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X-ray circular magnetic dichroism as a probe of spin reorientation transitions in Nd2Fe14B and Er2Fe14B systems
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1051-1052
mm
journal of magnetism and
magnetic materials ELSEVIER
X-ray circular magnetic dichroism as a probe of spin reorientation transitions in NdzFeaB and ErzFeIaBsystems J. Chaboy "'*, A. Marcelli b L.M. Garcla a, j. Bartolom6 a, M.D. Kuz'min H. Maruyama c, K. Kobayashi c, H. Kawata d, T. Iwazumi d
a
a lnstituto de Ciencia de Materiales de Arag6n, Facultad de Ciencias, CS1C, Unit,ersidad de Zaragoza, 50009 Zaragoza, Spain b LN.F.N., Laboratori Nazionali di Frascati, Casella Postale 13, 00044 Frascati, Italy c Department of Physics, Faculty of Science, Okayama Unit,ersity, Okayama, Japan d Photon Factory, National Laboratory for High Energy Physics, 1-l Oho, Tsukuba, Japan
Abstract We present the first experimental observation of spin reorientation phase transitions (SRT) with the X-Ray circular magnetic dichroism (XCMD) technique. Both the first-order SRT in ErzF%4B and the second-order one in Nd2FelaB have been clearly detected, demonstrating the feasibility of this technique for studying SRTs.
The discovery of new permanent magnet compounds of the type N d - F e - B has recently stimulated intensive studies of the RzFeI4B series (R = rare earth) to find the origin of the outstanding permanent magnet properties of NdzFe14B [1]. However, the temperature range in which these new alloys can be used as hard magnets is limited by the spin reorientation phase transition (SRT) that appears at low temperatures for R = N d , Ho, Yb, Er and Tm, destroying the uniaxial anisotropy. Although the behavior of the bulk magnetization has been well determined, the evolution of the magnetic moments on the microscopic scale is less well understood, especially in the cases of NdzF%4B and HozF%aB, where in the low-temperature phase the R and Fe moments can be considerably non collinear [2]. The limitation of conventional magnetic techniques, that they are unable to provide direct information on each magnetic sublattice individually, can be overcome by using X-ray circular magnetic dichroism (XCMD). In this work, XCMD is applied for the first time to study magnetic order-order phase transitions in the case of Nd2FelaB and Er2Fel4B compounds. Experiments were performed on the accumulator ring of TRISTAN at the beamline AR NE1 at KEK [3]. The XCMD spectra were recorded by reversing the sample magnetization for a fixed left circular polarization of the incoming radiation. The measurements were taken at different fixed temperatures: 300-325 K for the ErzFel4B
* Corresponding author. Fax: + 34 76-553773; email: jch [email protected].
sample, and from room temperature down to 50 K for NdeFe14 B. The samples were prepared starting from pure powder embedded in epoxy glue at T = 350 K under a magnetic field of 1.5 T. After the solidification of the glue, thin plates of the oriented powder were cut with the plane along the alignment direction to guarantee single magnetic domain conditions in the XCMD experiments. The spin-dependent absorption coefficient was determined as the difference of the normalized absorption coefficient for antiparallel ( p ~ ) and parallel (/~+) orientation of the photon spin and the projected magnetic field applied to the sample. The room-temperature XCMD spectra obtained at both the Nd L2-edge and at the Fe K-edge in the case of Nd2FelgB are shown in Fig. 1. In the case of the iron K-edge, the dichroic signal probes directly that magnetic moment on iron atoms is parallel to the net magnetization of the sample [4]. The XMCD effect at the L2-edge probes the spin polarization of the final 5d states of the photoabsorbing atom, modulated by the size of the spindependent transition matrix element, so that the sign of the XCMD signal is not directly linked to the sign of the difference of spin-polarized (up-down) 5d density of states [5]. In this sense, it has been experimentally observed for R2Co17 compounds that the sign of the XCMD signal is positive (R = Pr, Nd, Sm, Gd and Tb) or negative (Ho, Er and Tm) when the rare-earth magnetic moments are ferroand antiferromagnetically coupled to the Fe 3d moments for light and heavy rare-earths, respectively [6]. In the present compounds, the same relation holds: the Nd2Fe14B yields a positive XCMD signal while the Er2FeI4B signal is negative. The comparison of the XCMD signals at the Lz-edge of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO304-8853(94)OI258-X
1052
0.04
J. Chaboy et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1051-1052 .... I .... I'"'t'"'l
.... I
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--
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,o
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o
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--
-o.s
--
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the rare earth recorded at different temperatures is reported in Fig. 2 for both the Nd2Fe14B and ErzFe14B compounds. In the case of Er2Fe14B, the XCMD signal is zero at room temperature, when the magnetization is perpendicular to the c-axis and to the applied field, while it becomes negative as the transition temperature is reached, Ts = 315 K. On the other hand, in the case of NdzFe]4B the amplitude of the dichroic signal increases continuously as the temperature decreases. The character of the transition can be studied by considering the integrated intensity of the XCMD, as shown in Fig. 3, as a function of temperature. Its abrupt change in Er2FelaB agrees with the firstorder nature of the magnetic transition. In contrast, in NdzFe14B it grows in a continuous manner when cooling down through Ts, showing a rounded maximum at about Tm = 110 K, corresponding to the continuous increase in the tilting angle, 0T, between the magnetization and the c-axis. Because of the strong exchange between the Nd 4f
gOK
~IOOK
o.o*
<
-o.ol
O.OQ _o.o2 ]~.~, , , , ,,,,I,,,~ 8.VO 872 eV4 E n e r g y (keV)
925
9.28
g . 2 7 0.28
gJ~g
. . . .
