Multiferroic materials

Abstract

The present invention relates to new multiferroic materials. More particularly, the present invention relates to new multiferroic single phase ceramic materials as well as to thin films formed from these materials, methods of preparing these materials and their use as multiferroic materials in electronic components and devices.

Claims

1. A single phase ceramic material comprising a morphotropic phase boundary and a continuous percolating network of magnetic cations; wherein the material has the composition shown in formula (I) below:
(1-x)LM.sup.a.sub.(1-y)/2Fe.sub.yMg.sub.(1-y)/2O.sub.3-xQ (I) wherein: x is a value ranging from 0.01 to 0.4; y is a value ranging from 0.01 to 0.9; L is selected from Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; M.sup.a is selected from Ti, Hf or Zr; and Q is: a group of formula:
RM.sup.bO.sub.3 wherein: R is selected from Ca, Sr or Ba; and M.sup.b is selected from Ti or Hf or Zr; or a group selected from one or more of the following: [BiM.sup.dO.sub.3].sub.q[Bi(M.sup.e).sub.r(M.sup.f).sub.(1-r)O.sub.3].sub.(1-q), CaZrO.sub.3, SrZrO.sub.3, BaZrO.sub.3, KNbO.sub.3, NaNbO.sub.3, Ca.sub.2FeNbO.sub.6, PbZr.sub.1-pTi.sub.pO.sub.3, LnScO.sub.3, LnGaO.sub.3 or Ln.sub.2TiM.sup.cO.sub.6; wherein: Ln is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; p is a value ranging from 0 to 1; q is a value ranging from 0 to 1; r is a value ranging from 0 to 1; M.sup.c is selected from Mg, Ni, Cu, Mn, Co, Fe or Zn; M.sup.d is selected from Fe, Mn, or Cr, and M.sup.e and M.sup.f are independently selected from one or more of the following Zn, Ti, Sn, Mg, Nb, Ta, W, Li, Ni, Cu, Fe, Cr or Mn.

2. A single phase ceramic material according to claim 1, wherein the ceramic material comprising a morphotropic phase boundary is ferroelectric and the magnetic cations are present in an amount sufficient to impart ferromagnetic properties to the material.

3. A single phase material according to claim 1, wherein the material is multiferroic at a temperature between 243K and 473K.

4. A single phase ceramic material according to claim 1, wherein said material has the structure (Ia) shown below:
(1-x)BiM.sup.a.sub.(1-y)/2Fe.sub.yMg.sub.(1-y)/2O.sub.3-xRM.sup.bO.sub.3 (Ia) wherein: x is between 0.01 and 0.4; y is between 0.01 and 0.9; R is selected from Ca or Sr; and M.sup.a and M.sup.b are each independently selected from Zr or Ti.

5. A single phase ceramic material according to claim 4, wherein: i) M.sup.a is Ti; and/or ii) M.sup.b is Ti; and/or iii) R is Ca.

6. A single phase ceramic material according to claim 4, wherein: i) x is between 0.05 and 0.20; and/or ii) y is between 0.6 and 0.9.

7. A single phase ceramic material according to claim 4, wherein the single phase ceramic material is multiferroic at a temperature between 243K and 473K.

