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Crystal Growth and Anisotropic Magnetic Properties of RAg$_2$Ge$_2$ (R = Pr, Nd and Sm) Single Crystals

Devang A. Joshi, R. Nagalakshmi, R. Kulkarni, S. K. Dhar, A. Thamizhavel

arXiv:0808.2826v1cond-mat.str-el

TL;DR

This paper addresses the limited single-crystal evidence on anisotropic magnetism in RAg2Ge2 compounds. It grows Pr, Nd and Sm single crystals using Ag:Ge self-flux and measures their magnetic susceptibility and magnetization. All three compounds order antiferromagnetically, with composition-dependent easy axes and two metamagnetic transitions in NdAg2Ge2.

  • Problem

    The anisotropic magnetic properties of RAg2Ge2 compounds had not been thoroughly investigated using single crystals, while some polycrystalline ordering results remained uncertain.

    Source: §1 · p. 1

  • Method

    Single crystals of PrAg2Ge2, NdAg2Ge2 and SmAg2Ge2 were grown by high-temperature solution growth using an Ag:Ge eutectic flux and characterized magnetically.

    Source: §2 · p. 1

  • Results

    All three compounds order antiferromagnetically; the easy axis is [100] for PrAg2Ge2 and NdAg2Ge2, [001] for SmAg2Ge2, and NdAg2Ge2 shows transitions at 1.25 T and 3.56 T along [100].

    Source: §3 · pp. 2–3, §4 · p. 4

  • Takeaways & Limitations

    Single-crystal measurements establish composition-dependent anisotropic magnetism across RAg2Ge2 and confirm antiferromagnetic ordering in the studied compounds.

    Source: §3 · pp. 2–3, §4 · p. 4

    For SmAg2Ge2, measurements up to 6 T did not confirm whether a possible high-field metamagnetic transition occurs along [001].

    Source: §3 · p. 4

Abstract

from arXiv · show

We report the single crystal growth and anisotropic magnetic properties of the tetragonal RAg$_2$Ge$_2$ (R = Pr, Nd and Sm) compounds which crystallize in the ThCr$_2$Si$_2$ type crystal structure with the space group \textit{I4/mmm}. The single crystals of RAg$_2$Ge$_2$ (R = Pr, Nd and Sm) were grown by self-flux method using Ag:Ge binary alloy as flux. From the magnetic studies on single crystalline samples we have found that PrAg$_2$Ge$_2$ and NdAg$_2$Ge$_2$ order antiferromagnetically at 12 K and 2 K respectively, thus corroborating the earlier polycrystalline results. SmAg$_2$Ge$_2$ also orders antiferromagnetically at 9.2 K. The magnetic susceptibility and magnetization show a large anisotropy and the easy axis of magnetization for PrAg$_2$Ge$_2$ and NdAg$_2$Ge$_2$ is along the [100] direction where as it changes to [001] direction for SmAg$_2$Ge$_2$. Two metamagnetic transitions were observed in NdAg$_2$Ge$_2$ at $H_{\rm m1}$ = 1.25 T and $H_{\rm m2}$ =3.56 T for the field parallel to [100] direction where as the magnetization along [001] direction was linear indicating the hard axis of magnetization.

1. Introduction

RAg2Ge2 compounds belong to a widely studied structural family with notable magnetic phenomena, but the Pr, Nd and Sm members had not been thoroughly investigated. This work grows single crystals to examine their anisotropic magnetism and clarify earlier polycrystalline findings.

1. Introduction

  • The RAg2Ge2 compounds had not been investigated thoroughly despite extensive study of related ThCr2Si2-type materials.The broader family is studied for magnetic properties, heavy-fermion superconductivity, pressure-induced superconductivity, and metamagnetic transitions.Source: §1 · p. 1
  • Earlier polycrystalline studies reported antiferromagnetism in NdAg2Ge2 and an unconfirmed 12 K magnetic ordering in PrAg2Ge2.The single-crystal study was designed to investigate anisotropy and confirm these results.Source: §1 · p. 1
  • Single crystals of RAg2Ge2 with R = Pr, Nd and Sm were successfully grown to enable anisotropic magnetic measurements.Source: §1 · p. 1

2. Experimental Details

Because RAg2Ge2 melts incongruently, the crystals were grown by high-temperature solution growth using an Ag:Ge eutectic flux. The process used controlled homogenization, slow cooling, centrifugation, crystallographic orientation, and magnetic characterization.

