Magnetic frustration induced large magnetocaloric effect in the absence of long range magnetic order

Griffiths-like behavior and magnetocaloric properties of rare-earth silicide Tb2Co0.8Si3.2

Novel rare-earth silicide, Tb2Co0.8Si3.2compound, crystallizes in Lu2CoGa3structure, a distorted substitution variant of theAlB2structure. The compound exhibits a complex magnetic state, with a ferromagnetic transition at 58 K, followed by successive antiferromagnetic transitions at 24 K and 8 K, respectively. Isothermal and magnetic hysteresis studies indicate the prominence of competing antiferro and ferromagnetic interactions in the compound. However, this does not lead to the formation of spin glass behavior, as confirmed by AC magnetic susceptibility and heat capacity studies. In the paramagnetic state, the short-range ferromagnetic ordering of cobalt creates a Griffiths-like anomaly that is suppressed at higher magnetic fields. Investigation of magnetocaloric and magnetoresistance properties identifies the compound as a conventional second-order magnetocaloric material with negative magnetoresistance. Furthermore, the determination of Landau coefficients and subsequent analysis indicate that the isothermal entropy change of the compound can be calculated from these coefficients.

Investigating the magnetic and magnetocaloric behaviors of LiSm(PO3)4

We report a detailed study on the magnetic behaviors and magnetocaloric (MC) effect of a single crystal of lithium samarium tetraphosphate, LiSm(PO₃)₄. The analyses of temperature-dependent magnetization data have revealed magnetic ordering established with decreasing temperature below T _p, where T _p is the minimum of a dM/dT vs. T curve and varies as a linear function of the applied field H. The Curie temperature has been extrapolated from T _p(H) data, as H → 0, to be about 0.51 K. The establishment of magnetic-ordering causes a substantial change in the heat capacity C _p. Above T _p, the crystal exhibits paramagnetic behavior. Using the Curie-Weiss (CW) law and Arrott plots, we have found the crystal to have a CW temperature θ _CW ≈ -36 K, and short-range magnetic order associated with a coexistence of antiferromagnetic and ferromagnetic interactions ascribed to the couplings of magnetic dipoles and octupoles at the Γ₇ and Γ₈ states. An assessment of the MC effect has shown increases in value of the absolute magnetic-entropy change (|ΔS _m|) and adiabatic-temperature change (ΔT _ad) when lowering the temperature to 2 K, and increasing the magnetic-field H magnitude. Around 2 K, the maximum value of |ΔS _m| is about 3.6 J kg^-1 K^-1 for the field H = 50 kOe, and ΔT _ad is about 5.8 K for H = 20 kOe, with the relative cooling power (RCP) of ∼82.5 J kg^-1. In spite of a low MC effect in comparison to Li(Gd,Tb,Ho)(PO₃)₄, the absence of magnetic hysteresis reflects that LiSm(PO₃)₄ is also a candidate for low-temperature MC applications below 25 K.

Possible realization of three-dimensional quantum spin liquid behavior in HoVO4

The study of geometrically frustrated magnetic systems with unusual crystal field ground states offers a possibility of realizing the new aspects of physics of disordered systems. In this study, we report our results of structural, magnetic susceptibility, heat capacity measurements, along with density functional theory (DFT) calculations on HoVO₄; a compound in which the presence of a distorted kind of HoO₈polyhedral leads to multiple magnetic interaction paths. The observed broad maximum below 10 K in the temperature response of DC susceptibility curves implies the presence of short-range correlations. AC susceptibility rules out the possibility of any kind of spin freezing. Temperature dependent heat capacity measurement at zero field indicate towards the absence of long-range ordering, along with the presence of a broad maximum centered around 14 K. The residual heat capacity exhibits a characteristic power-law (T^α) behavior with the exponentαnearly equal to 2, which is analogous to that observed for other three-dimensional (3D) quantum spin liquid (QSL) systems. The DFT calculations signify the presence of dominant second and third nearest neighbor interactions, which in turn lead to magnetic frustration in our system. Our investigations suggest that HoVO₄can be a candidate for realizing a 3D QSL state.

