Synthetic antiferromagnet disk-shaped particle and suspension comprising such particles

Abstract

The synthetic antiferromagnet disk-shaped particle comprises a first ferromagnetic layer, a second ferromagnetic layer and a non-magnetic interlayer arranged between the first and the second ferromagnetic layer, wherein each of the first and the second ferromagnetic layer comprises a uniaxial magnetic anisotropy in the plane of the ferromagnetic layers such that the switching fields from an antiferromagnetic alignment of the first and the second ferromagnetic layer to a ferromagnetic alignment (H.sub.AF.fwdarw.F) and from the ferromagnetic alignment to the antiferromagnetic alignment (H.sub.F.fwdarw.AF) fulfill the condition $H_{\mathrm{AF}.fwdarw.F}-H_{F.fwdarw.\mathrm{AF}}>\frac{1}{4}.Math.H_{\mathrm{AF}.fwdarw.F}.$

Claims

1. A synthetic antiferromagnet disk-shaped particle comprising a first ferromagnetic layer, a second ferromagnetic layer, a non-magnetic interlayer arranged between the first and the second ferromagnetic layer, wherein each of the first and the second ferromagnetic layer comprises a uniaxial magnetic anisotropy in the plane of the ferromagnetic layers such that the switching fields from an antiferromagnetic alignment of the first and the second ferromagnetic layer to a ferromagnetic alignment (H.sub.AF.fwdarw.F) and from the ferromagnetic alignment to the antiferromagnetic alignment (H.sub.F.fwdarw.AF) fulfill the condition H.sub.AF.fwdarw.FH.sub.F.fwdarw.AF>.Math.H.sub.AF.fwdarw.F; wherein the uniaxial magnetic anisotropy in the plane of the ferromagnetic layers is larger than 5000 Joule per cubic meter; and wherein the synthetic antiferromagnet disk-shaped particle is characterized by a magnetic easy axis hysteresis loop in externally applied magnetic fields of 140 Millitesla or smaller.

2. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein an easy axis magnetic hysteresis loop has a substantially rectangular shape in the first and third quadrant of a coordinate system of magnetization versus magnetic field.

3. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein a ferromagnetic alignment of the first and the second ferromagnetic layers remains after the externally applied magnetic field is removed, but then decays back into an antiferromagnetically aligned ground state by thermal activation occurring at room temperature within a time period shorter than ten seconds; or the antiferromagnetically aligned ground state is re-established by application of an oscillatory magnetic field with a decaying field amplitude.

4. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein any one of the first or second ferromagnetic layer comprises Fe, Co, or Ni.

5. The synthetic antiferromagnet disk-shaped particle according to claim 4, wherein any one of the first or second ferromagnetic layer comprises an alloy comprising Fe, Co or Ni.

6. The synthetic antiferromagnet disk-shaped particle according to claim 5, wherein any one of the first or second ferromagnetic layer further comprises a rare earth element.

7. The synthetic antiferromagnet disk-shaped particle according to claim 6, wherein the rare earth element is samarium.

8. The synthetic antiferromagnet disk-shaped particle according to claim 4, wherein any one of the first or second ferromagnetic layer further comprises a rare earth element.

9. The synthetic antiferromagnet disk-shaped particle according to claim 8, wherein the rare earth element is samarium.

10. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein a diameter of the particle is between 10 nanometers and 5000 nanometers.

11. The synthetic antiferromagnetic disk-shaped particle according to claim 1, wherein a thickness of the particle is between 5 nanometers and 1000 nanometers.

12. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein the interlayer promotes an RKKY coupling between the first and the second ferromagnetic layer, thereby weakening the antiferromagnetic coupling between the first and the second ferromagnetic layer caused by magnetic stray fields of the first and second ferromagnetic layers.

13. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein the interlayer comprises platinum (Pt), iridium (Ir) or a PtIr alloy.

14. The synthetic antiferromagnet disk-shaped particle according to claim 13, wherein the interlayer comprises a Pt.sub.1-xIr.sub.x alloy.

15. The synthetic antiferromagnet disk-shaped particle according to claim 1, further comprising an additional layer capable of performing at least one of: oxidation protection of the first and the second ferromagnetic layer; chemical functionalization to obtain colloidal stability of the particle in liquids; biomedical functionalization; prevention of uptake of the particle by macrophages; release or reaction of a chemical substance comprised in the particle; and/or separation of the particle structure microfabricated on a substrate from the substrate.

