Spontaneous Spin-reorientation Research Articles

Effect of sintering temperature on microstructure and magnetic properties of ErFeO3 ceramics

ErFeO3 ceramics had been synthesized by the solid-state reaction method at a variety of sintering temperatures (1250, 1300, 1350, 1400, 1450, and 1500 °C). The microstructure, surface morphology, and elemental valence state of ErFeO3 polycrystalline ceramics were investigated using X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The study discovered that Fe3+ ions keep their valence, and all samples at temperatures ranging from 1300 to 1450 °C are in a pure phase. Within this range, increasing temperature widens the Fe-O-Fe bond angle, lengthens the Er-O bond, suppresses the super-exchange contact between the two ions via O2−, and diminishes the electromagnetic coupling effect. A Physical Property Measurement System (PPMS) was utilized to assess the magnetic properties of ErFeO3 polycrystalline ceramics. Ceramic samples sintered at 1300 °C showed spontaneous spin reorientation and a shift from ionic magnetic disorder to magnetic order. The magnetic hysteresis loops at temperatures before and after spin reorientation were examined. It was noted that the polycrystalline ceramic ErFeO3 underwent a transition in its magnetic configuration from Γ4 (Gx, Ay, Fz) to Γ2 (Fx, Cy, and Gz). As the sintering temperature increases, ErFeO3 polycrystalline ceramics lose their temperature adjustment point. This disrupts the spin reorientation of Er3+ and Fe3+ magnetic moments in the 89–102 K region. The secondary phase transition of ErFeO3 polycrystalline ceramics causes disorder, and the Néel temperature of Er3+ rises. This suggests that the sublattice field and magnetic ordering of Er3+ and Fe3+ are disturbed when the sintering temperature is raised. Consequently, 1300 °C is the optimal sintering temperature for ErFeO3 polycrystalline ceramics.

Ferrimagnetic structures with rare-earth induced spin-reorientation in the Mn self-doped perovskite (Er0.7Mn0.3)MnO3

The search for spin-reorientation (SR) phase transitions, a spontaneous rotation of ordered magnetic moments, in ferrimagnetic (FiM) materials, which carry a net magnetization, is of fundamental and practical interest for applications in the field of spintronics. In this work, we have investigated Mn self-doped (Er0.7Mn0.3)MnO3 solid solution with GdFeO3-type Pnma perovskite structure by combining specific heat, magnetic susceptibility and neutron powder diffraction measurements. We provide experimental evidence for FiM order below TC = 104 K, and a spontaneous SR transition at TSR = 11 K. A FiM structure appears in all (R1-xMnx)MnO3 compounds with R = Dy–Lu for x ≥ 0.2. But in this structural family, R = Er is the only material with a SR transition. FiM order in (Er0.7Mn0.3)MnO3 appears with ferromagnetic (FM) ordering of Mn3+ and Mn4+ cations at the B site along the a-direction, which are antiferromagnetically (AFM) coupled with Er3+ and Mn2+ cations at the A site. At TSR, ordered Er3+ change direction from the magnetically harder a-axis to the magnetically easy b-axis and induce a change of the whole FiM structure, including the direction of all Mn spins from the irreducible representation mGM3+ to mGM4+. We calculated the crystal-field (CF) anisotropy for Ho3+ in (Ho0.8Mn0.2)MnO3 and Er3+ in (Er0.7Mn0.3)MnO3, based on the point charge model; the results revealed large magnetic anisotropies. For the FiM structure of (Ho0.8Mn0.2)MnO3, the magnetically easy a-axis of Ho3+ keeps the high-temperature magnetic directions along the a-axis and gives rise to a pronounced magnetization reversal effect at low temperature because of a significant rise of Ho3+ ordered moments. On the other hand, for the FiM structure of (Er0.7Mn0.3)MnO3, the magnetically easy b-axis of Er3+ gives rise to a SR phase transition at TSR, which does not lead to magnetization reversal even though Er3+ ordered moments rise significantly at low temperatures.

