Strong non rare earth permanent magnets from double doped magnetic nanoparticles

Abstract

A magnetic nanoparticle, and composites thereof, comprising a ternary host compound comprising a transition metal oxide of size 2-30 nm having two transition metal dopants atom incorporated therein, such that the nanoparticle is converted from superparamagnetic or weak ferromagnetic to strong ferromagnetic material. The strong permanent magnets are formed from non-rare earth materials. The composite material can also include undoped nanoparticles.

Claims

1. A magnetic nanoparticle comprising a ternary host compound of a transition metal oxide having two different ionic charge states M.sub.1.sup.2+M.sub.2.sup.3+O.sub.4 of size 2-30 nm having two transition metal dopant atoms incorporated therein, wherein the dopant atom is chosen to be different from M but having the same ionic charge state the ternary host compound M.sub.1.sup.2+M.sub.2.sup.3+O.sub.4, where M.sub.1.sup.2+ in the host is replaced by a dopant selected from the remaining other host compounds transition metal ions of divalent charge state 2+, the transition metal M.sub.2.sup.3+ in the ternary host is replaced by a dopant selected from the group of: Cr.sup.3+ and Rh.sup.3+, such that the nanoparticle is converted from superparamagnetic or weak ferromagnetic to strong ferromagnetic material.

2. The nanoparticle as claimed in claim 1, wherein the ternary host is MFe.sub.2O.sub.4, and M is a transition metal selected from the group of: Fe.sup.2+, Mn.sup.2+, Co.sup.2+ or Ni.sup.2+.

3. The nanoparticle as claimed in claim 1 wherein the dopants for ionic charge state 2+ are selected from the group of: Fe.sup.2+, Mn.sup.2+, Co.sup.2+ and Ni.sup.2+.

4. A magnetic nanoparticle comprising: a ternary transition metal oxide host compound in the range of 2-30 nm having two transition metals M.sub.1.sup.2+M.sub.2.sup.3+O.sub.4 where M.sub.1 is a transition metal ion selected from divalent group Fe.sup.2+, Mn.sup.2+, Co.sup.2+ and Ni.sup.2+ and M.sub.2 is a transition metal ion selected from trivalent Fe.sup.3+, Cr.sup.3+ and Rh.sup.3+, the transition metal M.sub.1.sup.2+ in the host is replaced by a dopant selected from the remaining other host compounds transition metal ions of divalent charge state 2+ the transition metal M.sub.2.sup.3+ in the host is replaced by a dopant selected from the remaining other host compounds transition metal ions of trivalent charge state 3+, respectively the incorporation of dopant ions of charged state 2+ or 3+ replacing the corresponding host of the same charge-state, converting superparamagnetic or weak ferromagnetic nanoparticles to ferromagnetic nanoparticles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A, 1B, 1C 1D and 1E Depict the introduction of an magnetic dopant-spin shown as an arrow (FIG. 1B) in a magnetic nanoparticle (FIG. 1A) creates a magnetic nanoparticle where all the spins are aligned (FIG. 1C). These nanoparticles self-organize (FIG. 1D) to yield a nanomagnet (FIG. 1E) thus converting metallic oxide paramagnetic material to a strong ferromagnetic material to yield higher magnetic moment and larger coercivity.

(2) FIG. 2 Depicts the integration of soft phases and hard phases of magnetic nanomaterials are integrated, the resultant final product increases the figure of merit BH.sub.max (shown the area of M-H curve in the fourth quadrant)

(3) FIG. 3. Depicts the dependence of Coercivity in nanosize materials, with the sub-division of single domain and multi-domain regions. The critical size where Hc reduces to zero in nano-material such as metals or oxides. For example, for Fe.sub.3O.sub.4 this size about 30 nm while for CoFe.sub.2O.sub.4 this is about 15 nm.

(4) FIG. 4 Depicts the assembly of a DMNP based hard/soft permanent magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) One point should be emphasized that certain compounds such as Mn.sub.3O.sub.4 can be considered as ternary since Mn has two distinct oxidation states Mn.sup.2+ and Mn.sup.3+ along with O.sup.2−. Thus ferrites expressed as MFe.sub.2O.sub.4, a ternary compound where M could be Mn.sup.2+, Fe.sup.2+, Co.sup.2+ or Ni.sup.2+. To dope Fe.sub.2O.sub.4, we will need elements like Cr.sup.3+ or Rh.sup.3+. Stated another way, the ternary host compound can have two different ionic charge states of the same element M(1)M.sub.2(2)O4. the first dopant is chosen to replace M(1) and have the same ionic charge state as M(1) and the second dopant is chosen to replace and have the same ionic charge state as M.sub.2(2).

