Probing structural transformation and optical and magnetic properties in Cr doped GdMnO3: Jahn–Teller distortion, photoluminescence and magnetic switching effect

The systematic evolution of structure, photoluminescence and different magnetic transitions in GdMnO3 is reported after Cr doping. With increasing the Cr concentration from 10 to 40 at%, Rietveld refinement of X-ray diffraction patterns demonstrates that an O′ type orthorhombic structure transforms to O type, manifesting a reduction in lattice volume. The noticeable reduction in lattice volume is ascribed to the smaller size of the Cr3+ ion compared to Mn3+. The structural transformation is accompanied with a considerable decrease in the Jahn–Teller distortion factor evaluated from XRD, Raman and photoluminescence measurements. Magnetic studies reveal a considerable enhancement in Néel temperature (TN) from ∼42 K for x = 0 to 130 K for x = 0.4. Interestingly, we observe magnetization reversal (MR) with spin reorientation (TSR) for x = 0.3. The mechanism for such a magnetic behavior is discussed on the basis of competition between Mn, Cr and Gd. The incorporation of Cr not only constructively modifies the crystal structure and evokes the magnetic reversal phenomenon but also contributes towards the enhanced emission spectra. The promising structure and magnetic properties of Cr doped GdMnO3 offer potential pathways for spintronics and magnetic switching devices.


Introduction
Rare earth manganites (RMnO 3 , R ¼ rare earth) possess complex spin arrangements leading to unusual magnetic ordering such as antiferromagnetic (AFM) and cycloidal spin structures including spin reorientation (T SR ). [1][2][3][4][5] Since the last decade, a wide group of researchers has reported extensive investigations to comprehend the nature of spin, charge, orbital ordering or the exchange interaction of the transition metals. Recently, theoretical calculations along with experimental evidence reveal that such manganites exhibit distorted perovskites with an orthorhombic structure. The manganites of RMnO 3 type containing undersized trivalent R ions like GdMnO 3 demonstrate a ferroelectricity phenomenon induced due to competition between magnetic interactions evoking an antiferromagnetic (AFM) spin ordering that results into the lattice modulations. [6][7][8][9][10][11][12] GdMnO 3 show intriguing and captivating magnetic properties. GdMnO 3 crystallizes into an orthorhombic (O 0 ) structure showing canted AFM at $23 K along with Néel temperature (T N ) at $42 K. In general, the manganites exhibit inherent Jahn-Teller (J-T) distortion evoking unfavorable structural imperfections which essentially modify different physical properties. [13][14][15][16][17][18][19] Usually, the structural distortions can be perceived by employing sensitive characterization tools such as Raman, Xray absorption, and photoluminescence (PL) spectroscopy etc. In this regards, Modi et al. have reported that the room temperature Raman spectra of GdMn 1Àx Cr x O 3 (0 # x # 0.2) shows the reduction of Raman shi as doping concentration increases that related to anti-symmetric Jahn-Teller stretching mode and symmetric stretching mode. The broadening of Raman peaks can be consequence of lattice disorder as induced by Cr doping in Mn site. 20 While Li et al. have reported that there is a broadening of the Raman spectrum appeared from 400 to 900 cm À1 with increasing magnetic eld from 0 to 20 kOe in La 0.75 Ca 0. 25 MnO 3 . 21 The reason behind such broadening is associated with the structural changes. It is known that PL emissions of perovskite materials including CaTiO 3 and SrZrO 3 are affected considerably due to the intrinsic structure related distortions. 22,23 A wide group of researchers have reported the considerable tuning of magnetic properties, in particular aer doping various ions at Gd or Mn site. 24 In this context, Nandy et al. and Sarguna et al. discuss the signicant improvement of the magnetic properties by doping Na + and Y 3+ in Gd site. 25,26 The magnetic properties can also be modied if a non J-T active element replaces the J-T active element within the GdMnO 3 lattice. In this regard, including our previous studies, 24 Pal 4, we could show in our previous report that T SR could observe even when x ¼ 0.3. 