X-ray absorption near edge spectra (XANES) and magnetization of Zn doped MnV2O4 have been measured and from the magnetic measurement the critical exponents and magnetocaloric effect have been estimated. The XANES study indicates that Zn doping does not change the valence states in Mn and V. It has been shown that the obtained values of critical exponents β, γ and δ do not belong to universal class and the values are in between the 3D Heisenberg model and the mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperature are unambiguous and intrinsic to the system. All the samples show large magneto-caloric effect. The second peak in magneto-caloric curve of Mn0.95Zn0.05V2O4 is due to the strong coupling between orbital and spin degrees of freedom. But 10% Zn doping reduces the residual spins on the V-V pairs resulting the decrease of coupling between orbital and spin degrees of freedom.

The magnetocaloric effect (MCE) has attracted much interest because of its commercial uses in magnetic refrigeration.1 The materials showing second order magnetic transition exhibit conventional MCE and the magnetic contribution in it is of magnetic origin.2 Moreover, the materials involving first order magnetic transition show giant MCE with significant contribution from the lattice.3 Recently, in spinel ferromagnetic MnV2O4, which shows the orbital degeneracy, and the interplay of spin, orbital and lattice degrees of freedom, large magneto-caloric effect is reported around Tc (= 57 K).4 Other than the conventional spin-orbit coupling this interplay also arises due to the geometrical anisotropy in magnetic interaction (the so-called Kugel-Khomskii type coupling).5 Moreover, in these materials magnetic and structural these two phase transitions are separately obserevd.6–8 This MnV2O4 lies in the Mott insulator regime. MnV2O4 exhibits a magnetic transition at Tc = 57 K (second order transition), and then a structural phase transition from a cubic to a tetragonal phase at Ts = 53 K (first order transition), with the spin structure becoming non-collinear.9 It was also found that the structural transition could be induced by few tesla magnetic field.10,11 Moreover, the orbital ordering of MnV2O4 cannot be explained simply by anti-ferro orbital model.12 The large magneto-caloric effect in this compound is suggested to be related to the orbital entropy change due to the change of the orbital state of V3+ ions with an applied field around Tc (= 57 K).4 Furthermore, when Zn is doped on the Mn site the value of magneto-caloric effect increases and the maximum is observed at the lower transition temperature (TS) which has been attributed to orbital ordering.13 In this perspective the detail study of Zn doping on magneto-caloric effect of MnV2O4 will be interesting. Moreover, the critical behaviour, which has direct correlation with the MCE, of MnV2O4 might also be distinctive to provide interesting information about the magnetic spin ordering in this spinel Vanadate. Baek et al.14 have reported the critical exponents of this system. From the ac-susceptibility measurement they obtained very unusal values of the critical constants (β = 0.36 and γ = 0.59).14 In a recent paper Zhang et al. reported the critical constants β ∼ 0.349 (close to 3D Heisenberg model) and γ ∼ 0.909 (close to mean field model).15 Moreover, Garlea et al. determined the β value from the integrated intensity I(T) fitting.16 The I(T) can be described near the two phase transitions as I(T) ∞ (TC,S-T). The obtained value by them was close to 3D Heisenberg and 3D Ising models. To resolve the issue we have also studied the variation of critical constants with Zn doping using modified Arrot plots.

The polycrystalline Mn1-xZnxV2O4 samples were synthesized by solid state reaction technique. The MnO, ZnO and V2O3 powders were mixed and pressed into pellets. The pellets were sintered in evacuated quartz tube at 950 °C for 40 hours. The samples were characterized with X-ray powder diffraction (XRD) using Rigaku MiniFlex II DEXTOP X-ray Diffractometer. The XANES measurements have been carried out at the Energy-Scanning EXAFS beamline (BL-9) in transmission mode at the INDUS-2 Synchrotron Source (2.5 GeV, 100 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. This beamline operates in energy range of 4 KeV to 25 KeV. The beamline optics consist of a Rh/Pt coated collimating meridional cylindrical mirror and the collimated beam reflected by the mirror is monochromatized by a Si(111) (2d = 6.2709) based double crystal monochromator. The second crystal of DCM is a sagittal cylinder used for horizontal focusing. Three ionization chambers (300 mm length each) have been used for data collection in transmission mode, one ionization chamber for measuring incident flux (I0), second one for measuring transmitted flux (It) and the third ionization chamber for measuring EXAFS spectrum of a reference metal foil for energy calibration. Magnetic measurement was done using Magnetic Properties Measurement System (MPMS) SQUID (Quantum Design) magnetometer.