(6) The introduction of a magnetic impurity in magnetic nanoparticles creates the perfect magnetic nanoparticle where all the spins are aligned. We have thus converted transition metal oxide based paramagnetic material to a strong ferromagnetic material in the range 10-20 nm, in contrast to nanomaterial that was not doped. In doing so, we created a statistical distribution of hard (doped) and soft (undoped) magnets which forms the back bone of Nanocomposite of hard and soft (NC-HS) magnets. The alignment of all core and surface spins in nanocrystal of ˜20 nm, makes it highly resistive to reversal of magnetic field, yielding high coercivity needed for NC-HS magnets.

(7) If nanoparticles are in close proximity, exchange interactions between surface atoms can be significant. We have demonstrated that by incorporating a single magnetic dopant, surface spins are also aligned thereby eliminating the ‘magnetically-dead surface layer’ in nanoparticles. This makes the discovery of doped magnetic nanoparticles (DMNP) a breakthrough for the development of NC-HS based permanent magnets because the magnetic properties associated with surface spins is crucial to integration of soft and hard magnet for PM in the size range of 10-20 nm. Because of the exchange coupling at the soft/hard interface, the magnetic moments at the soft phase boundary have to align with the adjacent moment in hard phase. The regions close to the interface via exchange coupling create a chain of magnetic nanomagnets.

(8) We have developed a way to synthesize by a singular process, an agglomerate of soft and hard nanomagnet in the size of 20 nm with identical crystal structure. In the process, DMNP based nanomagnets are synthesized in presence of appropriate dopants. The dopant concentration is typically about 1% or less. In the case of Mn.sub.3O.sub.4, Fe.sup.2+ was used as the dopant. Results show that Mn.sub.3O.sub.4, Fe.sup.2+ nanoparticles show enhanced coercivity (200 Oe) and Curie temperature about 800 C. In general for developing NC-HS magnetic system, we need to have the following: 1. In this doping process of magnetic nanoparticles, statistically dopants get incorporated in less than 40% of the nanoparticles. We have a distribution of doped nanoparticles (hard magnets <40%) and undoped nanoparticles (soft magnets). 2. We have hard and soft magnetic nanoparticles of in the size range of 10 to 20 nm. 3. Both hard and soft magnets have the same crystal structure with the hard magnet having a well defined magnetic axis. (cf. FIG. 2). 4. As depicted in FIG. 2, the aligned magnetic axis, and hence the creation of macro-magnet is much simpler integration process. Alignment of magnetic axes of an ensemble of nanomagnets will result in large magnetic anisotropy and powerful magnets. 5 Since the magnetic axes of these nanomagnets are strongly correlated and well aligned, the magnetic properties of the present magnets will weakly depend on temperature i.e. better performance at high temperatures (˜500 C), desirable feature of future permanent magnets. 6. Nanopowder based materials will enable the molding of complex magnet shapes using additive manufacturing. 7 These nanomagnet powders are synthesized at temperatures between 50 C to 200 C enabling cost-effective, scalable and non-toxic process. 8. As a comparison to the rare-earth metallic system, the present nanomagnets are oxide-based i.e. no corrosion, as in the case of RE metal based magnets. 9 These transition metal oxides based magnets will be lighter than RE metals.

(9) Integration of doped ferromagnetic nanoparticle (hard) and undoped paramagnetic and/or superparamagnetic nanoparticle (soft) leads to NC-HS system. This is schematically shown in FIG. 5 as nanocomposite of hard (blue) and soft (green) magnets. Choice of Material