Simple spin-reorientation model for rare-earth orthoferrites and orthochromites

A simple theory of spontaneous spin reorientation in rare-earth orthoferrites RFeO3 and orthochromites RCrO3 induced by 4f-3d-interaction is presented. It is shown that both the temperature and the nature of the spin reorientation are the result of the competition between the second- and fourth-order spin anisotropy of the 3d-sublattice, the crystal field for 4f-ions, and also the 4f-3d interaction. A case of the substitution of the rare-earth ions with a non-magnetic impurity is also considered. Keywords: spin reorientation, 4f-3d interaction, weak ferromagnetism, orthoferrites, orthochromites.

Простая спин-переориентационная модель редкоземельных ортоферритов и ортохромитов

A simple theory of spontaneous spin reorientation in rare-earth orthoferrites RFeO3 and orthochromites RCrO3 induced by 4f-3d interaction is presented. It is shown that both the temperature and the nature of the spin reorientation are the result of the competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f-ions, and also the 4f-3d interaction. A case of the substitution of the rare-earth ions with a non-magnetic impurity is also considered.

Inclined magnetic structures and magnetic phase diagrams for the Pb2Fe2−xMnxGe2O9 (x = 0.16) single crystal

Single crystals of the Pb2Fe2−xMnxGe2O9 (x = 0.16) antiferromagnet have been grown. Using the specific heat measurements, a Néel temperature of TN = (42.0 ± 0.5) K for the synthesized crystals has been found. It has been shown using the magnetic measurements that, due to the competition between the magnetoanisotropic contributions of the iron and manganese subsystems in the crystals, near a temperature of Tc = 22 K, a spontaneous spin-reorientation transition occurs, the temperature of which in an applied magnetic field changes with the field value and orientation relative to the rhombic axes of the crystal. Based on the analysis of the temperature and field dependences of the magnetization obtained at different orientations of the magnetic field, it has been established that, below Tc, an inclined magnetic structure is formed in the crystal. The antiferromagnetic vector of the inclined structure rotates smoothly in the rhombic bc plane with increasing temperature from a direction close to the b axis at T = 4.2 K and tends to the rhombic c axis at T = Tc. The rotation of the antiferromagnetic vector occurs also at fixed temperatures T < Tc with increasing magnetic field. In the temperature range of Tc < T < TN, the antiferromagnetic vector is oriented along the rhombic c axis.Magnetic phase diagrams of states have been built for different magnetic field orientations relative to the rhombic axes of the crystal. The richest phase diagram is shown to correspond to the orientation H||c and contains, along with the above-listed states, one more inclined phase, in which the antiferromagnetic vector rotates toward the rhombic a axis direction with a change in temperature or magnetic field.

Simple Realistic Model of Spin Reorientation in 4f-3d Compounds

This is a simple but realistic microscopic theory of spontaneous spin reorientation in rare-earth perovskites, orthoferrites RFeO3 and orthochromites RCrO3, induced by the 4f-3d interaction, namely, the interaction of the well-isolated ground-state Kramers doublet or non-Kramers quasi-doublet of the 4f ion with an effective magnetic field induced by 3d sublattice. Both the temperature and the nature of the spin-reorientation transition are the result of competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f ions, and 4f-3d interaction.

Ferromagnetic metallic Sr-rich Ln1/2A1/2CoO3 cobaltites with spontaneous spin rotation

