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Orthogonal magnetic impurity in antiferromagnet YFeO3 single crystal: magnetic resonance and magnetisation measurements
M mpt
of Magnetism and Magnetic Materials 104-107 (1992) 1037-1038 North-Holland
Journal
T
M
z
M
Orthogonal magnetic impurity in antiferromagnet YFeO, single crystal: magnetic resonance and magnetisation measurements A.M. Balbashov a, A.G. Berezin a, Ju.V. Bobryshev a, J. Paches ’ and L. P&t b E.G. Rudashevsky
‘, P.Ju. Marchukov
‘, I.V. Nikolaev
‘,
’ General Physics Institute of the USSR Academy of Sciences, II 7942, VaciloLlst. 38, Moscow, USSR h Physical Institute of Czhecoslol)ak Academy of Sciences, Na Slocance 2, 180 40 Prague K-Lihen, CzechosloL,akia
Impurity modes with antiferromagnet YFeO,.
unusual dependence on a magnetic field have been observed experimentally in orthorhombic These modes are assigned to the electron transitions between the levels of a hSs,2 multiplet of the 3.5 X 10m4. The unusual behaviour of Fe3’. ‘impurity’ ions, which occupy the 4c-positions of the Y.if ions with concentration the modes is a consequence of the orthogonal state of the impurity.
Magnetic impurities in the antiferromagnets have been extensively studied (see e.g. ref. [l] and references there). However, investigating antiferromagnetic resonance (AFMR) in an antiferromagnet with the weak ferromagnetism we have observed an unusual field dependence of the impurity modes frequencies. The experiments were carried out on the yttrium orthoferrite YFeO, single crystal (space group P,,,,,-D$, T, = 645 K [2]), grown by the zone melting technique. Absorption of submillimeter radiation (75-400 CHz) was investigated at magnetic field up to 14 T and in 12-300 K temperature range. In addition to the wellknown low-frequency mode of antiferromagnetic resonance (AFMR) [3] we observed a group of well resolved and rather intensive resonance lines at temperature below 70 K (fig. 1). The frequencies of these lines (259, 263, 271, 2X3 and 299 GHz at zero field) did not depend on temperature. The strong anisotropy of the magnetic field dependence of the frequencies was found: they decrease linearly for the field parallel to
v
(GHz)
~ ~ .
T = 40.5 K
H (Tesla) o+ 0
YFeO,
2 G\
7
I
Fig. 1. Typical 0312~8853/92/$05.00
8
I
plot of impurity
158 GHz
9
1
H (T)
absorption
0 1992 - Elsevier
Science
I
I 4
I
I 8
I 12
Fig. 2. Frequency-field dependences of AFMR and impurity modes. The inset shows the relative orientation of the sublattice magnetisation M, and M,, impurity magnetisation y and applied magnetic field.
H 1) taxis
T = 40.5 K
.
lines. Publishers
the c-axis (fig. 2) and increase nearly quadratic for that to the b-axis. We would like to emphasize that in our experiments (H 11c-axis) we observed only the descending behaviour of the impurity modes. There were no additional ascending modes in this frequency range. From this result we conclude that the magnetic moment of the impurity is opposite to the magnetic field and therefore orthogonal to the antiferromagnetic axis (see inset in fig. 2). Since no magnetic impurities in the sample were measured by X-ray fluorescent analysis with the accuracy 10e5, we suppose that ‘impurities’ are Fe’+ tons, occupying the 4c-positions of the Y3+ B.V. All rights reserved
ions instead of their regular 4b-positions. The five resonance frequencies result from splitting of the ‘Si,: ground state of Fe”+ ‘impurity’ ion into six-nonequidistant levels due to the combined effect of the exchange interaction with the host matrix, of the crystal field and of the external field. In the 4c-position the effective field has only z-projection (along c-axis) [4] due to magnetic structure of the YFeO, host matrix which can be treated as consisting of two sublattices, antiferromagnetically ordered along the u-axis slightly canted towards the c-axis. This fact supports our suggestion that ‘impurity’ ions arc magnetized opposite to the weak ferromagnetic moment. It is worth to note that WC did not find other explanation for the five lines group in our case. These lines can not be assigned to the superfine interactions, since only 2.2 percent of the Fe ions may have the nonzero nuclear magnetic moment and their spin is i. The states of the Fe ions with changed valency (Fe’+ or Fe” ’ ) also result in only one lint to be seen in the submillimeter wave range, because of their nonzero orbital moment. The experimental data were described by the oneimpurity Hamiltonian (H is applied along the c-axis):
netic field changes the canting angle of the host matrix, thus altering the effective field and reducing the effect of the applied field due to the positive value of the k. The solid lines in fig. 2 which are fitted with the parameters: H,,, = 9.8 T, k = 0.536, D = 0.176 cm ‘, a3 = -0.017 cm-’ are in good agreement with the experimental data. The field H 11b dependcnces of the ‘impurity’ mode frequencies arc modified by the coupling with the close AFMR mode (296 GHz at H = 0). Nevertheless their behavior qualitatively agrees with the proposed model. Measurements of the spontaneous magnetisation along the c-axis also have revealed an anomaly (a small drop in the magnetisation curve at low temperatures), which enables us to calculate the concentration of ‘impurity’ ions: 3.5 x 10mJ. It is worth to note that in spite of such low amount of ‘impurity’ ions the rcsonancc can be strongly enhanced due to the coupling with the AFMR mode [5]. Because of this effect the resonance measurements arc expected to probe the crystal quality.
