Abstract
In this work the magnetic and superconducting properties of HoNi2B2C were investigated by using powder neutron diffraction and the specific heat and upper critical field (Hc2) measurements as a function of temperature. Below T=8 K, three distinct anomalies at the temperatures TN=5.2 K, TH1=5.6 K and TM=6.0 K were observed in the specific heat of HoNi2B2C, as reported in the literature. Our neutron data confirm the transitions to the Néel structure (qN=c*) at TN and to the modulated structure (qM=0.586a*) at TM. The peak at TH1=5.7 K in the specific heat data, whose exact nature was not known hitherto, is now attributed to the onset of a qH1=0.905c* magnetic helical structure as seen in our neutron data. Comparison between the thermal evolution of the magnetic structures and the temperature dependence of the upper critical field confirms that the first Hc2(T) depression at 6.1 K arises from the qM=0.586a* modulated magnetic structure. The second depression in Hc2(T) below 5.7 K can be ascribed to the qH1=0.905c* magnetic helical structure.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The coexistence between superconductivity and magnetism, which occurs in the rare-earth (R)-nickel-borocarbides RNi2B2C (R = Dy, Ho, Er or Tm), is a rare phenomenon since magnetism and superconductivity are mutually exclusive in most cases. Despite this coexistence, superconductivity is weakened by antiferromagnetism and this is best seen under an applied magnetic field: the upper critical field (Hc2(T)) is strongly reduced at the antiferromagnetic ordering temperature. HoNi2B2C is a complex case because there are not only one but three antiferromagnetic orders which coexist in a narrow temperature range. The correspondence between these antiferromagnetic orders and the reductions seen in the upper critical field has not yet been completely established in HoNi2B2C.
Main results. We confirm that the reduction seen in (Hc2(T)) in HoNi2B2C first at 6.1 K arises from the qM = 0.585a* modulated antiferromagnetic order, while another decrease at 5.7 K is ascribed to the qH1 = 0.905c* antiferromagnetic helical order. Both these antiferromagnetic orders are detrimental to superconductivity. We have also been able to correlate the peak seen in the specific heat data at TH1 = 5.7 K to the onset of the qH1 = 0.905c* antiferromagnetic order.
Wider implications. To know that both the qM = 0.585a* and qH1 = 0.905c* antiferromagnetic orders are detrimental to superconductivity puts constraints on the theories which will be devised to explain the Hc2(T) reductions in HoNi2B2C.