Continuous Magnetic Transition Research Articles

Critical exponents and magnetic entropy change across the continuous magnetic transition in (La, Pr)-Ba manganites

Critical exponents and magnetic entropy change across the continuous magnetic transition in (La, Pr)-Ba manganites

Low field control of spin switching and continuous magnetic transition in an ErFeO3 single crystal.

The magnetic behavior of a rare-earth orthoferrite ErFeO3 single crystal can be controlled by low magnetic fields from a few to hundreds of Oe. Here we investigated a high-quality ErFeO3 single crystal in the temperature range of 5-120 K, with two types of spin switching in the field-cooled-cooling (FCC) and field-cooled-warming (FCW) processes below the temperature of the spin reorientation (SR) transition from Γ4 to Γ2 at 98-88 K. The magnitude of the applied magnetic fields can regulate two types of spin switching along the a-axis of the ErFeO3 single crystal but does not affect the type and temperature range of the SR transition. An interesting "multi-step" type-II spin switching is observed in FCW under low magnetic fields (H < 18 Oe) just below the SR transition temperature, which is associated with the interaction and the change of magnetic configurations from rare-earth and iron magnetic sublattices. When the magnetic field is lower than 15 Oe, the type-II spin switching in the FCW process gradually changes to a continuous magnetic transition along the a-axis of the ErFeO3 single crystal. As the magnetic field is reduced to less than 17 Oe, the type-I spin switching in the FCW process also transforms into a continuous magnetic transition. Understanding the magnetic reversal effects will help us explore the potential applications of these magnetic materials for future information devices.

Suppression of superconducting parameters by correlated quasi-two-dimensional magnetic fluctuations

We consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside superconducting state and the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. An example of such material is RbEuFe$_{4}$As$_{4}$. We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is sensitive to the relation between the magnetic correlation length, $\xi_{h}$, and superconducting coherence length $\xi_{s}$ defining the 'scattering' ($\xi_{h}<\xi_{s}$) and 'smooth' ($\xi_{h}>\xi_{s}$) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations. Since the magnetic-scattering probability varies at the energy scale comparable with the gap, the quasiclassical approximation is not applicable in the crossover region and microscopic treatment is required. We find that the corrections to both the gap and superfluid density grow proportionally to $\xi_{h}$ until it remains much smaller than $\xi_{s}$. When $\xi_{h}$ exceeds $\xi_{s}$, both parameters have much weaker dependence on $\xi_{h}$. Moreover, the gap correction may decrease with increasing of $\xi_{h}$ in the vicinity of the magnetic transition. We also find that the crossover is unexpectedly broad: the standard scattering approximation becomes sufficient only when $\xi_{h}$ is substantially smaller than $\xi_{s}$.

Spin reorientation by Ni doping in Cu1−xNixCr2O4 spinels with x = 0 and 0.1, and evidence for canted magnetic Cr order above the onset of a ferromagnetic Cu

In ferrimagnetic spinels AB2O4 the magnetic structure is strongly influenced by lattice distortions, geometric frustration and the electronic properties of the cations at the A and B sites. Here, we report a comprehensive study on the temperature dependence of the magnetic structure of CuCr2O4 and Cu0.9Ni0.1Cr2O4 using neutron diffraction. CuCr2O4 undergoes a first continuous magnetic transition around 155 K into a canted long-range spin order on the Cr sublattice established by an antiferromagnetic and a ferromagnetic mode. Below 130 K a second transition occurs into a ferromagnetic order on the Cu sublattice resulting in a ferrimagnetic spin arrangement. Correlations between the appearance of magnetic modes and changes in the lattice geometry at different temperatures are discussed giving insight to magnetoelastic coupling. The occurrence of a ferromagnetic Cr mode above 130 K questions the common interpretation that a strong antiferromagnetic coupling between A and B spins in ferrimagnetic AB2O4 spinels is responsible for spin canting. From our neutron diffraction measurements of Cu1−xNixCr2O4 we identify a spin reorientation for the ferromagnetic modes with a nickel content between x = 0 and 0.1.

Magnetic properties and magnetocaloric effects in Er1 − xGdxCoAl intermetallic compounds**Project supported by the National Natural Science Foundation of China (Grant Nos. 51322605 and 51271192).

The magnetism and magnetocaloric effect in Er1 − xGdxCoAl (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) were investigated. The Er1 − xGdxCoAl compounds were synthesized by arc melting. With the increasing Gd content, the Néel temperature (TN) linearly increases from 14 K to 102 K, while the magnetic entropy change (−ΔSM) tends to decrease nonmonotonously. Under the field change from 0 T to 5 T, the −ΔSM of the compounds with x = 0.2–1 are stable around 10 J/kg ·K, then a cooling platform between 20 K and 100 K can be formed by combining these compounds. For x = 0.6, 0.8, 1.0, the compounds undergo two successive magnetic transitions, one antiferromagnetism to ferromagnetism and the other ferromagnetism to paramagnetism, with increasing temperature. The two continuous magnetic transitions in this series are advantageous to broaden the temperature span of half-peak width (δT) in the −ΔSM−T curve and improve the refrigeration capacity.

Observation of the second-order magnetic and reentrant spin-glass transitions in LiNi0.5Mn0.5O2

The structure and magnetic properties of LiNi0.5Mn0.5O2 were studied by synchrotron X-ray diffraction, dc and ac susceptibilities. The material showed a continuous magnetic transition from paramagnetism into ferrimagnetism, followed by a spin glass with the decrease of temperature. Using a criterion given by Banerjee to distinguish first-order magnetic transition from second-order ones, it is shown that the ferrimagnetic transition at 108 K belongs to the second-order type. The frequency dependence of peak intensity and the shift in ac susceptibility at 14 K suggest a reentrant spin glass transition in LiNi0.5Mn0.5O2.