8. A single phase ceramic material comprising: (i) a single phase ceramic material of formula I, wherein the material has the composition shown below:
(1-x)LM.sup.a.sub.(1-y)/2Fe.sub.yMg.sub.(1-y)/2O.sub.3-xQ (I) wherein: x is a value ranging from 0.01 to 0.4; y is a value ranging from 0.01 to 0.9; L is selected from Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; M.sup.a is selected from Ti, Hf or Zr; and Q is: a) a group of formula:
RM.sup.bO.sub.3 wherein: R is selected from Ca, Sr or Ba; and M.sup.b is selected from Ti, Hf or Zr; or b) a group selected from one or more of the following: [BiM.sup.dO.sub.3].sub.q[Bi(M.sup.e).sub.r(M.sup.f).sub.(1-r)O.sub.3].sub.(1-q), CaZrO.sub.3, SrZrO.sub.3, BaZrO.sub.3, KNbO.sub.3, NaNbO.sub.3, Ca.sub.2FeNbO.sub.6, PbZr.sub.1-pTi.sub.pO.sub.3, LnScO.sub.3, LnGaO.sub.3 or Ln.sub.2TiM.sup.cO.sub.6; wherein: Ln is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; p is a value ranging from 0 to 1; q is a value ranging from 0 to 1; r is a value ranging from 0 to 1; M.sup.c is selected from Mg, Ni, Cu, Mn, Co, Fe or Zn; M.sup.d is selected from Fe, Mn, or Cr; and M.sup.e and M.sup.f are independently selected from one or more of the following Zn, Ti, Sn, Mg, Nb, Ta, W, Li, Ni, Cu, Fe, Cr or Mn; and (ii) a further material, optionally selected from one or more of the following: BiTi.sub.3/8Fe.sub.2/8Ni.sub.3/8O.sub.3, [BiM.sup.dO.sub.3].sub.q[Bi(M.sup.e).sub.r(M.sup.f).sub.(1-r)O.sub.3].sub.(1-q), Bi(M.sup.e).sub.r(M.sup.f).sub.(1-r)O.sub.3, CaZrO.sub.3, SrZrO.sub.3, BaZrO.sub.3, KNbO.sub.3, NaNbO.sub.3, Ca.sub.2FeNbO.sub.6, LnScO.sub.3, LnGaO.sub.3 or Ln.sub.2TiM.sup.cO.sub.6; wherein Ln is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; p is a value ranging from 0 to 1; M.sup.c is selected from Mg, Ni, Cu or Zn; M.sup.d is selected from Fe, Mn, or Cr; and M.sup.e and M.sup.f are independently selected from one or more of the following Zn, Ti, Sn, Mg, Nb, Ta, W, Li, Ni, Cu, Fe, Cr or Mn.

9. A process for the preparation of a single phase ceramic material according to claim 1, wherein the single phase ceramic material comprises a morphotropic phase boundary and a continuous percolating network of magnetic cations, said process comprising sintering a material capable of forming a single phase ceramic material comprising a morphotropic phase boundary in the presence of a material capable of forming a continuous percolating network of magnetic cations within the morphotropic phase boundary of the ceramic material.

10. A process for forming a single phase ceramic material according to claim 8, comprising: (i) combining and milling materials of the formulae A to E shown below:
L.sub.2O.sub.3Formula A
RCO.sub.3Formula B
Fe.sub.2O.sub.3Formula C
M.sup.aO.sub.2Formula D
M.sup.bO.sub.2Formula E wherein L, R, M.sup.a and M.sup.b are as defined in claim 4; with a material of the formula F:
MgCO.sub.3.Mg(OH).sub.2.zH.sub.2O Formula F wherein z is an integer between 0-5; in the presence of a suitable organic solvent; (ii) evaporating the organic solvent and pressing the resulting powder to form a pellet; (iii) calcinating the pellet one or more times; and (iv) optionally sintering the pellet in the presence of a binder and MnO.sub.2.

11. A process according to claim 10, wherein: i) the organic solvent in step (i) is ethanol; and/or ii) the calcination of step (iii) is conducted at a temperature between 1190K and 1250K; and/or iii) the binder of step (iv) is polyvinyl butyral.

12. A thin film formed from a single phase ceramic material according to claim 1.

13. A process of forming a thin film according to claim 12, the process comprising depositing a thin film of a single phase ceramic material on to a substrate.

14. A process of forming a thin film according to claim 13, wherein the process of depositing the thin film is selected from pulsed layer deposition (PLD), atomic layer deposition (ALD), chemical vapour deposition (CVD) or physical vapour deposition (PVD).

15. An electronic component comprising a single phase ceramic material according to claim 1.

16. An electronic component according to claim 15, wherein the electronic component is selected from a memory device, a tunnel junction, a magnetic field sensor, a transmitter, a receiver, a transmitter-receiver module, a phase array system or a resonator.