The Pr$_{0.50}$Sr$_{0.5}$0CoO$_3$ perovskite exhibits unique magnetostructural properties among the rest of ferromagnetic/metallic Ln$_{0.50}$Sr$_{0.50}$CoO$_3$ compounds. The sudden orthorhombic-tetragonal (Imma $\to$ I4/mcm) structural transition produces an unusual magnetic behavior versus temperature and external magnetic fields. In particular, the symmetry change is responsible for a spontaneous spin rotation in this metallic oxide. We have studied half-doped Ln$_{0.50}$(Sr$_{1-x}$A$_x$)$_{0.50}$CoO$_3$ cobaltites varying the ionic radius rA of A-site cations (divalent cations and lanthanides) in order to complete the T-rA phase diagram. The influence of the structural distortion and the A-cations size for the occurrence of a spontaneous spin reorientation in the metallic state has been investigated. As the reorientation of the magnetization is driven by the temperature induced collapse of the orthorhombic distortion, a careful investigation of the structural symmetry is presented varying the structural distortion of the Sr-rich half-doped cobaltites by means of both compositional and temperature changes. The region in the phase diagram of these perovskites where the phase of magnetic symmetry Fm'm'm replaces that of Im'm'a symmetry was determined in this family of ferromagnetic/metallic cobaltites. In that region the magnetization direction has rotated 45 degrees within the a-b plane with respect to the second.

Magnetic structures and spin reorientation in the B-site disordered perovskite PrFe0.5Cr0.5O3

Through the detailed studies utilizing x-ray and neutron powder diffraction and magnetometry measurements, we report the magnetic phase transitions in the B-site disorder perovskite oxide PrFe0.5Cr0.5O3. The Fe3+/Cr3+ sublattice was observed to form a long-range magnetic order with the GxFz configuration below TN = 270 K from neutron powder diffraction. It then undergoes the second-order phase transition to FxGz spin configuration below 210 K and the transition ends at ~170 K. On the other hand, long range magnetic ordering with FxCy configuration at Pr3+ sublattice can be identified at base temperature. The magnetic phase diagram is derived with spontaneous spin ordering and reorientation at A- and B-sublattices.

Non-zero spontaneous magnetic moment along crystalline b-axis for rare earth orthoferrites

Rare earth orthoferrites demonstrate great application potentials in spintronics and optical devices due to their multiferroic and magnetooptical properties. In RFeO3, magnetic R3+ undergo a spontaneous spin reorientation in a temperature range determined by R (rare earth), where the magnetic structure changes from Γ2 to Γ4. The b-axis component of their magnetic moment, Mb, is reported in numerous neutron diffraction studies to remain zero at all temperatures. More sensitive magnetometer measurements reveal a small non-zero Mb, which is minute above ∼200 K. Mb increases with cooling and reaches values of ∼10–3 μB/f.u. at temperatures within or below the spin reorientation temperatures. Our results can be explained by assuming the Fe3+ spins as the origin of non-zero Mb, while R3+ spins suppress Mb. The representation analysis of point groups shows that non-zero Mb is associated with a small admixture of the Γ3 phase to Γ2 or Γ4. Such a mixing of the three magnetic phases requires at least a fourth order of the spin Hamiltonian for RFeO3 to describe the non-zero Mb.

Spontaneous Spin Reorientation in Gd–Co Amorphous Films

The phenomenon of spontaneous spin reorientation in Gd–Сo amorphous films, which results from the competition between the perpendicular magnetic anisotropy and shape anisotropy, is studied. It is shown that the reorientation occurs via the formation of a fine-grained nonuniform magnetic structure called hypercritical state in a wide temperature range. Magnetic domains in the spin-reorientation region are characterized using magnetic and magneto-optical measurements. Based on the experimental magnetometric data and results of simulating the temperature dependences of spontaneous magnetization, the phase diagram of the films containing 16–26 at % Gd is constructed, which, being plotted on coordinates elemental composition—temperature, exhibits the ranges of existence of the perpendicular anisotropy, transition magnetic anisotropy (hypercritical state), and planar magnetization state.

