Permanent magnets have long been applied for fixing prostheses in or on the body of a user. To this end a ferromagnetic material is anchored in the body and the prosthesis is provided with a magnet exerting a magnetic force on the ferromagnetic material. For a firm attachment of the prosthesis, the magnet needs to be strong, but in addition, the ferromagnetic material in the body must be very readily magnetizable. The strongest known permanent magnetic materials are based on compositions of iron or cobalt with rare earth metals. Said magnets are very sus- ceptible to corrosion so that they have to be encapsulated in, for example, stainless steel in order to prevent the release of corrosion products in the body. The possibility exists that during use but also during the fabrication of the prostheses, the stainless steel capsule becomes dam- aged, with the result that within a very short time the magnet system ceases to work.

In the article by Kiyoshi Watanabe, published in Materials Transactions, JIM, Vol. 32, No. 3 (1991), pp 292 to 298, it is reported that iron-platinum alloys with the addition of niobium possess good hard-magnetic properties.

It is, however, not possible with the method according to said article, to realize soft-magnetic properties. The im- portance of this will be explained later.

DE-A-4027681 discloses a method of fixing a den- tal prosthesis where a magnet is located in the dental

prosthesis and interacting with that, a head piece in the dental root. The magnet known from this publication com- prises 33 to 47 atom% of platinum, rest iron, while the head piece is comprised of soft-magnetic stainless steel.

The magnet further comprises preferably 0.1 to 10 atom% of an element selected from the group Ti, Mo, Nb, Ta, W, Cr and V. In this construction the implant may exhibit corro- sion because the material of the magnet is nobler.

It is the object of the invention to provide a ferromagnetic material that is suitable for fixing a pros- thesis in or on the body of a user, and which provides a sufficiently strong attachment of the prosthesis. It is desirable to be able to provide both material that has soft-magnetic properties and material that has hard- magnetic properties. The soft-magnetic material may then suitably be used as implant, while the prosthesis itself may then suitably the made in one piece from hard-magnetic materials. Soft-magnetic behaviour combined with highly spontaneous magnetisation and low remanence of the implant are important to create good adhesion.

The possibility of using the same material for both the implant and the prosthesis, while these only dif- fer with regard to the type of the magnetic properties, provides the further advantage that in the presence of body fluids no difference in the electrochemical potential can arise which could cause nerve irritation, or corro- sion.

To realize these objectives and advantages, the method according to the invention is characterized in that the alloy is melted in an oxidation-inhibiting atmosphere, avoiding any contact of the melt with surfaces that have a temperature in the region of or higher than the alloy's fusion temperature. The material obtained according to this method of the invention, which material is based on a conventional alloy of iron and platinum, having a platinum content above 30 atom%, is very corrosion-resistant and may be implanted in the body without protective encapsula- tion. In this respect the material obtained by the method

according to the invention differs from the material known from DE-A-4027681, which may indeed have the same general composition but which suffers from impurities that have a negative effect on the intended application as implant of soft-magnetic material.

The method according to the invention effec- tively prevents the diffusion of material between melt and said surfaces, for example, a crucible, so that the melt stays as much as possible free from impurities. The inven- tors believe that such impurities, if not avoided, will form so-called pinning centres for magnetic domeins or Bloch walls.

As oxidation-inhibiting atmosphere for melting the alloy, it is convenient to use an argon atmosphere. In a first embodiment of the method according to the inven- tion, the melting step preferably involves welding arc fu- sion in a cold crucible. The person skilled in the art is quite familiar with this form of fusion and it requires no further explanation. The temperature of the crucible is preferably approximately 100°C below the fusion temperature of the alloy. More preferably the temperature difference is at least approximately 200°C.

A second preferred embodiment of the method ac- cording to the invention is characterized in that the al- loy is molten by means of levitation fusion. Again, the person skilled in the art requires no further explanation.

Levitation fusion ensures that there is no contact between the melt and the crucible.

Basically the method according to the invention differs from the known method as described in the above- mentioned article by Watanabe and in DE-A-4026781, in that the manner of operation is completely clean. For example in the known prior art, fusion may be carried out in alu- minium crucibles and the product is subsequently poured into quarts ampoules, which results in undesirable con- tamination of the product obtained by said method.

In order to obtain the desired qualities it is further favourable after fusion to carry out an annealing

treatment at a temperature in the vicinity of 1300°C. Pref- erably the annealing treatment takes approximately one hour. It is further desirable to subsequently cool the al- loy to room temperature at a rate of at least approxi- mately 1000°C/minute. This provides a reproducibly homoge- nous solid solution of the components in the face-centred cubic gamma-iron structure of the alloy. The practically contamination-free material obtained by this method is magnetically soft, mechanically strong and very corrosion- resistant, and is especially suitable for use as implant.

A further elaboration of the method according to the invention is characterized in that a recrystallization treatment is carried out; in order to conveniently control the speed of the recrystallization treatment it is desir- able that the non-ferro metal be selected from the group Nb, Zr, Ga and Ti and that the amount be approximately 1 atom%. After the alloy is cooled down, the recrystalliza- tion is performed in a first preferred embodiment by heat- ing for 20 to 80 hours in the range of 575-650°, preferably approximately 625°C. In this way a face-centred tetragonal phase is provided, making the material obtained by the method magnetically hard, mechanically strong and very corrosion resistant. This material is especially suitable to be applied in or as the prosthesis.

To optimize the speed of the recrystallization treatment the non-ferro metal advantageously is 0.1 to 0.4 atom% Al, preferably 0.25 atom%, so that after cooling, the recrystallization treatment can be carried out by heating the alloy to approximately 525°C for approximately 20 hours. The recrystallization treatment may be further accelerated by placing the alloy during the recrystalliza- tion treatment under at least approximately 2 bars of uni- axial pressure, and by heating the alloy at a maximum tem- perature of approximately 450°C for 2 to 3 hours. This also provides a very energy-efficient recrystallization treat- ment.

In order to maintain the properties of the alloy obtained by the method according to the invention, it is desirable that after melting and before cooling, the alloy be cast into a predetermined form by means of vacuum cast- ing. A casting produced in this manner is very corrosion- resistant, is easy to polish, is mechanically strong, has a Vickers hardness of approximately 400, and, depending on whether or not it has undergone the above-mentioned re- crystallization treatment, may have both soft-and hard- magnetic properties.

The invention is also embodied in a permanent (hard)-magnetic product and a soft-magnetic product ob- tained by the method according to the invention.

To the person skilled in the art it will be ob- vious that within the framework of the invention diverse variants are possible, whose limitation is determined solely by the appended claims. Therefore the preceding specification is to be understood as elucidation of the appended claims without limiting these in any way whatso- ever.