24 Since Cr exhibits desired non J-T active characteristic, the interaction between R and Cr 3+ also suggests an unusual negative magnetization (NM) phenomenon. 28 The unexpected magnetic behavior of NM was rst predicted by Néel in the year, 1948. 29 In case of NM behavior, a denite temperature at which the value of magnetization becomes zero is termed as the compensation temperature (T comp ). Interestingly, the magnetization shows negative value along with some spin reorientation below T comp . In general, the NM phenomenon emerges either with changing the temperature or the degree of the magnetic eld. Therefore, such a phenomenon is unusual since it arises even under the inuence of applied magnetic eld having x direction. The exclusive properties of the NM have laid down various potential applications such as magnetic switching, magneto caloric and spintronics devices etc. [30][31][32] Usually, NM phenomenon has been reported in rare earth orthochromites, orthoferrites, and orthovanadates. 33 39 Modi et al. have reported the structural, electrical and magnetic properties of GdMn 1Àx Cr x O 3 when x # 0.2 having orthorhombic structure with space group Pbnm synthesized through solid state reaction technique. It has been observed a crossover of ZFC magnetization from positive value at x ¼ 0 to negative value at x ¼ 0.2. The negative magnetization is understood on the basis of competing WFM-FM interactions. 20 It is interesting to extend the Cr doping concentration above 0.2 to examine the unusual magnetic behavior in GdMnO 3 . Besides, Cr 3+ also plays an important role in negative magnetization and provides a prospective pathway to realize the magneto electric coupling and enhancement in multiferrocity. Surprisingly, there exists only a few reports which discuss NM phenomenon in rare earth manganites.
Therefore, in this work, we demonstrate the correlation between structural transformation and intriguing magnetic transitions in Cr doped GdMnO 3 varying dopant concentration from 10 to 40 at%. GdMn 1Àx Cr x O 3 (x ¼ 0.1, 0.2, 0.3 and 0.4) synthesized via facile sol-gel method reveals that O 0 type orthorhombic structure transforms to O type followed by a notable decrease in the J-T distortion factor with increasing Cr concentration. Raman and PL spectra endorse the dramatic decrease in the J-T distortion factor. Temperature and eld dependent magnetization including time dependent remanent magnetization measurements are carried out to evaluate unique properties like NM, compensation, magnetic switching etc. in these compounds.

Experimental details
GdMn 1Àx Cr x O 3 (x ¼ 0.1, 0.2, 0.3 and 0.4) were prepared via facile and robust sol-gel technique. For Cr doped GdMnO 3 sample, the stoichiometric amounts of gadolinium nitrate (Gd(NO 3 ) 3 $6H 2 O, Sigma Aldrich, >99.9%), manganese chloride (MnCl 2 $6H 2 O, Himedia, >98%) and chromium chloride (CrCl 3 $6H 2 O) were mixed with distilled water and citric acid. The ratio of cation to citric acid was kept constant at 1 : 1. First, the solution mixture was continuously stirred at 80 C for 20 minutes. Then, the dropwise addition of ethylene glycol to the mixture solution formed the gel. Aerwards, the obtained gel was dried at 100 C for 6 hours to produce precursor resin. Following grinding of resin using mortar and pestle, the powders were procured which appeared dark brown in color. GdMn 1Àx Cr x O 3 (x ¼ 0.1, 0.2, 0.3 and 0.4) samples were calcined at 1100 C for 5 hours in air. For structural characterization, the powders were studied using Rigaku make powder X-ray diffractometer (XRD) operating in the Bragg Brentano geometry equipped with a 3 kW rotating anode producing Cu K a radiation. The prominent vibrational modes emerging in GdMn 1Àx Cr x O 3 were investigated through Raman spectroscopy using Horiba Jobin Yvon. The excitation and emission spectra were collected with PL spectrometer (Hitachi F-4600). The magnetic property measurement system (MPMS) of Quantum Design, USA working between temperature range from 2 to 300 K were employed to characterize the magnetic properties.  