(10) Our earlier PCT application PCT/US2018/019458 on Fe.sup.2+ doping of Mn.sub.3O.sub.4 provided the following teachings: 1. Replacing Mn.sup.2+ with Fe.sup.2+ in Mn.sub.3O.sub.4 nanoparticles helps us align all the Mn.sup.2+ and Mn.sup.3+ spins in the same direction, thereby converting the Ferrimagnetic/Paramagnetic Mn.sub.3O.sub.4 to a strong ferromagnetic nanoparticle. 2. These nanoparticles with all aligned spins, both in the core and at the surface, provide for the first time, ferromagnetic nanoparticles that form nanorods i.e. nanomagnets. 3. To improve the ferromagnetic properties of DMNP, we chose a ferromagnetic system such as Cobalt ferrite nanoparticle that exhibit decent ferromagnetic materials. Cobalt ferrite is chemically expressed as CoFe.sub.2O.sub.4. Another way to express is as a mixture of two compounds CoO+Fe.sub.2O.sub.3, which identifies the charge of the ions (oxidation state) In CoO, Co charge state is CO.sup.2+. Hence in the doping scheme, we can replace Co.sup.2+ with Mn.sup.2+. By incorporating the dopant Mn.sup.2+ to replace Co.sup.2+, we expect the magnetic properties of doped Cobalt ferrite to improve significantly as obtained and tabulated in table 1. 4. In another embodiment we use a dopant like Cr.sup.3+ to replace Fe.sup.3+ in Fe.sub.2O.sub.3 This will constitute a ‘double doping’ scheme in a ternary nanoparticle of CoFe.sub.2O.sub.4. 5. These nanomagnets can be magnetically organized under a applied magnetic field, isostatic pressure and temperatures between 50 C to 500 C to create a bulk magnet. 6. Each of the processing steps, that are used to convert the magnetic nanopowder into a bulk magnet, will need the presence of an applied high magnetic field at all times. This is to keep the nanomagnet structure strongly aligned for a well directed magnetic axis. Ensemble of these nanomagnets with a unique magnetic axis, will result in a powerful permanent magnets. 7. Use of polymer/resin bonding material can be utilized to create bonded magnets and it is anticipated that they will yield reasonable-permanent magnets 8. The ternary system should have preferably highly anisotropic crystalline axis for improved magnetic properties. Such a structure is provided by a spinel or inverse spinel crystal structure. We decided the work with well known material Cobalt ferrite with formula CoFe.sub.2O.sub.4.

(11) To develop a permanent magnet from nanocrystal hard-soft (NC-HS) with DMNP, we have a choice ferrite system with inverse spinel structure with tetrahedral and Octahedral coordination.

(12) In the earlier PCT application we had demonstrated that paramagnetic Mn.sub.3O.sub.4 nanoparticles can be converted to a high temperature ferromagnetic nanoparticles. However, the coercivity Hc was limited to about 200 Oe. If the starting material was Fe.sub.3O.sub.4 and the dopant was Mn.sup.2+, the coercivity was increased to 500 Oe. We have demonstrated that system like cobalt ferrite the coercivity increased from 1,000 Oe to 3,500 Oe when it was doped with 5% Mn.sup.2+. The process to achieve high coercivity is summarized first in the table below.

(13) This application is directed to improvement of our earlier work by the incorporation of two dopants in a ternary compound which greatly improves the magnetic properties.

(14) TABLE-US-00001 TABLE 1 Doped magnetic nanoparticles for permanent magnets Chemical Coercivity Permanent System Formula Dopant Hc (Oe) Magnet Ternary with Mn.sub.3O.sub.4 Fe.sup.2+ .fwdarw. Mn.sup.2+  0 .fwdarw. 200 No same elements Fe.sub.3O.sub.4 Mn.sup.2+ .fwdarw. Fe.sup.2+  .sup. ~10 .fwdarw. 500  with two different oxidation states Ternary- CoFe.sub.2O.sub.4 Mn.sup.2+ .fwdarw. Co.sup.2+ 1000 .fwdarw. 3000 Moderate Compound as Single CoO Dopant Fe.sub.2O.sub.3 Ternary- CoFe.sub.2O.sub.4 Mn.sup.2+ .fwdarw. Co.sup.2+ 1000 .fwdarw. Strong Compound as Cr.sup.3+ or Rh.sup.3+ .fwdarw. 10000** Double dopants CoO Fe3+ Fe.sub.2O.sub.3 **These values of coercivity are anticipated for double doping