. . . . . aoo
310
]'
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~0
I . . . . 100
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Fig. 1. Room-temperature XCMD signals at the at the L2-edge of Nd (left panel) and at the iron K-edge in Nd2Fe14 B (right panel).
• 1
O.S oo
g~
E n e r g y (keV)
Fig. 2. Nd (left panel) and Er (right panel) L2-edge XCMD signals recorded at different temperatures in NdzFel4B and Er2Fe14B compounds. The units on the ordinate give the intensity of the XMCD signals: the zero of the scale is referred to the data taken at T = 80 and 300 K for Nd2FeI4B and ErzFe14B, respectively.
] 200
. . . .
I 300
Fig. 3. Temperature dependence of the Nd L2-edge integrated XCMD signal, (D), and the projection along the X-ray wave vector of the Nd magnetic moment taken from Ref. [8] (O), and Ref. [7] (~). Inset: temperature dependence of the Er Lz-edge XCMD integrated signal in Er2Fe]4B. and 5d moments, the XCMD signal is expected to reflect the temperature dependence of the average projection of the Nd magnetic moment in the direction of the beam. To estimate the temperature dependence of this projection we have considered the available data on t.ZNd and 0 x [7,8] and scaled the projection (/.ZNd COS0T) to the XCMD data. The remarkable similarity exhibited between the XCMD signal and the MSssbauer data (Fig. 3) demonstrates the feasibility of the XCMD technique for studying SRTs. Acknowledgements: We are indebted to T. Miyahara for his invaluable help during our time at KEK. This work has been performed with the approval of the Photon Factory Program Advisory Committee (Proposal N.91-215). J.Ch. and M.D.K. acknowledge FPI grants of the Ministerio de Educaci6n y Ciencia of Spain. This work was supported by DGICYT M A T 9 1 / 4 8 7 and M A T 9 3 / 0 2 4 0 C 0 4 grants and by INFN.
References [1] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [2] For a review, see J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819 and references therein. [3] S. Yamamoto, H. Kawata, H. Kitamura, M. Ando, N. Saki and N. Shiotani, Phys. Rev. Lett. 62 (1989) 2672. [4] C. Brouder and M. Hikam, Phys. Rev. B 43 (1991) 3089. [5] X. Wang, T.C. Leung, B.N. Harmon and P. Carra, Phys. Rev. B 47 (1993) 9087; J. Chaboy, A. Marcelli, L.M. Garc~a, J. Bartolom6, M.D. Kuz'min, H. Maruyama, K. Kobayashi, H. Kawata and T. Iwazumi, Europhys. Lett. 28 (1994) 135. [6] P. Fischer, G. Schutz and G. Wiesinger, Solid State Commun. 76 (1990) 777. [7] I. Nowik, K. Muraleedharan, G. Wonmann, B. Perscheid, G. Kaindl and N.C. Koon, Solid State Commun. 76 (1990) 967. [8] H. Onoedera, A. Fujita, H. Yamamoto, M. Sagawa and S. Hirosawa, J. Magn. Magn. Mater. 68 (1987) 6; 15.