Fig. 1 shows the XRD patterns of Mn1-xZnxV2O4 indicating the single phase of all the samples. We have refined the diffraction data with the Reitveld refinement program (the fitted curve has also been shown in Fig. 1) and the fitted parameters are shown in Table I. The lattice parameter “a” obtained from the Reitveld refinement of the XRD data is shown as a function Zn content in the inset of Figure 1. From the figure, it is clear that with increase of Zn content the lattice parameter decreases, which is due to lower ionic size of Zn2+(0.74 Å) compared to Mn2+(0.80 Å). The linear dependence of the lattice constant on Zn concentration indicates that there is no structural change in MnV2O4 with Zn doping and as a matter of fact Zn is doped into the crystal lattice following the Vegard's law.17 In order to probe the oxidation state of all the transition metals i.e. Mn, Zn and V, we have performed XANES measurements at their corresponding K edges. Figure 2 shows the edge step normalized XANES spectra for Mn K-edge for the Mn1-xZnxV2O4 (x = 0, 0.05 & 0.1) samples. The XANES spectra of all the samples are plotted with three standards Mn metal foil, MnO and Mn2O3 with +0, +2 and +3 oxidation state respectively. A typical K edge XANES spectra exhibits structured pre-edge region and the dominant peak called white line peak followed by main rising edge.18 Here, we emphasize only the main edge part of the XANES spectra for our purpose. The pre-edge features below the main edge are due to Mn 1s transition into unoccupied O 2p-Mn 3d (or Mn 3d/4p) hybridized states, which have p components projected at the Mn site as observed in many transition-metal oxides. The main edge feature at the Mn K-edge corresponds to the high-energy Mn 4p states. Energy position of the main edge in XANES spectra of a sample may be determined either as the energy corresponding to ∼0.5 absorption or maximum energy value of a first order differentiated spectrum.18 The main edge position of Mn1-xZnxV2O4 for x = 0, 0.05 and 0.1 samples coincides with MnO, for which Mn ion has been assigned to have a valence or effective charge of +2.

FIG. 1.

The X-ray diffraction pattern with the Reitveld refinement for the Mn1-xZnxV2O4 (with x = 0.0, 0.05.0.1). The inset shows the variation of lattice parameters with Zn concentration.

FIG. 1.

The X-ray diffraction pattern with the Reitveld refinement for the Mn1-xZnxV2O4 (with x = 0.0, 0.05.0.1). The inset shows the variation of lattice parameters with Zn concentration.

Close modal
Table I.

Structural parameters (lattice parameters, V-V bond lengths) of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1) samples obtained from Reitveld refinement of X-Ray Diffraction data. The structural data have been refined with Space group Fd-3m at 300 K.

 X = 0.0X = 0.05X = 0.1
a (Å) 8.5220(7) 8.5150(3) 8.5092(9) 
V-V (Å) 3.0130 (1) 3.0105 (1) 3.0105 (1) 
 X = 0.0X = 0.05X = 0.1
a (Å) 8.5220(7) 8.5150(3) 8.5092(9) 
V-V (Å) 3.0130 (1) 3.0105 (1) 3.0105 (1) 
FIG. 2.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0, 0.05 & 0.1 at Mn K-edge with along with reference Mn metal, MnO and Mn2O3 sample.

FIG. 2.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0, 0.05 & 0.1 at Mn K-edge with along with reference Mn metal, MnO and Mn2O3 sample.

