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169Tm Mössbauer effect in intermetallic compounds containing iron, cobalt and nickel
169Tm M O S S B A U E R E F F E C T I N I N T E R M E T A L L I C C O M P O U N D S C O N T A I N I N G I R O N , C O B A L T AND N I C K E L R. K. D A Y and J. B. D U N L O P CSIRO Division of Applied Physics, National Measurement Laboratory, Lindfield, N S W 2070, Australia
169Tin Mrssbauer measurements were made on thulium intermetallics. From TmFe2, the saturated electric quadrupole interaction was (523 _+ 10) MHz which enables the lattice contribution in pure thulium metal to be calculated as (40 _ 11) MHz. In TmCo2 the thulium moments were found to be quenched by 11% below the saturated value.
1. Introduction The MOssbauer effect in 169Tm has been used to investigate some thulium-3d transition metal intermetallics.
2. Experimental A MOssbauer source was prepared as described by Kalvius et al. [1]. A l0 wt% alloy of isotopically enriched 168Er in a l u m i n u m was arc-melted, vacuum cast into a 3 m m diameter cold copper mould and rolled to a thickness of 100 #m. A l0 m m diameter disc was irradiated in a neutron flux of 5 )< 1017 n m -2 s -1 for 9 days and then mounted on a constant acceleration drive capable of velocities of _+700 m m s-1. In addition to the 8.401 keV 169Tm ),-ray the source also emitted the characteristic Er L X-rays with the most intense of these having energies of 6.947, 7.809 and 8.188 keV. The intermetallics were melted in an argon arc furnace using 3 N pure thulium and 4 N pure 3d transition metals. Weight losses corresponding to 2 to 3% of thulium were observed and allowed for when weighing the starting materials. All the samples were analysed using an X-ray diffractometer. T m C o 2 and Tm2Fel7 required annealing at 1000 and 1200°C, respectively, for 24 h to reduce the second phase to below 3%. The high MOssbauer absorption cross-section of 169Tm required that special attention be given to the preparation of uniform absorbers in the range 50 to 80 g m -2 of thulium. The intermetallics investigated were brittle and could not be rolled into foils. The following preparation procedure was eventually adopted. The intermetallics were ground to < 50 # m and distributed on a layer of double-
sided transfer tape which had previously been shown to have negligible absorption of the 169Tm ),-ray. Such a layer contained 20 to 40 g m -2 of thulium and the required total thickness was achieved by adding further layers of tape and powder. The absorbers were clamped between 1 m m thick beryllium discs and mounted in a cryostat. The 3d transition elements Fe, Co and Ni have K absorption edges at 7.112, 7.712 and 8.339 keV, respectively. Comparing these numbers with the 169Tin ),-ray energy of 8.401 keV it was apparent that an increasingly strong photo-electric absorption of the ),-ray would occur in going from Fe through Co to Ni (30 g m -2 of Ni corresponds to one absorption length for 8.4 keV electro-magnetic radiation). In fact samples containing nickel were particularly effective at filtering out the t69Tm ),ray and transmitting the unwanted Er L X-rays. A single line absorber was prepared by dissolving 20 wt% of thulium in aluminum and rolling to a foil containing 70 g m -2 of thulium. At room temperature, the source-absorber combination gave a line width of 30 m m s - i and an absorption of 12%.
3. Results The following thulium-3d transition metal compounds were ,investigated: TmFe2, Tm2Fel7 , T m C o 2, T m C o 3, T m N i 2 and T m N i 3. Repeated attempts to prepare single phase samples of T m F e 3 were unsuccessful. N o M6ssbauer absorption was observed for T m N i 3 down to 4.2 K. For T m N i 2 a single line (with 0.7% absorption effect) was observed at 1.5 K. AC susceptibility measurements on this sample showed no evidence for magnetic order down to 1.6 K. F o r T m C o a a small broad absorption ( ~ 1% effect with a width of 114 m m s - 1) was
Journal of Magnetism and Magnetic Materials 15-18 (1980) 651-652 ©North Holland
651
652
R. K. Day, J. B. Dunlop~ 16Tm Mossbauer effect in intermetallics
TABLE 1 Magnetic hyperfine a n d electric quadrupole parameters Compound
TmFe 2 TmFe2 TmFe2* Tm2Fe w TmCo2 TmCo2 TmCo2
Temp.