24 Due to orthorhombic structure, we have expressed the lattice parameters as a z b z c/O2. For a < c/O2 < b, the perovskite adopts O type orthorhombic structure whereas the condition i.e. c/O2 < a < b satises the O 0 orthorhombic structure. Aer Rietveld renement, the obtained parameters are tabulated in Table 1. Following the tting parameters, the Rietveld renement appears to be fairly satisfactory looking at the difference plot shown as blue line and the small value of c 2 . The change in lattice parameters with composition of Cr is shown in Fig. 1(b). One may notice from Fig. 1(b) that with increasing Cr concentration while a and b decreases, c increases with reduction in lattice volume from 231.22 to 228.20 A 3 with increasing Cr from 0.1 to 0.4. The signicant decrease in lattice volume is attributed to the difference in ionic radii of Cr and Mn ions. The smaller ionic radius of Cr 3+ (0.61 A) than that of Mn 3+ (0.64 A) shows a noticeable decrease in lattice volume con-rming the substitution of Cr 3+ ions at Mn 3+ site in the lattice. Further, on the basis of parameters obtained from Rietveld renement, we show a representative unit cell of the crystal structure in three dimension (3D) indicating MnO 6 octahedra with spatial orientations and atom positions using Vesta soware depicted in Fig. 1(c) for x ¼ 0.3 and 0.4. 40 In MnO 6 octahedra, O1 atoms (red balls) reside at the two apical positions whereas O2 atoms (green balls) occupy four equatorial positions. The equatorial Mn-O2 bonds have two distinct bond lengths indicated as long (l) and short (s). If all of the Mn-O and average hMn-Oi bond lengths are calculated from XRD patterns, the coherent J-T distortion is estimated by following expression.

Results and discussion
The estimated values of s JT tabulated in Table 1. For GdMnO 3 , there exists a large J-T distortion factor i.e. s JT ¼ 0.2. 24 However, aer incorporating Cr into the host lattice, s JT is found to be $0.066 which reduces to 0.053 and 0.046 with increasing the Cr concentration from 0.1 to 0.2 and 0.3, respectively. For x ¼ 0.4, where O 0 to O orthorhombic structural transformation takes place, J-T distortion reduces to 0.037 which is almost 50% of the J-T factor observed in case of x ¼ 0.1. The considerable reduction in J-T distortion factor is ascribed to the replacement of J-T active element Mn 3+ ions by the non-J-T active, Cr 3+ ions in the lattice. Further, we observe that the difference in a and b i.e., |a À b| deceases from 0.4896 to 0.3685 when Cr concentration increases from 10 to 40 at%. Such decreasing trend clearly indicates that orthorhombic structure tends towards a more symmetrical structure i.e., the tetragonal structure.
To conrm the change in J-T distortion with increasing Cr concentration, we have undertaken of Raman spectroscopic measurement. Raman spectra of GdMn 1Àx Cr x O 3 (x ¼ 0.1, 0.2, 0.3 and 0.4) at room temperature are shown in Fig. 2. In case of x ¼ 0.1, the spectrum exhibits a tilting mode at 365 cm À1 along with the two J-T stretching modes i.e. anti-stretching (as) mode, A 1g at 474 cm À1 and stretching (s) mode, B 2g at 608 cm À1 in Fig. 2. 41 A 1g and B 2g modes arise due to in-plane antisymmetric vibration and stretching of O2, respectively which are correlated with J-T distortion. With increasing concentration of Cr from 0.2 to 0.3, both A 1g and B 2g are found to be red-shied indicating smaller Mn-O bond length (d Mn-O ) keeping the intensity of peaks almost same. Apparently, at x ¼ 0.4, the peak intensity drastically suppressed. Similar reduction in the peak intensity has been reported in GdMnO 3 aer increasing the Cr concentration to 0.2 by Modi et al. 20 Li et al. have also found the broadening of the Raman peaks within 400-900 cm À1 in La 0.75 Ca 0.25 MnO 3 when the magnetic eld increases from 0 to 2.5 kOe at room temperature. Such broadening of peaks indicates the reduction in orthorhombic distortion or MnO 6 octahedral distortion inducing structural changes. 