Room-temperature ferromagnetic-ferroelectric multiferroic material

Abstract

A multiferroic material for magnetic and electric switching including Iron selenide (Fe.sub.3Se.sub.4) nanoparticles and its derivatives or doped with at least one element selected from transitional metals, rare earths elements or combination of the two and chalcogens. Ferroelectric polarization and coupling of magnetic and ferroelectric behavior in the doped Fe3Se4 is observed at a temperature ranging from 15 to 30° C.

Claims

1. A multiferroic material consisting of doped Fe3Se4, wherein the Fe3Se4 is doped with at least one element selected from the group consisting of Vanadium, Gadolinium, Dysprosium, and chalcogens selected from Sulfur and Tellurium, or combinations thereof, wherein ferroelectric polarization and coupling of magnetic and ferroelectric behavior in the doped Fe3Se4 is observed at a temperature ranging from 15 to 30° C.

2. The material according to claim 1, wherein the doped Fe3Se4 is in form of nanoparticles, nanorods, thin film or bulk.

3. The material according to claim 1 for use in magnetic and electric switching.

4. A process for the preparation of multiferroic material, the multiferroic material consisting of Fe3Se4 doped with at least one element selected from the group consisting of Vanadium, Gadolinium, Dysprosium, and chalcogens, or combinations thereof, the process comprising the steps of: i) charging iron acetylacetonate and Selenium powder in a solvent under inert atmosphere to obtain a mixture; ii) charging into the mixture of step i), at least one element selected from the group consisting of Vanadium, Gadolinium, Dysprosium, and chalcogens selected from Sulfur and Tellurium, or combinations thereof, to obtain a charged mixture; iii) heating the charged mixture as obtained in step ii) for 1-4 hours at a temperature in the range of 120−300° C. to obtain a heated mixture; iv) cooling the heated mixture of step iii) to room temperature naturally to obtain cooled mixture; v) precipitating product from the cooled mixture of step (iv) by addition of alcohol; vi) washing the product with a mixture of hexane and 2-propanol to obtain doped Fe3Se4.

5. The process according to claim 4, wherein the alcohol is 2-propanol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Frequency dependence of (a) real part of permittivity (b) out of phase part of permittivity (c) loss factor (d) absolute permittivity measured with ac 1 V rms value at a temperature range from 200 K to 350 K.

(2) FIG. 2: Temperature dependence of real part of permittivity and loss tangent at frequency 1587 Hz. An anomaly can be seen (encircled) in ε′ around 317 K which is in close proximity of magnetic transition temperature.

(3) FIG. 3: Cole-Cole plot of as synthesized Fe.sub.3Se.sub.4 nanoparticles (ε″ versus ε′) measured by impedance spectroscopy at temperatures 185, 220, 295, 300, 317, 310, 326, 336 and 350 K respectively.

(4) FIG. 4: Ferroelectric polarization loop of Fe.sub.3Se.sub.4 nanoparticles at frequency 500 Hz and 100 V applied voltage (top panel). The frequency dependence of the loop taken at frequency 100, 200 and 500 Hz (bottom panel).

(5) FIG. 5: The plot of differential heat flow in TGA, specific heat capacity, dielectric permittivity and magnetization with temperature. All these parameters show a clear anomaly at around 317 K indicating towards spin-phonon-charge coupling.

(6) FIG. 6: Temperature dependence of Raman scattered signal from Fe.sub.3Se.sub.4 nanoparticles recorded from temperature 295 K to 333 K (top panel). The peak position and FWHM of two Raman modes are deduced from these plots and plotted against temperatures (Bottom panel). (a) and (b) shows the variation for 224 cm.sup.−1 mode and (c) and (d) shows the variation for 291 cm.sup.−1 mode.

(7) FIG. 7: Effect of external magnetic field on the Raman mode intensity is shown. (A) shows the Raman scattering signal without exposure. (B) Raman scattering signal with exposure to 100 Oe (Oersted) external magnetic field. (C) Raman scattering signal after the removal of magnetic field.

(8) FIG. 8: M-H hysteresis loop of V and Gd doped samples showing ferrimagnetic nature at room temperature.

(9) FIG. 9: Effect of external magnetic field on the Raman mode intensity is shown for two doped samples (V and Gd). Black data points shows the Raman scattering signal with exposure to 100 Oe external magnetic field. This indicates towards the presence of spin-phonon coupling in the system.

DETAILED DESCRIPTION OF THE INVENTION

(10) The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

(11) Room temperature is the terminology used in this document for temperature in the range of 15-30° C.

(12) In the view of above, the present invention provides a multiferroic material comprising Fe.sub.3Se.sub.4 nanoparticles characterized in that ferroelectric polarization in Fe.sub.3Se.sub.4 and coupling of magnetic and ferroelectric behavior is observed at room temperature.

