Magnetic properties of nickel and platinum quaternary borocarbides

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1 Hyperfine Interactions 104 (1997) Magnetic properties of nickel and platinum quaternary borocarbides L. Cristofolini a,a.lappas a,k.prassides a,k.vavekis a and M. Buchgeister b a School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, UK b Institut für Festkörper- und Werkstofforschung Dresden e.v., D Dresden, Germany The zero-field µ + SR technique is employed to study the magnetic properties of the nonsuperconducting borocarbides Sm 1 xlu xni 2B 2C(x=0.0, 0.2) and NdPt 1.5Au 0.6B 2C. Coherent ordering of the Sm electronic moments appears below T N = 9.86(3) and 4.15(3) Kforthex=0.0 and 0.2 compositions, respectively. No long range order of the Nd 3+ moments is found in NdPt 1.5Au 0.6B 2C down to 80 mk. Freezing of the electronic moments into a spin-glass state occurs at T f 1.2 K. A stretched exponential decay of the µ + spin polarisation is observed above T f with the exponent β smoothly decreasing on approaching T f from above, reminiscent of the behaviour in other spin glasses. 1. Introduction High-T c superconductivity in copper oxide materials has dominated superconductivity research in recent years. However, the discovery of critical temperatures as high as 23 K in a new family of quaternary borocarbides (Ln (Ni, Pd, Pt) B C) [1] has attracted considerable interest. In the layered LnNi 2 B 2 C(Ln=lanthanide) family, a magnetic pair-breaking effect associated with the lanthanide ions is clearly present with superconductivity found only for the smaller rare earth ions. However, when Ni is replaced by the non-magnetic Pt, the phase stability of the LnPt 2 B 2 C family decreases with decreasing rare-earth size, and superconductivity is now encountered only for the larger rare earths [2]. In the present work, we employ zero-field (ZF) and longitudinal-field (LF) µ + SR measurements to study the magnetic properties of the non-superconducting Sm 1 x Lu x Ni 2 B 2 C(x=0 and 0.2) and NdPt 1.5 Au 0.6 B 2 C (partial substitution by Au improves the phase purity of the system) phases. SmNi 2 B 2 C shows an antiferromagnetic (AF) ordering transition at 9.9 K, while gradual replacement of Sm 3+ by Lu 3+ leads to the suppression of AF and recovery of superconductivity at 6Kforx=0.6. The x = 0.2 composition exhibits a reduced AF ordering temperature ( 4.2 K)and a substantially increased width of the local field distribution. In contrast, measurements J.C. Baltzer AG, Science Publishers

2 62 L. Cristofolini et al. / Magnetic properties of nickel and platinum borocarbides to mk temperatures did not reveal any evidence for long range order of the Nd 3+ electronic moments in NdPt 1.5 Au 0.6 B 2 C; instead, there is a very pronounced increase in the depolarisation rate below 2.0 K that evidences the freezing of the Nd 3+ moments into a disordered random spin arrangement. 2. Experimental The Sm 1 x Lu x Ni 2 B 2 CandNdPt 1.5 Au 0.6 B 2 C samples were prepared from stoichiometric quantities of the elements using a standard arc furnace technique under Ar. Phase composition was checked by X-ray diffraction, revealing impurity phases to a level of < 5%. Additional characterisation was performed with a SQUID magnetometer, while the NdPt 1.5 Au 0.6 B 2 C sample was also studied by neutron diffraction at the Institut Laue Langevin, Grenoble, France down to 2 K. ZF and LF µ + SR data were collected at the Paul Scherrer Institute, Villigen, Switzerland, with both the General Purpose Spectrometer and the Low Temperature Facility using surface muons on the µ + SR dedicated beam line on the PSI 600 MeV proton accelerator. 3. Results and discussion 3.1. Nickel borocarbides, Sm 1 x Lu x Ni 2 B 2 C (x = 0.0, 0.2) In fig. 1, we show the ZF time-dependent µ + SR spectra of Sm 0.8 Lu 0.2 Ni 2 B 2 Cat temperatures of and 10.2 K. In both compositions, as the temperature is lowered, we observe that the fluctuations of the Sm electronic moments gradually slow down, a behaviour manifested in the temperature evolution of the relaxation rate of the ZF polarization which diverges at the freezing temperature, T N. In accord with the ZF data on SmNi 2 B 2 C [3], coherent ordering of the Sm 3+ magnetic moments is also found at low temperatures for Sm 0.8 Lu 0.2 Ni 2 B 2 C, as evidenced by the presence of a ZF spontaneous muon precession. The temperature dependence of the precession frequency, ν µ, is shown in fig. 2 for both samples. The lines through the data points represent fits in the range 0.6T N < T<T N to the critical law ν = ν 0 (1 T/T N ) β, with ν 0 = 16.2(2) and 14.9(6) MHz, β = 0.18(1) and 0.14(2), and T N = 9.86(3) and 4.15(3) KforSmNi 2 B 2 Cand Sm 0.8 Lu 0.2 Ni 2 B 2 C, respectively. The critical exponents β compare well with the prediction of 2D Ising models (β = 1/8) while ν µ remains essentially unchanged on doping. However, Lu doping affects substantially the width of the quasi-static (as revealed by LF measurements) field distribution at the base temperature which is reflected in the increase of the damping rate of the precessing component from 0.54(2) (x = 0.0, T=3.3K)to9.3(8)µs 1 (x=0.2,t=0.1k).

