Ferrimagnetic Transition Research Articles

The full magnon spectrum of yttrium iron garnet

The magnetic insulator yttrium iron garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used in microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the magnon spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here we have used time-of-flight inelastic neutron scattering to measure the full magnon spectrum throughout the Brillouin zone. We find that the existing models of the excitation spectrum fail to describe the optical magnon modes. Using a very general spin Hamiltonian, we show that the magnetic interactions are both longer-ranged and more complex than was previously understood. The results provide the basis for accurate microscopic models of the finite temperature magnetic properties of yttrium iron garnet, necessary for next-generation electronic devices.

Magnetic properties of (Nd,Ca)(Ba,La)Co2O5+δ tuned by the site-selected charge doping, oxygen disorder, and hydrostatic pressure

Comprehensive study of magnetic properties of the layer-ordered perovskites $\mathrm{N}{\mathrm{d}}_{1--x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{B}{\mathrm{a}}_{1--y}\mathrm{L}{\mathrm{a}}_{y}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ is presented as a function of the site-selected charge doping $\mathrm{[}c=(x--y)/2+\ensuremath{\delta}\ensuremath{-}0.5,x\ensuremath{\le}0.2$ and $y\ensuremath{\le}0.1,0.07l\ensuremath{\delta}l0.84]$, oxygen disorder, and hydrostatic pressure $Pl10\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$. The single-phase oxygen-ordered orthorhombic phase exhibiting complex ferrimagnetic, antiferromagnetic, and metal-insulator phase transitions was found for a narrow oxygen range around $\ensuremath{\delta}\ensuremath{\sim}0.5\ifmmode\pm\else\textpm\fi{}0.1$. Significant difference between impact of hole $(cg0)$ and electron $(cl0)$ doping was observed depending on the site of cation substitution. Gradual enhancement of the Curie temperature ${T}_{\mathrm{C}}$ was observed over the whole range of $c$ to be unaffected by the local oxygen vacancy disorder. Maximum of the N\'eel temperature ${T}_{\mathrm{N}}$ at $c=0$ was found rapidly disappearing at $c=0.05$ for Ca/Nd substitution while it was maintained for La/Ba substitution, indicating that the oxygen vacancy disorder, especially for $\ensuremath{\delta}g0.5$, has a larger effect on antiferromagnetic phase than the charge doping. The temperature of metal-insulator transition ${T}_{\mathrm{MIT}}$ was found practically unchanged by either charge doping or disorder. The application of hydrostatic pressure slightly suppressed ${T}_{\mathrm{C}}$ and increased ${T}_{\mathrm{N}}$ by stabilization of the antiferromagnetic phase with the largest observed value of $\mathrm{d}{T}_{\mathrm{N}}/\mathrm{d}P=5.75\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{kbar}$. Complex magnetic behavior affected by hydrostatic pressure was accounted for by ferro- and antiferromagnetic interactions resulting from the charge separation and spin transitions.

Effect of cobalt substitution on the magnetic properties of nickel chromite

It is of interest to study the magneto-structural coupling in geometrically frustrated antiferromagnets, where structural distortion elevates the ground state degeneracy, leading to a long-range magnetic order. In this regard a cubic spinel compound of the form AB2O4 is currently drawing much attention, where A refers to tetrahedral and B to octahedral sites. In the particular case of NiCr2O4 the material undergoes several structural phase transitions associated with the magnetic ordering. It is therefore necessary to study the magnetic behaviour of NiCr2O4 by substituting either A or B sites, or both systematically with suitable cations. The current work therefore aims at the modification of magnetic properties of NiCr2O4 by doping with Co2+ at A sites. In order to achieve the afore mentioned, (Ni1−xCox)Cr2O4 (0 ≤ x ≤ 1) were prepared using chemical co-precipitation techniques. X–ray diffraction (XRD) results indicate that the samples are in the expected phase without any trace of Cr2O3 impurities after calcination. Transmission electron microcopy (TEM) analyses of these samples show that the particles are mostly bi-pyramidal in shape, with sizes ranging from 50 nm to 100 nm. In the present study the ferrimagnetic transition temperatures (TC) of the various samples were determined utilizing magnetization as function of temperature measurements. TC for NiCr2O4 and CoCr2O4 was determined to be 82.4 ± 0.8 K and 99.5 ± 0.5 K, respectively. These values are higher than those previously reported in the literature for both these compounds. Substitution of Ni by Co, results in an increase in the TC, giving values of 89.2 ± 0.7 K and 90.6 ± 0.9 K for (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4, respectively. The (Ni0.5Co0.5)Cr2O4 sample demonstrated a high coercivity of 3.6 ± 0.1 T and a shift in the hysteresis loop observed under field cooled measurement, not previously reported in literature. X–ray photoelectron spectroscopy (XPS) of (Ni0.5Co0.5)Cr2O4 suggests that the oxidation states of Ni and Co are 2+, while that of Cr is 3+. In order to investigate the local structure around the cations, low temperature extended x–ray absorption fine structure (EXAFS) measurements were performed. From EXAFS it is confirmed that no redistribution of A and B site cations occur at low temperatures. Present findings demonstrate that the magnetic properties of NiCr2O4 can easily and dramatically be modified by doping Co2+ at A sites.