16. The synthetic antiferromagnet disk-shaped particle according to claim 1, wherein the uniaxial magnetic anisotropy in the plane of the first ferromagnetic layer is generated through deposition of the first ferromagnetic layer onto an obliquely sputtered tantalum layer or through alloying the first ferromagnetic layer with a rare earth element.

Description

(1) The invention is further described with regard to examples, which are illustrated by means of the following drawings, wherein:

(2) FIG. 1 shows a synthetic antiferromagnet disk-shaped particle;

(3) FIG. 2 illustrates an idealized view of the easy axis and hard axis hysteresis loops for magnetic fields applied in the plane of the ferromagnetic layer of a synthetic antiferromagnet particle;

(4) FIG. 3 illustrates an idealized view of the easy axis and hard axis hysteresis loops for magnetic fields applied in the plane of the ferromagnetic layer of another synthetic antiferromagnet particle;

(5) FIGS. 4-6 shows an operation mode of synthetic antiferromagnet particles in externally applied magnetic field (FIG. 4: random alignment; FIG. 5: alignment of the easy axis; FIG. 6: magnetic field sequence);

(6) FIG. 7 shows easy and hard axis hysteresis loops of a micropatterned disk-shaped antiferromagnet particle;

(7) FIG. 8-10 are illustrations of a synthetic antiferromagnet particle with ferromagnetic RKKY exchange between ferromagnetic layers and its hysteresis loop;

(8) FIG. 11 is a schematic layer structure of a functionalized synthetic antiferromagnet particle;

(9) FIG. 12 is an illustration of particle's specific heat loss in tumors.

(10) FIG. 1 illustrates a synthetic antiferromagnet particle 1. The particle 1 has a circular diameter and is disk-shaped. A first ferromagnetic layer 10 and a second ferromagnetic layer 11 are arranged above each other, separated by a non-magnetic interlayer 12.

(11) Arrows 20, 21 indicate the orientation of the magnetic moments of the first and the second ferromagnetic layers 10, 11 according to their uniaxial anisotropy K.sub.u in the plane of the ferromagnetic layers 10, 11. The arrows 20, 21 directing in opposite directions also indicate the antiferromagnetic alignment of the magnetic moments of the first and the second ferromagnetic layers 10, 11. The antiferromagnetic coupling may be obtained by the magnetic stray field of the first and second layers, which stray field is indicated by arrows 30.

(12) Dotted line 400 indicates the easy magnetization axis of both ferromagnetic layers 10, 11 of the particle 1. The magnetization is arranged in the planes of the ferromagnetic layers 10, 11. Dashed line 410 indicates the hard magnetization axis of both ferromagnetic layers 10, 11 of the particle 1. The hard magnetization axis is arranged perpendicular to the easy magnetization axis.

(13) The diameter 18 of the particle 1 may, for example, be about 500 nm. The height of the particle 1 may, for example, be about 14 nm. Thereby, the thicknesses of the first and second ferromagnetic layer 10, 11 may be about 6 nm and the thickness of the interlayer 12 may be about 2 nm.

(14) The first and second ferromagnetic layer 10, 11 are preferably made of Co, Fe, CoFe-alloy, FeB-alloy, CoB-alloy or FeCoB-alloy, possibly comprising, for example Ni. Preferably, the first and second ferromagnetic layers 10, 11 are CoFeB layers having a uniaxial anisotropy in the plane of each ferromagnetic layers of 20 kJ/m.sup.3 and a saturation magnetization of M.sub.s>1200 kA/m.

(15) The material of the interlayer preferably is Ta.

(16) In the M(H)-coordinate system shown in FIG. 2 the magnetization 14 of the particle dependent on the externally applied magnetic field H is depicted. The solid black line 40 illustrates an ideal easy-axis hysteresis loop in the plane of the ferromagnetic layers to obtain largest magnetization losses and an antiferromagnetic ground state of the particle with vanishing magnetic moment at zero magnetic field. The areas 44 circumscribed by the hysteresis loop 40 are representative for the hysteresis losses in the particle 1, for example as shown in FIG. 1. The hysteresis loops in the first 450 and in the third 452 quadrant of the M(H)-coordinate system have an exact rectangular shape for an idealized antiferromagnet particle.

