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Pressure induced loss of ferromagnetism in UPt
J~umM of M~nctism and Magnetic Materials 1 (1975) 5 8 - 6 3 N,,rth-th~lland Publishing Company
PRFSSURE INDUCED LOSS OF FERROMAGNETISM IN UPt* J.G. HUBER, M.B. MAPLE and O. WOHLLEBEN ?nxrirute .?'orI'urv and Applied Physical Sciences, Uni~rsi(r o1 Cahl;brnia, San Diego, La dolla. California 9203 7, U,S.A.
Tlte ~.,ak ferromagnetic saturation m o m e n t of the intetmetallic ~ompound UPt is reduced with respect to the atmospheric prexsure ~'alue by more th;m 90% at 20 kbar. The 30K Curie teml ~:*are is pressure independent, The paramagnetie susceptibility shows a maximum at 17K at all pressures. Plots of x - t vs. T above 30 K are nonlinear.
i. ~ntroductio~ Studies of the electronic properties of a metal under high pressure can be very informative; the appreciable and continuous pres~mre induced variation of the lattice parameter implies a controlled variation of the band structure ~4th some ol,vious advantages over the usual a~:eving technique. One property, metallic magnetism, origin:~ing on electronic wave functions with an inter. ~ e . ] i a l e de~rcc ~.t loca:tzauon, is very sensitive to the ba~d s~ruclurc: so much so that the magnetism of tran.q~,n vetals. ~\~r~nsta~ce. still defies attemg,ts at qu~n"~ "t~
:-
.
, ~ , , ~',,.. Or ol te|l even t ualitative, interpreta;I ion. Recent
i ~ : , ] ~ a l ad~aaccs [ t t !lave enabled us to study '-lie mzge¢ tic ~sceptibility o f metals under high pressure down t~ weak P~uli type spin paramagnetism. This ef~zer.~the highly interesting possibility of observing INe ~_;ansitien from itinerant ferromngnetism (e.g., Nil ~e Pauli spin paramagnetism (e.g., Pd) in a single For the clm~sical 3d ferrowagnets, the pressures needed lo induce such a transition are still beyond the timia of exi_,ting experimental techniques apparent,y be~au~a ~ e maximum pressure-induced change of the :c~ chert: I p & Vm~_x:~ O. 1 eV at I O0 kbar) : emains s~ai~ compared ~o the relevant band widths (:!;everal cV !. O~: zt-,c c~her hand, the 5f levels of actinidc and ¢>z af Ievels of rare earth metals are generally thought " R~sea:~ ~up~-~ed by lhc Air lorce Office of Scientific Re%*,~'~:h.-Xir | ,.~r~.x:Sy~terns Command, USAV, utder Atr:OSR ( ; ; a n t N , A~OSR-71-2073.
to be one and two orders o f magnitude narrower, respectively; i.e., of the order or smaller than practicable PAVmax. Actinide ferromagnets retain some of the delocalized character of transition metals; they do not lapse to the trivial limit of local magnetism generally encountered in rare earth metals. They therefore appeared to be good candidates for the study we had it, mind and we chose to investigate the magnetic properties of the intermetallic compound UPt under pressure. An off-stoichiometry sample, U0.54Pt0.46 , was reported to be a soft ferromagnet with a Curie temperature of about 30K and a satur~:tion moment of 0.8p B per uranium atom [2]. We took as a guide for the expected size of the pressure effect the empirical finding that superconductivity and magnetism of metallic uranium compounds are intimately related [2,3] and that uranium metal shows the most dramatic pressure dependence o f the superconducting transition temperature of all elements ~!4]. As shown below, ir is possible to suppress the saturation magne~',zation of UPt nearly completely by only 20 kbar. However, the UPt system turned out to be more complicated thzm anticipated with respect to the p,essure deper, dence of the Curie temperature, the magnetic structure of the high pressure state, and the concentration dependence of the saturation moment. We report here merely on the magnetiic susceptibility. Specific heat and electrical resistance measuremeats under pressure are needed for better understanding of ti,e results. Such work is in progress and will be reported later.
