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Magnetism, spin relaxation and hyperfine interactions of rare earths in RM6Al6(M = Cr, Mn, Cu)
MAGNETISM, SPIN RELAXATION AND HYPERFINE INTERACTIONS OF RARE EARTHS IN RMaAl&14 = Cr, Mn, Cu)* f. FELNER, M. SEHand 1. NOWIF: ResearchInstituteof Physics, The Hebrew University, Jerusafem, Israel (Ra&mi
18 F~b~5~
1981;accepted 20 May 1981)
Abstract-The systems R&A&(Rx rare earth or I’, M= Cr, Mn, Cu, Rh) were studied by magneti~tion measurements and by Mossbauer spectroscopy of lsJGd, j6’Dy, ‘“Er and “qb. The magmdzation studiesshow weakR-R antiferromagnetic exchange interactions in RCu&&(T.(Gd) = 21 K, less than 4 K for all other R) and strongcrystalline field effects. Similar phenomena are observedin RM&AbandRCr,A&,however, due to the presence of a Mn or Cr local moment the systems order ferrimagnetically. In RCr6Ab the order temperatures are low T, - 25 K, yet T,(GdCr6Ab)= 170K. The Mossbauer studies observations are consistent with the magnetization results. In the case where Er and Yb are not ordered at 4.1 K, Be spectra still show magnetic hyperfine structure however of paramagnetic nature. Thespectrayield the hype&e interaction spin Hamiltonian parameters and the spin relaxation rates. These turn out to be extremely slow (108-l@set-I), a very uncommon phenomenon for a con~ntrated Er or Yb metallic system.
compounds RM,Alt2_,, (R = rare earth, M= 3d elementand n = 4 or 6), have been found to crystallizein the ThMnfz (~4~~~~~ structure [1,2]. Some of their magnetic properties, in particular those of RFe.AL, have already been reported [2-4]. Here we extended the research of RFe6Al,[4]to similarsystems in which Fe is replaced by Cu, Mn and Cr. The RFe6A$ compounds order ferrimagneticallyat a relatively high temperature (Tc - 330K) due mainly to Fe-Fe and Fe-R exchange interactions.in RM&, (M = Cu, Mn, Cr) the exchange is mainly R-R and is weak leadingto very low antiferroma~etic ordering tempera~res. Only in GdCrsA$and Gd&A& the order is ferroma~etic~ The magneti~tion studies also reveal strong crystalline field effects on the rare earth ionic magnetic behaviour. Mossbauer studies of ‘=Gd, *6*Dy,‘=Er and “9’b show spectra which are in agreementwith the magnetizationstudies. One special feature in the ‘6‘Dy, ‘&Er and “OYb studies is the observation of magnetic hyperfine structure in the paramagnetic state. These could be analyzed within conventional Mossbauer spin.relaxation theories [5] to yield the paramagneticspin Hamiltonianparameters and the spin relaxation rates. considering that the systems contain rare earths, in a quite dense form and the systems are metallic,the spin-spin slow relaxation rates are surprising. There are only a few concent~ted metallic systems of Er and Yb in which paramagnetic,long spin relaxation time limit, magnetic hyperfine spectra are observed. These slow spin-spin relaxation rates are consistent with the observed strong crystalline fields acting on the rare earths ions. All experimental details concerning the research reported here can be found in our previous publications [1,2,41.
The
*Supportedin part by the fsraet A~demy of Sciences and %nanities, Commission for Basic Research.