(17) When the externally applied magnetic field reaches the saturation field H.sub.s of the ferromagnetic layer, the magnetic moments in the ferromagnetic layer are oriented into the direction of the magnetic field and the saturation magnetization M.sub.s is reached.

(18) In the first quadrant 450 of the M(H)-coordinate system the hysteresis loop for positive magnetic fields and positive magnetization values is shown. In the third quadrant 452 of the M(H)-coordinate system the hysteresis loop for negative magnetic fields and negative magnetization values is shown. For negative fields, the same magnetization process occurs for the other ferromagnetic layer such that a hysteresis loop occurs for the equivalent negative fields, and negative magnetization values.

(19) At zero magnetic field the particle represented in FIG. 2 is in its antiparallel configuration: the magnetic moments of the first and the second ferromagnetic layer 10, 11 are oriented in an antiparallel manner, for example as shown in FIG. 1. The particle has a vanishing total magnetization. The magnetization orientation characteristic for a synthetic antiferromagnet particle is re-established when the magnetic field is zero.

(20) In real particles, preferably, the saturation magnetization is substantially reached and the hysteresis loops have a substantially rectangular shape. Thus, preferably, for a field H>H.sub.AF.fwdarw.F>0 applied along the uniaxial anisotropy axis in the plane of the ferromagnetic layers, the alignment of the magnetizations of the first and the second ferromagnetic layer 10, 11 is substantially parallel, for example larger than 80 percent of the saturation magnetization (0.8.Math.M.sub.s). Preferably, the magnetization is even larger than 90 percent of the saturation magnetization (0.9.Math.M.sub.s).

(21) In the antiferromagnet disk-shaped particle according to the present invention, a hard axis magnetic saturation field of the particle is larger than an easy axis magnetic saturation field of the particle.

(22) The saturation field H.sub.s is the field required to align the magnetization of the ferromagnetic layers parallel to a field applied in the plane of the layer, perpendicular to the uniaxial anisotropy axis (hard axis magnetization process) This magnetization process is illustrated by the dashed line 41, showing no hysteresis losses.

(23) The particles of the present invention have ferromagnetic layers with a significant uniaxial anisotropy in the plane of the ferromagnetic layers.

(24) For a field aligned along this uniaxial anisotropy axis the ferromagnetic layer with a magnetization opposite to the direction of the applied field switches at a switching field H.sub.AF.fwdarw.F (0<H.sub.AF.fwdarw.F<H.sub.s and reaches a preferred magnetization M>0.9.Math.M.sub.3 within a small field interval HH.sub.AF.fwdarw.F when the field is increased. For a decreasing field the magnetization of the ferromagnetic layer remains close to the saturation value, preferably at M>0.8.Math.M.sub.s and switches back to antiferromagnetic alignment abruptly at a field H.sub.F.fwdarw.AF>0, which field is close to 0, within a field interval H<H.sub.F.fwdarw.AF, such that the area of the partial hysteresis loop is maximized.

(25) FIG. 3 illustrates idealized hysteresis loops of a synthetic antiferromagnet particle with even higher magnetization loss, showing remanence at zero field.

(26) The same reference numbers are used for the same or similar elements.

(27) The hysteresis loops extend over all four quadrants 450, 451, 452, 453 of the M(H)-coordinate system.

(28) The particle and its hysteresis loop is designed such that the antiferromagnetic coupling of the first and second antiferromagnetic layer 10, 11 is too weak to return the particle into its antiferromagnetic ground state when the external magnetic field H is turned to zero.

(29) The particle shown in the example of FIG. 3 is also designed that thermal decay into the ferromagnetic ground state occurs as shown by arrow 45. Thermal decay occurs at room temperature or body temperature in a time preferably below 10 seconds.

(30) Alternatively, an oscillatory magnetic field with a decaying field magnitude may be applied to the particle to return the particle into the antiferromagnetic ground state.

(31) For hyperthermia applications or also other applications, where the synthetic antiferromagnet particle is heated through hysteretic losses, preferably, a sequence of a magnetic alignment field and a magnetic oscillatory field is applied to the particle.

(32) This is outlined in more detail in FIGS. 4, 5 and 6.

