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Sign reversal of magnetization and tunable exchange bias field in NdCr1−xFexO3 (x=0.05–0.2)
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Sign reversal of magnetization and tunable exchange bias field in NdCr1 xFexO3 (x¼ 0.05–0.2) Tribedi Bora, S. Ravi n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 23 February 2015 Accepted 14 March 2015
Magnetization reversal and tunable exchange bias behavior are observed in NdCr1 xFexO3 compounds for x ¼ 0.05–0.20. The magnetic compensation temperature (Tcomp) is found to increase with increase in Fe concentration and its maximum value is 198 K for x ¼ 0.15 sample. The observed magnetization reversal is explained by considering the competition between the weak ferromagnetic component of Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ ions under the influence of negative internal magnetic field. The exchange anisotropy between the above two components of magnetic moments give rise to tunable positive and negative exchange bias fields. The sign reversal of exchange bias field also coincides with Tcomp. Bipolar switching of magnetization is demonstrated at T o Tcomp by just varying the positive magnetic field. & 2015 Published by Elsevier B.V.
Keywords: Neodymium chromites Antiferromagnetism Magnetization reversal Exchange bias
1. Introduction The rare earth orthochromites RCrO3 (R¼rare earth element) continue to be to focus of research due to their interesting physical properties and potential applications [1–4]. In these compounds, magnetic interactions are possible in three different networks such as Cr3 þ –Cr3 þ , Cr3 þ –R3 þ and R3 þ –R3 þ with O2 as intermediate ion [5]. Orthochromites are generally antiferromagnetic in nature with Néel temperature in the range 112–282 K depending on the type of rare earth element [5,6]. Substitutions on rare earth site using other rare earth elements and on Cr site using other transition elements give rise to interesting properties like magnetization reversal (MR) and exchange bias (EB) phenomena. Even though MR is known to appear in ferrimagnetic (FIM) compounds [7], recently many other compounds such as Sr2YbRuO6 [8], orthochromites [5,9–21], orthoferrites [22–24], othovanadates [25,26], molecular magnets [27,28] and intermetallic alloys [29,30] are shown to exhibit such behavior. Exchange bias phenomenon has been studied in heterostructure of bilayer/multilayer thin films of AFM/FM, AFM/FIM (ferrimagnetic), AFM/SG (spin glass), etc. [31]. Recently EB behavior has been reported even on bulk materials such as magnetic nano particles [32], charge ordered manganites [33], phase separated cobalites [34,35], orthochromites [13,14,18,19] and intermetallic compounds [29,30]. The EB field (HEB) is mostly reported to be either positive or negative but its n
Corresponding author. Fax: 91 3612690762. E-mail address: [email protected] (S. Ravi).
tunability is relatively rare. Several models have been invoked to explain the MR and EB phenomenon: core shell model [14], interaction between Dzayloshinsky–Moriya (DM) and single ion anisotropy [23], competition between the paramagnetic moments of dopant ions and the ferromagnetic component of host ions [13,19], AFM interaction between the FM components of two magnetic sublattices [36], etc. NdCrO3 is an orthorhombic perovskite compound having G-type antiferromagnetic structure with Néel temperature, TN 220 K along with a weak ferromagnetic component due to spin canting and in addition to that it exhibits spin reorientation transition at around TSR ¼35 K [37–39]. La substitution for Nd in NdCrO3 was found to suppress the TSR and increase the TN values [40]. In NdCrO3, both the cations are magnetic elements and it would be interesting to study the effect of doping of Fe3 þ ions having higher magnetic moment compared to the host Cr3 þ ions. In this paper, we report the preparation of NdCr1–xFexO3 (x ¼0–0.2) and study of their magnetic properties. MR was observed even for 5 atm% of Fe doping and it persists upto the doping level of 20 atm%. These samples also exhibit EB phenomenon and the maximum EB field was found to be þ 3.65 kOe for x¼ 0.10 sample.
2. Experimental details Polycrystalline samples of NdCr1 xFexO3 (x¼ 0, 0.05, 0.10, 0.15 and 0.20) were prepared by the standard sol–gel method. Nd2O3, Cr(NO3)3 9H2O, Fe(NO3)3 6H2O having 99.9% purity were taken
http://dx.doi.org/10.1016/j.jmmm.2015.03.060 0304-8853/& 2015 Published by Elsevier B.V.
Please cite this article as: T. Bora, S. Ravi, Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.03.060i
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Fig. 1. (a) XRD pattern along with the Rietveld refinement for x¼ 0.20 sample. (b) Variation of lattice parameters and unit cell volume with Fe concentration.
as starting compounds. Nd2O3 compound was dissolved in nitric acid and other two were dissolved in distilled water. An excess amount of citric acid was added to convert the mixture of above nitrate solution into citrate. Ethylene glycol was added to the citrate solution to promote the gel formation and the gel was converted into fine powder upon heating. The powder was presintered at 600 °C for 12 h and final sintering was done in pellet form at 1100 °C for 24 h. The prepared samples were characterized by recording X-ray diffraction patterns using Rigaku make TTRAX III diffractometer. Magnetization measurements were carried out using Lakeshore make vibrating sample magnetometer of model no. 7410. Microstructure and composition analysis were carried out by using Field Emission Scanning Electron Microscope with EDAX.