.@=g/_&Hefl-(l
[I] M.A. Ivanov,
+ D( s^; - 1/3S(
-k)H)
s + 1)) + a$;,
where S = $ is the ‘impurity’ spin, g = 2, He,, the effective ficld, D and uj anisotropy constants. The D-term in the Hamiltonian results from the interaction with the crystal field (for the S-state ions this term is small, of the order 0.1 cm ~’ and appears in the high order perturbation theory, see e.g. ref. 151) and also from the exchange coupling with the host matrix [61. The cubic term was introduced in ref. [7] to fit the experimental data in similar case (Mn’+). Coefficient k originates from the exchange interaction of the impurity and weak ferromagnetic moments, p and M, respectively. Our cxpcriments show that this intcraction is antiferromagnetic (k > 0). The external mag-
References
[2] [3]
[4]
[S] [h] [7] [X]
V.M. Loktev and Yu.G. Pogorelov. Phys. Rep. IS3 (1987) 209. M. Marezio, J.P. Remeika and P.D. Dernier. Acta Crystallogr. B26 (1970) 200X. A.M. Balbashov, A.G. Bererin. Yu.M. Gufan. G.S. Kolyadko. P.Yu. Marchukov and E.G. Rudashevsky. Sov. Phys. JETP 66 (1987) 174. K.P. Belov, A.K. Zvezdin, A.M. Kadomtseva and R.Z. Levitin. Orientational Transitions in Rare-Earth Magnets (Nauka. Moscow, 1979) p. 317. J. Kondo. Progr. Theor. Phys. 2X (1962) 1026. MA. Ivanov, V.Ya. Mitrofanov and A.Ya. Fishman. Phys. Stat. Sol. 61 (1974) 403. G. Mischler. P. Carrara and Y.M. D’Aubigne. Phys. Rev. B 1.5 (1977) 1568. AS. Prokhorov and E.G. Rudashevsky. Sov. JETP Lett. 22 (1975) 99.
of Magnetism and Magnetic Materials 104-107 (1992) 1037-1038 North-Holland
Journal
T
M
z
M
Orthogonal magnetic impurity in antiferromagnet YFeO, single crystal: magnetic resonance and magnetisation measurements A.M. Balbashov a, A.G. Berezin a, Ju.V. Bobryshev a, J. Paches ’ and L. P&t b E.G. Rudashevsky
‘, P.Ju. Marchukov
‘, I.V. Nikolaev
‘,
’ General Physics Institute of the USSR Academy of Sciences, II 7942, VaciloLlst. 38, Moscow, USSR h Physical Institute of Czhecoslol)ak Academy of Sciences, Na Slocance 2, 180 40 Prague K-Lihen, CzechosloL,akia
Impurity modes with antiferromagnet YFeO,.
unusual dependence on a magnetic field have been observed experimentally in orthorhombic These modes are assigned to the electron transitions between the levels of a hSs,2 multiplet of the 3.5 X 10m4. The unusual behaviour of Fe3’. ‘impurity’ ions, which occupy the 4c-positions of the Y.if ions with concentration the modes is a consequence of the orthogonal state of the impurity.
Magnetic impurities in the antiferromagnets have been extensively studied (see e.g. ref. [l] and references there). However, investigating antiferromagnetic resonance (AFMR) in an antiferromagnet with the weak ferromagnetism we have observed an unusual field dependence of the impurity modes frequencies. The experiments were carried out on the yttrium orthoferrite YFeO, single crystal (space group P,,,,,-D$, T, = 645 K [2]), grown by the zone melting technique. Absorption of submillimeter radiation (75-400 CHz) was investigated at magnetic field up to 14 T and in 12-300 K temperature range. In addition to the wellknown low-frequency mode of antiferromagnetic resonance (AFMR) [3] we observed a group of well resolved and rather intensive resonance lines at temperature below 70 K (fig. 1). The frequencies of these lines (259, 263, 271, 2X3 and 299 GHz at zero field) did not depend on temperature. The strong anisotropy of the magnetic field dependence of the frequencies was found: they decrease linearly for the field parallel to
v
(GHz)
~ ~ .
