Enhanced Magnetic Properties Through Alignment of Non-Magnetic Constituents

Abstract

The present invention relates to a method of producing permanent magnets free of rare-earth metals. Specifically the type of magnets produced by the present invention are rare-earth free magnets based on iron. More specifically, the magnets of the present invention are of the class hexaferrites. The present invention further relates to magnets produced by the method of the invention, which are highly aligned magnets with improved magnetic properties compared to commercially available analogues.

Claims

1. A non-ferromagnetic strontium hexaferrite precursor with a concentration of more than 15% by weight of anisotropic crystalline materials of the group consisting of six-line ferrihydrite (SLF), α-Fe.sub.2O.sub.3, and/or α-FeOOH, each with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm, wherein the non-ferromagnetic strontium hexaferrite precursor further comprises one or more sources of metals such as Strontium (Sr), Calcium (Ca), Barium (Ba), Magnesium (Mg), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), and Lanthanum (La).

2. The non-ferromagnetic strontium hexaferrite precursor according to claim 1, wherein said concentration of anisotropic crystalline materials makes up 100% by weight, such as 15-95% by weight.

3. The non-ferromagnetic strontium hexaferrite precursor according to any one of claims 1 to 2, wherein said concentration of anisotropic crystalline materials makes up 25 to 75% by weight.

4. The non-ferromagnetic strontium hexaferrite precursor of any one of claims 1 to 3, wherein the metal is strontium (Sr).

5. Use of the non-ferromagnetic strontium hexaferrite precursor according to any one of claims 1 to 4, for forming a strontium hexaferrite permanent magnet.

6. A method of producing a non-ferromagnetic strontium hexaferrite precursor comprising six-line ferrihydrite (SLF), the method comprising the steps of: a. mixing salts of Fe.sup.3+ and Sr.sup.2+ with a strong alkaline solution to form a gel, b. transferring said gel to an autoclave, and sealing said autoclave, c. placing said autoclave at a temperature ranging from 200-100° C. for more than 1 hour under autogenous pressure, and d. isolating the formed anisotropic crystalline non-ferromagnetic strontium hexaferrite precursor material comprising six-line ferrihydrite (SLF).

7. The method according to claim 6, wherein the salts of Fe.sup.3+ and Sr.sup.2+ are mixed in a Fe/Sr ratio is about 8.

8. The method according to any one of claims 6 to 7, wherein the reaction time is about 5 hours.

9. A non-ferromagnetic strontium hexaferrite precursor material comprising at least approximately 15wt % six-line ferrihydrite (SLF) crystallites having with a platelet or platelet-like morphology with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm obtainable by the method according to any one of claims 6 to 8.

10. A method of producing a non-ferromagnetic strontium hexaferrite precursor comprising Hematite (α-Fe.sub.2O.sub.3), the method comprising the steps of: a. mixing salts of Fe.sup.3+ with a strong alkaline solution to form a mixture comprising a dark red precipitate; b. transferring said mixture to an autoclave, and sealing said autoclave; c. placing said autoclave at a temperature of 200-100° C. for more than 1 hour under autogenous pressure; d. isolating the formed anisotropic crystalline non-magnetic precursor material; and e. adding a source of strontium (Sr) to the isolated material to obtain the non-ferromagnetic strontium hexaferrite precursor comprising Hematite (α-Fe.sub.2O.sub.3).

11. The method according to claim 10, wherein the isolation step d comprises washing the formed anisotropic crystalline non-magnetic precursor material with water to neutral pH (pH of 7), followed by washing with ethanol and drying at 85° C. until dry.

12. The method according to any one of claims 10 to 11, wherein the temperature is about 160° C.

13. The method according to any one of claims 10 to 12, wherein the reaction time under autogenous pressure is about 2 hours.

14. The method according to any one of claims 10 to 13, wherein the metal to base ratio Fe/OH is 4.

15. The method according to any one of claims 10 to 14, wherein the source of strontium is added to obtain an Fe/Sr molar ratio of 12.

16. A non-ferromagnetic strontium hexaferrite precursor material comprising at least approximately 15 wt % Hematite (α-Fe.sub.2O.sub.3) crystallites having with a platelet or platelet-like morphology with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm obtainable by the method according to any one of claims 10 to 15.

17. A method of producing a non-ferromagnetic strontium hexaferrite precursor comprising Goethite (α-FeOOH), the method comprising the steps of: a. mixing salts of Fe.sup.3+ with a strong alkaline solution to form a mixture comprising a dark red precipitate; b. transferring said mixture to an autoclave, and sealing said autoclave; c. placing said autoclave at a temperature of 200-100° C. for more than 1 hour under autogenous pressure; d. isolating the formed anisotropic crystalline non-magnetic precursor material; and e. adding a source of Strontium (Sr) to the isolated material to obtain the non-ferromagnetic strontium hexaferrite precursor comprising Goethite (α-FeOOH).

18. The method according to claim 17, wherein the isolation step d comprises washing the formed anisotropic crystalline non-magnetic precursor material with water to neutral pH (pH of 7), followed by washing with ethanol and drying at 85° C. until dry.

19. The method according to any one of claims 17 to 18, wherein the reaction temperature is about 100° C.

20. The method according to any one of claims 17 to 19, wherein the reaction time under autogenous pressure is about 10 hours.

21. The method according to any one of claims 17 to 20, wherein the step b. comprises said mixture having a pH of about 11.

22. The method according to any one of claims 17 to 21, wherein the source of strontium is added to obtain an Fe/Sr molar ratio of 12.