21 In the present case, the drastic reduction in peak intensity in x ¼ 0.4 thus conrms the structural transformation from O 0 to O type orthorhombic leading to signicant decrease in J-T distortion. The ngerprint of J-T distortion induced structural transformation in Cr doped GdMnO 3 is investigated further through photoluminescence studies. For a simplied comparative analysis, we have deconvoluted the emission spectra to reveal dominant emission peaks. The emission spectrum of pure GdMnO 3 shows four distinct peaks at $286, 308, 336 and 353 nm. In case of large J-T distortion, it is known that Mn 3+ exhibits four distinct energy levels at 3.5, 4, 8 and 8.5 eV corresponding to transition from O 2p to Mn(t 2g À JT), Mn(t 2g + JT), Mn(e g À JT), Mn(e g + JT), respectively. 42 The emission peak at $286 can be assigned to O 2p / Mn(t 2g + JT) transition of Mn 3+ . The peak at 308 nm is attributed to the band transition, 4 T 1g / 4 A 2g of Mn 4+ . Owing to broad nature of this peak, the energy level Mn(t 2g À JT) of Mn 3+ having comparable energy to Mn 4+ can also contribute towards this emission. The other peak located at 336 nm while emerges from relaxation of electrons from 4 T 1g energy band to 4 A 2g , peak at 353 nm originates from the electronic transition, 2 T 2g / 4 A 2g , corresponding to Mn 4+ . 43 Aer doping Cr (x ¼ 0.3 and 0.4), three peaks are found to be located at $294, 332 and 355 nm. Surprisingly, the emission peak at $286 nm observed in GdMnO 3 disappears completely by incorporating Cr in host lattice. The disappearance of above peak could be due to the J-T distortion factor of 0.2 observed in GdMnO 3 which reduces to 0.046 and 0.037 in x ¼ 0.3 and 0.4, respectively. Under reduced J-T distortion, the energy gap between Mn(t 2g À JT) and Mn(t 2g + JT) reduces signicantly. Considering this fact, the broad emission peak at $294 nm arises mainly from 4 T 1g / 4 A 2g of Mn 4+ whereas other peaks at $332 and 355 nm remain same. In this context, in our previous work, we have also shown the disappearance of peak at $286 nm aer doping Fe in GdMnO 3 where J-T distortion has been reduced by one order magnitude. 24 Moreira et al. have also mentioned the disappearance of PL emission peak in CaTiO 3 due to undistorted or ordered TiO 6 clusters. 23 J-T distortion factor less by one order magnitude thus inuences the PL spectra in Cr doped GdMnO 3 . Fig. 3(B) shows the emission spectra of GdMn 1Àx Cr x O 3 (x ¼ 0-0.4) under an excitation wavelength of 460, 582 and 625 nm. Aer exciting at wavelength of 460 nm, the emission spectrum of GdMnO 3 exhibits a broad peak ranging from 675-720 nm centered at $692 nm lying in red region. This emission peak is attributed to the spin-forbidden, 2 E g / 4 A 2g transition of Mn 4+ ion. Since Mn 4+ exhibits large effective positive charge, this transition is found to be dominated in the emission spectrum due to strong crystal eld of the host. 43 Aer incorporating Cr into GdMnO 3 , although the intensity of emission peak at $692 nm changes signicantly, no change in peak position is observed. Exciting GdMnO 3 with 582 nm, one broad emission having a maximum at 645 nm is observed indicating a blue-shi of $45 nm. Under an excitation wavelength of 625 nm, for GdMnO 3 , the broadband emission peak is found to be located  ¼ 0.1, 0.2, 0.3 and 0.4) nanoparticles at room temperature revealed from the structure refinement

Photoluminescence properties