3. Results and discussions NdCr1 xFexO3 (x¼ 0, 0.05, 0.10, 0.15 and 0.20) samples are found to be in single phase form as per XRD patterns. All XRD patterns were refined by choosing Pbnm space group in orthorhombic cell as per Rietveld method using Fullprof program [41]. Typical XRD pattern along with Rietveld refinement for x¼0.20 sample is shown in Fig. 1(a). The lattice parameters are found to increase systematically with increase in Fe concentration as shown in Fig. 1(b) and this behavior demonstrates the substitution of larger Fe3 þ ions (0.645 Å) at the site of Cr3 þ ions (0.615 Å). The reliability factors RBragg, Rf, Rp and χ2 for x ¼0.20 sample are found to be 3.3%, 4.8%, 9.7% and 6.1. The cation ratios Nd: Cr: Fe obtained from the EDAX analysis for x¼ 0.05 and 0.15 are found to be
Fig. 2. Magnetization versus temperature plots in ZFC and FC conditions for x ¼0 sample. The inset shows ZFC M–T plot in an expanded scale.
0.99:0.95:0.05 and 0.98:0.80:0.19 respectively. Temperature variation of magnetization in zero field cooled (ZFC) and field cooled (FC) conditions at an applied field of H¼ 2 kOe is shown in the inset of Fig. 2 for x ¼0 sample and in Fig. 3 for x ¼0.05 to 0.20 samples. The parent compound exhibits an AFM peak at TN ¼225 K as shown in Fig. 2 for clarity and for To TN, M value is found to increase with a typical paramagnetic behavior due to the presence of Nd3 þ ions. A large irreversibility is observed in the FC curve with the enhanced magnetization in the form of a plateau for T oTN due to the presence of canted Cr3 þ moment. A sharp rise in magnetization is observed around 50 K due to the spin reorientation of Cr3 þ ions [37,38]. The ZFC M–T plot of x¼ 0.05 sample follows the trend of x ¼0 sample but with a reduced TN of 219 K as shown in Fig. 3. Under FC condition, magnetic irreversibility is observed at T o TN in the form of a broad peak and for further decrease in temperature, magnetization decreases towards negative values by crossing over the ZFC magnetization curve at Tcross ¼163 K and passing through magnetic compensation (M ¼0) at Tcomp ¼102 K. A minimum magnetization (maximum negative value) of Mmin ¼ 2.45 emu/mol is observed at Tmin ¼ 80 K. On further decrease in temperature a sharp rise in magnetization towards positive value is observed at around TSR ≃ 60 K. M–T plots of other samples with higher Fe concentration 0.1 r xr0.2 also show similar behavior but with enhanced Tcomp values. Moreover, the broad peak observed in the vicinity of TN is found to diminish with increase in Fe concentration due to the widening of negative magnetization region. Tcomp value is found to vary from 102 K for x¼ 0.05 to 169 K for x ¼0.15 at H¼ 2 kOe. The negative magnetization values for x ¼0.10 and 0.15 samples are found to be about one order of magnitude larger than that of x¼ 0.05 sample. The Tcomp value for x ¼0.20 sample is found to be smaller than that of x¼ 0.15 sample and this can be understood in terms of lack of contribution of all doped Fe ions for magnetization reversal and some of the Fe ions are engaged in long range AFM interaction in Fe3 þ O2 Fe3 þ networks. Such networks are possible especially for x Z0.20. To understand the MR phenomenon, we have recorded the M–T curves of x¼0.05–0.20 samples at different applied fields in the range 50–5000 Oe. They are shown in Fig. 4 for H¼ 200 Oe. Prominent negative magnetization is observed for all these samples especially for low applied field. The magnitude of negative magnetization is found to decrease with increase in applied field and correspondingly the Tcomp value also decreases as shown in Fig. 5. MR was observed upto H ¼2000 Oe for x ¼0.05 and 0.20 samples and upto H ¼3000 Oe for x¼0.10 and 0.15 samples. For x ¼0.05 and 0.10 samples at H¼ 200 Oe, additional magnetic compensation is observed at low temperature i.e. at T ′comp ¼41 K and 31 K respectively. The maximum Tcomp value observed in the present series is found to be larger than that of transition element doped LaCrO3 [12,13] and it is mainly due to the presence of magnetic
Please cite this article as: T. Bora, S. Ravi, Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.03.060i
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Fig. 3. Magnetization versus temperature plots in ZFC and FC conditions for x ¼ 0.05 to 0.20 samples (a–d). The inset in (a) shows ZFC M–T plot for x¼ 0.05 in an expanded scale.
