nd0.5Sr0.5MnO3 system we did not observe any CE-type spin ordering; we have only observed A-type AFM phase in it [18]. Upon replacing Co by a small amount of Mn (10%) the FM–PM transition temperature is found to decrease for the sample with y¼0.1 (TC ¼215 K ). On further doping of Mn in the B site a strong reduction in the FM–PM transition temperature is observed. The TC for the sample y¼0.3 is found to be at around 125 K. Similarly, incorporation of very small amount of Co (5%) in the B-site of Nd0.5Sr0.5MnO3 is found to have detectable influence on the magnetic properties. We observe that both TN and TC are decreased to some extent in case of the sample with y¼0.95 compared to those in Nd0.5Sr0.5MnO3. For the sample with y¼0.9 the value of wR1 decreases gradually with the increase of temperature showing a hump at around 200 K. For further investigation of the detailed magnetic state, we have measured the nonlinear susceptibilities in terms of measuring second harmonic (wR2 ) of the samples as a function of temperature. The inversion symmetry of magnetization with respect to H demands that only odd harmonics will be present. Precisely, if M(H)¼ M( H) then w2 , w4 , w6 should be zero. So it is believed and also experimentally observed that if symmetry is broken (in case of FM state) even harmonics (generally,w2 ) can be observed [19]. The inversion symmetry is broken when

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3

Table 1 The lattice parameters and unit cell volume (V) of the samples obtained from the Rietveld refinement. Sample (y ¼)

0

0.1

0.3

0.9

0.95

1

˚ a (A)

5.3630(3)

5.3661(3)

5.3653(5)

5.3994(2)

5.4190(8)

5.4028(3)

˚ b (A) ˚ c (A)

7.5868(4)

7.5824(2)

7.6205(11)

7.6871(6)

7.6169(2)

7.6882(5)

5.4124(2)

5.4145(8)

5.4176(1)

5.3843(7)

5.4625(9)

5.3872(3)

V (A˚ 3)

220.219

220.304

221.505

223.479

225.470

223.772

280 hac = 3 Oe

y=

f = 555.3 Hz

8 7 6

260

0 ( 500) 0.1 ( 500) 0.3 ( 3000) 0.9 ( 20) 0.95 ( 10) 1

5 4 3

240 220

TC (K)

χ1R (emu/g)

10 9

2 1 0

200 180 160 140

-1 50

100

150

200

250

300

120

350

0.0

T (K)

0.2

0.4

0.6

0.8

1.0

y 1.5

f = 555.3 Hz

0.5

0.16

0.0

H = 100 Oe

0.14

-0.5

0.12

y= 1 0.95 0.3 ( 200) 0.1 ( 100) 0 ( 50)

-1.0 -1.5 -2.0 50

100

150

200 T (K)

250

300

Fig. 2. (a) The temperature variation of measured real part of linear ac magnetic susceptibility (wR1 ) of the samples. The magnitude of wR1 of some of the samples has been multiplied by arbitrary factors to display the curves in the same scale. The multiplication factors are given in the brackets. (b) The measured wR2 of the samples as a function of temperature. The numbers in the brackets are the multiplication factors.

spontaneous magnetization M0 is present in the sample. So, wherever there is a change of spontaneous magnetization an anomaly in w2 is observed. For example, w2 displays a strong peak around the ferromagnetic–paramagnetic transition due to the onset of spontaneous magnetization. From Fig. 2(b) we can observe that Nd0.5Sr0.5CoO3 shows a sharp peak in the wR2 vs. T curve at around the FM–PM transition region. Similarly, for y¼0.1 and 0.3 samples wR2 displays sharp peaks around their corresponding FM–PM transition temperatures. On the other hand, for Nd0.5Sr0.5MnO3, wR2 displays a positive hump around TN indicating the AFM transition followed by a negative peak around TC. For the sample with y¼0.95, we do not observe the low temperature hump, showing a destabilization of the AFM state. Only a peak is found around FM–PM transition region of this sample. In Fig. 3 we have plotted the FM–PM

