Order Magnetic Phase Transition Research Articles

Field‐induced phase transitions (FIPT) in molecular magnets

Ultrahigh magnetic fields (B = 100–1000 T) are valuable means for many-sided research in solid state physics. A brief description of the explosion method and some results of its application are presented. The magnetic coil compensation method has been used to study the first order phase transitions in a broad range of fields and temperatures. The Faraday rotation method has been applied to measure the second order magnetic phase transitions. A number of magnetic materials were studied experimentally and theoretically: first order phase transitions in RCo2 compounds, second order transitions in KMnF3, MnF2, etc. and steplike quantum magnetization process (transition from ferrimagnetic to ferromagnetic phase) in some mesoscopic magnets – Mn12Ac, Mn6Rad6 and V15. These magnetic measurements provide us with a direct and unique method of determination of the exchange interactions between magnetic ions in magnetic moleculas. The knowledge of the magnetic interacion energies is important for molecular engineering in order to design new magnetic nanoscale materials with desireable properties. The intramolecule exchange integrals of the magnetic nanoclusters Mn12Ac, Mn6Rad6 and V15 that have been determined by this method are presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic anisotropy of a DyNiAl single crystal

DyNiAl orders ferromagnetically below T C=30 K with the Dy magnetic moments oriented along the hexagonal c-axis, and undergoes an order–order magnetic phase transition at T 1=15 K where an antiferromagnetic component appears in the basal plane leading to a non-collinear magnetic structure. Magnetization measurements on a DyNiAl single crystal in fields up to 40 T revealed a large uniaxial magnetic anisotropy with anisotropy field of 25 T and field-induced transitions in fields applied along the basal plane. A low-field transition is observed in both magnetic phases and is associated with a rotation of the total magnetic moment toward the field direction to form a canted magnetic structure, which is stable in a wide field interval. A high-field transition is observed only in the low-temperature phase and corresponds to a spin flop of the antiferromagnetic component.

The order of magnetic phase transition in La(Fe 1− xCo x) 11.4Si 1.6 compounds

Magnetic transitions in La(Fe 1− x Co x ) 11.4Si 1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M( μ 0 H), using the Landau expansion of the magnetic free energy. For x⩽0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.

Theoretical investigations on giant magnetocaloric effect in MnAs 1− xSb x

In this work we apply a model to describe the magnetocaloric effect for the MnAs 1− x Sb x series of compounds, 0⩽ x⩽0.4. The behavior of the material under first order phase transitions is well described, and we are able to obtain the magnetocaloric potential for the series of compounds presenting first order magnetic phase transitions. Based on these results we predict the performance of a composite comprising a combination of compositions of this compound to work as active element in a magnetic refrigerator using an Erickson cycle spanning a great temperature range down from room temperature.

First-order transition from ferromagnetism to antiferromagnetism inCe(Fe0.96Al0.04)2:A magnetotransport study

The magnetotransport behaviour is investigated in detail across the first order magnetic phase transition from ferromagnetic to antiferromagnetic state in polycrystalline Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ sample. The study clearly brings out various generic features associated with a first order transition, viz., hysteresis, phase coexistence, supercooling and superheating, presence and limits of the metastable regimes. These results of magnetotransport study exhibit and support all the interesting thermomagnetic history effects that were observed in our earlier dc-magnetisation study on the same sample. Most notable here is the initial (or virgin) resistivity vs. field curve lying outside the hysteretic "butterfly shaped" magnetoresistivity loops obtained on cyclying the magnetic field between high enough positive and negative strengths. These findings, bearing one-to-one similarity with the data obtained in their magnetic counterpart (i.e., dc-magnetisation), have been ascribed an origin due to the arresting of this first order transition kinetics at low temperature and high magnetic field.

First-order transition from antiferromagnetism to ferromagnetism inCe(Fe0.96Al0.04)2

Results of dc magnetization study are presented showing interesting thermomagnetic history effects across the antiferromagnetic to ferromagnetic transition in Ce(Fe$_{0.96}$Al$_{0.04})_2$. Specifically, we observe (i)ZFC/FC irreversibility rising with increasing field; (ii) virgin curve lying outside the envelope M-H curve. We argue that these effects are quite different from the characteristics seen in spin-glasses or in hard ferromagnets; they can be understood as metastabilities associated with a first order magnetic phase transition.

Magnetic ordering and crystalline electric field splitting in Nd 3Pd 20Si 6

At the temperatures T N1≈2.4 K and T N2≈0.7 K, the cubic intermetallic compound Nd 3Pd 20Si 6 undergoes second order magnetic phase transitions. Here we report on the magnetic structures obtained from low-temperature neutron diffraction data by using a He dilution refrigerator, and on the crystalline electric field splitting of the Nd 3+ ground state multiplet 4I 9/2 obtained from inelastic neutron scattering data.