The magneticsusceptibilitycurves, some of which are shownin Figs, 1 and 2, were analyzed above the ordering temperature by a least square tit procedure to the formulax = x0f cl( T - 6). The values obtainedfor C and B are given in Table 1. The Neel points of RC&,AL,were deduced from the positionsof the maximain the susceptibility curves, (Fig, 1). For RMn,Ab the ordering temperature was determined as the point above which the external magnetic field dependence of the magnetic moment is linear. In RC&A&it is obvious from the shape of the magneti~tion curves and from the negative values of B that the compounds order an~fe~omagneti~~lydue to R-R exchange. The only surprising result is the row Curie constant of TbC&A&, It seems that crys~~iine fiefd effects play an important role in determiningthe magnetic properties. The ionic ground state Stark level of Tb3’ (‘Fs) in TbCuaAb(tetragonal point symmetry) is probably a nonmagnetic singlet. The crystalline field effects also explain the sharp drop in T,, in going from GdCyAl, (Gd3+,S state ion) to the other systems. The same phenomenonreveals itself also in the RMndAband RCr6A16 systems. In RM~A~ the Mn ion probably carries a focal moment,( - 1.4llB) as observed for YM%A&,However, since cry&&c fields reduce the rare earth cont~bution to the effective moment, it is ditlicult to determine the Mn con~bution. it seems that the Mn con~ibution increases towards the heavier rare earths. In GdMnsAb there is no Mn contribution. The magnetic structure of RMn& is not clear, since on one hand 19is negative and on the other hand there are no maxima in the susceptibility curves. The structure is probably ferrimagnetic. In R&Al6 the Cr ion like the Mn has a local moment in ErCrdA&and not in GdCraAk.The magneticstructure of RC%,At,is ferromagnetic as evidenced from the GdCr& curve (Fig. 2). The sharp drop of T, going
1091
1092
I.FELNER efd
25
50
75
100
K
1.
Fig, 1.
125
Magnetizationcurves of RCu& and RM&.
Table 1. Magn~$cpropertiesof the R&A& compounds Momentat Contpound
Ordering Temperature
K
Rare Earth
4.1K in 18 kOe
e
Curie constant
K
emu/moie
PB
Free Curie
Ion Const.
emu/m01 e
GdCu6A16
21
-22
7.8
TbCu6Al6
33.4(l)
-54
1.4
11.8
DyCu6AQ,
3.9(l)
-14
14.0
14.1
HoCu6Al6
1.4(i)
-11
14.3
14.1
ErCu6Al6
2.6(l)
-
3
11.3
11.5
TmCu6Al6
3.9(l)
-
5
6.7
7.1
YbCu6Al6
diamagnetic
LuCu6Al6
diamagnetic
(2)
(2)
1.1
-9
7.7
7.9
TbMn6A16
B
(2)
0.3
-20
2.6
11.8
(2)
2
- 7
13.7
14.1
-
13 0
11.5
Dyb6Al6
6
ErMn6A16
2.6(l)
TmHn6Al6
2
(11
I
1.9
YbMn6Alb YNn6Al6 GdCr6Alt
170
(51
DyCr6Al6
2u
(51
1.9
fS)
5.1
ErCr6Ai6 LuCr6Al6 GdRh6A16
15
5.5
4
(2)
4.5
1.4
i
I.2
-18
8.2
7.
-23
3.6
2.6
t.2 1.4
-44
I.5
0
IS8
7.8
7.9
!i - I
7.9
14.1
13.6
11.5
5.9
6.9
diamagnetic 30
%
7.9
IS
=gA16
Calculated P eff CM)
4’