T = 40.5 K
H (Tesla) o+ 0
YFeO,
2 G\
7
I
Fig. 1. Typical 0312~8853/92/$05.00
8
I
plot of impurity
158 GHz
9
1
H (T)
absorption
0 1992 - Elsevier
Science
I
I 4
I
I 8
I 12
Fig. 2. Frequency-field dependences of AFMR and impurity modes. The inset shows the relative orientation of the sublattice magnetisation M, and M,, impurity magnetisation y and applied magnetic field.
H 1) taxis
T = 40.5 K
.
lines. Publishers
the c-axis (fig. 2) and increase nearly quadratic for that to the b-axis. We would like to emphasize that in our experiments (H 11c-axis) we observed only the descending behaviour of the impurity modes. There were no additional ascending modes in this frequency range. From this result we conclude that the magnetic moment of the impurity is opposite to the magnetic field and therefore orthogonal to the antiferromagnetic axis (see inset in fig. 2). Since no magnetic impurities in the sample were measured by X-ray fluorescent analysis with the accuracy 10e5, we suppose that ‘impurities’ are Fe’+ tons, occupying the 4c-positions of the Y3+ B.V. All rights reserved
ions instead of their regular 4b-positions. The five resonance frequencies result from splitting of the ‘Si,: ground state of Fe”+ ‘impurity’ ion into six-nonequidistant levels due to the combined effect of the exchange interaction with the host matrix, of the crystal field and of the external field. In the 4c-position the effective field has only z-projection (along c-axis) [4] due to magnetic structure of the YFeO, host matrix which can be treated as consisting of two sublattices, antiferromagnetically ordered along the u-axis slightly canted towards the c-axis. This fact supports our suggestion that ‘impurity’ ions arc magnetized opposite to the weak ferromagnetic moment. It is worth to note that WC did not find other explanation for the five lines group in our case. These lines can not be assigned to the superfine interactions, since only 2.2 percent of the Fe ions may have the nonzero nuclear magnetic moment and their spin is i. The states of the Fe ions with changed valency (Fe’+ or Fe” ’ ) also result in only one lint to be seen in the submillimeter wave range, because of their nonzero orbital moment. The experimental data were described by the oneimpurity Hamiltonian (H is applied along the c-axis):
netic field changes the canting angle of the host matrix, thus altering the effective field and reducing the effect of the applied field due to the positive value of the k. The solid lines in fig. 2 which are fitted with the parameters: H,,, = 9.8 T, k = 0.536, D = 0.176 cm ‘, a3 = -0.017 cm-’ are in good agreement with the experimental data. The field H 11b dependcnces of the ‘impurity’ mode frequencies arc modified by the coupling with the close AFMR mode (296 GHz at H = 0). Nevertheless their behavior qualitatively agrees with the proposed model. Measurements of the spontaneous magnetisation along the c-axis also have revealed an anomaly (a small drop in the magnetisation curve at low temperatures), which enables us to calculate the concentration of ‘impurity’ ions: 3.5 x 10mJ. It is worth to note that in spite of such low amount of ‘impurity’ ions the rcsonancc can be strongly enhanced due to the coupling with the AFMR mode [5]. Because of this effect the resonance measurements arc expected to probe the crystal quality.
.@=g/_&Hefl-(l
[I] M.A. Ivanov,
+ D( s^; - 1/3S(
-k)H)
s + 1)) + a$;,
where S = $ is the ‘impurity’ spin, g = 2, He,, the effective ficld, D and uj anisotropy constants. The D-term in the Hamiltonian results from the interaction with the crystal field (for the S-state ions this term is small, of the order 0.1 cm ~’ and appears in the high order perturbation theory, see e.g. ref. 151) and also from the exchange coupling with the host matrix [61. The cubic term was introduced in ref. [7] to fit the experimental data in similar case (Mn’+). Coefficient k originates from the exchange interaction of the impurity and weak ferromagnetic moments, p and M, respectively. Our cxpcriments show that this intcraction is antiferromagnetic (k > 0). The external mag-
References
[2] [3]
[4]
[S] [h] [7] [X]
V.M. Loktev and Yu.G. Pogorelov. Phys. Rep. IS3 (1987) 209. M. Marezio, J.P. Remeika and P.D. Dernier. Acta Crystallogr. B26 (1970) 200X. A.M. Balbashov, A.G. Bererin. Yu.M. Gufan. G.S. Kolyadko. P.Yu. Marchukov and E.G. Rudashevsky. Sov. Phys. JETP 66 (1987) 174. K.P. Belov, A.K. Zvezdin, A.M. Kadomtseva and R.Z. Levitin. Orientational Transitions in Rare-Earth Magnets (Nauka. Moscow, 1979) p. 317. J. Kondo. Progr. Theor. Phys. 2X (1962) 1026. MA. Ivanov, V.Ya. Mitrofanov and A.Ya. Fishman. Phys. Stat. Sol. 61 (1974) 403. G. Mischler. P. Carrara and Y.M. D’Aubigne. Phys. Rev. B 1.5 (1977) 1568. AS. Prokhorov and E.G. Rudashevsky. Sov. JETP Lett. 22 (1975) 99.