AMORPHOUS RARE EARTH-IRON MATERIALS

Abstract: Iron and rare earth element containing amorphous materials which possess improved magnetostriction and display paramagnetic, ferromagnetic and random anisotropy characteristics at approximately room temperature where the materials have the general formula: Fe-X-RE wherein X is selected from B or Zr and provided that where X is B then RE is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Dy, Ho, Er, Tm, Yb and Lu, the selection of Pr or Nd being according to the formula (Pr,Nd)y(Fe80B20)(100-y) where y is between 10 and 50 and where X is Zr then RE is selected from the La, Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu. Materials of the invention can be used for generators of ultra-sound energy from 20 KHz up to MHz frequencies and where the materials possess high magnetostriction and high coupling coefficients they are suitable for use as receivers as well as generators.

Description: AMORPHOUS RARE EARTH-IRON MATERIALS The present invention relates to amorphous rare ■ earth-iron materials which possess magnetostrictive and magnetomechanical coupling properties. Rare Earth (RE)-iron crystalline compounds, such as TbFe 2 , are well known for their large magnetostrictive strains ( 2000 ppm) and good magetomechanical coupling (k-- about 0.7) at room temperature (A.E. Clark, J. Cullen, O.D.McMasters and E. Callen, AIP Conf. Proc. 2ϋ, 192 (1976)). A disadvantage of compounds of this type is that operation is limited by eddy currents through the skin effect which can only be overcome by bonding layers 1mm thick, raising the theoretical limit to about 60 or 80 KHz. The preparation of such laminates is time consuming and costly. In recent years interest in amorphous magnetoelastic materials has increased, for example, FeB compounds in which K 33 can exceed 0.9 after magnetic annealing (C. Modzelewski, H.T. Savage, L.T. Kabacoff and A.E. Clark, IEEE Trans.Mag 11(6) 2837 (1981)). Production by the spin melt technique provides ribbons 30 μm thick, which are ideally suited for laminations but their magnetostrictive strains are notoriously small (about 30 ppm), originating from itinerant electrons. Additions of 3d metals, such as Co, can reduce this magnetostriction even further for use in magnetic recording heads. Work performed on amorphous Fe-B structures containing Tb ions has increased the room temperature magnetostriction. With about 30 at;*. Tb strains of approximately 200ppm have been generated (D. Butler, R.D. Greenough and K.C. Pitman, J. de Phys. 49.(12) , 1341 (1988)) demonstrating that the magnetic strains from Tb ions in randomly orientated, uniaxial local fields do not collectively average zero. However, at lower levels of Tb content in Fe-B structures, less than about 18#, the effect is to decrease magnetostriction and permeability. Published European patent application no. 0191107 is concerned with amorphous material which operates magnetically. Various iron-zirconium and iron-boron compounds are included in a large range. One possible combination is iron-boron-lanthanum but no data is provided to show the properties of the alloy. The aim of EP 0191107 is to make materials having orientated magnetic moments which can be aligned in a magnetic field. This translates to maximising the entropy change at low temperatures. The random orientation of magnteic moments means that these materials will only display low magnetostriction. Published EP application no. 0184034 discusses a perpendicular magnetic recording medium and a method for producing the same. It is attempting to engineer magneto-optic properties which is inconsistent with the presence of magnetostrictive properties. The materials Fe-Zr-Pr and Fe-Zr-Nd are mentioned but these also include cobalt which, as previously mentioned, inhibits magnetostriction. Published UK application no. 2043698 relates to a thermomagnetic record carrier using anisotropic elements. Magnetostrictive compounds require highly isotropic elements. Gadolinium is included in the materials which acts to reduce magnetostriction in materials. Published EP application no. 0264153 is concerned with a magnetic material comprising iron, boron and a rare earth metal. Only low concentrations of Neodymium and Praseodymium (about 4#) are used giving the materials have hard magnetic properties which therefore do not possess high magnetostriction for which materials with soft magnetic properties are required. The materials are provided in the crystalline form. It is an aim of the present invention to produce materials with increased magnetostriction, the materials comprising iron-metal