59
J.G. Huber et aL/Pressure induced loss o f [erromagnetism in UPt
MII~IIATUREPRESSURECLAMP
mgCop iting ar
Jer I cm
¢lJ
31[
~ P u r e BeCu ! ~ Beryl¢o ,~5 ~6E
0-30
Fig. !. Miniature pressure clamps with pure BeCu body and small magnetically impure BeCu 25 and GE 0-30 inserts. Pressure range, 0-20 kbar, temperature range, 300-4.5 K.
2. Experimental details Our samples were prepared and analyzed as follows. Appropriate amounts o f uranium (Research Organic• Inorganic Chemical Corp., 99.9% purity) and platinum (Matthey Bishop Inc., 99.999% purity) were melted together in an argon atmosphere arc furnace. Weight losses were negligible. The samples were wrapped in tantalum foils, sealed i~l quartz tubes under one quarter atmosphere of helium and annealed at 900°C for one week. Thereafter they were cooled slowly down to room temperature. A microprobe analysis of the Uo.48Pto.~2 sample showed no composition inhomogeneity within an accuracy of 1%. X-ray identification of any phase other than the CrB structure in several unannealed as well as annealed Uo,50Pto.50 and U0.52Pto.48 samples proved futile. Extraneous lines were present, however. The X-ray pattern for a Un.52Pt0n48 sample at 4.5 K did not change noticeably
H (kGouss) Fig. 2. The magntetizationof Uo.s2Pto.4s at 3,94 kbar as Iunction of applied field at selected temperatures. The slopes of the straight lines fitted to the high field points define the differential paramag,netic susceptibilities while the zero field ma~,neUzation intercepts define the ~turatioa moments.
from that at room temperature. For our magnetization measurements the samples were pressurized in a miniature clamp. This method has been described elsewhere [ ! ]. The clamp design was modified slightly, though, to allow measurements at higher pressure down to liquid helium temperature. As seen in fig. 1, the clamp body along with various locking and supportive p~eces was made of pure BeCu. a material which is low in magnetic impurities but which yields plastically above 12 kbars. By using a small Berylco 25 insert cylindeT with tiny GE 0 - 3 0 pistons and back-up pieces, the amount of magnelically dirty but stronger material was minimized and the magnetization of the clamp at 4.5 K was kept within tolegabic limits. Inside the clam~, the central sample space had a bore o f approximately 2 ram, and a spherical sample was packed in indium within a teflon sleeve. BeCu rir,~s sealed the enclosure when the clamp was pre~urized.
60
J.(3. Huber et el./Pressure Induced loss of ferromagnetism in UPt
The indium served both as a quasi-hydrostatic pressure trangnitting medium and as a manometer. Its superconc~ucting transition temperature, and hence the pressate, was determined by an inductive susceptibility mecsurement in a separate experiment. Temperatures were derived from a calibrated plati. aura or germanium resistor, depending on the range. At F~x~ temperature and pressure magnetization curves were taken o f the clamp with sample with a Faraday magnetometer in fields up to 8 kGauss. The independently measured magnetization of the e m p ~ d a m p was subtracted point by point. The high field stope of ~ , resulting curves yielded the differential p a r a ~ e t i c susceptibility and the intercept of the tangtmt vd:th the ordinate g~ve the saturation magnetization. Exem?lary data may be seen in fig. 2.