Close modal

Figure 3 shows the edge step normalized XANES spectra for Zn K-edge for the Mn1-xZnxV2O4 for x = 0.05 and 0.1 along with two standards namely Zn metal and ZnO with +0 and +2 oxidation state respectively. This edge position in Mn1-xZnxV2O4 for x = 0.05 & 0.1 clearly shows that Zn is present in +2 oxidation state as its edge coincide with ZnO edge.

FIG. 3.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0.05 & 0.1 at Zn K-edge with along with Zn metal and ZnO standard sample.

FIG. 3.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0.05 & 0.1 at Zn K-edge with along with Zn metal and ZnO standard sample.

Close modal

Figure 4 shows the same edge step normalized XANES spectra for V K-edge for the Mn1-xZnxV2O4 (x = 0, 0.05 and 0.1) along with standards V metal and VOSO4 with +0 and +4 oxidation state respectively. A visual inspection of Figure 3, the main edge energy corresponding to V K edge is close to +4 but very far from +0. Energy positions of these samples, discussed above, well matches with the earlier reports for V3+.19 A table for their energy positions with the corresponding references is presented in Table II, indicating +3 oxidation state of Vanadium in our studied samples. From the above discussion it is clear that no change in the valence states of Mn and V is occurring due to doping of Zn.

FIG. 4.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0, 0.05 & 0.1 at V K-edge with along with V metal and VOSO4 standard sample.

FIG. 4.

Normalised XANES spectra of Mn1-xZnxV2O4 for x = 0, 0.05 & 0.1 at V K-edge with along with V metal and VOSO4 standard sample.

Close modal
Table II.

Vanadium 1st derivative spectra peak energies.

V metalV metal3 V3+2O33x = 0x = 0.05x = 0.10V4+OSO4
5464.60 5465 5475.7 5475.37 5475.32 5475.34 5478.13 
V metalV metal3 V3+2O33x = 0x = 0.05x = 0.10V4+OSO4
5464.60 5465 5475.7 5475.37 5475.32 5475.34 5478.13 

Fig. 5 shows the temperature dependence of magnetization of Mn1-xZnxV2O4 under zero field cooled (ZFC) condition at 100 Oe. The M-T curve of MnV2O4 exhibits a sharp paramagnetic-ferrimagnetic (PM-FM) phase transition. It is observed that the magnetization drops sharply for all the Zn doped MnV2O4 samples on cooling. This is due to the spin-pairing of V-V bonds.20 Inset of Fig. 5 shows the evolution of TC with Zn content for the Mn1-xZnxV2O4.

FIG. 5.

The temperature variation of magnetization of Mn1-xZnxV2O4 at 100 Oe magnetic field. Inset showing the TN (up arrow) and TS (down arrow) of all the samples.

FIG. 5.

The temperature variation of magnetization of Mn1-xZnxV2O4 at 100 Oe magnetic field. Inset showing the TN (up arrow) and TS (down arrow) of all the samples.

Close modal

The M(H) curves at different temperatures (around the Tc) have been shown in Fig. 6. According to the Scaling hypothesis,21 a second order magnetic phase transition near the Curie point is characterized by a set of critical exponents of β, γ and δ and the magnetic ordering can be studied

\begin{equation}({\rm H}/{\rm M})^{1/{\rm \gamma }} = {\rm C}({\rm T} - {\rm T}_{\rm c}) + {\rm C}_2 {\rm M}^{1/{\rm \beta }}\end{equation}
(H/M)1/γ=C(TTc)+C2M1/β
(1)

which combines the relations for the spontaneous magnetization below Tc

\begin{equation*}{\rm M}\sim({\rm T}_{\rm c} - {\rm T}),\end{equation*}
M(TcT),

and the inverse magnetic susceptibility above Tc

\begin{equation*}{\rm \chi }^{ - 1} \sim ({\rm T} - {\rm T}_{\rm c})\end{equation*}
χ1(TTc)

To find the correct values of β and γ, linear fits to the isotherms are made to get the intercepts giving M(T) and χ(T). These new values of β and γ are then used to make a new modified Arrott plot. New values of critical exponents thus obtained are re-introduced in the scaling of the modified Arrott plot. The process is repeated until the iteration converges, leading to an optimum fitting value.

FIG. 6.

