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Magnetic order of the heavy-fermion URu2Si2 in a field of 12 T
ELSEVIER
Physica B 234-236 (1997) 692-693
Magnetic order of the heavy-fermion URu2Si 2 in a field of 12 T o
a
N . H . v a n D i j k a, F. B o u r d a r o t a'*, B. F a k , F. L a p i e r r e a, L.P. R e g n a u l t a, P. B u r l e t a, J. B o s s y u, N. P y k a b, A.A. M e n o v s k y ~ aD~partement de Recherche Fondamentale sur la Matibre Condens~e, SPSMS, MDN, CEA/Grenoble, 17 rue des Martyrs, 38 054 Grenoble Cddex 9, France bILL, B.P. 156, 38 042 Grenoble, France Van der Waals-Zeeman Laboratory, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Abstract
We have performed elastic and inelastic neutron-scattering measurements on the heavy-fermion superconductor URu2Si2 in a magnetic field of 12 T along the c-axis of the tetragonal crystal structure. The small ordered moment and the energy gap in the magnetic excitation spectrum show a comparable transition temperature in a magnetic field of 12 T. This is in disagreement with recent predictions of a decoupling of the ordered moment and the energy gap in high magnetic fields.
Keywords: URuzSi2; Heavy fermions; Antiferromagnetism;Magnetic excitations
The heavy-fermion superconductor URu2Si2 shows unusual low-temperature properties with antiferromagnetic order at TN = 17.5 K and superconductivity below To = 1.2 K. Specific-heat measurements [1] show a large 2 anomaly at TN, indicating the formation of an energy gap in the magnetic excitation spectrum. Early neutron-scattering experiments [2] revealed an antiferromagnetically ordered structure with a tiny ordered moment of 0.03 __+0.011aa/U-atom, oriented along the c-axis of the tetragonal crystal structure. Polarized neutron-diffraction measurements [3] have confirmed the dipolar character of the ordered moment. The excitation spectrum determined by inelastic neutron scattering [4] is strongly anisotropic and has a minimum of 2.5 meV at Q = (0 0 1), or equivalently (1 0 0). The unusual combi* Corresponding author.
nation of a large energy gap in the magnetic excitation spectrum and a tiny ordered moment has led to considerable interest in the nature of the magnetic order. Recent neutron-scattering measurements I-5] in magnetic fields up to 8 T (BIIc) showed a relatively strong suppression of the ordered moment with an extrapolated critical field of 14.5 T at T = 4.5 K. In contrast, magnetoresistance measurements in magnetic fields up to 25 T 1-6] showed a relatively weak suppression of the magnetic gap with a critical field of 40 T (BIIc) at low temperatures. This critical field corresponds to the metamagnetic transition B*, where the heavyfermion state is suppressed [7]. The discrepancy in the extrapolated critical fields for the ordered moment and the magnetic gap has led to the suggestion that the ordered moment decouples from the magnetic gap in applied magnetic fields ['6]. In a magnetic field of 12 T the critical temperature of
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 1 0 9 4 - 0
N.H. van Dijk et al. / Physica B 234-236 (1997) 692 693 3,0
....
, ....
, ....
= ....
, ....
-~-
4000
, ....
O R u 2 S ia
2.5
"~ -- "~\z
a -- (1 0 0 )
2.0
a5oo { B -= 1 2 T
1.5
o
tu 1.0
3000
o.5
~
0.0
.... 0
' .... 5
' .... 10
t
'''['~ 15 T (K)
.... 20
2500
= .... 25
30
Fig. 1. Energy gap in the magnetic excitation spectrum (O) and magnetic Bragg peak intensity ( t ) of URu2Si2 at Q = (1 0 0) as function of temperature in a magnetic field of 12 T (Bllc). The background level for the Bragg peak (solid line) is about 2800 counts/(10 min), as determined above TN. The dashed and solid lines are guides to the eye.