3. Resatts Table ! shows the saturation moment P-s per uranium atom at atmosFberic pressure and 4.5 K for several samoles with ccmpc,sitions near ~tochiometric UPt. The raon~nt is a v~ry. sensi,;ve function of composit~,.-~ and heat treatment. Since it is eveqtwhere considerzb[y te~ than l ~ti3, UPt may be classified as a weak fer'~magr~.e~. The strong variation o f # s with composi~.~,~a is reminiscent of Invar [5 ], Ni3Ai [6] and ZrZn 2 ~ J. and in analog" with these systems, hints at a homo~ry range m the phase diagram. We have made no attempt to explore in detail the mfhzeace of vadous heat treatments and cold working ~n t~e m~aitude c~f the zero pre~ure saturation momen~ |%r a ~g~:encomposition. We merely gave all samph~s exactly the same heat treatment (described above) and took care not to cold work them in any *,~v thereafter. We note that U0.521~0A8, the sample with the smallest moment, is the only one which ~ la~e cryst~ites after arc-melting; i.e., this ~'~p.~e .~ems lo ~ cry,staltographically more stable ahaa fi~e others. Because of this crystallographic dis~ac~v~a and becauw "'e smallest moment L,~most con-
venient to work with, we concentrated our effort on this composition and on UO.S0Pt0.s0 as a representative of the mare ger,eral, larger moment category. The most interesting features in the pressure dependence of the magnetization curves are nearly independent o f sample composition. This justified their being reported here, even though detailed sample characterization is lacking. Fig. 3 shows the saturation magnetization of U032Pto.48 as function of temperature and pressure. The moment approaches zero asymptotically with increasing pressure. At 18.5 kbar, the maximum pressure employed, the saturation moment is reduced by more than 98% of the zero pressure value. Upon release of the pressure, the moment returned to its original value with an error of less than 10%, even after repeated pressure cycling. While the moment drops dramatically with increasing pressure, the Curie temperature, denned by the appearance of an intercept on the ordinate of the magnetization curves, remains independent of pressure within experimental accuracy. Fig. 4 shows the saturation magnetization of nominally stochiometric UPt. The initial moment is three times larger than for U0.s2Pt0.48 and the loss of the moment near 20 kbar is not as complete (94.5%). Also, a point of inflection appears in #s(P) near 5 kbar. However, again the moment vanishes very fast, asymptoticaUy with increasing pressure, and the Curie temperature remains pressure independent. Fig. 5 shows the pressure and temperature dependence of the high field differential susceptibility for U0.52Pt0.48. The data are taken from the same magnetization curves as the saturation moments in fig. 3 %.e, for example, fig. 2). A maximum in the susceptibility at 29K vanishes with pressure at a rate comparable to that of the saturation moment, while a second maximum near 17K is prominent at all pressures. The magnitude of this second maximt:m varies somewhat as a function of pressure; it is 20% larger at 9.5 kbar than at 0 or 18 kbar. A similar susceptibility surface in the p - T plane was obtained for the U0.s0Pt0.50 sample, where, however, the experimental scatter at low pressures is much larger
"c~e ~:~ar~:~or~mornem ~ r uranium aton~ of UxPt I-x at 4.5 K ,~ "s[ ~E
0.54 0.52
0.53 0.23
0.52 0.07
0.51 0.29
0.50 O.19
0.48 0.29
61
J.G. Huber et al./Pressure induced loss o f ferromagnetism in UPt
" ~ 0.08
U.s~P1,4e
~E •
t--
~= 0,06
0.04 0 =E
o~ "
0.02
25
4O T(*K)
,,O
Fig. 3. Saturation moment per uranium atom of Uo.saPto.4~ as function of temperature and pressure. because the larger saturation moment dominates tire magnetization curves. The values of the susceptibility for U0.50Ptq.5o are generally about twice as large as in fig. 5. In a limited survey of the paramagnetic su,cceptibility above 30K at several compositions and pressures we found that plots o f ×-1 vs. T are not striaght lines, they all curve toward the T azis with decreasing temperature. Our data for a sample of Uo,s2Pto.48 at 0 and "-" 10 kbar pressure are displayed in fig. 6.
4. Discussion
Figures 3 and 4 document the kind of data which are badly needed for a basic understanding of ferromagnetism in metals: continuous and nearly reversible reduction by two orders of magnitude of the saturation moment of a ferromagnetic metal as a function of the lattice constant without variation of chemical composition. The paramagnetic molar susceptibility at high pressures and 4.5 K is of the order of magnitude of mar, y nonmagnetic or zntiferromagnetic ,Jranium corn-
pounds [81 . While the simple fact of the loss of the ferromagnetic saturation m o m e n t is of general interest, many of the other features of figs. 3 to 5 make UPt appear to be a more complicated system than desirable for a general test case of theories of itinerant magneusm. The most unexpected result is the apparent pressure independence of the Curie temperature as seen directly in figs. 3 and 4 and indirectly in fig. 5. (The maximum near 29K in fig. 5 may be safely assumed to I:e associate, with spin fluctuations near the Curie temperature.) The pressure independence o f the Curie temperature of UPt is in sharp contrast to the .~ase of ZrZn 2 where the Curie temperature, as detected ! • low field a.c. suscepti. bility [9] as well as by static magnetization measurements [10], could be suppressed to zero witin 20 kbar. We are unable to give an explanation for this curiou.¢ behavior. Although it is unlikely that a ferromagnetic phase with T c "" 30K and a given crystal structure is converted continuously as function of pressure into a nonmagnetic or antiferromagnetic phase, with another crystal structure, this possibility cannot be excluded entirely. We kept the cells pressurh,.ed at a given value at room temperature for at least one night between
62
J.G. Huber et aL/Pressure induced loss of fereomagnetism in UPt
U.soPl.so
-'~~<,0"25~~020 _o "" 0.15 0.10~_ o_ 0,05 I,--.
o~._.