Magnetism in rare earth Co2 compounds under high pressures

We have studied electrical resistivity anomalies connected with magnetism in RECo2 (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at TC, whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although TC decreases with pressure in all four compounds, the TC(P) dependence for NdCo2 and TbCo2 differs considerably from this for HoCo2 and ErCo2. For the latter two, Tc vs P data deviate dramatically from the initial linear dependence above a critical pressure Pc to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at Tc for P>4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P>Pc the projected Co 3d density of states at EF decreases. Hence the Co moment collapses because the RE-Co-RE exchange channel becomes ineffective to induce the itinerant 3d electron metamagnetism. The localized RE moments, however, order at a “residual” TC due to the persisting Ruderman–Kittel–Kasuya–Yosida-type exchange interaction. For NdCo2 and TbCo2 the Tc values decrease with pressure by an exponential law but remain rather high in the highest applied pressures. Results of first-principles electronic structure calculations using the full-potential linearized augmented plane wave method are presented for HoCo2 and NdCo2 compounds, as well.

Evidence for competing order parameters in the paramagnetic phase of layered manganites (invited)

The magnetic field and temperature dependence of the magnetic susceptibility is studied for the ferromagnetic layered manganites SrO(La1−xSrxMnO3)2 in the composition range x=0.32–0.40. In the paramagnetic phase, the susceptibility exhibits an anomalous maximum at an intermediate magnetic field value. The size of this field-induced susceptibility enhancement increases dramatically with x from 10% for x=0.32 to 160% for x=0.40. The temperature dependence of the effect shows a maximum at T≈1.1 TC for all x. Quantitative analysis in terms of the Landau theory of phase transitions enables us to identify a distortion of the free energy F in the paramagnetic phase that is associated with the susceptibility anomaly. This free energy distortion corresponds to a magnetic system that approaches a first order magnetic phase transition as the temperature is lowered toward TC. Such a behavior is indicative of a second, competing order parameter, which is identified as the recently observed charge density wave. In the immediate vicinity of TC, the anomaly disappears and the system seems to undergo a more conventional second order paramagnetic–ferromagnetic phase transition.

High-magnetic-field behavior of the triangular-lattice antiferromagnetCuFeO2

The high magnetic field behaviour of the triangular lattice antiferromagnet CuFeO_2 is studied using single crystal neutron diffraction measurements in a field of up to 14.5 T and also by magnetisation measurements in a field of up to 12 T. At low temperature, two well-defined first order magnetic phase transitions are found in this range of applied magnetic field (H // c): at H_c1=7.6(3)/7.1(3) T and H_c2=13.2(1)/12.7(1) T when ramping the field up/down. In a field above H_c2 the magnetic Bragg peaks show unusual history dependence. In zero field T_N1=14.2(1) K separates a high temperature paramagnetic and an intermediate incommensurate structure, while T_N2=11.1(3) K divides an incommensurate phase from the low-temperature 4-sublattice ground state. The ordering temperature T_N1 is found to be almost field independent, while T_N2 decreases noticeably in applied field. The magnetic phase diagram is discussed in terms of the interactions between an applied magnetic field and the highly frustrated magnetic structure of CuFeO_2

Observation of the Initial Stage of the First Order Magnetic Phase Transition in Erbium Single Crystal

Observation of the Initial Stage of the First Order Magnetic Phase Transition in Erbium Single Crystal

A magnetic phase transition temperature calibration device

We have devised a technique of manufacturing temperature sensor calibration devices based on the magnetic properties of the pseudobinary compounds of formula (RxR1−x′)Co2, where R and R′ are heavy rare earth elements. The device is a solid sensor which provides an easily detectable first order magnetic phase transition at fixed temperature points. It is known that a first order magnetic phase transition from ferrimagnetism to paramagnetism is observed in compounds as TmCo2 (3.70 K), ErCo2 (32.05 K), HoCo2 (77.12 K), and DyCo2 (136.55 K). These transitions correspond to a large anomaly in the characteristic properties as function of temperature. In this work, we present the electrical resistivity and magnetization measurements of (Er, Ho)Co2 series and DyCo2 compounds showing the viability of the proposed devices from 32.05 up to 136.55 K. This range can be extended below and above by substitution of the chemical components and the stoichiometric composition. The number of transitions can be fixed by a convenient arrangement of several elements.

Temperature Calibration Devices Based on First Order Magnetic Phase Transition of (R<sub>1-x</sub>R'<sub>x</sub>)Co<sub>2</sub> Pseudo-Binaries Compounds

Temperature Calibration Devices Based on First Order Magnetic Phase Transition of (R<sub>1-x</sub>R'<sub>x</sub>)Co<sub>2</sub> Pseudo-Binaries Compounds

Novel Thermal Effects at the First Order Magnetic Phase Transition in Erbium, and a Comparison with Dysprosium

In low temperature studies of ultrapure erbium (and dysprosium) we have discovered unusual thermal effects at the first order magnetic transformation of erbium $(\ensuremath{\cong}19\mathrm{K})$. These include (1) superheating (i.e., the metal is colder after heat has been added to it than before the heat pulse), (2) supercooling, and (3) the existence of metastable intermediate phases during this phase transformation in erbium (four on heating and two on cooling). In comparison, dysprosium exhibits both superheating and supercooling, but no intermediate metastable phases are observed. Furthermore, none of these effects are observed in less pure metals.

