2.3 Magnetism and In-Situ Dynamic Observation of SmCo₅ Permanent Magnetic Alloy During Annealing at 600-1000°C
Investigators undertaking in study of magnetics and magnetic materials have attached importance to mechanism of the coercivity of SmCo5 permanent magnetic alloy. Their reason to do so is that the theoretical coercivity permanent magnetic alloy of SmCo₅ is 31,840 kA/m but in practice generally the coercivity of the SmCo5 permanent magnetic alloy produced in factory is only 1360 - 2036 kA/m, about seventeenth of the theoretical value. The SmCo5 produced in laboratory has the maximum magnetic performance of 4776 kA/m (Zhou, et al, 1995), merely being sixth to seventh of the theoretical value. In addition mechanism of the coercivity can not be explained by conventional addition theory. Granule size of the SmCo5 is about \(D = 5 - 20\ \mu m\) and the size of its single domain is about \(D = 0.3 - 1.6\ \mu m\), that the former is about seventeen times of the latter. According to theory of the single domain the coercivity should be proportional with the anisotropy constant \(K_{1}\) of magnetic crystals and the \(K_{1}\) of the SmCo5 should be lowered with lowering of temperature so that the mechanism of coercivity is not consistent with the theory of the single domain. Moreover, the coercivity valley appeared in SmCo5 permanent magnetic alloy by annealing at 750°C, i.e., degraded to the minimum, but the coercivity rose at temperature from 750 - 950°C (Pan, Ma, Li, 1993).
To study above mentioned problem clearly will enrich the coercivity theory and magnetics theory and promote development of the coercivity theory and magnetics theory.
The difference in this study from the method of previous studies is that the observation was carried out dynamically using transmission electronic microscope under the condition of raising temperature from room temperature to 960°C. This innovative method may systematically and precisely observe the relationship among the precipitation of new phase, phase transformation and the coercivity, so that it is better in continuity and systematism than the previous study in one or two temperature points using a massive specimen.
Specimen Preparation and Experimental Methods for Studying SmCo₅ Magnets
SmCo5 sample was melted by alloy melting method. The sample was made of the nominal composition and melted in an arc furnace at protection atmosphere of argon. The alloy obtained was grinded by vibration ball miller and pulverized into powder of around 5 μm. The powder orientation formed under 1.5 T magnetic field and then sintered. The sintered sample was sliced into 0.3 mm lamellae by a linear cutter and then thinned to films of about 100 nm by ionic thinning device. The filmy specimen was placed into the side insertion heating dais of 1000 kV and observed dynamically by the transmission electronic microscope in heating condition. In the meantime the observation was videotaped. The accelerating voltage was 1000 kV (Pan, Ma, Li, 1993).
Chemical Composition Analysis of SmCo₅ Permanent Magnetic Alloy
The chemical composition of SmCo5 Permanent Magnetic Alloy is shown in Table 2.3.
Magnetic Measurement Techniques for SmCo₅ Permanent Magnetst
The measurement was carried out by using CL6 - 1 magnetic parameter measuring instrument. The specimen was prepared as per composition of permanent magnetic alloy listed in Table 2.4 and 4 samples were selected for measurement. The result of measurement of magnetic performance is shown as Table 2.4. The specimen was annealed in a heat treatment furnace at temperature from room temperature to 1000°C, annealing at selected temperature of 250°C, 420°C, 500°C, 600°C, 750°C, 850°C, 900°C, 950°C, 1000°C for one hour and quenching to room temperature, respectively. The specimen was measured in magnetic parameter measurement instrument. The result of the measurement is described as variation curve as shown in Fig. 2.10. It can be seen from the figure that the coercivity of SmCo5 permanent magnetic alloy appeared linear variation after annealing and
Fig. 2.10 The coercivity of SmCo5 specimen after 1h annealing at different annealing temperatures
reached the minimum at 750°C. Afterwards the coercivity was restored until 900°C.
Magnetic Domain Structure of SmCo₅: Formation and Behavior
Using method of Kerr magnetic - optical effect to observe structure of magnetic domain obtained a figure of magnetic domain as shown in Fig. 2.11 to Fig. 2.13. The figures are photographs of structure of magnetic domain being magnified by 600 times and observed by Kerr magnetic - optical effect at room temperature.
The photograph shown in Fig. 2.11 is the pattern of planar magnetic domain in vertical to easily magnetisable axis. It can be seen from Fig. 2.11 that the volume of the positive domain equals to the negative domain and has no magnetism.
Fig. 2.11 Domain structure (600x) at thermal demagnetization
The Fig. 2.12 shows photographs of specimen of permanent magnetic alloy after magnetization under 3.0 T gauss magnetic field. The magnetization condition for Fig. 2.13 is the same as that of the Fig. 2.12. It can be seen from Fig. 2.13 that the volume of the positive domain decreased remarkably but the negative domain exhibited two traits: the first is the increase in number of the negative domain, the second is the widening of the negative domain. Pattern of the domain is like a labyrinthine.
Irreversible Loss of SmCo₅ Permanent Magnetic Alloy After Annealing at 25-1000°C
Experiment method was to hold the permanent magnetic alloy specimen for one hour at temperature 100°C, 200°C, 400°C, 500°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, respectively, and quench the specimen to room temperature. Afterwards, their magnetism was measured and the irreversible loss was calculated. The measurement result was drawn to the variation curve as shown in Fig. 2.14 (comparison between magnetic performance before and after the annealing) (Pan, Ma, Li, 1993; Pan, Ren, Tan, 1983).
It can be seen from Fig.2.14 that its irreversible loss is very small when the annealing temperature was below 500°C. The magnetic irreversible loss becomes the biggest when the specimen was annealing at 750°C for one hour. By annealing at 900 - 1000°C for one hour the magnetic irreversible loss again increases gradually and by annealing at 750 - 900°C the magnetic irreversible loss decreases gradually.
