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Magnon modes and impurity spin resonance in FeBr2 doped with 1% Mn2+
Solid State Communications, Vol. 27, pp. 1123—1125. © Pergamon Press Ltd. 1978. Printed in Great Britain.
0038—1098/78/0915—1123 $02.00/0
MAGNON MODES AND IMPURITY SPIN RESONANCE IN FeBr2 DOPED WITH 1% Mn2~ J. Tuchend1er~and J. MagariI~o Groupe de Physique des Solides de I’Ecole Normale Superieure,t 24 rue Lhomond, 75231 Paris 05, France and A.R. Fert and D. Bertrand Laboratoire de Physique des Solides associé au CNRS, INSA, Département de Physique, Avenue de Rangueil, 31077 Toulouse Cedex, France (Received 23 March 1978; in revised form 29 May 1978 by M Balkanski) We report results of far infrared magneto-absorption experiments in FeBr 2 doped with 1% MnBr2. Using radiations from several carcinotrons covering 2~impurity and the magnon modes the frequency range 77—600 GHz, modes we observe bothuniform the spectra corresponding to the Mn excitation energy gap of the A.F. magnons at zero of the hostlocalized crystal. The field is E 0 = 500 ±2 GHz. WE REPORT the study, by far infrared magnetoabsorption experiments in the range 77—600GHz, of the magnetic in FeBr2 single crystals containing 2~ionsexcitations with an atomic concentration of 1%. Our Mn main purpose was to obtain with the same experimental set up, both the low energy localized excitations associated with the substitutional impurity and the collective excitations of magnons associated with the host crystal. resonance in a Date pulsed field Impurity have beenspin carried out byexperiments Motokawa and [11 on Mn2’~ions in FeC1 2 using magnetic resonance techniques [2] in the 35—90GHz range. They deduce the local effective field. Mischler et al. [31using a conventional homodyne spectrometer 2~impurities in FeBr(EPR techniques) have studied Mn 2. They have shown the existence of a fine structure associated to the existence of additional coupling terms. On the other hand, the magnons of the host FeBr2 crystal have been studied in zero magnetic field by Fourier-transform spectroscopy [4] and by neutron scattering experiments [5]. Using Thomson-CSF carcinotrons, we have observed the localized impurity modes and the magnons by simple transmission techniques. We recall briefly that FeBr2 is a two sublattices antiferromagnet (TN = 14.2 K) [6] with ferromagnetic interactions between the ions within the layers and antiferromagnetic interactions between the layers characterized by the1 two para. andieffective interaction 1 [7]. FeBr metersj1 = 4.4cm 2 = 5.3cm 2 exhibits a strong single ion uniaxial anisotropy * Also Universitd Pierre et Marie Curie, Paris VI, France t Laboratoire associé au Centre National de la Recherche Scientifique. —
____________
D = 9.7 cm’ the easy axis being the c axis. The presence of this crystalline anisotropy, together with the antiferromagnetic interaction, involves the of a zero field gap in the magnon spectrum. Theexistence introduction of substitutional Mn2~ions in the FeBr 2 host with a concentration of 1% should modify weakly only the spectrum of the FeBr2 magnons. The impurity spectrum is explained by the fact that 2~has orbital angular the ground state of the free Mn momentum L = 0 and spin momentum S = ~ and that the exchange constants between the impurity and the host are much weaker than the host exchange constant (1 cm’ compared with 10 cm’). Therefore, the impurity spin behaves nearly as a paramagnetic spin in an effective molecular field. The experimental set up used in this experiment has been reported in detail elsewhere [8]. The only change is that the sample is mounted in the Faraday geometry. The direction of microwave propagation and the direction of the magnetic field are parallel to the c axis of the crystal. For the observation of the magnons in FeBr2 we have used a sample as thin as possible (e < 100pm). With thicker samples, radiations with circular polarization corresponding to the excited magnon are completely absorbed and the resonant line recorded is overabsorbed. In addition, with thicker samples, magnetostatic modes appear on the edge of the magnon peaks. For study impurity spin resonance 2~, wethe have usedofa the sample 400pm thick in orderofto Mn increase the number of Mn2~ions interacting with the radiation. Figure 1 shows typical recording traces at 4.2 and
1123
FeBr2 DOPED WITH 1% Mn2~
1124
1
4.2K
I’
3 4
Vol. 27, No. 11
600
500
16K
LU
—
400
Fe Br 2+1% MeBr2 1 42K
>C~)
uJ
GHz
LU
~ 300 D
-
ANTIFERROMAGNETIC
SATURATED PARAMAGNE TIC
I-
z200_
Cl)
HI
~
I
CD I— C)
I’
I
20
I-
I
30
MAGNETIC
40
z
100
FIELD (kG) I
2~in FeBr Fig. 1. Absorption spectra of Mn 1.6K for v = 79.3GHz.