Z
I,z,,I
IZ
.~
o
~0
I
I
I0 "
"15
,,
~m
I
~o
~.5
40
T(°K)
lOX O~='~=@.=.,...=m~ 4 4
\
Fig. 4. Saturation moment per uranium atom of Uo.soPto.s0 as functio~ of temperature and pressure.
x102l 20I
U.52
Pt.~e
-_~i.5i E
~
0.5
0!
T(OK}
,,0 \
\
|:~:. 5. The hi~,.h fie~,d differential paramagnetic susceptibility of Uo.s2Pto.4,~ as function of pressure and temperature,
J.G. Hub er
et
aL/Pressure induced loss of [erromagnetism in UPt
63
UPt would provide the answer most clearly. We are ai present involved in a study of' the resistivity a,d the specific heat of the UPt system. /
-T
I
Wethank R, Fitzgerald for the microprobe work and A,C. Lawson for X-ray analy~ts.
/ ,I
.50
References
,1
I00
,,
I
,,,
I,,,
150 :>00 TEMPERATURE (*KI
2 0
~iO0
Fig. 6. The inverse paramagnetic susceptibility of Uo.~2Pto.4s at 0 and *~ I 0 kbar as function of temperature.
runs. Thus room temperature annealing might have shifted the equilibrium between the two phases as dictated by the applied pressure. We could not check this point, since we did not have access to a high pressure X-ray camera. A continuous and reversible electronic phase transition is of course much more likely. Such transitions are common in metals containing uranium, most clearly in ~ uranium itself [4]. A second feature which complicates the analysis is the maxima of the susceptibility at 17K. Static susceptibility mea. surementz along cannot decide whether this maxima is associated with an antiferromagnetic transition or whether it is the kind of maximum attributed to para. magnons [11 ] and found in nonmagnetic metals which are nearly unstable toward cooperative moment formation; Pd [121, Sc [131, Y [14], and perhaps U2C 3 [151 for example. A neutron diffraction experiment on
[ 11 D. Wohlleben and M.B. Maple, Rev. Sci. Inst. 42 ( t g ? i ) 1573. [2] B.T. Matthias, C.W. Chu, E. Corenzwit and D. Wohileben. Prec. Natl. Academy of Sciences 64 (1969) 459. [31 B.T. Matthias, Journal de Physique 32 (1971) C1-607. [41 M.B. Maple and D. Wohlleben, Phys. Letters 38A (|972) 351. [5] R.W. Cochrane and G.M. G~aham, Canadian J. Phys. 48 (1970) 264. [6] F.R. deBoer, C.F. Schinkel, J. Biesterbos and S. Proost, J. Appi. Phys. 40 (1969) 1049. [7] G.S. Knapp, I:.Y. I:radin and H.V. (;ulbert, J. Appi. Phyr,. 42 (1971) 1341. [81 K.tt.J. Buschow and H..I. v~,~oDaM, AlP Conference Proceedings =5, ed~. (~.D. Grailani. Jr. and .t.J. Rh~nc (Nc~ York, 1972) !4.64. [9] T.F. Smith, J.A. MS,Josh and I...P. Wohlfarth. Phy~,. Rev. Letters 27 (1971) ! 732. I101 J.G. ttuber, M.B. Maple, D. W,~hlh'bct: u_~d (,.S. K~'.,~pp. Solid Stale Commun. (in pre~) [11 [ S. Misawa, Physics Letters 32A (~ 97~1 5,~i. 112[ F.E. Hoare and J.C. Matthews. Prot,. Roy. Soc. (l_ortdov~,~ A212 (1952~ t3~;A216 (i953~' 502. [i3 i b. Wohlleben, Ph.l, "~tlesi!,. Unw. ofCalif~,rnia (1968~. unpublished. [141 W.E. Gardner and J. Penfo!d, Physics Letters 26A (t968~ 204. [ 15 ] J.-L. Boutard and C.-It. de Novion, Solid Staxe Commun 14 (1974) 181.
