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Observation of spin-glass-like behavior in (Y, U)B4
Journal of Magnetism and Magnetic Materials 63 & 64 (1987) 193-195 North-Holland, Amsterdam
193
OBSERVATION OF SPIN-GLASS-LIKE BEHAVIOR IN (Y, U)B4 A WALLASH, J E CROW*, P SCHLOTTMANN Phystcs Department, Temple Umvers~ty, Phdadelph,a, PA 19122, USA M KURIC, S BLOOM, R P GUERTIN** Phystcs Department, Tufts Unwerslty, Medford, MA 02155, USA and S FONER Franets Bttter Naaonal Magnet Laboratory+. MIT, Cambndge, MA 02139, USA Low field magnetization measurements, including time dependent thermoremanent magnettzatlon, on U~Yt xB4, show spin-glass behavior for 0 1 < x <0 55 The paramagnetlc-spm glass phase boundary is reentrant, with the freezing temperature reaching about 14 K for x = 0 3 The temperature dependence of the rnagnetlc viscosityscales for the x = 0 2 and x = 0 4 samples
The spin-glass p h e n o m e n o n is associated with a large class of transition metal and rare earthbased metallic and Insulating systems [1] In this paper we report spin-glass behavior in a pseudobmary system where the magnetically active constituent IS an actlnlde ion, uranium, i e UxY~-xB4 for 0 l0 < x < 0 55 (Previously thts system was reported as ferromagnettc [2]) UxYI-xB4 ts particularly interesting for two reasons Ftrst, the magnetic m o m e n t associated with the U ions ts localized only over a hmlted range of U concentration, the same range over which spin glass behavior ts found This follows from the Hill plot [3] that for small U - U distances, the 5f-shell electron wave functtons overlap or are highly hybrtdlzed wtth the conductlon electrons so that localization of the fshell moment ~s not posstble In the pseudobinary system studted here, for example, at one extreme, UB4 IS only weakly paramagnetlc However, dilution wtth isomorphic YB4 causes the average U - U dtstance to increase, and localized magnetic behavtor results At very low U concentrations, the magnettc lnteracttons be* Supported by NSF DMR 8219782 ** Supported by NSF DMR 8502077 + Supported by NSF
tween the localized moments of the U tons weaken Secondly, because the spm-glass freezing temperature TG, vs U concentration, x, is reentrant [4], the same T6 may be obtamed for quite different values of x Thts feature allows mvesttgatton of spin-glass properttes of two samples wtth stmtlar magnetic mteraction strengths, but with qutte different concentrattons of magnetic ions Prehmlnary magnetic properttes and specific heat m UxYI-~B4 as well as detads of sample preparation were reported previously [4, 5] All samples had the smgle phase ThB4 structure, to the limit of our X-ray scattering analysis In this paper we focus on the very low field magnetic properties which gtve evtdence for the spin-glass properties Ftg 1 shows the temperature dependence of the heat capacity, C ( T ) , in the low temperature region for several UxYI-xB4 samples, three of which (x = 0 1, 0 2 and 0 3) are in the spin-glass regime The spm-glass freezing temperature, T6, determined by Arrott plot analysts, was about 3, 11 and 1 4 K , respectively [5] As discussed below, T~;, the freezmg temperatures determined by thermoremanent magnettzatton, were somewhat lower No large C ( T ) anomahes such as
0304-8853/87/$03 50 © Elsevier Science Publishers B V (North-Holland Physics Publishing Division)
194
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Fig 2 Low field magnetization for Uo ~Yo 6B4, showing the difference between the zero-field and field-cooled data