(33) In suspensions comprising a plurality of synthetic antiferromagnet particles, for example as shown in FIG. 1, no preferential particle orientation exists in the absence of an external magnetic field (FIG. 4). The particles in a liquid have a random orientation and rotate with a characteristic time TB, the Brownian relaxation time.

(34) In order to obtain the highest magnetic losses when applying oscillatory magnetic fields, the particles must be forced to align their easy magnetization axis with the uniaxial direction of the applied external field. Such an alignment is achieved by application of a magnetic field with a magnitude larger than the easy axis saturation field but smaller than the hard axis saturation field over a time duration t.sub.a longer than the Brown relaxation time of the particles.

(35) Thus, if an alignment field H.sub.a with H.sub.AF.fwdarw.F<H.sub.a<H.sub.s is applied for a time duration 52 of t.sub.a>.sub.B, the easy-axes of all particles align with the field (FIG. 5).

(36) Subsequently, an oscillatory field H.sub.osc with a magnitude larger than H.sub.AF.fwdarw.F to drive the magnetization of the particles through their easy-axis hysteresis loop is applied, preferably over a time duration 53 of t.sub.osc shorter than the Brown relaxation time of the particles.

(37) Afterwards, the particles are re-aligned again before the oscillatory field is again applied.

(38) In FIG. 6 the magnetic field sequence is shown that is required to align the particles with their easy axis and subsequently experience their easy axis hysteresis loops in the layer plane by the applied oscillatory field.

(39) The alignment field is applied to the particle 1 to achieve an alignment of, for example M>0.8M.sub.s, of the particle's easy magnetization axes along the alignment field. The oscillatory field with amplitude H.sub.osc and frequency f.sub.osc is applied during the time period t.sub.osc to drive the particles N=t.sub.osc.Math.f.sub.osc times through the easy axis hysteresis loop for heat generation before the particle returns to the antiferromagnetic ground state or is made to return to the antiferromagnetic ground state.

(40) FIG. 4 shows the status of the particles in the suspension before application of a magnetic field and after the particles have gained their random orientation and antiferromagnetic ground state after magnetic field application, e.g. after hyperthermia treatment. Typically, the random orientation of the particles is achieved in 10 ms to 100 ms after the magnetic field has been turned off, while the antiferromagnetic ground state is achieved in less than 10 seconds after the magnetic field has been turned off.

(41) The diameter of the synthetic antiferromagnet disk-shaped particles may be rather large, e.g. 500 nm, compared to superparamagnetic particles (<20 nm) commonly used for hyperthermia applications. Accordingly, the Brownian relaxation time may be rather large until the particles rotate back to an antiparallel ground state and random orientation in a liquid. Accordingly, there is plenty of time to drive the particles several times through their easy axis hysteresis loop in an applied oscillating field.

(42) FIG. 7 shows a hysteretic easy 40 and non-hysteretic hard-axis 41 hysteresis loops of a micropatterned antiferromagnet particle with a diameter of 500 nm. The first and second ferromagnetic layers are 6 nm thick CoFeB films, an amorphous ferromagnetic alloy with a saturation magnetization .sub.0M.sub.3=1.75 T, and an exchange stiffness A=15 pJ/m. The first and second ferromagnetic layers are antiferromagnetically coupled and each layer has a uniaxial anisotropy in the plane of the ferromagnetic layer of 20 kJ/m.sup.3. The first and second ferromagnetic layers are separated by a 2 nm non-magnetic interlayer, for example tantalum.

(43) The ferromagnetic state of the particle occurs after application of a magnetic field of slightly less than 50 mT along the easy axis. As may be seen in the drawing, the easy axis hysteresis loop has an almost rectangular shape in the first and the third quadrant of the M(H)-coordinate system and comprises slightly rounded edges close to the saturation magnetization.

(44) FIG. 8 and FIG. 9 show a disk-shaped synthetic antiferromagnet particle with a diameter 18 of 50 nm. The particle is shown in the antiferromagnetic ground state (FIG. 8) and in the ferromagnetic state after application of a field larger than H.sub.AF.fwdarw.F (FIG. 9) along the easy uniaxial anisotropy axis in the plane of ferromagnetic layer. The first and second ferromagnetic layers 10, 11 are 6 nm thick amorphous CoFeB films with a saturation magnetization .sub.0M.sub.s=1.75 T, and an exchange stiffness A=15 Wm, each having a uniaxial anisotropy in the plane of the ferromagnetic layer of 50 kJ/m.sup.3. The two ferromagnetic layers are separated by a 2 nm thick interlayer made of a Ft/Ir alloy with a ferromagnetic RKKY exchange of either 0.35 mJ/m.sup.2 or 0.39 mJ/m.sup.2, required to obtain a thermal decay of the ferromagnetic state back into the antiferromagnetic ground state in a reasonable short time, e.g. in less than 10s, preferably in about is. The two ferromagnetic layers are antiferromagnetically coupled with a homogeneous antiparallel magnetization 15 along the easy axis (ground state FIG. 8).