1.6
Magnetism,spin rdaxatiooand@perfineinteractions Of Fare earths
llun/sec
AtI MClX3C
AA
MClSc?C
wsec
%I
MC/S@C
W&
MC/SW
I. FELNERet al.
1094
from GdCrs& (Z’, = 170K) to DyCkAb (T, = 20 K) and the low Curie constant of DyCrs& again indicate the strong crystahine fieId effects. MOSSBAUER
STUDllC3
Mossbauer studies of “‘Gd in Gd&A& (M = Cr, Mn, Cu, Rh) at 77 K and 4.1 K were performed. The 77 K spectra yield the pure quadruple parameters and the 4.1 K spectra analyzed by full diagon~i~tion of the hyperfine structure Ham~to~n yield the magnetic
hyperfine field with its orientation relative to the tetragonal symmetry axis (Table 2). The 16’Dy Mossbauer studies were performed in the velocity range r 10mmjsec [4], observing only the two central lines of the magnetically split spectrum . This procedure yields accurate values for the hyperfine field (Table 2). The ‘%Er Mossbauer spectra were measured using a single line source in the form of Ho*.~Y,, Hz [4]. In the
1.000
0.998 0.996 0.994
0.996
I
I I
-20
0
-10
10
20
l.wo
0.998
0.998
-
4.2K
lIsaI* -80
t
I
,I
0
VELOCITY VELOCITY
‘Ll_...d
~..‘11..~I..
-40
40
80
(mm/set)
(mm/set)
Fig. 3. M&ssbauerspectraof ‘%d in CidM& at 4.1 K.
Fig. 4. Mksbauer spectra _- __of. . laEr .in__ErM& and “%‘b in YbM4A14
at4.1 K.
Magnetism,
spin reiaxation and hyl per&e interactions of rare earths
ErCkAI, and ErMrbAl, spectra (Fig. 3), one observes broadened absorption lines. This is most evident when compared to the ErFe6AL spectrum in which the Er ion is magnetically ordered to saturation [4] (the spectrum of ErCrsAL, indicates magnetic order in consistence with the magnetization studies). The spectra of ErCuaA& and ErMnsA&, were analyzed by Mossbauer spectra spin relaxation theory[5]. Gonzalez-Jimenez et 01.[5] give closed form formulas for the spectra of a 2+--f0’ transition for paramagnetic ions in an axiai crystalline field. These are exactly the conditions preva~ing in ES&A&, ErMGAl, and YbM%A&. The theoretical analysis assumes that the hype&e interaction spin Hamiltonian is given by H = AII+KI,+ &.(S+I- + s-J+) + -&41(21_ 1) (31; - Z(Zt 1)) and that the spin fluctuates transversely and longitudinally at rates W, and N$ respectively (The theoretical analysis yields these parameters, they are given In Table 2). We observe very slow spin relaxation rates (- 2 X tO’sec_’ for Er and 2 x IO”see-’ for Yb), though these are concentrated paramagnetic systems in which spinspin relaxation rates would be expected to be well above IO” set-‘. The reason for the slow spin-spin relaxation rates is probably strongly connected with the high crys-
1095
talline field effects. The crystalline fields lead to strong anisotropy which reduce spin relaxation rates [6], a phenomenon well known for Dy [7], Er 181and Yb [9] in insulators. However, in metallic systems it has seldom been observed [lo]. The spectrum of “‘Yb in YbCuaAl, shows only quadrupole splitting, thus consistent with the Yb being divalent as concluded also from the magnetization studies.
1. Fefner I., L Less. Common. Met& 72,241 (1980). 2. Felner I. and Nowik, I. J: Pitys. Chem. Solids 39,9S1 (1978); 40, 1035 (1979). 3. Buschow K. H. and Van der Kraan A. M., J. Phys. F. M&l Phys. 8,921 (1978). 4. Felner I., Seh M., Rakavy M. and Nowik I., /. Phvs. Chem. Solids 42,. 369 (1981); fiowik I., Felner I..and Skh M., J. Maa. & Mug. Mat. 15-U. 1215(19801. 5. Go&alez-Jimenez F., Imbert P: and’ Hartmann-Boutron F., Phys. Reu. 3 9,95 (1974). 6. Nowik I., Phys. Letters 15, 219 (1965). 7. Wickman H. H. and Nowik I., J. Phys. Chem. Solid 28,2099 (1967). 8. Hfiller A., WiedemanRW., Kienle and Hiifner S., Phys Letters 15,269 (1%5), 9. Nowik I. and Ofer S., 1. Phys. Chem. Solids 29,2f 17 (1969). 10. The concentrated metallic system ErRh4Bd exhibits such
phenomena, Shenoy G. K., Dunlap B. D., Frandin F. Y., Kimball C. W., Potzel W., Pr6bst F. and Kalvius G., J. App. Phys. 50, 1872(1979),also Phys. Reo. B21, 3886 (1980).
