_{2−x }Co

_{ x }Si

_{2}

^{1,a)}, Madhumita Halder

^{1}, A. K. Rajarajan

^{1}, A. K. Nigam

^{2}and S. Banerjee

^{3}

### Abstract

We report magnetic properties and magnetocaloric effect in intermetallic compounds NdMn_{2−x }Co_{ x }Si_{2} (*x* = 0.2, 0.4, 0.6, 0.8, and 1). dc magnetization study shows that these compounds undergo a ferromagnetic-like transition at around 45 K. Neutron diffraction study at 5 K for the *x* = 0.2 sample shows a canted-ferromagnetic state at lower temperature (5 K), and a collinear antiferromagnetic state at higher temperature (above ∼50 K). At 5 K, the Nd moments are aligned along the crystallographic *c*-axis and the Mn moments are canted to the *c*-axis. At higher temperatures (50, 100, 200, and 300 K), Nd sublattice does not order but Mn sublattice orders antiferromagnetically. A magnetocaloric effect (MCE) is found with a magnetic entropy change of 14.4 and 12.4 J kg^{−1} K^{−1} for the *x* = 0.2 and 0.4 samples, respectively, at 47.5 K under a field variation of 50 kOe. Various interesting phenomena such as metamagnetictransitions and domain wall pinning have been observed, and their role in obtaining a large MCE and an inverse MCE, respectively, has been brought out. The hysteresis (in magnetic field dependent dc magnetization study) reduces significantly at temperatures near and above the magnetic transition temperature (*T* _{ C }), which makes these materials important for their practical applications in magnetic refrigeration around *T* _{ C }.

M.H. acknowledges the help provided by D. D. Buddhikot for the magnetization measurements, and A. Das and A. B. Shinde for the neutron diffraction data collection. M.H. also thanks Homi Bhabha National Institute, Department of Atomic Energy, India, for the fellowship.

I. INTRODUCTION

II. EXPERIMENTAL DETAILS

III. RESULTS AND DISCUSSION

A. X-ray diffraction study

B. dc magnetization study and magnetocaloric effect

C. Neutron diffraction study

IV. SUMMARY AND CONCLUSION

### Key Topics

- Antiferromagnetism
- 28.0
- Domain walls
- 18.0
- Ferromagnetism
- 16.0
- Magnetic fields
- 16.0
- Intermetallic compounds
- 14.0

## Figures

(a) X-ray diffraction patterns for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at room-temperature. The (*hkl*) values corresponding to Bragg peaks are marked. (b) Variation of lattice constants and unit cell volume with Co concentration.

(a) X-ray diffraction patterns for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at room-temperature. The (*hkl*) values corresponding to Bragg peaks are marked. (b) Variation of lattice constants and unit cell volume with Co concentration.

Temperature dependence of FC and ZFC magnetization *M* for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at 300 Oe applied field. Inset shows the enlarged view of the FC and ZFC magnetization for the *x* = 0.8 and 1 samples.

Temperature dependence of FC and ZFC magnetization *M* for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at 300 Oe applied field. Inset shows the enlarged view of the FC and ZFC magnetization for the *x* = 0.8 and 1 samples.

(a) *M* vs *H* curves over all the four quadrants for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at 5 K. (b) The variation of magnetization *M* _{ S } (at 80 kOe) and coercive field with Co concentration. The solid lines are guide to eye.

(a) *M* vs *H* curves over all the four quadrants for *x* = 0.2, 0.4, 0.6, 0.8, and 1 samples at 5 K. (b) The variation of magnetization *M* _{ S } (at 80 kOe) and coercive field with Co concentration. The solid lines are guide to eye.

Magnetization isotherms (a) and (b) at temperatures below *T* _{ C } and low fields for the *x* = 0.2 and 0.4 samples, respectively. Inset of (a) and (b) shows magnetization isotherms at various temperatures below and above *T* _{ C }. The isotherms are at an interval of 5 K.