A Quantum Field Theoretical Analysis of Spin-Half Metamagnetism

A number of metallic compounds such as FeRh, Mn3GaC , and FeCl2 exhibit the interesting property of metamagnetism, i.e. the presence of order–order magnetic-phase transitions. The microscopic Hamiltonian that is commonly used to describe this class of system is of Heisenberg type with competing exchange interactions between the nearest- and next-nearest neighbours. Starting from this quantum Hamiltonian, through several transformations, it is possible to obtain an effective second-quantized Hamiltonian which has a two-body interaction term. A recent method of analyzing such Hamiltonians is then applied based on exact calculations for the corresponding nonlinear equation of motion. This is followed by quantization about the classical solutions. Stability conditions imposed on the classical solutions agree with earlier results.

Band metamagnetism and related phenomena in Er(Co1−xSix)2

We report on magnetic, transport, and cohesive properties of Er(Co1−xSix)2 compounds in varying external conditions (T,H,p). The localized Er 4f magnetic moments in ErCo2 order ferromagnetically below TC=33 K. The Co 3d band states form no moment above TC, but below TC the majority and minority 3d sub-bands split due to strong 4f−3d exchange interaction. Consequently, Co moments of ∼1 μB appear coupled antiparallel to the Er moments. This is accompanied by a ∼50% resistivity drop and an abrupt lattice expansion. The sharp magnetization, resistivity, and volume anomalies in the critical temperature region indicate a first order magnetic phase transition. Detailed comparative studies reveal that the onset of Co magnetizm appears at T0∼(TC−1) K. Si substitution for Co enhances both, TC (up to 66 K for x=0.1) and ΔT=TC−T0, but the lattice parameter remains unchanged. In opposite, both the magnetovolume and resistivity anomaly at T0 become reduced. Pressure effect on TC(T0) is large and negative, ∂ ln TC/∂p∼−28 Mbar−1 for all x⩽0.1. Also the resistivity drop at T0 and the magnetovolume effect (ωs=3 ΔL/L=4.2×10−3 in ErCo2) below T0 become reduced with increasing pressure. The latter is reflecting reduction of the Co moment gained below T0.

Effect of the Crystal Mosaicness on the Kinetics of the First Order Magnetic Phase Transition in Er

The time evolution of the first order magnetic phase transition in Er has been studied by X-ray diffraction with the single crystal sample which has a broad mosaic structure. The repetition of the heat cycles across the Curie temperature causes the broad mosaic structure, but this is found to exist only at the surface of the sample. The behavior of the time dependence of the phase transition in a region of broad mosaic structure of the sample differs qualitatively with that of the initial state of the crystal. The coexistence of the two magnetic phases has been observed in a certain narrow temperature range. A model for “pinning” the evolution of the stable phase by crystal defects has been proposed.

Antiferromagnetic Ground States in Some Commonly Known Lattices

Antiferromagnetism is a very interesting behaviours in some real materials and their alloys. It is important in high T_c materials as well. The main reasons for antiferromagnetism are the kinetic exchange[1,2]. In this study, I will consider commonly known lattices in two or three dimensions. Using the Heisenberg model I will show that there are two systems: Frustrated and unfrustrated. In unfrustrated case the spin structure is unique and it involves ferromagnetism as well. However, in frustrated case the spin structure is not unique and it gives more than one spin orderings. This behaviours is very interesting since it causes a first order magnetic phase transition[3,4]. Finally, I will summarize that the simplier lattice (the square, the S.C. and B.C.C.) are unfrustrated and the complicated lattices (triangular, F.C.C. and H.C.P.) are frustrated.

Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

Magnetic properties of ultrafine clusters of Fe5HO8·4H2O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R cr = 2 α β η/(1 - T cc/T 0), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T cc = 3/2 Nκ B T o 2 ηβ 2 (N is the number of iron atoms, κ B is the Boltzmann constant, T o is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R cr ∼ 1.5–2.0 nm and T JMT ∼ 4.2–6 K, respectively. Satisfactory agreement between the value R cr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.

An investigation of the magnetic properties of Pd2DySn

An investigation is reported of the magnetic ordering in Pd2DySn using magnetisation, specific heat and neutron scattering. A first order magnetic phase transition is identified at T=5K leading to an antiferromagnetic ordering below. An alternative magnetic order is suggested for temperatures between 5K and 12K.