Fig. 4. ZFC and FC M–T curves of x ¼0.05 to 0.20 samples at H¼ 200 Oe along with fitted data.
rare earth ion (Nd3 þ ). Similarly, larger Tcomp values of 252 K and 230 K are reported in other orthochromites containing magnetic rare earth ions, i.e. SmCr0.5Fe0.5O3 [17] and La0.15Pr0.85CrO3 [19] respectively. The competition between the weak ferromagnetic component of canted Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ ions under the influence of the negative internal magnetic field due to the AFM ordered Cr3 þ ions gives rise to the magnetization reversal. According to Cook et al. [5], the net magnetization by considering the weak ferromagnetic component and the paramagnetic component under the influence of the negative
internal magnetic field is, M = MCr + 3þ
C (H + HI ) where, (T − θC )
MCr is the
canted FM component of Cr ions, HI is the internal magnetic field and C is the Curie constant. The FC M–T curves were fitted to the above equation within the temperature range of Tmin oT oTN and the fitted data are shown as solid line in Fig. 4. For T oTmin, the magnetization data could not be fitted to the above equation due to the onset of spin reorientation transition. The estimated θC values for x¼0.05 sample is found to be in the range of 40 K to 65 K depending on the applied field. These values for x¼ 0.10, 0.15 and 0.20 are found to be in the range of 119 to 139 K, 98 K to 167 K and 162 K to 176 K respectively. Plots of HI
Please cite this article as: T. Bora, S. Ravi, Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.03.060i
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Fig. 5. Tcomp as a function of applied magnetic field.
and MCr as a function of H are shown in Fig. 6. HI and MCr values are found to almost saturate for H Z0.5 kOe, however for x ¼0.15 sample, they increase linearly. Bipolar switching of magnetization at T ¼100 K for x ¼0.05 and 0.20 samples are shown in Fig. 7. Here, the samples were initially FC to 100 K through TN by applying H¼ 200 Oe. By keeping the temperature fixed H was increased from 200 Oe to 4000 Oe and then decreased to 200 Oe. Such a field variation flips the magnetization from negative to positive and back to negative. Subsequent cycling of the fields between the above two values give rise to reproducible magnetization reversal. Thus by varying the magnitude of positive applied field, bipolar switching of magnetization can be achieved. Similar type of switching behavior is also observed for x ¼0.10 and 0.15 samples for H ¼200 Oe to 7000 Oe. In order to study the exchange bias behavior, M–H loops were recorded under FC condition (H¼5000 Oe) in the temperature range 30–20 K at 5 K interval. Typical M–H loops for x ¼0.15
sample in expanded scale are shown in Fig. 8 at selected temperatures. For x¼ 0.05 sample, as the sample is cooled from TN, the M–H loops are found to shift towards negative field axis and for further cooling, the center of M–H loop approaches towards origin. For Tr 150 K, the loops are found to shift towards positive field axis. Thus the present sample exhibits exchange bias field that can be tuned by varying the temperature. Similar behavior was observed for x ¼0.10 sample but the shifting in negative field axis was noticed only in a narrow temperature range close to TN. M–H loops of x¼ 0.15 sample (Fig. 8) also show shifting of the loop but mostly along the positive H axis and similar behavior was observed for x¼ 0.20 sample also. The exchange bias field was determined using the relation HEB = (H+ + H −)/2; where H þ and H are the field values corresponding to M¼0 at ascending and descending branches of the M– H loop respectively. Fig. 9(a) shows HEB versus temperature plot for x¼ 0.05 sample. As the temperature is decreased from TN, HEB decreases towards negative value and approaches a maximum negative value of 0.85 kOe at 200 K. For further decrease in temperature, HEB increases towards positive values and approaches HEB ¼0 axis at T ≃ Tcomp ¼160 K. For To Tcomp, a broad positive peak is observed at TP ¼85 K with a maximum positive HEB value of þ2.17 kOe and it is close to the temperature at which maximum negative magnetization was observed in the M–T plot (see Fig. 4(a)). For T oTP, the HEB values fall towards negative value by crossing the HEB ¼ 0 axis for the second time at T ≃ T ′comp ¼ 40 K. Unlike x ¼0.05 sample, x¼ 0.10 sample exhibits positive HEB values in a wide temperature range below TN with a peak value of HEB ¼3.65 kOe at 85 K. Small negative HEB values are observed in a narrow temperature range close to TN. The observed maximum HEB value is comparable to that reported in La0.15Pr0.85CrO3 [19], NdMnO3 [36], LaCr0.85Mn0.15O3 [13] and La0.8Ce0.2CrO3 [14]
Fig. 6. (a) HI and (b) MCr as a function of applied field for NdCr1 xFexO3 samples.
