Magnetic Properties And Exchange Bias Of (Mn_1-xCo_x)₆₅Sn₃₅ Alloy Ribbons

The Ni₂In-type(Mn_1-xCo_x)₆₅Sn₃₅ series alloys undergoes a second-order magnetic phase transition from ferromagnetic/ferromagnetic state to paramagnetic state near the Curie temperature.The temperature range of phase transition is very wide and the magnetic properties are easily controlled by Co-content.There exists a large magnetocaloric effect near phase transition,which has potential applications in the field of magnetic refrigeration.Giant exchange bias effect has been found in the field-cooling samples below the blocking temperature T_B.In this paper we focus on the magnetocaloric effect and exchange bias effect of(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons from the perspective of basic research.The main contents are as following:In(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons,the content of Co atoms has a profound effect on the structural and magnetic properties.The radius of Co atom is slightly smaller than that of Mn atom,and both of their electronegativity and electron concentration are similar.Therefore,the Co atom has a high solid solubility in the alloys and can maintain a good single phase at x=0.3.The cell volume shrinks and lattice constants decrease with the introduction of Co atom.The preferential occupancy order of Co atoms in the lattice has a direct influence on Curie temperature and magnetic moment of the alloys.The magnetic transition about(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons refers to the second-order ferromagnetic/ferrimagnetic to paramagnetic transition accompanied by the change of magnetic order degree and a large magnetocaloric effect.The Curie temperature varies in the temperature range of 88-216 K with the change of Co content in this alloy ribbons.In this paper,the isothermal magnetization curves were measured in the(Mn_0.95Co_0.05)₆₅Sn₃₅5 and(Mn_0.7Co_0.3)₆₅Sn₃₅5 alloy ribbons around the Curie temperature.Then,the corresponding magnetic entropy change was calculated by use the Maxwell relation.The peak values of maximum magnetic entropy change under?₀?H=0-7 T magnetic field change are-1.96 and 4.75 Jkg^-1K^-1,respectively.The refrigeration temperature range is 40 K and 35 K,respectively.The values of refrigeration capacity reaches up to 166.2 and 78.4Jkg^-1.These values indicate that these alloys have a great potential in the magnetic refrigeration.A splitting has been observed at low temperature of the thermomagnetic curves of the(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons.There is a blocking temperature on the ZFC M-T curves.Below the blocking temperature,variation in magnetization is a little while variation in magnetization is rapid above the blocking temperature.It is known from AC susceptibility that this phenomenon show a characteristic that the alloys have a spin-glass-like state at a low temperature.Under the modes of field cooling,exchange bias effect has been observed below the blocking temperature of the(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons.The spin-glass-like state and exchange bias effect of alloys are related to the occupancy of Co atoms in alloys.When Co-substitution is below x=0.15,Co atoms occupy 2?d?sites merely.The alloy exhibits complete spin disorder below the blocking temperature.Exchange bias originates from the interfacial interaction between ferrimagnetic and spin glass state.Around x=0.18,there appears the ferrimagnetic compensation.The Curie temperature and saturated magnetization shows the minimal velue.In this case,there exists the multiple interaction among the spin glass,ferrimagnetic,and antiferromagnetic state,which induces the maximal exchange bias effect.With further increasing Co-substitution,Co atoms start to occupy 2?a?sites simultaneously.The Co-Co pairs give rise to the appearance of ferromagnetic domains,for which the exchange bias effect is attributed to the interfacial interaction between ferromagnetic and spin glass state.Thus,both the exchange bias and coercive fields reduce gradually.In this work,the temperature effect,the influence of mximum measurement field,cooling-field effect,the magnetic training effect and the effect of co-doping with other elements on the exchange bias in(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons have been systematically investigated.The temperature range of the exchange bias of the alloy system depends on the blocking temperature of the spin glass state.The maximum value of the coercive field also appears near the blocking temperature.When the measurement field is small,small loop effects tend to occur.Under the condition of constant temperature,the bias field of the alloy increases firstly and then decreases with the change of cooling field,and reaches the maximum value at 0.2 T.The magnetic trainning effect decreases with the increase of the number of cycles,which is consistent with the Bink empirical formula.Co-doping of Fe with Co can lower the Curie temperature of the alloy and the Ni-replacement introduces an antiferromagnetic interaction into the alloy,which have little effect at increasing the exchange bias field.In addition to this,the paper accumulated a wealth of experience in exploring the exchange bias effects of(Mn_1-xCo_x)₆₅Sn₃₅ alloy ribbons.