G r o u n d state
Quadrupole
Total
(K)
splitting a s (MHz)
parameter P (MHz)mate
splitting (ram s - 1)
1.26 4.2 4.2 4.2 1.4 3.3 4.2
2493 2487 2480 2495 2004 1929 1916
520 525 530 526 404 394 380
1182 _+ 3 1180 _+ 3 1160 i183 __. 4 951 4- 3 915 +__6 909 ___ 9
___ 6 _ 5 _ 9 +_ 6 +__ 13 ___ 19
___ 10 ___ 8 _ 13 ___ 11 ___ 14 _+ 26
*Cohen [5].
observed at room temperature. At lower temperatures the splitting increased but the percentage absorption was too low for data analysis. The results for TmFe2, T m C o 2 and Tm2Fel7 are set out in table 1 and have been taken from computer fits to the data using the simple nuclear Hamiltonian 9C = a I z + P [ I ~ - I ( I + 1)/3]
together with a ratio of ground to excited state nuclear moments /x~//~g -- - 2.214 + 0.014. Our value for /~,//~g is based upon the analysis of 23 sets of experimental data and may be compared to the recently reported value of - 2 . 2 2 3 + 0.013 of Wit and Niesen [2]. The errors quoted in table 1 are statistical. When the errors in the velocity calibration and the ratio of It~//zg are included, the additional errors in the total splitting and in a g a r e 0.5 and 0.7%, respectively. The results in table 1, together with the previously reported [2] ground state splitting, (2253.7 _ 60) MHz, and excited state quadrupole parameter, (483.2 + 3) MHz for pure thulium metal, allow several observations to be made. (1) In the cubic compound T m F e 2 there is no lattice contribution (Pc) to the quadrupole parameter P, and crystal field quenching effects should be negligible because of the high value for the magnetic ordering temperature ( T c = 555 K)*. Hence we believe that the measured P is equal to the saturation value of P4f, the 4f electron contribution. P4t (saturation) = (523 _ 10) MHz. From this we can estimate the lattice contribution of P in pure thulium metal to be (40 +__ 11) MHz which is *Recent calculations ( B o w d e n - - p r i v a t e communication) indicate quenching in T m F e 2 m a y be a b o u t 1%.
in reasonable agreement with the values quoted by Bleaney [3] who reviewed previous M6ssbauer work. The ground state splitting, as, in T m F e 2 and TmzFel7 is 10.5% greater than for thulium due to the transferred hyperfine fields from the iron sites and is typical of the values found in other rare earth iron intermetallics. (2) The case of TmEFel7 is somewhat strange as no large lattice contribution to P is observable in this hexagonal material. However, the experimental data were fitted with a single spectrum, whereas there are two crystallographically inequivalent sites. (3) In TmCo 2, the quadrupole parameter P is reduced by 22% from the saturation value. It would thus appear that significant crystal field quenching is taking place in this compound. Bowden et al. [4] using second order perturbation theory have shown that in the presence of quenching 8 a / a = (sP/e)[(2J - 1)/(6(J - 2))]. For T m 3+ J = 6 thus ( 2 J - 1 ) / ( 6 ( J - 2 ) ) = 0.46 and 8 a / a = 0 . 4 6 ( S P / P ) . Experimentally 8 P / P = 0.23 ___0.03, therefore the expected reduction in the 4f contribution to the magnetic hyperfine field is 10.6%. The saturated value of ag is 2253.7 MHz (from measurements on thulium), hence we expect ag to be 2015 for TmCo 2 which agrees with the measured value of 2004.3 MHz. The transferred hyperfine field in TmCo z is expected to be much less than in T m F e z because of the small induced moment at the cobalt sites. (4) The temperature dependence of the magnetic hyperfine field, as, for TmCo 2 is very interesting. We have measured the ac susceptibility (Xac) of our sample and found a sharp peak at 4.9 K, while the hyperfine field decreased by only 4% from 1.4 to 4.2 K. There seems a possibility that the magnetic ordering transition is of the first order.
References [1] G. M. Kalvius, F. E. W a g n e r and W. Potzel, J. de Phys. 37, Suppl. C6 (1976) 657. [2] H. P. Wit and L. Niesen, Hyperfine Interactions 1 (1976) 501. [3] B. Bleaney, Magnetic Properties of Rare Earth Metals, ed. R. J. Elliott (Plenum, London, 1972) p. 383. [4] G. J. Bowden, R. K. D a y a n d M. Sarwar, Intern. Conf. on Magnetism, Vol. IV (Nauka, Moscow, 1973) p. 475. [5] R. L. Cohen, Phys. Rev. 134A (1964) 94.