Effects of a high DC magnetic field on spin reorientation in dysprosium- yttrium iron garnets at low temperatures

In this work, magnetic properties of dysprosium doped yttrium iron garnet single crystals DyxY3-xFe5O12 (1 ≤ x ≤ 3) are studied in the 2-50 K temperature range under high D.C. magnetic fields H up to 16 T. When the sample is allowed to rotate freely on itself, critical points are observed and form a critical line of second order type which increases linearly with T in the magnetic phase diagram. This line is independent of the content x delimiting a canted phase which is issued from the low symmetry angular &amp;lt;uuw&amp;gt; phase occurring from the spontaneous spin reorientation below TSR (H = 0) = 14.62 K. In comparison with the free ion, the observed reduction at 0 K of the Dy3+ moments is also discussed.

Magnetic and Magnetodielectric Properties of Ho0.5Nd0.5Fe3(BO3)4

The magnetic and magnetodielectric properties of Ho0.5Nd0.5Fe3(BO3)4 ferroborate with the competing Ho–Fe and Nd–Fe exchange couplings have been experimentally and theoretically investigated. Step anomalies in the magnetization curves at the spin-reorientation transition induced by the magnetic field B ║ c have been found. The spontaneous spin-reorientation transition temperature TSR ≈ 8 K has been refined. The measured magnetic properties and observed features are interpreted using a single theoretical approach based on the molecular field approximation and calculations within the crystal field model of the rare-earth ion. Interpretation of the experimental data includes determination of the crystal field parameters for Ho3+ and Nd3+ ions in Ho0.5Nd0.5Fe3(BO3)4 and parameters of the Ho–Fe and Nd–Fe exchange couplings.

Магнитные и магнитодиэлектрические свойства Ho-=SUB=-0.5-=/SUB=-Nd-=SUB=-0.5-=/SUB=-Fe-=SUB=-3-=/SUB=-(BO-=SUB=-3-=/SUB=-)-=SUB=-4-=/SUB=-

AbstractThe magnetic and magnetodielectric properties of Ho_0.5Nd_0.5Fe_3(BO_3)_4 ferroborate with the competing Ho–Fe and Nd–Fe exchange couplings have been experimentally and theoretically investigated. Step anomalies in the magnetization curves at the spin-reorientation transition induced by the magnetic field B ║ c have been found. The spontaneous spin-reorientation transition temperature T _SR ≈ 8 K has been refined. The measured magnetic properties and observed features are interpreted using a single theoretical approach based on the molecular field approximation and calculations within the crystal field model of the rare-earth ion. Interpretation of the experimental data includes determination of the crystal field parameters for Ho^3+ and Nd^3+ ions in Ho_0.5Nd_0.5Fe_3(BO_3)_4 and parameters of the Ho–Fe and Nd–Fe exchange couplings.

Spontaneous spin-reorientation transition in (NdSmDy)(FeCo)B alloys

A magnetic transition accompanied by a sharp decrease in magnetization has been detected in an (NdSmDy)(FeCo)B alloy at temperature T = 110 K. It is found that the sample undergoes a spin-reorientation transition accompanied with a change in the type of magnetic anisotropy. The “easy axis”-type anisotropy corresponds to high temperatures T > 110 K. A magnetic structure of the type of “the cone of easy-magnetization axes” forms at low temperatures T < 110 K.

Anisotropic magnetoresistivity and magnetic entropy change in HoAl2

We report a correlation between magnetoresistivity Δρmag and magnetic entropy change ΔSmag for specified crystallographic directions of a HoAl2 single crystal. This ferromagnetic compound shows differentiated behavior among the members of the RAl2 series due to the presence of spontaneous spin reorientation. We discussed this effect in order to establish a relationship between the two quantities studied. The calculations were performed using a self-consistent model based on a Hamiltonian containing exchange, Zeeman and crystal field terms. We found that the normalized relation {Δρmag} = (T/TC)m{ΔSmag}, with m=0.1 for the [100] and [110] directions matches satisfactorily the two quantities.