PRFSSURE INDUCED LOSS OF FERROMAGNETISM IN UPt* J.G. HUBER, M.B. MAPLE and O. WOHLLEBEN ?nxrirute .?'orI'urv and Applied Physical Sciences, Uni~rsi(r o1 Cahl;brnia, San Diego, La dolla. California 9203 7, U,S.A.
Tlte ~.,ak ferromagnetic saturation m o m e n t of the intetmetallic ~ompound UPt is reduced with respect to the atmospheric prexsure ~'alue by more th;m 90% at 20 kbar. The 30K Curie teml ~:*are is pressure independent, The paramagnetie susceptibility shows a maximum at 17K at all pressures. Plots of x - t vs. T above 30 K are nonlinear.
i. ~ntroductio~ Studies of the electronic properties of a metal under high pressure can be very informative; the appreciable and continuous pres~mre induced variation of the lattice parameter implies a controlled variation of the band structure ~4th some ol,vious advantages over the usual a~:eving technique. One property, metallic magnetism, origin:~ing on electronic wave functions with an inter. ~ e . ] i a l e de~rcc ~.t loca:tzauon, is very sensitive to the ba~d s~ruclurc: so much so that the magnetism of tran.q~,n vetals. ~\~r~nsta~ce. still defies attemg,ts at qu~n"~ "t~
:-
.
, ~ , , ~',,.. Or ol te|l even t ualitative, interpreta;I ion. Recent
i ~ : , ] ~ a l ad~aaccs [ t t !lave enabled us to study '-lie mzge¢ tic ~sceptibility o f metals under high pressure down t~ weak P~uli type spin paramagnetism. This ef~zer.~the highly interesting possibility of observing INe ~_;ansitien from itinerant ferromngnetism (e.g., Nil ~e Pauli spin paramagnetism (e.g., Pd) in a single For the clm~sical 3d ferrowagnets, the pressures needed lo induce such a transition are still beyond the timia of exi_,ting experimental techniques apparent,y be~au~a ~ e maximum pressure-induced change of the :c~ chert: I p & Vm~_x:~ O. 1 eV at I O0 kbar) : emains s~ai~ compared ~o the relevant band widths (:!;everal cV !. O~: zt-,c c~her hand, the 5f levels of actinidc and ¢>z af Ievels of rare earth metals are generally thought " R~sea:~ ~up~-~ed by lhc Air lorce Office of Scientific Re%*,~'~:h.-Xir | ,.~r~.x:Sy~terns Command, USAV, utder Atr:OSR ( ; ; a n t N , A~OSR-71-2073.
to be one and two orders o f magnitude narrower, respectively; i.e., of the order or smaller than practicable PAVmax. Actinide ferromagnets retain some of the delocalized character of transition metals; they do not lapse to the trivial limit of local magnetism generally encountered in rare earth metals. They therefore appeared to be good candidates for the study we had it, mind and we chose to investigate the magnetic properties of the intermetallic compound UPt under pressure. An off-stoichiometry sample, U0.54Pt0.46 , was reported to be a soft ferromagnet with a Curie temperature of about 30K and a satur~:tion moment of 0.8p B per uranium atom [2]. We took as a guide for the expected size of the pressure effect the empirical finding that superconductivity and magnetism of metallic uranium compounds are intimately related [2,3] and that uranium metal shows the most dramatic pressure dependence o f the superconducting transition temperature of all elements ~!4]. As shown below, ir is possible to suppress the saturation magne~',zation of UPt nearly completely by only 20 kbar. However, the UPt system turned out to be more complicated thzm anticipated with respect to the p,essure deper, dence of the Curie temperature, the magnetic structure of the high pressure state, and the concentration dependence of the saturation moment. We report here merely on the magnetiic susceptibility. Specific heat and electrical resistance measuremeats under pressure are needed for better understanding of ti,e results. Such work is in progress and will be reported later.
59
J.G. Huber et aL/Pressure induced loss o f [erromagnetism in UPt
MII~IIATUREPRESSURECLAMP
mgCop iting ar
Jer I cm
¢lJ
31[
~ P u r e BeCu ! ~ Beryl¢o ,~5 ~6E
0-30
Fig. !. Miniature pressure clamps with pure BeCu body and small magnetically impure BeCu 25 and GE 0-30 inserts. Pressure range, 0-20 kbar, temperature range, 300-4.5 K.