Correlation between the nucleation of a Griffiths-like Phase and Colossal Magnetoresistance across the compositional metal-insulator boundary in La1-xCaxMnO3

Detailed measurements of the magnetic and transport properties of single crystals La1−xCaxMnO3 (0.18 ≤ x ≤ 0.27) are summarized; comparisons between which (i) not only confirm that Griffiths Phase-like(GP) features are not a prerequisite for CMR, but also demonstrate that the presence of GP-like characteristics do not guarantee the appearance of CMR; (ii) indicate that whereas continuous magnetic transitions occur for 0.18 ≤ x ≤ 0.25, the universality class of these transitions belongs to that of nearest-neighbour 3D Heisenberg model only for x ≤ 0.20, beyond which complications due to GP-like behaviour occur.

The evolution of Griffiths-phase-like features and colossal magnetoresistance inLa1−xCaxMnO3 (0.18 ≤ x ≤ 0.27) across the compositional metal–insulator boundary

Detailed measurements of the magnetic and transport properties of single crystals ofLa1−xCaxMnO3 (0.18 ≤ x ≤ 0.27) are summarized, and lead to the following conclusions. Whiletemperature-dependent (magneto-) resistance measurements narrow thecompositionally modulated metal–insulator (M–I) transition to lie between0.19 ≤ xc ≤ 0.20 in the series studied, comparisons between the latter magnetic data provide thefirst unequivocal demonstration that (i) the presence of Griffiths-phase-like (GP)features do not guarantee colossal magnetoresistance (CMR), while confirming(ii) that neither are the appearance of such features a prerequisite for CMR.These data also reveal that (iii) whereas continuous magnetic transitions occur for0.18 ≤ x ≤ 0.25, theuniversality class of these transitions belongs to that of a nearest-neighbour 3D Heisenberg model onlyfor x≤0.20, beyond which complications due to GP-like behaviour occur. The implications of thevariation (or lack thereof) in critical exponents and particularly critical amplitudes andtemperatures across the compositionally mediated M–I transition support theassertion that the dominant mechanism underlying ferromagnetism across theM–I transition changes from ferromagnetic super-exchange (SE) stabilized byorbital ordering in the insulating phase to double-exchange (DE) in the orbitallydisordered metallic regime. The variations in the acoustic spin-wave stiffness,D, and thecoercive field, HC, support this conclusion. These SE and DE interaction mechanisms are demonstrated tonot only belong to the same universality class but are also characterized by comparablecoupling strengths. Nevertheless, their percolation thresholds are manifestly different inthis system.

Influence of the nature of the magnetic phase transition on the associated magnetocaloric effect in the Ni–Mn–Ga system

A summary of extensive measurements of magnetisation isotherms and of the field- and temperature-dependent AC susceptibility for Ni–Mn–Ga alloys is presented. Detailed analysis of these data provide a quantitative appraisal of the magnetic phase transitions in this system through the establishment of metamagnetic boundaries at first-order magnetic/martensitic structural transitions and crossover lines at continuous magnetic transitions. At the “tuned” composition Ni 55.2Mn 18.6Ga 26.2 a giant magnetocaloric effect with Δ S m=−20.4×10 4 ergs g −1 K −1 (−20.4 J kg −1 K −1) (at 317 K in a 50 kOe (5 T) field) is achieved, and this is shown to result from the essential coincidence of two transitions exhibiting both first- and second-order characteristics simultaneously.

The Ising model on a dynamically triangulated disk with a boundary magnetic field

We use Monte Carlo simulations to study a dynamically triangulated disk with Ising spins on the vertices and a boundary magnetic field. For the case of zero magnetic field we show that the model possesses three phases. For one of these the boundary length grows linearly with disk area, while the other two phases are characterized by a boundary whose size is on the order of the cut-off. A line of continuous magnetic transitions separates the two small boundary phases. We determine the critical exponents of the continuous magnetic phase transition and relate them to predictions from continuum 2-d quantum gravity. This line of continuous transitions appears to terminate on a line of discontinuous phase transitions dividing the small boundary phases from the large boundary phase. We examine the scaling of bulk magnetization and boundary magnetization as a function of boundary magnetic field in the vicinity of this tricritical point.

Magnetic structure and critical properties of FeGe2.

A neutron study of the tetragonal antiferromagnet ${\mathrm{FeGe}}_{2}$ has shown the existence of two continuous magnetic transitions at temperatures of \ensuremath{\sim}263 and \ensuremath{\sim}289 K. The upper temperature corresponds to a transition from paramagnetism to a basal-plane spiral structure propagating along the cell edges in that plane. At the lower temperature the spiral structure is transformed into the simple collinear structure previously reported in the literature. Typical critical behavior is observed at the upper temperature for individual satellite peaks. The spiral propagation vector decreases continuously to zero at the lower critical point, exhibiting power-law behavior with an exponent of ${0.40}_{7}$\ifmmode\pm\else\textpm\fi{}0.005. Heat-capacity measurements reveal two \ensuremath{\lambda}-type anomalies with critical exponents in the expected range. The phase diagram has been analyzed using mean-field and renormalization-group considerations. A model based on zero basal-plane spin anisotropy yields a magnetic structure which agrees with the observed structure of the intermediate phase. The effect of an external field has also been treated theoretically.