3 L. Cristofolini et al. / Magnetic properties of nickel and platinum borocarbides 63 Fig. 1. Time evolution of the ZF µ + spin polarisation in Sm 0.8Lu 0.2Ni 2B 2CatT=0.100 and 10.2 K. Fig. 2. Temperature dependence of the ZF muon precession frequency, ν µ, and the local field at the muon site, B µ,forsmni 2B 2CandSm 0.8Lu 0.2Ni 2B 2C. The solid lines are fits of the data to critical laws as discussed in the text Platinum borocarbides, NdPt 1.5 Au 0.6 B 2 C In fig. 3, we show the ZF time-dependent µ + SR spectra of NdPt 1.5 Au 0.6 B 2 Cat and 9.9 K. The ZF µ + spin polarisation at 2.0 K and above was best described by a stretched exponential relaxation function: P µ (t) =Aexp( (λt) β ). The time constant λ shows little variation at high temperature, but on cooling it rapidly increases and diverges below about 2.0 K (fig. 4), implying a rapid slowing down of the fluctuating random moments. The principally dynamic character of the depolarization down to 2.0 K was confirmed by collecting LF (up to 10.0 kg) data (1)

4 64 L. Cristofolini et al. / Magnetic properties of nickel and platinum borocarbides Fig. 3. Time evolution of the ZF µ + spin polarisation in NdPt 1.5Au 0.6B 2CatT=0.100 and 9.9 K. The lines correspond to the fits described in the text. Fig. 4. Temperature evolution of the relaxation rate, λ in NdPt 1.5Au 0.6B 2C. Filled and open circles are GPS and LTF data in the paramagnetic regime; diamonds are data for the spin glass phase. Inset: Evolution of the static depolarization time constant, s in the SG phase. Lines are guides to the eye. which were also fitted by the same power-law function. In addition, the exponent β decreases smoothly from 1.32(5) at 300 K (implying a relaxation partly due to static nuclear dipolar fields and partly to rapidly fluctuating paramagnetic moments) to 0.44(1) at 2.0 K (fig. 5). This behaviour resembles closely that of Ising spin glasses above their freezing temperature [4].

5 L. Cristofolini et al. / Magnetic properties of nickel and platinum borocarbides 65 Fig. 5. The stretched exponential coefficient β as a function of temperature in NdPt 1.5Au 0.6B 2C. At 1.2 K and below, the data were best fitted with a Uemura spin glass function which includes coexisting static and dynamic random local fields: [ 1 P µ (t)=a 3 exp( λt) + 2 ( ( s t) 2 ( ) 1 )exp ] λt +( s t) 3 2. (2) λt +( s t) 2 The dynamic depolarisation rate, λ (fig. 4), diverges as T approaches the freezing temperature from below. The static contribution, s, to the depolarisation is shown in the inset of fig. 4 and corresponds to a static local field spread in which the HWHM of each cartesian component is 270 G at 80 mk. Assuming that in the paramagnetic regime the field distribution width is the same as that measured at base temperature ( s 24 µs 1 ), we can extract the temperature evolution of the correlation time (τ c ) of the fluctuating magnetic moments. τ c increases from 5.3(7) sat T=9.9Kto2.0(5) at T = 2.0 K. Near the freezing temperature, the evolution of the correlation time is satisfactorily described by a simple Arrhenius expression which results in an activation energy, E a /k B = 3.1(5) K and a pre-exponential factor, τ 0 = 4.2(4) s. We note that in agreement with the present µ + SR data, neutron diffraction measurements on the same sample showed no evidence of magnetic Bragg scattering down to 2 K [5]. 4. Conclusions In conclusion, muon spectroscopy has been used to show the appearance of ordered magnetic moments at the Sm sites in the borocarbide phases, SmNi 2 B 2 Cand Sm 0.8 Lu 0.2 Ni 2 B 2 C. The local field, B µ, at the muon site is essentially unaffected by

6 66 L. Cristofolini et al. / Magnetic properties of nickel and platinum borocarbides Lu substitution (roughly 8% reduction upon 20% Lu doping), but the local field spread increases drastically (by a factor of 20). Similar measurements on NdPt 1.5 Au 0.6 B 2 C reveal a different behaviour with no onset of long range AF order down to 80 mk. Instead, a spin glass phase is observed below 1.2 K (with a static random local field spread of 270 G). On approaching T f from above, the µ + -spin dynamics are described by a stretched exponential decay with rapid growth of the correlation times, τ c, reminiscent of the spin freezing process in Ising spin glasses. Acknowledgement We thank EPSRC for financial support and the Paul Scherrer Institute for provision of beam time. References [1] R. Nagarajan et al., Phys. Rev. Lett. 72 (1994) 274; R.J. Cava et al., Nature 367 (1994) 252. [2] R.J. Cava et al., Phys. Rev. B 49 (1994) [3] K. Prassides et al., Europhys. Lett. 29 (1995) 641. [4] I.A. Campbell et al., Phys. Rev. Lett 72 (1994) 1291; A. Lappas et al., J. Magn. Magn. Mater (1995) [5] L. Cristofolini et al., unpublished results.