Gd (x, y, z) (0.9833 (6)  at $690 nm. In case of x ¼ 0-0.2, the intensity of peak is nearly same which diminishes for x ¼ 0.3 and 0.4. The peak position of emission i.e. $692 nm of Mn 4+ can vary signicantly depending on the host material and excitation wavelength. For example, while this peak is observed at $617 nm in Na 2 SiF 6 , same peak is found to be red shied by $100 nm showing prominent emission at 723 nm in SrTiO 3 . 44 The excitation spectra have been taken by monitoring l em ¼ 692 nm depicted in Fig. 3(C). In GdMnO 3 , the excitation spectrum is comprised of a strong peak at $460 nm along with a shoulder peak at $490 nm. This excitation peak is primarily attributed to 4 A 2g / 2 T 2g electronic transition of Mn 4+ ion in the lattice. Two more sharp peaks are observed to be centered at $582 and $625 nm. The former one originates because of 4 A 2g / 4 T 2g transitions whereas the latter one is ascribed to the local vibration-activated 2 E g 4 4 A 2g transitions of Mn 4+ ion. 44 We do not observe any change in the intensity of peak at $460 nm with increasing Cr concentration. However, one can see an appreciable enhancement in the intensity of excitation peaks at $582 and 625 nm for x ¼ 0.1 and 0.2 which reduces when x ¼ 0.3 and 0.4. It is known that energy bands of Cr 3+ are almost similar to Mn 4+ which may result into overlapping of different energy bands. Therefore, we have proposed an energy band diagram to explain the luminescence behavior of GdMn 1Àx Cr x O 3 showing various energy levels such as O 2p, Mn(t 2g À JT), Mn(t 2g + JT), Mn(e g À JT), Mn(e g + JT) of Mn 3+ under J-T effect, 2 E g , 4 T 2g , 2 T 2g , exciting with 200 nm, the excited electrons do not reach to Mn(e g À JT) and Mn(e g + JT) of Mn 3+ due to large energy gap. However, the energetic electrons are only transferred to Mn(t 2g À JT) and Mn(t 2g + JT) which relax to O 2p showing emissions at 286, 294/308, 332 and 355 nm. Aer exciting with 460 nm, the electrons are not excited to t 2g of Mn 3+ or 4 T 1g of Mn 4+ /Cr 3+ due to signicant energy gap. In pure GdMnO 3 , the excited electrons reach at 2 T 2g of Mn 4+ which follow multistep relaxation via 4 T 2g to 2 E g and eventually come back to 4 A 2g inducing strong emission peak at $692 nm. In the presence of Cr 3+ ion, the similar energy levels of Cr 3+ and Mn 4+ can inuence the intensity of this emission peak by providing additional energetic electrons. Under excitation with higher wavelength of 582 nm, the excited electrons residing at 4 T 2g of Mn 4+ /Cr 3+ de-excited to 2 E g and nally relax to 4 A 2g showing a similar emission band at $645 nm. However, at l ex ¼ 625 nm, this emission band arises due to absorption/emission process occurring between 4 A 2g and 2 E g energy levels. Thus, it is established that in Cr doped GdMnO 3 , the photoluminescence properties of Mn 4+ can be modied by Cr 3+ ion due to the presence of additional energetic electrons providing improved emissions in red region under different excitation wavelengths. Further, decreasing temperature, while M ZFC increases and attains the maximum magnetization at temperature T P , which is broad in nature, M FC increases continuously. In the case of x ¼ 0.2, however, aer bifurcation of M ZFC and M FC at $63 K, M ZFC crosses over the M FC at temperature $31 K, indicating a magnetic phase transition. Upon reducing temperature further, M ZFC attains maxima ($2.2 emu g À1 ) at T P $ 19 K, whereas M FC increases continuously. For x ¼ 0.3, although bifurcation is observed at $95 K, T P remains same as in case of x ¼ 0.2 except a decrease in M max to $1.56 emu g À1 . On the other hand, M FC in the case of x ¼ 0.3 shows a maximum of 0.45 emu g À1 at T max $36 K followed by decrease in magnetization attaining a minimum value of $0.32 emu g À1 at $28 K. The temperature at which minimum magnetization observed is known to be spin-reorientation temperature (T SR ). Although T SR, is not detectable from ZFC and FC plot in x ¼ 0.2, it is observed at $25 K by plotting (M ZFC À M FC )/M ZFC vs. T (shown in the inset Fig. 4(a)). In case of x ¼ 0.4, aer bifurcation of M ZFC and M FC at $110 K, M ZFC attains a maxima of 0.71 emu g À1 at $19 K same as the T P of x ¼ 0.2. Besides, with decreasing temperature, M FC increases showing a M max $1 emu g À1 at $42 K and further decreasing temperature, M FC crosses the temperature axis (T ¼ 0) at $27 K, known