(13) In preferred embodiment, said material is in the form of nanoparticles, nanorods, thin film or bulk.

(14) The present invention provides a very novel and unexpected phenomenon of observing spontaneous and reversible ferroelectric polarization in monoclinic iron selenide Fe.sub.3Se.sub.4. The spin-charge-phonon coupling is probed by dielectric impedance spectroscopy and Raman scattering experiment in the vicinity of magnetic transition temperature. The P-E hysteresis loop measurement at different temperature in a broad frequency range confirms the existence of spontaneous reversible polarization in this material. Thus Fe.sub.3Se.sub.4 may be termed as magnetoelectric multiferroic at room temperature. The dielectric spectroscopy of Fe.sub.3Se.sub.4 shows a clear anomaly around magnetic phase transition (T.sub.c).

(15) The invention further provides nanorods of monoclinic iron selenide (Fe.sub.3Se.sub.4) grown by high temperature thermal decomposition in organic solvents, with average diameter ˜50 nm. X-ray diffraction pattern showed that nanorods are monoclinic in structure with space group C2/m. Temperature dependence of magnetization shows the transition temperature around 323 K below which it goes into a ferrimagnetic phase. Ferroelectric hysteresis measurements at room temperature (˜298 K) shows closed loops in a broad frequency range. P-E loop taken with frequency 500 Hz and at potential 100 V is shown in FIG. 4.

(16) The invention provides observation of ferroelectric order in Fe.sub.3Se.sub.4 nanoparticles at room temperature. These particles also show signatures of spin-charge coupling as an anomaly is observed in dielectric permittivity around magnetic transition temperature 323 K. Also, strong dependence of intensity of Raman spectra on external magnetic field indicates towards the presence of spin-phonon-charge coupling in the system. Ferroelectric polarization measurements revealed hysteresis loops in a broad frequency range. The microscopic origin of the coupling between spin-charge-phonon is not clearly understood. Vigorous theoretical calculations are required to probe this mechanism in this compound. It is observed that the coexistence of both magnetic and charge ordering at room temperature proposes Fe.sub.3Se.sub.4 as a possible room temperature multiferroic compound.

(17) The dielectric response from the sample is measured in a frequency range 1 Hz to 10.sup.6 Hz spanning temperature range 150 K to 350 K at 1 V rms. The frequency dependence of various parameters is plotted as a function of frequency in FIG. 1. An anomaly is observed around temperature 317 K in both the real and imaginary part of c verses temperature curve plotted for a broad range of frequency. Above 317 K the value of c decreases sharply indicative of a ferroelectric to paraelectric transition.

(18) In order to understand the nature of dielectric relaxation in these nanorods, complex Argand plane plot ε″ and ε′, also known as Cole-Cole plot, is examined. FIG. 3 shows the plots between the real permittivity (ε′) and imaginary permittivity (ε″) part of the impedance at different temperatures. It is observed from the Cole-Cole plot that the center of the semicircle arcs are below the x-axis implying that the electrical response from the sample departs from the ideal Debye's relaxation process. Cole-Cole plots at various temperatures are given in figure. As can be inferred from the figure at very low temperatures (plots at 185 K, 220 K) there is only one semicircle which comes from the contribution from the grains. As the temperature increases and approaches near the Curie temperature another semicircle appears at lower frequency region associated with the contribution from the grain boundary. At Curie temperature both the contributions from grain and grain boundary is prominent but when the temperature is raised much above (plots at 336 K and 350 K), only the contributions due to grain boundary is prominent.

(19) FIG. 6 shows the Raman spectrums of as prepared Fe.sub.3Se.sub.4 nanoparticles taken at temperatures from 295 K to 333 K. The spectrum consists of sharp peaks at 224, 291, 409 cm.sup.−1. The peak at 224 and 291 cm.sup.−1 can be ascribed to the Fe—Se vibration modes as it is close to the reported values of 220 and 285 cm.sup.−1 for the Fe—Se vibration in β-Fe.sub.7Se.sub.8 having similar monoclinic structure.

(20) As the temperature is increased, the peaks, 224, 291 cm.sup.−1, show significant change in the peak position and peak width (FWHM). When the peak position and FWHM is plotted with temperature, a clear incongruity is seen around the magnetic/ferroelectric transition temperature (˜323 K) (FIG. 6). From the figure, it is appreciated that both the Raman modes softens as temperature is increased from 295 K. As the temperature further reaches the magnetic/ferroelectric ordering temperature the Raman mode starts hardening and peak shifts towards higher wavenumber and then immediately after transition temperature decreases sharply towards lower wavenumber. This anomaly in Raman modes observed near magnetic transition temperature provided a significant input indicating spin-phonon coupling in the system. This kind of anomaly near the magnetic transition temperature has been observed previously in case of pure selenium element. The effect of external magnetic field on these Raman modes is also studied. At room temperature, (which is close to the magnetic transition temperature) even a very small external magnetic field (0.01 T) distorts the spectra significantly and intensity of Raman modes decreases sharply (FIG. 7).