Competing magnetic interactions and magnetocaloric effect in Ho5Sn3

The rare-earth intermetallic compound Ho₅Sn₃demonstrates fascinating magnetic properties, which include temperature-driven multiple magnetic transitions and field-driven metamagnetism. We address the magnetic character of this exciting compound through a combined experimental and theoretical studies. Ho₅Sn₃orders antiferromagnetically below 28 K, and shows further spin reorientation transitions at 16 K and 12 K. We observe a sizable amount of low-temperature magnetocaloric effect (MEC) in Ho₅Sn₃with a maximum value of entropy change ΔS= -9.5 J Kg^-1 K^-1for an applied field ofH= 50 kOe at around 30 K. The field hysteresis is almost zero above 15 K where the MEC is important. Interestingly, ΔSis found to change its sign from positive to negative as the temperature is increased above about 8 K, which can be linked to the multiple spin reorientation transitions. The signature of the metamagnetism is visible in the ΔSversusHplot. The magnetic ground state, obtained from the density functional theory based calculation, is susceptible to the effective Coulomb interaction (U_eff) between electrons. Depending upon the value ofU_eff, the ground state can be ferromagnetic or antiferromagnetic. The compound shows large relaxation (14% change in magnetisation in 60 min) in the field cooled state with a logarithmic time variation, which may be connected to the competing magnetic correlations observed in our theoretical calculations. The competing magnetic ground states are equally evident from the small value of the paramagnetic Curie-Weiss temperature.

Complex magnetic properties associated with competing local and itinerant magnetism in [Formula: see text]

Ternary intermetallic compound [Formula: see text] has been synthesized in single phase and characterized by x-ray diffraction, scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX) analysis, magnetization, heat capacity, neutron diffraction and muon spin rotation/relaxation ([Formula: see text]SR) measurements. The polycrystalline compound was synthesized in single phase by introducing necessary vacancies in Co/Si sites. Magnetic, heat capacity, and zero-field neutron diffraction studies reveal that the system undergoes magnetic transition below [Formula: see text]4 K. Neutron diffraction measurement further reveals that the magnetic ordering is antiferromagnetic in nature with an weak ordered moment. The high temperature magnetic phase has been attributed to glassy in nature consisting of ferromagnetic clusters of itinerant (3d) Co moments as evident by the development of internal field in zero-field [Formula: see text]SR below 50 K. The density-functional theory (DFT) calculations suggest that the low temperature magnetic transition is associated with antiferromagnetic coupling between Pr 4f and Co 3d spins. Pr moments show spin fluctuation along with unconventional orbital moment quenching due to crystal field. The evolution of the symmetry and the crystalline electric field environment of Pr-ions are also studied and compared theoretically between the elemental Pr and when it is coupled with other elements such as Co. The localized moment of Pr 4f and itinerant moment of Co 3d compete with each other below [Formula: see text]20 K resulting in an unusual temperature dependence of magnetic coercivity in the system.

Geometrical frustration and cluster spin glass with random graphs

We develop a based on a sparse random graph to account for the interplay between geometric frustration and disorder in cluster magnetism. Our theory allows introduction of the cluster network connectivity as a controllable parameter. Two types of inner cluster geometry are considered: triangular and tetrahedral. The theory was developed for general, nonuniform intracluster interactions, but in the present paper the results presented correspond to uniform, antiferromagnetic (AF) intraclusters interaction J_{0}/J. The clusters are represented by nodes on a finite connectivity random graph, and the intercluster interactions are randomly Gaussian distributed. The graph realizations are treated in replica theory using the formalism of order parameter functions, which allows one to calculate the distribution of local fields and, as a consequence, the relevant observable. In the case of triangular cluster geometry, there is the onset of a classical spin liquid state at a temperature T^{*}/J and then, a cluster spin glass (CSG) phase at a temperature T_{/}J. The CSG ground state is robust even for very weak disorder or large negative J_{0}/J. These results does not depend on the network connectivity. Nevertheless, variations in the connectivity strongly affect the level of frustration f_{p}=-Θ_{CW}/T_{f} for large J_{0}/J. In contrast, for the nonfrustrated tetrahedral cluster geometry, the CSG ground state is suppressed for weak disorder or large negative J_{0}/J. The CSG boundary phase presents a reentrance which is dependent on the network connectivity.

