Magnetovolume Coupling Research Articles

Magnetism of 3d electrons in YCo12-xFexB6 compounds

The magnetic properties of the polycrystalline YCo12-xFexB6 alloys are thoroughly investigated through the scope of the Kuz’min model describing the thermal evolution of the spontaneous magnetization. The analysis of this behavior reveals a strong composition dependence of the magnetization as well as its thermal evolution. This is interpreted as an evolution from first neighbor ferromagnetic (FM) interactions with progressively reinforced antiferromagnetic (AFM) interactions. The magnetization in the ordered state decreases when increasing Fe content from 0.45 µB/f.u. down to 0.11 µB/f.u. for x = 0 and x = 4, respectively. The Inoue-Shimizu s-d model has also been used close to the ordering temperature to investigate the evolution of the Landau coefficients and study in detail the order of the magnetic transition and have some insights on the magnetovolume coupling in this series of compounds. It revealed that Fe for Co substitution causes the already weak magnetovolume exchange to become even weaker.

Simulating the giant magnetocaloric effect-from mean-field theory to microscopic models

Magnetocaloric materials are recognized as one of the major classes of magnetic materials for energy applications, and can be either employed as refrigerants in heat-pumping devices, or in thermomagnetic generators for energy conversion/harvesting. For both applications, having a material that presents a first-order magnetic phase transition is advantageous, as this typically leads to enhanced values of magnetization change in temperature (relevant to energy conversion) and of the magnetocaloric effect (relevant to heat-pumping). We present a brief overview of selected models applied to the simulation of applied magnetic field and temperature-dependent magnetization and magnetic entropy change of first-order magnetic phase transition systems, covering mean-field models such as the Landau theory of phase transitions and the Bean-Rodbell model, up to more recent developments using a Ising-like microscopic model with magnetovolume coupling effects. We highlight the fundamental and practical limitations of employing these models and compare predicted thermodynamic properties.

Tailoring Negative Thermal Expansion via Tunable Induced Strain in La-Fe-Si-Based Multifunctional Material.

Zero thermal expansion (ZTE) composites are typically designed by combining positive thermal expansion (PTE) with negative thermal expansion (NTE) materials acting as compensators and have many diverse applications, including in high-precision instrumentation and biomedical devices. La(Fe1-x,Six)13-based compounds display several remarkable properties, such as giant magnetocaloric effect and very large NTE at room temperature. Both are linked via strong magnetovolume coupling, which leads to sharp magnetic and volume changes occurring simultaneously across first-order phase transitions; the abrupt nature of these changes makes them unsuitable as thermal expansion compensators. To make these materials more useful practically, the mechanisms controlling the temperature over which this transition occurs and the magnitude of contraction need to be controlled. In this work, ball-milling was used to decrease particles and crystallite sizes and increase the strain in LaFe11.9Mn0.27Si1.29Hx alloys. Such size and strain tuning effectively broadened the temperature over which this transition occurs. The material's NTE operational temperature window was expanded, and its peak was suppressed by up to 85%. This work demonstrates that induced strain is the key mechanism controlling these materials' phase transitions. This allows the optimization of their thermal expansion toward room-temperature ZTE applications.

Theoretical Description of the Magnetocaloric Effect With Second-Order Magnetic Phase Transitions in La0.65Ce0.05Sr0.3Mn1−xCuxO3 (x = 0 and x = 0.15) Manganite Using the Bean–Rodbell Model of Magneto–volume Coupling

Theoretical Description of the Magnetocaloric Effect With Second-Order Magnetic Phase Transitions in La0.65Ce0.05Sr0.3Mn1−xCuxO3 (x = 0 and x = 0.15) Manganite Using the Bean–Rodbell Model of Magneto–volume Coupling

Pressure Dependence of Magnetic Properties in La(Fe,Si)13 : Multistimulus Responsiveness of Caloric Effects by Modeling and Experiment