Fig. 7. Bipolar switching of magnetization for (a) x¼ 0.05 and (b) x¼ 0.20 samples at T ¼100 K.
Please cite this article as: T. Bora, S. Ravi, Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.03.060i
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Fig. 8. M–H loops recorded at different temperatures for x ¼ 0.15 sample.
Fig. 9. Temperature variation of HEB for (a) x¼ 0.05, (b) x¼ 0.10, (c) x ¼ 0.15 and (d) x ¼0.20 samples.
compounds. For a comparison, temperature variations of ΔM /MZFC are shown in Fig.10 for different H values, where ΔM is the difference between the FC (MFC) and ZFC (MZFC) magnetization at a particular temperature. All curves pass through the temperature axis at a particular temperature known as Tcross. A large negative peak at To Tcross is observed for all the samples and in addition to that a small positive peak is observed at T 4Tcross especially for x ¼0.05 and x ¼0.10 samples. These peaks coincide with respective positive and negative peaks of HEB vs. T plots. The observed HEB and its temperature dependence can be explained by considering the exchange anisotropy between the ferromagnetic components of Cr3 þ ions (MCr) and the paramagnetic moments of Nd3 þ and Fe3 þ ions (MNd þMFe) under the negative
internal field. The negative peak observed in the temperature range Tcomp oT o TN especially for x ¼0.05 sample is due to the dominant MCr aligned along the field direction compared to MNd þMFe which are aligned along HI, i.e. opposite to the direction of applied field. On the other hand for T o Tcomp, HEB shows a broad positive peak because MNd þMFe dominates over the MCr component. The dominant behavior of MNd þMFe at low temperature is consistent with general temperature dependence of paramagnetic moment. The HEB behavior of x¼ 0.10 sample can be also understood based on the above argument; however in this sample, negative HEB is observed in a very narrow temperature range. It is mainly due to the enhanced Tcomp value, which is quite close to TN. The observation of only positive HEB values for x ¼0.15
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Fig. 10. Temperature variation of relative irreversible magnetization for (a) x¼ 0.05, (b) x ¼0.10, (c) x ¼0.15 and (d) x¼ 0.20 samples.
and 0.20 sample is due to the larger contribution of MNd þ MFe as a result of higher concentration of Fe doping and the corresponding larger Tcomp value quite close to TN. Thus in this report, the sign reversal of HEB especially for x ¼0.05 and 0.10 and its mechanism is discussed. Such a sign reversal of HEB has been reported in other orthochromites [14,18,19] and intermetallic compounds [29,30]; however the mechanism is found to differ depending on the nature of the sample and its microstructure. EB was explained by considering various models such as core shell structure [14], competition between the DM interaction and single ion magnetic anisotropy [22], unidirectional magnetic anisotropy due to DM interaction [18] and antiparallel coupling between the magnetic spins of Cr3 þ ions and rare earth ions [19]. Sign reversal of different sub components of magnetic moment of rare earth ions and conduction electron polarization is reported to contribute EB and its sign reversal in intermetallic compounds [29,30]. Since, we are not dealing with nano-particles, core shell structure is unlikely to play a role in the present set of samples. Because of the presence of considerable paramagnetic moment, the present data could not be fitted to the model involving the competition between DM interaction and single ion anisotropy [25]. The measured M–T data could be fitted to the equation [as per Ref. [5]] containing the canted FM component of Cr3 þ ions and paramagnetic moments of Fe3 þ and Nd3 þ ions. HEB is explained in terms of exchange anisotropy between MCr and MNd þ MFe and its sign reversal is due to one component overtaking the other as the temperature is varied.
4. Conclusions We have prepared single phase polycrystalline samples NdCr1 xFexO3 (x¼ 0–0.2) by the sol–gel method and they are found to crystallize in orthorhombic structure with Pbnm space group. The lattice parameters increase systematically with Fe concentration. Magnetization reversal has been observed even for 5 atm% of Fe doping and this phenomenon persists upto x ¼0.20. The magnetic compensation temperature is found to increase with doped Fe concentration and its maximum value is 198 K for x ¼0.15 sample. The magnetization reversal is explained by
considering the competition between the canted FM component of Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ in negative internal field. The maximum HI value is found to be 29.6 kOe for x¼ 0.15 sample. Tunable exchange bias field has been observed due to the exchange anisotropy between the above two components of magnetic moment. The sign reversal of HEB is observed due to the switching of domination of one component over the other in different temperature region. The maximum HEB value is found to be 3.65 kOe for x ¼0.10 sample.
Acknowledgments Authors are thankful to Department of Science and Technology (DST), New Delhi, for FIST XRD facility (Ref. no. SR/FST/PSII-020/ 2009) and for the research grant, vide Ref. no. SR/S2/CMP-0078/ 2010. Authors are thankful to Dr. M. Kar, IIT Patna for his help in carrying out microstructural studies.