to be compensation temperature, T comp . Below T comp , the M FC becomes negative and attains a minimum magnetization, À5.83 emu g À1 , at temperature $2 K. It is important to note that when x is 0.4, an interesting property of magnetization reversal is observed with applying magnetic eld of 500 Oe. Further, decreasing the applied magnetic eld to 50 and 100 Oe, we could observe the magnetization reversal in x ¼ 0.3 as well which is absent under an applied eld of 500 Oe. Moreover, except at x ¼ 0.1, we show that with increase in Cr concentration upto 0.4, one can observe T SR and T comp with varying the applied eld from 50 to 500 Oe. The mechanism of the magnetization reversal can be understood in terms of negative exchange interaction between two components such as Cr and Gd ions. The ferromagnetic order emerging due to canted AFM ordering of Cr 3+ produces an internal eld at paramagnetically ordered Gd 3+ ions which are aligned to that of net Cr 3+ moment. When external eld is applied, the component of Cr 3+ (M Cr ) is pointed along the external eld direction while Gd 3+ ion experiencing the internal eld opposite to that of external eld. Therefore, the net magnetization of the system, M s ¼ M Cr À M Gd varies with temperature and external eld. In the region T > T comp , the M Cr dominates over M Gd resulting into a net positive magnetization in eld direction giving a maxima in M FC (shown in Fig. 4(b)). At a critical temperature magnetization corresponding to M Gd and M Cr cancel out, resulting a zero magnetization known as T comp . Decreasing the temperature further the magnetization becomes negative as the moment of Gd 3+ ion is increased while M Cr 3+ remains the same.

Magnetic properties
As a consequence, M Gd 3+ dominates over the magnetization corresponding to M Cr 3+ . A typical M FC magnetization vs.
temperature plot of Cr 3+ and Gd 3+ under 50 Oe at T > T comp and T < T comp is shown in Fig. 4(b). While with applying low eld, we could observe a magnetization maxima in FC curve and magnetization reversal followed by spin reorientation transition, with application of high eld (500 Oe), the rotation of moment of M Gd 3+ ion takes place along the external eld direction since the applied eld is large enough to overcome the internal eld generates by Cr 3+ ions. Hence a positive magnetization is observed in whole temperature range. With varying Cr concentration, the applied eld strength also changes which essentially evokes a positive magnetization in FC mode. For instance, in x ¼ 0.1 and x ¼ 0.2, the magnetization is always positive independent of applied eld. However, in x ¼ 0.3 while the FC magnetization is positive under 500 Oe, we do not observe positive magnetization upto 500 Oe for x ¼ 0.4. Magnetization reversal has been reported in orthochromites, orthoferrites and orthovanadates. [45][46][47] The T SR is also observed at  To determine the magnetic transition temperature, 1/c vs. temperature curves are plotted for GdMn 1Àx Cr x O 3 (x ¼ 0. 1-0.4) shown in Fig. 6. The antiferromagnetic Néel temperature (T N ) is found to be increased from $49 and $130 K when x is varied from 0.1 to 0.4. The estimated values are much higher than that of GdMnO 3 (i.e. T N $ 42 K). 24 It is interesting that as we dope Cr 3+ in Mn 3+ site, the T N is increased. This may be understood in terms of dilution of the Mn-Mn interaction due to Cr-Mn and Cr-Cr exchange interactions. In addition, from 1/c vs. temperature plots, the effective magnetic moment (m eff ) is calculated aer tting the data in paramagnetic region using Curie-Weiss law i.e. 1/c ¼ (T À Q)/C, where C is Curie constant and Q is Curie-Weiss temperature (insets of Fig. 6). The negative value of Q indicates the antiferromagnetic coupling between the atoms in all samples. The m eff is estimated theoretically using the formula, m eff ¼ [(m Gd ) 2 + (1 À x) (m Mn ) 2 + x(m Cr ) 2 ] 1/2 . The theoretical values of m eff are found to be 9. 