Switching from pyroelectric to ferroelectric order in Ni-doped CaBaCo4O7

We report ferroelectric ordering in Ni substituted CaBaCo4O7. Magnetization showed ferrimagnetic transition at 60 K and an additional transition is found ~ 82 K, further, enhanced antiferromagnetic interactions and decrease in saturation magnetization are noticed with Ni substitution. The dielectric and pyroelectric measurements illustrate a strong coupling between spin and charge degrees of freedom; ferroelectric behavior is confirmed with enhanced ordering temperature (~82 K) and saturation polarization (250 muC/m2. Neutron diffraction has revealed an increase in c-lattice parameter in Ni sample and all the Co/Ni moments are reoriented in a- direction; evidently a non-collinear ferrimagnetic to collinear ferrimagnetic spin order is observed. The coupling between the triangular and Kagome layers weakens and leads to up-up-down-down AFM ordering in the Kagoma layer. This can be viewed as a 2D-collinear layer with unequal bond distances and most likely responsible for the switching of electric polarization.

Magnetic properties of Cd1–x Fe x Cr2S4 (x = 0.5–0.8) solid solutions

Cd1–x Fe x Cr2S4 (x = 0.5–0.8) spinel solid solutions have been synthesized and their magnetic properties have been measured using a Quantum Design PMS-9 instrument at temperatures from 5 to 300 K in a static (7960 A/m) and an ac (100, 1000, and 10000 Hz) magnetic field (79.6 A/m peak) at an external field H = 0. At low temperatures, all of the solid solutions have extended ranges where their dynamic magnetic susceptibility around their ferrimagnetic transition is frequency-dependent. The obtained results are discussed in terms of models that take into account the instability of the long-range magnetic order in the endmembers of the solid solution system: FeCr2S4 and CdCr2S4.

Direct observation of magnon-phonon coupling in yttrium iron garnet

The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of $\sim$560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.

Effects of Cu doping on the electronic structure and magnetic properties of MnCo2O4 nanostructures

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping () on the electronic structure and magnetic properties of the spinel O4. A detailed comparison is given for the and cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ()A]B i.e. substitutes for on the octahedral B-sites. For the bulk samples, the ferrimagnetic K for is lowered to K for the sample, this decrease being due to the effect of Cu doping. For the nanosize () sample, the lower K ( K) is observed using analysis, this lowering being due to finite size effects. For , fits of dc paramagnetic susceptibility data of versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: K (4.1 K), K (16.3 K) and K (13.8 K) for . The temperature dependence of ac susceptibilities and at different frequencies shows that in bulk samples of and , the transition at TC is the normal second order transition. But for the nanosize and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at TC is followed by a re-entrant spin-glass transition at lower temperatures K (138 K) for (). Analysis of the ac susceptibilities, and , versus T data is done in terms of two scaling laws: (i) Vogel–Fulcher law ; and (ii) power law of critical slowing-down . These fits confirm the existence of glassy behavior below TSG with the parameters (8.91), (9.6 × 10) and K (∼138 K) for the samples (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in versus (AT-line) further supports the existence of glassy states in nanosize samples. For , the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with x ⩾ 0.1 have the coercivity HC and remanence . Since the results reported here in these nanostructures are significantly different from those in bulk , further investigations of their magnetic structures using neutron diffraction are warranted.