23. A non-ferromagnetic strontium hexaferrite precursor material comprising at least approximately 15 wt % Goethite (α-FeOOH) crystallites having with a needle or needle-like morphology with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm obtainable by the method according to any one of claims 17 to 22.

24. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: a. providing a non-ferromagnetic strontium hexaferrite precursor according to any of claims 1 to 4, 9, 16, and 23; b. compacting said precursor at a temperature between 700-1000° C. by spark plasma sintering (SPS) in the absence of any applied magnetic field, thereby inducing a phase change, and c. isolating the thus formed strontium hexaferrite permanent magnet.

25. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: a. providing a non-ferromagnetic strontium hexaferrite precursor according to any of claims 1 to 4, 9, 16, and 23; b. compacting said precursor by the application of pressure c. calcining said compacted precursor at a temperature between 700-1220° C. in the absence of any applied magnetic field, thereby inducing a phase change, and d. isolating the thus formed strontium hexaferrite permanent magnet.

26. A strontium hexaferrite permanent magnet comprising highly aligned strontium hexaferrite, characterized by its most intense Bragg reflections being the (006), (008), (107) and (0014) reflections, and wherein the (008)/[(110+008)] integrated intensity ratio is at least 0.5 as obtained by X-ray powder diffraction using Co-Kα radiation.

27. The strontium hexaferrite permanent magnet according to claim 26, further comprising pronounced Bragg reflections along the (00l) reflection, as evidenced by powder X-ray diffraction obtained using Co-Kα radiation.

28. The strontium hexaferrite permanent magnet according to any of claims 26 to 27, characterized by comprising an X-ray powder diffraction pattern with characteristic peaks expressed in 2-theta angle (°) for (006) at 2θ=26.9°±0.2°, (110) at 2θ=35.4°±0.2°, (008) at 2θ=36.3°±0.2°, (107) at 2θ=37.7°±0.2° and (0014) at 2θ=65.8°±0.2° as obtained using Co-Kα radiation.

29. The strontium hexaferrite permanent magnet according to any of claims 26 to 15 28, characterized by comprising a hysterisis squareness ratio (M.sub.r/M.sub.s) which is between 0.65 and 1.0, such as between 0.70 and 1.0, such as between 0.75 and 1.0, such as between 0.80 and 1.0, such as between 0.85 and 1.0, such as between 0.95 and 1.0.

30. The strontium hexaferrite permanent magnet according to any of claims 26 to 29, characterized by comprising a hysteretic squareness ratio (M.sub.r/M.sub.s) is between 0.85 and 1.0, such as between 0.95 and 1.0.

31. A strontium hexaferrite permanent magnet comprising strontium hexaferrite according to any one of claims 26 to 30, obtainable by the method according to any one of claims 24 to 25.

32. Use of a strontium hexaferrite permanent magnet according to any one of claims 26 to 31 as a magnetic component in a device.

Description

DESCRIPTION OF FIGURES

[0176] FIG. 1: standard magnetic M versus H hysteresis loop of A) a non-aligned magnet and B) a highly aligned magnet. M.sub.s, M.sub.r, and H.sub.c are indicated on the figures

[0177] FIG. 2: A) schematic representation of the platelet shaped crystallites of the non-magnet precursor, and B) schematic representation of the needle shaped crystallites of the non-magnetic precursor. The A and C lengths are indicated by arrows.

[0178] FIG. 3: powder X-ray diffraction pattern of the non-magnetic precursors as prepared according to Example 1. The graph also shows the results of the Rietveld analysis performed according to the specification of Example 1. Bragg peak indicators of each constituent of the Rietveld analysis are shown and marked with arrows.

[0179] FIG. 4: TEM images of the non-magnetic precursors prepared according to Example 1 at a) 100° C., b) 150° C., and c) 200° C.

[0180] FIG. 5: powder X-ray diffraction pattern of the non-magnetic Hematite precursor as prepared from FeCl.sub.3.Math.6H.sub.2O according to Example 2.

[0181] FIG. 6: HR-TEM image showing platy morphology of hematite particles. Some crystallites are tilted, which causes them to appear darker. Pale Goethite needles are also observed in the TEM image.

[0182] FIG. 7: Powder X-Ray diffraction pattern of the non-magnetic Goethite precursor as prepared according to Example 3. The graph also shows the results of the Rietveld analysis performed according to the specification of Example 3. Bragg peak indicators are shown under the diffraction pattern.

[0183] FIG. 8: TEM image of the non-magnetic precursors α-FeOOH prepared according to Example 3 at 100° C. for 12 hours

[0184] FIG. 9: powder X-ray diffraction pattern of the hexaferrite permanent magnet labelled SPS-3 obtained by SPS according to Example 4 with indicators of relevant Bragg reflections. The graph also shows the results of the Rietveld analysis performed according to the specification of Example 4. Bragg peak indicators are shown under the diffraction pattern.

[0185] FIG. 10: powder X-ray diffraction pattern of the hexaferrite permanent magnets obtained by compacting and subsequent calcination according to Example 5 with indicators of relevant Bragg reflections. The graph also shows the results of the Rietveld analysis performed according to the specification of Example 5. Bragg peak indicators are shown under each diffraction pattern. The top and bottom samples differ only in their non-magnetic precursor production temperature. The permanent strontium hexaferrite magnet obtained by either way has the same relevant Bragg reflections and Bragg peak indicators.

[0186] FIG. 11: magnetic hysteresis loops of the hexaferrite permanent magnets prepared by SPS according to Example 4, labelled SPS-1, SPS-2 and SPS-3, with a zoom on M.sub.r and H.sub.c region.