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Sign reversal of magnetization and tunable exchange bias field in NdCr1 xFexO3 (x¼ 0.05–0.2) Tribedi Bora, S. Ravi n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 23 February 2015 Accepted 14 March 2015
Magnetization reversal and tunable exchange bias behavior are observed in NdCr1 xFexO3 compounds for x ¼ 0.05–0.20. The magnetic compensation temperature (Tcomp) is found to increase with increase in Fe concentration and its maximum value is 198 K for x ¼ 0.15 sample. The observed magnetization reversal is explained by considering the competition between the weak ferromagnetic component of Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ ions under the influence of negative internal magnetic field. The exchange anisotropy between the above two components of magnetic moments give rise to tunable positive and negative exchange bias fields. The sign reversal of exchange bias field also coincides with Tcomp. Bipolar switching of magnetization is demonstrated at T o Tcomp by just varying the positive magnetic field. & 2015 Published by Elsevier B.V.
Keywords: Neodymium chromites Antiferromagnetism Magnetization reversal Exchange bias
1. Introduction The rare earth orthochromites RCrO3 (R¼rare earth element) continue to be to focus of research due to their interesting physical properties and potential applications [1–4]. In these compounds, magnetic interactions are possible in three different networks such as Cr3 þ –Cr3 þ , Cr3 þ –R3 þ and R3 þ –R3 þ with O2 as intermediate ion [5]. Orthochromites are generally antiferromagnetic in nature with Néel temperature in the range 112–282 K depending on the type of rare earth element [5,6]. Substitutions on rare earth site using other rare earth elements and on Cr site using other transition elements give rise to interesting properties like magnetization reversal (MR) and exchange bias (EB) phenomena. Even though MR is known to appear in ferrimagnetic (FIM) compounds [7], recently many other compounds such as Sr2YbRuO6 [8], orthochromites [5,9–21], orthoferrites [22–24], othovanadates [25,26], molecular magnets [27,28] and intermetallic alloys [29,30] are shown to exhibit such behavior. Exchange bias phenomenon has been studied in heterostructure of bilayer/multilayer thin films of AFM/FM, AFM/FIM (ferrimagnetic), AFM/SG (spin glass), etc. [31]. Recently EB behavior has been reported even on bulk materials such as magnetic nano particles [32], charge ordered manganites [33], phase separated cobalites [34,35], orthochromites [13,14,18,19] and intermetallic compounds [29,30]. The EB field (HEB) is mostly reported to be either positive or negative but its n
Corresponding author. Fax: 91 3612690762. E-mail address: [email protected] (S. Ravi).
tunability is relatively rare. Several models have been invoked to explain the MR and EB phenomenon: core shell model [14], interaction between Dzayloshinsky–Moriya (DM) and single ion anisotropy [23], competition between the paramagnetic moments of dopant ions and the ferromagnetic component of host ions [13,19], AFM interaction between the FM components of two magnetic sublattices [36], etc. NdCrO3 is an orthorhombic perovskite compound having G-type antiferromagnetic structure with Néel temperature, TN 220 K along with a weak ferromagnetic component due to spin canting and in addition to that it exhibits spin reorientation transition at around TSR ¼35 K [37–39]. La substitution for Nd in NdCrO3 was found to suppress the TSR and increase the TN values [40]. In NdCrO3, both the cations are magnetic elements and it would be interesting to study the effect of doping of Fe3 þ ions having higher magnetic moment compared to the host Cr3 þ ions. In this paper, we report the preparation of NdCr1–xFexO3 (x ¼0–0.2) and study of their magnetic properties. MR was observed even for 5 atm% of Fe doping and it persists upto the doping level of 20 atm%. These samples also exhibit EB phenomenon and the maximum EB field was found to be þ 3.65 kOe for x¼ 0.10 sample.
2. Experimental details Polycrystalline samples of NdCr1 xFexO3 (x¼ 0, 0.05, 0.10, 0.15 and 0.20) were prepared by the standard sol–gel method. Nd2O3, Cr(NO3)3 9H2O, Fe(NO3)3 6H2O having 99.9% purity were taken
http://dx.doi.org/10.1016/j.jmmm.2015.03.060 0304-8853/& 2015 Published by Elsevier B.V.
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Fig. 1. (a) XRD pattern along with the Rietveld refinement for x¼ 0.20 sample. (b) Variation of lattice parameters and unit cell volume with Fe concentration.
as starting compounds. Nd2O3 compound was dissolved in nitric acid and other two were dissolved in distilled water. An excess amount of citric acid was added to convert the mixture of above nitrate solution into citrate. Ethylene glycol was added to the citrate solution to promote the gel formation and the gel was converted into fine powder upon heating. The powder was presintered at 600 °C for 12 h and final sintering was done in pellet form at 1100 °C for 24 h. The prepared samples were characterized by recording X-ray diffraction patterns using Rigaku make TTRAX III diffractometer. Magnetization measurements were carried out using Lakeshore make vibrating sample magnetometer of model no. 7410. Microstructure and composition analysis were carried out by using Field Emission Scanning Electron Microscope with EDAX.