Increase in the Magnetic Ordering Temperature (Tc) as a Function of the Applied Pressure for A2Mn[Mn(CN)6] (A = K, Rb, Cs) Prussian Blue Analogues.

Magnetization measurements under pressure reveal that the external hydrostatic pressure significantly increases in the ferrimagnetic transition temperature, Tc, for A2Mn[Mn(CN)6] (A = K, Rb, Cs). In the case of monoclinic A = K and Rb, dTc/dp values are 21.2 and 14.6 K GPa-1, respectively, and Tc increases by 53 and 39%, respectively, from ambient pressure to 1.0 GPa. The cubic A = Cs compound also shows a monotonous increase with an initial rate of 4.22 K GPa-1 and about 11.4 K GPa-1 above 0.6 GPa, and an overall Tc increase by 26% at 1.0 GPa. The increase in Tc is attributed to deformation of the structure such that the MnII-N≡C angle decreases with increasing pressure. The smaller the alkali cation, the greater the decrease in the MnII-N≡C angle induced by pressure and the larger the increase of dTc/dp. This is in accordance with the ambient-pressure structures for A2Mn[Mn(CN)6] (A = K, Rb, Cs), which have decreasing MnII-N≡C angles that correlate to the observed increasing Tcs as K > Rb > Cs. The large increase in Tc for the A = K compound is the highest class among several cyano-bridged metal complexes. The tuning of the transition temperature by such a weak pressure may lead to additional applications such as switching devices.

Critical behavior of multiferroic sulpho spinel compounds: MCr2S4 (M = Co [formula omitted] Fe)

Near the paramagnetic to ferrimagnetic transition temperature, the dc magnetization data of two sulpho spinels, CoCr2S4 and FeCr2S4, are analyzed by the modified Arrott plot, the Kouvel-Fisher method, log M vs log H, and the scaling analysis. Critical exponents β = 0.3946±0.0064, 0.421±0.04, γ = 1.101±0.004, 1.041±0.015, and δ = 3.77±0.06, 3.512±0.14 are obtained around the critical temperature ∼ 224.4 K and 166.5 K for CoCr2S4 and FeCr2S4, respectively. Scaling analysis indicates that calculated critical exponents, as well as critical temperatures, are intrinsic to the systems. The values of critical exponents of CoCr2S4 are between those predicted for a three dimensional Heisenberg model and those predicted by mean field theory. Probably, the observed large spin-phonon coupling and magnetocrystalline anisotropy caused by Co ion may play the key role in this critical behavior of CoCr2S4. The critical exponents of FeCr2S4 are close to the mean field theory except for δ, which is higher than the theoretical value. The higher value of δ may arise due to the incomplete ferrimagnetic transition and presence of short range magnetic ordering above TC for FeCr2S4.

Magnetic properties and magnetodielectric effect in Y-type hexaferrite Ba0.5Sr1.5Zn2-xMgxFe11AlO22

The effect of Mg substitution on structural, magnetic, and dielectric properties of Y-type hexaferrite Ba0.5Sr1.5Zn2-xMgxFe11AlO22 (x = 0, 0.4, 0.8, 1.2, 1.6, 2.0) prepared by the solid state reaction method was investigated. The X-ray diffraction patterns are analyzed by Rietveld refinement method and the hexagonal structure with R3−m was confirmed. The magnetic transition temperatures associated with the longitudinal conical to proper screw phase transition (T1) and proper screw to ferrimagnetic phase transition (T2) can be modulated by varying Mg content. For x ≤ 1.2 samples, the transition temperature T2 monotonically increases with increasing Mg substitution, suggesting the possibility that the helical spin order of Mg substituted Y-type hexaferrites is sustained up to higher temperatures than that of undoped samples. The saturation magnetization at room temperature increases to a maximum with Mg content x = 1.2 and then decreases with x. The variations of magnetic properties can be attributed to the difference in the preferential site for Zn and Mg and different distribution of Fe3+ ions over tetrahedral and octahedral sites. The results of magnetic field dependence of dielectric constant, i.e., magnetodielectric effect, at various temperatures imply that the doped samples show magnetically induced ferroelectricity up to 250 K. The intrinsic magnetodielectric effect is observed in all the samples. It is worthy to note that the magnetodielectric effect measured below 200 K increases with increasing Mg content, which is probably attributed to the variation of magnetic structures. The Mg-doped hexaferrites Ba0.5Sr1.5Zn2-xMgxFe11AlO22 may be potential candidates for application in magnetoelectric devices.

