Intense ferromagnetic fluctuations preceding magnetoelastic first-order transitions in giant magnetocaloric LaFe13−xSix

Abstract

First-order magnetic transitions are of both fundamental and technological interest given that a number of emergent phases and functionalities are thereby created [1][2][3][4][5] . Of particular interest are giant magnetocaloric effects, which are attributed to first-order magnetic transitions and have attracted broad attention for solid-state refrigeration applications 6 . While the conventional wisdom is that atomic lattices play an important role in first-order magnetic transitions 7,8 , a coherent microscopic description of the lattice and spin degrees of freedom is still lacking. Here, we study the magnetic phase transition dynamics on the intermetallic LaFe 13-x Si x , which is one of the most classical giant magnetocaloric systems, in both frequency and time domains utilizing neutron scattering and ultrafast X-ray diffraction. We have observed a strong magnetic diffuse scattering in the paramagnetic state preceding the first-order magnetic transition, corresponding to picosecond ferromagnetic fluctuations. Upon photon-excitation, the ferromagnetic state is completely suppressed in 0.9 ps and recovered in 20 ps.The ultrafast dynamics suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.First-order magnetic transitions (FOMTs) take place with abrupt changes of several physical quantities at transition points with respect to temperature, magnetic field, electric field, pressure, stress, etc [1][2][3][4][5] . Such features offer a great opportunity for applications in a variety of functional sensors with high intrinsic signal-to-noise ratios.More importantly, the multiple degrees of freedom are often strongly coupled at FOMTs so that some unprecedented functionalities become possible based on crossing control. FOMTs can be categorized into magneto-structural and magnetoelastic transitions 9 . The former involves two magnetic phases accommodating different

Emergence of a hidden magnetic phase in LaFe11.8Si1.2 investigated by inelastic neutron scattering as a function of field and temperature

“…INS was used as a tool to explore the possibility of spin fluctuations emerging about T c that would result in enhancement of the heat capacity and renormalization of the energy barrier. It should be noted that whilst previous measurements [15,16,28,29] indicated presence of spin fluctuations for Q ≤ 0.8 Å and a phonon peak near 27 meV in the ferromagnetic state, neither had the low Q resolution (to 0.1 Å−1 ) presented here.…”

“…[11] It has also been argued that the reduced hysteresis is a consequence of spin fluctuations lowering (renormalizing) the energy barriers one might normally expect of a first order phase transition. [12], [13] More recently, evidence has started to emerge of PM spin fluctuations in LFS [14], [15], [16] but a full understanding remains incomplete. In this work we present inelastic neutron scattering (INS) data for LaFe 11.8 Si 1.2 above and below the Curie temperature, T c , and find that quasi-elastic excitations, appear in the PM state, persisting to higher temperatures.…”

Direct observation of magnetoelastic coupling in a molecular spin qubit: new insights from crystal field neutron scattering data

“…This portion of the spectrum contains information on relaxation dynamics of zero energy processes. It has long been used for studying the diffusive dynamics of protons 40 and other elements, 41 but has also been used to study magnetic dynamics including the spin state fluctuations preceding phase transitions, 42 and the lifetime of zero-energy processes in CF split systems 43 and molecular magnets. 44 A key result used in the current work, is that the energy and lifetime of an excitation are not completely independent, as the lifetime of inelastic phenomena, such as CF excitations, is encoded into the inelastic spectrum as an energy broadening.…”

“…44 In order to design SMMs for use in real world applications, not only are the electronic properties important, but clearly the vibrational properties are too, studies of which are still lacking. Here we discuss the candidate qubit [Ho(W 5 O 18 ) 2 ] 9-, 50 a SMM shown to have so-called atomic clock transitions 42 that protect the molecule from dipolar relaxation at key external fields. However, thermal effects remain a known issue for this compound.…”

“…This portion of the spectrum contains information on relaxation dynamics of zero energy processes. It has long been used for studying the diffusive dynamics of protons 40 and other elements, 41 but has also been used to study magnetic dynamics including the spin state fluctuations preceding phase transitions, 42 and the lifetime of zero-energy processes in CF split systems 43 and molecular magnets. 44 …”