For a better understanding of multistimuli-responsive caloric materials with a first-order transition and for optimization of their functional properties, it is necessary to predict the behavior of the material under changes of both magnetic field and pressure. Here, we design and build a special device that can provide a self-consistent set of parameters needed for the comprehensive characterization of multistimuli-responsive functional magnetic materials. Using this scientific instrument, a data set of simultaneously measured magnetization, ${M(T)}_{H}$, and volume magnetostriction, $\ensuremath{\omega}(T{)}_{H}$, values are obtained for ${\mathrm{LaFe}}_{11.4}{\mathrm{Si}}_{1.6}$ with a first-order transition. Furthermore, based on simultaneously measured M(T) and \ensuremath{\omega}(T) dependencies obtained at ambient pressure, we develop an approach that allows the behavior of magnetization under different pressures, ${M(T)}_{P}$, to be described analytically. Additional parameters, such as compressibility, \ensuremath{\kappa}(T); thermal expansion coefficient, \ensuremath{\alpha}(T); and magnetoelastic interaction or effective magnetovolume coupling constant, ${C}_{MV}$, are determined. For verification of our developed model, direct measurements of magnetization under external pressure (up to P = 1 GPa) are carried out on the same sample as that used for simultaneous measurement of magnetization and magnetovolume effect. A comparison of simulated ${M(T)}_{P}$ dependencies with experimental $M(T{)}_{P}$ confirms that our approach provides a more realistic behavior of transition temperature under pressure, ${T}_{C}(P)$, than that of the ${T}_{C}(P)$ predicted by the Bean-Rodbell model; thus, this approach is more suitable for predicting the behavior of multistimuli-responsive caloric materials with first-order transitions under changes of both magnetic field and pressure.

Strong temperature – Dependence of Ni -alloying influence on the stacking fault energy in austenitic stainless steel

Using ab initio alloy theory, we calculate the impact of Ni on the stacking fault energy in austenitic stainless steel as a function of temperature. We show that the influence of Ni strongly couples with temperature. While a positive effect on the stacking fault energy is obtained at ambient temperature, the opposite negative effect is disclosed at elevated temperatures. An important rationale behind is demonstrated to be the variation of magneto-volume coupling induced by Ni alloying. The alloy influence on the finite temperature evolution of Ni impact is evaluated for elements Cr, Mo and N.

Magnetocaloric materials with ultra-small magnetic nanoparticles working at room temperature

Through the use of the Monte Carlo simulations utilising the mean-field approach, we show that a dense assembly of separated ultra-small magnetic nanoparticles embedded into a non-magnetic deformable matrix can be characterized by a large isothermal magnetic entropy change even upon applying a weak magnetic field with values much smaller than one Tesla. We also show that such entropy change may be very significant in the vicinity of the room temperature which effect normally requires an application of a strong external magnetic field. The deformable matrix chosen in this work as a host for magnetic nanoparticles adopts a thin film form with a large surface area to volume ratio. This in turn in combination with a strong magneto-volume coupling exhibited by this material allows us to show its suitability to be used in the case of a variety of applications utilising local cooling/heating such as future magnetic refrigerants.

Enhanced magnetopiezoelectric effect at the Néel temperature in CaMn2Bi2

We have experimentally studied a magnetopiezoelectric effect (MPE), i.e., dynamic distortion caused by AC electric currents, in the antiferromagnetic conductor ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ at low temperatures down to $77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ using laser Doppler vibrometry. When AC electric currents are applied to ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, dynamic displacement signals which increase in proportion to the current amplitude are clearly observed at $77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, but their magnitude and temperature dependence strongly depend on Joule heating effects caused by the applied electric currents. Especially, poor thermal contact of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ samples to the sample holder produces second-harmonic displacement signals owing to thermal expansion due to the Joule heating, and tends to affect the temperature dependence of the MPE signals. In a measurement run without any heating effects, a sharp enhancement of the MPE signal is observed at the N\'eel temperature of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, which suggests that fluctuations of itinerant Mn moments near the N\'eel temperature play an important role in the MPE. We speculate that electric currents produce nonequilibrium antiferromagnetic moments via the antiferromagnetic Edelstein effect and the induced antiferromagnetic momemts enhance the MPE signal with the help of a large magnetovolume coupling near the N\'eel temperature. The MPE efficiency as a piezoelectric response reaches 300--$500\phantom{\rule{0.16em}{0ex}}\mathrm{pC}/\mathrm{N}$ at a maximum, which is comparable to high values of piezoelectric coefficients detected in the piezoelectric (ferroelectric) ceramics.

Multisite Exchange-Enhanced Barocaloric Response in Mn3NiN

We have studied the barocaloric effect (BCE) in the geometrically frustrated antiferromagnet Mn$_{3}$NiN across the N\'{e}el transition temperature. Experimentally we find a larger barocaloric entropy change by a factor of 1.6 than that recently discovered in the isostructural antiperovskite Mn$_{3}$GaN despite greater magnetovolume coupling in the latter. By fitting experimental data to theory we show that the larger BCE of Mn$_{3}$NiN originates from multi-site exchange interactions amongst the local Mn magnetic moments and their coupling with itinerant electron spins. Using this framework, we discuss the route to maximise the BCE in the wider Mn$_{3}$AN family.

