- Email: [email protected]
Mo-doped NdFeB permanent magnets
NtuuSm
Pergamon
-. Vol. 6. pi. 953-956.1995 CGp)TigltO1995Ekfi~scienceLtd PfiatediutheusA. Allri@b3mavai 0965-977319s s9so + .oo
0965-9773(95)00218-9
MICROMAGNETIC ANALYSIS OF NANOCRYSTALLINE Nb/Mo-DOPED NdFeB PERMANENT MAGNETS
Ga- AND
G. Rieger, J. Bauer, M. Seeger and H. Kronmiiller Max-Planck-Institut flir Metallforschung, Institut fiir Physik, Heisenbergstral3e1, D-70569 Stuttgart, Germany Abstract- Nanocrystalline as well as microcrystalline melt-spun ribbons of NdFeB alloys with small additions of Ga and Nb or MO were prepared using the single-roller technique. Average grain sizes of 25 nm up to 500 nm were obtained by varying the wheel speed and the parameters of the heat treatment after rapid quenching. Systematic investigationsof the relationship between microstructural ejj?ects and magnetic properties were carried out according to micromagnetic concepts. The theory of nucleation in single-domain particles is able to explain the magnetization reversal process, if several deteriorating effects of the microstructure are taken into account. The additives Ga and NbIMo both improve the coercive held. Ga is known to form new intergranular phases leading to a better decoupling of the grains. High-melting NblMo-containing precipitates act as nucleation centers. Ga- and NblMo-dopedsamples show therefore small magnetization reversal intervals and a high reversibility of the first part of the demagnetization curve. Furthermore the concept of microstructural parameters QK and N,f is applied to an extensive investigation of the temperature dependence of coercivity. For different microstructures it turns out that a~ reaches saturation as required by theory whereas a wide spectrum of Ng is found. Nevertheless a strong correlation of both CxKand NCff restricts the improvement of the coercive field. Best critical fields at room-temperature up to 2.5 T have been obtained for annealed Gaand Nb-doped samples with a fine-grained nanocrystalline and uniform microstructure.
INTRODUCTION Macroscopic magnetic properties are determined not only by intrinsic properties but also by the microstructure of the hard magnetic material. Micromagnetic concepts are able to describe the main microstructural effects (1,2). The critical field (defined as the maximum of xirr) is written as a modified Brown’s expression (2)
whereati accounts for misaligned grains,crK describes inhomogcncitics of the ma~nctocrystallinc anisotropy near grain-boundary regions and Ng is an effective local dcmagnctization laclor. In isotropic melt-spun magnets the mean QW reaches 0.6 due to the random distribution of the easy axes. The beginning of nucleation in some grains causes strong demagnetization liclds in neighbduring grains via dipolar coupling. Therefore CX~ generally determines the critical lield. The basic formula for a micromagnctic analysis of the tcmpcraturc dcpcndcncc ol’II,:,, is then given as 953
G RIEGER,J BAUER,M SEEGERANDH KRONMULLER
954
POHcrit(T)= JsV1 where H.~[KI(T),K~(T),J,(T)]
&IJH~(T)
_ N
JsU-2
(2)
en*
is given in (3).
EXPERIMENTAL Ribbon flakes of alloys in the NdFeBGa, NdFeBGaNb, NdFeBGaMo systems were prepared by melt-spinning on a rotating copper wheel in helium atmosphere.Annealing treatmentswere carried out at temperaturesbetween600°C and 760°C and annealingtimes of 7 to 60 m in. The magneticmeasurementswere madewith a QuantumDesign SQUID- magnetometerand a PAR VSM having a superconductingmagnetwith a maximum applied field of 9 T. Electron m icrographswere madeusing scanning(SEM) and transmissionelectronm icroscopy (TEM) technique.
