Crystal Growth,magnetic Transitions And Magnetoelectric Properties Of Quasi One-dimensional Spin Chain Magnetic Materials

Low-dimensional magnetic material is an ideal model system for studying magnetism in condensed matters.In this system,the exchange interactions between magnetic ions are relatively simple for computation and theoretical calculation.On the other hand,quantum fluctuation and magnetic frustration in a low-dimensional system give rise to exotic ground state,phase transitions and excited quantum states,such as Haldane gap,spin liquid,spin-Peierls transition and magnetization plateau.Therefore,study on the low-dimensional magnetic material is helpful and important to understand various magnetic phenomena and quantum effects in condensed matters.In this work,we mainly investigate several quasi-one-dimensional spin-chain materials including the Ising spin-chaina-CoV₂O₆,the zigzag spin-chains FeNbO₄ and MnWO₄.We successfully grew high-quality single crystals of these materials by the flux method.Using the commercial SQUID/PPMS and pulsed high magnetic field facilities,we investigate the low-temperature magnetization steps ina-CoV₂O₆,magnetic phase transitions in FeNbO₄and multiferroic properties in MnWO₄.The thesis is organized as follows:In chapter 1,we introduce background of the low-dimensional spin system and some relative physical phenomena such as frustration,magnetization plateau,Haldane gap,field-induced multiferroicity.Recent progress on several low-dimensional materials are briefly reviewed.In chapter 2,we introduce the single-crystal growth,measurement principles and techniques in pulsed high magnetic fields,which include magnetization,electric polarization,electron spin resonance and magnetocaloric effect.In chapter 3,we focus our interest on the 1/3 magnetization plateau in the Ising spin-chain compounda-CoV₂O₆.High-quality crystals were grown by the molten salt method.The M(H)curves show thata-CoV₂O₆ exhibits one-third magnetization plateaus at 5-13 K when field is applied along the c axis.Through measurements in various temperatures,we construct an overall H-T phase diagram ofa-CoV₂O₆ and observe clear evidence of multiple steps in magnetization below 5 K.The evolution of the 1/3-plateau is further studied via rotation of magnetic field directions.The results show that(1)when field is rotated within the ab plane the plateau disappears completely;(2)when field deviates from the c axis to the ab plane,the critical fields of the plateau increase gradually together with decrease of the magnetic moments of this plateau.From the ESR measurements,we deduce a large g-factor(g=8.9),indicating a strong spin-orbital coupling ina-CoV₂O₆.Thus,the temperature and field-angular dependences of 1/3magnetization plateau are originated from this spin-orbital coupling effect.In chapter 4,we employ both DC and pulsed fields to study the 1/3 magnetization plateau and multiple steps ina-CoV₂O₆.Magnetization in pulsed fields is quite different from that in DC fields,showing multiple steps in an intermediate range of 4.2-6 K,negative magnetization below 4.2 K and asymmetric magnetization in negative fields below 11 K.We demonstrate that these unusual behaviors in magnetization are caused by the spin dynamics and the anomalous magnetocaloric effect(MCE)ina-CoV₂O₆,i.e.,abrupt changes of sample temperature in adiabatic conditions.We successfully separate the influence between the intrinsic slow spin dynamics and the quasi-extrinsic temperature change.From the MCE data,we find that some irreversible behavior is originated from the slow spin dynamics.Two different slow dynamics associated with the metastable steps are observed:one is sensitive to the slow field sweep rate at the order of?m T/s and weakly temperature dependent,while the other responds to the rapid field sweep rate of?k T/s and dominates at lowest temperature.We also distinguish that the metastable transition at H₄ is the first order and crucial for the ferrimagnetic to ferromagnetic transition.This study is useful to the understanding of multistep magnetization ina-CoV₂O₆ and sheds light on recent experimental findings in Ca₃Co₂O₆ and related compounds.In chapter 5,we investigate the magnetic anisotropy and phase transitions in zigzag chain FeNbO₄.Single crystals of FeNbO₄ and isostructural Ni WO₄ were grown by a flux method.Magnetic susceptibility and specific heat show that FeNbO₄ undergoes antiferromagnetic orderings below 44 K.High-field magnetization data reveal a spin-flop transition at 12 T when magnetic field is applied along the magnetic easy axis(a-axis).ESR study on the single crystals reveals three resonance modes which can be described by the molecule-field theory.The zero-frequency resonance field at 11 T corresponds to the spin-flop transition,demonstrating a close correlation between the high-field magnetization and the ESR spectra.In chapter 6,we mainly study the magnetic phase transitions and multiferroic properties in the zigzag chain MnWO₄.This compounds also belongs to type II multiferroic material.High magnetic field can induce low(AF2)and high(IV)ferroelectric phases when field is applied along the magnetic easy axis(H//x).An interesting issue is that these two ferroelectric phases are always opposite even if different bias voltages are applied.This result is puzzling and still a matter of fact till now.We grew MnWO₄ single crystals by the flux method.By means of magnetization and electric polarization in pulsed fields,we summarize the H-T phase diagrams for the three crystallographic directions.We further investigate the AF2 and IV phases via rotating magnetic field directions:for 30°<q<55°,the polarization of the high-field IV phase is opposite to that of AF2,whereas they change to have the same direction for 55°<q<75°.This is manifest that MnWO₄ is a low-dimensional frustrated magnet with a chirality.Under control of bias voltage,we further study the behaviors of polarization reversal in this compound.An unusual magnetoelectric memory effect is revealed similar to the observations in Ni₃V₂O₈.