18 F~b~5~
1981;accepted 20 May 1981)
Abstract-The systems R&A&(Rx rare earth or I’, M= Cr, Mn, Cu, Rh) were studied by magneti~tion measurements and by Mossbauer spectroscopy of lsJGd, j6’Dy, ‘“Er and “qb. The magmdzation studiesshow weakR-R antiferromagnetic exchange interactions in RCu&&(T.(Gd) = 21 K, less than 4 K for all other R) and strongcrystalline field effects. Similar phenomena are observedin RM&AbandRCr,A&,however, due to the presence of a Mn or Cr local moment the systems order ferrimagnetically. In RCr6Ab the order temperatures are low T, - 25 K, yet T,(GdCr6Ab)= 170K. The Mossbauer studies observations are consistent with the magnetization results. In the case where Er and Yb are not ordered at 4.1 K, Be spectra still show magnetic hyperfine structure however of paramagnetic nature. Thespectrayield the hype&e interaction spin Hamiltonian parameters and the spin relaxation rates. These turn out to be extremely slow (108-l@set-I), a very uncommon phenomenon for a con~ntrated Er or Yb metallic system.
compounds RM,Alt2_,, (R = rare earth, M= 3d elementand n = 4 or 6), have been found to crystallizein the ThMnfz (~4~~~~~ structure [1,2]. Some of their magnetic properties, in particular those of RFe.AL, have already been reported [2-4]. Here we extended the research of RFe6Al,[4]to similarsystems in which Fe is replaced by Cu, Mn and Cr. The RFe6A$ compounds order ferrimagneticallyat a relatively high temperature (Tc - 330K) due mainly to Fe-Fe and Fe-R exchange interactions.in RM&, (M = Cu, Mn, Cr) the exchange is mainly R-R and is weak leadingto very low antiferroma~etic ordering tempera~res. Only in GdCrsA$and Gd&A& the order is ferroma~etic~ The magneti~tion studies also reveal strong crystalline field effects on the rare earth ionic magnetic behaviour. Mossbauer studies of ‘=Gd, *6*Dy,‘=Er and “9’b show spectra which are in agreementwith the magnetizationstudies. One special feature in the ‘6‘Dy, ‘&Er and “OYb studies is the observation of magnetic hyperfine structure in the paramagnetic state. These could be analyzed within conventional Mossbauer spin.relaxation theories [5] to yield the paramagneticspin Hamiltonianparameters and the spin relaxation rates. considering that the systems contain rare earths, in a quite dense form and the systems are metallic,the spin-spin slow relaxation rates are surprising. There are only a few concent~ted metallic systems of Er and Yb in which paramagnetic,long spin relaxation time limit, magnetic hyperfine spectra are observed. These slow spin-spin relaxation rates are consistent with the observed strong crystalline fields acting on the rare earths ions. All experimental details concerning the research reported here can be found in our previous publications [1,2,41.
The
*Supportedin part by the fsraet A~demy of Sciences and %nanities, Commission for Basic Research.