169Tin Mrssbauer measurements were made on thulium intermetallics. From TmFe2, the saturated electric quadrupole interaction was (523 _+ 10) MHz which enables the lattice contribution in pure thulium metal to be calculated as (40 _ 11) MHz. In TmCo2 the thulium moments were found to be quenched by 11% below the saturated value.
1. Introduction The MOssbauer effect in 169Tm has been used to investigate some thulium-3d transition metal intermetallics.
2. Experimental A MOssbauer source was prepared as described by Kalvius et al. [1]. A l0 wt% alloy of isotopically enriched 168Er in a l u m i n u m was arc-melted, vacuum cast into a 3 m m diameter cold copper mould and rolled to a thickness of 100 #m. A l0 m m diameter disc was irradiated in a neutron flux of 5 )< 1017 n m -2 s -1 for 9 days and then mounted on a constant acceleration drive capable of velocities of _+700 m m s-1. In addition to the 8.401 keV 169Tm ),-ray the source also emitted the characteristic Er L X-rays with the most intense of these having energies of 6.947, 7.809 and 8.188 keV. The intermetallics were melted in an argon arc furnace using 3 N pure thulium and 4 N pure 3d transition metals. Weight losses corresponding to 2 to 3% of thulium were observed and allowed for when weighing the starting materials. All the samples were analysed using an X-ray diffractometer. T m C o 2 and Tm2Fel7 required annealing at 1000 and 1200°C, respectively, for 24 h to reduce the second phase to below 3%. The high MOssbauer absorption cross-section of 169Tm required that special attention be given to the preparation of uniform absorbers in the range 50 to 80 g m -2 of thulium. The intermetallics investigated were brittle and could not be rolled into foils. The following preparation procedure was eventually adopted. The intermetallics were ground to < 50 # m and distributed on a layer of double-
sided transfer tape which had previously been shown to have negligible absorption of the 169Tm ),-ray. Such a layer contained 20 to 40 g m -2 of thulium and the required total thickness was achieved by adding further layers of tape and powder. The absorbers were clamped between 1 m m thick beryllium discs and mounted in a cryostat. The 3d transition elements Fe, Co and Ni have K absorption edges at 7.112, 7.712 and 8.339 keV, respectively. Comparing these numbers with the 169Tin ),-ray energy of 8.401 keV it was apparent that an increasingly strong photo-electric absorption of the ),-ray would occur in going from Fe through Co to Ni (30 g m -2 of Ni corresponds to one absorption length for 8.4 keV electro-magnetic radiation). In fact samples containing nickel were particularly effective at filtering out the t69Tm ),ray and transmitting the unwanted Er L X-rays. A single line absorber was prepared by dissolving 20 wt% of thulium in aluminum and rolling to a foil containing 70 g m -2 of thulium. At room temperature, the source-absorber combination gave a line width of 30 m m s - i and an absorption of 12%.
3. Results The following thulium-3d transition metal compounds were ,investigated: TmFe2, Tm2Fel7 , T m C o 2, T m C o 3, T m N i 2 and T m N i 3. Repeated attempts to prepare single phase samples of T m F e 3 were unsuccessful. N o M6ssbauer absorption was observed for T m N i 3 down to 4.2 K. For T m N i 2 a single line (with 0.7% absorption effect) was observed at 1.5 K. AC susceptibility measurements on this sample showed no evidence for magnetic order down to 1.6 K. F o r T m C o a a small broad absorption ( ~ 1% effect with a width of 114 m m s - 1) was
Journal of Magnetism and Magnetic Materials 15-18 (1980) 651-652 ©North Holland
651
652
R. K. Day, J. B. Dunlop~ 16Tm Mossbauer effect in intermetallics
TABLE 1 Magnetic hyperfine a n d electric quadrupole parameters Compound
TmFe 2 TmFe2 TmFe2* Tm2Fe w TmCo2 TmCo2 TmCo2
Temp.