(21) The earlier results in Fe.sub.3Se.sub.4 by single crystals (phys. stat. sol. (a) 20, K29 1973) show that for beyond magnetic ordering temperature the interatomic spacing rearranges such that the cation-cation overlapping disappears partially or completely. This may be the reason behind the anomaly in Raman modes near transition temperature.

(22) In one embodiment, the present invention provides a multiferroic material comprising Fe.sub.3Se.sub.4 nanoparticles doped with at least one element selected from transitional metals, rare earths elements and chalcogens.

(23) In preferred embodiment, the Fe site is doped with transition metal elements and rare earth elements. More preferably, the Fe site is doped with transition metal elements, rare earth elements selected from Cr (Chromium), Co (Cobalt), Mn (Manganese), V (Vanadium), Gd (Gadolinium), Dy (Dysprosium) and the like.

(24) In another preferred embodiment, the anion site (Se) is doped with chalcogens. More preferably, the anion site (Se) is doped with chalcogens selected from S (Sulfur), Te (Tellurium).

(25) The results also show the coexistence of magnetization and ferroelectric polarization in doped samples. All the doped samples give ferrimagnetic nature in the magnetization hysteresis measurements at room temperature (FIG. 8). Also, the spin-phonon coupling is observed in doped samples similar to the pristine sample of Fe.sub.3Se.sub.4. When an external magnetic field is applied, the intensity of Raman modes decreases to a great extent (FIG. 9).

(26) In another embodiment, the present invention provides use of iron selenide alone or doped with at least one element selected from transitional metal, rare earths and chalcogens for magnetic and electric switching at room temperature.

(27) The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

(28) Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

EXAMPLE

(29) Magnetization measurements were done using the VSM attachment of PPMS from Quantum design systems equipped with 9 T superconducting magnet. Ferroelectric hysteresis loop measurements were done on pellets made by cold pressing the sample powder in zero field. Temperature dependent dielectric spectroscopy was performed using Novocontrol Beta NB Impedence Analyzer with a home-built sample holder to couple with a helium closed-cycle-refrigerator (Janis Inc.). The powdered sample was compressed in the form of circular pellet of diameter 13 mm and a custom designed sample holder was used to form parallel plate capacitor geometry.

Example 1

Synthesis of Fe.SUB.3.Se.SUB.4 .Nanoparticles

(30) Iron acetyl acetonate Fe(acac).sub.3(0.53 g, 1.5 mmol) and Se powder (0.158 g, 2 mmol) were added to 15 ml of oleylamine in a 100 ml three-neck flask under N.sub.2 atmosphere. The mixture was heated to 120° C. and kept for 1 h. Then, temperature was increased up to 200° C. and kept for 1 h. Finally, the solution temperature was raised to 300° C. and kept for 1 h. After 1 h, the heat source was removed and solution was allowed to cool down naturally to room temperature. The Fe.sub.3Se.sub.4 nanoparticles were precipitated by the addition of 20 ml of 2-propanol. The precipitate was then centrifuged and washed with solution containing hexane and 2-propanol in 3:2 ratio.

Example 2

Synthesis of Doped Sample

(31) To observe the effect of anion and cation site doping on the properties of Fe.sub.3Se.sub.4, few samples were prepared. The Fe site was doped with transition metal elements (Cr, Co, Mn, V) and rare earth elements (Gd, Dy). Similarly the anion site (Se) was doped with other chalcogens (S, Te). The samples were synthesized by taking stoichiometric amount of all the precursors at the beginning of the reaction.

(32) Some of the advantages of the invention include that invention reports for the first time, the presence of spontaneous and reversible ferroelectric polarization in Fe.sub.3Se.sub.4 and their various derivatives (various cation/anion dopings and morphology) and coupling of magnetic and ferroelectric behavior at room temperature. These compound are relatively cheap and usually rare-earth free which is an added advantage for a multiferroic material. The simple crystal structure of this compound is an added advantage. Due to the presence of magnetoelectric coupling in Fe.sub.3Se.sub.4, the said material may find application in data storage, four state memory devices, magnetoelectric switching, spin valves, spintronics, catalytic activity, sensing devices, gas sensing etc, transducers and pyroelectric devices.