M/H (emu g-1Oe-1)

χ2R (10-4emu g-1Oe-1)

Fig. 3. Variation of TC of the samples as a function of y. Lines are the guide to the eye.

hac = 3 Oe

1.0

0.10

y= 0 0.1 0.3 ( 0.5) 0.9 ( 100) 0.95 ( 5) 1 ( 0.5)

0.08 0.06 0.04 0.02 0.00 -0.02 50

100

150

200

250

300

T (K) Fig. 4. The temperature variation of the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations normalized by the applied filed (H ¼ 100 Oe) as a function of temperature.

transition temperatures (TC) of the samples as a function of Mn concentration (y). This plot clearly displays a non-monotonic dependence of TC with Mn doping level. The transition temperature TC first decreases with increasing y and again increases as y increases. Furthermore, the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations of the samples has also been measured as a function of temperature as shown in the Fig. 4. From ZFC curves we observe that the obtained value of transition temperatures are quite similar as found from ac susceptibility data. We observe that the samples with higher Co content (y¼0, 0.1, 0.3) display a strong bifurcation in the ZFC–FC magnetizations. This behavior is

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S. Kundu, T.K. Nath / Journal of Magnetism and Magnetic Materials 325 (2013) 1–6

also found in many cobaltite systems and attributed to the anisotropic nature of these materials. However, Nd0.5Sr0.5MnO3 displays a very small bifurcation between ZFC–FC magnetizations due to the antiferromagnetic nature of the sample in the low temperature regime. On small substitution of Co in Nd0.5Sr0.5 MnO3 we find a large separation between the ZFC–FC magnetizations in the samples y¼0.95 and 0.9. This indicates the possible destabilization of the AFM order in the low temperature regime. We have also measured the temperature and field dependent resistivity of the samples. Fig. 5(a) displays the variation of resistivity as a function of temperature for the samples. The samples with y ¼0 and 0.1 exhibit overall metallic behavior with a change of slope in resistivity around their corresponding FM– PM regime. The sample y¼0.1 displays low temperature resistivity upturn below 60 K. All the other samples display an insulating/semiconducting behavior in the measured temperature range. On further Mn substitution in Nd0.5Sr0.5CoO3 the metallic behavior is suppressed and the sample displays insulating behavior in the measured temperature range. The insulating behavior in Nd0.5Sr0.5MnO3 is due to its AFM ordering. The samples with y¼0.95, 0.9 and 0.5 exhibit similar insulating nature and increase in resistivity with the increase of Co content. We find that this increment is a few orders of magnitude compared to Nd0.5Sr0.5 MnO3 sample. The resistivity studies reveal that both, the Mn substitution in Nd0.5Sr0.5CoO3 and Co substitution in Nd0.5Sr0.5 MnO3, make the samples more resistive. To have a clear view about the change of resistivity with Mn concentration, we have

107 106

ρ (Ohm-cm)

105 104

y=

103 102 101

1 0.95 0.9 0.5 0.3 0.1 0

10-3

100 10-1

10-4

10-2

0

50

100

150

200

250

300

T (K) 10000

ρ10 K (104 Ohm-cm)

100 1 0.01 1E-4 1E-6 1E-8 0.0

0.2

0.4

0.6

0.8

1.0

y Fig. 5. (a) The temperature dependent resistivity of the samples measured at zero magnetic field. (b) The resistivity of the samples at 10 K as a function of y. Lines are the guide to the eye. The position of the arrow shows a lower limit of the resistivity for the sample with y¼0.5.

