Abstract:
Ferromagnetic material for temperature sensitive elements or parts has a direction of easy magnetization which varies depending upon temperature. The material has the formula: 
     
       Nd.sub.1-u R.sub.u (Co.sub.1-x M.sub.x).sub.z 
     
     wherein R is one or more rare earth elements, M is at least one element selected from the group consisting of B, Al, Si, Ti, V, Cr, Mn, Fe, Ni, Cu, Zr, Nb, Ta, Mo, W, Hf, Pd, Sn and Pb, 0≦u≦0.5, 0&lt;x&lt;0.4 and 4.4≦z≦5.5.

Description:
This is a continuation of co-pending application Ser. No. 480,031, filed on 29 Mar. 1983, which itself is a continuation of Ser. No. 290,973, filed 7 Aug. 1981, both abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to material for temperature sensitive elements or parts, and particularly to a material for temperature sensitive elements consisting of a ferromagnetic material of a rare earth cobalt compound of which the magnetic anisotropy varies depending upon the temperature. 
     When a ferromagnetic body of a rare earth cobalt compound is rotatable and is positioned between two permanent magnets 2a and 2b, as illustrated in FIG. 1, the ferromagnetic body 1 turns toward a fixed direction against the magnetic field generated by the permanent magnets 2a and 2b, due to the magnetic anisotropy of the ferromagnetic body 1. As the ferromagnetic body 1 is gradually heated, the body 1 of some kinds of rare earth compounds does not rotate, but the body 1 of other kinds of rare earth compounds starts rotating at a temperature of T 1 , rotates by an angle of 90 degrees, and stops at a temperature of T 2 . The rotation phenomenon of the ferromagnetic body is generated by variation of the easy direction of magnetization of the body by an angle of 90 degrees due to the spin reorientation depending upon temperature. 
     The variance of the direction of easy magnetization of the rare earth cobalt compound will now be explained in detail. 
     RCo 5  type compounds (R being a rare earth element) have the crystal structure of the hexagonal system, as illustrated in FIG. 2a. In FIG. 2a, the small circle indicates the cobalt element and the large circle having dots indicates the rare earth element. When the direction of easy magnetization of the RCo 5  type compound is parallel to the c-axis ([0001]direction) of the crystal, the state is indicated by the symbol &#34;A&#34; in FIGS. 2b and 3. When the direction of easy magnetization is in the basal plane ((0001)plane) of the crystal, the state is indicated by the symbol &#34;P&#34; in FIGS. 2b and 3. When the direction of easy magnetization is present between the c-axis and the basal plane, for example on a surface of an imaged cone, the state being intermediate between the A state and P state is indicated by the symbol &#34;C&#34; in FIGS. 2b and 3. Temperature dependence of the direction of easy magnetization of RCo 5  type rare earth cobalt compounds is shown in FIG. 3 (cf. the Bulletin of the Japan Institute of Metals, Vol. 16, No. 2, 1977, page 83). 
     As is obvious from FIG. 3, when the rare earth element is praseodymium (Pr), neodymium (Nd), terbium (Tb) or holmium (Ho), the direction of easy magnetization varies, depending upon temperature. Particularly, the direction of easy magnetization of NdCo 5  and TbCo 5  can vary from the P state to the A state via the C state. As to the rest of the RCo 5  type compounds, the direction of easy magnetization is constant in the A state. The broken lines in FIG. 3 denote the undetermined or presumed state of the direction of easy magnetization. 
     As to the R 2  Co 17  type rare earth cobalt compounds, temperature dependence of the direction of easy magnetization is shown in FIG. 4 (cf. the same page of the above mentioned reference). In FIG. 4, the symbols A, C and P and the broken lines have the same meaning as explained above. The direction of easy magnetization of the Lu 2  Co 17  compound only can vary from the P state to the C state. There is no R 2  Co 17  type compound of which the direction of easy magnetization can vary from the P state to the A state via the C state. 
     The direction of easy magnetization of Y 1-x  Nd x  Co 5  compound varies depending upon temperature, as illustrated in FIG. 5, when the molar ratio parameter &#34;x&#34; is 0.25, 0.50, 0.75 and 1. In FIG. 5, the symbol &#34;β&#34; indicated at the ordinate means the angle between the c-axis of the crystal and the direction of easy magnetization. As can be seen from FIG. 5, a transition temperature range wherein the angle β varies from 90 degrees to zero degrees (i.e. the direction of easy magnetization varies from the P state to the A state) can change, depending on the composition of the rare earth elements (i.e. the molar ratio &#34;x&#34;). In this case, for example, the transition temperature range of NdCo 5  (&#34;x&#34; being 1) is from 230° to 285° K. (i.e. from -43° to 12° C.). 
     Furthermore, the direction of easy magnetization of the DyCo z  compound varies depending upon temperature, as is illustrated in FIG. 6, when the molar ratio parameter &#34;z&#34; is 4.4, 4.6, 5.0 and 5.3. As can be seen from FIG. 6, the transition temperature range can be changed, depending the composition of the dysprosium cobalt compound (i.e. the molar ratio &#34;z&#34;). The data of FIG. 6 were obtained as a result of the present inventor&#39;s experiments. Test pieces of DyCo z  compounds were produced in accordance with the process for producing a magnetic body proposed by the present inventors as U.S. Pat. Nos. 4,347,201 and 4,459,248 (European Patent Application No. 79302389.6 i.e., EP-A-0010960). The process is disclosed in column 6, lines 10-18, and 50-52, and column 10, lines 20-17, of U.S. Pat. No. 4,347,201, and in column 6, lines 11-19 and 51-53 and column 10, lines 20-27, of U.S. Pat. No. 4,459,248). The DyCo z  compound has a disadvantage, i.e. a relative low saturation magnetization, as shown in Table 1, therefore, when the DyCo z  compound body is used as a switch element of a temperature sensitive device, the switching property of the switch element is low so that the device has a disadvantageously large size. 
     