(15) Enhancement of coercivity by double doping is expected to increase the magneto-crystalline anisotropy which is the key factor for improving the performance of permanent magnets including from spring exchange effect. The above table designates the different doping possibilities that will yield nanomagnets which when integrated efficiently will yield high performing permanent magnets that will supersede the performance of rare-earth Nd based magnets. We are proposing a Fe.sub.3O.sub.4 nanoparticles (which can be considered as a ternary compound as described above) be doped with Mn.sup.2+ or Co.sup.2+ for Fe.sup.2+ in FeO and dopant Cr.sup.3+ to replace Fe.sup.3+ in Fe.sub.2O.sub.3. Thus, dopants such as Mn.sup.2+ or Co.sup.2+ and Cr.sup.3+ are incorporated at both substitutional sites Fe.sup.2+ and Fe.sup.3+, respectively in Fe.sub.3O.sub.4. All above compounds have been chosen because they have spinel structures that consist of a tetrahedral and octahedral coordinated structure. When Co in tetrahedral is replaced by Mn.sup.2+, we propose that the spin-axis gets more aligned with the crystalline axis. In case of replacement of Fe.sup.3+ in octahedral with Rh.sup.3+ or Cr.sup.3+ we expect more alignment of the magnetic axis with crystalline axis. Thus double doping would proportionally increase the net coercivity as proposed above.

(16) In all cases of doping, we must maintain charge neutrality of the dopant atom with respect to the host atom. Additionally, the ion size must match. For example, Mn.sup.2+ has ionic radius size of 0.8 pm (picometer), which corresponds well with the Co.sup.2+ ionic radius size of 0.74 pm. Similarly, Fe.sup.3+ ionic radius size of 64 pm corresponds well with possible dopants Rh.sup.3+ (ionic radius 0.67 pm) or Cr.sup.3+ (ionic radius size 0.69 pm). Double doping using Mn.sup.2+ for Co.sup.2+ and Rh.sup.3 for Fe.sup.3 respectively, could have a strong effect on the spin alignment and subsequently on the coercivity of doped Co ferrite system.

(17) Preparation of Mn Doped Co Ferrite

(18) Dopant incorporation in nanocrystals in the size range of 5-30 nm is not an equilibrium process rather it is more statistical, in particular, when we are want to incorporate a single Mn.sup.2+ at Co.sup.2+ site in Co-ferrite. Normally Co-ferrite is ferromagnetic To incorporate a dopant in a nanoparticle host, certain basic requirements are to be satisfied (as mentioned above) (i) the charge of the dopant ion must be the same as the host ion it replaces; (ii) the ionic radius of the dopant ion should be similar to host ion; and (iii) the magnetic moment of the dopant ion should be significant so as to generate a substantial magnetic field at nearest neighbor atoms when confined in the nanoparticle. Mn.sup.2+ as a dopant satisfies all the conditions to replace Co.sup.2+ in CoFe.sub.2O.sub.4 and impacts the ferromagnetic nanoparticles. As an example, we used a process where we dissolve FeCl.sub.2 and CoCl.sub.2 in 2 to 1 molar ratio in deionize water to which we mix 5% of MnCl.sub.2. After mixing the above the temperature is raised to 80° C. and then NaOH is added to the above solution drop-wise until reaching a pH˜12 at which time a precipitate appears. After stirring for over 12 hours at room temperature the precipitate was separated using a centrifuge. The precipitate was washed multiple times and dried at 80 C which was characterized for magnetic and structural properties. We performed a series of experiments to assess the magnetic properties of this powder of these nanoparticles. The vibrating sample magnetometer (VSM) measurements showed that a saturation magnetization of 69.3 emu/gm with saturation field greater than 2.1 T (21,000 Oe). At 300 K temperature, coercivity was measured to be 3133 Oe (table1).

(19) Permanent Magnets from DMNP Nanopowder

(20) How to increase the coercivity and BH.sub.max to a value beyond the values that rare-earth (RE) permanent magnet possess. To achieve, we advance the process whereas we could make simultaneously doped and undoped magnetic nanoparticles and fabricate NC-HS magnets. In order to increase the energy product BH.sub.max beyond the 200 KJ/m.sup.3 from our nanomagnets it is necessary that we i) Increase the coercivity to a value about 10,000 Oe and ii) increase the remanence magnetization >10 kG.

(21) Our DMNP nanopowder consist of nanoparticles with following properties; i) the size of these nanoparticles vary from 5 to 30 nm and agglomerated nanorods as much as 5 micron in size. ii) These nanorods are made of nanoparticles which are either ferromagnetic and/or superparamagnetic. The wide variation of size and strength of magnetization can be enlisted as two category of magnetic materials, soft phase and hard phase. when a hard phase to be exchange-coupled with a soft phase, remanence magnetization can be increased to yield a value >10 kG. We have fortunately both of these phases in our DMNP nanopowder. A Combination of soft and hard phase when used to properly assemble permanent magnets could not only yield higher energy product BH.sub.max but also a better temperature dependence.