Ordering phenomena in a heterostructure of frustrated and unfrustrated triangular-lattice Ising layers.

We study critical and magnetic properties of a bilayer Ising system consisting of two triangular planes A and B, with the antiferromagnetic (AF) coupling J_{A} and the ferromagnetic (FM) one J_{B} for the respective layers, which are coupled by the interlayer interaction J_{AB} by using Monte Carlo simulations. When J_{A} and J_{B} are of the same order, the unfrustrated FM plane orders first at a high temperature T_{c1}∼J_{B}. The spontaneous FM order then exerts influence on the other frustrated AF plane as an effective magnetic field, which subsequently induces a ferrimagnetic order in this plane at low temperatures below T_{c2}. When short-range order is developed in the AF plane while the influence of the FM plane is still small, there appears a preemptive Berezinskii-Kosterlitz-Thouless-type pseudocritical crossover regime just above the ferrimagnetic phase transition point, where the short-distance behavior up to a rather large length scale exponentially diverging in ∝J_{A}/T is controlled by a line of Gaussian fixed points at T=0. In the crossover region, a continuous variation in the effective critical exponent 4/9≲η^{eff}≲1/2 is observed. The phase diagram by changing the ratio J_{A}/J_{B} is also investigated.

Variety of elastic anomalies in an orbital-active nearly itinerant cobalt vanadate spinel

We perform ultrasound velocity measurements on a single crystal of nearly-metallic spinel Co$_{1.21}$V$_{1.79}$O$_4$ which exhibits a ferrimagnetic phase transition at $T_C \sim$ 165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above $T_C$ but also the ferrimagnetic phase below $T_C$, which should be driven by the nearly-itinerant character of the orbitally-degenerate V 3$d$ electrons. In the paramagnetic phase above $T_C$, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above $T_C$, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly-itinerant character of the V 3$d$ electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally-degenerate 3$d$ electrons. In the ferrimagnetic phase below $T_C$, the elastic moduli exhibit distinct anomalies at $T_1\sim$ 95 K and $T_2\sim$ 50 K, with a sign change of the magnetoresistance at $T_1$ (positive below $T_1$) and an enhancement of the positive magnetoresistance below $T_2$, respectively. These observations below $T_C$ suggest the successive occurrence of an orbital glassy order at $T_1$ and a structural phase transition at $T_2$, where the rather localized character of the V 3$d$ electrons evolves below $T_1$ and is further enhanced below $T_2$.

Room temperature magnetoresistance and exchange bias in “314-type” oxygen-vacancy ordered SrCo0.85Fe0.15O2.62

Herein, we report the magneto-transport and exchange bias effect in a “314-type” oxygen vacancy ordered material with composition SrCo0.85Fe0.15O2.62. This material exhibits a ferrimagnetic transition above room temperature, at 315 K. The negative magnetoresistance starts to appear from room temperature (−1.3% at 295 K in 70 kOe) and reaches a sizable value of 58% at 4 K in 70 kOe. A large exchange bias effect is observed below 315 K when the sample is cooled in the presence of a magnetic field. The coexistence of nearly compensated and ferrimagnetic regions in the layered structure leads to a magnetoresistance and an exchange bias in this sample. The appearance of a sizable magnetoresistance and a giant exchange bias effect, especially near room temperature, indicates that “314-type” cobaltates are a promising class of material systems for the exploration of materials with potential applications as magnetic sensors or in the area of spintronics.

Magnetic ordering of Mo2NiB2-type {Gd, Tb, Dy)2Co2Al compounds by magnetization and neutron diffraction study