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those associated with long range ferromagnetic order were observed near the freezmg temperatures for any of the samples However, for the x = 0 2 sample, the insert shows an addmonal heat capacity m the region of TG (11 K) This heat capacity is presumably of magnetic origin, perhaps of an mnerant nature The lattice heat capactty for the x = 0 2 sample was estimated from higher temperature heat capacity data on the same sample The data of fig 1 are typical of spin-glasses m the region of their freezing temperatures, the complete absence of A-type anomahes ts inconsistent wtth the origmal interpretation of ferromagnetic ordermg of locahzed spins In fig 2 we show the magnetizatton tn very low applied fields for U 0 2 Y 0 6 B 4 Stmllar data, all measured with a vtbratIng sample magnetometer in small dc fields, were obtained for several other samples The data for the lower curve were obtamed by coohng m zero field followed by a slow warmup in a 10 G apphed field Data for the upper curve were obtained from slow field coolmg In a constant 10 G field This lrreverstble behavtor ts very similar to data observed for other spin-glass systems, e g , Cu~_~Mn~ (0 01 < x < 0 02) [6] The magnetic history in the data for fig 2 suggests that the low field magnettc properties
may be time dependent as observed for other spm-glasses In fig 3 we show the thermoremanent magnetization, ~ RM v% the logarithm of time for the same sample as m fig 2 These data were taken by coohng the sample to the mdlcated temperature m a 20 G field from well above TG, turnmg off the field at t = 0, and then observing the time dependent magnettzatton For T < 4 5 K and T > 9 7 5 K OhRM was essentially independent of time The data were fit to the formula M = M n - S l n t , where S represents the magnetic viscosity [7] S attains its maximum at about 0 8 7 ~ , T ~ , = 9 7 5 K for the x = 0 4 sample and 9 50 K for the x = 0 2 sample We note that these temperatures are very close to
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LOG(TIME) Flg 3 Time dependence netlzatlon for U,~ 4Yo 6B4
of the thermoremanent mag-
A Wallash et al / Spm-glass-hke behavtor m (Y, U)B4
I0 O8
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195
s a m p l e D a t a for b o t h the x = 0 2 and x = 0 4 samples are s h o w n in fig 4, in the n o r m a h z e d f o r m S/Sn, ax vs T / T ' G T h e solid h n e In fig 4 IS a guide to the eye, d r a w n t h r o u g h the x = 0 2 sample data T h e u n c e r t a i n t y in Smax IS e s t i m a t e d to be a b o u t 10% T h i s n o r m a l i z e d plot suggests u m v e r s a h t y of the m a g n e t i c viscosity vs t e m p e r a t u r e in spin-glass samples h a v i n g widely different c o n c e n t r a t i o n s of m a g n e t i c ions W e are e x a m i n i n g samples with o t h e r c o n c e n t r a t i o n s to test this u n i v e r s a l i t y A future p u b h c a t l o n will e m p h a s i z e high m a g n e t i c field at high hydrostatic pressure results
References the t e m p e r a t u r e s at which the v e r y low field c o o l e d a n d zero field c o o l e d m a g n e t i z a t i o n d w e r g e as T decreases As p o m t e d o u t in the m t r o d u c t o r y p a r a g r a p h , the freezing t e m p e r a t u r e vs x for U~Y1 xB4 is r e e n t r a n t , a t t a i n i n g a m a x i m u m at x = 0 3 a n d then d e c r e a s i n g to zero at a b o u t x -- 0 55 C o n s e q u e n t l y , the x = 0 2 a n d x = 0 4 samples have v e r y n e a r l y the same m a g n e t i c i n t e r a c t i o n s t r e n g t h , a l t h o u g h the c o n c e n t r a t i o n of the m a g n e t i c a l l y active e l e m e n t dltters b y a factor of two T i m e d e p e n d e n t t h e r m o r e m a n e n t m a g n e t i z a t i o n data were also o b t a i n e d for the x = 0 2
[1] For a review of experimental results see C Y Huang, J Magn Magn Mat 51 (1985) 1 [2] A L Giorgl, E G Szklarz, RW WhlteandHH Hdl, J Less-Common Metals 34 (1974) 348 [3] H H Hdl, Plutonmm 1970 and other Actmldes, ed W N Miner (The Metallurgical Society of the AIME, New York, 1970) p 2 [4] A Wallash, J E Crow and Z Flsk, J Appl Phys 57 (1985) 3143 A Wallash, J E Crow and Z Fmk, J Magn Magn Mat 47 & 48 (1985) 552 [5] A Wallash, J E Crow and Z Fisk, J Magn Magn Mat 54-57 (1986) 547 [6] S Nagota, P H Keesom and H R Harrison, Phys Rev B19 (1979) 1633 [7] C N Guy, J Phys F8 (1978) 1309, 1505