[0187] FIG. 12: magnetic hysteresis loops of the hexaferrite permanent magnets prepared by compaction and subsequent calcination according to Example 5.

[0188] FIG. 13: powder X-ray diffraction pattern of the hexaferrite permanent magnets obtained by compacting and subsequent calcination according to Example 8 with indicators of relevant Bragg reflections. The graph also shows the results of the Rietveld analysis performed according to the specification of Example 8. Bragg peak indicators of each constituent of the precursor are shown under each diffraction pattern.

[0189] FIG. 14: magnetic hysteresis loops of the hexaferrite permanent magnets prepared by compaction and subsequent calcination according to Example 8 with a zoom on the M.sub.r and H.sub.c region. A) comparison of magnetic hysteresis for all samples of Example 8 produced by either dry mixing (DM) or wet mixing (WM) and compacted at either low pressure (220 MPa) or high pressure (870 MPa). B) comparison of magnetic hysteresis for an aligned hexaferrite sample (WM 870 MPa) and a hexaferrite sample with random orientation.

EXAMPLES

Example 1: Preparation and Structural Characterization of Non-Ferromagnetic Strontium Hexaferrite Precursor Comprising Anisotropic Crystallites of Six-Line Ferrihydrite (SLF) Precursor to be Used for Subsequent Production of Strontium Hexaferrite Permanent Magnets

[0190] A solution of 3 M Fe(NO.sub.3).sub.3.Math.9H.sub.2pl O and 0.75 M Sr(NO.sub.3).sub.2 in a molar ratio of 8:1, respectively, was co-precipitated by the slow addition of 8 M NaOH to form a gel. Following complete addition of NaOH, the gel was allowed to stir for approximately 3 hours. The gel was then transferred to a 175 ml teflon-lined steel autoclave and placed inside an oven operating ata temperature of 200° C., 150° C., or 100° C. for 5 hours. The thus obtained samples were identified as SLF-200, SLF-150 and SLF-100 according to the synthesis temperature.

[0191] The precursors were examined by X-ray powder diffraction (Rigaku SmartLab diffractometer, Co-Kα radiation) and analyzed by Rietveld refinement using the FullProf Suite software package, including parameters such as zero point correction, scale factor, lattice constants, Lorentzian crystallite size parameters Y and SZ, and preferred orientations. The XRD patterns of the 3 precursor samples are shown in (FIG. 3). A Thompson-Cox-Hastings pseudo-Voigt function was used to describe the peak profile. Rietveld refinement retrieved a composition of the three samples. From this, the amount of six-line ferrihydrite (SLF) present in the non-magnetic precursor is approximately 15, 38 and 30% for SLF-100, SLF-150 and SLF-200 respectively. In the present analysis, the SLF phase was refined based on the Dritts model. SLF-100 showed poor crystallinity when compared with that of SLF-150 and SLF-200 as a consequence of the lower hydrothermal temperature used in the reaction. Rietveld refinement of the as-synthesized samples result in crystallites with platelet like shape, and an average A-length of 11 nm compared to C-length of 5.5 nm can be extracted.

[0192] Transmission electron microscopy (TEM) and spatially resolved elemental analysis were recorded for SLF-150 and SLF-200 on a FEI TALOS F200A TEM microscope operating at 200 kV, for approximately 200 particles and analysed using the Gatan software. The TEM micrographs confirmed the presence of microscopic platelets present in the as-synthesized powers of SLF-150 and SLF-200, see FIG. 4. Lognormal fit to the size extracted from the TEM images results in particle dimensions of A=23 nm and C=7 nm for 150° C., while the 200° C. gave dimensions of A=30 nm and C=8 nm.

[0193] Since these non-magnetic nano-crystallites are not interacting with each other by way of magnetism, the crystallites can easily align along their faces when optimum pressure is applied either in cold compaction or using a spark plasma sintering (SPS). The alignment can happen at room temperature, because no magnetic interaction needs to be broken. However, for the conversion into SrFe.sub.12O.sub.19 elevated temperatures are needed.

Example 2: Preparation and Structural Characterization of Non-Ferromagnetic Precursor Comprising Anisotropic Crystallites of Hematite (α-Fe.SUB.2.O.SUB.3.) for Subsequent Preparation of Strontium Hexaferrite Permanent Magnets

[0194] Hematite (α-Fe.sub.2O.sub.3) precursors having a platelet morphology can be synthesized via a wet chemical approach as described in the following. Other reagents known to a person skilled in the art may be substituted to produce similar hematite crystallite precursors.

Hematite Precursor Made from FeCl.SUB.3..Math.6H.SUB.2.O

[0195] A 60 mL aqueous solution of 2 M of FeCl.sub.3.Math.6H.sub.2O, was added dropwise to a 60 mL, 8 M NaOH solution whilst stirring, resulting in a dark red precipitate forming upon addition. The precipitate solution was transferred to a Teflon-lined stainless-steel autoclave, then heated at 160° C. for 2 hours. Once cooled the precipitate was washed using deionised water and centrifugated until the supernatant was pH 7. It was finally washed once with ethanol, placed in drying oven at 85° C. until dry, then ground with a pestle and mortar. The thus obtained precursors were further utilized in Examples 8 and 10.

