Impact of co-doping (Gd and Mn) on the magnetic properties has been systematically investigated in SmCrO3 compound. For the synthesized compound Sm0.9Gd0.1Cr0.85Mn0.15O3 (SGCMO), below the Neel transition temperature and under low applied magnetic field, temperature induced magnetization reversal at 105 K (crossover temperature) was noticed in the field cooled magnetization curve. Magnetization reversal attained maximum value of -1.03 emu/g at 17 K where spin reorientation occurred. The magnetization reversal disappeared under higher applied field. From the M-H plots an enhancement in the magnetization was observed due to Gd doping. Magnetocaloric effect at low temperatures measured through the magnetic entropy change was found sixteen times higher for this compound as compared to pristine SmCrO3 and twice to that of SmCr0.85Mn0.15O3 compound. The study reveals the importance of co-doping in tailoring the magnetic properties of rare-earth chromites.

In recent years, rare-earth orthochromites (RCrO3) have been extensively explored because they exhibit temperature induced magnetization reversal (TIMR) phenomenon.1–5 This is useful in thermomagnetic switches and thermally assisted random access memories.6,7 TIMR is defined as a temperature induced crossover of magnetization from a positive to negative value. It has also been reported in other compounds viz. manganites, orthoferrites, spinel ferrites and orthovanadates8–11etc. In different magnetic systems, magnetization reversal (MR) can occur due to several mechanisms such as, negative exchange coupling among ferromagnetic (antiferromagnetic) sublattices,12–14 negative exchange coupling among ferromagnetic/canted-antiferromagnetic15 and paramagnetic sublattices,16–19 imbalance of spin and orbital moments,20 interfacial exchange coupling between ferromagnetic and antiferromagnetic phases21–23 etc. SmCrO3 (SCO) has a distorted orthorhombic structure (Pnma space group) and belongs to the family of rare-earth orthochromites (RCrO3). In SCO compound, Cr3+ spins orders antiferromagnetically (TN) ∼ 192 K. Dielectric studies on SCO samples showed colossal dielectric constant of the order of ∼ 104 at room temperature which was associated to the grain boundary effects.24 It also exhibited ferroelectric nature; confirmed through Raman and dielectric spectroscopic techniques.24,25 These properties make SCO a multiferroic material. Observation of TIMR in SCO is still controversial. For example, Wu et al.26 and Dash et al.5 have not observe any sign of MR in this compound whereas Gupta et al.27 reported giant temperature dependent magnetization reversal in SCO compound along with a large exchange-bias and coercivity.27 Regardless of the magnetic nature of pristine SCO, manganese (Mn) doping in SCO induces magnetization reversal5,28 and also enhances the magnetocaloric effect (MCE) at low temperatures,28 which is responsible for magnetic refrigeration. MCE investigation in chromites has gained significant attention recently in view of its utilization as cheaper, safer, more efficient, and environmental friendly cooling technology for future. Not much work has been reported on co-doping of Gd3+ and Mn3+ magnetic ions in SCO. In this paper, we report the co-doping effect in SCO compound.

Polycrystalline samples of SmCrO3 were prepared with Gd (10%) and Mn (15%) co-doping [(Sm0.9Gd0.1)Cr0.85Mn0.15O3] by solid state reaction process. The structural properties of compounds were analysed by Rietveld refinement using Fullprof software. Magnetic properties of the samples were studied using physical property measurement system (PPMS) system. Details of the sample preparation and characterizations are given in Ref. 28.

Fig. 1 shows the XRD pattern of the SGCMO compound analysed using Rietveld refinement. As can be seen, the material exhibits single phase nature with all the characteristic peaks belonging to orthorhombic perovskite Pnma space group. The lattice parameters obtained from the refinement are a = 5.5203(2) Å, b = 7.6292(3) Å, c = 5.3631(2) Å. The lattice volume (225.87Å3) of the SGCMO compound is smaller than that of SmCr0.85Mn0.15O3 (SCMO) compound.28 This is attributed to the smaller ionic radius of Gd3+ (rGd3+ = 1.053 Å) ion in comparison to Sm3+ (rSm3+ = 1.079 Å) ion.29 This allows the unit cell to shrink and the ensuing decrease in volume. The tolerance factor (t) has a greater role in deciding the structure of the unit cell. We have calculated the Goldschmidt tolerance factor using the formula, t=(RA+RO)/2(RB+RO), where RA and RB are the average ionic radii of A-site and B-site cations, and RO is the ionic radius of anion (O2+) and the calculated value is 0.87. This is in accordance with the proposed range of t in between, 0.75 < t < 0.9, representing an orthorhombically distorted structure.30 Further, the distortion of orthorhombic unit cell from the ideal cubic structure, is defined by the orthorhombic strain factor S = 2(ac)/(a + c), and the calculated value is 0.0288.

FIG. 1.

Rietveld refined fitted powder XRD pattern of SGCMO compound.

FIG. 1.

Rietveld refined fitted powder XRD pattern of SGCMO compound.