or iron-metalloid compounds with rare earth elements. It is also an aim of the present invention to provide materials which display paramagnetic, ferromagnetic and random anisotropy characteristics at approximately room temperature. Accordingly there are provided amorphous materials containing iron and rare earth elements characterised in that the materials have the general formula: Fe-X-RE wherein X is selected from B or Zr and provided that where X is B then RE is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Dy, Ho, Er, Tm, Yb and Lu the selection of Pr or Nd being according to the formula (Pr,Nd) y (Fe 0 8o B 0 20 ) (1 . y) where y is between 0.1 and 0.5 and where X is Zr then RE is selected from the La, Ce, Pr. Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu. Preferably where X is B then RE is Sm and where X is Zr, RE is preferably Tb. X may be chosen so as to reduce the melting/solidifying temperature with a eutectic point which indicates their utility for melt-spinning into amorphous ribbons. Certain materials of the present invention such as Fe-Zr-Tb demonstrate increased magnetostriction over similar materials and enable crystalline and amorphous characteristics to be present. Consequently, the amorphous compounds of the present invention are able to exhibit an increase in Curie temperature, permeability, magnetostriction and magnetomechanical coupling. These properties could not be obtained with crystalline counterpart materials because the magnetic crystalline anisotropy would be too large. The amorphous materials of the present invention tend to have improved mechanical properties, such as being less brittle and stronger, even if they are only 30 μ thick, compared to the crystalline analogues and hence they can be laminated into stacks. Crystalline devices which are conventionally used in transformer cores possess slow reaction time. Materials of the present invention may be used to generate ultra-sound and under optimal conditions frequency ranges of from about 20KHz upto MHz are attainable. Where these materials possess high magnetostriction and high coupling coefficients they may be suitable as receivers as well as generators. It should be noted that the general formula given above can include ratios of Fe to X to RE other 1:1:1. For the materials Fe 0 8o B 0 20 and Fe 0 90 Zr 0 10 including additionally upto 50 at# and 10 at# of rare earth (RE) element respectively it is found that as the RE content is raised, the relative permeability increases, as do the Curie temperatures, magnetostriction and magnetomechanical coupling. Amorphous materials having the formula Fe 0 8o B Q 20 containing upto 50% of RE display magnetic strains of about lOOppm. A magnetic phase diagram for Fe-Zr-RE derived from magnetisation data indicates the transition to a random anisotropy phase, evident at low temperatures in Fe 0 90 Zr 0 10 rapidly rises to near room temperature with the addition of Tb and approaches the Curie temperature at about 9% Tb. The preferred range of RE in Fe-Zr-RE where RE is Tb is approximately 7 to 9 at% . Such materials can be formed in the amorphous state whilst maintaining properties such as high permeability (μ R about 20) while developing strains of about 90 to 120ppm with magnetomechanical couplings of about 0.25 without annealing. With annealing, especially magnetic annealing, the magnetomechanical coupling may be increased to about 0.90. As such these materials are in an ideal form for use in high frequency operation, especially if formed into stacks about 30 μm thick. Amorphous materials of the present invention can be made according to techniques known by those skilled in the art. Generally molten alloy/compound is cooled at 1000000 degrees per second from just above the melt temperature to below the solidification point. This may be achieved by either spraying or injection of a thin stream of molten alloy onto a copper wheel rotating at about 3500 rpm or by applying a stream of molten alloy by injection into a rotating drum of water to form wires. Amorphous (melt spun) materials of the present invention can be made into ribbons about 30 μm thick. Stacks can be made by laminating, i.e. bonding, the ribbons together, so that a variety of shapes and sizes can be made (similar to transformer cores) . These stacks are less affected by skin effect and eddy current losses. The invention will now be described by way of example only and with reference to the accompanying Drawings of which: Figure 1 shows the anisotropic magnetostriction ( s ) at room temperature in RE-Fe-B or RE-Fe-Zr compounds as a function of RE content; Figure 2 shows the magnetomechanical coupling coefficient, K-, as a function of applied field in unannealed Sm 0 05 (Fe 0 8o B 0 20 ) 0 g5 ; Figure 3 shows the magnetic phase diagram for Tb y (Fe 0 g Zr 0 1 ) χ _ derived from magnetisation measurements; Figure 4 shows variations in maximum differential relative permeability (3) and coercivity (o) in Tb y (Fe 0 g Zr 0 _ 1 ) 1 _ at room temperature; Figure 5 shows the room temperature magnetostriction (B) and magnetomechanical coupling (o) as a function of Tb content in Fe o . 