                       TABLE 1______________________________________           Saturation Magnetization (T)Material        at Room Temperature______________________________________DyCo.sub.5      0.437NdCo.sub.5      1.228TbCo.sub.5      0.236Thermorite*     0.26Magnetic Shunt Alloy**           0.24______________________________________ *Mn--Zn system ferrite having a Curie point of 90° C.; **Fe--Ni system alloy steel having a Curie point of 50° C.; 
    
     As can be seen in Table 1, the saturation magnetization of a NdCo 5  compound is the largest among the RCo 5  compounds of which the direction of easy magnetization can vary from the P state to the A state via the C state. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide material for temperature sensitive elements or parts which have a high saturation magnetization and a transition temperature range shifted to higher temperature as compared with that of conventional rare earth cobalt compounds. 
     It is another object of the present invention to keep or raise the level of the saturation magnetization of the NdCo 5  compound. 
     It is still another object of the present invention to provide material for a temperature sensitive element having the direction of easy magnetization which can vary from the P state to the A state within a desired temperature range, preferably, at the ambient temperature and above. 
     According to the present invention, material for temperature sensitive elements or parts of which the direction of easy magnetization varies, depending upon temperature, has the formula: 
     
         Nd.sub.1-u R.sub.u (Co.sub.1-x M.sub.x).sub.z 
    
     wherein R is one or more rare earth elements, M is at least one element selected from the group consisting of B, Al, Si, Ti, V, Cr, Mn, Fe, Ni, Cu, Zr, Nb, Ta, Mo, W, Hf, Pd, Sn and Pb, 0≦u≦0.5, 0&lt;x&lt;0.4 and 4.4≦z≦5.5. 
     If the molar ratio &#34;x&#34; is 0.4 or above, the saturation magnetization of the above mentioned material is remarkably lowered or the degree of orientation of the material (hereinafter explained) is worsened. It is preferable that the range of the molar ratio &#34;x&#34; is from 0.03 to 0.25. 
     When a part of the cobalt of the above mentioned material is replaced with the above combination of Fe and another element, the saturation magnetization of the material tends to decrease. However, when a part of the cobalt is replaced with Fe and another element, it is possible to suppress the tendency to decrease the saturation magnetization. The material containing Fe and another element, which may partly replace the cobalt, is indicated by the following formula: 
     