17. An electronic device comprising an electronic component according to claim 15.

18. An electronic device according to claim 17, wherein the electronic device is a tunable microwave device.

19. The electronic component according to claim 16, wherein the memory device is selected from MRAM, FERAM or MERAM.

Description

(1) Further examples of the invention are described hereinbelow, by way of example only, with reference to the accompanying figures, in which:

(2) FIG. 1 shows the X-ray diffraction pattern of Examples 1-6 (bottom trace to top trace respectively).

(3) FIG. 2 shows Pawley fits to PXRD data from Example 4 using (f) a single R3c unit cell, (g) superimposed R3c and Pna2.sub.1 unit cells, and (h) a single monoclinic Pa unit cell.

(4) FIG. 3 shows the room temperature P(E) measurements on Examples 1, 4 and 6, confirming ferroelectricity.

(5) FIG. 4 shows the magnetic isotherms for Example 4.

(6) FIG. 5 shows (a) the linear ME effect in Example 4 at room temperature and (b) the variation of linear ME and magnetisation with T for Example 4 showing T.sub.N=370 K

(7) FIG. 6 show (a) the dielectric permittivity (left axis) and loss (right axis) on Example 1 (black) and Example 4 (red), and (b) the P(E) loop on Example 4 showing ferroelectric switching at 473 K.

METHODS AND EQUIPMENT

(8) Unless stated otherwise, all reagents and solvents were commercially available and used as received.

(9) Sample Preparation

(10) Powder samples of (1-x)BiTi.sub.(1-y)/2Fe.sub.yMg.sub.(1-y)/2O.sub.3-(x)CaTiO.sub.3, in the compositional range x=0.15, y=0.60-0.90, were synthesised by a conventional solid-state reaction. The binary oxides Bi.sub.2O.sub.3, CaCO.sub.3, Fe.sub.2O.sub.3, TiO.sub.2 (pre-dried at 473 K) and MgCO.sub.3.Mg(OH).sub.2.xH.sub.2O (x3, used as received) were weighed in stoichiometric amounts and ball milled in ethanol for 20 hours. The mixtures obtained after evaporating ethanol were pelletised and calcined at 1208 K for 12 hours in a platinum-lined alumina crucible. These pellets were then re-ground thoroughly and re-pelletised, and subjected to a second calcination at 1213 K for 12 hours in platinum-lined alumina crucibles. The resulting powders were found to contain only the target phase with no minority phases visible by PXRD. Dense pellets (>95% of crystallographic density) suitable for property measurements were produced from these powders by the following protocol: first, 2 wt % polyvinyl butyral binder and 0.2 wt % MnO.sub.2 were added to the samples, and this mixture was ball-milled for 20 hours. The resulting mixture was then pelletised (8 mm diameter) with a uniaxial press, followed by pressing at 210.sup.8 Pa in a cold isostatic press. These pellets were loaded into a Pt-lined alumina boat. A programmable tube furnace was used to heat the reaction under flowing oxygen to 943 K for 1 hour, followed by 1228 K for 3 hours, followed by 1173 K for 12 hours before cooling to room temperature at 5 K min.sup.1. The resulting pellets were found to contain no minority phases by PXRD. Their densities were measured using an Archimedes balance.

(11) Powder X-Ray Diffraction (PXRD)

(12) All data were collected using a PANalytical X'Pert Pro diffractometer in Bragg-Brentano geometry with a monochromated Co K.sub.1 source (=1.78896 ) and position-sensitive X'Celerator detector. Each sample was contained in a back-filled sample holder and rotated during the measurement. A programmable divergence slit was used to provide a constant illuminated area throughout the angular range. Data were collected in the angular range 52130 in steps of 0.0167. Pawley refinements were carried out using the software package Topas Academic (version 5). For each PXRD pattern, background was modelled using a Chebyschev polynomial function with 12 refined parameters. Lattice parameters, a sample height correction, peak profile functions and model-independent peak intensities were refined. Peak profiles were modelled with a modified Thompson-Cox-Hastings pseudo-Voigt function. When fitting data to a single phase (R3c or Pa cells), a Stephens anisotropic strain broadening function was refined. In two-phase (R3c+Pna2.sub.1) refinements this function was refined only for the rhombohedral (R3c) phase.