25, 9.20, 9.15  K, the magnetization with a slim loop increases linearly with increasing eld which does not saturate upto 70 kOe (inset of Fig. 7). Further, with decreasing the temperature to 4 K, the area under the loop enhances and the loops are symmetrical along magnetic and magnetization axes, indicate the absence of exchange bias. Aer analyzing the hysteresis loops measured at 4 and 20 K, the calculated values of maximum magnetization (M max ), coercivity (H c ) and remanence (M r ) are given in Table 3. One may note that M max increases with increase in Cr concentration except at x ¼ 0.3 corroborates with m eff calculated from Curie-Weiss tting and is in contrast with the theoretical value of m eff . The increase in M max thus supports the presence of Mn 4+ and oxygen vacancies. It is presumed that as the concentration of Mn 4+ decreases compared to Mn 3+ with increase in Cr concentration, magnetization increases as observed in the present case. Generally, the magnetization hysteresis loop and its linear increasing nature at high eld appear due to the weak ferromagnetic ordering which primarily induced by deviation of the collinearity of the moments in an antiferromagnet. Mao  The magnetization reversal has been further investigated by tting the FC magnetization curves employing following equation: where M, M Cr , C, H, H int and Q is called as the total magnetization, magnetization due to the Cr 3+ ions, Curie constant, applied magnetic eld, internal magnetic eld from Cr 3+ ions, and Weiss temperature, respectively. The ttings of M FC at 100 Oe are shown in Fig. 8 and the estimated parameters are tabulated in Table 2 Fig. 9.
Owing to the existence of characteristic features like magnetic reversal, Cr doped GdMnO 3 (x ¼ 0.3 and x ¼ 0.4) has been explored for distinct magnetic switching effect. At 100 Oe, the sample is cooled down to 10 K in FC mode and magnetic switching measurements are recorded shown in Fig. 10. First, the magnetization is measured for 180 s at 10 K. Then, the magnetic eld is quickly increased to 500 Oe and 1700 for x ¼ 0.3 and 0.4, respectively followed by measuring the magnetization for constant time interval of 180 s. Apparently, these samples exhibit a promising magnetic switching effect. The measurement cycles are repeated for several times to examine the switching reproducibility which indicated reversible and continuous switching of the magnetization over cycles. The switching between the positive and negative magnetization states can be triggered through changing the magnitude of the eld with xed eld direction. Moreover, this can be tuned in a predictable way. The magnetic switching effects driven by external magnetic eld render promising application of these H C (Oe) M r (emu g À1 )

Conclusions
In summary, we systematically examined the structural evolution and rich sequence of magnetic transitions in Cr doped GdMnO 3 synthesized using sol-gel method. As Cr concentration is increased from 0.1 to 0.4, we observed the structural transformation from O 0 to O type orthorhombic one along with the reduction in lattice volume. The decrease in lattice volume was due to the smaller ionic radius of Cr 3+ ion compared to Mn 3+ . The structural transformation was manifested by the reduction in J-T distortion factor estimated by the bond length    (2)).
obtained from Rietveld renement. Raman spectra supported the observed reduction in J-T distortion factor as reected showing decrease in the intensity of asymmetric stretching bonds at 487 and 610 cm À1 . Further, the emission peak at $286 nm in PL spectra disappeared indicating decrease in J-T distortion factor. PL study demonstrated the emission spectra related to Mn 4+ energy levels which improved aer incorporating 10 at% of Cr 3+ . Magnetic measurements showed an increase in T N from $42 K for x ¼ 0 to $130 K when x reached 0.4. Besides magnetization reversal with spin reorientation and magnetic switching effect are also observed as x reached 0.3. These materials can be used in magnetic switching, magneto caloric and spintronics devices.

Conflicts of interest
There are no conicts to declare.