2 at 4.2 and
10
20
30
MAGNETIC FIELD
1.6K obtained for the frequency v = 79.3 GHz. We observe essentially two lines at 24.5 kG and 28.4 kG due to Mn2’~ions substituted in one or the other of the two magnetic sublattices a and 13 of FeBr 2 antiferromagnet. They correspond to transitions between the lowest 2~ion ~ and ~ states of the Zeeman structure of a Mn substituted in the a and the 13 sublattices. The other L~m= 1 transition lines between higher states of Mn2~ —
40
50
60
(kG)
Fig. 2. Some observed resonance frequencies are plotted vs the applied field. Results at 15.5 GHz the are crosses from + are 2~modes for one sublattice Mischler eta!. [3]. and Forthe thedots MnS for the other sublattice. ~
=
2900
±30
G
However due to the exper-
imental error in our experD’/g~1pB = —770 ± 20G iments the value of B2 is a3/g~~pB = 70 ±3 G meaningless. —
—
appear clearly in Fig. 1 but are of lower intensity because of the weaker occupation numbers of these higher states. It should be noted that at 1.6K the small absorption which almost coincides with the 1/2 1/2 transition should not be taken as a resonance line since it should be one of magnitude smaller than at we 2~order ions substituted in the j3 sublattice 4.2 K. For Mn do not observe the other transition lines because they —
-~
should appear field higher than the metamagnetic transition field for of aFeBr 2 (29 kG) [7]. The positions of our absorption lines at 79.3 GHz are in complete agreement with the EPR lines obtained at 15.5GHz by Mischler eta!. [3], if we take for the impurity gç1 factor the value g~1= 1 .93 ±0.02. Mischler et al. [3] show that this experimental spectrum is well described by the effective spin Hamiltonian of an impurity withS’ = ~eff
=
+ D’[S~ ~S’(S’+ 1)] + ea3S~ ~B4°O4°+ g~p~H~S~ —
+
where e takes the values + 1 or — 1 according to the 2~is substituted. magnetic into whichare: the Mn The valuessublattice of the coefficients
B~/g~ 1p~ = 15 ±4 G Figure 2 shows the magnetic field dependence of the uniform magnon energies of the host. At first, we observe an energy gap at zero field E0 = 500 ±2GHz. This value is slightly weaker than those obtained by neutron scattering experiments [5]: E0 = 530GHz or Fourier spectroscopy E0 = 513on± 2GHz. However,transform these experiments were[4]: performed pure FeBr 2. 2~ions The introduction of the substitutional Mn modifies weakly the magnon spectrum. In experiments on the Mn~Fe1_~Cl2 alloys, we have noticed a variation of the gap, z~E’0= 10GHz forx = 0.01. This correction would give a very good agreement with the Fourier transform results. In the A.F. phase, the two modes are split by the magnetic field, the field dependence being described by an effective factor ~AF = 3.85 ±0.05. In the saturated paramagnetic phase, our results and the previous data by Fert eta!. [9] enable us to obtain the effective factor = 3.60 ±0.05.
gs.p.
During thephases metamagnetic transition
1125 Vol. 27, No.11 FeBr2 DOPED WITH 1%Mn2~ transmitted signals resulting from the diffusion of the adapted to the study of the excitation spectra of mixed light by the various domains. In fact, the beginning of compounds. this weakening is sharply marked on the recordings. in conclusion, millimeter and submihimeter transmission resonance experiments allow a simple study of Acknowledgement The authors wish to express their both the magnon modes of the host FeBr 2 and the thanks to S. Legrand (CEN Saclay) for the preparation localized defect modes. The set-up is therefore perfectly of the crystals from which the samples were prepared. —
REFERENCES 1.
MOTOKAWA M. & DATE M.,J. Phys. Soc. Japan 23, 1216 (1967).
2.
DATEM.,J.Phys.Soc.Japan 16, 1337 (1961).
3.
MISCHLER G., CARRARA P. & MERLE D’AUBIGN~Y., Phys. Rev. BiS, 1568 (1977).
4.
PETITGRAND D. & MEYER P.,J. Phys. 37, 1417 (1976).
5.
YELON W.B. & VETTIER C.,J. Phys. C8, 2760 (1975).
6.
WILKINSON M.K., CABLE J.W., WOLLEN E.0. & KOEHLER W.C., Phys, Rev. 113,497 (1959).
7.
FERT A.R., Unpublished thesis, Toulouse III (1973).
8.
MAGARItJO J., TUCHENDLER J., D’HAENENS J.P. & LINZ A., Phys. Rev. B13, 2805 (1976).
9.