The magnetic ordering of Mo2NiB2-type {Gd, Tb, Dy}2Co2Al (Immm, No. 71, oI10) compounds has been established using bulk magnetic measurements and neutron diffraction study. Polycrystalline Gd2Co2Al, Tb2Co2Al and Dy2Co2Al undergo ferrimagnetic transitions (TC) at 78K, 98K and 58K, respectively, and low-temperature field induced transition (Tm) around 15K, 20K and 15K, respectively. Between Tm and TC Gd2Co2Al, Tb2Co2Al and Dy2Co2Al are soft ferrimagnets. Below Tm Gd2Co2Al is soft ferrimagnet, whereas Tb2Co2Al and Dy2Co2Al exhibit permanent magnet properties with residual magnetization per rare earth of 4.95B and 4.8B, respectively, and large coercive field of 72kOe and 22kOe, respectively, at 2K.The magnetocaloric effects of Gd2Co2Al, Tb2Co2Al and Dy2Co2Al were calculated in terms of isothermal magnetic entropy change and they reach maximum values of −10.4J/kgK, −7.6J/kgK and −6.6J/kgK for a field change of 50kOe near 75K, 98K and 58K, respectively. Low-temperature transition of Gd2Co2Al is followed by the magnetic entropy change of −2.9J/kg K in a field change of 50kOe at 15K. Low temperature magnetic ordering with enhanced anisotropic effects in Tb2Co2Al and Dy2Co2Al is accompanied by a positive magnetocaloric effect with isothermal magnetic entropy changes of +19.9J/kgK at 20K (field change 0–50kOe) and +2.7J/kgK at 15K (field change 0–10kOe), respectively.Neutron diffraction study shows that, in zero applied field, Tb2Co2Al exhibits c-axis ferrimagnetic ordering with magnetic space group Immm′ and propagation vector K0=[0, 0, 0] below TCND∼111K with MTb=8.86(15)B and MCo=0.26(2)B at 2K.

Effect of Al Substitution in Structural and Magnetic Properties of MnCr2O4

Single-phase samples of Mn(Cr1−x Al x )2O4 (x = 0 – 0.30) with cubic spinel structure were prepared and the lattice constant is found to decrease from a = 8.4396 A for x = 0 to a = 8.3801 A for x = 0.30. The substitution of Al at Cr site is confirmed from the blue shift of Raman modes. Magnetization measurements and analysis show all the prepared samples exhibit ferrimagnetic transition with transition temperature in the range of 46 K for x = 0 to 33 K for x = 0.30. The saturation magnetization (M s ) and the estimated anisotropy constant (K) show an anomalous behavior up to x = 0.10 and beyond that they decrease monotonously. They are explained by considering different site preferences of Al 3+ ions as the doping concentration is increased. The theoretical and experimental effective magnetic moment of the samples is found to be comparable and it decreases with increase in Al concentration.

Effect of Al Substitution on Structural, Magnetic, and Magnetocaloric Properties of Er6Fe23−xAlx (x = 0 and 3) Intermetallic Compounds

The structural, magnetic, and magnetocaloric properties of Er6Fe23−xAlx (x = 0 and 3) intermetallic compounds have been studied systematically. Samples were prepared using the arc furnace by annealing at 1073 K for one week. Rietveld analysis of XRD shows the formation of pure crystalline phase with cubic Fm-3m structure. Refinement results show that the unit cell volume decreases with increasing Al content. The Curie temperature Tc of the prepared samples was found to be strongly dependent on the aluminum content. This reduces magnetization and the ferrimagnetic phase transition temperature (Tc) from 481 K (for x = 0) to 380 K (for x = 3), is due to the substitution of magnetic element (Fe) by non-magnetic atoms (Al). With the increase of the Al content, a decrease in the values of magnetic entropy is observed. The magnitude of the isothermal magnetic entropy (|∆SM|) at the Tc decreases from 1.8 J/kg·K for x = 0 to 0.58 J/kg·K for x = 3 for a field change 14 kOe. Respectively, the relative cooling power (RCP) decreases with increasing Al content reaching 42 Jkg−1 for x = 0 to 28 Jkg−1 for x = 3.

Impedance spectroscopy and ac conductivity mechanism in Sm doped Yttrium Iron Garnet

Abstract Polycrystalline samples of Y 3- x Sm x Fe 5 O 12 for x =0.0 to 3.0 were prepared in single phase form with cubic structure. The analysis of XRD patterns using Reitveld refinement reveals that these samples crystallize in cubic structure with Ia 3 d space group and lattice constant increases systematically with Sm concentration. The depressed and asymmetric semicircle of complex impedance spectra suggests that the relaxation of charge carriers is of non-Debye type and it is controlled by thermally activated charge carriers. Both grains and grain boundaries are found to contribute for the measured complex impedance spectrum, however at higher temperature (T ≥ 543 K), the relaxation process is dominated by the grain boundaries. The temperature dependence of relaxation frequency and conductivity exhibit a change of slope in the vicinity of ferrimagnetic transition temperature which is attributed to the magneto-electric coupling. The analysis of ac conductivity spectra in terms of Jonscher power law suggests that the conduction process is governed by small polaron hopping model for x ≤ 0.5 and for higher concentration overlapping of large polaron tunneling model is found to play a dominant role. The dielectric constant is found to increase with increase with increase in Sm concentration.