[0196] The precursors were examined by X-ray powder diffraction (Rigaku SmartLab diffractometer, Co-Kα radiation) and analyzed by Rietveld refinement using the FullProf Suite software package, including parameters such as zero point correction, scale factor, lattice constants, Lorentzian crystallite size parameters Y and SZ, and preferred orientations. The XRD patterns of the 160° C. precursor is shown in FIG. 5. A Thompson-Cox-Hastings pseudo-Voigt function was used to describe the peak profile.

[0197] The particle morphology analysis with TEM revealed hexagonal platelets with A-sizes in the order of 1-2 μm shown in FIG. 6. The hexagonal platelets of hematite are either oriented with the platelet plane parallel to the view (light plate shape) or perpendicular (dark elongated shape). The observed light needles are goethite (α-FeOOH).

Example 3: Preparation and Structural Characterization of Non-Ferromagnetic Precursor Comprising Anisotropic Crystallites of Goethite (α-FeOOH) for Subsequent Preparation of Strontium Hexaferrite Permanent Magnets

Goethite (α-FeOOH) Precursor Made from Fe(NO.SUB.3.).SUB.3..Math.9H.SUB.2.O

[0198] A 40 mL aqueous solution of 1 M of Fe(NO.sub.3).sub.3.Math.9H.sub.2O was made and 8 M NaOH solutionwas added dropwise to obtained an [OH.sup.−]/[NO.sub.3.sup.−] ratio of 1.33, resulting in a dark red precipitate forming upon addition of NaOH. The precipitate solution was allowed to stir for one hour. The precipitate solution was transferred to a Teflon-lined stainless steel autoclave. The pH was maintained at 11. The autoclave was heated at 100° C. for 10 hours. Once cooled the precipitate was washed using deionised water and centrifuged until the supernatant was pH 7. It was then washed once with ethanol, placed in drying at 85° C. until dry, then ground with pestle and mortar. The thus obtained precursors were further utilized in Example 10.

[0199] The precursors were examined by X-Ray powder diffraction (Rigaku Smart Lab Diffractometer, Co-Ka radiation) and analysed by Reitveld refinement using FullProf suite software package, including parameters such as zero point correction, scale factor, lattice constants, Lorentzian crystallite size parameters Y and SZ and preferred orientations. The XRD patterns of the 100° C. precursor is shown in FIG. 7. A Thomas-Cox-Hastings pseudo-Voigt function was used to describe the peak profile. A TEM image of the goethite sample is shown in FIG. 8, clearly revealing the needle-like morphology of the crystallites

Example 4: Preparation and Structural Characterisation of Aligned Magnetic M-Type Strontium Hexaferrite Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Six-Line Ferrihydrite (SLF) and Spark Plasma Sintering (SPS).

[0200] The non-magnetic precursors prepared according to Example 1 were compacted into bulk strontium hexaferrite magnets using spark plasms sintering (SPS). 0.8 g of the precursor powder was weighed and placed in a graphite die of 12.6 mm inner diameter and enclosed between two graphite punches. Graphite paper (˜0.2 μm) was placed between the powders and the graphite die and between the die and the upper and lower punch. The die was subsequently inserted in a vacuum chamber of a SPS Synthex Inc. 1500 model, Dr. Sinter Lab series spark plasma sintering system. In the chamber, a uniaxial pressure was applied to the punches while a pulsed dc current was passed through the die, sintering the powders contained within. A maximum temperature of 750° C. and maximum pressure of 100 MPa were applied during the sintering process, which was completed after 8 minutes. The die was freely cooled before the sintered pellet was extracted. Finally, the graphite paper that adhered to the pellet was removed by polishing. The thus produced pellet, consisting of aligned M-type strontium hexaferrite permanent magnets, having a diameter of 12.6 mm and a thickness of ˜1 mm. The SPS pellets were named SPS-1, SPS-2 and SPS-3 corresponding to the SLF-100, SLF-150 and SLF-200 precursor samples respectively.

[0201] The SPS pellets were subjected to X-ray diffraction analysis using the same Rietveld parameters presented in Example 1. The powder X-ray diffractograms collected on the obtained SPS pellets reveal a major suppression of the Bragg reflections with (hk0). This selective Bragg reflection suppression is due to the crystallographic orientation of the crystallites within the sample, this is also referred to as crystallographic preferred orientation or texture and it is due to the anisotropic shape of the crystallites. The anisotropy effect is especially pronounced in the SPS-3 sample, where almost a complete suppression is observed for any reflection other than (00l) as shown in FIG. 9. The preferred orientation was implemented in the structural modelling of the powder diffraction data using the March-Dollase multiaxial function. According to the Rietveld refinements, SPS-3 exhibits the largest degree of preferred orientation while SPS-1 has the lowest degree of preferred orientation. Other relevant physical properties are summarized further below in Table 1.

Example 5: Preparation and Structural Characterisation of Aligned Magnetic M-Type Strontium Hexaferrite Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Six-Line Ferrihydrite (SLF) and Cold Compaction Followed by Calcination

[0202] The precursor powders described in example 1 were subjected to conventional co-compaction followed by calcination of the compacted pellets. 2 pellets of 0.15 g for each of the precursor powders SLF-100, SLF-150, and SLF-200 were prepared for sintering at 750° C., 900° C., and 1050° C. The pellets were compacted with similar conditions of 2 tonnes of pressure for five minutes corresponding to 700 MPa. After compaction to dense pellets, the samples were sintered at 750° C., 900° C., and 1050° C. for 2 hours. During the sintering, the samples were held in an Al.sub.2O.sub.3 crucible. The compacted and sintered pellets (identified as T.sub.Hyd(100), T.sub.Hyd(150), and T.sub.Hyd(200) in reference to the precursor) were subjected to X-ray diffraction analysis employing the Rietveld analysis parameters as presented in Example 1, see FIG. 10. Relevant physical properties are summarized further below in Table 1.

