According to the Catholic magisterium, are sedevacantists by definition schismatics?
The short answer is yes.
Under Canon Law schism is a crime against religion and the unity of the Catholic Church and thus a sedevacantist incurs a latae sententiae excommunication (c. 1364.1).
Sedevacantism is a word not found in Canon Law because it does not have a term for Catholics who refuse to summit to the authority of the pope, or the bishops he has appointed as cardinals. Canon 751 tells us that schism is the withdrawal of submission to the pope or from communion of the members of the Church subject to the Supreme Pontiff. This is were sedevacantists fit in.
Can. 751 Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him.
The reason you do not hear about Sedevacantists being excommunicated rests in the fact that the penalty of schism is incurred automaticly in canon law. For a Catholic to incur a latae sententiae pentalty all the conditions in canon 1323 must be met first. The concept of latae sententiae has no parallel in modern criminal law in most countries; but in a nutshell, a person who incurs a latae sententiae penalty does so ipso facto, without any judge or ecclesiastical authority imposing it on him.
In the 1983 Code of Canon Law (CIC) eight other sins carry the penalty of automatic excommunication: apostasy, heresy, schism (CIC 1364:1), violating the sacred species (CIC 1367), physically attacking the pope (CIC 1370:1), sacramentally absolving an accomplice in a sexual sin (CIC 1378:1), consecrating a bishop without authorization (CIC 1382), and directly violating the seal of confession (1388:1). - Apart from abortion, are there other sins that incur automatic excommunication?
From a Catholic perspective this is a logical conclusion when dealing with sedevacantists. How can one be in full communion with the Universal Catholic Church, if you refuse to acknowledge the pope, the Magisterium of the Church and the leaders of the Catholic Church and yet remain in full communion with the entire Catholic Church.
Sources and further reading:
Can You Be a Catholic and a Sedevacantist?
Decree Remitting the Excommunication "Latae Sententiae" of the Bishops of the Society of St. Pius X
Have Pro-Abortion Politicians Excommunicated Themselves?