plotted the measured resistivity of the samples at 10 K as a function of y as shown in the Fig. 5(b). This clearly displays the non-monotonic dependence of resistivity on y. The measured magnetoresistance (MR¼[r(H)  r(0)]/r(0)) of the samples at a lowest temperature of 3 K are shown in the Fig. 6(a) and (b). We have presented our measured data with J99H geometry. Measurement in J?H geometry produces similar data and hence signifies the absence of any anisotropic magnetoresistance in our samples. The samples with y¼ 0 and 0.1 do not show any appreciable MR. The MR of these samples is below 1% and positive. Interestingly, a monotonic enhancement of negative MR is found with the increase of Mn substitution in Nd0.5Sr0.5 CoO3. The highest value of MR at 8 T is about  25% in case of y¼0.3 and  77% as found in y¼0.9. For the samples with y¼0.95 and 1 the MR almost reaches 100%. The variation of the highest value of MR at 3 K is clearly shown in the Fig. 6(c). This figure shows a monotonic increase of MR with increasing y. Interestingly, we observe a hysteric behavior in the sample with y¼0.9. For y¼0.95 and 1 this hysteric nature is enhanced. Moreover, we find that on field cycling, the finally obtained value of MR (H¼0 T) is much smaller than the initial value for these two samples. This indicates that once the field is applied the initial resistive state is never recovered by reducing the field to zero. The high field phase remains arrested even when the field becomes zero.

4. Discussions From the magnetic measurements we have found that the FM–PM transition temperature decreases on substitution of Mn for Co in the Co-rich samples. It is true that any substitution is related to crystallographic distortion due to the mismatch in the ionic radii of the elements. It is also a well discussed issue that in manganites, A-site disorder (or chemical pressure) strongly influences the FM–PM transition temperature [20]. So, there is a possibility that a similar effect is responsible for changing the TC of the samples. However, the work by Moritomo et al. shows that the effect of any chemical pressure on B-site substituted half doped manganites is insufficient to account the observed properties [21]. On the other hand, Mn can act as impurity in the long range Co–O–Co networks (in the Co rich samples). The outcome is the enhanced phase separation in the systems. In such a phase separated state FM-metallic clusters and AFM-insulating clusters may coexist [22]. Presumably, such enhanced phase separation destroys the comparably long range ferromagnetic order in Nd0.5Sr0.5CoO3 reducing the FM–PM transition temperature on Mn substitution. This phase separation effect induces a concomitant increase in the resistivity with the increase of Mn doping in the Co-rich compounds. The measured magnetoresistance of the Co-rich samples is evidently small compared to that in the Mnrich samples. For instance, y¼0 and 0.1 samples displays negligibly small MR with positive value. This positive MR is a common property of a material and originates due to the Lorentz force on the charge carriers. All the other samples displays negative MR. There are several origins of commonly observed negative MR in the perovskite manganites. For granular materials the low-field MR generally comes from the spin polarized tunneling of electrons [23]. Double exchange mechanism (intrinsic MR) [4] and phase separation effect also lead to negative MR [24]. Small value of MR in the Co-rich samples (y¼0, 0.1) is possibly due to the fact that these materials are not highly spin polarized and phase separated to that extent. The sample with y¼0.3 (and also y¼0.9) shows significant negative MR with a linear nature of MR vs. H curve. This enhancement of negative MR is, we think, due to the enhancement of the phase separation effect and spin polarized tunneling with the increase of Mn substitution. We find that the

S. Kundu, T.K. Nath / Journal of Magnetism and Magnetic Materials 325 (2013) 1–6

5

5 0

0 0 0.1 0.3

T=3K

MR (%)

MR(%)

-10

-20

y=

-5

-15 -20

-40

T=3K

y=

0.9 0.95 1.0

-60 -80

-25

-100

-30 -100 -80 -60 -40 -20 0

20 40 60 80 100

-100 -80 -60 -40 -20 0

H (k Oe)

20 40 60 80 100

H (k Oe)

0

MR(%)