Magnetovolume coupling in transformation behaviour of Mn–Ni–Sn metamagnetic shape memory alloys

The shift rate of transformation temperature as a function of the magnetic field is one of the crucial parameters predetermining the promising functional properties originated from the martensitic transformation (MT) of the Heusler metamagnetic shape memory alloys (MetaMSMAs). In the present work, we interrelated such a shift of MT temperature with the volume magnetostriction constant and estimated this constant using the recent experimental data obtained for two Mn–Ni–Sn-based MetaMSMAs. As a consequence, the role of the magnetovolume coupling in the transformational behaviour of these two alloys and the nature of magnetic state inherent to their austenitic and martensitic phases have been revealed. It has been shown that the shift of MT temperature under the strong magnetic field and the magnetization jump accompanying MT can be caused by both the forced and spontaneous volume magnetostriction if magnetoelastic constant is large and positive.

Plastic deformation modes in paramagnetic γ-Fe from longitudinal spin fluctuation theory

Using an efficient first-principles computational scheme, we calculate the intrinsic stacking fault energy (γisf) and the unstable stacking fault energy (γusf) of paramagnetic γ-Fe as a function of temperature. The formation energies are derived from free energies accounting for thermal longitudinal spin fluctuations (LSFs). LSFs are demonstrated to be important for the accurate description of the temperature-dependent magnetism, intrinsic and unstable stacking fault energies, and have a comparatively large effect on γisf of γ-Fe. Dominated by the magneto-volume coupling at thermal excitations, γisf of γ-Fe exhibits a positive correlation with temperature, while γusf declines with increasing temperature. The predicted stacking fault energy of γ-Fe is negative at static condition, crosses zero around 540 K, and reaches 71.0 mJ m−2 at 1373 K, which is in good agreement with the experimental value. According to the plasticity theory formulated in terms of the intrinsic and unstable stacking fault energies, twinning remains a possible deformation mode even at elevated temperatures. Both the large positive temperature slope of γisf and the predicted high-temperature twinning are observed in the case of austenitic stainless steels.

The role of competing magnetic interactions on the abnormal expansion properties in manganese antiperovskites, Mn3+xA1−xN (A = Ni, Sn)

The role of competing magnetic interactions in Mn-doped manganese antiperovskites Mn3+xA1−xN (A = Ni and Sn) has been explored using magnetometry and dilatometry, with particular focus on their relation to abnormal expansion properties. Rare negative magnetisation phenomena are found in zero-field-cooled conditions for both A = Sn and Ni when Mn is doped on the A site. Such behaviour is evidence of both competing magnetic sublattices and magnetocrystalline anisotropy. By comparison with other compounds with the antiferromagnetic Γ5g magnetic structure, we suggest that competing interactions along with magnetovolume coupling influence the near-zero thermal expansion behaviour at intermediate temperatures below TN in the A = Ni family.

Behaviour of the Young's modulus at the magnetocaloric transition in La(Fe,Co,Si)13

Magnetic solid state cooling applications require families of samples where the magnetic transition is cascaded across the working range of the fridge. Although magnetic properties are widely studied, information relating to the mechanical properties of such systems is less prevalent. Here we study the mechanical properties of a series of magnetocaloric La(Co,Fe,Si)13 samples where the Co content is varied to produce a range of transition temperatures. It was found that at room temperature the flexural strength decreases and the Young's modulus increases with increasing Co content. Interestingly we find a significant reduction of Young's modulus at temperature around the magnetic transition temperature. This reduction was less pronounced with increasing Co content. We associate the softening with the magnetovolume coupling known to exist in these materials.

Stacking fault energy of C-alloyed steels: The effect of magnetism

First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic γ-Fe and FeCrNi austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution.

On the rich magnetic phase diagram of (Ni, Co)–Mn–Sn Heusler alloys

We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)–Mn–Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations.

Magnetovolume effect, macroscopic hysteresis, and moment collapse in the paramagnetic state of cubic MnGe under pressure

Itinerant magnets generally exhibit pressure induced transitions towards non magnetic states. Using synchrotron based X-ray diffraction and emission spectroscopy, the evolution of the lattice and spin moment in the chiral magnet MnGe was investigated in the paramagnetic state and under pressures up to 38 GPa. The collapse of spin-moment takes place in two steps. A first-order transition with a huge hysteresis around 7 GPa transforms the system from the high-spin at ambient pressure to a low-spin state. The coexistence of spin-states and observation of history-depending irreversibility is explained as effect of long-range elastic strains mediated by magnetovolume coupling. Only in a second transition, at about 23 GPa, the spin-moment collapses.