Magnetization as a function of applied magnetic field for the Mn1-xZnxV2O4 (with x = 0.0, 0.05.0.1) at different temperatures (Isotherms have been measured every 2 K interval around Curie temperature).

FIG. 6.

Magnetization as a function of applied magnetic field for the Mn1-xZnxV2O4 (with x = 0.0, 0.05.0.1) at different temperatures (Isotherms have been measured every 2 K interval around Curie temperature).

Close modal

Fig. 7 shows the final result for the Mn1-xZnxV2O4 samples. We have taken M(H) isotherms from 50 K to 70 K in every 2 K interval. The calculated values of β and γ are, respectively, 0.393 and 1.01 for x = 0 sample. For x = 0.05 the values are respectively, 0.40 and 1.02 whereas for x = 0.1 the values of β and γ are respectively, 0.42 and 1.07. Below 54 K it deviates from linearity due to the first order transition at ∼52 K.

FIG. 7.

Final results for critical constants of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1). (colour online)

FIG. 7.

Final results for critical constants of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1). (colour online)

Close modal

The critical values we obtained are not from the universality class. According to the scaling theory the magnetization equation can be written as M(H,ɛ)ɛ−β = f±(H/ɛβ+γ), where ɛ is the reduced temperature, f+ for T > Tc and f for T < Tc are regular functions.22 The equation states that the plot between M|ɛ|−β vs H|ɛ|−(β+γ) gives two universal curves: one for T > Tc and other for T < Tc. As shown in Fig. 8, the curves are divided in two parts one above Tc and one below Tc. The inset of Fig. 8 also shows log-log plot and this also falls into two classes one above Tc and one below Tc, in agreement with the scaling theory. Therefore the FM behaviour around Curie temperature get renormalized following the scaling equation of state indicating that the calculated critical exponents are reliable. Moreover, exponents often show various systematic trends or crossover phenomenon's one approaches Tc.23,24 This occurs due to the presence of various competing couplings and/or disorder. For this reason, it is useful to introduce temperature-dependent effective exponents for ɛ ≠ 0. It can be mentioned that effective exponents are non universal properties, and they are defined as:

\begin{equation}{\rm \beta }^{{\rm eff}} {\rm (\varepsilon }) = \frac{{d[\ln Ms(\varepsilon)]}}{{d[ln\varepsilon]}}\cdot {\rm \gamma }^{\rm eff} ({\rm \varepsilon }) = \frac{{d\left[ {ln\chi _0^{ - 1} (\varepsilon)} \right]}}{{d[ln\varepsilon]}}\end{equation}
β eff (ɛ)=d[lnMs(ɛ)]d[lnɛ]·γ eff (ɛ)=dlnχ01(ɛ)d[lnɛ]
(2)

We have calculated the βeff and γeff by using the Eq. (2) (not shown), which do not match with universality class.

FIG. 8.

Universal curves and inset shows the log–log plot of universal curves of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1).

FIG. 8.

Universal curves and inset shows the log–log plot of universal curves of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1).

Close modal

It is observed from the above discussion that the critical exponents of the present investigated sample are not consistent with the universality class. The similar behaviour has been observed in perovskite Pr0.5Sr0.5MnO3 (β = 0.397and γ = 1.331)25 and in La0.7Sr0.3MnO3 (β = 0.45 and γ = 1.2)26 due to phase separation. Other than perovskite, Gd80Au20 also shows unusual critical exponents (β = 0.44(2) and γ = 1.29(5)) arise due to the dilution of global spin with the substitution of non-magnetic ions.27 In the present case very close to the second order PM-FM transition (at ∼ 58 K) there exists a first order structural transition (∼52 K) which is associated with the collinear to non-collinear spin transition. This may cause a large spin fluctuation which may be responsible for the unusual critical exponents between the actual material and the theoretical model.