3. Results and discussions NdCr1 xFexO3 (x¼ 0, 0.05, 0.10, 0.15 and 0.20) samples are found to be in single phase form as per XRD patterns. All XRD patterns were refined by choosing Pbnm space group in orthorhombic cell as per Rietveld method using Fullprof program [41]. Typical XRD pattern along with Rietveld refinement for x¼0.20 sample is shown in Fig. 1(a). The lattice parameters are found to increase systematically with increase in Fe concentration as shown in Fig. 1(b) and this behavior demonstrates the substitution of larger Fe3 þ ions (0.645 Å) at the site of Cr3 þ ions (0.615 Å). The reliability factors RBragg, Rf, Rp and χ2 for x ¼0.20 sample are found to be 3.3%, 4.8%, 9.7% and 6.1. The cation ratios Nd: Cr: Fe obtained from the EDAX analysis for x¼ 0.05 and 0.15 are found to be
Fig. 2. Magnetization versus temperature plots in ZFC and FC conditions for x ¼0 sample. The inset shows ZFC M–T plot in an expanded scale.
0.99:0.95:0.05 and 0.98:0.80:0.19 respectively. Temperature variation of magnetization in zero field cooled (ZFC) and field cooled (FC) conditions at an applied field of H¼ 2 kOe is shown in the inset of Fig. 2 for x ¼0 sample and in Fig. 3 for x ¼0.05 to 0.20 samples. The parent compound exhibits an AFM peak at TN ¼225 K as shown in Fig. 2 for clarity and for To TN, M value is found to increase with a typical paramagnetic behavior due to the presence of Nd3 þ ions. A large irreversibility is observed in the FC curve with the enhanced magnetization in the form of a plateau for T oTN due to the presence of canted Cr3 þ moment. A sharp rise in magnetization is observed around 50 K due to the spin reorientation of Cr3 þ ions [37,38]. The ZFC M–T plot of x¼ 0.05 sample follows the trend of x ¼0 sample but with a reduced TN of 219 K as shown in Fig. 3. Under FC condition, magnetic irreversibility is observed at T o TN in the form of a broad peak and for further decrease in temperature, magnetization decreases towards negative values by crossing over the ZFC magnetization curve at Tcross ¼163 K and passing through magnetic compensation (M ¼0) at Tcomp ¼102 K. A minimum magnetization (maximum negative value) of Mmin ¼ 2.45 emu/mol is observed at Tmin ¼ 80 K. On further decrease in temperature a sharp rise in magnetization towards positive value is observed at around TSR ≃ 60 K. M–T plots of other samples with higher Fe concentration 0.1 r xr0.2 also show similar behavior but with enhanced Tcomp values. Moreover, the broad peak observed in the vicinity of TN is found to diminish with increase in Fe concentration due to the widening of negative magnetization region. Tcomp value is found to vary from 102 K for x¼ 0.05 to 169 K for x ¼0.15 at H¼ 2 kOe. The negative magnetization values for x ¼0.10 and 0.15 samples are found to be about one order of magnitude larger than that of x¼ 0.05 sample. The Tcomp value for x ¼0.20 sample is found to be smaller than that of x¼ 0.15 sample and this can be understood in terms of lack of contribution of all doped Fe ions for magnetization reversal and some of the Fe ions are engaged in long range AFM interaction in Fe3 þ O2 Fe3 þ networks. Such networks are possible especially for x Z0.20. To understand the MR phenomenon, we have recorded the M–T curves of x¼0.05–0.20 samples at different applied fields in the range 50–5000 Oe. They are shown in Fig. 4 for H¼ 200 Oe. Prominent negative magnetization is observed for all these samples especially for low applied field. The magnitude of negative magnetization is found to decrease with increase in applied field and correspondingly the Tcomp value also decreases as shown in Fig. 5. MR was observed upto H ¼2000 Oe for x ¼0.05 and 0.20 samples and upto H ¼3000 Oe for x¼0.10 and 0.15 samples. For x ¼0.05 and 0.10 samples at H¼ 200 Oe, additional magnetic compensation is observed at low temperature i.e. at T ′comp ¼41 K and 31 K respectively. The maximum Tcomp value observed in the present series is found to be larger than that of transition element doped LaCrO3 [12,13] and it is mainly due to the presence of magnetic
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Fig. 3. Magnetization versus temperature plots in ZFC and FC conditions for x ¼ 0.05 to 0.20 samples (a–d). The inset in (a) shows ZFC M–T plot for x¼ 0.05 in an expanded scale.