Magnetization isotherms (a) and (b) at temperatures below *T* _{ C } and low fields for the *x* = 0.2 and 0.4 samples, respectively. Inset of (a) and (b) shows magnetization isotherms at various temperatures below and above *T* _{ C }. The isotherms are at an interval of 5 K.

Magnetic entropy change −Δ*S* _{ M } vs temperature for (a) *x* = 0.2 and (b) 0.4 samples at different values of applied fields. The inset of (a) and (b) shows the −Δ*S* _{ M } vs *T* curve for *x* = 0.2 and 0.4 samples, respectively, for Δ*H* = 50 kOe. The shaded area corresponds to the relative cooling power.

Magnetic entropy change −Δ*S* _{ M } vs temperature for (a) *x* = 0.2 and (b) 0.4 samples at different values of applied fields. The inset of (a) and (b) shows the −Δ*S* _{ M } vs *T* curve for *x* = 0.2 and 0.4 samples, respectively, for Δ*H* = 50 kOe. The shaded area corresponds to the relative cooling power.

(a) and (b) *M* vs *H* curves over all the four quadrants for *x* = 0.6 sample at different temperatures. Inset shows the variation of coercive field with temperature for *x* = 0.6 sample.

(a) and (b) *M* vs *H* curves over all the four quadrants for *x* = 0.6 sample at different temperatures. Inset shows the variation of coercive field with temperature for *x* = 0.6 sample.

*M* vs *H* curves over all the four quadrants for (a) *x* = 0.2, and (b) *x*= 0.8, samples at various temperatures.

*M* vs *H* curves over all the four quadrants for (a) *x* = 0.2, and (b) *x*= 0.8, samples at various temperatures.

(a), (b), and (c): Neutron diffraction patterns for the *x* = 0.2 sample at 5, 50, and 300 K, respectively. The open circles represent the observed patterns. The solid lines represent the Rietveld refined patterns. The difference between observed and calculated patterns is also shown at the bottom of each panel by solid lines. The vertical bars indicate the allowed Bragg peaks position for chemical (top row) and magnetic (bottom row) phases. (d) The temperature dependence of the antiferromagnetic (111) Bragg peak.

(a), (b), and (c): Neutron diffraction patterns for the *x* = 0.2 sample at 5, 50, and 300 K, respectively. The open circles represent the observed patterns. The solid lines represent the Rietveld refined patterns. The difference between observed and calculated patterns is also shown at the bottom of each panel by solid lines. The vertical bars indicate the allowed Bragg peaks position for chemical (top row) and magnetic (bottom row) phases. (d) The temperature dependence of the antiferromagnetic (111) Bragg peak.

(a) Variation of lattice constants and *d* _{Mn–Mn} with temperature for the *x* = 0.2 sample. The solid lines are guide to eye. (b) Temperature dependence of the full magnetic moment of Mn (oriented at an angle of +30(1)° (clockwise) and −30(1)° (counter clockwise) to the *c*-axis for two consecutive layers in the canted-ferromagnetic phase, and in a + − + − sequence along the *c*-axis in the type-I antiferromagnetic phase) for the *x* = 0.2 sample. The dotted line separates the canted-ferromagnetic (CFM) and type-I antiferromagnetic (AFM) regions.

(a) Variation of lattice constants and *d* _{Mn–Mn} with temperature for the *x* = 0.2 sample. The solid lines are guide to eye. (b) Temperature dependence of the full magnetic moment of Mn (oriented at an angle of +30(1)° (clockwise) and −30(1)° (counter clockwise) to the *c*-axis for two consecutive layers in the canted-ferromagnetic phase, and in a + − + − sequence along the *c*-axis in the type-I antiferromagnetic phase) for the *x* = 0.2 sample. The dotted line separates the canted-ferromagnetic (CFM) and type-I antiferromagnetic (AFM) regions.

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