Example 6: Characterization of Magnetic Properties in Aligned M-Type Strontium Hexaferrite Magnets Obtained from Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Six-Line Ferrihydrite (SLF) and Spark Plasma Sintering (SPS).

[0203] Magnetic properties of the SPS pellets were measured using a Quantum Design Physical Properties Measurement System (PPMS) equipped with a vibrating sample magnetometer (VSM). For the measurements, the SPS pellet was placed in a tubular brass sample holder and held between two quartz rods. Room temperature magnetic hysteresis loop measured on the SPS compacted pellets are presented in FIG. 11 nd was measured using standard experimental settings well-known to a person of skill in the art.

[0204] The alignment of the crystallites in a magnet can also be evaluated indirectly in the magnetism of said magnet based on the hysteretic squareness ratio defined by M.sub.r/M.sub.s. The Stoner Wohlfrath model states that an ideally aligned magnet gives a ratio equal to unity, while it decreases to 0.5 for completely randomly oriented crystallites. Table 2 found further below summarizes this and other key properties of the presented samples.

Example 7: Characterization of Magnetic Properties in Aligned M-Type Strontium Hexaferrite Magnets Obtained from Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Six-Line Ferrihydrite (SLF) and Cold Compaction Followed by Calcination.

[0205] In a similar manner to Example 6, VSM magnetic measurements were conducted on the permanent magnets obtained according to Example 5 (T.sub.Hyd(100), T.sub.Hyd(150), and T.sub.Hyd(200)) to extract the magnetic hysteresis loops (FIG. 12). Extracted physical properties are summarized below in Table 2.

TABLE-US-00001 TABLE 1 Overview of structural and magnetic properties of strontium hexaferrite permanent magnet prepared from the SLF-precursors of Example 1 using cold compaction or SPS compaction (Example 6). Sintering Pressure G.sub.1 008/ H.sub.C BH.sub.max Sample Temp (° C.) (MPa) pref. (008 + 110) M.sub.r/M.sub.s (kA/m) (kJ/m.sup.3) T.sub.Hyd(100) 750 700 0.907 0.322 0.563 502 9 900 700 0.752 0.508 0.651 199 11 T.sub.Hyd(150) 750 700 0.832 0.416 0.639 511 11 900 700 0.845 0.404 0.610 509 12 1050 700 0.665 0.681 0.756 78 9.28 T.sub.Hyd(200) 750 700 0.737 0.566 0.654 470 12 900 700 0.537 0.835 0.747 279 21 1050 700 0.494 0.906 0.806 100 18 SPS-1 750 100 0.694 0.623 0.659 474 11 SPS-2 750 100 0.387 0.951 0.898 265 27 SPS-3 750 100 0.322 0.981 0.926 247 33

[0206] The parameter G.sub.1 stems from the Rietveld refinement and gives a qualitative measure for the preferred orientation in the sample. The lower the number the higher the preferred orientation. The number can vary between 1 and 0, where 1 is a randomly oriented sample and 0 is close to a single crystal. Another method for looking at preferred orientation is to look at the integrated area of some of the characterizing peaks found in the diffraction patterns, e.g. the (110) and (008) peaks. Here (008) gives information on crystallites being oriented parallel with the wanted direction, while (110) is crystal planes perpendicular to the wanted direction. The following index is used (008)/[(110)+(008)] to quantify the alignment and should preferably be between 1 and 0.5 with 1 representing the most aligned sample.

[0207] M.sub.r/M.sub.s ratio from Table 1 shows the squareness ratio for the differently prepared samples. For all the presented magnets found in Table 2, it is evident that the temperature used in producing the non-magnetic anisotropic precursor has a direct influence on the hysteretic squareness ratio, with a higher synthesis temperature resulting in a squareness ratio closer to unity. This in turn shows that sample alignment can be controlled by the synthesis temperature, provided that initial nanoparticles synthesized are thin platelets or needles.

[0208] The (BH).sub.max of all samples were analyzed and it was found that SPS-3 showed the highest value of (BH).sub.max with 33(4) kJ/m.sup.3, followed by SPS-2 (27 kJ/m.sup.3) and T.sub.Hyd(200) (21 kJ/m.sup.3). These samples also show the highest degree of crystallite alignment as evidenced by G.sub.1 and [008/(008+110)] value.

[0209] The magnetic properties as evaluated by magnetic hysteresis showed an enhanced maximum energy product of the aligned M-type strontium hexaferrite obtained using anisotropic non-magnet precursors of (BH).sub.max=33 kJ/m.sup.3 compared to those found in commercially available La—Co free hexaferrites, obtained by dry processing [(BH).sub.max=26-29 kJ/m.sup.3].

Example 8: Preparation and Structural Characterisation of Aligned Magnetic M-Type Strontium Hexaferrite Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Hematite (α-Fe.SUB.2.O.SUB.3.) and Cold Compaction Followed by Calcination

[0210] Different ways of mixing the precursor powder from Example 2 with SrCO.sub.3 and Sr(OH).sub.2 was tested. Different pressures was also applied in the cold compaction to check the effect on the alignment.

Dry Mixing

[0211] A precursor from Example 2 (α-Fe.sub.2O.sub.3 70 wt. % and α-FeOOH 30 wt. %) 4.003 g (50.1 mmol) was weighed off and hand mixed in an agate pestle and mortar with 0.619 g (4.18 mmol) of SrCO.sub.3 for 15 minutes. A mixing ratio of 1:10 molar ratio of Sr:Fe was used.