2. Experimental details Our samples were prepared and analyzed as follows. Appropriate amounts o f uranium (Research Organic• Inorganic Chemical Corp., 99.9% purity) and platinum (Matthey Bishop Inc., 99.999% purity) were melted together in an argon atmosphere arc furnace. Weight losses were negligible. The samples were wrapped in tantalum foils, sealed i~l quartz tubes under one quarter atmosphere of helium and annealed at 900°C for one week. Thereafter they were cooled slowly down to room temperature. A microprobe analysis of the Uo.48Pto.~2 sample showed no composition inhomogeneity within an accuracy of 1%. X-ray identification of any phase other than the CrB structure in several unannealed as well as annealed Uo,50Pto.50 and U0.52Pto.48 samples proved futile. Extraneous lines were present, however. The X-ray pattern for a Un.52Pt0n48 sample at 4.5 K did not change noticeably
H (kGouss) Fig. 2. The magntetizationof Uo.s2Pto.4s at 3,94 kbar as Iunction of applied field at selected temperatures. The slopes of the straight lines fitted to the high field points define the differential paramag,netic susceptibilities while the zero field ma~,neUzation intercepts define the ~turatioa moments.
from that at room temperature. For our magnetization measurements the samples were pressurized in a miniature clamp. This method has been described elsewhere [ ! ]. The clamp design was modified slightly, though, to allow measurements at higher pressure down to liquid helium temperature. As seen in fig. 1, the clamp body along with various locking and supportive p~eces was made of pure BeCu. a material which is low in magnetic impurities but which yields plastically above 12 kbars. By using a small Berylco 25 insert cylindeT with tiny GE 0 - 3 0 pistons and back-up pieces, the amount of magnelically dirty but stronger material was minimized and the magnetization of the clamp at 4.5 K was kept within tolegabic limits. Inside the clam~, the central sample space had a bore o f approximately 2 ram, and a spherical sample was packed in indium within a teflon sleeve. BeCu rir,~s sealed the enclosure when the clamp was pre~urized.
60
J.(3. Huber et el./Pressure Induced loss of ferromagnetism in UPt
The indium served both as a quasi-hydrostatic pressure trangnitting medium and as a manometer. Its superconc~ucting transition temperature, and hence the pressate, was determined by an inductive susceptibility mecsurement in a separate experiment. Temperatures were derived from a calibrated plati. aura or germanium resistor, depending on the range. At F~x~ temperature and pressure magnetization curves were taken o f the clamp with sample with a Faraday magnetometer in fields up to 8 kGauss. The independently measured magnetization of the e m p ~ d a m p was subtracted point by point. The high field stope of ~ , resulting curves yielded the differential p a r a ~ e t i c susceptibility and the intercept of the tangtmt vd:th the ordinate g~ve the saturation magnetization. Exem?lary data may be seen in fig. 2.