Fig. 4. ZFC and FC M–T curves of x ¼0.05 to 0.20 samples at H¼ 200 Oe along with fitted data.
rare earth ion (Nd3 þ ). Similarly, larger Tcomp values of 252 K and 230 K are reported in other orthochromites containing magnetic rare earth ions, i.e. SmCr0.5Fe0.5O3 [17] and La0.15Pr0.85CrO3 [19] respectively. The competition between the weak ferromagnetic component of canted Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ ions under the influence of the negative internal magnetic field due to the AFM ordered Cr3 þ ions gives rise to the magnetization reversal. According to Cook et al. [5], the net magnetization by considering the weak ferromagnetic component and the paramagnetic component under the influence of the negative
internal magnetic field is, M = MCr + 3þ
C (H + HI ) where, (T − θC )
MCr is the
canted FM component of Cr ions, HI is the internal magnetic field and C is the Curie constant. The FC M–T curves were fitted to the above equation within the temperature range of Tmin oT oTN and the fitted data are shown as solid line in Fig. 4. For T oTmin, the magnetization data could not be fitted to the above equation due to the onset of spin reorientation transition. The estimated θC values for x¼0.05 sample is found to be in the range of 40 K to 65 K depending on the applied field. These values for x¼ 0.10, 0.15 and 0.20 are found to be in the range of 119 to 139 K, 98 K to 167 K and 162 K to 176 K respectively. Plots of HI
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Fig. 5. Tcomp as a function of applied magnetic field.
and MCr as a function of H are shown in Fig. 6. HI and MCr values are found to almost saturate for H Z0.5 kOe, however for x ¼0.15 sample, they increase linearly. Bipolar switching of magnetization at T ¼100 K for x ¼0.05 and 0.20 samples are shown in Fig. 7. Here, the samples were initially FC to 100 K through TN by applying H¼ 200 Oe. By keeping the temperature fixed H was increased from 200 Oe to 4000 Oe and then decreased to 200 Oe. Such a field variation flips the magnetization from negative to positive and back to negative. Subsequent cycling of the fields between the above two values give rise to reproducible magnetization reversal. Thus by varying the magnitude of positive applied field, bipolar switching of magnetization can be achieved. Similar type of switching behavior is also observed for x ¼0.10 and 0.15 samples for H ¼200 Oe to 7000 Oe. In order to study the exchange bias behavior, M–H loops were recorded under FC condition (H¼5000 Oe) in the temperature range 30–20 K at 5 K interval. Typical M–H loops for x ¼0.15
sample in expanded scale are shown in Fig. 8 at selected temperatures. For x¼ 0.05 sample, as the sample is cooled from TN, the M–H loops are found to shift towards negative field axis and for further cooling, the center of M–H loop approaches towards origin. For Tr 150 K, the loops are found to shift towards positive field axis. Thus the present sample exhibits exchange bias field that can be tuned by varying the temperature. Similar behavior was observed for x ¼0.10 sample but the shifting in negative field axis was noticed only in a narrow temperature range close to TN. M–H loops of x¼ 0.15 sample (Fig. 8) also show shifting of the loop but mostly along the positive H axis and similar behavior was observed for x¼ 0.20 sample also. The exchange bias field was determined using the relation HEB = (H+ + H −)/2; where H þ and H are the field values corresponding to M¼0 at ascending and descending branches of the M– H loop respectively. Fig. 9(a) shows HEB versus temperature plot for x¼ 0.05 sample. As the temperature is decreased from TN, HEB decreases towards negative value and approaches a maximum negative value of 0.85 kOe at 200 K. For further decrease in temperature, HEB increases towards positive values and approaches HEB ¼0 axis at T ≃ Tcomp ¼160 K. For To Tcomp, a broad positive peak is observed at TP ¼85 K with a maximum positive HEB value of þ2.17 kOe and it is close to the temperature at which maximum negative magnetization was observed in the M–T plot (see Fig. 4(a)). For T oTP, the HEB values fall towards negative value by crossing the HEB ¼ 0 axis for the second time at T ≃ T ′comp ¼ 40 K. Unlike x ¼0.05 sample, x¼ 0.10 sample exhibits positive HEB values in a wide temperature range below TN with a peak value of HEB ¼3.65 kOe at 85 K. Small negative HEB values are observed in a narrow temperature range close to TN. The observed maximum HEB value is comparable to that reported in La0.15Pr0.85CrO3 [19], NdMnO3 [36], LaCr0.85Mn0.15O3 [13] and La0.8Ce0.2CrO3 [14]
Fig. 6. (a) HI and (b) MCr as a function of applied field for NdCr1 xFexO3 samples.
Fig. 7. Bipolar switching of magnetization for (a) x¼ 0.05 and (b) x¼ 0.20 samples at T ¼100 K.