Electronic Energy Spectrum Analysis of SmCo₅ Permanent Magnetic Allo
The electronic energy spectrum of SmCo5 alloy was measured at room temperature. Conditions for measurement of AES was: energy of incident electronic beam 3 keV, the beam current 1 μA, test voltage 60 V, multiplying voltage 1200 V, time constant 0.03 s, magnifying multiple 40 times and vacuum degree (2.66 - 3.99)×10⁻⁵ Pa ((2 - 3)×10⁻⁷ Torr). Main measurement condition of XPS was: using radiation of magnesium target as light source, voltage 8 kV, electric current 30 mA and flux energy 50 eV.
Fig. 2.15 shown the experimental result of optical electron energy spectrum measured for SmCo5 specimen after annealing at different temperature from 25°C to 900°C as per above mentioned conditions. It can be seen from the figure that the proportion of samarium and cobalt changed along with rising of temperature so that the samarium atoms enriched area or cobalt atoms enriched area appeared in interior of the alloy. The element segregating of samarium and cobalt atoms caused the alloy to be SmCo5 phase partly and the other phases (Sm2Co7 and Sm2Co17) in the other part. No matter the Sm2Co7 or Sm2Co17 their magnetic crystal anisotropy are all lower than that of SmCo5. Inhomogeneous of the solid solution magnetic crystal had a much lower anisotropy. This inhomogeneous area reduced gradually and the SmCo5 phase increased when the temperature was above 750°C and as the result the coercivity measured after quenched was enhanced. When this inhomogeneous was reduced to zero the homogeneous solid solution was quenched to room temperature as per proportion of SmCo5 phase and could hold the 1 : 5 phase. Then the coercivity could still be restored because the second phase could not be formed by element segregation but under an important premise that the oxygen should not be too high in the alloy. It can be seen from Fig. 2.15 that the oxygen did not increase along with rising of temperature so that the Sm2O3 was not formed in the alloy. An optic - electron energy spectrum experiment was designed to test the variation status of the elements of samarium, cobalt and oxygen. The specimen was annealing at high temperature of 750°C (heating under protection gas) for one hour and then quenched to room temperature. Afterwards, the optic - electron energy spectrum experiment was carried out. The distribution curve of elements of samarium, cobalt and oxygen of SmCo5 after annealing at 750°C for one hour was described as shown in Fig. 2.16 in accordance with peeling measured distribution of elements of samarium, cobalt and oxygen. The abscissa is the peeling time (min) and the ordinate is the atomic fraction. It can be seen from the figure that there was no peak value of oxygen.
To explain the problem, two specimens of the samarium - enriched SmCo5 alloy (containing high samarium content) were annealed at 750°C for one hour. The optic - electron energy spectrum experiment of distribution of elements of samarium, cobalt and oxygen and the measured curve is shown in Fig. 2.16. The optic - electron energy spectrum experiment of distribution of elements of samarium, cobalt and oxygen and the measured curve is shown in Fig. 2.17 for specimens annealed at different temperature from 25°C to 900°C for one hour.
The rare earth elements are easy to combine with oxygen to form rare earth oxides (such as Sm2O3) because of activity of the rare earth elements. Therefore, to
make a judge is to check whether appearing of the peak value. It can be seen from Fig. 2.17 and Fig. 2.18 that there is no peak value of the oxygen.
In-Situ and Dynamic Observation of Eutectoid Decomposition in SmCo₅ Using Electron Microscopy
The above mentioned SmCo5 experimental specimen was sliced into lamellae of 0.25 mm in direction perpendicular to easy magnetisable axis. Then the lamellae were thinned to about 100 nm by electrolysis polishing and ionic thinning. The specimen film was placed into the side inserting and heating dais of JEM - 1000 and heated under condition of a vacuum degree of 266×10⁻⁷ Pa (2×10⁻⁷ Torr) and an accelerating voltage of 1000 kV. Then the dynamic observation was carried out. The Fig. 2.19 was the electronic micrograph (the graph of bright field) and the electronic diffraction pattern in P area of electronic micrograph being taken directly in thermal state inside of the electronic microscope when temperature of the filmy specimen was 500°C. Through diffraction and calculation of the precipitated tilted striate texture and approximate round black piece, respectively, it was testified: the precipitated tilted striate texture is Sm2Co7; the approximate round black piece texture is Sm2Co17. This result indicated that the eutectoid decomposition existed in SmCo5 permanent magnetic alloy at 500°C. The experiment also observed that the SmCo5 permanent magnetic alloy eutectoid decomposed remarkably into phases of Sm2Co7 and Sm2Co17 at 400°C, 600°C and 750°C, even appeared composition of the other phase.
In-Situ and Dynamic Observation of SmCo₅ in Thermal State Using Transmission Electron Microscopy
The phase precipitation was observed in the specimen of SmCo5 after holding at 750°C for 10 min. This precipitate phase was found as Sm2Co17, Sm2Co7 and other phase texture with different composition through electronic diffraction. The obvious defects in the newly precipitated Sm2Co17 phase could be observed clearly after holding for 20min. The Fig. 2.20 (a), (b) shown these defects by magnified 100 thousands times in the texture of precipitated Sm2Co17 phase.
It was observed that the precipitated phase varied, dissolved, grown and aggregated, drastically after holding 30 min. The Fig. 2.21 (a), (b) shown the elements segregation appeared in the precipitated Sm2Co17 phase by holding at 750°C for 50 min. The black piece in the photograph was the precipitated Sm2Co17 phase. The element fluctuation and segregation of the precipitated Sm2Co17 phase were shown in Fig. 2.22 when holding for 60 min within the electronic microscope. It