9 Zr o . ι - Figure 6 shows the comparison of the effects on the relative permeabilities due to the addition of Tb to (Fe 0 8o B 0 20 ) -or ( Fe o. 9 o Zr o.iO' 1 • Figure 7 shows a graph of magnetostricive properties of RE y (Fe 0 . 82 B o.i 8 --ι- y alloys, where Ol y iO. , as a function of RE content where RE is Praseodymium, Neodymium and Dysprosium; Figure 8 shows a graph of the magnetisation of the materials of figure 7 taken at lOOOKA/m; and, Figure 9 shows a graph of the permeability of the materials of figure 7 taken at lOOOKA/m. Metal-metalloid (Fe 0 82 B 0 l8 ) and metal-metal (Fe 0 g Zr Q λ ) compounds were chosen to host the RE since both are known to spin easily into the amorphous phase. The RE was selected from the highly anisotropic Tb and Sm or from Gd for comparative purposes and due to its different density, itinerant electron effects. These were used to produce compounds with start compositions RE y (Fe 0 82 B 0 l8 ) 1 . for 0 <. <. 0.5 and RE χ (Fe 0 go Zr 0 10 ) 1 . x for 0 <. χ <. 0.1 were prepared either by arc melting or by induction heating in a cold boat. Subsequent spinning was conducted in an inert gas atmosphere with a copper wheel and the products examined with X-ray diffraction for signs of crystallinity. Magnetostriction measurements were made at room temperature with strain gauges and at low temperatures with a capacitance dilatometer (as described by N. Tsuya, K. Arai, Y. Shiraga, Jap. J. App. Phys. 13.. l8θ8 (1974)) to avoid the use of heat cured strain gauge adhesives. DC magnetic fields were applied in parallel and then perpendicular to the strain measurement direction to obtain the anisotropic magnetostrictive strain coefficient, s . Magnetisation data and B-H curves were obtained with a vibrating sample magnetometer with DC applied fields. Relative permeabilities, μ R , were derived by numerical differentiation of the B-H curves. Magnetomechanical coupling coefficients k were measured either by the resonance/antiresonance technique or combining separate measurements of the magnetostrictive strain coefficient 33 (=d /dH) , permeability at constant stress (μ ) pand elastic compliance at constant field, S H where K 2 3 = d 2 _ 3 /(μ S H ) . With reference to figure 1, Sm in Fe-B at room temperature shows a large magnetostriction. This magnetostriction is similar to that shown by the prior art Fe-B-Tb compounds described above. The Sm in Fe-B materials are different in demonstrating negative magnetostriction and confirm the ability of the RE ions to generate bulk magnetic strains. In the case of Fe-Zr the anisotropy of the Tb ions was found to be effective with the smallest amounts of Tb ions, giving an immediate increase in magnetostriction whereas at least about 18% of Tb was required to raise the magnetostriction in Fe-B. With lower amounts of Tb the magnetostriction and permeability may actually be reduced in comparison to Fe-B without Tb. The rates of increase of about 10ppm/ati Tb are approximately equal for the two types of compound which are shown in Figure 1. Figure 2 shows the magnetomechanical coupling coefficient k 33 for the composition Sm Q 05 (Fe 0 8 o B o. 2 o * *o.95* The v& l ues observed in figure 2 are abnormally high for measurements on 'as spun' samples. A feature of the Fe-B-RE compounds produced is the strength of the exchange interaction with corresponding Curie temperatures, T c of about 500 to 600K so that competition between anisotropy and exchange only becomes critical at or below about 50K. The exchange interaction in Fe 0 q Zr Q j is much weaker, evident in the absence of an ordered magnetic phase at room temperature and consequently additions of RE have a substantial effect on the magnetic phase diagram as shown in figure 3- >From magnetisation measurements three phases are identified with the Curie temperatures, T , and spin freezing temperatures, T f , associated with the onset of a random anisotropy phase, tending to converge as the Tb content is increased. It is believed that the structure of the ferromagnetic phase is probably ferromagnetic with a wandering axis but