Close modal

1. Magnetization reversal

Figure 2(a) demonstrates the temperature dependence of magnetization (MT) curves under the zero field-cooled (ZFC) and field-cooled (FC) modes at different applied fields. An antiferromagnetic transition occurs at TN ∼162 K that is calculated from the minimum of the differentiation of magnetization versus temperature plot. It is interesting to compare here the TN of the present compound with that for SCMO (∼168 K) and SCO (∼192 K).5,27,28 The TN of SCMO sample decreased with respect to that for undoped SCO because of the development of double-exchange interaction between Mn3+ (t2g3eg1)/Cr3+ (t2g3eg0) ions via O2- ions.5 In present study, adding 10% Gd ion on Sm-site in SCMO compound, further lowers TN. This can be interpreted by the effect of Gd doping on the average A-site ionic radius i.e.Ravg.=(xGd×RGd2)+((1x)Sm×RSm2). The calculated values of Ravg.SGCMO, Ravg.SCMO and Ravg.SCO are 1.076 Åand 1.079 Å(for the last two compounds), respectively. The magnetic properties of orthochromites significantly depend upon Ravg, and TN decreases with decreasing Ravg.31,32

FIG. 2.

(a)-(d) Temperature depend magnetization curves under FC and ZFC modes at different fields (50-5000 Oe). The solid blue line shows the fit to the Eq. (1).

FIG. 2.

(a)-(d) Temperature depend magnetization curves under FC and ZFC modes at different fields (50-5000 Oe). The solid blue line shows the fit to the Eq. (1).

Close modal

Below TN in FC mode (under H = 50 Oe applied field) magnetization increases with decreasing temperature. It attains maximum values Mmax ∼ 0.081 emu/g with a positive polarity at the peak temperature Tpeak ∼ 147 K. On further cooling, magnetization continuously decreases. It crosses zero magnetization (M = 0) at a crossover temperature (Tcross1 ∼ 105 K). Below Tcross1, there is a change in the magnetization polarity i.e. temperature induced magnetization reversal occurs. Magnetization continues to decrease further until it approaches a minimum value of Mmin ∼ -1.03 emu/g at 17 K. This temperature is defined as the spin reorientation temperature (TSR). The negative value of magnetization indicates direction of net magnetic moment is against the applied magnetic field. The magnetic moments of Sm3+, Gd3+ and Mn3+ ions get aligned antiparallel to weak ferromagnetic (WFM) components of the Cr3+ spins resulting in magnetization reversal phenomenon. This happens because the internal field produced by canted Cr3+ spins rotates the magnetization of other magnetic ions. Further below TSR, magnetization again starts increasing, a possible justification for this behavior is the rotation of the net magnetic moment from one easy direction to the other.2 A careful observation at the magnetization behavior under ZFC mode reveals that it remains positive below Tcomp1 [Fig. 2a]. Figs. (2b–2c) illustrate the M(T) plots under higher applied magnetic field in the range of 1000 ≤ H ≤ 2500 Oe. When the applied magnetic field is 1000 Oe, magnetization again crosses zero value at Tcomp2 and becomes positive in FC mode below TSR. At much higher applied magnetic fields, i.e., 5000 Oe (Fig. 2d), the rotation of the moments of Mn3+ and Sm3+/Gd3+ ions along the field takes place since the applied field dominates the internal field, and hence, magnetization becomes positive in the entire range of measured temperature. It is worth mentioning here that in case of SCMO compound the magnetization in FC mode becomes positive under 2500 Oe applied field, whereas in the present case higher field is required to suppress the negative magnetization.28 The FC magnetization value at the lowest temperature measured for SGCMO sample is twice to that for SCMO compound,28 which can be understood in terms of the contribution from Gd3+-Gd3+ ions ordering.

The various FC magnetization curves acquired under different fields from 50 Oe to 2500 Oe, were fitted using the following equation:2 

M=MCr+C(H+HI)Tθ
(1)

where M, MCr, C, HI, H and θ stand for the total magnetization, weak FM component of canted Cr3+ ions, a Curie constant, an internal field from Cr3+ ion, an applied field and a Weiss temperature. The fitting is shown by solid line in Figs. 2(a–c). The obtained fitting parameters are summarized in Table I. The negative value of internal field confirms the assumption that it is opposite to the applied field and its value being larger than the applied magnetic field allows the magnetization of ions to get aligned antiparallel to that of Cr3+ ions when the applied field is smaller. The reason for this is the antisymmetric exchange or pseudodipolar coupling.33 These are high order interactions which arise from the effect of spin-orbit coupling, introduced as a perturbation on the spin-only ground state of the magnetic ions. The values of MCr and HI increase with increasing external field that can be understood as an enhancement in the AFM ordering.

TABLE I.

Fitting parameters for M-T curves in FC mode shown in figs. 2a–2c.