-20 -40 -60

T=3K H = 80 k Oe

-80 -100 0.0

0.2

0.4

0.6

0.8

1.0

y Fig. 6. (a) and (b) displays the measured magnetoresistance of the samples measured at 3 K as a function of magnetic field. The MR is shown for several cycles of magnetic field between 8 T to  8 T field. (c) The plot of the MR at highest field (8 T) and at 3 K as a function of y. Lines are the guide to the eye.

nature of MR vs. H is quite different from what is generally found in manganites due to the effect of spin polarized tunneling and double exchange mechanism. Tunneling type MR generally display a sharp drop in MR in the low- H regime rather than a linear nature [25]. In the high field regime MR shows slow variation with H due to the double exchange. This indicates that although spin polarized tunneling is presumably present in the above discussed samples, this is not the dominating one and contributions from other mechanisms (like phase separation) is considerably high. For the Mn-rich samples (y ¼1, 0.95, 0.9), the main observed features are the destabilization of the AFM order and enhancement of resistivity with the increasing Co content in the samples. This effect of the destabilization of AFM and charge orbital order in half doped manganites on B-site impurity doping is a very widely discussed issue in literature [21,26–28]. It is generally found that the impurity doping induces a ferromagnetic phase in charge ordered systems. There are various mechanisms predicted by different groups. For example, the nature of the coupling between the host and dopant, formation of impurity ‘d’ band, change in the eg electron density, modification of the Jahn–Teller distortion etc. have been discussed in details. On such substitution, metal–insulator transition has also been observed [28]. However, in our case we do not observe any metallic state in the samples with y¼1, 0.95, 0.9. This indicates that the instability in the AFM phase in our case (for Nd0.5Sr0.5MnO3) is not accompanied with an emergence of long-range ferromagneticmetallic state. Rather, the observed phenomena can be best explained through the disorder effect and phase separation scenario. Incorporation of Co impurity in the Mn–O–Mn network disturbs the zigzag chain of the Mn ions destabilizing the order in the B-site. This is manifested in the increase in resistivity on

increasing Co content in the Mn-rich samples. Although on such B-site doping possibility of phase separation cannot be ruled out, clearly, the presence of FM-metallic clusters is highly improbable. The nature of MR observed for Nd0.5Sr0.5MnO3 and for y ¼0.95, 0.9 revealing hysteresis as well as the arrest of phase has been recently investigated in terms of kinetic arrest [29,30]. The phase below TN has been termed as a mixture of equilibrium AFMinsulating and metastable FM-metallic phase which appears due to the arrest of kinetics across TN. On application of field the AFMinsulating phase converts into FM phase but remains arrested when the field is reduced to zero [30]. Furthermore, all the electronic and magnetic properties of the samples will depend on the nature of the interaction between the Co and Mn ions via oxygen. In these samples there are a large number of magnetic species present. For examples, Co may exist in six possible states, namely, as Co3 þ with three different spin states and as Co4 þ with three different spin states. On the other hand, Mn can exist in Mn3 þ and Mn4 þ states. So, understandably, there are many possible ways for Co-Mn interaction to take place via oxygen. So it appears that a complete understanding of the magnetic state of such system is not possible through only bulk magnetization and susceptibility measurements and one needs to adopt appropriate experimental techniques.

5. Conclusions In summary, we have investigated the effect of Mn substitution in Nd0.5Sr0.5Co(1  y)MnyO3 with y¼0, 0.1, 0.3, 0.5, 0.9, 0.95 and 1. The relative proportion of Co and Mn strongly influence the magnetic transition temperature, resistivity and magnetoresistance of the samples. We have found that the increase in Mn

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content in Nd0.5Sr0.5CoO3 or increase in Co content in Nd0.5Sr0.5 MnO3 increases the resistivity of the samples and decreases the ferromagnetic–paramagnetic transition temperature. This is attributed to the effect of phase separation and B-site disorder effect. The AFM state in Nd0.5Sr0.5MnO3 is found to be destabilized with increasing Co content. Although no indication of a metallic state has been found. The enhancement of MR with the increase of Mn concentration has been attributed to the effect of phase separation.

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