mm
journal of magnetism and
magnetic materials ELSEVIER
X-ray circular magnetic dichroism as a probe of spin reorientation transitions in NdzFeaB and ErzFeIaBsystems J. Chaboy "'*, A. Marcelli b L.M. Garcla a, j. Bartolom6 a, M.D. Kuz'min H. Maruyama c, K. Kobayashi c, H. Kawata d, T. Iwazumi d
a
a lnstituto de Ciencia de Materiales de Arag6n, Facultad de Ciencias, CS1C, Unit,ersidad de Zaragoza, 50009 Zaragoza, Spain b LN.F.N., Laboratori Nazionali di Frascati, Casella Postale 13, 00044 Frascati, Italy c Department of Physics, Faculty of Science, Okayama Unit,ersity, Okayama, Japan d Photon Factory, National Laboratory for High Energy Physics, 1-l Oho, Tsukuba, Japan
Abstract We present the first experimental observation of spin reorientation phase transitions (SRT) with the X-Ray circular magnetic dichroism (XCMD) technique. Both the first-order SRT in ErzF%4B and the second-order one in Nd2FelaB have been clearly detected, demonstrating the feasibility of this technique for studying SRTs.
The discovery of new permanent magnet compounds of the type N d - F e - B has recently stimulated intensive studies of the RzFeI4B series (R = rare earth) to find the origin of the outstanding permanent magnet properties of NdzFe14B [1]. However, the temperature range in which these new alloys can be used as hard magnets is limited by the spin reorientation phase transition (SRT) that appears at low temperatures for R = N d , Ho, Yb, Er and Tm, destroying the uniaxial anisotropy. Although the behavior of the bulk magnetization has been well determined, the evolution of the magnetic moments on the microscopic scale is less well understood, especially in the cases of NdzF%4B and HozF%aB, where in the low-temperature phase the R and Fe moments can be considerably non collinear [2]. The limitation of conventional magnetic techniques, that they are unable to provide direct information on each magnetic sublattice individually, can be overcome by using X-ray circular magnetic dichroism (XCMD). In this work, XCMD is applied for the first time to study magnetic order-order phase transitions in the case of Nd2FelaB and Er2Fel4B compounds. Experiments were performed on the accumulator ring of TRISTAN at the beamline AR NE1 at KEK [3]. The XCMD spectra were recorded by reversing the sample magnetization for a fixed left circular polarization of the incoming radiation. The measurements were taken at different fixed temperatures: 300-325 K for the ErzFel4B
* Corresponding author. Fax: + 34 76-553773; email: jch [email protected].
sample, and from room temperature down to 50 K for NdeFe14 B. The samples were prepared starting from pure powder embedded in epoxy glue at T = 350 K under a magnetic field of 1.5 T. After the solidification of the glue, thin plates of the oriented powder were cut with the plane along the alignment direction to guarantee single magnetic domain conditions in the XCMD experiments. The spin-dependent absorption coefficient was determined as the difference of the normalized absorption coefficient for antiparallel ( p ~ ) and parallel (/~+) orientation of the photon spin and the projected magnetic field applied to the sample. The room-temperature XCMD spectra obtained at both the Nd L2-edge and at the Fe K-edge in the case of Nd2FelgB are shown in Fig. 1. In the case of the iron K-edge, the dichroic signal probes directly that magnetic moment on iron atoms is parallel to the net magnetization of the sample [4]. The XMCD effect at the L2-edge probes the spin polarization of the final 5d states of the photoabsorbing atom, modulated by the size of the spindependent transition matrix element, so that the sign of the XCMD signal is not directly linked to the sign of the difference of spin-polarized (up-down) 5d density of states [5]. In this sense, it has been experimentally observed for R2Co17 compounds that the sign of the XCMD signal is positive (R = Pr, Nd, Sm, Gd and Tb) or negative (Ho, Er and Tm) when the rare-earth magnetic moments are ferroand antiferromagnetically coupled to the Fe 3d moments for light and heavy rare-earths, respectively [6]. In the present compounds, the same relation holds: the Nd2Fe14B yields a positive XCMD signal while the Er2FeI4B signal is negative. The comparison of the XCMD signals at the Lz-edge of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO304-8853(94)OI258-X
1052
0.04
J. Chaboy et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1051-1052 .... I .... I'"'t'"'l
.... I
....
_:'1
"1 .... I .... I'q
--
0.02
,o
05
-a o,~.5
o
j,
-T 0
§
e.5 0.00
--
-o.s
--
-I,o
o c~
~ O.lO
z.o
1.s O.O5
1
I.... I.... [.... I.... I, _ 0 . 0 2 .... .... l .... lllllll,111lll -20-10 0 10 2 0 3 0 ~_0 EOI°(eV) . . . . . .