Please cite this article as: T. Bora, S. Ravi, Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.03.060i
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Fig. 8. M–H loops recorded at different temperatures for x ¼ 0.15 sample.
Fig. 9. Temperature variation of HEB for (a) x¼ 0.05, (b) x¼ 0.10, (c) x ¼ 0.15 and (d) x ¼0.20 samples.
compounds. For a comparison, temperature variations of ΔM /MZFC are shown in Fig.10 for different H values, where ΔM is the difference between the FC (MFC) and ZFC (MZFC) magnetization at a particular temperature. All curves pass through the temperature axis at a particular temperature known as Tcross. A large negative peak at To Tcross is observed for all the samples and in addition to that a small positive peak is observed at T 4Tcross especially for x ¼0.05 and x ¼0.10 samples. These peaks coincide with respective positive and negative peaks of HEB vs. T plots. The observed HEB and its temperature dependence can be explained by considering the exchange anisotropy between the ferromagnetic components of Cr3 þ ions (MCr) and the paramagnetic moments of Nd3 þ and Fe3 þ ions (MNd þMFe) under the negative
internal field. The negative peak observed in the temperature range Tcomp oT o TN especially for x ¼0.05 sample is due to the dominant MCr aligned along the field direction compared to MNd þMFe which are aligned along HI, i.e. opposite to the direction of applied field. On the other hand for T o Tcomp, HEB shows a broad positive peak because MNd þMFe dominates over the MCr component. The dominant behavior of MNd þMFe at low temperature is consistent with general temperature dependence of paramagnetic moment. The HEB behavior of x¼ 0.10 sample can be also understood based on the above argument; however in this sample, negative HEB is observed in a very narrow temperature range. It is mainly due to the enhanced Tcomp value, which is quite close to TN. The observation of only positive HEB values for x ¼0.15
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Fig. 10. Temperature variation of relative irreversible magnetization for (a) x¼ 0.05, (b) x ¼0.10, (c) x ¼0.15 and (d) x¼ 0.20 samples.
and 0.20 sample is due to the larger contribution of MNd þ MFe as a result of higher concentration of Fe doping and the corresponding larger Tcomp value quite close to TN. Thus in this report, the sign reversal of HEB especially for x ¼0.05 and 0.10 and its mechanism is discussed. Such a sign reversal of HEB has been reported in other orthochromites [14,18,19] and intermetallic compounds [29,30]; however the mechanism is found to differ depending on the nature of the sample and its microstructure. EB was explained by considering various models such as core shell structure [14], competition between the DM interaction and single ion magnetic anisotropy [22], unidirectional magnetic anisotropy due to DM interaction [18] and antiparallel coupling between the magnetic spins of Cr3 þ ions and rare earth ions [19]. Sign reversal of different sub components of magnetic moment of rare earth ions and conduction electron polarization is reported to contribute EB and its sign reversal in intermetallic compounds [29,30]. Since, we are not dealing with nano-particles, core shell structure is unlikely to play a role in the present set of samples. Because of the presence of considerable paramagnetic moment, the present data could not be fitted to the model involving the competition between DM interaction and single ion anisotropy [25]. The measured M–T data could be fitted to the equation [as per Ref. [5]] containing the canted FM component of Cr3 þ ions and paramagnetic moments of Fe3 þ and Nd3 þ ions. HEB is explained in terms of exchange anisotropy between MCr and MNd þ MFe and its sign reversal is due to one component overtaking the other as the temperature is varied.
4. Conclusions We have prepared single phase polycrystalline samples NdCr1 xFexO3 (x¼ 0–0.2) by the sol–gel method and they are found to crystallize in orthorhombic structure with Pbnm space group. The lattice parameters increase systematically with Fe concentration. Magnetization reversal has been observed even for 5 atm% of Fe doping and this phenomenon persists upto x ¼0.20. The magnetic compensation temperature is found to increase with doped Fe concentration and its maximum value is 198 K for x ¼0.15 sample. The magnetization reversal is explained by
considering the competition between the canted FM component of Cr3 þ ions and the paramagnetic moments of Nd3 þ and Fe3 þ in negative internal field. The maximum HI value is found to be 29.6 kOe for x¼ 0.15 sample. Tunable exchange bias field has been observed due to the exchange anisotropy between the above two components of magnetic moment. The sign reversal of HEB is observed due to the switching of domination of one component over the other in different temperature region. The maximum HEB value is found to be 3.65 kOe for x ¼0.10 sample.
Acknowledgments Authors are thankful to Department of Science and Technology (DST), New Delhi, for FIST XRD facility (Ref. no. SR/FST/PSII-020/ 2009) and for the research grant, vide Ref. no. SR/S2/CMP-0078/ 2010. Authors are thankful to Dr. M. Kar, IIT Patna for his help in carrying out microstructural studies.
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