the magnetic gap is 16.5 K, while the critical temperature of the ordered moment is estimated at 9 K. We have performed elastic and inelastic neutronscattering measurements on a single crystalline URu2Si2 sample with a mass of 5 g, which was grown in a tri-arc furnace with the Czochralski method and annealed for 8 days at a temperature of 950°C. The measurements were performed on the IN8 triple-axis spectrometer at the ILL in Grenoble. We have used a 40'-40'-40'-40' collimation, a final wave vector of 2.662 1 and a 7 cm thick PG(0 0 2) filter, placed after the sample, in order to reduce higher-order contributions. We measured both the ordered moment and the magnetic gap at Q = (1 0 0) in a magnetic field of 12 T (Nilc) in order to investigate the predicted decoupling of the critical temperatures. The measured magnetic gap and the magnetic Bragg peak intensity at Q = (1 0 0) are shown in Fig. 1 as functions of temperature in a magnetic field of 12 T. It is clear that the critical temperatures
693
of the magnetic gap and the ordered moment are of the same order of magnitude. The weak field dependence of the critical temperature for the magnetic gap is in agreement with the magnetoresistance measurements [6]. The observation that the N6el temperature, determined from the magnetic Bragg peaks at 12 T, is close to its value in zero field does not support a substantial decoupling of the ordered moment and the magnetic gap in magnetic fields up to 12 T. Apparently, the ordered moment shows an unusual field dependence which is relatively strong at low fields and relatively weak at higher fields. This suggests that the ordered moment may be governed by two different energy scales. An unusual temperature dependence of the ordered moment has earlier been reported for unannealed samples [8]. In conclusion, the ordered moment remains coupled to the energy gap in the magnetic excitation spectrum in magnetic fields up to 12 T (Bllc), while their field dependences are rather different. The discrepancy between the large energy gap in the magnetic excitation spectrum and the tiny ordered moment imposes serious questions whether the dipolar ordered moment is the (main) order parameter which drives the transition. One of us (NHvD) acknowledges a TMR grant of the European Community (No.ERB4001GT950831).
References Ill [2] [3] [4] [5] [6] [7] [8]
T.T.M. Palstra et al., Phys. Rev. Lett. 55 (1985) 2727. C. Broholm et al., Phys. Rev. Lett. 58 (1987) 1467. M.B. Walker et al., Phys. Rev. Lett. 71 (1993) 2630. C. Broholm et al., Phys. Rev. B 43 (1991) 12 809. T.E. Mason et al., J. Phys.: Condens. Matter 7 (1995) 5089. S.A.M. Mentink et al., Phys. Rev. B 53 (1996) R6014. A. de Visser et al., Solid State Commun. 64 (1987) 527. B. F~k et al., J. Magn. Magn. Mater. 154 (1996) 339.
Physica B 234-236 (1997) 692-693
Magnetic order of the heavy-fermion URu2Si 2 in a field of 12 T o
a
N . H . v a n D i j k a, F. B o u r d a r o t a'*, B. F a k , F. L a p i e r r e a, L.P. R e g n a u l t a, P. B u r l e t a, J. B o s s y u, N. P y k a b, A.A. M e n o v s k y ~ aD~partement de Recherche Fondamentale sur la Matibre Condens~e, SPSMS, MDN, CEA/Grenoble, 17 rue des Martyrs, 38 054 Grenoble Cddex 9, France bILL, B.P. 156, 38 042 Grenoble, France Van der Waals-Zeeman Laboratory, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Abstract
We have performed elastic and inelastic neutron-scattering measurements on the heavy-fermion superconductor URu2Si2 in a magnetic field of 12 T along the c-axis of the tetragonal crystal structure. The small ordered moment and the energy gap in the magnetic excitation spectrum show a comparable transition temperature in a magnetic field of 12 T. This is in disagreement with recent predictions of a decoupling of the ordered moment and the energy gap in high magnetic fields.
Keywords: URuzSi2; Heavy fermions; Antiferromagnetism;Magnetic excitations
The heavy-fermion superconductor URu2Si2 shows unusual low-temperature properties with antiferromagnetic order at TN = 17.5 K and superconductivity below To = 1.2 K. Specific-heat measurements [1] show a large 2 anomaly at TN, indicating the formation of an energy gap in the magnetic excitation spectrum. Early neutron-scattering experiments [2] revealed an antiferromagnetically ordered structure with a tiny ordered moment of 0.03 __+0.011aa/U-atom, oriented along the c-axis of the tetragonal crystal structure. Polarized neutron-diffraction measurements [3] have confirmed the dipolar character of the ordered moment. The excitation spectrum determined by inelastic neutron scattering [4] is strongly anisotropic and has a minimum of 2.5 meV at Q = (0 0 1), or equivalently (1 0 0). The unusual combi* Corresponding author.