(45) The ferromagnetic state is shown in FIG. 9 with the two ferromagnetic layers 10, 11 having a parallel magnetization 15 along the easy axis and in the direction of the applied magnetic field. The ferromagnetic state occurs after application of a field of about 80 mT along the easy axis 40.

(46) In FIG. 10, the easy axis hysteresis loops of the particle of FIG. 8 and FIG. 9 is shown with the interlayer 12 causing a ferromagnetic RKKY interaction J.sub.FM required to obtain a thermal decay of the ferromagnetic state back into the antiferromagnetic ground state in a reasonable short time, e.g. less than 10s, preferably in about is. The ferromagnetic RKKY exchange of the interlayer partially compensates the antiferromagnetic coupling arising from the stray field of the two ferromagnetic layers 10, 11.

(47) The hysteresis loops for RKKY interaction for two different strengths of the interaction is shown. The dark line shows the hysteresis loop for the particle having a RKKY interaction of J.sub.FM.=0.35 mJ/m.sup.2. The light line shows the hysteresis loop for the particle having a RKKY interaction of J.sub.FM.=0.39 mJ/m.sup.2.

(48) At zero field a remanent magnetization nearly equal to the saturation magnetization is observed and the particle remains locked in the ferromagnetic state. The ferromagnetic state is less stable than the antiferromagnetic state and because of the partial compensation of the antiferromagnetic coupling from the stray field by the ferromagnetic RKKY exchange, the energy barrier between the states is sufficiently small to allow a thermal decay to the antiferromagnetic ground state within about 1 s.

(49) FIG. 11 is a schematic illustration of an example of a layered particle structure of a functionalized disk-shaped antiferromagnet particle 1. The particle is manufactured by a layer-wise deposition of different materials on a substrate 13, for example on a wafer, such as a silicon or glass wafer. Directly above the substrate, a sacrificial layer 14 is deposited to allow the particle construction to be removed from the substrate after manufacture of the particle 1. Preferably, a sacrificial layer 14 is a water soluble layer or a layer that can be etched for example in citric acid. For example, the sacrificial layer is a 5 nm thick layer, for example comprising magnesium or copper or MgO. Also other solvents may be used to remove the sacrificial layer 14.

(50) The further layers arewhen seen from bottom to top: a first or bottom functionalization layer 19 on top of the sacrificial layer 14, a seed layer 16, a first ferromagnetic layer 10, a non-magnetic interlayer 12, a second ferromagnetic layer 11 and a second or top functionalization layer 17.

(51) The seed layer 16 is, for example, a 2 nm to 6 nm thick layer, for example comprising or being made of Ta. The seed layer 16 is provided to promote an appropriate growth of the first ferromagnetic layer 10 and to obtain specific magnetic properties. Such specific magnetic properties, may, for example, be a sufficiently small or absent perpendicular magnetic anisotropy to keep the magnetization of the ferromagnetic layers 10, 11 in the plane, and an in-plane uniaxial magnetic anisotropy K.sub.u, in the ferromagnetic layer plane, to align the magnetization along this direction, to obtain a single-domain state, and to design switching fields H.sub.AF.fwdarw.F and H.sub.F.fwdarw.AF optimized to obtain largest magnetization loss within the boundaries defined by the biological discomfort level for oscillatory fields.

(52) The seed layer 16 may be deposited, for example by sputtering under an oblique angle, to give the seed layer 16 a specific structure. The seed layer 16 may, for example, be a nano-crystalline tantalum layer.

(53) The first ferromagnetic layer 10 grown on or deposited onto such a structured seed layer 16 then shows a uniaxial anisotropy in the plane of the first ferromagnetic layer.