The magneticsusceptibilitycurves, some of which are shownin Figs, 1 and 2, were analyzed above the ordering temperature by a least square tit procedure to the formulax = x0f cl( T - 6). The values obtainedfor C and B are given in Table 1. The Neel points of RC&,AL,were deduced from the positionsof the maximain the susceptibility curves, (Fig, 1). For RMn,Ab the ordering temperature was determined as the point above which the external magnetic field dependence of the magnetic moment is linear. In RC&A&it is obvious from the shape of the magneti~tion curves and from the negative values of B that the compounds order an~fe~omagneti~~lydue to R-R exchange. The only surprising result is the row Curie constant of TbC&A&, It seems that crys~~iine fiefd effects play an important role in determiningthe magnetic properties. The ionic ground state Stark level of Tb3’ (‘Fs) in TbCuaAb(tetragonal point symmetry) is probably a nonmagnetic singlet. The crystalline field effects also explain the sharp drop in T,, in going from GdCyAl, (Gd3+,S state ion) to the other systems. The same phenomenonreveals itself also in the RMndAband RCr6A16 systems. In RM~A~ the Mn ion probably carries a focal moment,( - 1.4llB) as observed for YM%A&,However, since cry&&c fields reduce the rare earth cont~bution to the effective moment, it is ditlicult to determine the Mn con~bution. it seems that the Mn con~ibution increases towards the heavier rare earths. In GdMnsAb there is no Mn contribution. The magnetic structure of RMn& is not clear, since on one hand 19is negative and on the other hand there are no maxima in the susceptibility curves. The structure is probably ferrimagnetic. In R&Al6 the Cr ion like the Mn has a local moment in ErCrdA&and not in GdCraAk.The magneticstructure of RC%,At,is ferromagnetic as evidenced from the GdCr& curve (Fig. 2). The sharp drop of T, going
1091
1092
I.FELNER efd
25
50
75
100
K
1.
Fig, 1.
125
Magnetizationcurves of RCu& and RM&.
Table 1. Magn~$cpropertiesof the R&A& compounds Momentat Contpound
Ordering Temperature
K
Rare Earth
4.1K in 18 kOe
e
Curie constant
K
emu/moie
PB
Free Curie
Ion Const.
emu/m01 e
GdCu6A16
21
-22
7.8
TbCu6Al6
33.4(l)
-54
1.4
11.8
DyCu6AQ,
3.9(l)
-14
14.0
14.1
HoCu6Al6
1.4(i)
-11
14.3
14.1
ErCu6Al6
2.6(l)
-
3
11.3
11.5
TmCu6Al6
3.9(l)
-
5
6.7
7.1
YbCu6Al6
diamagnetic
LuCu6Al6
diamagnetic
(2)
(2)
1.1
-9
7.7
7.9
TbMn6A16
B
(2)
0.3
-20
2.6
11.8
(2)
2
- 7
13.7
14.1
-
13 0
11.5
Dyb6Al6
6
ErMn6A16
2.6(l)
TmHn6Al6
2
(11
I
1.9
YbMn6Alb YNn6Al6 GdCr6Alt
170
(51
DyCr6Al6
2u
(51
1.9
fS)
5.1
ErCr6Ai6 LuCr6Al6 GdRh6A16
15
5.5
4
(2)
4.5
1.4
i
I.2
-18
8.2
7.
-23
3.6
2.6
t.2 1.4
-44
I.5
0
IS8
7.8
7.9
!i - I
7.9
14.1
13.6
11.5
5.9
6.9
diamagnetic 30
%
7.9
IS
=gA16
Calculated P eff CM)
4’
1.6
Magnetism,spin rdaxatiooand@perfineinteractions Of Fare earths
llun/sec
AtI MClX3C
AA
MClSc?C
wsec
%I
MC/S@C
W&
MC/SW
I. FELNERet al.
1094
from GdCrs& (Z’, = 170K) to DyCkAb (T, = 20 K) and the low Curie constant of DyCrs& again indicate the strong crystahine fieId effects. MOSSBAUER
STUDllC3
Mossbauer studies of “‘Gd in Gd&A& (M = Cr, Mn, Cu, Rh) at 77 K and 4.1 K were performed. The 77 K spectra yield the pure quadruple parameters and the 4.1 K spectra analyzed by full diagon~i~tion of the hyperfine structure Ham~to~n yield the magnetic
hyperfine field with its orientation relative to the tetragonal symmetry axis (Table 2). The 16’Dy Mossbauer studies were performed in the velocity range r 10mmjsec [4], observing only the two central lines of the magnetically split spectrum . This procedure yields accurate values for the hyperfine field (Table 2). The ‘%Er Mossbauer spectra were measured using a single line source in the form of Ho*.~Y,, Hz [4]. In the
1.000
0.998 0.996 0.994
0.996
I
I I
-20
0
-10
10
20
l.wo
0.998
0.998
-
4.2K
lIsaI* -80
t
I
,I
0
VELOCITY VELOCITY
‘Ll_...d
~..‘11..~I..