Low-temperature magnetic phase transitions in the multiferroic Nd0.9Dy0.1Fe3(BO3)4. Part 1. Transitions induced by magnetic fields directed along the trigonal symmetry axis. Spontaneous transitions with temperature changes

The elastic and magnetic characteristics of single-crystal Nd0.9Dy0.1Fe3(BO3)4 are studied at low temperatures in zero magnetic field and in external fields H||C3. The temperature dependences of the acoustic mode velocities and the magnetic susceptibility manifest a transition of the magnetic subsystem into a magnetically ordered state and two successive, spontaneous spin-reorientation phase transitions. The possibility of a spontaneous transition into an incommensurate (spiral) magnetic phase in the crystal is discussed. It is shown that an external magnetic field directed along the trigonal axis of the crystal induces a sequence of spin-reorientation phase transitions. An H-T phase diagram (H||C3) is constructed for this compound.

A New Method of Intergrain Exchange Interaction Energy Determination in Nanostructured Alloys with Spontaneous Spin-Reorientation Transition

A new method of intergrain exchange interaction energy determination in nanostructured magnetically hard alloys with spontaneous spin-reorientation transition has been developed and verified. It is based on the knowing of the material magnetic anisotropy constants values at its “easy cone” state and the external negative magnetic field strength H, which reversibly “tears” the exchange coupling between nanograins. It should be more than exchange field and smaller than coercivity value. Using this method the exchange interaction energy parameter for MQP-B+ alloy was estimated. Its value is in agreement with estimated one by FMR technique for film system Fe-Nd2Fe14B and close to that obtained from the remanent magnetization temperature dependences measurements in negative magnetic fields.

Tailoring surface phase transition and magnetic behaviors in BiFeO3 via doping engineering.

The charge-spin interactions in multiferroic materials (e.g., BiFeO3) have attracted enormous attention due to their high potential for next generation information electronics. However, the weak and deficient manipulation of charge-spin coupling notoriously limits their commercial applications. To tailor the spontaneous charge and the spin orientation synergistically in BiFeO3 (BFO), in this report, the 3d element of Mn doping engineering is employed and unveils the variation of surface phase transition and magnetic behaviors by introducing chemical strain. The spontaneous ferroelectric response and the corresponding domain structures, magnetic behaviors and spin dynamics in Mn-doped BFO ceramics have been investigated systematically. Both the surface phase transition and magnetization were enhanced in BFO via Mn doping. The interaction between the spontaneous polarization charge and magnetic spin reorientation in Mn-doped BFO are discussed in detail. Moreover, our extensive electron paramagnetic resonance (EPR) results demonstrate that the 3d dopant plays a paramount role in the surface phase transition, which provides an alternative route to tune the charge-spin interactions in multiferroic materials.

Magnetoelastic properties of Nd0.9Dy0.1Fe3(BO3)4 ferroborate

The low-temperature elastic characteristics of single-crystal Nd0.9Dy0.1Fe3(BO3)4 are studied. A transition of the magnetic system into an antiferromagnetic ordered state and a spontaneous spin-reorientation phase transition are observed in the temperature behavior of the speed and absorption of acoustic modes. Two anomalies in the speed and absorption of transverse acoustic modes are discovered at the lowest experimental temperature of 1.7 K in an external magnetic field directed along the trigonal axis of the crystal. These features are associated with two reorientation phase transitions that follow one another.

Magnetic susceptibility and heat capacity of Ho0.5Nd0.5Fe3(BO3)4

The magnetic properties of trigonal Ho0.5Nd0.5Fe3(BO3)4 substituted compound with the competitive Ho–Fe and Nd–Fe exchange interactions have been investigated theoretically. The spontaneous spin-reorientation transition near 9K has been described. The increase in temperature of the spin-reorientation transition in Ho0.5Nd0.5Fe3(BO3)4 to 9K in comparison with 5K in HoFe3(BO3)4 was explained. The calculations were performed using a molecular-field approximation and a crystal-field model for the rare-earth subsystem. A good agreement between the experimental and calculated temperature dependences of magnetic susceptibility and heat capacity has been achieved.