Magnetic and Mssbauer Studies on GdCoB and DyCoB

1. Introduction

Intermetallic borides containing rareearth (R) and transitionmetal (T) elements, expressed as RCoB, have attracted considerable interest because of the coexistence of strong magnetic interactions and a rigid boron network. Among them, gadolinium cobalt boride (GdCoB) and dysprosium cobalt boride (DyCoB) are especially noteworthy. Both compounds crystallise in the orthorhombic TiNiSitype structure (space group Pnma) and display antiferromagnetic ordering at low temperatures, but subtle differences in their magnetic behaviour arise from the distinct 4f electron configurations of Gd (F) and Dy (H/).

This page summarises the main results obtained from magnetic susceptibility, magnetisation, heatcapacity, and ^57Fe Mssbauer spectroscopy studies carried out on polycrystalline GdCoB and DyCoB samples. The purpose is to illustrate how complementary techniques provide a consistent picture of the magnetic ground state, spinreorientation phenomena and the hyperfine interactions that are sensitive to the rareearth sublattice.

2. Sample preparation and structural characterisation

Highpurity elements (Gd/Dy, Co, B) were arcmelted in an argon atmosphere, turned over and remelted several times to ensure homogeneity. The resulting ingots were annealed at 900C for 120h and subsequently quenched. Powder Xray diffraction confirmed a single orthorhombic phase with lattice parameters:

• GdCoB: a = 6.846, b = 4.410, c = 7.049
• DyCoB: a = 6.824, b = 4.398, c = 7.032

3. Magnetic susceptibility and magnetisation

3.1 Temperature dependence

Zerofieldcooled (ZFC) and fieldcooled (FC) magnetic susceptibility () were measured in a 0.1T field from 2K to 300K. Both compounds obey a CurieWeiss law above 100K: \[ \chi = \frac{C}{T-\theta_{p}} \] The effective moments derived from the Curie constant are 7.94_B for GdCoB and 10.6_B for DyCoB, in close agreement with the freeion values of Gd and Dy, confirming that Co carries a negligible magnetic moment.

The Weiss temperatures are modestly negative (_p 12K for GdCoB, 9K for DyCoB), indicating predominant antiferromagnetic (AFM) exchange.

3.2 Ordering temperatures

A clear cusp in (T) marks the Nel temperature (T_N). For GdCoB, T_N = 18.5K, whereas DyCoB orders at a slightly lower temperature, T_N = 16.2K. The lower T_N of DyCoB is ascribed to the stronger crystalfield (CF) anisotropy of Dy, which reduces the effective exchange field.

3.3 Fielddependent magnetisation

Isothermal M(H) curves recorded at 2K display a linear increase up to about 2T, followed by a spinflop transition. In GdCoB the critical field is H_SF 3.1T, with a subsequent gradual approach to saturation (M_sat 7_B/Gd). DyCoB shows a more pronounced hysteresis and a higher critical field H_SF 4.5T, reflecting the larger magnetocrystalline anisotropy. Above 7T both compounds display a metamagnetic transition to a fieldpolarised state.

Figure 1: Representative M(H) curves at 2K. The arrows indicate spinflop fields.

4. Heatcapacity measurements

Specificheat (C_p) data recorded between 2K and 50K reveal sharp type anomalies at the magnetic ordering temperatures, confirming bulk AFM transitions. The magnetic entropy change S_m calculated from C_p(T) integrates to Rln(2J+1) for each ion (17.3JmolK for Gd, 23.0JmolK for Dy), indicating that the full 4f multiplet participates in the ordering.

A second, broader feature appears around 68K in DyCoB, which is absent in GdCoB. This lowtemperature anomaly is assigned to a crystalfield induced splitting of the Dy groundstate doublet, leading to a Schottkytype contribution.

5. ^57Fe Mssbauer spectroscopy

To probe the hyperfine field at the transitionmetal site, a small amount (2at%) of ^57Fe was substituted for Co. Mssbauer spectra were collected at temperatures ranging from 4.2K to 30K.

5.1 Paramagnetic region

Above T_N the spectra consist of a single narrow line with isomer shift 0.10mms, typical for Fe in a nonmagnetic metallic environment. No quadrupole splitting is resolved, reflecting the nearly cubic symmetry of the Fe site in the TiNiSi framework.

5.2 Magnetically ordered region

Below T_N a sextet appears, signalling the development of an internal magnetic hyperfine field (B_hf) at the Fe nucleus. For GdCoB, B_hf grows rapidly on cooling, reaching 31T at 4.2K. In DyCoB the maximum B_hf is slightly lower, 28T, consistent with the reduced ordered moment of Dy.

The temperature dependence of B_hf follows the Brillouintype orderparameter behaviour: \[ B_{\mathrm{hf}}(T)=B_0\left[1-\left(\frac{T}{T_{N}}\right)^{\alpha}\right]^{\beta} \] with 0.34 for GdCoB and 0.30 for DyCoB, values close to the threedimensional Heisenberg exponent, confirming that longrange AFM order dominates the lowtemperature phase.

5.3 Spinreorientation evidence

In DyCoB, an additional linebroadening occurs between 9K and 13K, which can be fitted by assuming a distribution of hyperfine fields. This is interpreted as a gradual spinreorientation transition, also hinted at by the fielddependent magnetisation data. No comparable effect is observed for GdCoB, where the Gd 4f spin is essentially isotropic.

Figure 2: Representative ^57Fe Mssbauer spectra at selected temperatures.

6. Discussion

The combined magnetic and Mssbauer investigations reveal that GdCoB and DyCoB share a common antiferromagnetic framework driven by indirect RR exchange mediated by CoB conduction electrons. However, the magnitude of the ordered moment, the critical fields for spinflop, and the lowtemperature hyperfine behaviour differ markedly because of the contrasting 4f orbital character.

• GdCoB: The halffilled 4f shell of Gd yields a nearly isotropic Sstate (L=0). Consequently, the magnetic anisotropy is weak, the spinflop occurs at lower fields, and the hyperfine field at Fe reaches a larger value, reflecting a more robust exchange field.
• DyCoB: The strong spinorbit coupling and large orbital contribution (L=5) produce pronounced crystalfield effects. These manifest as a reduced Nel temperature, higher spinflop field, a secondary Schottky peak in C_p, and a distribution of hyperfine fields indicative of a temperaturedependent reorientation of Dy moments.

The Mssbauer data also underscore the efficiency of Fe as a local probe of the 4f magnetic ordering even when present in minute concentrations. The linear correlation between B_hf and the sublattice magnetisation enables a quantitative assessment of the order parameter, complementing bulk magnetisation and calorimetry.

7. Conclusions

Both GdCoB and DyCoB order antiferromagnetically at low temperatures (18K and 16K respectively).
Magnetic susceptibility and heatcapacity confirm bulk ordering and reveal crystalfield effects that are pronounced in DyCoB.
Magnetisation measurements identify spinflop transitions at 3T (GdCoB) and 4.5T (DyCoB) and a metamagnetic polarisation above 7T.
^57Fe Mssbauer spectroscopy detects sizable hyperfine fields (31T for GdCoB, 28T for DyCoB) whose temperature dependence follows a Heisenbergtype order parameter.
A lowtemperature hyperfinefield distribution in DyCoB signals a spinreorientation process absent in GdCoB.

The study demonstrates how magnetic and Mssbauer techniques together provide a detailed microscopic understanding of rareearth transitionmetal borides, and it establishes a solid basis for further investigations of anisotropic magnetic phenomena in related RCoB families.