2. Experimental Details

  • An Ag:Ge alloy with 75.5:24.5 composition and a 651 °C eutectic point was selected as the flux because direct-melt growth was precluded.The constituent elements had high melting points, and a fourth-element flux could introduce a new phase.Source: §2 · p. 1
  • The charge was homogenized at 1050 °C for 48 hours, cooled to the eutectic temperature over three weeks, and centrifuged at 750 °C to remove the crystals.The starting-material ratio was 1:16.25:6.75, and the crystals were typically 5 × 3 × 0.4 mm3 platelets.Source: §2 · p. 2
  • Laue diffraction oriented the tetragonal crystals, which were cut along principal directions for susceptibility and magnetization measurements from 1.8–300 K and up to 12 T.Measurements used SQUID magnetometry for fields up to 7 T and VSM measurements up to 12 T.Source: §2 · p. 2

3. Results and Discussion

Single-crystal measurements confirm phase-pure RAg2Ge2 samples and reveal strongly anisotropic magnetic behavior across Pr, Nd, and Sm compounds. PrAg2Ge2 and NdAg2Ge2 favor [100], whereas SmAg2Ge2 shows distinctive [001]-axis ordering and field response.

Sample characterization

  • Powder diffraction and EDAX confirm phase-pure, ThCr2Si2-type SmAg2Ge2 with lattice constants a = 4.226(2) Å and c = 11.045(8) Å.The refinement places Sm, Ag, and Ge at the 2a, 4d, and 4e sites, respectively.Source: §3 · p. 2

PrAg2Ge2

  • PrAg2Ge2 has [100] as its easy magnetization axis, with a susceptibility anomaly at TN = 12 K and complex behavior below ordering.The [100] susceptibility rises more rapidly at low temperature, while the [001] response drops suddenly at TN.Source: §3 · p. 2
  • PrAg2Ge2 magnetization remains nearly linear, reaching 1.17 µB/Pr at 8 T along [100] and 0.39 µB/Pr at 12 T along [001].Both values remain below the Pr saturation moment of 3.2 µB, indicating that higher fields are needed for saturation.Source: §3 · pp. 2–3

NdAg2Ge2

  • NdAg2Ge2 orders antiferromagnetically at 2 K and has [100] as its easy axis, with µeff = 3.62 µB/Nd indicating trivalent Nd.The susceptibility along [100] exceeds that along [001].Source: §3 · p. 3
  • NdAg2Ge2 exhibits metamagnetic transitions at Hm1 = 1.25 T and Hm2 = 3.56 T along [100], reaching 2.1 µB/Nd at 12 T.Along [001], magnetization is anomaly-free up to 12 T and reaches 1.26 µB/Nd, identifying [001] as the hard axis.Source: §3 · p. 3

SmAg2Ge2

  • SmAg2Ge2 orders antiferromagnetically at TN = 9.2 K along [001], while its 2 K magnetization shows no anomaly up to 6 T.The authors suggest a possible high-field spin-flop or spin-flip transition along [001], but state that high-field measurements are needed; measured moments are 0.018 µB/Sm along [100] and 0.008 µB/Sm along [001].Source: §3 · pp. 3–4

4. Summary and Conclusion

Single-crystal measurements establish antiferromagnetic order and strong directional dependence across PrAg2Ge2, NdAg2Ge2, and SmAg2Ge2. The easy axis is [100] for Pr and Nd but [001] for Sm, while Pr ordering does not follow usual de Gennes scaling.

  • All three RAg2Ge2 compounds order antiferromagnetically, with easy-axis magnetization along [100] for Pr and Nd and [001] for Sm.Source: §4 · p. 4
  • PrAg2Ge2 orders at 12 K, above NdAg2Ge2 at 2 K and close to SmAg2Ge2 at 10 K.Source: §4 · p. 4
  • The ordering temperatures do not obey usual de Gennes scaling, with crystal-field effects proposed as a possible explanation for PrAg2Ge2.Source: §4 · p. 4
  • Single-crystal data clearly establish antiferromagnetic ordering in PrAg2Ge2, which was not discernible in earlier neutron scattering on polycrystalline samples.Source: §4 · p. 4
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