Studies on magnetocaloric effect of Tb2Ni0.90Si2.94 compound

A comparative study has been carried out on the magnetocaloric properties of as-cast and annealed Tb₂Ni_0.90Si_2.94 intermetallic compound. While the as-cast material exhibits ferromagnetic cluster-glass behaviour below 9.9 K coexisting with antiferromagnetic (AFM) interaction, the annealed system shows AFM ordering below 13.5 K and spin freezing occurs below 4 K. The compound exhibits moderate magnetocaloric performance with maximum isothermal entropy changes (-ΔS _M) 8.8 and 10.9 J kg^-1 K^-1, relative cooling power (RCP) 306 and 365 J kg^-1, along with adiabatic temperature change (ΔT _ad) 5.5 and 8.15 K for 70 kOe magnetic field change in as-cast and annealed forms, respectively. The estimated magnetic entropy change is found to be larger for annealed sample in comparison to that of as-cast analogue. However, the full width at half maxima (FWHM) of -ΔS _M(T) behaviour is larger in as-cast compound due to the presence of inherent structural disorder which reduces with thermal annealing. A positive isothermal entropy change (-ΔS _M) and adiabatic temperature change (ΔT _ad) is observed for the as-cast compound in the measured field and temperature region. In contrast, the annealed system exhibits inverse magnetocaloric effect in the low field and temperature region where AFM interactions dominate. Magnetocaloric effect (MCE) is used as a tool to establish a subtle correlation between the observed magnetocaloric effect and the reported magnetic properties of the system.

Quadrupolar ordering and exotic magnetocaloric effect in RB4 (R = Dy, Ho)

The interplay of charge, spin, orbital and lattice degrees of freedom has recently received great interest due to its potential to improve the magnetocaloric effect (MCE) for the purpose of magnetic cooling applications. Here, a new mechanism for a large entropy change with low magnetic fields in rare-earth tetraborides, especially for Ho_1-xDy_xB₄ (x = 0.0, 0.5, and 1.0), is proposed. For x = 0.0, 0.5, and 1.0, the maximum entropy changes of the giant inverse MCE are found to be 22.7 J/kgK, 19.6 J/kgK, and 19.0 J/kgK with critical fields of 25 kOe, 40 kOe, and 50 kOe, respectively. For all compounds, systematic study on how the entropy changes as a function of the field and temperature is performed to investigate their correlation with consecutive double transitions, i.e., the magnetic dipolar order at T = T_N and the quadrupolar order at T = T_Q (T_Q < T_N). Based on Landau theory, it is found that this behaviour is attributed to the strong coupling between magnetic dipoles and quadrupoles in the presence of strong spin-orbit coupling and geometric frustration. Our work offers new insights into both academic and industrial interests in the discovery of giant MCE with various applications for magnetic cooling systems.

Magnetocaloric Effect in Cu5-NIPA Molecular Magnet: A Theoretical Study

We calculated the magnetocaloric properties of the molecular nanomagnet Cu5-NIPA, consisting of five spins S = 1 / 2 arranged in two corner-sharing triangles (hourglass-like structure without magnetic frustration). The thermodynamics of the system in question was described using the quantum Heisenberg model solved within the field ensemble (canonical ensemble) using exact numerical diagonalization. The dependence of the magnetic entropy and magnetic specific heat on the temperature and the external magnetic field was investigated. The isothermal entropy change for a wide range of initial and final magnetic fields was discussed. Due to plateau-like behavior of the isothermal entropy change as a function of the temperature, a high degree of tunability of magnetocaloric effect with the initial and final magnetic field was demonstrated.

Unusual bidirectional frequency dependence of dynamical susceptibility in hexagonal intermetallic Pr2Ni0.95Si2.95

In this study, the synthesis of a novel ternary intermetallic compound Pr₂Ni_0.95Si_2.95 forming in single phase only by deliberately introducing vacancies in Ni/Si site is reported. The detailed studies on dc magnetization, heat capacity, ac magnetization & associated dynamical scaling, different types of non-equilibrium dynamical behaviour, viz., magnetic relaxation behaviour as a function of wait time and temperature, aging phenomena, and magnetic memory effect firmly establish that the compound exhibits spin freezing behaviour below 3.3 K (T_f). However, below T_f, temperature dependence of ac susceptibility data exhibit an additional peak that shows reverse frequency dependence to that generally observed in a glassy system. The unusual bidirectional frequency dependence in a single magnetic system is of significant interest and rarely reported in literature. Competing exchange interaction arising from c/a ~ 1 and crystallographic randomness driven magnetic phase separation has been argued to be responsible for such observation. The reverse frequency shift of the low temperature peak has been described on the basis of a simple phenomenological model proposed in this work.