3. Resatts Table ! shows the saturation moment P-s per uranium atom at atmosFberic pressure and 4.5 K for several samoles with ccmpc,sitions near ~tochiometric UPt. The raon~nt is a v~ry. sensi,;ve function of composit~,.-~ and heat treatment. Since it is eveqtwhere considerzb[y te~ than l ~ti3, UPt may be classified as a weak fer'~magr~.e~. The strong variation o f # s with composi~.~,~a is reminiscent of Invar [5 ], Ni3Ai [6] and ZrZn 2 ~ J. and in analog" with these systems, hints at a homo~ry range m the phase diagram. We have made no attempt to explore in detail the mfhzeace of vadous heat treatments and cold working ~n t~e m~aitude c~f the zero pre~ure saturation momen~ |%r a ~g~:encomposition. We merely gave all samph~s exactly the same heat treatment (described above) and took care not to cold work them in any *,~v thereafter. We note that U0.521~0A8, the sample with the smallest moment, is the only one which ~ la~e cryst~ites after arc-melting; i.e., this ~'~p.~e .~ems lo ~ cry,staltographically more stable ahaa fi~e others. Because of this crystallographic dis~ac~v~a and becauw "'e smallest moment L,~most con-
venient to work with, we concentrated our effort on this composition and on UO.S0Pt0.s0 as a representative of the mare ger,eral, larger moment category. The most interesting features in the pressure dependence of the magnetization curves are nearly independent o f sample composition. This justified their being reported here, even though detailed sample characterization is lacking. Fig. 3 shows the saturation magnetization of U032Pto.48 as function of temperature and pressure. The moment approaches zero asymptotically with increasing pressure. At 18.5 kbar, the maximum pressure employed, the saturation moment is reduced by more than 98% of the zero pressure value. Upon release of the pressure, the moment returned to its original value with an error of less than 10%, even after repeated pressure cycling. While the moment drops dramatically with increasing pressure, the Curie temperature, denned by the appearance of an intercept on the ordinate of the magnetization curves, remains independent of pressure within experimental accuracy. Fig. 4 shows the saturation magnetization of nominally stochiometric UPt. The initial moment is three times larger than for U0.s2Pt0.48 and the loss of the moment near 20 kbar is not as complete (94.5%). Also, a point of inflection appears in #s(P) near 5 kbar. However, again the moment vanishes very fast, asymptoticaUy with increasing pressure, and the Curie temperature remains pressure independent. Fig. 5 shows the pressure and temperature dependence of the high field differential susceptibility for U0.52Pt0.48. The data are taken from the same magnetization curves as the saturation moments in fig. 3 %.e, for example, fig. 2). A maximum in the susceptibility at 29K vanishes with pressure at a rate comparable to that of the saturation moment, while a second maximum near 17K is prominent at all pressures. The magnitude of this second maximt:m varies somewhat as a function of pressure; it is 20% larger at 9.5 kbar than at 0 or 18 kbar. A similar susceptibility surface in the p - T plane was obtained for the U0.s0Pt0.50 sample, where, however, the experimental scatter at low pressures is much larger
"c~e ~:~ar~:~or~mornem ~ r uranium aton~ of UxPt I-x at 4.5 K ,~ "s[ ~E
0.54 0.52
0.53 0.23
0.52 0.07
0.51 0.29
0.50 O.19
0.48 0.29
61
J.G. Huber et al./Pressure induced loss o f ferromagnetism in UPt
" ~ 0.08
U.s~P1,4e
~E •
t--
~= 0,06
0.04 0 =E
o~ "
0.02
25
4O T(*K)
,,O
Fig. 3. Saturation moment per uranium atom of Uo.saPto.4~ as function of temperature and pressure. because the larger saturation moment dominates tire magnetization curves. The values of the susceptibility for U0.50Ptq.5o are generally about twice as large as in fig. 5. In a limited survey of the paramagnetic su,cceptibility above 30K at several compositions and pressures we found that plots o f ×-1 vs. T are not striaght lines, they all curve toward the T azis with decreasing temperature. Our data for a sample of Uo,s2Pto.48 at 0 and "-" 10 kbar pressure are displayed in fig. 6.
4. Discussion
Figures 3 and 4 document the kind of data which are badly needed for a basic understanding of ferromagnetism in metals: continuous and nearly reversible reduction by two orders of magnitude of the saturation moment of a ferromagnetic metal as a function of the lattice constant without variation of chemical composition. The paramagnetic molar susceptibility at high pressures and 4.5 K is of the order of magnitude of mar, y nonmagnetic or zntiferromagnetic ,Jranium corn-
pounds [81 . While the simple fact of the loss of the ferromagnetic saturation m o m e n t is of general interest, many of the other features of figs. 3 to 5 make UPt appear to be a more complicated system than desirable for a general test case of theories of itinerant magneusm. The most unexpected result is the apparent pressure independence of the Curie temperature as seen directly in figs. 3 and 4 and indirectly in fig. 5. (The maximum near 29K in fig. 5 may be safely assumed to I:e associate, with spin fluctuations near the Curie temperature.) The pressure independence o f the Curie temperature of UPt is in sharp contrast to the .~ase of ZrZn 2 where the Curie temperature, as detected ! • low field a.c. suscepti. bility [9] as well as by static magnetization measurements [10], could be suppressed to zero witin 20 kbar. We are unable to give an explanation for this curiou.¢ behavior. Although it is unlikely that a ferromagnetic phase with T c "" 30K and a given crystal structure is converted continuously as function of pressure into a nonmagnetic or antiferromagnetic phase, with another crystal structure, this possibility cannot be excluded entirely. We kept the cells pressurh,.ed at a given value at room temperature for at least one night between
62
J.G. Huber et aL/Pressure induced loss of fereomagnetism in UPt
U.soPl.so
-'~~<,0"25~~020 _o "" 0.15 0.10~_ o_ 0,05 I,--.
o~._.