193
OBSERVATION OF SPIN-GLASS-LIKE BEHAVIOR IN (Y, U)B4 A WALLASH, J E CROW*, P SCHLOTTMANN Phystcs Department, Temple Umvers~ty, Phdadelph,a, PA 19122, USA M KURIC, S BLOOM, R P GUERTIN** Phystcs Department, Tufts Unwerslty, Medford, MA 02155, USA and S FONER Franets Bttter Naaonal Magnet Laboratory+. MIT, Cambndge, MA 02139, USA Low field magnetization measurements, including time dependent thermoremanent magnettzatlon, on U~Yt xB4, show spin-glass behavior for 0 1 < x <0 55 The paramagnetlc-spm glass phase boundary is reentrant, with the freezing temperature reaching about 14 K for x = 0 3 The temperature dependence of the rnagnetlc viscosityscales for the x = 0 2 and x = 0 4 samples
The spin-glass p h e n o m e n o n is associated with a large class of transition metal and rare earthbased metallic and Insulating systems [1] In this paper we report spin-glass behavior in a pseudobmary system where the magnetically active constituent IS an actlnlde ion, uranium, i e UxY~-xB4 for 0 l0 < x < 0 55 (Previously thts system was reported as ferromagnettc [2]) UxYI-xB4 ts particularly interesting for two reasons Ftrst, the magnetic m o m e n t associated with the U ions ts localized only over a hmlted range of U concentration, the same range over which spin glass behavior ts found This follows from the Hill plot [3] that for small U - U distances, the 5f-shell electron wave functtons overlap or are highly hybrtdlzed wtth the conductlon electrons so that localization of the fshell moment ~s not posstble In the pseudobinary system studted here, for example, at one extreme, UB4 IS only weakly paramagnetlc However, dilution wtth isomorphic YB4 causes the average U - U dtstance to increase, and localized magnetic behavtor results At very low U concentrations, the magnettc lnteracttons be* Supported by NSF DMR 8219782 ** Supported by NSF DMR 8502077 + Supported by NSF
tween the localized moments of the U tons weaken Secondly, because the spm-glass freezing temperature TG, vs U concentration, x, is reentrant [4], the same T6 may be obtamed for quite different values of x Thts feature allows mvesttgatton of spin-glass properttes of two samples wtth stmtlar magnetic mteraction strengths, but with qutte different concentrattons of magnetic ions Prehmlnary magnetic properttes and specific heat m UxYI-~B4 as well as detads of sample preparation were reported previously [4, 5] All samples had the smgle phase ThB4 structure, to the limit of our X-ray scattering analysis In this paper we focus on the very low field magnetic properties which gtve evtdence for the spin-glass properties Ftg 1 shows the temperature dependence of the heat capacity, C ( T ) , in the low temperature region for several UxYI-xB4 samples, three of which (x = 0 1, 0 2 and 0 3) are in the spin-glass regime The spm-glass freezing temperature, T6, determined by Arrott plot analysts, was about 3, 11 and 1 4 K , respectively [5] As discussed below, T~;, the freezmg temperatures determined by thermoremanent magnettzatton, were somewhat lower No large C ( T ) anomahes such as
0304-8853/87/$03 50 © Elsevier Science Publishers B V (North-Holland Physics Publishing Division)
194
Wallash et al / ~pm-glass-hke behat~mr in ( Y U ) B j
A
g.
lobBY-~
' " r''
(~q;Ux)la£
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X=05
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05
4
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X=O0
0
_
~
•
COOLED
.
i
,
L
.
.
.
.
I0 T(K)
Fig l Low temperature heat capacity, U~ Y~ xB4 samples The x = 0 I 0 2 and 0 the spin-glass regime The insert shows the netic heat capacity, C - C~ A ~ for the x = 0
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FIELD
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12
14
T(F)
.
