Spontaneous Spin-reorientation Transition Research Articles

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.

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.

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.

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.

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.

Magnetic properties of the Nd0.95Dy0.05Fe3(BO3)4 ferroborate with small substitution in the rare-earth element subsystem

The magnetic properties of a substituted Nd0.95Dy0.05Fe3(BO3)4 ferroborate single crystal with competing Nd-Fe and Dy-Fe exchange interactions are studied experimentally and theoretically. A spontaneous spin-reorientation transition is detected near T = 4.3 K, and anomalies are observed in the low-temperature magnetization curves along trigonal axis c and in basal plane ab. The measured properties and the detected effects are interpreted in terms of a general theoretical approach, which is based on the molecular field approximation and crystal field calculations for a rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility in the range 2–300 K, the anomalies in the magnetization curves for B ‖ c and B ⊥ c in fields up to 1.5 T, and the field and temperature dependences of magnetization in fields up to 9 T are described. The effect of small substitution in the rare-earth subsystem on the magnetic properties is analyzed. The crystal field parameters and the parameters of the R-Fe and Fe-Fe exchange interactions are determined from the experimental data.

Magnetic Properties of Nd&lt;sub&gt;0.6&lt;/sub&gt;Dy&lt;sub&gt;0.4&lt;/sub&gt;Fe&lt;sub&gt;3&lt;/sub&gt;(Bo&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;

The magnetic properties of trigonal Nd0.6Dy0.4Fe3(BO3)4 substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated. It has been shown that in Nd0.6Dy0.4Fe3(BO3)4 a spontaneous spin-reorientation transition from an ease-axis state to an easy-plane occurs near 31 K. Anomalies of the magnetization curves are observed in a spin-flop transition induced by the magnetic field B||c. The calculations were performed using a molecular-field approximation and a crystal-field model for the rare-earth subsystem. Extensive experimental data on the magnetic properties of Nd0.6Dy0.4Fe3(BO3)4 have been interpreted and good agreement between theory and experiment has been achieved using the obtained theoretical dependences.

Magnetic properties of Nd 0.9Dy 0.1Fe 3(BO 3) 4

The magnetic properties of trigonal Nd 0.9Dy 0.1Fe 3(BO 3) 4 substituted compound with the competitive Nd–Fe and Dy–Fe exchange interactions have been investigated. It has been shown that in Nd 0.9Dy 0.1Fe 3(BO 3) 4 a spontaneous spin-reorientation transition from an ease-axis state to an easy-plane occurs near 8 K. Anomalies of the magnetization curves are observed in a spin-flop transition induced by the magnetic field B‖c. The calculations were performed using a molecular-field approximation and a crystal-field model for the rare-earth subsystem. Extensive experimental data on the magnetic properties of Nd 0.9Dy 0.1Fe 3(BO 3) 4 have been interpreted and good agreement between theory and experiment has been achieved using the obtained theoretical dependences.

Magnetic properties of HoFe3(BO3)4

The magnetic properties of an antiferromagnet with trigonal symmetry, namely, HoFe3(BO3)4, have been investigated theoretically. The calculations have been performed in the molecular field approximation and in the framework of the crystal field model for the rare-earth subsystem. Extensive experimental data on the magnetic properties of HoFe3(BO3)4 have been interpreted and good agreement between theory and experiment has been achieved using the obtained theoretical dependences. The spontaneous spin-reorientation transition and the spin-reorientation transition induced by a magnetic field B ‖ a from the easy-axis to easy-plane state, as well as the spin-flop transition in a magnetic field B ‖ c, have been described. It has been shown that the spontaneous spin-reorientation transition is a magnetic analog of the Jahn-Teller effect. The temperature dependences of the initial magnetic susceptibility at temperatures ranging from 2 to 300 K, the nonlinear curves of magnetization for B ‖ c and B ⊥ c in a magnetic field up to 1.2 T (which indicate the occurrence of first-order phase transitions), and their evolution with variations in the temperature have been described, as well as the temperature and field dependences of the magnetization in a magnetic field up to 9 T. The parameters of the trigonal crystal field for the rare-earth ion Ho3+ and the parameters of the Fe-Fe and Ho-Fe exchange interactions have been determined in the course of interpretation of the experimental data.

The influence of spontaneous and field induced spin reorientation transitions on the magnetocaloric properties in rare earth intermetallic compounds: Application to TbZn

We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type TbZn compound. Two successive peaks in the magnetocaloric quantities are observed and attributed to different types of phase transitions. For the magnetic field applied in the ⟨110⟩ direction, the first peak is ascribed to a spontaneous first-order spin reorientation transition (SRT) at T1=63 K, and the second one to the ferroparamagnetic phase transition. The application of an external magnetic field of 2 T along this direction leads to a tablelike behavior in the magnetocaloric quantities (ΔST and ΔTS) as a consequence of two successive second-order SRTs at TSR1=71 K and at TSR2=160 K. Applying a magnetic field of 5 T suppress the flat behavior but a high refrigeration capacity of 352 J/kg is predicted in a wide temperature range from 62 to 258 K. When the magnetic field is applied along the ⟨100⟩ direction an inverse magnetocaloric effect is observed in the temperature range below T1. The system was studied throughout a Hamiltonian that takes into account the Zeeman, exchange, and crystal field interactions.