         Nd.sub.1-u R.sub.u (Co.sub.1-q-y Fe.sub.q M.sub.y).sub.z 
    
     wherein R is one or more other rare earth elements besides Nd, M is at least one element selected from the group consisting of B, Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zr, Nb, Ta, Mo, W, Hf, Pd, Sn and Pb, 0≦u≦0.5, 0&lt;q≦0.2, 0≦y≦0.3, x=q+y and 4.4≦z≦5.5. It is preferable that M is Al. 
     According to the present invention, the molar ratio &#34;z&#34; of the cobalt and M element(s) to the rare earth element(s) is from 4.4 to 5.5. As the molar ratio &#34;z&#34; increases, the transition beginning temperature T 1  and the transition ending temperature T 2  of the material of the present invention are shifted toward a higher temperature, as illustrated in FIG. 6 (hereinafter explained). If the molar ratio &#34;z&#34; is above 5.5, the degree of orientation of a thermal sensitive element of the material is worsened. As the molar ratio &#34;z&#34; decreases, the temperatures T 1  and T 2  decrease. The decrease of the temperatures T 1  and T 2  is undesirable, if the transition temperature range is brought below the ambient temperature. However, since the decrease of the temperatures T 1  and T 2  can be compensated with the addition of Al and the like, it is possible to use material having a molar ratio &#34;z&#34; of 4.4 or more. 
     Furthermore, it is possible to replace a part of the Nd with another rare earth element, such as Sm, Pr, up to a molar ratio &#34;u&#34; of 0.5. If the molar ratio &#34;u&#34; is above 0.5, the saturation magnetization is low so that such material is unsuitable for a temperature sensitive element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a rotatable ferromagnetic body and two permanent magnets; 
     FIGS. 2a and 2b illustrate a crystal structure and states of the direction of easy magnetization of an RCo 5  type rare earth cobalt compound, respectively; 
     FIG. 3 is a graph showing the temperature dependence of the direction of easy magnetization of RCo 5  type compounds; 
     FIG. 4 is a graph showing the temperature dependence of the direction of easy magnetization of R 2  Co 17  type compounds; 
     FIG. 5 is a graph showing the temperature dependence of the direction of easy magnetization of Y 1-x  Nd x  Co 5  compounds; 
     FIG. 6 is a graph showing the temperature dependence of the direction of easy magnetization of DyCo z  compounds; 
     FIGS. 7 through 39 are graphs showing the temperature dependence of the direction of easy magnetization of NdR(CoM) compounds, which have compositions described in Table 2, respectively; 
     FIG. 40 is a graph showing the relationship between the transition beginning and ending temperatures T 1  and T 2  and the molar ratio &#34;z&#34;; 
     FIG. 41 is a perspective view of a sintered body to be measured by the X-ray diffraction method; 
     FIG. 42 is a graph showing a diffraction pattern of a sintered body of Sm(CoFeCu) 6 .8 compound; and 
     FIG. 43 is a graph showing a diffraction pattern of a sintered body of DyCo 5  compound. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be explained by examples and comparative experiments. 
     Example 1 
     Starting materials of neodymium and, if necessary, another rare earth element, cobalt and at least one element of B, Al, Si, Ti, V, Cr, Mn, Fe, Ni, Cu, Zr, Nb, Ta, Mo, W, Hf, Pd, Sn and Pb were caused to be molten at a temperature of from 1300° to 1500° C. under an inert gas atmosphere by an arc-melting or induction melting method. The melt was cast into a mold to form an ingot having a predetermined composition. The ingot was ground to a fine powder having a grain size of a single magnetic domain. The grains of fine powder were oriented by applying a magnetic field at 150° C. to arrange the direction of easy magnetization of each grain of the fine powder in one direction. Then, the fine powders were sintered at a temperature above 1000° C. and heat-treated to produce a test piece of a temperature sensitive element, as disclosed in the above U.S. Pat. Nos. 4,347,201 and 4,459,248. Composition, transition beginning temperature T 1 , transition ending temperature T 2  and saturation magnetization of the obtained test pieces are shown in Table 2. At the temperature T 1  the direction of easy magnetization of the test piece begins to leave from the basal plane of the crystal, as the temperature of the test piece rises. At the temperature T 2  the direction of easy magnetization reaches the c-axis of the crystal. The basal plane and the c-axis form a right angle. Namely, as the temperature of the test piece rises, the direction of easy magnetization varies from the P state to the A state via the C state. In Table 2, enumerated drawings show the temperature dependence of the direction of easy magnetization of each of the test pieces. 
     