Magnetostriction of La0.75Sm0.25Mn2Si2 compound

The magnetostriction and magnetization are studied for a quasi-single crystalline sample of the La0.75Sm0.25Mn2Si2 compound which has the spontaneous first-order antiferromagnetic to ferromagnetic (AF–F) phase transition at a temperature of 160 K. For the AF state, the field-induced AF–F transition is observed in magnetic fields directed both along the easy c-axis and in the basal plane. The transition is accompanied by both the volume change ΔV/V = 2.8 × 10−3 and an anisotropic lattice distortion. The anisotropic magnetostriction is negative in magnetic fields applied along the c-axis and positive, in the basal plane. In small magnetic fields, both the volume and anisotropic magnetostrictions are proportional to the square of magnetization. In magnetic fields applied along the c-axis, the field-induced canting of the magnetic moments leads to much larger magnetoelastic deformations than in the basal plane. The values of the magnetovolume coupling constants both along the c-axis and in the basal plane are in agreement with those estimated from the thermodynamic relationships. Very small volume and linear strains are observed for the ferromagnetic state at 250 K. The obtained results suggest that the magnetoelastic effects in these compounds are mainly due to the change of mutual orientations of the Mn magnetic moments of adjacent layers.

Thermodynamics of the 2-D Ising Model From a Random Path Sampling Method

We explore the thermodynamics of the 2-D Ising model from a random path sampling method. We show how convergence of free energy and magnetization is obtained in a number of sweeps orders of magnitude below the number of total configurations. The effects of external parameters, such as magnetic field (H) and temperature (T) are accounted in a computationally simple way, allowing very detailed (H, T)-dependent calculations. The effects of more complex interactions can also be considered, particularly magnetovolume coupling, which has proven to be difficult to tackle using traditional approaches. We show its effect on magnetic entropy change (magnetocaloric effect) in this microscopic scenario. As magnetic refrigeration technology matures, the microscopic optimization of magnetovolume effects to enhance the magnetocaloric properties may become a valuable tool in the search of new magnetic refrigerants.

Electronic structure and the magneto-caloric effect

We present a theoretical investigation of the entropy changes upon the application of an external field leading to the magneto-caloric effect (MCE). The case of localized magnetic moments is treated within the Weiss molecular field model, but special emphasis is given to cases of itinerant electron magnetism. These are described within the Landau theory of phase transitions and the temperature dependence is included via spin fluctuations. Since the parameters of the Landau expansion can be calculated from first-principles calculations of the electronic and magnetic structure, an immediate connection to the electronic band structure and its properties becomes possible. We study ordinary ferromagnets, including magneto-volume coupling and itinerant electron metamagnets, where in a small external field range large changes of the magnetic moments occur. We find that such metamagnetic systems are the most promising candidates for a large MCE in itinerant electron systems. We apply our expressions to several transition metals and their alloys, as well as to the metamagnets YCo2 and Fe2P, and find reasonable agreement with available experimental data.

Critical magnetic behavior of magnetocaloric materials with the Gd5Si4-type structure

An extensive investigation on the magnetism of Gd5(SixGe1−x)4 compounds which stabilize in the Gd5Si4-type structure is here presented, focusing on the Gd5Si4 and Gd5Si2Ge2 compositions. Through Arrott plot analysis, the temperature dependencies of the spontaneous magnetizations are retrieved and the magnetic interaction coefficients (Jexch) are estimated. The obtained Jexch value for Gd5Si4 is 15% higher than that of Gd5Si2Ge2. Such enhancement cannot be attributed to a magneto-volume coupling only, being mainly associated with the magnetic polarization of the (Si,Ge)-(Si,Ge) interslab connections. The critical exponents were estimated (δ=3.1 and 2.9, γ=1.01 and 1.06, β=0.48 and 0.56, and n=0.67 and 0.63 for Gd5Si4 and Gd5Si2Ge2, respectively) demonstrating that both compositions belong to the same universal class. This conclusion is reinforced by the construction of a universal logarithmic scaling plot of the M(H) isothermal curves. Hence, we conclude that the magnetism of the Gd5Si4-type structure compounds is simply dominated by long range ferromagnetic interactions. This contrasts to other Gd5(SixGe1−x)4 compositions where a strong competition antiferro versus ferromagnetic interactions and short versus long range order are observed. Finally, the Gd5Si4 composition has shown a reduction of 50% on the magnetic entropy change maxima when compared with the Gd5Si2Ge2 compound studied, despite its higher magnetic interaction coefficient value.