Wet Mixing

[0212] To avoid the grinding having an effect of the shape of the crystallites, the precursor material in Example 2 was wet mixed with Sr(OH).sub.2. A mass of 0.476 g (3.91 mmol) of anhydrous Sr(OH).sub.2 was dissolved in 30 mL deionised water until a homogeneous solution was achieved at 85° C. 3.750g (47.0 mmol) of precursor from Example 2 was added and stirred continuously for 45 mins at 85° C. The resultant slurry was poured onto a petri dish and was dried on a hot plate at 35° C. overnight. The dried mixture was scraped out and ground in a pestle and mortar for 2 minutes. A mixing ratio of 1:10 molar ratio of Sr:Fe was used.

Cold Compaction and Calcination

[0213] Four pellets of 0.4 g were prepared, two of the wet mixed (WM) and two of the dry mixed (DM). All pellets were uniaxial cold pressed in a 12.6 mm diameter pressing die. The low pressure pellets was made were made at 2.5 tonnes/cm.sup.2 of pressure for 5 minutes corresponding to a pressure of 220 MPa. The dry mixed and wet mixed pellets were named DM 220 MPa and WM 220 MPa respectively. High pressure pellets were pressed at 10 tonnes/cm.sup.2 of pressure for 5 minutes corresponding to 870 MPa. The dry mixed and wet mixed pellets named DM 870 MPa and WM 870 MPa respectively. After compaction to dense pellets all samples were calcined at 1100° C. for 2 hours. During the calcination the samples were held in an Al.sub.2O.sub.3 crucible.

[0214] After pressing and calcination, the pellets were subjected to X-ray diffraction analysis using the same Rietveld parameters presented in Example 4. The powder X-ray patterns collected on the as-obtained calcined pellets reveal a major suppression of the Bragg reflections with (hk0). FIG. 13 shows the powder diffraction pattern, where the preferred orientation was refined using the March-Dollase function as already explained. The XRD data reveal a clear preferred orientation of the sample. A summary of relevant physical properties is found further below in Table 3.

Example 9: Characterization of Magnetic Properties in Aligned M-Type Strontium Hexaferrite Magnets Obtained from Using Non-Ferromagnetic Strontium Hexaferrite Precursor of Platelet Shaped Hematite (α-Fe.SUB.2.O.SUB.3.) and Cold Compaction Followed by Calcination

[0215] In a similar manner to Example 6, VSM magnetic measurements were conducted on the permanent magnets obtained according to Example 8 (DM 220 MPa, DM 870 MPa, WM 220 MPa, and WM 870 MPa)) to extract the magnetic hysteresis loops (FIG. 14).

[0216] Extracted physical properties are summarized below in Table 3.

TABLE-US-00002 TABLE 2 Overview of structural and magnetic properties of strontium hexaferrite permanent magnet prepared from the hematite-precursors of Example 2 using cold compaction and calcination (Example 8). Pressure G.sub.1 H.sub.c BH.sub.Max Sample (MPa) pref M.sub.r/M.sub.s (kAm.sup.−1) (kJm.sup.−3) Random orientation 0.98 0.54 297 11(2) Dry mixed 220 0.59 0.71 154 15(2) Dry mixed 870 0.53 0.74 122 15(2) Wet mixed 220 0.64 0.70 225 18(2) Wet mixed 870 0.54 0.75 185 21(2)

[0217] Table 3 illustrates the possibility and potential of producing rare-earth-free permanent magnets retaining a substantial maximum energy product from anisotropic non-magnetic precursors in a simple two-step process combining cold compaction and subsequent calcination. Such a simple two-step process is readily scalable to large production facilities due to its cheap costs and easy handling.

[0218] Example 10: Preparation and Structural Characterisation of Aligned Magnetic M-Type Strontium Hexaferrite Using Non-Ferromagnetic Strontium Hexaferrite Precursor Mixture of Platelet Shaped Hematite (α-Fe.sub.2O.sub.3) and of Needles Shaped Goethite (α-FeOOH) and Cold Compaction Followed by Calcination

[0219] Different ratios of hematite and goethite was mixed with corresponding amounts of SrCO.sub.3. The hematite and goethite was prepared separately as in Example 2 and 3. The three powders were mixed as in Example 8 dry mixing.

[0220] Five pellets of ˜0.4 g were prepared, with the following hematite and goethite ratios 100% hematite, 75% hematite and 25% goethite, 50% hematite and 50% goethite, 25% hematite and 75% goethite, and finally 100% goethite. All pellets were uniaxial cold pressed in a 6 mm diameter pressing die. Applying a pressure of 10 tonnes/cm.sup.2 of pressure for 5 minutes corresponding to 870 MPa. The samples were named (100% /0% hematite/goethite, 75%/25% hematite/goethite, 50%/50% hematite/goethite, 25%/75% hematite/goethite, 0%/100% hematite/goethite). After compaction to dense pellets all samples were calcined at 1100° C. for 2 hours. During the calcination the samples were held in an Al.sub.2O.sub.3 crucible.

Example 11: Characterization of Magnetic Properties in Aligned M-Type Strontium Hexaferrite Magnets Obtained from Using Non-Ferromagnetic Strontium Hexaferrite Precursor Mixture of Platelet Shaped Hematite (α-Fe.SUB.2.O.SUB.3.) and of Needles Shaped Goethite (α-FeOOH) and Cold Compaction Followed by Calcination

[0221] In a similar manner to Example 6, VSM magnetic measurements were conducted on the permanent magnets obtained according to Example 10 (100%/0% hematite/goethite, 75%/25% hematite/goethite, 50%/50% hematite/goethite, 25%/75% hematite / goethite, 0%/100% hematite/goethite). Extracted physical properties are summarized below in Table 4.