is not confirmed. At room temperature each phase can be obtained by changing the Tb content and the corresponding maximum relative permeabilities, coercivities, anisotropic magnetostriction and coupling coefficients are shown in figures 4 and 5- As the Tb content is increased, there are simultaneous increases in T c , μ R , and k 33 with peak values of strain (about l4θppm) coupling (0.25) and permeability (75) all in the range 7 <. Tb <.9 at%. Addition of Tb to Fe-Zr has an opposite effect on T. when compared to 10 the addition of Tb to Fe-B with an increase in the latter and a decrease in the former. This difference is thought to relate to the dependence of the strength of the exchange interaction on the mean atomic spacings (Bethe-Slater curve) and different atomic radii of constituent elements. The addition of Tb to Fe-Zr increases the mean spacing and reduces 'frustration 1 effects while the addition of Tb to Fe-B at first increases atomic separation, slightly weakening the exchange interaction until the Tb content is sufficiently high to cause the Tb ions -to strengthen the sperimagnetic exchange. This provides a explanation of the opposite effects on the maximum relative permeabilities as shown in figure 6. It is thought that as the Fe-Fe exchange in Fe Q g Zr 0 1 is very weak, the addition of Tb immediately introduces a dominant Tb-Fe exchange, forming ordered clusters. However the Fe-B compounds near the stoichiometric composition, Fe g0 B 20 , already possess short range order with Fe-Fe exchange comparable to that in " crystalline Fe 3 B and much stronger than in Fe 0 g Zr 0 t . Hence it is believed that in the RE-Fe-B system, magnetostrictive strains from the RE ions will only occur if the Tb-Fe exchange is comparable to or greater than the Fe-Fe exchange, ie. when the Tb content approaches the TbFe 2 eutectic. Figure 7 shows the magnetostriction of Nd-Fe-B, Pr-Fe-B and Dy-Fe-B for rare earth composition of 0-50%. The inital effect of adding Pr is shown to yield a large improvement in magnetostriction from about 32ppm at 0% of RE to about 54ppm at 10-20 of RE. At anything more than about 35 RE the effect to reduce magnetostriction below that if no RE were present. The addition of Dy results in a large initial decrease in magnetostriction upto about 10% by composition. Between 10% and 30% Dy there is a large increase in magnetostriction upto a level of 53ppm. Anything in the range of about 22% to 35% Dy gives an improvement in magnetostriction over other compositions. Figures 8 and 9 show respectively the magnetisation and permeability of the same alloys as described in figure 7•

Claims: CLAIMS 1. Amorphous materials containing iron and - rare earth elements characterised in that the materials have the general formula: Fe-X-RE wherein X is selected from B or Zr and provided that where X is B then RE is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Dy, Ho, Er, Tm, Yb and Lu the selection of Pr or Nd being according to the formula (Pr,Nd) y (Fe 0 8o B 0 20 ) (1 _ y) where y is between 0.1 and 0.5 and_ where X is Zr then RE is selected from the La, Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu. 2. Amorphous materials according to claim 1 characterised in that where X is B then RE is Sm. 3. Amorphous materials according to claim 1 characterised in that when X is Zr then RE is Tb. 4. Amorphous materials according to claim 1 characterised in that the materials Fe 0 8o B 0 20 and Fe 0 g0 Zr 0 10 including additionally upto 50 at% and 10 at% of RE element respectively. 5- Amorphous materials according to claim 3 characterised in that Tb is present in Fe-Zr-RE in the range of 7 to 9 at%. 6. Amorphous materials according to any one of claims 1 to 5 characterised in that the materials are subjected to annealing. 7. Amorphous materials according to claim 6 characterised in that the annealing is magnetic annealing. 8. Amorphous materials according to any one of claims 1 to 7 characterised in that the materials are formed into laminated stacks or ribbons. 9. Amorphous materials according to claim 7 characterised in that the stacks or ribbons are about 30 um thick. 10. Amorphous materials specifically as herein described and with reference to the accompanying drawings.

Inventors: CLEGG SARAH JANE, GREENOUGH RODNEY DAVID, GREGORY TIFFANY JANE, PURDY JONATHAN HARRY, CLEGG, SARAH, JANE, GREENOUGH, RODNEY, DAVID, GREGORY, TIFFANY, JANE, PURDY, JONATHAN, HARRY
Applicants: SECR DEFENCE BRIT, UNIV HULL, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI, THE UNIVERSITY OF HULL
Application year: 1992
Main classification: H01L41
IPC codes: H01F1, H01L41