# Patent

## Title
Magnetically hard material

## Abstract

The invention relates to a modified strontium ferrite of the general chemical formula: Sr1-xLaxFe12-yCoyO19; in which x=y=0.01-1.00 or x=0.15 and y=x/1.6n to =x/2.6n, where n is the Fe2O3:SrO ratio used and may be 5-6. According to the invention, lanthanum and cobalt are added in an amount such that 0.14≦x≦0.145 and 0.14≦y≦0.145. The modified strontium ferrite thus prepared has excellent magnetic properties with regard to the remanence BR, the coercive force HCJ and the ratio of HK to HCJ.

## Background
FIELD OF THE INVENTION
The invention relates to magnetically hard material and magnets produced therefrom. In particular, the invention relates to a strontium ferrite having improved magnetic properties.
BACKGROUND OF THE INVENTION
Strontium ferrites of the general formula SrFe12O19 have long been used in large quantity as magnetically hard ferrites. It is known that the magnetic properties can be influenced by adding various elements, such as, for example, cobalt and lanthanum. Thus, for example, cobalt is used for increasing the remanence BR and lanthanum for increasing the coercive force HCJ and the breaking strength. Such a modified strontium ferrite has the general formula: Sr1-xLaxFe12-yCoyO19 in which x=y=0.01-1.00 or x=0.15 and y=x/1.6n to =x/2.6n, and n is the Fe2O3:SrO ratio used and can assume a value of 5-6.
Apart from the exact amount of the respective additives, such a ferrite is greatly influenced by the preparation process, in particular by the conditions during sintering. Thus, it is generally found that an increase in the sintering temperature leads to an increase in the remanence BR and to a decrease in the coercive force HCJ. Conversely, a low sintering temperature has a positive effect on the coercive force HCJ but reduces the remanence BR. Such modified strontium ferrites are disclosed inter alia, in WO 99/34376 A1, EP 1465214 A1, US 20040151661 A1 or WO 03/056578 A1.
Moreover, EP 0964411 A1 describes a modified strontium ferrite of the abovementioned form, in which x=y=0.15 is fulfilled. This strontium ferrite has a high coercive force HCJ of 4500 Oe. No information is given regarding the remanence BR. From this publication, it is evident that small changes in the proportions of cobalt, of lanthanum or of oxygen lead to considerable changes in the magnetic properties. Moreover, the magnetic properties can be influenced by whether lanthanum and cobalt are added to the strontium ferrite before the calcination or after the calcination.
WO 98/03864 moreover discloses such a modified strontium ferrite, extensive investigations showing that the relationship between the concentration of cobalt and lanthanum and the magnetic properties is very complex. According to this publication, a modified strontium ferrite in which the proportion of cobalt and lanthanum in which x=y=0.3 is preferred.
It was therefore an object of the invention to provide a modified strontium ferrite which has improved magnetic properties, in particular which has a high remanence BR and at the same time a high coercive force HCJ, and in which the squareness of the demagnetization curve, expressed by HK/HCJ, is high. HK is the field strength at which the remanence is irreversibly reduced by 5%.
SUMMARY OF THE INVENTION
In accordance with the present invention, such a modified strontium ferrite is determined by the features of claim 1. The dependent claims relate to further advantageous aspects of the invention.
More specifically, a modified strontium ferrite of the general formula Sr1-xLaxFe12-yCoyO19, wherein 0.14≦x≦0.145; 0.14≦y≦0.145 is disclosed.
In one embodiment of the modified strontium ferrite x=y=0.142 is fulfilled. In such embodiments, BR≧(5100−0.1844*Oe−1*Ha) G; in which BR is the remanent magnetization in G, HCJ being the coercive force in Oe. Further, the modified strontium ferrite wherein the ratio of HK to HCJ is >88%, preferably >90%.
In preferred embodiments, the modified strontium ferrite wherein a sintering process at a temperature between 1225° C. and 1245° C. was carried out during the preparation. Further the modified strontium ferrite wherein lanthanum oxide (La2O3) and cobalt oxide (Co3O4) are added after the calcination step.
A magnetically hard article of the present invention wherein it was produced from a modified strontium ferrite according to the above embodiments.
The process for the preparation of a modified strontium ferrite of the type noted above can be made using the following steps:
initial introduction of a mixture of iron oxide and strontium carbonate;
calcination of the mixture at a temperature of 1300° C.+/−20° C.;
precomminution of the calcined strontium ferrite in the dry state;
admixing of lanthanum oxide and cobalt oxide in a specified amount;
wet milling of the calcined strontium ferrite with the additives lanthanum oxide and cobalt oxide to a particle size in the range of 0.7-1.0 μm;
pressing in a magnetic field, drying and sintering of the blank thus obtained, at a temperature in the range between 1220° C. and 1250° C., preferably between 1225° C. and 1245° C.;
all such that lanthanum oxide and cobalt oxide are added in an amount such that, in the prepared sintered modified strontium ferrite, a composition according to the following formula results: Sr1-xLaxFe12-yCoyO19, in which 0.14≦x≦0.145; and 0.14≦y≦0.145.
Below, embodiments of the invention are described in more detail with reference to the attached figures.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a flow diagram of the preparation process for the modified strontium ferrite of the invention; and
FIG. 2 shows a diagram for illustrating the advantageous magnetic properties of the strontium ferrite modified according to the invention.



DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT
While the present invention is susceptible of embodiment in various forms, there is shown in the drawings a number of presently preferred embodiments that are discussed in greater detail hereafter. It should be understood that the present disclosure is to be considered as an exemplification of the present invention, and is not intended to limit the invention to the specific embodiments illustrated. It should be further understood that the title of this section of this application (“Detailed Description of an Illustrative Embodiment”) relates to a requirement of the United States Patent Office, and should not be found to limit the subject matter disclosed herein.
As shown in FIG. 1, the modified strontium ferrite according to the invention is prepared as follows. In a first step, an iron oxide is mixed with a strontium oxide or strontium carbonate. The mixture thus prepared is calcined in a second step at a temperature of 1300° C.+/−20° C.
In a third step, the calcined material is comminuted in the dry state to mean particle sizes of 10-30 μm. Lanthanum oxide (La2O3) and cobalt oxide (Co3O4) are added to this ferrite powder as additives.
Wet milling to particle sizes of 0.7-1.0 μm is effected as the next step.
This suspension is pressed in a magnetic field, dried and then sintered at temperatures between 1220° C. and 1250° C., preferably between 1225° C. and 1245° C. Lanthanum oxide (La2O3) and cobalt oxide (Co3O4) are added as additives to the suspension obtained as a result of the wet milling.
The blank is then dried in a magnetic field, pressed, and then sintered at temperatures between 1220° C. and 1250° C., preferably between 1225° C. and 1245° C.
First Example
A first example according to the invention was prepared using the process described above. For this purpose, lanthanum oxide and cobalt oxide in a specified ratio were added to the already calcined and precomminuted strontium ferrite during the wet milling. Thereafter, the suspension was pressed in a magnetic field of 3000 Oe, then dried, and sintered at 1225° C. The prepared product then had a composition of the formula Sr1-xLaxFe12-yCoyO19, in which x=y=0.142.
The main magnetic characteristics, namely the remanence BR and the polarization coercive force HCJ were then measured at 23° C. The measurement was effected according to the method DIN EN 60404-5.
Here, values of 4240 G for the remanence BR, of 4738 Oe for the coercive force HCJ and of 88% for the ratio of HK to HCJ were found.
Second Example
The second example was identical to the first example, with the only difference that the sintering temperature was not 1225° C. but 1230° C.
The value for the remanence BR was 4300 G, the value for the coercive field HCJ was 4612 Oe and the ratio HK to HCJ was 90%.
Third Example
Once again, a modified strontium ferrite of the same composition was prepared identically to the first and second examples. The sintering temperature here was 1235° C. The resulting value for the remanence BR was 4390 G. The value for the coercive force HCJ was 4361 Oe. The ratio of HK to HCJ was 91%.
Fourth Example
In a fourth example, a sintering temperature of 1240° C. was now applied to an identical modified strontium ferrite. The value of the remanence BR increased to 4480 G while the coercive force HCJ decreased to 3808 Oe. The ratio of HK to HCJ was still 91%.
Fifth Example
In a fifth example, a sintering temperature of 1245° C. was now chosen. A modified strontium ferrite of the same composition as in the first four examples was used for this purpose. The value of the remanence BR was 4560 G. The value for the coercive force HCJ decreased to 3003 Oe. The ratio HK to HCJ was 92%.
All these modified strontium ferrites according to the invention show excellent squareness through high values of HK to HCJ with simultaneously very high values for both the remanence and the coercive force HCJ. The hard ferrite magnets thus produced have substantially better magnetic properties than the conventional strontium ferrite magnets.
First Comparative Example
For comparison, a modified strontium ferrite of the formula Sr1-xLaxFe12-yCoyO19, in which x=y=0.15 was set, was now prepared. This modified strontium ferrite was otherwise prepared in the same manner as the strontium ferrites according to the invention, in particular as the strontium ferrite according to the invention of the second example. Here too, the sintering temperature was 1230° C.
A remanence BR of 4070 G resulted. The coercive force HCJ was 4637 Oe. The ratio of HK to HCJ decreased to 73%.
Second Comparative Example
A modified strontium ferrite having the same composition as the first comparative example was once again prepared, the sintering temperature being 1235° C. The resulting value of the remanence BR was 4170 G. The value for the coercive force HCJ was 4361 Oe. The ratio of HK to HCJ was 85%.