(54) During deposition of the first and of the second ferromagnetic layers 10, 11 a uniaxial magnetic field may be applied. The strength of this applied magnetic field is sufficiently strong to align the magnetic moments during the deposition along the field direction to obtain an induced uniaxial anisotropy in the layer plane in the ferromagnetic layers 10, 11. During the deposition of the second ferromagnetic layer 11, the applied field must be sufficiently strong to align the magnetization of the second ferromagnetic layer 11 during its growth in the presence of the stray field of the first ferromagnetic layer 10.

(55) The first and second ferromagnetic layers 10, 11 are, for example, 6 nm thick, for example comprising an Fe-alloy, Co-alloy or a CoFe alloy, for example CoFeB, CoFe, FeB, CoB.

(56) In order to further increase the magnetic anisotropy induced, for example, by a magnetic field applied during sputtering of the ferromagnetic layers 10, 11, for example a CoFeB layer is alloyed with a rare earth metal, in particular Sm. Amorphous Sm (20%) Co (80%) layer can develop a significant uniaxial anisotropy in the layer plane induced by an applied field during layer growth or by an annealing process in a field following the deposition. An amorphous structure of the ferromagnetic layers arises from the Sm in the Sm(20%)Co(80%) alloy. Thus, the obtainable anisotropy in the plane of the ferromagnetic layers may be tuned by selecting the Sm/B-ratio in the CoFeBSm alloy or also in a Sm/CoFe ratio in an CoFeSm alloy. A uniaxial anisotropy in the layer plane of 50 kJ/m.sup.3 may be obtained in amorphous CoSm or CoFeSm or CoFeBSm layers of e.g. 6 nm thickness.

(57) The non-magnetic interlayer 12 may have a thickness of e.g. 2 nm and is made, for example, of Ta, Ta.sub.1-xPt.sub.x, Ir.sub.1-xPt.sub.x.

(58) The bottom functionalization layer 19 and the top functionalization layer 17 may protect the particle layer construction, from for example oxidation. More particularly, the two functionalization layers 17, 19 are layers permitting a biological, biomedical, physical or chemical functionalization of the particle, i.e. the binding of specific molecules to these layers, for example to obtain colloidal stability of the particle in liquids, to provide attachment capability of the particle to specific cells, to prevent uptake of the particle by macrophages, to enable release or reaction of a chemical substance comprised in the particle, for example a drug, upon heating of the particle. Functionalization layers may have a thickness of, for example, 2 nm to 10 nm. Materials for functionalization layers are, for example, Au, C, Ti-oxide, Ta, Si-oxide. The bottom and the top functionalization layer 17, 19 may be identical or may be different. The bottom and the top functionalization layer 17, 19 may be designed and chosen depending on the desired function of the synthetic antiferromagnet particle.

(59) A chemical or biochemical functionalization is preferably obtained after dissolution of the particle from the wafer by adding specific molecules to the functionalization layers 17 or 19 that are comprised of materials to obtain a suitable bonding to the molecules.

(60) FIG. 12 illustrates the specific heating power required to obtain a temperature rise of 15 K within a tumor of a given radius. The curved lines 25, 26, 27 show the requirements for the particle's specific heat loss. Solid line 25 represents a particle concentration of 100 mg/cm.sup.3, dotted line 26 represents a particle concentration of 10 mg/cm.sup.3 and dashed line 27 represents a particle concentration of 1 mg/cm.sup.3.

(61) The typical specific power losses (SLP) for typical superparamagnetic particles (MNP) is indicated by the dashed area 21. The horizontal line 22 shows the SLP for hypothetical (best) CoFe nanoparticles exhibiting a rectangular hysteresis loop operated at the biological discomfort level Hf=5.Math.10.sup.9 Am.sup.1s.sup.1 providing about 5900 W/g. Thus, also tumors of very small sizes are available for hyperthermia treatment with these particles.

(62) The shaded rectangle 23 indicates tumors with a diameter of about 1 mm that can be treated with best CoFe nanoparticles with a particle concentration of 100 mg/cm.sup.3 that can be obtained by direct injection of the particles into the tumor. Magnetic nanoparticle concentrations of 10-100 mg/cm.sup.3 per tumor tissue are practicable. The shaded area 24 highlights the minimal diameter of tumors accessible with particle concentrations of 1 mg/cm.sup.3 of the best CoFe particles that can be expected for an antibody-targeted approach.