Role of random magnetic anisotropy on the valence, magnetocaloric and resistivity properties in a hexagonal Sm2Ni0.87Si2.87 compound

In this work, we report the effect of random magnetic anisotropy (RMA) on the valence, magnetocaloric and resistivity properties in a glassy intermetallic material Sm₂Ni_0.87Si_2.87. On the basis of detailed studies on the valence band and core level electronic structure, we have established that both the Sm³⁺ and Sm²⁺ ions are present in the system, suggesting the compound to be of mixed valence in nature. The significant observation of positive magnetic entropy change in zero-field cooled measurement has been argued due to the presence of RMA that develops due to local electronic environmental variations between the rare-earth ions in the system. The quantum interference effect caused by the elastic electron-electron interaction is responsible for the resistivity upturn at low-temperature for this disordered metallic conductor.

Observation of short range order driven large refrigerant capacity in chemically disordered single phase compound Dy2Ni0.87Si2.95

In this work, we report the successful synthesis of a new intermetallic compound Dy2Ni0.87Si2.95 forming in single phase only with a chemically disordered structure. The random distribution of Ni/Si and crystal defects create a variation in the local electronic environment between the magnetic Dy ions. In the presence of both disorder and competing exchange interactions driven magnetic frustration, originating due to c/a ∼ 1, the compound undergoes spin freezing behaviour below 5.6 K. In the non-equilibrium state below the spin freezing behaviour, the compound exhibits aging phenomena and magnetic memory effects. In the magnetically short-range ordered region, much above the freezing temperature, an unusual occurrence of considerable magnetic entropy change, -ΔSmaxM ∼ 21 J kg-1 K-1 with large cooling power RCP ∼ 531 J kg-1 and adiabatic temperature change, ΔTad ∼ 10 K for a field change of 70 kOe, is observed for this short range ordered cluster-glass compound without any magnetic hysteresis loss.

Chemical disorder driven reentrant spin cluster glass state formation and associated magnetocaloric properties of Nd2Ni0.94Si2.94

In this work, we report the synthesis of a new ternary intermetallic compound, Nd₂Ni_0.94Si_2.94, that forms in single phase only in a defect crystal structure. The compound exhibits an antiferromagnetic transition below 7.2 K (T_N) followed by a spin cluster freezing behaviour below 2.85 K (T_f), which makes the compound a reentrant spin cluster glass system. The detailed studies of dc and ac magnetization, heat capacity, non-equilibrium dynamical behaviour, viz., aging effect, temperature and field dependent magnetic relaxation and magnetic memory effect establish the compound to be a cluster-glass material below freezing temperature. The interplay between competing exchange coupling (c/a ≃ 1.04 ⇒ J_NN ≃ J_NNN) and chemical disorder driven variation in the electronic environment among the Nd ions has been argued to be responsible for such a metastable state formation. A considerable value of MCE parameters (-ΔS ∼ 11.4 J kg^-1 K^-1, RCP ∼ 160 J kg^-1 and ΔT_ad ∼ 5.2 K for a field change of 70 kOe) is obtained for this magnetically frustrated glassy compound.

Low-field induced large magnetocaloric effect in Tm2Ni0.93Si2.93: influence of short-range magnetic correlation

In this work, we report the successful synthesis of a new intermetallic compound Tm₂ [Formula: see text] [Formula: see text] that forms in single phase only in defect crystal structure. The compound does not show any long range magnetic ordering down to 2 K. The material exhibits a large magnetic entropy change ([Formula: see text] J [Formula: see text] K^-1) and adiabatic temperature change ([Formula: see text] K) at 2.2 K for a field change of 20 kOe which can be realized by permanent magnets, thus being very beneficial for application purpose. In the absence of long-range magnetic ordering down to 2 K, the metastable nature of low-temperature spin dynamics and short-range magnetic correlations are considered to be responsible for such a large magnetocaloric effect over a wide temperature region.