Strong magnetocaloric effect induced by spin reorientation transitions in epitaxial Ho thin films

Abstract

Magnetocaloric effect (MCE) in antiferromagnetic holmium (Ho) film is studied near the spin reorientation temperatures. A series of magnetization isotherms from 60 K to 150 K around the Neel temperature, T N ≈ 130 K were recorded for both in-plane and out-of-plane magnetic field orientations. A change in entropy, ∆S M of-5 J/kg K was found near T N for an in-plane field orientation. A large change in ∆S M of-11.8 J/kg K due to a fan-helix spin transition at Т = 90 K is observed for an in-plane field orientation. Spin transition at both T N and at the fan-helix transition exhibit larger MCE in the in-plane field orientations in comparison with the perpendicular orientation. The value of the refrigerant capacity extracted from the temperature dependence of ΔS M (T) is found to be larger for the "in-plane" orientation by a factor of two. I.

Influence of Dimensional Effects on the Curie Temperature of Dy and Ho Thin Films

“…There has been growing interest in the study of rare-earth nanostructures, which was initiated by finding a number of novel effects, such as the giant magnetocaloric effect [8]. Such effects are determined by the magnetic properties of nanostructures and their variations with temperature or magnetic field.…”

Magnetic relaxation between ferromagnetic and helix spin configurations in holmium films

“…Usually, stationary spin states of Ho are the focus of researchers. All these states are well-known in the literature [1][2][3][4][5][6][9][10][11][12]. Variations in thicknesses and mechanical strains cause a significant change in the spin state map presented in field-temperature coordinates.…”

“…A neutron diffraction study indicated that a film with a thickness of approximately 100 nm exhibits properties distinct from those of the bulk material, yet remains suitable for examining the periodicity of a spin helix and various incommensurate spin structures [2]. X-ray diffraction (XRD) was recorded to confirm the hexagonal structure and orientation of the c axis [3].…”

“…This natural material is uniquely characterized by spiral, fan, helix, gliding and others spin configurations within its hexagonal crystal lattice. The fundamental origin of the competitive incommensurate and chiral phases in holmium has been extensively discussed in many publications [1][2][3][4]. The magnetic anisotropy at interfaces, surface anisotropy and the effects of mechanical stresses in thin Ho films further complicate the situation compared to bulk samples [2,[5][6][7][8].…”

“…Additionally, the constants associated with the bulk, surface and interface anisotropies are also highly sensitive to the mechanical stresses. The period of the incommensurate spin structures in Ho is typically very large, ∼10-100 nm and even more [3]. Consequently, the common designation 'thin films', usually referring to metallic films less than 10 nm thick, does not apply to Ho films.…”