0.0
the rare earth recorded at different temperatures is reported in Fig. 2 for both the Nd2Fe14B and ErzFe14B compounds. In the case of Er2Fe14B, the XCMD signal is zero at room temperature, when the magnetization is perpendicular to the c-axis and to the applied field, while it becomes negative as the transition temperature is reached, Ts = 315 K. On the other hand, in the case of NdzFe]4B the amplitude of the dichroic signal increases continuously as the temperature decreases. The character of the transition can be studied by considering the integrated intensity of the XCMD, as shown in Fig. 3, as a function of temperature. Its abrupt change in Er2FelaB agrees with the firstorder nature of the magnetic transition. In contrast, in NdzFe14B it grows in a continuous manner when cooling down through Ts, showing a rounded maximum at about Tm = 110 K, corresponding to the continuous increase in the tilting angle, 0T, between the magnetization and the c-axis. Because of the strong exchange between the Nd 4f
gOK
~IOOK
o.o*
<
-o.ol
O.OQ _o.o2 ]~.~, , , , ,,,,I,,,~ 8.VO 872 eV4 E n e r g y (keV)
925
9.28
g . 2 7 0.28
gJ~g
. . . .
. . . . . aoo
310
]'
32o
~0
I . . . . 100
T(Z)
Fig. 1. Room-temperature XCMD signals at the at the L2-edge of Nd (left panel) and at the iron K-edge in Nd2Fe14 B (right panel).
• 1
O.S oo
g~
E n e r g y (keV)
Fig. 2. Nd (left panel) and Er (right panel) L2-edge XCMD signals recorded at different temperatures in NdzFel4B and Er2Fe14B compounds. The units on the ordinate give the intensity of the XMCD signals: the zero of the scale is referred to the data taken at T = 80 and 300 K for Nd2FeI4B and ErzFe14B, respectively.
] 200
. . . .
I 300
Fig. 3. Temperature dependence of the Nd L2-edge integrated XCMD signal, (D), and the projection along the X-ray wave vector of the Nd magnetic moment taken from Ref. [8] (O), and Ref. [7] (~). Inset: temperature dependence of the Er Lz-edge XCMD integrated signal in Er2Fe]4B. and 5d moments, the XCMD signal is expected to reflect the temperature dependence of the average projection of the Nd magnetic moment in the direction of the beam. To estimate the temperature dependence of this projection we have considered the available data on t.ZNd and 0 x [7,8] and scaled the projection (/.ZNd COS0T) to the XCMD data. The remarkable similarity exhibited between the XCMD signal and the MSssbauer data (Fig. 3) demonstrates the feasibility of the XCMD technique for studying SRTs. Acknowledgements: We are indebted to T. Miyahara for his invaluable help during our time at KEK. This work has been performed with the approval of the Photon Factory Program Advisory Committee (Proposal N.91-215). J.Ch. and M.D.K. acknowledge FPI grants of the Ministerio de Educaci6n y Ciencia of Spain. This work was supported by DGICYT M A T 9 1 / 4 8 7 and M A T 9 3 / 0 2 4 0 C 0 4 grants and by INFN.
References [1] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [2] For a review, see J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819 and references therein. [3] S. Yamamoto, H. Kawata, H. Kitamura, M. Ando, N. Saki and N. Shiotani, Phys. Rev. Lett. 62 (1989) 2672. [4] C. Brouder and M. Hikam, Phys. Rev. B 43 (1991) 3089. [5] X. Wang, T.C. Leung, B.N. Harmon and P. Carra, Phys. Rev. B 47 (1993) 9087; J. Chaboy, A. Marcelli, L.M. Garc~a, J. Bartolom6, M.D. Kuz'min, H. Maruyama, K. Kobayashi, H. Kawata and T. Iwazumi, Europhys. Lett. 28 (1994) 135. [6] P. Fischer, G. Schutz and G. Wiesinger, Solid State Commun. 76 (1990) 777. [7] I. Nowik, K. Muraleedharan, G. Wonmann, B. Perscheid, G. Kaindl and N.C. Koon, Solid State Commun. 76 (1990) 967. [8] H. Onoedera, A. Fujita, H. Yamamoto, M. Sagawa and S. Hirosawa, J. Magn. Magn. Mater. 68 (1987) 6; 15.