(22) To assemble the permanent magnets, currently two procedures are followed. Use of sintering at high temperatures with large isostatic or uniaxial pressure. Post magnetization under high magnetic field yields the final permanent magnet. Alternatively, the particles are mixed with a binding agent, hot pressed and then post magnetization under high magnetic field. In our case, we are using the latter scheme referred to as bonded permanent magnet. This has the advantage that we can simultaneously align the nanorods and compress them under the applied magnetic field to create a well-directed magnetic axis.

(23) The evolution of next generation permanent magnets can be seen as coming from ternary DMNP system where Co.sup.2+ in CoFe.sub.2O.sub.4 is replaced by Mn.sup.2+ and concurrently, Fe.sup.3+ is replaced by Rh.sup.3+ or Cr.sup.3+. This double doping will align the magnetic axis with crystalline axis, eliminating the canting angle between the two. Such a magnetic alignment involving tetrahedral as well as octahedral coordination in spinel structure of CoFe.sub.2O.sub.4 could increase the performance of permanent magnet due to optimization of enhanced coercivity and magneto-crystalline anisotropy due to double doping.

(24) Advantages of Double Doped Co-Ferrite

(25) 1. DMNP based nanomagnets have already a well aligned magnetic axis, and hence the creation of macro-magnet is much simpler integration process. Alignment of magnetic axes of an ensemble of nanomagnets will result in large magnetic anisotropy and powerful magnets. 2. Since the magnetic axes of these nanomagnets are strongly correlated and well aligned, the magnetic properties of our magnets will depend weakly on temperature i.e. they would perform better at high temperatures, a desirable feature. 3. Nanopowder will enable to mold complex magnet shapes. 4. These nanomagnet powders are synthesized at temperatures between 50 C to 200 C a cost-effective, scalable growth process. 5. As a comparison to the rare-earth metallic system, our nanomagnets are oxide-based i.e. no corrosion, as in the case of RE metal based magnets. 6. These transition metal oxides based magnets will be lighter than RE metals.

(26) Summarized above are the properties of current magnetic materials and the critical parameters that control the performance of magnets in use. In particular, rare-earth magnets are expensive and dominate the market for high performance applications. Our discovery of doped magnetic nanoparticles (DMNP) using commonly available magnetic materials and a magnetic dopant, has resulted in nanosize magnetic material with enhanced magnetic parameters that will be in the same range as the best rare-earth Nd-magnets as listed in the table below. These are Hc, the magnetic-resistance to turning the direction of magnetization 180°, Curie point T.sub.e, the temperature at which the material loses its magnetic properties since the alignment of spins disappears at higher temperatures, and Maximum Energy Product (BH.sub.max is an Energy Density) is a commonly used for figure of merit of magnets.

(27) Our nanomagnets will have additional advantages over the conventional magnets, since they are small, light weight, possess large magnetic anisotropy and are thermally stable.

(28) This invention provides a new class of magnetic materials by double doping of ternary host (introducing two impurity atoms) of known paramagnetic, ferrimagnetic or ferromagnetic nanomaterials to develop a high temperature nanomagnet. By introducing two dopants, in a ferromagnetic ternary system such as CoFe.sub.2O.sub.4 where Co.sup.2+ can be replaced by Mn.sup.2+ and Fe.sup.3+ is replaced by Cr.sup.3+. Thus in ternary nanomaterials we achieve, by double doping, high coercivity and remanence magnetization to create a high energy product permanent magnet that will operate at high temperatures. In fact, the doping of magnetic nanoparticles has created a true nanocomposite of hard and soft magnets which when integrated will surpass the performance of RE-based permanent magnets. These nanomagnetic hard and soft nanomaterials when integrated into bulk magnets, will yield powerful paramagnets for future electric vehicles, magnetic storage devices, sensitive electro-mechanical sensors and many more applications.

(29) The present invention has been described with respect to the above exemplary embodiments, However, as those skilled in the art will recognize, modifications and variation sin the specific details which have been described may be resorted to without departing from the spirits and scope of the invention as defined in the appended claims.