The values for the remanence in the first and second comparative examples are clearly substantially lower than in the case of the modified strontium ferrites according to the invention. Moreover, the ratio of HK to HCJ is substantially poorer.
Third Comparative Example
Once again, a modified strontium ferrite was prepared by the same process as in the examples according to the invention. However, it was specified here that x=y=0.1.
At a sintering temperature of 1230° C. in this comparative example, values of 4190 G for the remanence BR and of 4499 Oe for the coercive force HCJ resulted. The value of the ratio of HK to HCJ was 88%.
In this comparative example, both the remanence BR and the coercive force HCJ are clearly lower than in the example according to the invention at the same sintering temperature.
Fourth Comparative Example
Once again, a modified strontium ferrite having the same composition as in the third comparative example was prepared. The sintering temperature was adjusted to 1240° C. The value of the remanence BR was 4300 G, and 4159 Oe was obtained for the coercive force HCJ. The ratio of HK to HCJ was 90%.
Although a very good value for the coercive force HCJ was achieved in this comparative example, the remanence BR was substantially below the value of the corresponding example according to the invention.
Fifth Comparative Example
A further modified strontium ferrite with the same composition as in the third and fourth comparative examples was prepared and was sintered at a temperature of 1245° C. The resulting value of the remanence BR was 4350 G. The value of the coercive force HCJ was 3845 and the ratio of HK to HCJ was still 90%.
The comparison of the third to fifth comparative example with the embodiments according to the invention shows that, according to the invention, in each case substantially better pairs of values for the remanence and the coercive force are achieved with a comparably good ratio of HK to HCJ.
Sixth Comparative Example
In the sixth comparative example, a modified strontium ferrite of the formula Sr1-xLaxFe12-yCoyO19, in which x=y=0.16 was chosen, was prepared.
The strontium ferrite thus prepared was sintered at a temperature of 1245° C. The resulting value for the remanence BR was 4540 G. The coercive force HCJ was 3116 Oe. The ratio HK to HCJ was 94%.
The modified strontium ferrite thus prepared had good values for remanence and squareness but the value for the coercive force is very much smaller than in the example according to the invention.
Seventh Comparative Example
Once again, a modified strontium ferrite having the same composition as in the sixth comparative example was prepared and was sintered at 1235° C. Here, the resulting value of the remanence BR was 4270 G. The value for HCJ was 4021 Oe. The ratio HK to HCJ was 90%.
In the case of this composition of the strontium ferrite according to Comparative Examples 6 and 7, it was found that the dependence of the magnetic properties on the sintering temperature is very pronounced. As a result of this, even with relatively small variations in sintering temperature, the advantageous properties can no longer be guaranteed.
Eighth Comparative Example
A modified strontium ferrite of the formula Sr1-xLaxFe12-yCoyO19 was once again prepared. The value for x=y was set at 0.138. At a sintering temperature of 1230° C., the resulting value for the remanence BR was 4080 G. The value for HCJ was 4524 Oe. The ratio of HK to HCJ was 74%.
Ninth Comparative Example
A ninth comparative example was measured with the same composition as in the eighth comparative example but at a sintering temperature of 1235° C. Here, the value of the remanence BR was 4120 G while the coercive force HCJ was 4298 Oe. The ratio of remanence BR to saturation magnetization Bs was equal to 83%.
Although the composition of the modified strontium ferrite of the eighth and ninth comparative examples is relatively close to the embodiments according to the invention, the values for the remanence BR, for the coercive force HCJ and also for the ratio of HK to HCJ at the same sintering temperatures are substantially poorer than the case of the invention. Surprisingly, these values are also substantially poorer than in the third to fifth comparative examples. From this it is evident that even small changes in the composition can have substantial effects on the magnetic properties.
In the first to fifth examples according to the invention, it is found that a modified strontium ferrite having outstanding properties can be obtained over the entire range of the sintering temperature from 1225° C. to 1245° C. With regard to the remanence BR, to the coercive force HCJ and to the ratio of remanence BR to saturation magnetization BS, the strontium ferrites prepared according to the invention are excellent and are superior to those of the comparative examples.
Overall, it is found that, with the modified strontium ferrite according to the invention, it is possible, at a ratio of HK to HCJ of 88% or more, to achieve a pair of values for the coercive force HCJ and the remanence BR which satisfies the following equation:

BRG−1+0.1844*Oe−1*HCJ≧5100.

As an overview, Table 1 shows the composition and the magnetic characteristics of the embodiments according to the invention and of the comparative examples.














TABLE 1








BR in
HCJ in
Sintering
HK/HCJ *
BRG−1 + 0.1844 *



x = y =
G
Oe
temperature
100
Oe−1 * HCJ






















E1
0.142
4240
4738
1225
88
5113


E2
0.142
4300
4612
1230
90
5150


E3
0.142
4390
4361
1235
91
5194


E4
0.142
4480
3808
1240
91
5182


E5
0.142
4560
3003
1245
92
5113


C1
0.150
4070
4637
1230
73
4925


C2
0.150
4170
4361
1235
85
4974


C3
0.100
4190
4499
1230
88
5019


C4
0.100
4300
4159
1240
90
5066


C5
0.100
4350
3845
1245
90
5059


C6
0.160
4540
3116
1245
94
5059


C7
0.160
4270
4021
1235
90
5011


C8
0.138
4080
4524
1230
74
4914


C9
0.138
4120
4298
1235
83
4912









Although an illustrative embodiment of the invention has been shown and described, it is to be understood that various modifications and substitutions may be made by those skilled in the art without departing from the novel spirit and scope of the invention.

## Claims

1. Modified strontium ferrite of the general formula


Sr1-xLaxFe12-yCoyO19, wherein x=y=0.142


sintered at a temperature of between 1225° C. and 1245° C.

2. Modified strontium ferrite according to claim 1, having a ratio of HK to HCJ of >88%.

3. Modified strontium ferrite according to claim 1, wherein lanthanum oxide (La2O3) and cobalt oxide (Co3O4) are added after a calcination step.

4. Magnetically hard article, wherein it was produced from a modified strontium ferrite according to claim 1.

5. Modified strontium ferrite according to claim 1, having a ratio of HK to HCJ of >90%.