Study of magnetic compensation behavior in Mn(Cr1−xFex)2O4

We report the preparation of single phase samples of Mn(Cr1−xFex)2O4 for x=0–0.50 and study of their structural and magnetic properties. These samples are found to crystallize in cubic structure with Fd3¯m space group with typical lattice constant varying from a=8.4396Å for x=0 to 8.4588Å for x=0.50. The ferrimagnetic transition temperature as per magnetic measurements is found to increase from 46K for x=0 to 402K for x=0.50 due to the strengthening of the superexchange interaction. The Saturation magnetization value is found to decrease monotonously with increase in Fe concentration beyond x=0.10 and approaches magnetic compensation at x=0.40. In addition to composition induced magnetic compensation, we have observed magnetic compensation due to change in temperature for x=0.40 with a compensation temperature of 267K. This is explained by considering the substitution of Fe3+ ions in one of the Cr3+ sites and the different temperature dependence of two sublattice moments.

Eu-doping effects on the structural and multiferroic properties of YMn0.5Cr0.5O3

Y1−xEuxMn0.5Cr0.5O3 (x = 0.1–1.0) oxides were synthesized in a polycrystalline form by a conventional solid-state reaction method. X-ray diffraction patterns revealed that the whole series of compounds featured a simple distorted perovskite structure with the orthorhombic space group Pbnm. With increasing Eu content, the shorter a and c axes increased, while the longest b axis changed little. The ferrimagnetic transition temperature TN increased gradually to x = 0.8 before declining with a further increase in x. A clear correlation between TN and the negative molecular field coefficient within the Cr3+ sublattice indicated that an antiferromagnetic exchange interaction between the Cr3+ cations plays a dominant role in the magnetic ordering. By reversing the sign of the poling field, the occurrence of electric polarization reversal at temperatures around TN implied a possible multiferroic state in the samples. From the dielectric point of view, the disappearance of the dielectric constant anomaly at TN might be related to the high AC conductivity of the samples, which is a common feature of orthochromites.

Structural, magnetic, and dielectric properties of multiferroic Co1−xMgxCr2O4 nanoparticles

We examined the structural, magnetic, and dielectric properties of Co1−xMgxCr2O4 nanoparticles with composition x=0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1 in detail. X-ray diffraction (XRD) revealed normal spinel structure for all the samples. Rietveld refinement fitting results of the XRD showed no impurity phases which signifies the formation of single phase Co1−xMgxCr2O4 nanoparticles. The average crystallite size showed a peak behaviour with maxima at x=0.6. Raman and Fourier transform infrared (FTIR) spectroscopy also confirmed the formation of single phase normal spinel for all the samples and exhibited dominant vibrational changes for x≥0.6. For x=0 (CoCr2O4), zero field cooled/field cooled (ZFC/FC) magnetization curves showed paramagnetic (PM) to ferrimagnetic (FiM) transition at Tc=97K and a conical spiral magnetic order at Ts=30K. The end members CoCr2O4 (x=0) and MgCr2O4 (x=1) are FiM and antiferromagnetic (AFM), respectively. Tc and Ts showed decreasing trend with increasing x, followed by an additional AFM transition at TN=15K for x=0.6. The system finally stabilized and changed to highly frustrated AFM structure at x=1 due to formation of pure MgCr2O4. High field FC curves (5T) depicted nearly no effect on spiral magnetic state, which is attributed to strong exchange B-B magnetic interactions at low temperatures. Dielectric parameters showed a non-monotonous behaviour with Mg concentration and were explained with the help of Maxwell-Wagner model and Koop’s theory. Dielectric properties were improved for nanoparticles with x=0.6 and is attributed to their larger average crystallite size. In summary, Mg doping has significantly affects the structural, magnetic, and dielectric properties of CoCr2O4 nanoparticles, which can be attributed to variations in local magnetic exchange interactions and variation in average crystallite size of these chromite nanoparticles.