nation of a large energy gap in the magnetic excitation spectrum and a tiny ordered moment has led to considerable interest in the nature of the magnetic order. Recent neutron-scattering measurements I-5] in magnetic fields up to 8 T (BIIc) showed a relatively strong suppression of the ordered moment with an extrapolated critical field of 14.5 T at T = 4.5 K. In contrast, magnetoresistance measurements in magnetic fields up to 25 T 1-6] showed a relatively weak suppression of the magnetic gap with a critical field of 40 T (BIIc) at low temperatures. This critical field corresponds to the metamagnetic transition B*, where the heavyfermion state is suppressed [7]. The discrepancy in the extrapolated critical fields for the ordered moment and the magnetic gap has led to the suggestion that the ordered moment decouples from the magnetic gap in applied magnetic fields ['6]. In a magnetic field of 12 T the critical temperature of
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 1 0 9 4 - 0
N.H. van Dijk et al. / Physica B 234-236 (1997) 692 693 3,0
....
, ....
, ....
= ....
, ....
-~-
4000
, ....
O R u 2 S ia
2.5
"~ -- "~\z
a -- (1 0 0 )
2.0
a5oo { B -= 1 2 T
1.5
o
tu 1.0
3000
o.5
~
0.0
.... 0
' .... 5
' .... 10
t
'''['~ 15 T (K)
.... 20
2500
= .... 25
30
Fig. 1. Energy gap in the magnetic excitation spectrum (O) and magnetic Bragg peak intensity ( t ) of URu2Si2 at Q = (1 0 0) as function of temperature in a magnetic field of 12 T (Bllc). The background level for the Bragg peak (solid line) is about 2800 counts/(10 min), as determined above TN. The dashed and solid lines are guides to the eye.
the magnetic gap is 16.5 K, while the critical temperature of the ordered moment is estimated at 9 K. We have performed elastic and inelastic neutronscattering measurements on a single crystalline URu2Si2 sample with a mass of 5 g, which was grown in a tri-arc furnace with the Czochralski method and annealed for 8 days at a temperature of 950°C. The measurements were performed on the IN8 triple-axis spectrometer at the ILL in Grenoble. We have used a 40'-40'-40'-40' collimation, a final wave vector of 2.662 1 and a 7 cm thick PG(0 0 2) filter, placed after the sample, in order to reduce higher-order contributions. We measured both the ordered moment and the magnetic gap at Q = (1 0 0) in a magnetic field of 12 T (Nilc) in order to investigate the predicted decoupling of the critical temperatures. The measured magnetic gap and the magnetic Bragg peak intensity at Q = (1 0 0) are shown in Fig. 1 as functions of temperature in a magnetic field of 12 T. It is clear that the critical temperatures
693
of the magnetic gap and the ordered moment are of the same order of magnitude. The weak field dependence of the critical temperature for the magnetic gap is in agreement with the magnetoresistance measurements [6]. The observation that the N6el temperature, determined from the magnetic Bragg peaks at 12 T, is close to its value in zero field does not support a substantial decoupling of the ordered moment and the magnetic gap in magnetic fields up to 12 T. Apparently, the ordered moment shows an unusual field dependence which is relatively strong at low fields and relatively weak at higher fields. This suggests that the ordered moment may be governed by two different energy scales. An unusual temperature dependence of the ordered moment has earlier been reported for unannealed samples [8]. In conclusion, the ordered moment remains coupled to the energy gap in the magnetic excitation spectrum in magnetic fields up to 12 T (Bllc), while their field dependences are rather different. The discrepancy between the large energy gap in the magnetic excitation spectrum and the tiny ordered moment imposes serious questions whether the dipolar ordered moment is the (main) order parameter which drives the transition. One of us (NHvD) acknowledges a TMR grant of the European Community (No.ERB4001GT950831).
References Ill [2] [3] [4] [5] [6] [7] [8]
T.T.M. Palstra et al., Phys. Rev. Lett. 55 (1985) 2727. C. Broholm et al., Phys. Rev. Lett. 58 (1987) 1467. M.B. Walker et al., Phys. Rev. Lett. 71 (1993) 2630. C. Broholm et al., Phys. Rev. B 43 (1991) 12 809. T.E. Mason et al., J. Phys.: Condens. Matter 7 (1995) 5089. S.A.M. Mentink et al., Phys. Rev. B 53 (1996) R6014. A. de Visser et al., Solid State Commun. 64 (1987) 527. B. F~k et al., J. Magn. Magn. Mater. 154 (1996) 339.