In order to further investigate we have also estimated the magneto caloric effect of all the samples. The magnetic entropy change is given by

\begin{equation}\left| {\Delta Sm} \right| = \sum\nolimits_i {\frac{{Mi - Mi + 1}}{{Ti + 1 - Ti}}\Delta Hi}\end{equation}
ΔSm=iMiMi+1Ti+1TiΔHi
(3)

Where Mi is the Magnetization at Temperature Ti.28 The obtained |ΔSm| has been plotted as a function of temperature in Fig. 9. In the inset of Fig. 6 the magnetic field variation of |ΔSm| shows the H2/3 dependency (the value of the exponent for x = 0, 0.05 and 0.1 are respectively, 0.633, 0.67 and 0.666). This is consistent with the relation between magnetic entropy and the magnetic field near the magnetic phase transition which is given by29 

\begin{equation}| {\Delta {\rm S_m}} | = - 1.07{\rm qR}({\rm g\mu }_{\rm B} {\rm JH}/k{\rm T}_{\rm c})^{2/3}\end{equation}
|ΔSm|=1.07 qR (gμB JH /kTc)2/3
(4)

where q is the number of magnetic ions, R is the gas constant, and g is the Landau factor. In Fig. 9 it is observed that as Zn content increases the magnetic entropy value decreases, which is also consistent with the Eq. (4). As with increase of Zn content the magnetic Mn ions decrease. Moreover, in Mn1-xZnxV2O4 for x = 0 and 0.1 only one peak is found and that is at Tc but for x = 0.05 two peaks are observed (one is at Tc and another is at Too). The observed behaviour for x = 0 and x = 0.05 is consistent with those reported.4,13 It has been explained that the large magneto caloric effect in MnV2O4 is due to the change of the orbital state of V3+ ions with applied field around Tc which leads to the change in orbital entropy.4 The observed second peak in x = 0.05 is suggested to be due to the strong coupling between orbital and spin degrees of freedom.13 But in that case in MnV2O4 sample also we should get two peaks. Moreover, in the present investigation x = 0.1 sample does not show second peak. It might be the fact that in MnV2O4 the two transitions are very close to each other and because of that two peaks overlap into a single peak. Moreover for 10% Zn doping the chemical pressure increases which reduces the residual spins on the V-V pairs which leads to the decrease of long range magnetic ordering. As a matter of fact the coupling between orbital and spin degrees of freedom decreases. This might be the reason of diminishing the peak at low temperature when 10% Zn is doped

FIG. 9.

Magneto caloric effect of Mn1-xZnxV2O4 at 2T and 4T magnetic fields. Inset shows the fitting of ΔS (entropy change) vs magnetic field (H) curve of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1).

FIG. 9.

Magneto caloric effect of Mn1-xZnxV2O4 at 2T and 4T magnetic fields. Inset shows the fitting of ΔS (entropy change) vs magnetic field (H) curve of Mn1-xZnxV2O4 (with x = 0, 0.05, 0.1).

Close modal

The XANES study indicates that no change in the valence states of Mn and V is occurring due to doping of Zn and V remains in 3+ state. The giant magneto-caloric effect value is observed in these spinel vanadates and the entropy change (MCE value) decreases with increase of Zn content. It has been shown that the obtained values of the critical exponents β, γ and δ do not belong to universal class and the values are in between the 3D Heisenberg model and mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperatures are unambiguous and intrinsic to the system. The observed double peaks in MCE of Mn0.95Zn0.05V2O4 are due to the strong coupling between orbital and spin degrees of freedom. In this composition (x = 0.05) the orbital ordering becomes maximum which in effect increase the coupling between orbital and spin degrees of freedom at Too leading the second peak in magneto-caloric behavior in x = 0.05 sample. When Zn content increases (viz. x = 0.1) the chemical pressure increases which reduces the residual spins on the V-V pairs which leads to the decrease of long range magnetic ordering. As a consequence coupling between orbital and spin degrees of freedom decreases.

SC is grateful to the funding agencies DST (Grant No.: SR/S2/CMP-26/2008) and CSIR (Grant No.: 03(1142)/09/EMR-II) and BRNS, DAE (Grant No.: 2013/37P/43/BRNS) for financial support. PS is grateful to CSIR, India for providing Research Fellowship. Authors are also grateful to D. Budhikot for his help in magnetization measurement.

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