20
Fig 2 Low field magnetization for Uo ~Yo 6B4, showing the difference between the zero-field and field-cooled data
C(T), for four 3 samples are m addmonal mag2 sample
those associated with long range ferromagnetic order were observed near the freezmg temperatures for any of the samples However, for the x = 0 2 sample, the insert shows an addmonal heat capacity m the region of TG (11 K) This heat capacity is presumably of magnetic origin, perhaps of an mnerant nature The lattice heat capactty for the x = 0 2 sample was estimated from higher temperature heat capacity data on the same sample The data of fig 1 are typical of spin-glasses m the region of their freezing temperatures, the complete absence of A-type anomahes ts inconsistent wtth the origmal interpretation of ferromagnetic ordermg of locahzed spins In fig 2 we show the magnetizatton tn very low applied fields for U 0 2 Y 0 6 B 4 Stmllar data, all measured with a vtbratIng sample magnetometer in small dc fields, were obtained for several other samples The data for the lower curve were obtamed by coohng m zero field followed by a slow warmup in a 10 G apphed field Data for the upper curve were obtained from slow field coolmg In a constant 10 G field This lrreverstble behavtor ts very similar to data observed for other spin-glass systems, e g , Cu~_~Mn~ (0 01 < x < 0 02) [6] The magnetic history in the data for fig 2 suggests that the low field magnettc properties
may be time dependent as observed for other spm-glasses In fig 3 we show the thermoremanent magnetization, ~ RM v% the logarithm of time for the same sample as m fig 2 These data were taken by coohng the sample to the mdlcated temperature m a 20 G field from well above TG, turnmg off the field at t = 0, and then observing the time dependent magnettzatton For T < 4 5 K and T > 9 7 5 K OhRM was essentially independent of time The data were fit to the formula M = M n - S l n t , where S represents the magnetic viscosity [7] S attains its maximum at about 0 8 7 ~ , T ~ , = 9 7 5 K for the x = 0 4 sample and 9 50 K for the x = 0 2 sample We note that these temperatures are very close to
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Uo4Yo6B 4 FIELD COOLED IN 20G
-03- 5
. . . . 590K
b-
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m2 Q5 <(
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783 K
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LOG(TIME) Flg 3 Time dependence netlzatlon for U,~ 4Yo 6B4
of the thermoremanent mag-
A Wallash et al / Spm-glass-hke behavtor m (Y, U)B4
I0 O8
o Uoz¥osB4 • Uo 4Yo~ B4
OE
FIELD COOLED IOG
.i O2 0 o 'O2
O4
O6
T/T~
O8
~0
Fig 4 Normahzed plot of the magnetic vmcoslty, S/So,dx vs T/T'~,, for two spin-glass samples T~, is defined as the upper temperature at which the magnetic wscoslty, S, vanishes
195
s a m p l e D a t a for b o t h the x = 0 2 and x = 0 4 samples are s h o w n in fig 4, in the n o r m a h z e d f o r m S/Sn, ax vs T / T ' G T h e solid h n e In fig 4 IS a guide to the eye, d r a w n t h r o u g h the x = 0 2 sample data T h e u n c e r t a i n t y in Smax IS e s t i m a t e d to be a b o u t 10% T h i s n o r m a l i z e d plot suggests u m v e r s a h t y of the m a g n e t i c viscosity vs t e m p e r a t u r e in spin-glass samples h a v i n g widely different c o n c e n t r a t i o n s of m a g n e t i c ions W e are e x a m i n i n g samples with o t h e r c o n c e n t r a t i o n s to test this u n i v e r s a l i t y A future p u b h c a t l o n will e m p h a s i z e high m a g n e t i c field at high hydrostatic pressure results
References the t e m p e r a t u r e s at which the v e r y low field c o o l e d a n d zero field c o o l e d m a g n e t i z a t i o n d w e r g e as T decreases As p o m t e d o u t in the m t r o d u c t o r y p a r a g r a p h , the freezing t e m p e r a t u r e vs x for U~Y1 xB4 is r e e n t r a n t , a t t a i n i n g a m a x i m u m at x = 0 3 a n d then d e c r e a s i n g to zero at a b o u t x -- 0 55 C o n s e q u e n t l y , the x = 0 2 a n d x = 0 4 samples have v e r y n e a r l y the same m a g n e t i c i n t e r a c t i o n s t r e n g t h , a l t h o u g h the c o n c e n t r a t i o n of the m a g n e t i c a l l y active e l e m e n t dltters b y a factor of two T i m e d e p e n d e n t t h e r m o r e m a n e n t m a g n e t i z a t i o n data were also o b t a i n e d for the x = 0 2
[1] For a review of experimental results see C Y Huang, J Magn Magn Mat 51 (1985) 1 [2] A L Giorgl, E G Szklarz, RW WhlteandHH Hdl, J Less-Common Metals 34 (1974) 348 [3] H H Hdl, Plutonmm 1970 and other Actmldes, ed W N Miner (The Metallurgical Society of the AIME, New York, 1970) p 2 [4] A Wallash, J E Crow and Z Flsk, J Appl Phys 57 (1985) 3143 A Wallash, J E Crow and Z Fmk, J Magn Magn Mat 47 & 48 (1985) 552 [5] A Wallash, J E Crow and Z Fisk, J Magn Magn Mat 54-57 (1986) 547 [6] S Nagota, P H Keesom and H R Harrison, Phys Rev B19 (1979) 1633 [7] C N Guy, J Phys F8 (1978) 1309, 1505