TABLE-US-00003 TABLE 4 Overview of structural and magnetic properties of strontium hexaferrite permanent magnet prepared from the hematite-precursors of Example 2 and goethite precursor of Example 3 using cold compaction at 870 MPa and calcination at 1100° C. for 2 hours. Hematite/ G.sub.1 008/ M.sub.r M.sub.s H.sub.C BH.sub.max Geothite pref. (008 + 110) Am.sup.2/kg Am.sup.2/kg M.sub.r/M.sub.s (kA/m) (kJ/m.sup.3) 100%/0%  0.61 0.72 51.8 74.1 0.70 208 21 75%/25% 0.61 0.74 51.2 74.3 0.70 181 19 50%/50% 0.58 0.80 51.9 74.0 0.70 202 20 25%/75% 0.50 0.88 54.6 73.0 0.75 174 21  0%/100% 0.43 0.94 59.7 72.3 0.80 157 22

[0222] Table 4 illustrates the possibility and potential of producing rare-earth-free permanent magnets retaining a substantial maximum energy product from anisotropic non-magnetic precursors in a simple two-step process combining cold compaction and subsequent calcination. Such a simple two-step process is readily scalable to large production facilities due to its cheap costs and easy handling.

ITEMS 1

[0223] 1. A non-ferromagnetic precursor with a concentration of more than 15% by weight of anisotropic crystalline materials of the group consisting of six-line ferrihydrite (SLF), +-Fe.sub.2O.sub.3, and/or α-FeOOH, each with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm. [0224] 2. The precursor according to item 1, wherein said concentration of anisotropic crystalline materials makes up 100% by weight, such as 15-95% by weight. [0225] 3. The precursor according to any of the preceding items, wherein said concentration of anisotropic crystalline materials makes up 25 to 75% by weight. [0226] 4. The precursor according to any of items 1-3, wherein said anisotropic crystalline material is six-line ferrihydrite (SLF), [0227] 5. The precursor according to any of items 1-3, wherein said anisotropic crystalline material is α-Fe.sub.2O.sub.3. [0228] 6. The precursor according to any of items 1-3, wherein said anisotropic crystalline material is α-FeOOH. [0229] 7. The precursor of any of the preceding items, further comprising one or more sources of metals such as Calcium (Ca), Strontium (Sr), Barium (Ba), Magnesium (Mg), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), and Lanthanum (La). [0230] 8. The non-ferromagnetic precursor of any of the preceding items, for use in formation of a strontium hexaferrite permanent magnet. [0231] 9. A method of producing a non-ferromagnetic precursor, comprising the steps of: [0232] a. mixing salts of Fe.sup.3+ with a strong alkaline solution to form a gel, [0233] b. transferring said gel to an autoclave, and sealing said autoclave, [0234] c. placing said autoclave at a temperature of equal to or less than 200° C. for more than 1 hour under autogenous pressure, and [0235] d. isolating the formed anisotropic crystalline non-magnetic precursor material. [0236] 10. The method according to item 9, wherein step a. may further comprise the addition of a source of a divalent metal such Calcium (Ca), Strontium (Sr), Barium (Ba), Magnesium (Mg), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), and Zinc (Zn). [0237] 11. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: [0238] a. providing a non-ferromagnetic precursor according to any of items 1-7; [0239] b. compacting said precursor at a temperature between 700-1000° C. by spark plasma sintering (SPS) in the absence of any applied magnetic field, thereby inducing a phase change, and [0240] c. isolating the thus formed M-type permanent magnet. [0241] 12. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: [0242] a. providing a non-ferromagnetic precursor according to any of items 1-7; [0243] b. compacting said precursor by the application of pressure [0244] c. calcining said compacted precursor at a temperature between 700-1220° C. in the absence of any applied magnetic field, thereby inducing a phase change, and [0245] d. isolating the M-type permanent magnet. [0246] 13. The method of producing a strontium hexaferrite permanent according to any of items 11-12, wherein the anisotropic crystalline material of said precursor is largely α-Fe.sub.2O.sub.3. [0247] 14. The method of producing a strontium hexaferrite permanent according to any of items 11-12, wherein the anisotropic crystalline material of said precursor is largely six-line ferrihydrite (SLF). [0248] 15. The method of producing a strontium hexaferrite permanent according to any of items 11-12, wherein the anisotropic crystalline material of said precursor is largely α-FeOOH. [0249] 16. A strontium hexaferrite permanent magnet comprising highly aligned strontium hexaferrite, characterized by its most intense Bragg reflections being the (006), (008), (107) and (0014) reflections, and wherein the (008)/[(110+008)] integrated intensity ratio is at least 0.5 as obtained by X-ray powder diffraction using Co-Kα radiation. [0250] 17. The strontium hexaferrite permanent magnet according to item 16, further comprising pronounced Bragg reflections along the (00l) reflection, as evidenced by powder X-ray diffraction obtained using Co-Kα radiation. [0251] 18. The strontium hexaferrite permanent magnet according to any of items 16-17, further comprising an X-ray powder diffraction pattern with characteristic peaks expressed in 2-theta angle (°) for (006) at 2θ=26.9°±0.2°, (110) at 2θ=35.4°±0.2°, (008) at 2θ=36.3°±0.2°, (107) at 2θ=37.7°±0.2° and (0014) at 2θ=65.8°±0.2° as obtained using Co-Kα radiation. [0252] 19. The strontium hexaferrite permanent magnet according to any of items 16-18, with a powder X-ray pattern substantially similar to FIG. 9. [0253] 20. The strontium hexaferrite permanent magnet according to any of items 16-19, further comprising a hysterisis squareness ratio (M.sub.R/M.sub.s) which is between 0.65 and 1.0, such as between 0.70 and 1.0, such as between 0.75 and 1.0, such as between 0.80 and 1.0, such as between 0.85 and 1.0, such as between 0.95 and 1.0. [0254] 21. The strontium hexaferrite permanent magnet according to any of items 16-20, further comprising a hysteretic squareness ratio (M.sub.r/M.sub.s) is between 0.85 and 1.0, such as between 0.95 and 1.0. [0255] 22. Use of a strontium hexaferrite permanent magnet according to any of the preceding items as a magnetic component in a device. [0256] 23. The use according to the preceding item wherein said device is any one of: [0257] a. storage devices, [0258] b. generators, [0259] c. stators in electric motors and/or electric engines, [0260] d. audio devices, [0261] e. magnetic imaging scanners, [0262] f. magnetic brakes, [0263] g. linear motors, [0264] h. electrodynamic bearings, or [0265] i. magnetic toys.