RESULTS AND DISCUSSION Magnetic properties at room temperature The variation of the processingparameters,i.e. different wheel speeds,leads to a systematic change of the initial magnetization curve in rapidly solidified permanent magnets (4). By normalizing and subtracting the reversible rotations, one can determine the fraction of multidomain particles easily (Fig. lb). Tab. 1 gives the results for varying wheel speeds v, and annealingtimes ta. They agreewell with averagegrain sizes taken from m icrographs. Whereasfor as-quenched GaNb/Mo-doped specimensthe fraction of multidomain particles mostly vanishes already at wheel speeds of v, = 20 m /s, we found always a low-field stage in the initial magnetization curve for Ga-doped specimensup to the highest used wheel speedsv, = 50 m /s indicating a non-vanishing fraction of multidomain particles (Fig. lb). The explanation of this behaviour seemsto be the ability of Nb and M O to act as nucleationcentersduring rapid solidification. Thus the crystallization is controlled and restricted leading to a uniform m icrostructure. High critical fields of poHctir = 2.3 T for as-quenchedsamplesare reached. Optimized heat treatmentenhancesthe rt value of GaNb-dopedsamplesup to 2.5 T but sometimesreducesit for Ga-dopedsamplesdue to inhomogeneousgrain sizes and growth. From temperaturedependent measurementsthis must be assignedto strong demagnetizationfields. In conclusion the coercive fields are enhancedby two main effects: 1. The intergranularphasesare modified. New Ga-containingphaseswith a higher viscosity lead to a better decoupling of the grains. This phenomenonis mainly known from investigationson sintered magnets(5,6,7). TABLE 1 Fraction of Multidomain Particles and Grain Size composition
wheel speed
vs [m/s1
NdlsFe74B7Gal NdlyFe73B7GatMoz
Ndr7l%B7.sGal.sNbz
30 40 30 14 30
annealing treatment
as-quenched as-quenched as-quenched 7OO”C,12 min
fraction of multidomain particles [%] 48 20 0 44 0 27
grain size < d >[nm]
250 100 20 50-400 20 20-50
NANOCRYSTAUINE
Ga-
AND
Nb/Mo-Dwm NdFeB PERMANENT MAGNETS
955
Figure 1: a) Room temperaturehysteresisloops of Ga-, and GaMo-dopedsamples. b) Initial magnetizationcurves of Ga-, and GaMo-dopedsamples sinteredmagnets(5,6,7). 2. The better surface, shapeand arrangementof the grains in this doped melt-spun magnets enhancesthe coercive field accordingto eq.(1). Reversibility of the demagnetization curve
Reversible rotations determine the magnetization processeson the recoil and part of the demagnetizationcurve of an isotropic magnet. Irreversible processes(spontaneousrotations) should therefore not occur until a maximum field Hr near the critical field is reached. Large Hz and small field intervals of magnetizationreversal indicate a uniform magnetic behaviour of the assembly of particles and therefore the uniformity of the microstructure. Hysteresis loops of Nb- or MO-dopedsamplesare characterizedby large HE and small magnetization reversal field-intervals as comparedto sampleswithout Nb/Mo (Fig. la). The full reversibility of the demagnetizationcurve of a MO-dopedspecimenhas beenproved up to a reversedfield of H-31.7 T. This is strongly related to the absenceof a gradlent of the grain-size in the crossse%on of the ribbons betweenfree side and wheel side which is usually found (Fig. 2). Moreover the distribution of grain-sizeswithin GaNb/Mo-doped samplesis narrow in comparison to the broaddistribution in Ga-dopedsamples. Microstructural parameters
The microstructuralparametersq andNcff are determinedby the temperaturedependenceof HCti, (Fig. 3a) accordingto eq.(2). They representa physical link betweenprocessingparameters
and macroscopicmagneticproperties. We found no significant influence of the wheel speedand thereforeof the grain size over 2-3 orders of magnitude. Annealing treatmentswhich influence mainly the shapeand surfaceof the grains and less their size lead at least to the samecoercivity enhancementas comparedto the wheel speedvariation. Annealing treatmentslead to a systematic increaseof both microstructural parametersCXKand Ncf (Fig. 3a). This must be interpretedon the one hand as a result of more perfect grain surfaces and on the other hand as a result of more polyhedral grain shapeswhich lead to enhanceddemagnetizationfields. Plotting a~ versus N,ff provides a detailed picture of the microstructural effects controlled by the melt-spinning
956
G RIEGER,J RVJER,M SEEGERANDH KRONMCJUER
Figure 2: Micrographof a) NdlrFe7dB7Gai,30 m /s, annealed(SEM), b) Ndr7Fe7sBTGal Me, 30 m /s,as-quenched(TEM).