                                           TABLE 2__________________________________________________________________________                       Saturation                               FIG. NumberSample              T.sub.1 Magnetization                               of theNo. Composition     (°C.)                       (T)     Drawing__________________________________________________________________________ 1  Nd(Co.sub.0.97 B.sub.0.03).sub.5               -5˜13                       0.8      7 2  Nd(Co.sub.0.92 Al.sub.0.08).sub.5               15˜36                       1.05     8 3  Nd(Co.sub.0.88 Al.sub.0.12).sub.5               28˜47                       0.92     9 4  Nd(Co.sub.0.97 Al.sub.0.03).sub.5                1˜22                       1.33    10 5  Nd(Co.sub.0.97 Si.sub.0.03).sub.5               12˜30                       0.76    11 6  Nd(Co.sub.0.97 V.sub.0.03).sub.5                0˜19                       1.03    12 7  Nd(Co.sub.0.97 Cr.sub.0.03).sub.5               -10˜7                       1.02    13 8  Nd(Co.sub.0.97 Mn.sub.0.03).sub.5               -38˜-15                       1.08    14 9  Nd(Co.sub.0.75 Cu.sub.0.25).sub.5               -5˜25                       0.95    1510  Nd(Co.sub.0.97 Zr.sub.0.03).sub.5               -11˜5                       1.15    1611  Nd(Co.sub.0.97 Nb.sub.0.03).sub.5               -15˜14                       1.19    1712  Nd(Co.sub.0.97 Mo.sub.0.03).sub.5               -2˜15                       1.12    1813  Nd(Co.sub.0.97 Pd.sub.0.03).sub.5               -12˜11                       0.86    1914  Nd(Co.sub.0.97 Sn.sub.0.03).sub.5               -25˜11                       0.81    2015  Nd(Co.sub.0.95 Ni.sub.0.05).sub.5               -11˜13                       1.06    2116  Nd(Co.sub.0.95 Fe.sub.0.05).sub.5               -4˜12                       1.15    2217  Nd(Co.sub.0.90 Fe.sub.0.10).sub.5               -2.5˜12                       1.20    2318  Nd(Co.sub.0.97 Hf.sub.0.03).sub.5               -12.5˜2.5                       1.12    2419  Nd(Co.sub.0.97 Ta.sub.0.03).sub.5               -12.5˜8                       1.15    2520  Nd(Co.sub.0.97 W.sub.0.03).sub.5                0˜15                       1.08    2621  Nd(Co.sub.0.97 Pb.sub.0.03).sub.5               -10˜17.5                       0.78    2722  Nd(Co.sub.0.97 Ti.sub.0.03).sub.5                -4˜14.5                       1.00    2823  Nd(Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.5               29˜48                       1.18    2924  Nd(Co.sub.0.82 Fe.sub.0.10 Al.sub.0.08).sub.5               49˜61                       1.24    3025  Nd(Co.sub.0.83 Fe.sub.0.05 Al.sub.0.12).sub.5               46˜64                       0.93    3126  Nd(Co.sub.0.78 Fe.sub.0.10 Al.sub.0.12).sub.5               75˜85                       1.07    3227  Nd(Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.4.6               26˜45                       1.12    3328  Nd(Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.5.3               36˜54                       1.20    3429  Nd(Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.5.5               37˜58                       1.21    3530  Nd.sub.0.9 Sm.sub.0.1 (Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.5               -30˜-5                       1.17    3631  Nd.sub.0.9 Sm.sub.0.1 (Co.sub.0.83 Fe.sub.0.05 Al.sub.0.12).sub.5.3               40˜61                       1.20    3732  Nd.sub.0.9 Pr.sub.0.1 (Co.sub.0.87 Fe.sub.0.05 Al.sub.0.08).sub.5               10.5˜30                       1.18    3833  Nd.sub.0.9 Pr.sub.0.1 (Co.sub.0.83 Fe.sub.0.05 Al.sub.0.12).sub.5                 31˜47.5                       1.06    39*   NdCo.sub.5      -7˜13                       1.2     --__________________________________________________________________________ *Comparative example 
    