Z
I,z,,I
IZ
.~
o
~0
I
I
I0 "
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,,
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I
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~.5
40
T(°K)
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\
Fig. 4. Saturation moment per uranium atom of Uo.soPto.s0 as functio~ of temperature and pressure.
x102l 20I
U.52
Pt.~e
-_~i.5i E
~
0.5
0!
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,,0 \
\
|:~:. 5. The hi~,.h fie~,d differential paramagnetic susceptibility of Uo.s2Pto.4,~ as function of pressure and temperature,
J.G. Hub er
et
aL/Pressure induced loss of [erromagnetism in UPt
63
UPt would provide the answer most clearly. We are ai present involved in a study of' the resistivity a,d the specific heat of the UPt system. /
-T
I
Wethank R, Fitzgerald for the microprobe work and A,C. Lawson for X-ray analy~ts.
/ ,I
.50
References
,1
I00
,,
I
,,,
I,,,
150 :>00 TEMPERATURE (*KI
2 0
~iO0
Fig. 6. The inverse paramagnetic susceptibility of Uo.~2Pto.4s at 0 and *~ I 0 kbar as function of temperature.
runs. Thus room temperature annealing might have shifted the equilibrium between the two phases as dictated by the applied pressure. We could not check this point, since we did not have access to a high pressure X-ray camera. A continuous and reversible electronic phase transition is of course much more likely. Such transitions are common in metals containing uranium, most clearly in ~ uranium itself [4]. A second feature which complicates the analysis is the maxima of the susceptibility at 17K. Static susceptibility mea. surementz along cannot decide whether this maxima is associated with an antiferromagnetic transition or whether it is the kind of maximum attributed to para. magnons [11 ] and found in nonmagnetic metals which are nearly unstable toward cooperative moment formation; Pd [121, Sc [131, Y [14], and perhaps U2C 3 [151 for example. A neutron diffraction experiment on
[ 11 D. Wohlleben and M.B. Maple, Rev. Sci. Inst. 42 ( t g ? i ) 1573. [2] B.T. Matthias, C.W. Chu, E. Corenzwit and D. Wohileben. Prec. Natl. Academy of Sciences 64 (1969) 459. [31 B.T. Matthias, Journal de Physique 32 (1971) C1-607. [41 M.B. Maple and D. Wohlleben, Phys. Letters 38A (|972) 351. [5] R.W. Cochrane and G.M. G~aham, Canadian J. Phys. 48 (1970) 264. [6] F.R. deBoer, C.F. Schinkel, J. Biesterbos and S. Proost, J. Appi. Phys. 40 (1969) 1049. [7] G.S. Knapp, I:.Y. I:radin and H.V. (;ulbert, J. Appi. Phyr,. 42 (1971) 1341. [81 K.tt.J. Buschow and H..I. v~,~oDaM, AlP Conference Proceedings =5, ed~. (~.D. Grailani. Jr. and .t.J. Rh~nc (Nc~ York, 1972) !4.64. [9] T.F. Smith, J.A. MS,Josh and I...P. Wohlfarth. Phy~,. Rev. Letters 27 (1971) ! 732. I101 J.G. ttuber, M.B. Maple, D. W,~hlh'bct: u_~d (,.S. K~'.,~pp. Solid Stale Commun. (in pre~) [11 [ S. Misawa, Physics Letters 32A (~ 97~1 5,~i. 112[ F.E. Hoare and J.C. Matthews. Prot,. Roy. Soc. (l_ortdov~,~ A212 (1952~ t3~;A216 (i953~' 502. [i3 i b. Wohlleben, Ph.l, "~tlesi!,. Unw. ofCalif~,rnia (1968~. unpublished. [141 W.E. Gardner and J. Penfo!d, Physics Letters 26A (t968~ 204. [ 15 ] J.-L. Boutard and C.-It. de Novion, Solid Staxe Commun 14 (1974) 181.