External Field (Oe)MCr (emu/g)HI (Oe)θ (K)
50 0.587 -70 -27 
1000 0.703 -1068 -34 
2500 0.813 -2546 -81 
External Field (Oe)MCr (emu/g)HI (Oe)θ (K)
50 0.587 -70 -27 
1000 0.703 -1068 -34 
2500 0.813 -2546 -81 

2. M-H loop and magnetocaloric measurements

The isothermal magnetization M-H curves measured under H = ± 90 kOe in ZFC mode at 2 K, 20 K and 80 K are shown in Fig. 3a. The loops are well symmetrically closed without any saturation. The maximum loop opening occurs near the spin reorientation temperature (enlarged view of M-H curve at low field in the inset Fig. 3a). Such behavior is attributed to the coexistence of high field antiferromagnetic and low field weak ferromagnetic components. The magnetization increases almost linearly for all the loops in the region of larger magnetic field. Therefore, the high-field part of the M-H curves evolution can be represented as M(H) = χAFMH + Ms. Here χAFMH is the antiferromagnetic contribution and Ms is the saturation magnetization of the weak ferromagnetic phase. The values of Ms at different temperatures can be calculated by subtracting the high field linear AFM contribution from the total magnetization. The values for the present case are 15.8 emu/g, 5.25 emu/g and 0.17 emu/g at 2 K, 20 K and 80 K, respectively. The Ms value equal to 1.83 emu/g at 20 K was reported for SCMO compound by Kumar et al.28 which indicates a contribution of Gd3+ ions in enhancing the magnetization of the present SGCMO sample. The coercive field was found to decrease for SGCMO compound.

FIG. 3.

(a) M-H curves at 2 K, 20 K and 80 K. Inset shows the enlarged view of M-H curve at lower applied field, (b) M-H curve (in first quadrant) and (c) −ΔS versus T.

FIG. 3.

(a) M-H curves at 2 K, 20 K and 80 K. Inset shows the enlarged view of M-H curve at lower applied field, (b) M-H curve (in first quadrant) and (c) −ΔS versus T.

Close modal

To investigate the effect of substitution on the magnetocaloric properties, the isothermal magnetization M-H curves of SGCMO compound were measured in first quadrant with applied field up to 90 kOe. The data were acquired between 2 and 102 K temperature range with an interval of ΔT = 5 K and the plots are shown in Fig. 3(b). The MCE behavior is measured through the magnetic entropy change defined as:34 

ΔS(T)ΔH=HIHF(M(T,H)T)HdH
(2)

numerical integration of the above Eq. by trapezoidal rule yields35 

ΔS(Tav)ΔH=δH2δT(δM1+2i=2n1δMi+δMn)
(3)

where average temperature Tav(=(Tj+ Tj+1)/2) from the two magnetization isotherms measured at Tj and Tj+1 in a magnetic field changing by ΔH = HF - HI a constant step δH. While δT = Tj+1Tj is the temperature difference between the two isotherms, n is the number of points measured for each of the two isotherms with the magnetic field changing from H1 = HI to Hn = HF at δH = ΔH/(n−1). δMi = [M(Tj+1)iM(Tj)i] is the difference in the magnetization at Tj+1 and Tj for each magnetic-field step from 1 to n. Fig. 3(c) shows the temperature dependent −ΔS for different field variations from 0-90 kOe. The value of −ΔS increases sharply with decreasing temperature and reaches a maximum value of 3.25 J kg-1 K-1 at 4.5 K for 0 - 50 kOe field variation. Doping of 10% Gd at the Sm-site in SCMO compound was found to noticeably improve the MCE properties. On comparing the −ΔS values of the present compound with that for the SCO and SCMO compounds reported by Gupta et al.36 and Kumar et al.,28 respectively; we observed that SGCMO yielded almost sixteen times higher −ΔS than SCO and twice that of SCMO material. Several factors are responsible for enhancing the −ΔS properties; (i) SGCMO sample exhibits higher magnetization and lower coercive field therefore less energy would be lost in thermal process as compare to the SCMO sample thus contributes in increasing −ΔS value; (ii) according to Eq. (2) the value of −ΔS depends not only on the magnetic moments but on the slope (dM/dT) also. The larger these values are the higher is the MCE; (iii) large −ΔS value in SGCMO may have contributions arising from the Gd3+-Gd3+ magnetic interactions as has been reported by Yin et al. where −ΔS value reached maximum at 2 K in GdCrO3 single crystal.37 

We successfully achieved Gd and Mn co-doping in SCO compound and obtained single phase sample. At low magnetic fields (≤50 Oe) a single compensation temperature around 105 K was observed in the field cooled magnetization curve. Below this temperature, magnetization reversal was observed with maximum value of -1.03 emu/g reaching at 17 K where a spin reorientation occurs. With the increase in applied magnetic field strength, field cooled magnetization increased towards the positive values. There were two crossover temperatures observed under 1000 Oe applied field. Substitution induced effect was observed in the form of enhancement in magnetization and the magnetocaloric properties at low temperatures.

The author S. Kumar would like to thank the University Grant Commission, New Delhi, for providing the Rajiv Gandhi National Fellowship (RGNF), whereas I. Coondoo acknowledges the financial support from FCT, Portugal, through SFRH/BPD/81032/2011. N. Kumar acknowledges the financial support at UPR, Puerto Rico from DOD Grant (AFOSR-FA9550-16-1-0295).

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