     In Table 2, the saturation magnetization is indicated by intensity of magnetization at a magnetic field intensity of 1.2 MA/m. 
     Example 2 
     Test pieces of Nd(Co 0 .87 Fe 0 .05 Al 0 .08) z  were produced in the same manner as that mentioned in Example 1. The molar ratio &#34;z&#34; was 4.6(sample 27), 4.8, 5.0(sample 23), 5.3(sample 28) and 5.5(sample 29). The temperatures T 1  and T 2  are shown in FIG. 40. As can be seen from FIG. 40, the transition temperature range of the material indicated by the above formula varies, depending upon the molar ratio &#34;z&#34;. 
     Example 3 
     When the degree of orientation of a sintered body 20 (FIG. 41) is measured by the X-ray diffraction method, X-rays (indicated by a solid arrow) irradiate a bottom surface to obtain a diffraction pattern. If the c-axis of the material of the sintered body 20 is arranged in a predetermined direction (e.g. a certain diameter direction, indicated by a broken arrow in FIG. 41) of the bottom surface, peaks from the (h k·0) type lattice plane only appear in the diffraction pattern, and there are no peaks from the (00·m) type lattice plane which is at right angles to the c-axis. For example, powders of Sm((Co 0 .78 Fe 0 .08 Cu 0 .14) 6 .8 are pressed in a magnetic field, and then are sintered to form a body. The sintered body is measured by the X-ray diffraction method to obtain a diffraction pattern, as illustrated in FIG. 42. The sintered body is a permanent magnet having a good rectangular hysteresis loop and has the c-axis arranged in one direction. As can be seen from FIG. 42, when the degree of orientation of the sintered body is superior, peaks of the (h k·0) plane only appear in the diffraction pattern. When a sintered body of DyCo 5  compound (in FIG. 6) is measured by the X-ray diffraction method, a diffraction pattern is obtained having peaks diffracted from the (h k.0) plane, as illustrated in FIG. 43. Therefore, it is found that the degree of orientation of the sintered body is inferior. When the orientation of the sintered body is disordered, the peak of the (111) plane sensitively appears in the diffraction pattern. In FIG. 43, the peak of the (200) plane is near (on the left side of) the peak of the (111) plane, and is of a lesser degree. The high ratio of the heights of the two peaks I 111  /I 200  indicates the degree of orientation. 
     The samples 4, 6, 7, 8, 9 and 10 (in Table 2) of the Nd(Co 0 .97 M 0 .03) 5  compound were measured by the X-ray diffraction method to obtain the degree of orientation thereof in Table 3. 
     
                       TABLE 3______________________________________Sample      Element of M inNo.         NdCoM compound                     I.sub.111 /I.sub.200______________________________________4           Al            0.106           V             0.627           Cr            0.368           Mn            0.3810          Zr            0.6711          Nb            0.58______________________________________ 
    
     As can be seen from Tables 2 and 3, as the degree of orientation of the material becomes superior, i.e. the ratio of I 111  /I 200  becomes small, and the saturation magnetization becomes large.