ITEMS 2

[0266] 1. A non-ferromagnetic precursor with a concentration of more than 15% by weight of anisotropic crystalline materials of the group consisting of six-line ferrihydrite (SLF), α-Fe.sub.2O.sub.3, and/or α-FeOOH, each with an average aspect ratio A/C≥2, a size C ranging from 2-200 nm, and a size A ranging from 4-2000 nm. [0267] 2. The precursor according to item 1, wherein said concentration of anisotropic crystalline materials makes up 100% by weight, such as 15-95% by weight. [0268] 3. The precursor according to any of the preceding items , wherein said concentration of anisotropic crystalline materials makes up 25 to 75% by weight. [0269] 4. The precursor of any of the preceding items, further comprising one or more sources of metals such as Calcium (Ca), Strontium (Sr), Barium (Ba), Magnesium (Mg), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), and Lanthanum (La). [0270] 5. The non-ferromagnetic precursor of any of the preceding items, for use in formation of a strontium hexaferrite permanent magnet. [0271] 6. A method of producing a non-ferromagnetic precursor, comprising the steps of: [0272] a. mixing salts of Fe.sup.3+ with a strong alkaline solution to form a gel, [0273] b. transferring said gel to an autoclave, and sealing said autoclave, [0274] c. placing said autoclave at a temperature of equal to or less than 200° C. for more than 1 hour under autogenous pressure, and [0275] d. isolating the formed anisotropic crystalline non-magnetic precursor material. [0276] 7. The method according to items 6, wherein step a. may further comprise the addition of a source of a divalent metal such Calcium (Ca), Strontium (Sr), Barium (Ba), Magnesium (Mg), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), and Zinc (Zn). [0277] 8. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: [0278] a. providing a non-ferromagnetic precursor according to any of items 1-4; [0279] b. compacting said precursor at a temperature between 700-1000° C. by spark plasma sintering (SPS) in the absence of any applied magnetic field, thereby inducing a phase change, and [0280] c. isolating the thus formed M-type permanent magnet. [0281] 9. A method of producing a strontium hexaferrite permanent magnet comprising the steps of: [0282] a. providing a non-ferromagnetic precursor according to any of items 1-4; [0283] b. compacting said precursor by the application of pressure [0284] c. calcining said compacted precursor at a temperature between 700-1220° C. in the absence of any applied magnetic field, thereby inducing a phase change, and [0285] d. isolating the M-type permanent magnet. [0286] 10. A strontium hexaferrite permanent magnet comprising highly aligned strontium hexaferrite, characterized by its most intense Bragg reflections being the (006), (008), (107) and (0014) reflections, and wherein the (008)/[(110+008)] integrated intensity ratio is at least 0.5 as obtained by X-ray powder diffraction using Co-Kα radiation. [0287] 11. The strontium hexaferrite permanent magnet according to item 10, further comprising pronounced Bragg reflections along the (00l) reflection, as evidenced by powder X-ray diffraction obtained using Co-Kα radiation. [0288] 12. The strontium hexaferrite permanent magnet according to any of items 10-11, further comprising an X-ray powder diffraction pattern with characteristic peaks expressed in 2-theta angle (°) for (006) at 2θ=26.9°±0.2°, (110) at 2θ=35.4°±0.2°, (008) at 2θ=36.3°±0.2°, (107) at 2θ=37.7°±0.2° and (0014) at 2θ=65.8°±0.2° as obtained using Co-Kα radiation. [0289] 13. The strontium hexaferrite permanent magnet according to any of items 10-12, further comprising a hysterisis squareness ratio (M.sub.R/M.sub.s) which is between 0.65 and 1.0, such as between 0.70 and 1.0, such as between 0.75 and 1.0, such as between 0.80 and 1.0, such as between 0.85 and 1.0, such as between 0.95 and 1.0. [0290] 14. The strontium hexaferrite permanent magnet according to any of items 10-13, further comprising a hysteretic squareness ratio (M.sub.r/M.sub.s) is between 0.85 and 1.0, such as between 0.95 and 1.0. [0291] 15. Use of a strontium hexaferrite permanent magnet according to any of the preceding items as a magnetic component in a device.