FERT A.R., LEOTIN J., OUSSET J.C., BERTRAND D., CARRARA P. & ASKENAZY S., Solid State Commun. 18, 327 (1976).
0038—1098/78/0915—1123 $02.00/0
MAGNON MODES AND IMPURITY SPIN RESONANCE IN FeBr2 DOPED WITH 1% Mn2~ J. Tuchend1er~and J. MagariI~o Groupe de Physique des Solides de I’Ecole Normale Superieure,t 24 rue Lhomond, 75231 Paris 05, France and A.R. Fert and D. Bertrand Laboratoire de Physique des Solides associé au CNRS, INSA, Département de Physique, Avenue de Rangueil, 31077 Toulouse Cedex, France (Received 23 March 1978; in revised form 29 May 1978 by M Balkanski) We report results of far infrared magneto-absorption experiments in FeBr 2 doped with 1% MnBr2. Using radiations from several carcinotrons covering 2~impurity and the magnon modes the frequency range 77—600 GHz, modes we observe bothuniform the spectra corresponding to the Mn excitation energy gap of the A.F. magnons at zero of the hostlocalized crystal. The field is E 0 = 500 ±2 GHz. WE REPORT the study, by far infrared magnetoabsorption experiments in the range 77—600GHz, of the magnetic in FeBr2 single crystals containing 2~ionsexcitations with an atomic concentration of 1%. Our Mn main purpose was to obtain with the same experimental set up, both the low energy localized excitations associated with the substitutional impurity and the collective excitations of magnons associated with the host crystal. resonance in a Date pulsed field Impurity have beenspin carried out byexperiments Motokawa and [11 on Mn2’~ions in FeC1 2 using magnetic resonance techniques [2] in the 35—90GHz range. They deduce the local effective field. Mischler et al. [31using a conventional homodyne spectrometer 2~impurities in FeBr(EPR techniques) have studied Mn 2. They have shown the existence of a fine structure associated to the existence of additional coupling terms. On the other hand, the magnons of the host FeBr2 crystal have been studied in zero magnetic field by Fourier-transform spectroscopy [4] and by neutron scattering experiments [5]. Using Thomson-CSF carcinotrons, we have observed the localized impurity modes and the magnons by simple transmission techniques. We recall briefly that FeBr2 is a two sublattices antiferromagnet (TN = 14.2 K) [6] with ferromagnetic interactions between the ions within the layers and antiferromagnetic interactions between the layers characterized by the1 two para. andieffective interaction 1 [7]. FeBr metersj1 = 4.4cm 2 = 5.3cm 2 exhibits a strong single ion uniaxial anisotropy * Also Universitd Pierre et Marie Curie, Paris VI, France t Laboratoire associé au Centre National de la Recherche Scientifique. —
____________
D = 9.7 cm’ the easy axis being the c axis. The presence of this crystalline anisotropy, together with the antiferromagnetic interaction, involves the of a zero field gap in the magnon spectrum. Theexistence introduction of substitutional Mn2~ions in the FeBr 2 host with a concentration of 1% should modify weakly only the spectrum of the FeBr2 magnons. The impurity spectrum is explained by the fact that 2~has orbital angular the ground state of the free Mn momentum L = 0 and spin momentum S = ~ and that the exchange constants between the impurity and the host are much weaker than the host exchange constant (1 cm’ compared with 10 cm’). Therefore, the impurity spin behaves nearly as a paramagnetic spin in an effective molecular field. The experimental set up used in this experiment has been reported in detail elsewhere [8]. The only change is that the sample is mounted in the Faraday geometry. The direction of microwave propagation and the direction of the magnetic field are parallel to the c axis of the crystal. For the observation of the magnons in FeBr2 we have used a sample as thin as possible (e < 100pm). With thicker samples, radiations with circular polarization corresponding to the excited magnon are completely absorbed and the resonant line recorded is overabsorbed. In addition, with thicker samples, magnetostatic modes appear on the edge of the magnon peaks. For study impurity spin resonance 2~, wethe have usedofa the sample 400pm thick in orderofto Mn increase the number of Mn2~ions interacting with the radiation. Figure 1 shows typical recording traces at 4.2 and
1123
FeBr2 DOPED WITH 1% Mn2~
1124
1
4.2K
I’
3 4
Vol. 27, No. 11
600
500
16K
LU
—
400
Fe Br 2+1% MeBr2 1 42K
>C~)
uJ
GHz
LU
~ 300 D
-
ANTIFERROMAGNETIC
SATURATED PARAMAGNE TIC
I-
z200_
Cl)
HI
~
I
CD I— C)
I’
I
20
I-
I
30
MAGNETIC
40
z
100
FIELD (kG) I
2~in FeBr Fig. 1. Absorption spectra of Mn 1.6K for v = 79.3GHz.