G r o u n d state
Quadrupole
Total
(K)
splitting a s (MHz)
parameter P (MHz)mate
splitting (ram s - 1)
1.26 4.2 4.2 4.2 1.4 3.3 4.2
2493 2487 2480 2495 2004 1929 1916
520 525 530 526 404 394 380
1182 _+ 3 1180 _+ 3 1160 i183 __. 4 951 4- 3 915 +__6 909 ___ 9
___ 6 _ 5 _ 9 +_ 6 +__ 13 ___ 19
___ 10 ___ 8 _ 13 ___ 11 ___ 14 _+ 26
*Cohen [5].
observed at room temperature. At lower temperatures the splitting increased but the percentage absorption was too low for data analysis. The results for TmFe2, T m C o 2 and Tm2Fel7 are set out in table 1 and have been taken from computer fits to the data using the simple nuclear Hamiltonian 9C = a I z + P [ I ~ - I ( I + 1)/3]
together with a ratio of ground to excited state nuclear moments /x~//~g -- - 2.214 + 0.014. Our value for /~,//~g is based upon the analysis of 23 sets of experimental data and may be compared to the recently reported value of - 2 . 2 2 3 + 0.013 of Wit and Niesen [2]. The errors quoted in table 1 are statistical. When the errors in the velocity calibration and the ratio of It~//zg are included, the additional errors in the total splitting and in a g a r e 0.5 and 0.7%, respectively. The results in table 1, together with the previously reported [2] ground state splitting, (2253.7 _ 60) MHz, and excited state quadrupole parameter, (483.2 + 3) MHz for pure thulium metal, allow several observations to be made. (1) In the cubic compound T m F e 2 there is no lattice contribution (Pc) to the quadrupole parameter P, and crystal field quenching effects should be negligible because of the high value for the magnetic ordering temperature ( T c = 555 K)*. Hence we believe that the measured P is equal to the saturation value of P4f, the 4f electron contribution. P4t (saturation) = (523 _ 10) MHz. From this we can estimate the lattice contribution of P in pure thulium metal to be (40 +__ 11) MHz which is *Recent calculations ( B o w d e n - - p r i v a t e communication) indicate quenching in T m F e 2 m a y be a b o u t 1%.
in reasonable agreement with the values quoted by Bleaney [3] who reviewed previous M6ssbauer work. The ground state splitting, as, in T m F e 2 and TmzFel7 is 10.5% greater than for thulium due to the transferred hyperfine fields from the iron sites and is typical of the values found in other rare earth iron intermetallics. (2) The case of TmEFel7 is somewhat strange as no large lattice contribution to P is observable in this hexagonal material. However, the experimental data were fitted with a single spectrum, whereas there are two crystallographically inequivalent sites. (3) In TmCo 2, the quadrupole parameter P is reduced by 22% from the saturation value. It would thus appear that significant crystal field quenching is taking place in this compound. Bowden et al. [4] using second order perturbation theory have shown that in the presence of quenching 8 a / a = (sP/e)[(2J - 1)/(6(J - 2))]. For T m 3+ J = 6 thus ( 2 J - 1 ) / ( 6 ( J - 2 ) ) = 0.46 and 8 a / a = 0 . 4 6 ( S P / P ) . Experimentally 8 P / P = 0.23 ___0.03, therefore the expected reduction in the 4f contribution to the magnetic hyperfine field is 10.6%. The saturated value of ag is 2253.7 MHz (from measurements on thulium), hence we expect ag to be 2015 for TmCo 2 which agrees with the measured value of 2004.3 MHz. The transferred hyperfine field in TmCo z is expected to be much less than in T m F e z because of the small induced moment at the cobalt sites. (4) The temperature dependence of the magnetic hyperfine field, as, for TmCo 2 is very interesting. We have measured the ac susceptibility (Xac) of our sample and found a sharp peak at 4.9 K, while the hyperfine field decreased by only 4% from 1.4 to 4.2 K. There seems a possibility that the magnetic ordering transition is of the first order.
References [1] G. M. Kalvius, F. E. W a g n e r and W. Potzel, J. de Phys. 37, Suppl. C6 (1976) 657. [2] H. P. Wit and L. Niesen, Hyperfine Interactions 1 (1976) 501. [3] B. Bleaney, Magnetic Properties of Rare Earth Metals, ed. R. J. Elliott (Plenum, London, 1972) p. 383. [4] G. J. Bowden, R. K. D a y a n d M. Sarwar, Intern. Conf. on Magnetism, Vol. IV (Nauka, Moscow, 1973) p. 475. [5] R. L. Cohen, Phys. Rev. 134A (1964) 94.