(13) FIG. 1 shows the X-ray diffraction pattern of Examples 1-6 (bottom trace to top trace respectively).

(14) FIG. 2 shows Pawley fits to PXRD data from Example 4 using (f) a single R3c unit cell, (g) superimposed R3c and Pna2.sub.1 unit cells, and (h) a single monoclinic Pa unit cell.

(15) The compositions x=0.15, y=0.60 and y=0.80 (Examples 1 and 4) were selected for detailed property studies. Pawley refinements on these compositions show that a model with both R and O phases, and a single-phase monoclinic model in a 2a.sub.p2a.sub.p2a.sub.p unit cell (space group Pa, which is a polar sub-group of both R3c and Pna2.sub.1; refined lattice parameters shown in Table 1), both produce superior fits to that obtained by a purely rhombohedral model (Table 2) showing that these compositions exist in the morphotropic phase boundary region towards the rhombohedral limit, and hence that they are long-range ordered polar non-cubic materials.

(16) TABLE-US-00001 TABLE 1 Refined lattice parameters obtained from Pawley fits to PXRD data of compositions x = 0.15, y = 0.60 and 0.80 (Examples 1 and 4), modelled in a 2a.sub.p 2a.sub.p 2a.sub.p cell in space group Pa. Refined Lattice Parameters Volume Composition a () b () c () y () (.sup.3) x = 0.15, 5.6037(3) 7.9047(6) 5.5666(1) 89.433(7) 246.56(2) y = 0.60 (Example 1) x = 0.15, 5.6019(7) 7.903(1) 5.5641(1) 89.40(1) 246.32(5) y = 0.80 (Example 4)

(17) TABLE-US-00002 TABLE 2 Comparison of agreement factors obtained from Pawley fits to PXRD data in three candidate models, for the two compositions x = 0.15, y = 0.60 and 0.80 (Examples 1 and 4). Weighted Profile Goodness of fit R-factor (R.sub.wp) (.sup.2) R3c + R3c + Composition R3c Pna2.sub.1 Pa R3c Pna2.sub.1 Pa x = 0.15, y = 0.60 7.421 6.373 6.169 2.104 1.583 1.499 (Example 1) x = 0.15, y = 0.80 6.883 6.136 5.969 1.821 1.477 1.392 (Example 4)
Electrical Measurements

(18) For electric poling, gold was sputtered on both sides of thin discs (150-200 m with 20 m tolerance). For P(E) measurements, silver conductive paint (RS components) was applied on both sides of thin discs and cured at 393 K for 10 minutes. The disc was loaded in a PVDF sample holder. Silicone oil was used as dielectric medium to avoid air breakdown. Measurements were conducted using a Radiant ferroelectric tester system.

(19) FIG. 3 shows the P(E) measurements on thin discs of Examples 1, 4 and 6. Well saturated loops confirm ferroelectricity in these compositions.

(20) The maximum polarisation (P.sub.max) for Examples 1, 4 and 6 are 57 C/cm.sup.2, 46 C/cm.sup.2 and 18 C/cm.sup.2 respectively.

(21) Impedance Measurements

(22) Impedance and phase angles were measured using an Agilent LCR meter E4980 by applying an AC voltage of 0.5 V.

(23) Magnetic Measurements

(24) Magnetic measurements were carried out using MPMS XL-7 and MPMS3 systems (Quantum Design, USA). For this, powder or pellet samples were loaded into a polycarbonate capsule and fixed into a straight drinking plastic straw and then loaded into a SQUID dc probe. The isothermal magnetisation data were decomposed using the general function:

(25) $M\left(H\right)=\underset{i}{.Math.}{m}_i\left(H\right)$

(26) Where m.sub.i is a generic function describing a single magnetic component taking the form:

(27) $m_i\left(H\right)=a.Math.\tanh \left(\frac{H-b}{c}\right)+d$

(28) Where a represent the saturation magnetisation, b the coercive field, c a parameter describing the squareness of the loop, and d a linear term including paramagnetic, diamagnetic and antiferromagnetic contributions for the individual magnetic component. Above the magnetic ordering temperature of the perovskite phase only one component was used to describe the isothermal magnetisation assigned to a high Fe content impurity. Below the perovskite magnetic ordering temperature two components were used.