30m /s cyO.92N,,=0.76
%I
Ldrw”/s
Figure 3: a) Determinationof the microstructuralparameters. b) Correlationbetweenthe microstructuralparameters. technique. The dashedline in Fig. 3b reflects the strong correlationbetweenall processing parameters,e.g. the annealingtreatment. REFERENCES 1. W .F.Brown, liiicrumagnetics, IntersciencePub.,J. W iley & Sons,New York (1963) 2. H. Kronmliller,Phys. Stat. Sol. (b) 144,385 (1987) 3. G. Martinekand H. KronmtillerJ. Magn. Magn. Mat. 86,177 (1990) 4. M . Gronefeldand H. Kronmtiller,J. Magn. Magn. Mat. &, L267 (1990) 5. M . Endoh,M . Tokunaga,and H. Harada,IEEE. Trans. Magn. MAC-a, 2290 (1987) 6. J. Fidler, and K.G. Knoch,J. Magn. Magn. Mat. @ , 48 (1989) 7. M . Seeger,J. Bauer,H. Kronmliller,J. Bemardi,J. Fidler, J. Magn. Magn. Mat.(1994)
Pergamon
-. Vol. 6. pi. 953-956.1995 CGp)TigltO1995Ekfi~scienceLtd PfiatediutheusA. Allri@b3mavai 0965-977319s s9so + .oo
0965-9773(95)00218-9
MICROMAGNETIC ANALYSIS OF NANOCRYSTALLINE Nb/Mo-DOPED NdFeB PERMANENT MAGNETS
Ga- AND
G. Rieger, J. Bauer, M. Seeger and H. Kronmiiller Max-Planck-Institut flir Metallforschung, Institut fiir Physik, Heisenbergstral3e1, D-70569 Stuttgart, Germany Abstract- Nanocrystalline as well as microcrystalline melt-spun ribbons of NdFeB alloys with small additions of Ga and Nb or MO were prepared using the single-roller technique. Average grain sizes of 25 nm up to 500 nm were obtained by varying the wheel speed and the parameters of the heat treatment after rapid quenching. Systematic investigationsof the relationship between microstructural ejj?ects and magnetic properties were carried out according to micromagnetic concepts. The theory of nucleation in single-domain particles is able to explain the magnetization reversal process, if several deteriorating effects of the microstructure are taken into account. The additives Ga and NbIMo both improve the coercive held. Ga is known to form new intergranular phases leading to a better decoupling of the grains. High-melting NblMo-containing precipitates act as nucleation centers. Ga- and NblMo-dopedsamples show therefore small magnetization reversal intervals and a high reversibility of the first part of the demagnetization curve. Furthermore the concept of microstructural parameters QK and N,f is applied to an extensive investigation of the temperature dependence of coercivity. For different microstructures it turns out that a~ reaches saturation as required by theory whereas a wide spectrum of Ng is found. Nevertheless a strong correlation of both CxKand NCff restricts the improvement of the coercive field. Best critical fields at room-temperature up to 2.5 T have been obtained for annealed Gaand Nb-doped samples with a fine-grained nanocrystalline and uniform microstructure.
INTRODUCTION Macroscopic magnetic properties are determined not only by intrinsic properties but also by the microstructure of the hard magnetic material. Micromagnetic concepts are able to describe the main microstructural effects (1,2). The critical field (defined as the maximum of xirr) is written as a modified Brown’s expression (2)
whereati accounts for misaligned grains,crK describes inhomogcncitics of the ma~nctocrystallinc anisotropy near grain-boundary regions and Ng is an effective local dcmagnctization laclor. In isotropic melt-spun magnets the mean QW reaches 0.6 due to the random distribution of the easy axes. The beginning of nucleation in some grains causes strong demagnetization liclds in neighbduring grains via dipolar coupling. Therefore CX~ generally determines the critical lield. The basic formula for a micromagnctic analysis of the tcmpcraturc dcpcndcncc ol’II,:,, is then given as 953
G RIEGER,J BAUER,M SEEGERANDH KRONMULLER
954
POHcrit(T)= JsV1 where H.~[KI(T),K~(T),J,(T)]
&IJH~(T)
_ N
JsU-2
(2)
en*
is given in (3).
EXPERIMENTAL Ribbon flakes of alloys in the NdFeBGa, NdFeBGaNb, NdFeBGaMo systems were prepared by melt-spinning on a rotating copper wheel in helium atmosphere.Annealing treatmentswere carried out at temperaturesbetween600°C and 760°C and annealingtimes of 7 to 60 m in. The magneticmeasurementswere madewith a QuantumDesign SQUID- magnetometerand a PAR VSM having a superconductingmagnetwith a maximum applied field of 9 T. Electron m icrographswere madeusing scanning(SEM) and transmissionelectronm icroscopy (TEM) technique.