-40
40
80
(mm/set)
(mm/set)
Fig. 3. M&ssbauerspectraof ‘%d in CidM& at 4.1 K.
Fig. 4. Mksbauer spectra _- __of. . laEr .in__ErM& and “%‘b in YbM4A14
at4.1 K.
Magnetism,
spin reiaxation and hyl per&e interactions of rare earths
ErCkAI, and ErMrbAl, spectra (Fig. 3), one observes broadened absorption lines. This is most evident when compared to the ErFe6AL spectrum in which the Er ion is magnetically ordered to saturation [4] (the spectrum of ErCrsAL, indicates magnetic order in consistence with the magnetization studies). The spectra of ErCuaA& and ErMnsA&, were analyzed by Mossbauer spectra spin relaxation theory[5]. Gonzalez-Jimenez et 01.[5] give closed form formulas for the spectra of a 2+--f0’ transition for paramagnetic ions in an axiai crystalline field. These are exactly the conditions preva~ing in ES&A&, ErMGAl, and YbM%A&. The theoretical analysis assumes that the hype&e interaction spin Hamiltonian is given by H = AII+KI,+ &.(S+I- + s-J+) + -&41(21_ 1) (31; - Z(Zt 1)) and that the spin fluctuates transversely and longitudinally at rates W, and N$ respectively (The theoretical analysis yields these parameters, they are given In Table 2). We observe very slow spin relaxation rates (- 2 X tO’sec_’ for Er and 2 x IO”see-’ for Yb), though these are concentrated paramagnetic systems in which spinspin relaxation rates would be expected to be well above IO” set-‘. The reason for the slow spin-spin relaxation rates is probably strongly connected with the high crys-
1095
talline field effects. The crystalline fields lead to strong anisotropy which reduce spin relaxation rates [6], a phenomenon well known for Dy [7], Er 181and Yb [9] in insulators. However, in metallic systems it has seldom been observed [lo]. The spectrum of “‘Yb in YbCuaAl, shows only quadrupole splitting, thus consistent with the Yb being divalent as concluded also from the magnetization studies.
1. Fefner I., L Less. Common. Met& 72,241 (1980). 2. Felner I. and Nowik, I. J: Pitys. Chem. Solids 39,9S1 (1978); 40, 1035 (1979). 3. Buschow K. H. and Van der Kraan A. M., J. Phys. F. M&l Phys. 8,921 (1978). 4. Felner I., Seh M., Rakavy M. and Nowik I., /. Phvs. Chem. Solids 42,. 369 (1981); fiowik I., Felner I..and Skh M., J. Maa. & Mug. Mat. 15-U. 1215(19801. 5. Go&alez-Jimenez F., Imbert P: and’ Hartmann-Boutron F., Phys. Reu. 3 9,95 (1974). 6. Nowik I., Phys. Letters 15, 219 (1965). 7. Wickman H. H. and Nowik I., J. Phys. Chem. Solid 28,2099 (1967). 8. Hfiller A., WiedemanRW., Kienle and Hiifner S., Phys Letters 15,269 (1%5), 9. Nowik I. and Ofer S., 1. Phys. Chem. Solids 29,2f 17 (1969). 10. The concentrated metallic system ErRh4Bd exhibits such
phenomena, Shenoy G. K., Dunlap B. D., Frandin F. Y., Kimball C. W., Potzel W., Pr6bst F. and Kalvius G., J. App. Phys. 50, 1872(1979),also Phys. Reo. B21, 3886 (1980).