(29) For Example 4, the magnetic isotherm (M(H)) data collected below T.sub.N=370 K showed a hysteresis with finite coercive field (250 Oe) consistent with ferromagnetic behaviour.

(30) FIG. 4 shows the magnetic isotherms obtained for Example 4.

(31) Magnetoelectric Measurements

(32) Details of the magnetoelectric measurements set-up and protocol are described elsewhere [18]. In this experiment, a sinusoidal electric field E=E.sub.ac cos t (=2 f where f is frequency, E.sub.ac is the electric field amplitude) is applied across the disc and the first harmonic of the complex ac magnetic moment, m(t)=(mi.m)cos t is measured. The measurements were performed in the absence of any dc magnetic and electric fields. In this scenario, the real part of the electrically induced magnetic moment [18] is:

(33) $m^{}=\left(E_{ac}\right)\frac{V}{{}_0}$
where V is the sample volume. This moment involves only the linear ME () effect, whereas the higher order effects are zero. To demonstrate the linear ME effect on y=0.6 and 0.8, the electric field amplitude E.sub.ac was varied and the induced moment was recorded. Linear ME susceptibility () was calculated from a plot of volume ac magnetization amplitude M.sub.ac (=m/V) vs E.sub.ac following the relation [19]:

(34) $.Math..Math.={}_0.Math.\frac{M_{ac}}{E_{ac}}.$

(35) All measurements were performed at f=1 Hz. The sensitivity of the experimental set-up used here is m=VM.sub.ac>510.sup.12 Am.sup.2. Prior to ME measurements, discs were poled externally using aixPES (aixACCT Systems) at a field of 100 kV/cm for 15 minutes from 343 K to room temperature. Discs were then loaded into a modified dc SQUID probe at 300 K and subjected to a magnetic field of 2 T for 30 minutes. After removal of electric and magnetic fields, electrodes were short circuited for 15 minutes before conducting ME measurements at 300 K. For ME measurements at 10 and 150 K (for x=0.15, y=0.6), the sample was cooled down to the measurement temperature in the presence of an electric field (3.5 kV/cm) and a magnetic field (2 T) and the protocol for 300 K measurement was followed. For the temperature dependence of , an electric field (3.5 kV/cm for y=0.6 and 2.7 kV/cm for y=0.8 respectively) and a magnetic field of 2 T were applied at 300 K followed by cooling to 10 K at a rate of 1 K min.sup.1, and the data was collected at 1 Hz. The temperature was stabilized for 5 minutes at each step prior to measurement. The room temperature bulk d.c. resistivity of x=0.15, y=0.6 is 33 G.Math.m, and that of x=0.15, y=0.8 is 21 G.Math.m. The leakage currents observed for y=0.6 and y=0.8 are 0.35 nA (320 K) and 0.23 nA (360 K) respectively at the maximum measurement fields. These values are too low to cause any artifacts in the ME measurements. The upper limit of temperature in this measurement set-up is 360 K.

(36) To confirm whether the two order parameters (polarisation (P) and magnetisation (M)) are coupled, magnetoelectric measurements were performed on a disc poled both electrically and magnetically. The linear magnetoelectric (ME) coupling was measured as the slope of the magnetisation (.sub.0M) induced by the applied electric field (E) plot as shown in FIG. 5(a).

(37) The room temperature linear ME coefficient () is 0.26 ps/m as shown in FIG. 5(a).

(38) FIG. 6 shows (a) the dielectric permittivity (left axis) and loss (right axis) on Example 1 (black) and Example 4 (red), and (b) the P(E) loop on Example 4 showing ferroelectric switching at 473 K.

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