Observation of spontaneous spin reorientation in Nd1 − x Dy x Fe3(BO3)4 ferroborates with a competitive R-Fe exchange

The magnetic, magnetoelectric, and magnetoelastic properties of DyFe3(BO3)4 single crystals and the Nd0.75Dy0.25Fe3(BO3)4 substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated in magnetic fields up to 210 kOe. It has been shown that the antiferromagnetic state in Nd0.75Dy0.25Fe3(BO3)4 below the Neel temperature is easy-plane and a spontaneous spin-reorientation transition to a uniaxial state occurs near 25 K. Anomalies are observed in various physical characteristics in a spin-flop transition induced by the magnetic field along the trigonal axis. The H-T phase diagrams are constructed; they are in good agreement with a simple model taking into account the anisotropy of the exchange splitting and g factors of the ground doublets of the Dy3+ and Nd3+ ions. The features of the magnetoelectric effect in various magnetic phases have been studied. Spontaneous electric polarization has been revealed in the region of the existence of an easy-plane antiferromagnetic state in the substituted compound. A qualitative theoretical justification of the possibility of the appearance of the spontaneous spin-reorientation transition in the Nd0.75Dy0.25Fe3(BO3)4 substituted compound is given.

Magnetic properties of Lu 2Co 17−xSi x single crystals

The Co-sublattice anisotropy in Lu 2Co 17 consists of four competitive contributions from Co atoms at crystallographically different sites in the Th 2Ni 17-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu 2Co 17− x Si x ( x=0−3.4) grown by the Czochralski method. The SRT in Lu 2Co 17 was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T SR1≈680 K and “easy-cone”–“easy-axis” at T SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu 2Co 16Si ( T SR1≈75 K and T SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu 2Co 17− x Si x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.

Magnetic anisotropy of Tb 1−xGd xMn 6Sn 6 compounds

Magnetization curves of Tb 1− x Gd x Mn 6Sn 6 compounds (0⩽ x⩽1) have been measured for aligned powder samples in the temperature range 4.2–300 K in pulsed magnetic fields up to 30 T. Temperature and concentration dependences of the magnetocrystalline anisotropy constants K 1 and K 2 and concentration dependence of the temperature of spontaneous spin-reorientation transition have been determined. Using these data, we estimated the contribution of the manganese and terbium atoms to the magnetic anisotropy of Tb 1− x Gd x Mn 6Sn 6 and analyzed the origin of the appearance of field-induced first-order magnetic phase transition in these compounds.

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe3(BO3)4

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe3(BO3)4 was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.

Fe spin structure in Tb/Fe multilayers

The Fe spin structure in ultrahigh-vacuum-deposited hcp Tb(t Tb )/bcc Fe(t Fe ) multilayers (t Tb and t Fe are the Tb and Fe layer thicknesses respectively) with perpendicular magnetic anisotropy (PMA) and large coercivity at low temperatures was investigated by 57 Fe Mossbauer spectroscopy, the polar magneto-optical Kerr effect, and superconducting quantum interference device magnetometry. A spontaneous reversible temperature-driven Fe spin reorientation transition from orientation parallel to the film plane to an out-of plane orientation (with Fe spins tilted by an angle relative to the film normal direction) was observed upon cooling below the reorientation temperature T R . At a fixed value of t Tb , T R was found to follow a t -1 Fe behaviour, indicating that the PMA originates from the interfaces. Upon cooling, the perpendicular remanent magnetization shows a step-like increase near the magnetic ordering temperature of Tb, contrary to (T), which exhibits no such anomaly. Apparently, strong coupling of Fe layers via magnetic Tb layers affects the perpendicular magnetic domain structure at remanence. Further, 57 Fe probe layer Mossbauer results concerning structure, composition and Fe spin orientation within Fe-on-Tb and Tb-on-Fe interfaces are reported.

Influence of external fields on spin reorientation transitions in uniaxial ferromagnets. II. Ultrathin ferromagnetic films

The field-dependent spin reorientation transition (SRT) in ultrathin ferromagnetic films is discussed. A rather general treatment is presented which involves the extension of the anisotropy-flow concept to systems in an applied magnetic field. Special features of field-induced SRT's are deduced from the general analysis. Emphasis is laid on the experimental implications, whereby general features are quantitatively described which are as characteristic as fingerprints and serve to set up a natural classification of the SRT's in external fields. Special attention is dedicated to resolving the substantial differences between in-field and spontaneous SRT's; ignoring these differences may result in grave misinterpretations of experimental findings.

Thermal expansion and magnetoelastic coupling in single-crystal HoCo 3Ni 2

The thermal expansion of single-crystal HoCo 3Ni 2, which belongs to the uniaxial hexagonal hard magnets RECo 5, has been measured along the a- and c-axes in the temperature range 6.5–270 K. The spontaneous spin-reorientation transition that occurs in HoCo 3Ni 2 between T SR1 = 160 K and T SR2 = 200 K is found to be accompanied by appreciable changes in the lattice parameters. The magnetostriction constants λ α,0 1, λ α,0 2, λ α,2 1 and λ α,2 2 of HoCo 3Ni 2 are determined and compared with prediction of the point-charge model.