2 at 4.2 and
10
20
30
MAGNETIC FIELD
1.6K obtained for the frequency v = 79.3 GHz. We observe essentially two lines at 24.5 kG and 28.4 kG due to Mn2’~ions substituted in one or the other of the two magnetic sublattices a and 13 of FeBr 2 antiferromagnet. They correspond to transitions between the lowest 2~ion ~ and ~ states of the Zeeman structure of a Mn substituted in the a and the 13 sublattices. The other L~m= 1 transition lines between higher states of Mn2~ —
40
50
60
(kG)
Fig. 2. Some observed resonance frequencies are plotted vs the applied field. Results at 15.5 GHz the are crosses from + are 2~modes for one sublattice Mischler eta!. [3]. and Forthe thedots MnS for the other sublattice. ~
=
2900
±30
G
However due to the exper-
imental error in our experD’/g~1pB = —770 ± 20G iments the value of B2 is a3/g~~pB = 70 ±3 G meaningless. —
—
appear clearly in Fig. 1 but are of lower intensity because of the weaker occupation numbers of these higher states. It should be noted that at 1.6K the small absorption which almost coincides with the 1/2 1/2 transition should not be taken as a resonance line since it should be one of magnitude smaller than at we 2~order ions substituted in the j3 sublattice 4.2 K. For Mn do not observe the other transition lines because they —
-~
should appear field higher than the metamagnetic transition field for of aFeBr 2 (29 kG) [7]. The positions of our absorption lines at 79.3 GHz are in complete agreement with the EPR lines obtained at 15.5GHz by Mischler eta!. [3], if we take for the impurity gç1 factor the value g~1= 1 .93 ±0.02. Mischler et al. [3] show that this experimental spectrum is well described by the effective spin Hamiltonian of an impurity withS’ = ~eff
=
+ D’[S~ ~S’(S’+ 1)] + ea3S~ ~B4°O4°+ g~p~H~S~ —
+
where e takes the values + 1 or — 1 according to the 2~is substituted. magnetic into whichare: the Mn The valuessublattice of the coefficients
B~/g~ 1p~ = 15 ±4 G Figure 2 shows the magnetic field dependence of the uniform magnon energies of the host. At first, we observe an energy gap at zero field E0 = 500 ±2GHz. This value is slightly weaker than those obtained by neutron scattering experiments [5]: E0 = 530GHz or Fourier spectroscopy E0 = 513on± 2GHz. However,transform these experiments were[4]: performed pure FeBr 2. 2~ions The introduction of the substitutional Mn modifies weakly the magnon spectrum. In experiments on the Mn~Fe1_~Cl2 alloys, we have noticed a variation of the gap, z~E’0= 10GHz forx = 0.01. This correction would give a very good agreement with the Fourier transform results. In the A.F. phase, the two modes are split by the magnetic field, the field dependence being described by an effective factor ~AF = 3.85 ±0.05. In the saturated paramagnetic phase, our results and the previous data by Fert eta!. [9] enable us to obtain the effective factor = 3.60 ±0.05.
gs.p.
During thephases metamagnetic transition
1125 Vol. 27, No.11 FeBr2 DOPED WITH 1%Mn2~ transmitted signals resulting from the diffusion of the adapted to the study of the excitation spectra of mixed light by the various domains. In fact, the beginning of compounds. this weakening is sharply marked on the recordings. in conclusion, millimeter and submihimeter transmission resonance experiments allow a simple study of Acknowledgement The authors wish to express their both the magnon modes of the host FeBr 2 and the thanks to S. Legrand (CEN Saclay) for the preparation localized defect modes. The set-up is therefore perfectly of the crystals from which the samples were prepared. —
REFERENCES 1.
MOTOKAWA M. & DATE M.,J. Phys. Soc. Japan 23, 1216 (1967).
2.
DATEM.,J.Phys.Soc.Japan 16, 1337 (1961).
3.
MISCHLER G., CARRARA P. & MERLE D’AUBIGN~Y., Phys. Rev. BiS, 1568 (1977).
4.
PETITGRAND D. & MEYER P.,J. Phys. 37, 1417 (1976).
5.
YELON W.B. & VETTIER C.,J. Phys. C8, 2760 (1975).
6.
WILKINSON M.K., CABLE J.W., WOLLEN E.0. & KOEHLER W.C., Phys, Rev. 113,497 (1959).
7.
FERT A.R., Unpublished thesis, Toulouse III (1973).
8.
MAGARItJO J., TUCHENDLER J., D’HAENENS J.P. & LINZ A., Phys. Rev. B13, 2805 (1976).
9.
FERT A.R., LEOTIN J., OUSSET J.C., BERTRAND D., CARRARA P. & ASKENAZY S., Solid State Commun. 18, 327 (1976).