RESULTS AND DISCUSSION Magnetic properties at room temperature The variation of the processingparameters,i.e. different wheel speeds,leads to a systematic change of the initial magnetization curve in rapidly solidified permanent magnets (4). By normalizing and subtracting the reversible rotations, one can determine the fraction of multidomain particles easily (Fig. lb). Tab. 1 gives the results for varying wheel speeds v, and annealingtimes ta. They agreewell with averagegrain sizes taken from m icrographs. Whereasfor as-quenched GaNb/Mo-doped specimensthe fraction of multidomain particles mostly vanishes already at wheel speeds of v, = 20 m /s, we found always a low-field stage in the initial magnetization curve for Ga-doped specimensup to the highest used wheel speedsv, = 50 m /s indicating a non-vanishing fraction of multidomain particles (Fig. lb). The explanation of this behaviour seemsto be the ability of Nb and M O to act as nucleationcentersduring rapid solidification. Thus the crystallization is controlled and restricted leading to a uniform m icrostructure. High critical fields of poHctir = 2.3 T for as-quenchedsamplesare reached. Optimized heat treatmentenhancesthe rt value of GaNb-dopedsamplesup to 2.5 T but sometimesreducesit for Ga-dopedsamplesdue to inhomogeneousgrain sizes and growth. From temperaturedependent measurementsthis must be assignedto strong demagnetizationfields. In conclusion the coercive fields are enhancedby two main effects: 1. The intergranularphasesare modified. New Ga-containingphaseswith a higher viscosity lead to a better decoupling of the grains. This phenomenonis mainly known from investigationson sintered magnets(5,6,7). TABLE 1 Fraction of Multidomain Particles and Grain Size composition
wheel speed
vs [m/s1
NdlsFe74B7Gal NdlyFe73B7GatMoz
Ndr7l%B7.sGal.sNbz
30 40 30 14 30
annealing treatment
as-quenched as-quenched as-quenched 7OO”C,12 min
fraction of multidomain particles [%] 48 20 0 44 0 27
grain size < d >[nm]
250 100 20 50-400 20 20-50
NANOCRYSTAUINE
Ga-
AND
Nb/Mo-Dwm NdFeB PERMANENT MAGNETS
955
Figure 1: a) Room temperaturehysteresisloops of Ga-, and GaMo-dopedsamples. b) Initial magnetizationcurves of Ga-, and GaMo-dopedsamples sinteredmagnets(5,6,7). 2. The better surface, shapeand arrangementof the grains in this doped melt-spun magnets enhancesthe coercive field accordingto eq.(1). Reversibility of the demagnetization curve
Reversible rotations determine the magnetization processeson the recoil and part of the demagnetizationcurve of an isotropic magnet. Irreversible processes(spontaneousrotations) should therefore not occur until a maximum field Hr near the critical field is reached. Large Hz and small field intervals of magnetizationreversal indicate a uniform magnetic behaviour of the assembly of particles and therefore the uniformity of the microstructure. Hysteresis loops of Nb- or MO-dopedsamplesare characterizedby large HE and small magnetization reversal field-intervals as comparedto sampleswithout Nb/Mo (Fig. la). The full reversibility of the demagnetizationcurve of a MO-dopedspecimenhas beenproved up to a reversedfield of H-31.7 T. This is strongly related to the absenceof a gradlent of the grain-size in the crossse%on of the ribbons betweenfree side and wheel side which is usually found (Fig. 2). Moreover the distribution of grain-sizeswithin GaNb/Mo-doped samplesis narrow in comparison to the broaddistribution in Ga-dopedsamples. Microstructural parameters
The microstructuralparametersq andNcff are determinedby the temperaturedependenceof HCti, (Fig. 3a) accordingto eq.(2). They representa physical link betweenprocessingparameters
and macroscopicmagneticproperties. We found no significant influence of the wheel speedand thereforeof the grain size over 2-3 orders of magnitude. Annealing treatmentswhich influence mainly the shapeand surfaceof the grains and less their size lead at least to the samecoercivity enhancementas comparedto the wheel speedvariation. Annealing treatmentslead to a systematic increaseof both microstructural parametersCXKand Ncf (Fig. 3a). This must be interpretedon the one hand as a result of more perfect grain surfaces and on the other hand as a result of more polyhedral grain shapeswhich lead to enhanceddemagnetizationfields. Plotting a~ versus N,ff provides a detailed picture of the microstructural effects controlled by the melt-spinning
956
G RIEGER,J RVJER,M SEEGERANDH KRONMCJUER
Figure 2: Micrographof a) NdlrFe7dB7Gai,30 m /s, annealed(SEM), b) Ndr7Fe7sBTGal Me, 30 m /s,as-quenched(TEM).
30m /s cyO.92N,,=0.76
%I
Ldrw”/s
Figure 3: a) Determinationof the microstructuralparameters. b) Correlationbetweenthe microstructuralparameters. technique. The dashedline in Fig. 3b reflects the strong correlationbetweenall processing parameters,e.g. the annealingtreatment. REFERENCES 1. W .F.Brown, liiicrumagnetics, IntersciencePub.,J. W iley & Sons,New York (1963) 2. H. Kronmliller,Phys. Stat. Sol. (b) 144,385 (1987) 3. G. Martinekand H. KronmtillerJ. Magn. Magn. Mat. 86,177 (1990) 4. M . Gronefeldand H. Kronmtiller,J. Magn. Magn. Mat. &, L267 (1990) 5. M . Endoh,M . Tokunaga,and H. Harada,IEEE. Trans. Magn. MAC-a, 2290 (1987) 6. J. Fidler, and K.G. Knoch,J. Magn. Magn. Mat. @ , 48 (1989) 7. M . Seeger,J. Bauer,H. Kronmliller,J. Bemardi,J. Fidler, J. Magn. Magn. Mat.(1994)